store and expose parent_thread_id on Threads (#25113)

## Why

This PR
https://github.com/openai/codex/pull/24161#discussion_r3325692763
revealed a subagent data modeling issue, where we overloaded
`forked_from_id` to also mean `parent_thread_id`. That's incorrect since
guardian and review subagents can be a subagent and NOT fork the main
thread's history.

The solution here is to explicitly store a new `parent_thread_id` on
`SessionMeta`, alongside `forked_from_id` which already exists. While
we're at it, also expose it in the app-server protocol on the `Thread`
object.

A thread->subagent relationship and a fork of thread history are
orthogonal concepts.

## What Changed

- Added top-level `parent_thread_id` persistence on `SessionMeta` and
runtime/session plumbing through `SessionConfiguredEvent`,
`CodexSpawnArgs`, `SessionConfiguration`, `ThreadConfigSnapshot`,
`TurnContext`, and `ModelClient`.
- Made turn metadata, request headers, analytics, and subagent-start
events read the separate runtime/top-level parent field instead of
deriving general parent lineage from `SessionSource` or
`forked_from_thread_id`.
- Passed parent lineage separately at delegated subagent, review,
guardian, agent-job, and multi-agent spawn construction sites;
copied-history fork lineage remains derived only from `InitialHistory`.
- Persisted and exposed parent lineage through rollout/thread-store
projections and app-server v2 `Thread.parentThreadId`.
- Updated app-server README text and regenerated app-server schema
fixtures for the additive `parentThreadId` response field.

codex-rs/analytics/src/analytics_client_tests.rs

@@ -160,11 +160,13 @@ fn sample_thread_with_metadata(
     ephemeral: bool,
     source: AppServerSessionSource,
     thread_source: Option<AppServerThreadSource>,
+    parent_thread_id: Option<String>,
 ) -> Thread {
     Thread {
         id: thread_id.to_string(),
         session_id: format!("session-{thread_id}"),
         forked_from_id: None,
+        parent_thread_id,
         preview: "first prompt".to_string(),
         ephemeral,
         model_provider: "openai".to_string(),
@@ -195,6 +197,7 @@ fn sample_thread_start_response(
             ephemeral,
             AppServerSessionSource::Exec,
             Some(AppServerThreadSource::User),
+            /*parent_thread_id*/ None,
         ),
         model: model.to_string(),
         model_provider: "openai".to_string(),
@@ -240,6 +243,7 @@ fn sample_thread_resume_response(
         model,
         AppServerSessionSource::Exec,
         Some(AppServerThreadSource::User),
+        /*parent_thread_id*/ None,
     )
 }
 
@@ -249,9 +253,16 @@ fn sample_thread_resume_response_with_source(
     model: &str,
     source: AppServerSessionSource,
     thread_source: Option<AppServerThreadSource>,
+    parent_thread_id: Option<String>,
 ) -> ClientResponsePayload {
     ClientResponsePayload::ThreadResume(ThreadResumeResponse {
-        thread: sample_thread_with_metadata(thread_id, ephemeral, source, thread_source),
+        thread: sample_thread_with_metadata(
+            thread_id,
+            ephemeral,
+            source,
+            thread_source,
+            parent_thread_id,
+        ),
         model: model.to_string(),
         model_provider: "openai".to_string(),
         service_tier: None,
@@ -1755,6 +1766,7 @@ async fn compaction_event_ingests_custom_fact() {
                         agent_role: None,
                     }),
                     Some(AppServerThreadSource::Subagent),
+                    Some(parent_thread_id.to_string()),
                 )),
             },
             &mut events,
@@ -2456,7 +2468,7 @@ fn subagent_thread_started_thread_spawn_serializes_parent_thread_id() {
         SubAgentThreadStartedInput {
             session_id: "session-root".to_string(),
             thread_id: "thread-spawn".to_string(),
-            parent_thread_id: None,
+            parent_thread_id: Some(parent_thread_id.to_string()),
             product_client_id: "codex-tui".to_string(),
             client_name: "codex-tui".to_string(),
             client_version: "1.0.0".to_string(),
@@ -2534,11 +2546,14 @@ fn subagent_thread_started_other_serializes_expected_shape() {
 
 #[test]
 fn subagent_thread_started_other_serializes_explicit_parent_thread_id() {
+    let parent_thread_id =
+        codex_protocol::ThreadId::from_string("33333333-3333-4333-8333-333333333333")
+            .expect("valid thread id");
     let event = TrackEventRequest::ThreadInitialized(subagent_thread_started_event_request(
         SubAgentThreadStartedInput {
             session_id: "session-root".to_string(),
             thread_id: "thread-guardian".to_string(),
-            parent_thread_id: Some("parent-thread-guardian".to_string()),
+            parent_thread_id: Some(parent_thread_id.to_string()),
             product_client_id: "codex-tui".to_string(),
             client_name: "codex-tui".to_string(),
             client_version: "1.0.0".to_string(),
@@ -2553,7 +2568,7 @@ fn subagent_thread_started_other_serializes_explicit_parent_thread_id() {
     assert_eq!(payload["event_params"]["subagent_source"], "guardian");
     assert_eq!(
         payload["event_params"]["parent_thread_id"],
-        "parent-thread-guardian"
+        "33333333-3333-4333-8333-333333333333"
     );
 }
 
@@ -2642,7 +2657,7 @@ async fn subagent_thread_started_inherits_parent_connection_for_new_thread() {
                 SubAgentThreadStartedInput {
                     session_id: "session-root".to_string(),
                     thread_id: "thread-review".to_string(),
-                    parent_thread_id: None,
+                    parent_thread_id: Some(parent_thread_id.to_string()),
                     product_client_id: "parent-client".to_string(),
                     client_name: "parent-client".to_string(),
                     client_version: "1.0.0".to_string(),

codex-rs/analytics/src/client_tests.rs

@@ -124,6 +124,7 @@ fn sample_thread(thread_id: &str) -> Thread {
         id: thread_id.to_string(),
         session_id: format!("session-{thread_id}"),
         forked_from_id: None,
+        parent_thread_id: None,
         preview: "first prompt".to_string(),
         ephemeral: false,
         model_provider: "openai".to_string(),

codex-rs/analytics/src/events.rs

@@ -1048,9 +1048,7 @@ pub(crate) fn subagent_thread_started_event_request(
         thread_source: Some(ThreadSource::Subagent),
         initialization_mode: ThreadInitializationMode::New,
         subagent_source: Some(subagent_source_name(&input.subagent_source)),
-        parent_thread_id: input
-            .parent_thread_id
-            .or_else(|| subagent_parent_thread_id(&input.subagent_source)),
+        parent_thread_id: input.parent_thread_id,
         created_at: input.created_at,
     };
     ThreadInitializedEvent {
@@ -1060,19 +1058,7 @@ pub(crate) fn subagent_thread_started_event_request(
 }
 
 pub(crate) fn subagent_source_name(subagent_source: &SubAgentSource) -> String {
-    match subagent_source {
-        SubAgentSource::Review => "review".to_string(),
-        SubAgentSource::Compact => "compact".to_string(),
-        SubAgentSource::ThreadSpawn { .. } => "thread_spawn".to_string(),
-        SubAgentSource::MemoryConsolidation => "memory_consolidation".to_string(),
-        SubAgentSource::Other(other) => other.clone(),
-    }
-}
-
-pub(crate) fn subagent_parent_thread_id(subagent_source: &SubAgentSource) -> Option<String> {
-    subagent_source
-        .parent_thread_id()
-        .map(|parent_thread_id| parent_thread_id.to_string())
+    subagent_source.kind().to_string()
 }
 
 fn analytics_hook_status(status: HookRunStatus) -> HookRunStatus {

codex-rs/analytics/src/reducer.rs

@@ -55,7 +55,6 @@ use crate::events::codex_hook_run_metadata;
 use crate::events::codex_plugin_metadata;
 use crate::events::codex_plugin_used_metadata;
 use crate::events::plugin_state_event_type;
-use crate::events::subagent_parent_thread_id;
 use crate::events::subagent_source_name;
 use crate::events::subagent_thread_started_event_request;
 use crate::facts::AnalyticsFact;
@@ -267,20 +266,18 @@ impl ThreadMetadataState {
         session_id: String,
         session_source: &SessionSource,
         thread_source: Option<ThreadSource>,
+        parent_thread_id: Option<String>,
         initialization_mode: ThreadInitializationMode,
     ) -> Self {
-        let (subagent_source, parent_thread_id) = match session_source {
-            SessionSource::SubAgent(subagent_source) => (
-                Some(subagent_source_name(subagent_source)),
-                subagent_parent_thread_id(subagent_source),
-            ),
+        let subagent_source = match session_source {
+            SessionSource::SubAgent(subagent_source) => Some(subagent_source_name(subagent_source)),
             SessionSource::Cli
             | SessionSource::VSCode
             | SessionSource::Exec
             | SessionSource::Mcp
             | SessionSource::Custom(_)
             | SessionSource::Internal(_)
-            | SessionSource::Unknown => (None, None),
+            | SessionSource::Unknown => None,
         };
         Self {
             session_id,
@@ -516,10 +513,7 @@ impl AnalyticsReducer {
         input: SubAgentThreadStartedInput,
         out: &mut Vec<TrackEventRequest>,
     ) {
-        let parent_thread_id = input
-            .parent_thread_id
-            .clone()
-            .or_else(|| subagent_parent_thread_id(&input.subagent_source));
+        let parent_thread_id = input.parent_thread_id.clone();
         let parent_connection_id = parent_thread_id
             .as_ref()
             .and_then(|parent_thread_id| self.threads.get(parent_thread_id))
@@ -1238,6 +1232,7 @@ impl AnalyticsReducer {
         let session_source: SessionSource = thread.source.into();
         let session_id = thread.session_id;
         let thread_id = thread.id;
+        let parent_thread_id = thread.parent_thread_id;
         let Some(connection_state) = self.connections.get(&connection_id) else {
             return;
         };
@@ -1245,6 +1240,7 @@ impl AnalyticsReducer {
             session_id.clone(),
             &session_source,
             thread.thread_source.map(Into::into),
+            parent_thread_id,
             initialization_mode,
         );
         self.threads.insert(

codex-rs/app-server-protocol/schema/json/ServerNotification.json

@@ -3366,6 +3366,13 @@
             "null"
           ]
         },
+        "parentThreadId": {
+          "description": "The ID of the parent thread. This will only be set if this thread is a subagent.",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "path": {
           "description": "[UNSTABLE] Path to the thread on disk.",
           "type": [

codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.schemas.json

@@ -15513,6 +15513,13 @@
               "null"
             ]
           },
+          "parentThreadId": {
+            "description": "The ID of the parent thread. This will only be set if this thread is a subagent.",
+            "type": [
+              "string",
+              "null"
+            ]
+          },
           "path": {
             "description": "[UNSTABLE] Path to the thread on disk.",
             "type": [

codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.v2.schemas.json

@@ -13337,6 +13337,13 @@
             "null"
           ]
         },
+        "parentThreadId": {
+          "description": "The ID of the parent thread. This will only be set if this thread is a subagent.",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "path": {
           "description": "[UNSTABLE] Path to the thread on disk.",
           "type": [

codex-rs/app-server-protocol/schema/json/v2/ThreadForkResponse.json

@@ -1036,6 +1036,13 @@
             "null"
           ]
         },
+        "parentThreadId": {
+          "description": "The ID of the parent thread. This will only be set if this thread is a subagent.",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "path": {
           "description": "[UNSTABLE] Path to the thread on disk.",
           "type": [

codex-rs/app-server-protocol/schema/json/v2/ThreadListResponse.json

@@ -851,6 +851,13 @@
             "null"
           ]
         },
+        "parentThreadId": {
+          "description": "The ID of the parent thread. This will only be set if this thread is a subagent.",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "path": {
           "description": "[UNSTABLE] Path to the thread on disk.",
           "type": [

codex-rs/app-server-protocol/schema/json/v2/ThreadMetadataUpdateResponse.json

@@ -851,6 +851,13 @@
             "null"
           ]
         },
+        "parentThreadId": {
+          "description": "The ID of the parent thread. This will only be set if this thread is a subagent.",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "path": {
           "description": "[UNSTABLE] Path to the thread on disk.",
           "type": [

codex-rs/app-server-protocol/schema/json/v2/ThreadReadResponse.json

@@ -851,6 +851,13 @@
             "null"
           ]
         },
+        "parentThreadId": {
+          "description": "The ID of the parent thread. This will only be set if this thread is a subagent.",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "path": {
           "description": "[UNSTABLE] Path to the thread on disk.",
           "type": [

codex-rs/app-server-protocol/schema/json/v2/ThreadResumeResponse.json

@@ -1036,6 +1036,13 @@
             "null"
           ]
         },
+        "parentThreadId": {
+          "description": "The ID of the parent thread. This will only be set if this thread is a subagent.",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "path": {
           "description": "[UNSTABLE] Path to the thread on disk.",
           "type": [

codex-rs/app-server-protocol/schema/json/v2/ThreadRollbackResponse.json

@@ -851,6 +851,13 @@
             "null"
           ]
         },
+        "parentThreadId": {
+          "description": "The ID of the parent thread. This will only be set if this thread is a subagent.",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "path": {
           "description": "[UNSTABLE] Path to the thread on disk.",
           "type": [

codex-rs/app-server-protocol/schema/json/v2/ThreadStartResponse.json

@@ -1036,6 +1036,13 @@
             "null"
           ]
         },
+        "parentThreadId": {
+          "description": "The ID of the parent thread. This will only be set if this thread is a subagent.",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "path": {
           "description": "[UNSTABLE] Path to the thread on disk.",
           "type": [

codex-rs/app-server-protocol/schema/json/v2/ThreadStartedNotification.json

@@ -851,6 +851,13 @@
             "null"
           ]
         },
+        "parentThreadId": {
+          "description": "The ID of the parent thread. This will only be set if this thread is a subagent.",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "path": {
           "description": "[UNSTABLE] Path to the thread on disk.",
           "type": [

codex-rs/app-server-protocol/schema/json/v2/ThreadUnarchiveResponse.json

@@ -851,6 +851,13 @@
             "null"
           ]
         },
+        "parentThreadId": {
+          "description": "The ID of the parent thread. This will only be set if this thread is a subagent.",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "path": {
           "description": "[UNSTABLE] Path to the thread on disk.",
           "type": [

codex-rs/app-server-protocol/schema/typescript/v2/Thread.ts

@@ -17,6 +17,10 @@ sessionId: string,
  * Source thread id when this thread was created by forking another thread.
  */
 forkedFromId: string | null,
+/**
+ * The ID of the parent thread. This will only be set if this thread is a subagent.
+ */
+parentThreadId: string | null,
 /**
  * Usually the first user message in the thread, if available.
  */

codex-rs/app-server-protocol/src/protocol/common.rs

@@ -2328,6 +2328,7 @@ mod tests {
                     id: "67e55044-10b1-426f-9247-bb680e5fe0c8".to_string(),
                     session_id: "67e55044-10b1-426f-9247-bb680e5fe0c7".to_string(),
                     forked_from_id: None,
+                    parent_thread_id: None,
                     preview: "first prompt".to_string(),
                     ephemeral: true,
                     model_provider: "openai".to_string(),
@@ -2370,6 +2371,7 @@ mod tests {
                         "id": "67e55044-10b1-426f-9247-bb680e5fe0c8",
                         "sessionId": "67e55044-10b1-426f-9247-bb680e5fe0c7",
                         "forkedFromId": null,
+                        "parentThreadId": null,
                         "preview": "first prompt",
                         "ephemeral": true,
                         "modelProvider": "openai",

codex-rs/app-server-protocol/src/protocol/v2/tests.rs

@@ -142,6 +142,7 @@ fn thread_resume_response_round_trips_initial_turns_page() {
             id: "thr_123".to_string(),
             session_id: "thr_123".to_string(),
             forked_from_id: None,
+            parent_thread_id: None,
             preview: String::new(),
             ephemeral: false,
             model_provider: "openai".to_string(),
@@ -3581,6 +3582,7 @@ fn thread_lifecycle_responses_default_missing_optional_fields() {
     let fork: ThreadForkResponse = serde_json::from_value(response).expect("thread/fork response");
 
     assert_eq!(start.instruction_sources, Vec::<AbsolutePathBuf>::new());
+    assert_eq!(start.thread.parent_thread_id, None);
     assert_eq!(resume.instruction_sources, Vec::<AbsolutePathBuf>::new());
     assert_eq!(fork.instruction_sources, Vec::<AbsolutePathBuf>::new());
     assert_eq!(start.active_permission_profile, None);

codex-rs/app-server-protocol/src/protocol/v2/thread_data.rs

@@ -108,6 +108,8 @@ pub struct Thread {
     pub session_id: String,
     /// Source thread id when this thread was created by forking another thread.
     pub forked_from_id: Option<String>,
+    /// The ID of the parent thread. This will only be set if this thread is a subagent.
+    pub parent_thread_id: Option<String>,
     /// Usually the first user message in the thread, if available.
     pub preview: String,
     /// Whether the thread is ephemeral and should not be materialized on disk.

codex-rs/app-server/README.md

@@ -134,7 +134,7 @@ Example with notification opt-out:
 - `thread/resume` — reopen an existing thread by id so subsequent `turn/start` calls append to it. Accepts the same permission override rules as `thread/start`.
 - `thread/fork` — fork an existing thread into a new thread id by copying the stored history; if the source thread is currently mid-turn, the fork records the same interruption marker as `turn/interrupt` instead of inheriting an unmarked partial turn suffix. The returned `thread.forkedFromId` points at the source thread when known. Accepts `ephemeral: true` for an in-memory temporary fork, emits `thread/started` (including the current `thread.status`), and auto-subscribes you to turn/item events for the new thread. Experimental clients can pass `excludeTurns: true` when they plan to page fork history via `thread/turns/list` instead of receiving the full turn array immediately. Accepts the same permission override rules as `thread/start`.
 - `thread/start`, `thread/resume`, and `thread/fork` responses include the legacy `sandbox` compatibility projection. Experimental clients can read `runtimeWorkspaceRoots` for the thread-scoped runtime roots and `activePermissionProfile` for the named or implicit built-in profile identity/provenance when known.
-- `thread/list` — page through stored rollouts; supports cursor-based pagination and optional `modelProviders`, `sourceKinds`, `archived`, `cwd`, and `searchTerm` filters. Each returned `thread` includes `status` (`ThreadStatus`), defaulting to `notLoaded` when the thread is not currently loaded.
+- `thread/list` — page through stored rollouts; supports cursor-based pagination and optional `modelProviders`, `sourceKinds`, `archived`, `cwd`, and `searchTerm` filters. Each returned `thread` includes `status` (`ThreadStatus`), defaulting to `notLoaded` when the thread is not currently loaded. Subagent threads also include `parentThreadId` when the immediate control/spawn parent is known.
 - `thread/loaded/list` — list the thread ids currently loaded in memory.
 - `thread/read` — read a stored thread by id without resuming it; optionally include turns via `includeTurns`. The returned `thread` includes `status` (`ThreadStatus`), defaulting to `notLoaded` when the thread is not currently loaded.
 - `thread/turns/list` — experimental; page through a stored thread’s turn history without resuming it; supports cursor-based pagination with `sortDirection`, `itemsView`, `nextCursor`, and `backwardsCursor`.
@@ -424,7 +424,7 @@ Later, after the idle unload timeout:
 
 ### Example: Read a thread
 
-Use `thread/read` to fetch a stored thread by id without resuming it. Pass `includeTurns` when you want thread history loaded into `thread.turns`. The returned thread includes `agentNickname` and `agentRole` for AgentControl-spawned thread sub-agents when available.
+Use `thread/read` to fetch a stored thread by id without resuming it. Pass `includeTurns` when you want thread history loaded into `thread.turns`. The returned thread includes `parentThreadId`, `agentNickname`, and `agentRole` for subagent threads when available.
 
 ```json
 { "method": "thread/read", "id": 22, "params": { "threadId": "thr_123" } }

codex-rs/app-server/src/bespoke_event_handling.rs

@@ -2170,6 +2170,7 @@ mod tests {
             thread_id,
             rollout_path: None,
             forked_from_id: None,
+            parent_thread_id: None,
             preview: "fallback preview".to_string(),
             name: Some("Rollback thread".to_string()),
             model_provider: "openai".to_string(),

codex-rs/app-server/src/request_processors/thread_processor.rs

@@ -4012,6 +4012,7 @@ pub(crate) fn thread_from_stored_thread(
         id: thread_id.clone(),
         session_id: thread_id,
         forked_from_id: thread.forked_from_id.map(|id| id.to_string()),
+        parent_thread_id: thread.parent_thread_id.map(|id| id.to_string()),
         preview: thread.preview,
         ephemeral: false,
         model_provider: if thread.model_provider.is_empty() {
@@ -4220,6 +4221,7 @@ fn build_thread_from_snapshot(
         id: thread_id.to_string(),
         session_id,
         forked_from_id: None,
+        parent_thread_id: config_snapshot.parent_thread_id.map(|id| id.to_string()),
         preview: String::new(),
         ephemeral: config_snapshot.ephemeral,
         model_provider: config_snapshot.model_provider_id.clone(),

codex-rs/app-server/src/request_processors/thread_processor_tests.rs

@@ -395,6 +395,7 @@ mod thread_processor_behavior_tests {
             thread_id,
             rollout_path: Some(PathBuf::from("/tmp/thread.jsonl")),
             forked_from_id: None,
+            parent_thread_id: None,
             preview: "preview".to_string(),
             name: None,
             model_provider: "openai".to_string(),
@@ -681,6 +682,7 @@ mod thread_processor_behavior_tests {
                 },
             },
             session_source: SessionSource::Cli,
+            parent_thread_id: None,
             thread_source: None,
         };
 

codex-rs/app-server/src/request_processors/thread_resume_redaction.rs

@@ -172,6 +172,7 @@ mod tests {
             id: "thread-1".to_string(),
             session_id: "session-1".to_string(),
             forked_from_id: None,
+            parent_thread_id: None,
             preview: "preview".to_string(),
             ephemeral: false,
             model_provider: "mock_provider".to_string(),

codex-rs/app-server/src/request_processors/thread_summary.rs

@@ -305,6 +305,7 @@ pub(crate) fn summary_to_thread(
         id: thread_id.clone(),
         session_id: thread_id,
         forked_from_id: None,
+        parent_thread_id: None,
         preview,
         ephemeral: false,
         model_provider,

codex-rs/app-server/src/thread_status.rs

@@ -891,6 +891,7 @@ mod tests {
             id: thread_id.to_string(),
             session_id: thread_id.to_string(),
             forked_from_id: None,
+            parent_thread_id: None,
             preview: String::new(),
             ephemeral: false,
             model_provider: "mock-provider".to_string(),

codex-rs/app-server/tests/common/lib.rs

@@ -38,6 +38,7 @@ pub use responses::create_final_assistant_message_sse_response;
 pub use responses::create_request_permissions_sse_response;
 pub use responses::create_request_user_input_sse_response;
 pub use responses::create_shell_command_sse_response;
+pub use rollout::create_fake_parented_rollout_with_source;
 pub use rollout::create_fake_rollout;
 pub use rollout::create_fake_rollout_with_source;
 pub use rollout::create_fake_rollout_with_text_elements;

codex-rs/app-server/tests/common/rollout.rs

@@ -118,6 +118,53 @@ pub fn create_fake_rollout_with_source(
     model_provider: Option<&str>,
     git_info: Option<GitInfo>,
     source: SessionSource,
+) -> Result<String> {
+    create_fake_rollout_with_source_and_parent_thread_id(
+        codex_home,
+        filename_ts,
+        meta_rfc3339,
+        preview,
+        model_provider,
+        git_info,
+        source,
+        /*parent_thread_id*/ None,
+    )
+}
+
+/// Create a minimal rollout file with an explicit session source and control parent.
+#[allow(clippy::too_many_arguments)]
+pub fn create_fake_parented_rollout_with_source(
+    codex_home: &Path,
+    filename_ts: &str,
+    meta_rfc3339: &str,
+    preview: &str,
+    model_provider: Option<&str>,
+    git_info: Option<GitInfo>,
+    source: SessionSource,
+    parent_thread_id: ThreadId,
+) -> Result<String> {
+    create_fake_rollout_with_source_and_parent_thread_id(
+        codex_home,
+        filename_ts,
+        meta_rfc3339,
+        preview,
+        model_provider,
+        git_info,
+        source,
+        Some(parent_thread_id),
+    )
+}
+
+#[allow(clippy::too_many_arguments)]
+fn create_fake_rollout_with_source_and_parent_thread_id(
+    codex_home: &Path,
+    filename_ts: &str,
+    meta_rfc3339: &str,
+    preview: &str,
+    model_provider: Option<&str>,
+    git_info: Option<GitInfo>,
+    source: SessionSource,
+    parent_thread_id: Option<ThreadId>,
 ) -> Result<String> {
     let uuid = Uuid::new_v4();
     let uuid_str = uuid.to_string();
@@ -133,6 +180,7 @@ pub fn create_fake_rollout_with_source(
     let meta = SessionMeta {
         id: conversation_id,
         forked_from_id: None,
+        parent_thread_id,
         timestamp: meta_rfc3339.to_string(),
         cwd: PathBuf::from("/"),
         originator: "codex".to_string(),
@@ -217,6 +265,7 @@ pub fn create_fake_rollout_with_text_elements(
     let meta = SessionMeta {
         id: conversation_id,
         forked_from_id: None,
+        parent_thread_id: None,
         timestamp: meta_rfc3339.to_string(),
         cwd: PathBuf::from("/"),
         originator: "codex".to_string(),

codex-rs/app-server/tests/suite/conversation_summary.rs

@@ -124,6 +124,7 @@ async fn get_conversation_summary_by_thread_id_reads_pathless_store_thread() ->
         .create_thread(CreateThreadParams {
             thread_id,
             forked_from_id: None,
+            parent_thread_id: None,
             source: SessionSource::Cli,
             thread_source: None,
             base_instructions: BaseInstructions::default(),

codex-rs/app-server/tests/suite/v2/client_metadata.rs

@@ -1,7 +1,7 @@
 use anyhow::Result;
 use app_test_support::McpProcess;
+use app_test_support::create_fake_parented_rollout_with_source;
 use app_test_support::create_fake_rollout;
-use app_test_support::create_fake_rollout_with_source;
 use app_test_support::to_response;
 use codex_app_server_protocol::JSONRPCResponse;
 use codex_app_server_protocol::RequestId;
@@ -9,6 +9,7 @@ use codex_app_server_protocol::ReviewDelivery;
 use codex_app_server_protocol::ReviewStartParams;
 use codex_app_server_protocol::ReviewStartResponse;
 use codex_app_server_protocol::ReviewTarget;
+use codex_app_server_protocol::SessionSource as ApiSessionSource;
 use codex_app_server_protocol::ThreadForkParams;
 use codex_app_server_protocol::ThreadForkResponse;
 use codex_app_server_protocol::ThreadResumeParams;
@@ -198,7 +199,7 @@ async fn turn_start_sends_fork_lineage_in_turn_metadata_for_thread_fork_v2() ->
 }
 
 #[tokio::test]
-async fn review_start_sends_fork_lineage_in_turn_metadata_for_thread_fork_v2() -> Result<()> {
+async fn review_start_sends_parent_lineage_in_turn_metadata_for_thread_fork_v2() -> Result<()> {
     skip_if_no_network!(Ok(()));
 
     let review_payload = serde_json::json!({
@@ -276,8 +277,9 @@ async fn review_start_sends_fork_lineage_in_turn_metadata_for_thread_fork_v2() -
         request.header("x-openai-subagent").as_deref(),
         Some("review")
     );
+    assert!(metadata.get("forked_from_thread_id").is_none());
     assert_eq!(
-        metadata["forked_from_thread_id"].as_str(),
+        metadata["parent_thread_id"].as_str(),
         Some(review_thread_id.as_str())
     );
     let review_request_thread_id = metadata["thread_id"]
@@ -297,7 +299,7 @@ async fn review_start_sends_fork_lineage_in_turn_metadata_for_thread_fork_v2() -
 }
 
 #[tokio::test]
-async fn turn_start_sends_subagent_lineage_after_cold_thread_resume_v2() -> Result<()> {
+async fn turn_start_sends_other_subagent_lineage_after_cold_thread_resume_v2() -> Result<()> {
     skip_if_no_network!(Ok(()));
 
     let server = responses::start_mock_server().await;
@@ -320,20 +322,15 @@ async fn turn_start_sends_subagent_lineage_after_cold_thread_resume_v2() -> Resu
 
     let parent_thread_id = CoreThreadId::new();
     let parent_thread_id_str = parent_thread_id.to_string();
-    let subagent_thread_id = create_fake_rollout_with_source(
+    let subagent_thread_id = create_fake_parented_rollout_with_source(
         codex_home.path(),
         "2025-01-05T12-00-00",
         "2025-01-05T12:00:00Z",
         "Saved subagent message",
         Some("mock_provider"),
         /*git_info*/ None,
-        SessionSource::SubAgent(SubAgentSource::ThreadSpawn {
-            parent_thread_id,
-            depth: 1,
-            agent_path: None,
-            agent_nickname: None,
-            agent_role: None,
-        }),
+        SessionSource::SubAgent(SubAgentSource::Other("guardian".to_string())),
+        parent_thread_id,
     )?;
 
     let mut mcp = McpProcess::new(codex_home.path()).await?;
@@ -352,6 +349,11 @@ async fn turn_start_sends_subagent_lineage_after_cold_thread_resume_v2() -> Resu
     .await??;
     let ThreadResumeResponse { thread, .. } = to_response::<ThreadResumeResponse>(resume_resp)?;
     assert_eq!(thread.id, subagent_thread_id);
+    assert_eq!(thread.parent_thread_id, Some(parent_thread_id_str.clone()));
+    assert_eq!(
+        thread.source,
+        ApiSessionSource::SubAgent(SubAgentSource::Other("guardian".to_string()))
+    );
 
     let turn_req = mcp
         .send_turn_start_request(TurnStartParams {
@@ -386,7 +388,7 @@ async fn turn_start_sends_subagent_lineage_after_cold_thread_resume_v2() -> Resu
         metadata["parent_thread_id"].as_str(),
         Some(parent_thread_id_str.as_str())
     );
-    assert_eq!(metadata["subagent_kind"].as_str(), Some("thread_spawn"));
+    assert_eq!(metadata["subagent_kind"].as_str(), Some("guardian"));
     assert_eq!(metadata["thread_id"].as_str(), Some(thread.id.as_str()));
     assert_eq!(metadata["turn_id"].as_str(), Some(turn.id.as_str()));
     assert!(metadata.get("forked_from_thread_id").is_none());

codex-rs/app-server/tests/suite/v2/thread_list.rs

@@ -1,5 +1,6 @@
 use anyhow::Result;
 use app_test_support::McpProcess;
+use app_test_support::create_fake_parented_rollout_with_source;
 use app_test_support::create_fake_rollout;
 use app_test_support::create_fake_rollout_with_source;
 use app_test_support::create_final_assistant_message_sse_response;
@@ -1049,7 +1050,7 @@ async fn thread_list_filters_by_subagent_variant() -> Result<()> {
 
     let parent_thread_id = ThreadId::from_string(&Uuid::new_v4().to_string())?;
 
-    let review_id = create_fake_rollout_with_source(
+    let review_id = create_fake_parented_rollout_with_source(
         codex_home.path(),
         "2025-02-02T09-00-00",
         "2025-02-02T09:00:00Z",
@@ -1057,6 +1058,7 @@ async fn thread_list_filters_by_subagent_variant() -> Result<()> {
         Some("mock_provider"),
         /*git_info*/ None,
         CoreSessionSource::SubAgent(SubAgentSource::Review),
+        parent_thread_id,
     )?;
     let compact_id = create_fake_rollout_with_source(
         codex_home.path(),
@@ -1109,6 +1111,10 @@ async fn thread_list_filters_by_subagent_variant() -> Result<()> {
         .map(|thread| thread.id.as_str())
         .collect();
     assert_eq!(review_ids, vec![review_id.as_str()]);
+    assert_eq!(
+        review.data[0].parent_thread_id,
+        Some(parent_thread_id.to_string())
+    );
 
     let compact = list_threads(
         &mut mcp,

codex-rs/app-server/tests/suite/v2/thread_read.rs

@@ -1358,6 +1358,7 @@ async fn seed_pathless_store_thread(
         .create_thread(CreateThreadParams {
             thread_id,
             forked_from_id: None,
+            parent_thread_id: None,
             source: ProtocolSessionSource::Cli,
             thread_source: None,
             base_instructions: BaseInstructions::default(),

codex-rs/app-server/tests/suite/v2/thread_resume.rs

@@ -1791,6 +1791,7 @@ stream_max_retries = 0
     let session_meta = SessionMeta {
         id: conversation_id,
         forked_from_id: None,
+        parent_thread_id: None,
         timestamp: "2025-01-05T12:00:00Z".to_string(),
         cwd: repo_path.clone(),
         originator: "codex".to_string(),

codex-rs/app-server/tests/suite/v2/thread_unarchive.rs

@@ -211,6 +211,7 @@ async fn thread_unarchive_preserves_pathless_store_metadata() -> Result<()> {
         .create_thread(CreateThreadParams {
             thread_id,
             forked_from_id: Some(parent_thread_id),
+            parent_thread_id: None,
             source: SessionSource::Cli,
             thread_source: None,
             base_instructions: BaseInstructions::default(),

codex-rs/core/src/agent/agent_resolver.rs

@@ -32,5 +32,5 @@ fn register_session_root(session: &Arc<Session>, turn: &Arc<TurnContext>) {
     session
         .services
         .agent_control
-        .register_session_root(session.conversation_id, &turn.session_source);
+        .register_session_root(session.conversation_id, turn.parent_thread_id);
 }

codex-rs/core/src/agent/control.rs

@@ -54,6 +54,7 @@ pub(crate) enum SpawnAgentForkMode {
 pub(crate) struct SpawnAgentOptions {
     pub(crate) fork_parent_spawn_call_id: Option<String>,
     pub(crate) fork_mode: Option<SpawnAgentForkMode>,
+    pub(crate) parent_thread_id: Option<ThreadId>,
     pub(crate) environments: Option<Vec<TurnEnvironmentSelection>>,
 }
 
@@ -262,12 +263,12 @@ impl AgentControl {
                 .await?
             }
             (Some(session_source), None) => {
-                let forked_from_thread_id = thread_spawn_parent_thread_id(&session_source);
                 Box::pin(state.spawn_new_thread_with_source(
                     config.clone(),
                     self.clone(),
                     session_source,
-                    forked_from_thread_id,
+                    options.parent_thread_id,
+                    /*forked_from_thread_id*/ None,
                     /*thread_source*/ Some(ThreadSource::Subagent),
                     /*persist_extended_history*/ false,
                     /*metrics_service_name*/ None,
@@ -309,6 +310,7 @@ impl AgentControl {
                 }
             };
             let thread_config = new_thread.thread.codex.thread_config_snapshot().await;
+            let parent_thread_id = thread_config.parent_thread_id;
             emit_subagent_session_started(
                 &new_thread
                     .thread
@@ -319,7 +321,7 @@ impl AgentControl {
                 client_metadata,
                 new_thread.thread.codex.session.session_id(),
                 new_thread.thread_id,
-                /*parent_thread_id*/ None,
+                parent_thread_id,
                 thread_config,
                 subagent_source.clone(),
             );
@@ -490,6 +492,7 @@ impl AgentControl {
                 self.clone(),
                 session_source,
                 /*thread_source*/ Some(ThreadSource::Subagent),
+                /*parent_thread_id*/ Some(parent_thread_id),
                 /*forked_from_thread_id*/ Some(parent_thread_id),
                 /*persist_extended_history*/ false,
                 inherited_shell_snapshot,
@@ -638,6 +641,7 @@ impl AgentControl {
             .history
             .ok_or_else(|| CodexErr::ThreadNotFound(thread_id))?
             .items;
+        let parent_thread_id = stored_thread.parent_thread_id;
 
         let resumed_thread = state
             .resume_thread_with_history_with_source(ResumeThreadWithHistoryOptions {
@@ -649,6 +653,7 @@ impl AgentControl {
                 }),
                 agent_control: self.clone(),
                 session_source,
+                parent_thread_id,
                 inherited_shell_snapshot,
                 inherited_exec_policy,
             })
@@ -841,9 +846,9 @@ impl AgentControl {
     pub(crate) fn register_session_root(
         &self,
         current_thread_id: ThreadId,
-        current_session_source: &SessionSource,
+        current_parent_thread_id: Option<ThreadId>,
     ) {
-        if thread_spawn_parent_thread_id(current_session_source).is_none() {
+        if current_parent_thread_id.is_none() {
             self.state.register_root_thread(current_thread_id);
         }
     }
@@ -1218,7 +1223,8 @@ impl AgentControl {
         child_thread_id: ThreadId,
         session_source: Option<&SessionSource>,
     ) {
-        let Some(parent_thread_id) = session_source.and_then(thread_spawn_parent_thread_id) else {
+        let Some(parent_thread_id) = session_source.and_then(SessionSource::parent_thread_id)
+        else {
             return;
         };
         let Some(state_db_ctx) = thread.state_db() else {
@@ -1263,10 +1269,6 @@ impl AgentControl {
     }
 }
 
-fn thread_spawn_parent_thread_id(session_source: &SessionSource) -> Option<ThreadId> {
-    session_source.parent_thread_id()
-}
-
 fn agent_matches_prefix(agent_path: Option<&AgentPath>, prefix: &AgentPath) -> bool {
     if prefix.is_root() {
         return true;

codex-rs/core/src/agent/control_tests.rs

@@ -74,6 +74,15 @@ fn assistant_message(text: &str, phase: Option<MessagePhase>) -> ResponseItem {
     }
 }
 
+#[test]
+fn register_session_root_skips_threads_with_explicit_parent() {
+    let control = AgentControl::default();
+
+    control.register_session_root(ThreadId::new(), Some(ThreadId::new()));
+
+    assert_eq!(control.state.agent_id_for_path(&AgentPath::root()), None);
+}
+
 fn spawn_agent_call(call_id: &str) -> ResponseItem {
     ResponseItem::FunctionCall {
         id: None,

codex-rs/core/src/client.rs

@@ -173,6 +173,7 @@ struct ModelClientState {
     provider: SharedModelProvider,
     auth_env_telemetry: AuthEnvTelemetry,
     session_source: SessionSource,
+    parent_thread_id: Option<ThreadId>,
     model_verbosity: Option<VerbosityConfig>,
     enable_request_compression: bool,
     include_timing_metrics: bool,
@@ -321,6 +322,7 @@ impl ModelClient {
         installation_id: String,
         provider_info: ModelProviderInfo,
         session_source: SessionSource,
+        parent_thread_id: Option<ThreadId>,
         model_verbosity: Option<VerbosityConfig>,
         enable_request_compression: bool,
         include_timing_metrics: bool,
@@ -344,6 +346,7 @@ impl ModelClient {
                 provider: model_provider,
                 auth_env_telemetry,
                 session_source,
+                parent_thread_id,
                 model_verbosity,
                 enable_request_compression,
                 include_timing_metrics,
@@ -637,7 +640,7 @@ impl ModelClient {
 
     fn build_responses_identity_headers(&self) -> ApiHeaderMap {
         let mut extra_headers = self.build_subagent_headers();
-        if let Some(parent_thread_id) = parent_thread_id_header_value(&self.state.session_source)
+        if let Some(parent_thread_id) = parent_thread_id_header_value(self.state.parent_thread_id)
             && let Ok(val) = HeaderValue::from_str(&parent_thread_id)
         {
             extra_headers.insert(X_CODEX_PARENT_THREAD_ID_HEADER, val);
@@ -664,7 +667,7 @@ impl ModelClient {
         if let Some(subagent) = subagent_header_value(&self.state.session_source) {
             client_metadata.insert(X_OPENAI_SUBAGENT_HEADER.to_string(), subagent);
         }
-        if let Some(parent_thread_id) = parent_thread_id_header_value(&self.state.session_source) {
+        if let Some(parent_thread_id) = parent_thread_id_header_value(self.state.parent_thread_id) {
             client_metadata.insert(
                 X_CODEX_PARENT_THREAD_ID_HEADER.to_string(),
                 parent_thread_id,
@@ -1733,10 +1736,8 @@ fn subagent_header_value(session_source: &SessionSource) -> Option<String> {
     }
 }
 
-fn parent_thread_id_header_value(session_source: &SessionSource) -> Option<String> {
-    session_source
-        .parent_thread_id()
-        .map(|parent_thread_id| parent_thread_id.to_string())
+fn parent_thread_id_header_value(parent_thread_id: Option<ThreadId>) -> Option<String> {
+    parent_thread_id.map(|parent_thread_id| parent_thread_id.to_string())
 }
 
 const RESPONSE_STREAM_CHANNEL_CAPACITY: usize = 1600;

codex-rs/core/src/client_tests.rs

@@ -61,6 +61,13 @@ use tracing_subscriber::registry::LookupSpan;
 use tracing_subscriber::util::SubscriberInitExt;
 
 fn test_model_client(session_source: SessionSource) -> ModelClient {
+    test_model_client_with_parent(session_source, /*parent_thread_id*/ None)
+}
+
+fn test_model_client_with_parent(
+    session_source: SessionSource,
+    parent_thread_id: Option<ThreadId>,
+) -> ModelClient {
     let provider = create_oss_provider_with_base_url("https://example.com/v1", WireApi::Responses);
     let thread_id = ThreadId::new();
     ModelClient::new(
@@ -70,6 +77,7 @@ fn test_model_client(session_source: SessionSource) -> ModelClient {
         /*installation_id*/ "11111111-1111-4111-8111-111111111111".to_string(),
         provider,
         session_source,
+        parent_thread_id,
         /*model_verbosity*/ None,
         /*enable_request_compression*/ false,
         /*include_timing_metrics*/ false,
@@ -272,13 +280,16 @@ fn build_subagent_headers_sets_internal_memory_consolidation_label() {
 #[test]
 fn build_ws_client_metadata_includes_window_lineage_and_turn_metadata() {
     let parent_thread_id = ThreadId::new();
-    let client = test_model_client(SessionSource::SubAgent(SubAgentSource::ThreadSpawn {
-        parent_thread_id,
-        depth: 2,
-        agent_path: None,
-        agent_nickname: None,
-        agent_role: None,
-    }));
+    let client = test_model_client_with_parent(
+        SessionSource::SubAgent(SubAgentSource::ThreadSpawn {
+            parent_thread_id,
+            depth: 2,
+            agent_path: None,
+            agent_nickname: None,
+            agent_role: None,
+        }),
+        Some(parent_thread_id),
+    );
 
     client.advance_window_generation();
 
@@ -520,6 +531,7 @@ fn model_client_with_counting_attestation(
         /*installation_id*/ "11111111-1111-4111-8111-111111111111".to_string(),
         provider,
         SessionSource::Exec,
+        /*parent_thread_id*/ None,
         /*model_verbosity*/ None,
         /*enable_request_compression*/ false,
         /*include_timing_metrics*/ false,

codex-rs/core/src/codex_delegate.rs

@@ -74,6 +74,8 @@ pub(crate) async fn run_codex_thread_interactive(
 ) -> Result<Codex, CodexErr> {
     let (tx_sub, rx_sub) = async_channel::bounded(SUBMISSION_CHANNEL_CAPACITY);
     let (tx_ops, rx_ops) = async_channel::bounded(SUBMISSION_CHANNEL_CAPACITY);
+    let conversation_history = initial_history.unwrap_or(InitialHistory::New);
+    let forked_from_thread_id = conversation_history.forked_from_id();
     let CodexSpawnOk { codex, .. } = Box::pin(Codex::spawn(CodexSpawnArgs {
         config,
         installation_id: parent_session.installation_id.clone(),
@@ -84,9 +86,10 @@ pub(crate) async fn run_codex_thread_interactive(
         plugins_manager: Arc::clone(&parent_session.services.plugins_manager),
         mcp_manager: Arc::clone(&parent_session.services.mcp_manager),
         extensions: Arc::clone(&parent_session.services.extensions),
-        conversation_history: initial_history.unwrap_or(InitialHistory::New),
+        conversation_history,
         session_source: SessionSource::SubAgent(subagent_source.clone()),
-        forked_from_thread_id: Some(parent_session.conversation_id),
+        forked_from_thread_id,
+        parent_thread_id: Some(parent_session.conversation_id),
         thread_source: Some(ThreadSource::Subagent),
         agent_control: parent_session.services.agent_control.clone(),
         dynamic_tools: Vec::new(),

codex-rs/core/src/codex_thread.rs

@@ -7,6 +7,7 @@ use crate::session::SessionSettingsUpdate;
 use crate::session::SteerInputError;
 use codex_features::Feature;
 use codex_otel::SessionTelemetry;
+use codex_protocol::ThreadId;
 use codex_protocol::config_types::ApprovalsReviewer;
 use codex_protocol::config_types::CollaborationMode;
 use codex_protocol::config_types::Personality;
@@ -68,6 +69,7 @@ pub struct ThreadConfigSnapshot {
     pub personality: Option<Personality>,
     pub collaboration_mode: CollaborationMode,
     pub session_source: SessionSource,
+    pub parent_thread_id: Option<ThreadId>,
     pub thread_source: Option<ThreadSource>,
 }
 

codex-rs/core/src/guardian/review.rs

@@ -156,8 +156,8 @@ pub(crate) fn is_guardian_reviewer_source(
 ) -> bool {
     matches!(
         session_source,
-        codex_protocol::protocol::SessionSource::SubAgent(SubAgentSource::Other(name))
-            if name == GUARDIAN_REVIEWER_NAME
+        codex_protocol::protocol::SessionSource::SubAgent(SubAgentSource::Other(label))
+            if label == GUARDIAN_REVIEWER_NAME
     )
 }
 

codex-rs/core/src/personality_migration_tests.rs

@@ -45,6 +45,7 @@ async fn write_rollout_with_user_event(dir: &Path, thread_id: ThreadId) -> io::R
         meta: SessionMeta {
             id: thread_id,
             forked_from_id: None,
+            parent_thread_id: None,
             timestamp: TEST_TIMESTAMP.to_string(),
             cwd: std::path::PathBuf::from("."),
             originator: "test_originator".to_string(),

codex-rs/core/src/realtime_context_tests.rs

@@ -32,6 +32,7 @@ fn stored_thread(cwd: &str, title: &str, first_user_message: &str) -> StoredThre
         thread_id: ThreadId::new(),
         rollout_path: Some(PathBuf::from("/tmp/rollout.jsonl")),
         forked_from_id: None,
+        parent_thread_id: None,
         preview: first_user_message.to_string(),
         name: (!title.is_empty()).then(|| title.to_string()),
         model_provider: "test-provider".to_string(),

codex-rs/core/src/session/mod.rs

@@ -402,6 +402,7 @@ pub(crate) struct CodexSpawnArgs {
     pub(crate) conversation_history: InitialHistory,
     pub(crate) session_source: SessionSource,
     pub(crate) forked_from_thread_id: Option<ThreadId>,
+    pub(crate) parent_thread_id: Option<ThreadId>,
     pub(crate) thread_source: Option<ThreadSource>,
     pub(crate) agent_control: AgentControl,
     pub(crate) dynamic_tools: Vec<DynamicToolSpec>,
@@ -467,6 +468,7 @@ impl Codex {
             conversation_history,
             session_source,
             forked_from_thread_id,
+            parent_thread_id,
             thread_source,
             agent_control,
             dynamic_tools,
@@ -595,6 +597,7 @@ impl Codex {
             app_server_client_version: None,
             session_source,
             forked_from_thread_id,
+            parent_thread_id,
             thread_source,
             dynamic_tools,
             persist_extended_history,

codex-rs/core/src/session/review.rs

@@ -90,6 +90,7 @@ pub(super) async fn spawn_review_thread(
         sess.session_id().to_string(),
         sess.thread_id().to_string(),
         forked_from_thread_id,
+        parent_turn_context.parent_thread_id,
         &session_source,
         parent_turn_context.thread_source,
         review_turn_id.clone(),
@@ -113,6 +114,7 @@ pub(super) async fn spawn_review_thread(
         reasoning_effort,
         reasoning_summary,
         session_source,
+        parent_thread_id: parent_turn_context.parent_thread_id,
         thread_source: parent_turn_context.thread_source,
         environments: parent_turn_context.environments.clone(),
         available_models,

codex-rs/core/src/session/session.rs

@@ -97,6 +97,8 @@ pub(crate) struct SessionConfiguration {
     pub(super) session_source: SessionSource,
     /// Immediate history source copied into this thread, when this thread was forked.
     pub(super) forked_from_thread_id: Option<ThreadId>,
+    /// Immediate control/spawn parent for this thread, when it has one.
+    pub(super) parent_thread_id: Option<ThreadId>,
     /// Optional analytics source classification for this thread.
     pub(super) thread_source: Option<ThreadSource>,
     pub(super) dynamic_tools: Vec<DynamicToolSpec>,
@@ -187,6 +189,7 @@ impl SessionConfiguration {
             personality: self.personality,
             collaboration_mode: self.collaboration_mode.clone(),
             session_source: self.session_source.clone(),
+            parent_thread_id: self.parent_thread_id,
             thread_source: self.thread_source,
         }
     }
@@ -511,6 +514,10 @@ impl Session {
             .forked_from_thread_id
             .or_else(|| initial_history.forked_from_id());
         session_configuration.forked_from_thread_id = forked_from_id;
+        let parent_thread_id = session_configuration
+            .parent_thread_id
+            .or_else(|| initial_history.get_resumed_parent_thread_id());
+        session_configuration.parent_thread_id = parent_thread_id;
 
         let event_persistence_mode = if session_configuration.persist_extended_history {
             ThreadEventPersistenceMode::Extended
@@ -550,6 +557,7 @@ impl Session {
                             CreateThreadParams {
                                 thread_id,
                                 forked_from_id,
+                                parent_thread_id,
                                 source: session_source,
                                 thread_source: session_configuration.thread_source,
                                 base_instructions: BaseInstructions {
@@ -1031,6 +1039,7 @@ impl Session {
                     installation_id.clone(),
                     session_configuration.provider.clone(),
                     session_configuration.session_source.clone(),
+                    session_configuration.parent_thread_id,
                     config.model_verbosity,
                     config.features.enabled(Feature::EnableRequestCompression),
                     config.features.enabled(Feature::RuntimeMetrics),
@@ -1040,7 +1049,7 @@ impl Session {
                 .with_prompt_cache_key_override(
                     crate::guardian::prompt_cache_key_override_for_review_session(
                         &session_configuration.session_source,
-                        session_configuration.forked_from_thread_id,
+                        session_configuration.parent_thread_id,
                     ),
                 ),
                 code_mode_service: crate::tools::code_mode::CodeModeService::new(),
@@ -1083,6 +1092,7 @@ impl Session {
                     session_id,
                     thread_id,
                     forked_from_id,
+                    parent_thread_id,
                     thread_source: session_configuration.thread_source,
                     thread_name: session_configuration.thread_name.clone(),
                     model: session_configuration.collaboration_mode.model().to_string(),

codex-rs/core/src/session/tests.rs

@@ -433,6 +433,7 @@ fn test_model_client_session() -> crate::client::ModelClientSession {
         /*installation_id*/ "11111111-1111-4111-8111-111111111111".to_string(),
         ModelProviderInfo::create_openai_provider(/* base_url */ /*base_url*/ None),
         codex_protocol::protocol::SessionSource::Exec,
+        /*parent_thread_id*/ None,
         /*model_verbosity*/ None,
         /*enable_request_compression*/ false,
         /*include_timing_metrics*/ false,
@@ -3096,6 +3097,7 @@ async fn set_rate_limits_retains_previous_credits() {
         app_server_client_version: None,
         session_source: SessionSource::Exec,
         forked_from_thread_id: None,
+        parent_thread_id: None,
         thread_source: None,
         dynamic_tools: Vec::new(),
         persist_extended_history: false,
@@ -3201,6 +3203,7 @@ async fn set_rate_limits_updates_plan_type_when_present() {
         app_server_client_version: None,
         session_source: SessionSource::Exec,
         forked_from_thread_id: None,
+        parent_thread_id: None,
         thread_source: None,
         dynamic_tools: Vec::new(),
         persist_extended_history: false,
@@ -3449,6 +3452,7 @@ async fn attach_thread_persistence(session: &mut Session) -> PathBuf {
         CreateThreadParams {
             thread_id: session.conversation_id,
             forked_from_id: None,
+            parent_thread_id: None,
             source: SessionSource::Exec,
             thread_source: None,
             base_instructions: BaseInstructions::default(),
@@ -3729,6 +3733,7 @@ pub(crate) async fn make_session_configuration_for_tests() -> SessionConfigurati
         app_server_client_version: None,
         session_source: SessionSource::Exec,
         forked_from_thread_id: None,
+        parent_thread_id: None,
         thread_source: None,
         dynamic_tools: Vec::new(),
         persist_extended_history: false,
@@ -4473,6 +4478,7 @@ async fn session_new_fails_when_zsh_fork_enabled_without_packaged_zsh() {
         app_server_client_version: None,
         session_source: SessionSource::Exec,
         forked_from_thread_id: None,
+        parent_thread_id: None,
         thread_source: None,
         dynamic_tools: Vec::new(),
         persist_extended_history: false,
@@ -4583,6 +4589,7 @@ pub(crate) async fn make_session_and_context() -> (Session, TurnContext) {
         app_server_client_version: None,
         session_source: SessionSource::Exec,
         forked_from_thread_id: None,
+        parent_thread_id: None,
         thread_source: None,
         dynamic_tools: Vec::new(),
         persist_extended_history: false,
@@ -4677,6 +4684,7 @@ pub(crate) async fn make_session_and_context() -> (Session, TurnContext) {
             /*installation_id*/ "11111111-1111-4111-8111-111111111111".to_string(),
             session_configuration.provider.clone(),
             session_configuration.session_source.clone(),
+            session_configuration.parent_thread_id,
             config.model_verbosity,
             config.features.enabled(Feature::EnableRequestCompression),
             config.features.enabled(Feature::RuntimeMetrics),
@@ -4818,6 +4826,7 @@ async fn make_session_with_config_and_rx(
         app_server_client_version: None,
         session_source: SessionSource::Exec,
         forked_from_thread_id: None,
+        parent_thread_id: None,
         thread_source: None,
         dynamic_tools: Vec::new(),
         persist_extended_history: false,
@@ -4922,6 +4931,7 @@ async fn make_session_with_history_source_and_agent_control_and_rx(
         app_server_client_version: None,
         session_source: session_source.clone(),
         forked_from_thread_id: None,
+        parent_thread_id: None,
         thread_source: None,
         dynamic_tools: Vec::new(),
         persist_extended_history: false,
@@ -5944,6 +5954,7 @@ async fn shutdown_complete_does_not_append_to_thread_store_after_shutdown() {
         CreateThreadParams {
             thread_id: session.conversation_id,
             forked_from_id: None,
+            parent_thread_id: None,
             source: SessionSource::Exec,
             thread_source: None,
             base_instructions: BaseInstructions::default(),
@@ -6426,6 +6437,7 @@ where
         app_server_client_version: None,
         session_source: SessionSource::Exec,
         forked_from_thread_id: None,
+        parent_thread_id: None,
         thread_source: None,
         dynamic_tools,
         persist_extended_history: false,
@@ -6520,6 +6532,7 @@ where
             /*installation_id*/ "11111111-1111-4111-8111-111111111111".to_string(),
             session_configuration.provider.clone(),
             session_configuration.session_source.clone(),
+            session_configuration.parent_thread_id,
             config.model_verbosity,
             config.features.enabled(Feature::EnableRequestCompression),
             config.features.enabled(Feature::RuntimeMetrics),

codex-rs/core/src/session/tests/guardian_tests.rs

@@ -693,6 +693,7 @@ async fn guardian_subagent_does_not_inherit_parent_exec_policy_rules() {
             GUARDIAN_REVIEWER_NAME.to_string(),
         )),
         forked_from_thread_id: None,
+        parent_thread_id: None,
         thread_source: None,
         agent_control: AgentControl::default(),
         dynamic_tools: Vec::new(),

codex-rs/core/src/session/turn_context.rs

@@ -5,6 +5,7 @@ use crate::environment_selection::ResolvedTurnEnvironments;
 use codex_model_provider::SharedModelProvider;
 use codex_model_provider::create_model_provider;
 use codex_protocol::SessionId;
+use codex_protocol::ThreadId;
 use codex_protocol::models::AdditionalPermissionProfile;
 use codex_protocol::openai_models::ToolMode;
 use codex_protocol::protocol::ThreadSource;
@@ -62,6 +63,7 @@ pub struct TurnContext {
     pub(crate) reasoning_effort: Option<ReasoningEffortConfig>,
     pub(crate) reasoning_summary: ReasoningSummaryConfig,
     pub(crate) session_source: SessionSource,
+    pub(crate) parent_thread_id: Option<ThreadId>,
     pub(crate) thread_source: Option<ThreadSource>,
     pub(crate) environments: ResolvedTurnEnvironments,
     /// The session's absolute working directory. All relative paths provided
@@ -232,6 +234,7 @@ impl TurnContext {
             reasoning_effort,
             reasoning_summary: self.reasoning_summary,
             session_source: self.session_source.clone(),
+            parent_thread_id: self.parent_thread_id,
             thread_source: self.thread_source,
             environments: self.environments.clone(),
             #[allow(deprecated)]
@@ -506,6 +509,7 @@ impl Session {
             session_id.to_string(),
             thread_id.to_string(),
             session_configuration.forked_from_thread_id,
+            session_configuration.parent_thread_id,
             &session_configuration.session_source,
             session_configuration.thread_source,
             sub_id.clone(),
@@ -529,6 +533,7 @@ impl Session {
             reasoning_effort,
             reasoning_summary,
             session_source,
+            parent_thread_id: session_configuration.parent_thread_id,
             thread_source: session_configuration.thread_source,
             environments,
             #[allow(deprecated)]

codex-rs/core/src/thread_manager.rs

@@ -187,6 +187,7 @@ pub(crate) struct ResumeThreadWithHistoryOptions {
     pub(crate) initial_history: InitialHistory,
     pub(crate) agent_control: AgentControl,
     pub(crate) session_source: SessionSource,
+    pub(crate) parent_thread_id: Option<ThreadId>,
     pub(crate) inherited_shell_snapshot: Option<Arc<ShellSnapshot>>,
     pub(crate) inherited_exec_policy: Option<Arc<crate::exec_policy::ExecPolicyManager>>,
 }
@@ -601,6 +602,7 @@ impl ThreadManager {
             Arc::clone(&self.state.auth_manager),
             self.agent_control(),
             session_source,
+            /*parent_thread_id*/ None,
             forked_from_thread_id,
             thread_source,
             options.dynamic_tools,
@@ -685,6 +687,7 @@ impl ThreadManager {
             auth_manager,
             self.agent_control(),
             session_source,
+            /*parent_thread_id*/ None,
             /*forked_from_thread_id*/ None,
             thread_source,
             Vec::new(),
@@ -713,6 +716,7 @@ impl ThreadManager {
             InitialHistory::New,
             Arc::clone(&self.state.auth_manager),
             self.agent_control(),
+            /*parent_thread_id*/ None,
             /*forked_from_thread_id*/ None,
             /*thread_source*/ None,
             Vec::new(),
@@ -746,6 +750,7 @@ impl ThreadManager {
             auth_manager,
             self.agent_control(),
             session_source,
+            /*parent_thread_id*/ None,
             /*forked_from_thread_id*/ None,
             thread_source,
             Vec::new(),
@@ -916,6 +921,7 @@ impl ThreadManager {
             history,
             Arc::clone(&self.state.auth_manager),
             self.agent_control(),
+            /*parent_thread_id*/ None,
             forked_from_thread_id,
             thread_source,
             Vec::new(),
@@ -1039,6 +1045,7 @@ impl ThreadManagerState {
             config,
             agent_control,
             self.session_source.clone(),
+            /*parent_thread_id*/ None,
             /*forked_from_thread_id*/ None,
             /*thread_source*/ None,
             /*persist_extended_history*/ false,
@@ -1056,6 +1063,7 @@ impl ThreadManagerState {
         config: Config,
         agent_control: AgentControl,
         session_source: SessionSource,
+        parent_thread_id: Option<ThreadId>,
         forked_from_thread_id: Option<ThreadId>,
         thread_source: Option<ThreadSource>,
         persist_extended_history: bool,
@@ -1073,6 +1081,7 @@ impl ThreadManagerState {
             Arc::clone(&self.auth_manager),
             agent_control,
             session_source,
+            parent_thread_id,
             forked_from_thread_id,
             thread_source,
             Vec::new(),
@@ -1096,6 +1105,7 @@ impl ThreadManagerState {
             initial_history,
             agent_control,
             session_source,
+            parent_thread_id,
             inherited_shell_snapshot,
             inherited_exec_policy,
         } = options;
@@ -1108,6 +1118,7 @@ impl ThreadManagerState {
             Arc::clone(&self.auth_manager),
             agent_control,
             session_source,
+            parent_thread_id,
             /*forked_from_thread_id*/ None,
             thread_source,
             Vec::new(),
@@ -1130,6 +1141,7 @@ impl ThreadManagerState {
         agent_control: AgentControl,
         session_source: SessionSource,
         thread_source: Option<ThreadSource>,
+        parent_thread_id: Option<ThreadId>,
         forked_from_thread_id: Option<ThreadId>,
         persist_extended_history: bool,
         inherited_shell_snapshot: Option<Arc<ShellSnapshot>>,
@@ -1145,6 +1157,7 @@ impl ThreadManagerState {
             Arc::clone(&self.auth_manager),
             agent_control,
             session_source,
+            parent_thread_id,
             forked_from_thread_id,
             thread_source,
             Vec::new(),
@@ -1167,6 +1180,7 @@ impl ThreadManagerState {
         initial_history: InitialHistory,
         auth_manager: Arc<AuthManager>,
         agent_control: AgentControl,
+        parent_thread_id: Option<ThreadId>,
         forked_from_thread_id: Option<ThreadId>,
         thread_source: Option<ThreadSource>,
         dynamic_tools: Vec<codex_protocol::dynamic_tools::DynamicToolSpec>,
@@ -1182,6 +1196,7 @@ impl ThreadManagerState {
             auth_manager,
             agent_control,
             self.session_source.clone(),
+            parent_thread_id,
             forked_from_thread_id,
             thread_source,
             dynamic_tools,
@@ -1204,6 +1219,7 @@ impl ThreadManagerState {
         auth_manager: Arc<AuthManager>,
         agent_control: AgentControl,
         session_source: SessionSource,
+        parent_thread_id: Option<ThreadId>,
         forked_from_thread_id: Option<ThreadId>,
         thread_source: Option<ThreadSource>,
         dynamic_tools: Vec<codex_protocol::dynamic_tools::DynamicToolSpec>,
@@ -1258,6 +1274,7 @@ impl ThreadManagerState {
             conversation_history: initial_history,
             session_source,
             forked_from_thread_id,
+            parent_thread_id,
             thread_source,
             agent_control,
             dynamic_tools,

codex-rs/core/src/tools/handlers/agent_jobs.rs

@@ -211,6 +211,7 @@ async fn run_agent_job_loop(
                             "agent_job:{job_id}"
                         )))),
                         SpawnAgentOptions {
+                            parent_thread_id: Some(session.conversation_id),
                             environments: Some(turn.environments.to_selections()),
                             ..Default::default()
                         },

codex-rs/core/src/tools/handlers/multi_agents/spawn.rs

@@ -124,6 +124,7 @@ async fn handle_spawn_agent(
         SpawnAgentOptions {
             fork_parent_spawn_call_id: args.fork_context.then(|| call_id.clone()),
             fork_mode: args.fork_context.then_some(SpawnAgentForkMode::FullHistory),
+            parent_thread_id: Some(session.conversation_id),
             environments: Some(turn.environments.to_selections()),
         },
     ))

codex-rs/core/src/tools/handlers/multi_agents_v2/list_agents.rs

@@ -30,7 +30,7 @@ impl ToolExecutor<ToolInvocation> for Handler {
         session
             .services
             .agent_control
-            .register_session_root(session.conversation_id, &turn.session_source);
+            .register_session_root(session.conversation_id, turn.parent_thread_id);
         let agents = session
             .services
             .agent_control

codex-rs/core/src/tools/handlers/multi_agents_v2/spawn.rs

@@ -144,6 +144,7 @@ async fn handle_spawn_agent(
             SpawnAgentOptions {
                 fork_parent_spawn_call_id: fork_mode.as_ref().map(|_| call_id.clone()),
                 fork_mode,
+                parent_thread_id: Some(session.conversation_id),
                 environments: Some(turn.environments.to_selections()),
             },
         ),

codex-rs/core/src/turn_metadata.rs

@@ -261,6 +261,7 @@ impl TurnMetadataState {
         session_id: String,
         thread_id: String,
         forked_from_thread_id: Option<ThreadId>,
+        parent_thread_id: Option<ThreadId>,
         session_source: &SessionSource,
         thread_source: Option<ThreadSource>,
         turn_id: String,
@@ -278,18 +279,15 @@ impl TurnMetadataState {
             )
             .to_string(),
         );
-        let (parent_thread_id, subagent_kind) = match session_source {
-            SessionSource::SubAgent(subagent_source) => (
-                subagent_source.parent_thread_id().or(forked_from_thread_id),
-                Some(subagent_source.kind().to_string()),
-            ),
+        let subagent_kind = match session_source {
+            SessionSource::SubAgent(subagent_source) => Some(subagent_source.kind().to_string()),
             SessionSource::Cli
             | SessionSource::VSCode
             | SessionSource::Exec
             | SessionSource::Mcp
             | SessionSource::Custom(_)
             | SessionSource::Internal(_)
-            | SessionSource::Unknown => (None, None),
+            | SessionSource::Unknown => None,
         };
         let base_metadata = TurnMetadataBag {
             request_kind: None,

codex-rs/core/src/turn_metadata_tests.rs

@@ -122,6 +122,7 @@ fn turn_metadata_state_uses_platform_sandbox_tag() {
         "session-a".to_string(),
         "thread-a".to_string(),
         /*forked_from_thread_id*/ None,
+        /*parent_thread_id*/ None,
         &SessionSource::Exec,
         Some(ThreadSource::User),
         "turn-a".to_string(),
@@ -163,6 +164,7 @@ fn turn_metadata_state_uses_explicit_subagent_thread_source() {
         "session-a".to_string(),
         "thread-a".to_string(),
         /*forked_from_thread_id*/ None,
+        /*parent_thread_id*/ None,
         &SessionSource::Exec,
         Some(ThreadSource::Subagent),
         "turn-a".to_string(),
@@ -191,6 +193,7 @@ fn turn_metadata_state_includes_root_fork_lineage() {
         "session-a".to_string(),
         "thread-a".to_string(),
         Some(source_thread_id),
+        /*parent_thread_id*/ None,
         &SessionSource::Exec,
         Some(ThreadSource::User),
         "turn-a".to_string(),
@@ -223,6 +226,7 @@ fn turn_metadata_state_includes_thread_spawn_subagent_parent_without_fork() {
         "session-a".to_string(),
         "thread-a".to_string(),
         /*forked_from_thread_id*/ None,
+        Some(parent_thread_id),
         &SessionSource::SubAgent(SubAgentSource::ThreadSpawn {
             parent_thread_id,
             depth: 1,
@@ -261,6 +265,7 @@ fn turn_metadata_state_includes_forked_thread_spawn_subagent_lineage() {
         "session-a".to_string(),
         "thread-a".to_string(),
         Some(parent_thread_id),
+        Some(parent_thread_id),
         &SessionSource::SubAgent(SubAgentSource::ThreadSpawn {
             parent_thread_id,
             depth: 1,
@@ -291,38 +296,46 @@ fn turn_metadata_state_includes_forked_thread_spawn_subagent_lineage() {
 }
 
 #[test]
-fn turn_metadata_state_includes_known_parent_for_other_subagent() {
+fn turn_metadata_state_includes_known_parent_for_non_thread_spawn_subagents_without_fork() {
     let temp_dir = TempDir::new().expect("temp dir");
     let cwd = temp_dir.path().abs();
     let permission_profile = PermissionProfile::read_only();
     let parent_thread_id =
         ThreadId::from_string("44444444-4444-4444-8444-444444444444").expect("thread id");
+    let sources = [
+        (SubAgentSource::Review, "review"),
+        (SubAgentSource::Other("guardian".to_string()), "guardian"),
+        (
+            SubAgentSource::Other("agent_job:job-1".to_string()),
+            "agent_job:job-1",
+        ),
+    ];
 
-    let state = TurnMetadataState::new(
-        "session-a".to_string(),
-        "thread-a".to_string(),
-        Some(parent_thread_id),
-        &SessionSource::SubAgent(SubAgentSource::Other("guardian".to_string())),
-        Some(ThreadSource::Subagent),
-        "turn-a".to_string(),
-        cwd,
-        &permission_profile,
-        WindowsSandboxLevel::Disabled,
-        /*enforce_managed_network*/ false,
-    );
+    for (subagent_source, subagent_kind) in sources {
+        let state = TurnMetadataState::new(
+            "session-a".to_string(),
+            "thread-a".to_string(),
+            /*forked_from_thread_id*/ None,
+            Some(parent_thread_id),
+            &SessionSource::SubAgent(subagent_source),
+            Some(ThreadSource::Subagent),
+            "turn-a".to_string(),
+            cwd.clone(),
+            &permission_profile,
+            WindowsSandboxLevel::Disabled,
+            /*enforce_managed_network*/ false,
+        );
 
-    let header = state.current_header_value().expect("header");
-    let json: Value = serde_json::from_str(&header).expect("json");
+        let header = state.current_header_value().expect("header");
+        let json: Value = serde_json::from_str(&header).expect("json");
 
-    assert_eq!(
-        json["forked_from_thread_id"].as_str(),
-        Some("44444444-4444-4444-8444-444444444444")
-    );
-    assert_eq!(
-        json["parent_thread_id"].as_str(),
-        Some("44444444-4444-4444-8444-444444444444")
-    );
-    assert_eq!(json["subagent_kind"].as_str(), Some("guardian"));
+        assert!(json.get("forked_from_thread_id").is_none());
+        assert_eq!(
+            json["parent_thread_id"].as_str(),
+            Some("44444444-4444-4444-8444-444444444444")
+        );
+        assert_eq!(json["subagent_kind"].as_str(), Some(subagent_kind));
+    }
 }
 
 #[test]
@@ -335,6 +348,7 @@ fn turn_metadata_state_includes_turn_started_at_unix_ms_after_start() {
         "session-a".to_string(),
         "thread-a".to_string(),
         /*forked_from_thread_id*/ None,
+        /*parent_thread_id*/ None,
         &SessionSource::Exec,
         Some(ThreadSource::User),
         "turn-a".to_string(),
@@ -364,6 +378,7 @@ fn turn_metadata_state_includes_model_and_reasoning_effort_only_in_request_meta(
         "session-a".to_string(),
         "thread-a".to_string(),
         /*forked_from_thread_id*/ None,
+        /*parent_thread_id*/ None,
         &SessionSource::Exec,
         /*thread_source*/ None,
         "turn-a".to_string(),
@@ -412,6 +427,7 @@ fn turn_metadata_state_marks_user_input_requested_during_turn_only_for_mcp_reque
         "session-a".to_string(),
         "thread-a".to_string(),
         /*forked_from_thread_id*/ None,
+        /*parent_thread_id*/ None,
         &SessionSource::Exec,
         /*thread_source*/ None,
         "turn-a".to_string(),
@@ -464,6 +480,7 @@ fn turn_metadata_state_ignores_client_reserved_metadata_before_start() {
         "session-a".to_string(),
         "thread-a".to_string(),
         /*forked_from_thread_id*/ None,
+        /*parent_thread_id*/ None,
         &SessionSource::Exec,
         Some(ThreadSource::User),
         "turn-a".to_string(),
@@ -511,6 +528,7 @@ fn turn_metadata_state_merges_client_metadata_without_replacing_reserved_fields(
         "session-a".to_string(),
         "thread-a".to_string(),
         Some(source_thread_id),
+        Some(parent_thread_id),
         &SessionSource::SubAgent(SubAgentSource::ThreadSpawn {
             parent_thread_id,
             depth: 1,
@@ -612,6 +630,7 @@ fn turn_metadata_state_overlays_compaction_only_on_compaction_requests() {
         "session-a".to_string(),
         "thread-a".to_string(),
         /*forked_from_thread_id*/ None,
+        /*parent_thread_id*/ None,
         &SessionSource::Exec,
         Some(ThreadSource::User),
         "turn-a".to_string(),
@@ -671,6 +690,7 @@ async fn turn_metadata_state_preserves_lineage_after_git_enrichment() {
         "session-a".to_string(),
         "thread-a".to_string(),
         Some(parent_thread_id),
+        Some(parent_thread_id),
         &SessionSource::SubAgent(SubAgentSource::ThreadSpawn {
             parent_thread_id,
             depth: 1,

codex-rs/core/tests/responses_headers.rs

@@ -105,6 +105,7 @@ async fn responses_stream_includes_subagent_header_on_review() {
         /*installation_id*/ TEST_INSTALLATION_ID.to_string(),
         provider.clone(),
         session_source,
+        /*parent_thread_id*/ None,
         config.model_verbosity,
         /*enable_request_compression*/ false,
         /*include_timing_metrics*/ false,
@@ -233,6 +234,7 @@ async fn responses_stream_includes_subagent_header_on_other() {
         /*installation_id*/ TEST_INSTALLATION_ID.to_string(),
         provider.clone(),
         session_source,
+        /*parent_thread_id*/ None,
         config.model_verbosity,
         /*enable_request_compression*/ false,
         /*include_timing_metrics*/ false,
@@ -350,6 +352,7 @@ async fn responses_respects_model_info_overrides_from_config() {
         /*installation_id*/ TEST_INSTALLATION_ID.to_string(),
         provider.clone(),
         session_source,
+        /*parent_thread_id*/ None,
         config.model_verbosity,
         /*enable_request_compression*/ false,
         /*include_timing_metrics*/ false,

codex-rs/core/tests/suite/client.rs

@@ -488,6 +488,7 @@ async fn resume_replays_legacy_js_repl_image_rollout_shapes() {
             item: RolloutItem::SessionMeta(SessionMetaLine {
                 meta: SessionMeta {
                     id: ThreadId::default(),
+                    parent_thread_id: None,
                     timestamp: "2024-01-01T00:00:00Z".to_string(),
                     cwd: ".".into(),
                     originator: "test_originator".to_string(),
@@ -618,6 +619,7 @@ async fn resume_replays_image_tool_outputs_with_detail() {
             item: RolloutItem::SessionMeta(SessionMetaLine {
                 meta: SessionMeta {
                     id: ThreadId::default(),
+                    parent_thread_id: None,
                     timestamp: "2024-01-01T00:00:00Z".to_string(),
                     cwd: ".".into(),
                     originator: "test_originator".to_string(),
@@ -902,6 +904,7 @@ async fn send_provider_auth_request(server: &MockServer, auth: ModelProviderAuth
         /*installation_id*/ "11111111-1111-4111-8111-111111111111".to_string(),
         provider,
         SessionSource::Exec,
+        /*parent_thread_id*/ None,
         config.model_verbosity,
         /*enable_request_compression*/ false,
         /*include_timing_metrics*/ false,
@@ -2358,6 +2361,7 @@ async fn azure_responses_request_includes_store_and_reasoning_ids() {
         /*installation_id*/ "11111111-1111-4111-8111-111111111111".to_string(),
         provider.clone(),
         SessionSource::Exec,
+        /*parent_thread_id*/ None,
         config.model_verbosity,
         /*enable_request_compression*/ false,
         /*include_timing_metrics*/ false,

codex-rs/core/tests/suite/client_websockets.rs

@@ -2155,6 +2155,7 @@ async fn websocket_harness_with_provider_options(
         /*installation_id*/ TEST_INSTALLATION_ID.to_string(),
         provider.clone(),
         SessionSource::Exec,
+        /*parent_thread_id*/ None,
         config.model_verbosity,
         /*enable_request_compression*/ false,
         runtime_metrics_enabled,

codex-rs/core/tests/suite/personality_migration.rs

@@ -61,6 +61,7 @@ async fn write_rollout_with_user_event(dir: &Path, thread_id: ThreadId) -> io::R
         meta: SessionMeta {
             id: thread_id,
             forked_from_id: None,
+            parent_thread_id: None,
             timestamp: TEST_TIMESTAMP.to_string(),
             cwd: std::path::PathBuf::from("."),
             originator: "test_originator".to_string(),
@@ -109,6 +110,7 @@ async fn write_rollout_with_meta_only(dir: &Path, thread_id: ThreadId) -> io::Re
         meta: SessionMeta {
             id: thread_id,
             forked_from_id: None,
+            parent_thread_id: None,
             timestamp: TEST_TIMESTAMP.to_string(),
             cwd: std::path::PathBuf::from("."),
             originator: "test_originator".to_string(),

codex-rs/core/tests/suite/responses_api_proxy_headers.rs

@@ -130,8 +130,9 @@ async fn responses_api_parent_and_subagent_requests_include_identity_headers() -
             .header("x-codex-turn-metadata")
             .ok_or_else(|| anyhow!("child request missing x-codex-turn-metadata"))?,
     )?;
+    assert!(child_turn_metadata.get("forked_from_thread_id").is_none());
     assert_eq!(
-        child_turn_metadata["forked_from_thread_id"].as_str(),
+        child_turn_metadata["parent_thread_id"].as_str(),
         Some(parent_thread_id)
     );
 

codex-rs/core/tests/suite/review.rs

@@ -136,8 +136,9 @@ async fn review_op_emits_lifecycle_and_review_output() {
             .expect("review request turn metadata"),
     )
     .expect("review request turn metadata json");
+    assert!(turn_metadata.get("forked_from_thread_id").is_none());
     assert_eq!(
-        turn_metadata["forked_from_thread_id"].as_str(),
+        turn_metadata["parent_thread_id"].as_str(),
         Some(parent_thread_id.as_str())
     );
 

codex-rs/core/tests/suite/rollout_list_find.rs

@@ -184,6 +184,7 @@ async fn find_locates_rollout_file_written_by_recorder() -> std::io::Result<()>
         RolloutRecorderParams::new(
             thread_id,
             /*forked_from_id*/ None,
+            /*parent_thread_id*/ None,
             SessionSource::Exec,
             /*thread_source*/ None,
             BaseInstructions::default(),

codex-rs/core/tests/suite/sqlite_state.rs

@@ -212,6 +212,7 @@ async fn backfill_scans_existing_rollouts() -> Result<()> {
                 meta: SessionMeta {
                     id: thread_id,
                     forked_from_id: None,
+                    parent_thread_id: None,
                     timestamp: "2026-01-27T12:00:00Z".to_string(),
                     cwd: codex_home.to_path_buf(),
                     originator: "test".to_string(),

codex-rs/exec/src/event_processor_with_human_output_tests.rs

@@ -211,6 +211,7 @@ async fn config_summary_entries_include_runtime_workspace_roots() {
         session_id: SessionId::new(),
         thread_id: ThreadId::new(),
         forked_from_id: None,
+        parent_thread_id: None,
         thread_source: None,
         thread_name: None,
         model: "gpt-5.4".to_string(),

codex-rs/exec/src/lib.rs

@@ -1070,6 +1070,7 @@ fn session_configured_from_thread_start_response(
     session_configured_from_thread_response(
         &response.thread.session_id,
         &response.thread.id,
+        response.thread.parent_thread_id.as_deref(),
         response.thread.thread_source.map(Into::into),
         response.thread.name.clone(),
         response.thread.path.clone(),
@@ -1092,6 +1093,7 @@ fn session_configured_from_thread_resume_response(
     session_configured_from_thread_response(
         &response.thread.session_id,
         &response.thread.id,
+        response.thread.parent_thread_id.as_deref(),
         response.thread.thread_source.map(Into::into),
         response.thread.name.clone(),
         response.thread.path.clone(),
@@ -1123,6 +1125,7 @@ fn review_target_to_api(target: ReviewTarget) -> ApiReviewTarget {
 fn session_configured_from_thread_response(
     session_id: &str,
     thread_id: &str,
+    parent_thread_id: Option<&str>,
     thread_source: Option<codex_protocol::protocol::ThreadSource>,
     thread_name: Option<String>,
     rollout_path: Option<PathBuf>,
@@ -1140,11 +1143,16 @@ fn session_configured_from_thread_response(
         .map_err(|err| format!("session id `{session_id}` is invalid: {err}"))?;
     let thread_id = ThreadId::from_string(thread_id)
         .map_err(|err| format!("thread id `{thread_id}` is invalid: {err}"))?;
+    let parent_thread_id = parent_thread_id
+        .map(ThreadId::from_string)
+        .transpose()
+        .map_err(|err| format!("parent thread id is invalid: {err}"))?;
 
     Ok(SessionConfiguredEvent {
         session_id,
         thread_id,
         forked_from_id: None,
+        parent_thread_id,
         thread_source,
         thread_name,
         model,

codex-rs/exec/src/lib_tests.rs

@@ -307,6 +307,7 @@ fn turn_items_for_thread_returns_matching_turn_items() {
         id: "thread-1".to_string(),
         session_id: "thread-1".to_string(),
         forked_from_id: None,
+        parent_thread_id: None,
         preview: String::new(),
         ephemeral: false,
         model_provider: "openai".to_string(),
@@ -585,12 +586,33 @@ async fn session_configured_from_thread_response_preserves_thread_source() {
     );
 }
 
+#[tokio::test]
+async fn session_configured_from_thread_response_preserves_parent_thread_id() {
+    let codex_home = tempdir().expect("create temp codex home");
+    let cwd = tempdir().expect("create temp cwd");
+    let config = ConfigBuilder::default()
+        .codex_home(codex_home.path().to_path_buf())
+        .fallback_cwd(Some(cwd.path().to_path_buf()))
+        .build()
+        .await
+        .expect("build config");
+    let parent_thread_id = ThreadId::new();
+    let mut response = sample_thread_start_response();
+    response.thread.parent_thread_id = Some(parent_thread_id.to_string());
+
+    let event = session_configured_from_thread_start_response(&response, &config)
+        .expect("build bootstrap session configured event");
+
+    assert_eq!(event.parent_thread_id, Some(parent_thread_id));
+}
+
 fn sample_thread_start_response() -> ThreadStartResponse {
     ThreadStartResponse {
         thread: codex_app_server_protocol::Thread {
             id: "67e55044-10b1-426f-9247-bb680e5fe0c8".to_string(),
             session_id: "67e55044-10b1-426f-9247-bb680e5fe0c7".to_string(),
             forked_from_id: None,
+            parent_thread_id: None,
             preview: String::new(),
             ephemeral: false,
             model_provider: "openai".to_string(),

codex-rs/exec/tests/event_processor_with_json_output.rs

@@ -111,6 +111,7 @@ fn session_configured_produces_thread_started_event() {
         session_id: SessionId::from(thread_id),
         thread_id,
         forked_from_id: None,
+        parent_thread_id: None,
         thread_source: None,
         thread_name: None,
         model: "codex-mini-latest".to_string(),

codex-rs/mcp-server/src/outgoing_message.rs

@@ -299,6 +299,7 @@ mod tests {
                 session_id: codex_protocol::SessionId::new(),
                 thread_id,
                 forked_from_id: None,
+                parent_thread_id: None,
                 thread_source: None,
                 thread_name: None,
                 model: "gpt-4o".to_string(),
@@ -344,6 +345,7 @@ mod tests {
             session_id: codex_protocol::SessionId::new(),
             thread_id,
             forked_from_id: None,
+            parent_thread_id: None,
             thread_source: None,
             thread_name: None,
             model: "gpt-4o".to_string(),
@@ -411,6 +413,7 @@ mod tests {
             session_id: codex_protocol::SessionId::new(),
             thread_id,
             forked_from_id: None,
+            parent_thread_id: None,
             thread_source: None,
             thread_name: None,
             model: "gpt-4o".to_string(),

codex-rs/memories/write/src/runtime.rs

@@ -171,7 +171,8 @@ impl MemoryStartupContext {
         context: &StageOneRequestContext,
     ) -> anyhow::Result<(String, Option<TokenUsage>)> {
         let installation_id = resolve_installation_id(&config.codex_home).await?;
-        let session_source = self.thread.config_snapshot().await.session_source;
+        let config_snapshot = self.thread.config_snapshot().await;
+        let session_source = config_snapshot.session_source;
         let model_client = ModelClient::new(
             Some(Arc::clone(&self.auth_manager)),
             SessionId::from(self.thread_id), // We use thread_id to detach this query from the foreground user session.
@@ -179,6 +180,7 @@ impl MemoryStartupContext {
             installation_id,
             config.model_provider.clone(),
             session_source,
+            config_snapshot.parent_thread_id,
             config.model_verbosity,
             config.features.enabled(Feature::EnableRequestCompression),
             config.features.enabled(Feature::RuntimeMetrics),

codex-rs/protocol/src/protocol.rs

@@ -2451,6 +2451,11 @@ impl InitialHistory {
             .and_then(|meta| meta.thread_source)
     }
 
+    pub fn get_resumed_parent_thread_id(&self) -> Option<ThreadId> {
+        self.get_resumed_session_meta()
+            .and_then(|meta| meta.parent_thread_id)
+    }
+
     fn get_resumed_session_meta(&self) -> Option<&SessionMeta> {
         match self {
             InitialHistory::New | InitialHistory::Cleared | InitialHistory::Forked(_) => None,
@@ -2716,6 +2721,8 @@ pub struct SessionMeta {
     pub id: ThreadId,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub forked_from_id: Option<ThreadId>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub parent_thread_id: Option<ThreadId>,
     pub timestamp: String,
     pub cwd: PathBuf,
     pub originator: String,
@@ -2750,6 +2757,7 @@ impl Default for SessionMeta {
         SessionMeta {
             id: ThreadId::default(),
             forked_from_id: None,
+            parent_thread_id: None,
             timestamp: String::new(),
             cwd: PathBuf::new(),
             originator: String::new(),
@@ -3421,6 +3429,8 @@ pub struct SessionConfiguredEvent {
     pub thread_id: ThreadId,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub forked_from_id: Option<ThreadId>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub parent_thread_id: Option<ThreadId>,
     /// Optional analytics source classification for this thread.
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub thread_source: Option<ThreadSource>,
@@ -3490,6 +3500,7 @@ impl<'de> Deserialize<'de> for SessionConfiguredEvent {
             #[serde(default)]
             thread_id: Option<ThreadId>,
             forked_from_id: Option<ThreadId>,
+            parent_thread_id: Option<ThreadId>,
             #[serde(default)]
             thread_source: Option<ThreadSource>,
             #[serde(default)]
@@ -3530,6 +3541,7 @@ impl<'de> Deserialize<'de> for SessionConfiguredEvent {
             session_id: wire.session_id,
             thread_id: wire.thread_id.unwrap_or_else(|| wire.session_id.into()),
             forked_from_id: wire.forked_from_id,
+            parent_thread_id: wire.parent_thread_id,
             thread_source: wire.thread_source,
             thread_name: wire.thread_name,
             model: wire.model,
@@ -5329,6 +5341,7 @@ mod tests {
                 session_id,
                 thread_id,
                 forked_from_id: None,
+                parent_thread_id: None,
                 thread_source: None,
                 thread_name: None,
                 model: "codex-mini-latest".to_string(),

codex-rs/rollout/src/list.rs

@@ -62,6 +62,8 @@ pub struct ThreadItem {
     pub git_origin_url: Option<String>,
     /// Session source from session metadata.
     pub source: Option<SessionSource>,
+    /// Immediate control/spawn parent thread id from session metadata.
+    pub parent_thread_id: Option<ThreadId>,
     /// Random unique nickname from session metadata for AgentControl-spawned sub-agents.
     pub agent_nickname: Option<String>,
     /// Role (agent_role) from session metadata for AgentControl-spawned sub-agents.
@@ -95,6 +97,7 @@ struct HeadTailSummary {
     git_sha: Option<String>,
     git_origin_url: Option<String>,
     source: Option<SessionSource>,
+    parent_thread_id: Option<ThreadId>,
     agent_nickname: Option<String>,
     agent_role: Option<String>,
     model_provider: Option<String>,
@@ -778,6 +781,7 @@ async fn build_thread_item(
             git_sha,
             git_origin_url,
             source,
+            parent_thread_id,
             agent_nickname,
             agent_role,
             model_provider,
@@ -799,6 +803,7 @@ async fn build_thread_item(
             git_sha,
             git_origin_url,
             source,
+            parent_thread_id,
             agent_nickname,
             agent_role,
             model_provider,
@@ -1101,6 +1106,7 @@ async fn read_head_summary(path: &Path, head_limit: usize) -> io::Result<HeadTai
             RolloutItem::SessionMeta(session_meta_line) => {
                 if !summary.saw_session_meta {
                     summary.source = Some(session_meta_line.meta.source.clone());
+                    summary.parent_thread_id = session_meta_line.meta.parent_thread_id;
                     summary.agent_nickname = session_meta_line.meta.agent_nickname.clone();
                     summary.agent_role = session_meta_line.meta.agent_role.clone();
                     summary.model_provider = session_meta_line.meta.model_provider.clone();

codex-rs/rollout/src/metadata_tests.rs

@@ -35,6 +35,7 @@ async fn extract_metadata_from_rollout_uses_session_meta() {
     let session_meta = SessionMeta {
         id,
         forked_from_id: None,
+        parent_thread_id: None,
         timestamp: "2026-01-27T12:34:56Z".to_string(),
         cwd: dir.path().to_path_buf(),
         originator: "cli".to_string(),
@@ -87,6 +88,7 @@ async fn extract_metadata_from_rollout_returns_latest_memory_mode() {
     let session_meta = SessionMeta {
         id,
         forked_from_id: None,
+        parent_thread_id: None,
         timestamp: "2026-01-27T12:34:56Z".to_string(),
         cwd: dir.path().to_path_buf(),
         originator: "cli".to_string(),
@@ -347,6 +349,7 @@ fn write_rollout_in_sessions_with_cwd(
     let session_meta = SessionMeta {
         id,
         forked_from_id: None,
+        parent_thread_id: None,
         timestamp: event_ts.to_string(),
         cwd,
         originator: "cli".to_string(),

codex-rs/rollout/src/recorder.rs

@@ -81,6 +81,7 @@ pub enum RolloutRecorderParams {
     Create {
         conversation_id: ThreadId,
         forked_from_id: Option<ThreadId>,
+        parent_thread_id: Option<ThreadId>,
         source: SessionSource,
         thread_source: Option<ThreadSource>,
         base_instructions: BaseInstructions,
@@ -156,6 +157,7 @@ impl RolloutRecorderParams {
     pub fn new(
         conversation_id: ThreadId,
         forked_from_id: Option<ThreadId>,
+        parent_thread_id: Option<ThreadId>,
         source: SessionSource,
         thread_source: Option<ThreadSource>,
         base_instructions: BaseInstructions,
@@ -164,6 +166,7 @@ impl RolloutRecorderParams {
         Self::Create {
             conversation_id,
             forked_from_id,
+            parent_thread_id,
             source,
             thread_source,
             base_instructions,
@@ -652,6 +655,7 @@ impl RolloutRecorder {
             RolloutRecorderParams::Create {
                 conversation_id,
                 forked_from_id,
+                parent_thread_id,
                 source,
                 thread_source,
                 base_instructions,
@@ -673,6 +677,7 @@ impl RolloutRecorder {
                 let session_meta = SessionMeta {
                     id: session_id,
                     forked_from_id,
+                    parent_thread_id,
                     timestamp,
                     cwd: config.cwd().to_path_buf(),
                     originator: originator().value,
@@ -1020,6 +1025,7 @@ fn fill_missing_thread_item_metadata(item: &mut ThreadItem, state_item: ThreadIt
         git_sha,
         git_origin_url,
         source,
+        parent_thread_id,
         agent_nickname,
         agent_role,
         model_provider,
@@ -1049,6 +1055,9 @@ fn fill_missing_thread_item_metadata(item: &mut ThreadItem, state_item: ThreadIt
     if item.source.is_none() {
         item.source = source;
     }
+    if item.parent_thread_id.is_none() {
+        item.parent_thread_id = parent_thread_id;
+    }
     if item.agent_nickname.is_none() {
         item.agent_nickname = agent_nickname;
     }
@@ -1690,6 +1699,7 @@ fn thread_item_from_state_metadata(item: codex_state::ThreadMetadata) -> ThreadI
                 .or_else(|_| serde_json::from_value(Value::String(item.source)))
                 .unwrap_or(SessionSource::Unknown),
         ),
+        parent_thread_id: None,
         agent_nickname: item.agent_nickname,
         agent_role: item.agent_role,
         model_provider: Some(item.model_provider),

codex-rs/rollout/src/recorder_tests.rs

@@ -85,6 +85,7 @@ async fn state_db_init_backfills_before_returning() -> anyhow::Result<()> {
         meta: SessionMeta {
             id: thread_id,
             forked_from_id: None,
+            parent_thread_id: None,
             timestamp: "2026-01-27T12:34:56Z".to_string(),
             cwd: home.path().to_path_buf(),
             originator: "test".to_string(),
@@ -369,6 +370,7 @@ async fn recorder_materializes_on_flush_with_pending_items() -> std::io::Result<
         RolloutRecorderParams::new(
             thread_id,
             /*forked_from_id*/ None,
+            /*parent_thread_id*/ None,
             SessionSource::Exec,
             /*thread_source*/ None,
             BaseInstructions::default(),
@@ -449,6 +451,7 @@ async fn persist_reports_filesystem_error_and_retries_buffered_items() -> std::i
         RolloutRecorderParams::new(
             thread_id,
             /*forked_from_id*/ None,
+            /*parent_thread_id*/ None,
             SessionSource::Exec,
             /*thread_source*/ None,
             BaseInstructions::default(),
@@ -974,6 +977,7 @@ fn fill_missing_thread_item_metadata_preserves_identity_and_prefers_state_git_fi
         git_sha: Some("filesystem-sha".to_string()),
         git_origin_url: Some("https://example.com/filesystem.git".to_string()),
         source: None,
+        parent_thread_id: None,
         agent_nickname: None,
         agent_role: None,
         model_provider: None,
@@ -991,6 +995,7 @@ fn fill_missing_thread_item_metadata_preserves_identity_and_prefers_state_git_fi
         git_sha: Some("state-sha".to_string()),
         git_origin_url: Some("https://example.com/state.git".to_string()),
         source: Some(SessionSource::Exec),
+        parent_thread_id: None,
         agent_nickname: Some("state-agent".to_string()),
         agent_role: Some("state-role".to_string()),
         model_provider: Some("state-provider".to_string()),

codex-rs/rollout/src/session_index_tests.rs

@@ -27,6 +27,7 @@ fn write_rollout_with_metadata(path: &Path, thread_id: ThreadId) -> std::io::Res
             meta: SessionMeta {
                 id: thread_id,
                 forked_from_id: None,
+                parent_thread_id: None,
                 timestamp,
                 cwd: ".".into(),
                 originator: "test_originator".into(),

codex-rs/rollout/src/tests.rs

@@ -606,6 +606,7 @@ async fn test_list_conversations_latest_first() {
                 git_sha: None,
                 git_origin_url: None,
                 source: Some(SessionSource::VSCode),
+                parent_thread_id: None,
                 agent_nickname: None,
                 agent_role: None,
                 model_provider: Some(TEST_PROVIDER.to_string()),
@@ -623,6 +624,7 @@ async fn test_list_conversations_latest_first() {
                 git_sha: None,
                 git_origin_url: None,
                 source: Some(SessionSource::VSCode),
+                parent_thread_id: None,
                 agent_nickname: None,
                 agent_role: None,
                 model_provider: Some(TEST_PROVIDER.to_string()),
@@ -640,6 +642,7 @@ async fn test_list_conversations_latest_first() {
                 git_sha: None,
                 git_origin_url: None,
                 source: Some(SessionSource::VSCode),
+                parent_thread_id: None,
                 agent_nickname: None,
                 agent_role: None,
                 model_provider: Some(TEST_PROVIDER.to_string()),
@@ -750,6 +753,7 @@ async fn test_pagination_cursor() {
                 git_sha: None,
                 git_origin_url: None,
                 source: Some(SessionSource::VSCode),
+                parent_thread_id: None,
                 agent_nickname: None,
                 agent_role: None,
                 model_provider: Some(TEST_PROVIDER.to_string()),
@@ -767,6 +771,7 @@ async fn test_pagination_cursor() {
                 git_sha: None,
                 git_origin_url: None,
                 source: Some(SessionSource::VSCode),
+                parent_thread_id: None,
                 agent_nickname: None,
                 agent_role: None,
                 model_provider: Some(TEST_PROVIDER.to_string()),
@@ -820,6 +825,7 @@ async fn test_pagination_cursor() {
                 git_sha: None,
                 git_origin_url: None,
                 source: Some(SessionSource::VSCode),
+                parent_thread_id: None,
                 agent_nickname: None,
                 agent_role: None,
                 model_provider: Some(TEST_PROVIDER.to_string()),
@@ -837,6 +843,7 @@ async fn test_pagination_cursor() {
                 git_sha: None,
                 git_origin_url: None,
                 source: Some(SessionSource::VSCode),
+                parent_thread_id: None,
                 agent_nickname: None,
                 agent_role: None,
                 model_provider: Some(TEST_PROVIDER.to_string()),
@@ -882,6 +889,7 @@ async fn test_pagination_cursor() {
             git_sha: None,
             git_origin_url: None,
             source: Some(SessionSource::VSCode),
+            parent_thread_id: None,
             agent_nickname: None,
             agent_role: None,
             model_provider: Some(TEST_PROVIDER.to_string()),
@@ -1052,6 +1060,7 @@ async fn test_get_thread_contents() {
             git_sha: None,
             git_origin_url: None,
             source: Some(SessionSource::VSCode),
+            parent_thread_id: None,
             agent_nickname: None,
             agent_role: None,
             model_provider: Some(TEST_PROVIDER.to_string()),
@@ -1251,6 +1260,7 @@ async fn test_updated_at_uses_file_mtime() -> Result<()> {
             meta: SessionMeta {
                 id: conversation_id,
                 forked_from_id: None,
+                parent_thread_id: None,
                 timestamp: ts.to_string(),
                 cwd: ".".into(),
                 originator: "test_originator".into(),
@@ -1403,6 +1413,7 @@ async fn test_timestamp_only_cursor_skips_same_second_filesystem_ties() {
                 git_sha: None,
                 git_origin_url: None,
                 source: Some(SessionSource::VSCode),
+                parent_thread_id: None,
                 agent_nickname: None,
                 agent_role: None,
                 model_provider: Some(TEST_PROVIDER.to_string()),
@@ -1420,6 +1431,7 @@ async fn test_timestamp_only_cursor_skips_same_second_filesystem_ties() {
                 git_sha: None,
                 git_origin_url: None,
                 source: Some(SessionSource::VSCode),
+                parent_thread_id: None,
                 agent_nickname: None,
                 agent_role: None,
                 model_provider: Some(TEST_PROVIDER.to_string()),

codex-rs/state/src/extract.rs

@@ -322,6 +322,7 @@ mod tests {
                     forked_from_id: Some(
                         ThreadId::from_string(&Uuid::now_v7().to_string()).expect("thread id"),
                     ),
+                    parent_thread_id: None,
                     timestamp: "2026-02-26T00:00:00.000Z".to_string(),
                     cwd: PathBuf::from("/child/worktree"),
                     originator: "codex_cli_rs".to_string(),
@@ -479,6 +480,7 @@ mod tests {
                 meta: SessionMeta {
                     id: thread_id,
                     forked_from_id: None,
+                    parent_thread_id: None,
                     timestamp: "2026-02-26T00:00:00.000Z".to_string(),
                     cwd: PathBuf::from("/workspace"),
                     originator: "codex_cli_rs".to_string(),

codex-rs/state/src/runtime/threads.rs

@@ -1318,6 +1318,7 @@ mod tests {
             meta: SessionMeta {
                 id: thread_id,
                 forked_from_id: None,
+                parent_thread_id: None,
                 timestamp: metadata.created_at.to_rfc3339(),
                 cwd: PathBuf::new(),
                 originator: String::new(),
@@ -1377,6 +1378,7 @@ mod tests {
             meta: SessionMeta {
                 id: thread_id,
                 forked_from_id: None,
+                parent_thread_id: None,
                 timestamp: created_at,
                 cwd: PathBuf::new(),
                 originator: String::new(),

codex-rs/thread-store/src/in_memory.rs

@@ -328,6 +328,7 @@ fn stored_thread_from_state(
             .and_then(|metadata| metadata.rollout_path.clone())
             .or(rollout_path),
         forked_from_id: created.forked_from_id,
+        parent_thread_id: created.parent_thread_id,
         preview: metadata
             .and_then(|metadata| metadata.preview.clone())
             .unwrap_or_default(),

codex-rs/thread-store/src/local/create_thread.rs

@@ -30,6 +30,7 @@ pub(super) async fn create_thread(
         RolloutRecorderParams::new(
             params.thread_id,
             params.forked_from_id,
+            params.parent_thread_id,
             params.source,
             params.thread_source,
             params.base_instructions,

codex-rs/thread-store/src/local/helpers.rs

@@ -122,6 +122,7 @@ pub(super) fn stored_thread_from_rollout_item(
         thread_id,
         rollout_path: Some(item.path),
         forked_from_id: None,
+        parent_thread_id: item.parent_thread_id,
         preview,
         name: None,
         model_provider: item

codex-rs/thread-store/src/local/mod.rs

@@ -1023,6 +1023,7 @@ mod tests {
         CreateThreadParams {
             thread_id,
             forked_from_id: None,
+            parent_thread_id: None,
             source: SessionSource::Exec,
             thread_source: None,
             base_instructions: BaseInstructions::default(),

codex-rs/thread-store/src/local/read_thread.rs

@@ -235,6 +235,7 @@ async fn read_thread_from_rollout_path(
     })?;
     if let Ok(meta_line) = read_session_meta_line(path.as_path()).await {
         thread.forked_from_id = meta_line.meta.forked_from_id;
+        thread.parent_thread_id = meta_line.meta.parent_thread_id;
         if let Some(model_provider) = meta_line
             .meta
             .model_provider
@@ -287,6 +288,7 @@ async fn stored_thread_from_sqlite_metadata(
         .ok()
         .map(|meta_line| meta_line.meta);
     let forked_from_id = session_meta.as_ref().and_then(|meta| meta.forked_from_id);
+    let parent_thread_id = session_meta.as_ref().and_then(|meta| meta.parent_thread_id);
     let preview = metadata
         .preview
         .clone()
@@ -298,6 +300,7 @@ async fn stored_thread_from_sqlite_metadata(
         thread_id: metadata.id,
         rollout_path: Some(metadata.rollout_path),
         forked_from_id,
+        parent_thread_id,
         preview,
         name,
         model_provider: if metadata.model_provider.is_empty() {
@@ -361,6 +364,7 @@ fn stored_thread_from_meta_line(
         thread_id: meta_line.meta.id,
         rollout_path: Some(path),
         forked_from_id: meta_line.meta.forked_from_id,
+        parent_thread_id: meta_line.meta.parent_thread_id,
         preview: String::new(),
         name: None,
         model_provider: meta_line

codex-rs/thread-store/src/types.rs

@@ -72,6 +72,8 @@ pub struct CreateThreadParams {
     pub thread_id: ThreadId,
     /// Source thread id when this thread is created as a fork.
     pub forked_from_id: Option<ThreadId>,
+    /// The ID of the parent thread. This will only be set if this thread is a subagent.
+    pub parent_thread_id: Option<ThreadId>,
     /// Runtime source for the thread.
     pub source: SessionSource,
     /// Optional analytics source classification for this thread.
@@ -362,6 +364,8 @@ pub struct StoredThread {
     pub rollout_path: Option<PathBuf>,
     /// Source thread id when this thread was forked from another thread.
     pub forked_from_id: Option<ThreadId>,
+    /// The ID of the parent thread. This will only be set if this thread is a subagent.
+    pub parent_thread_id: Option<ThreadId>,
     /// Best available user-facing preview, usually the first user message.
     pub preview: String,
     /// Optional user-facing thread name/title.

codex-rs/tui/src/app/loaded_threads.rs

@@ -120,6 +120,7 @@ mod tests {
             id: thread_id.to_string(),
             session_id: thread_id.to_string(),
             forked_from_id: None,
+            parent_thread_id: None,
             preview: String::new(),
             ephemeral: false,
             model_provider: "openai".to_string(),

codex-rs/tui/src/app/tests.rs

@@ -2767,6 +2767,7 @@ async fn inactive_thread_started_notification_initializes_replay_session() -> Re
                 id: agent_thread_id.to_string(),
                 session_id: agent_thread_id.to_string(),
                 forked_from_id: None,
+                parent_thread_id: None,
                 preview: "agent thread".to_string(),
                 ephemeral: false,
                 model_provider: "agent-provider".to_string(),
@@ -2856,6 +2857,7 @@ async fn inactive_thread_started_notification_preserves_primary_model_when_path_
                 id: agent_thread_id.to_string(),
                 session_id: agent_thread_id.to_string(),
                 forked_from_id: None,
+                parent_thread_id: None,
                 preview: "agent thread".to_string(),
                 ephemeral: false,
                 model_provider: "agent-provider".to_string(),
@@ -2914,6 +2916,7 @@ async fn thread_read_session_state_does_not_reuse_primary_permission_profile() {
         id: read_thread_id.to_string(),
         session_id: read_thread_id.to_string(),
         forked_from_id: None,
+        parent_thread_id: None,
         preview: "read thread".to_string(),
         ephemeral: false,
         model_provider: "read-provider".to_string(),
@@ -5011,6 +5014,7 @@ async fn thread_rollback_response_discards_queued_active_thread_events() {
                 id: thread_id.to_string(),
                 session_id: thread_id.to_string(),
                 forked_from_id: None,
+                parent_thread_id: None,
                 preview: String::new(),
                 ephemeral: false,
                 model_provider: "openai".to_string(),

codex-rs/tui/src/app/thread_session_state.rs

@@ -411,6 +411,7 @@ mod tests {
             id: read_thread_id.to_string(),
             session_id: read_thread_id.to_string(),
             forked_from_id: None,
+            parent_thread_id: None,
             preview: "read thread".to_string(),
             ephemeral: false,
             model_provider: "read-provider".to_string(),

codex-rs/tui/src/app_server_session.rs

@@ -2279,6 +2279,7 @@ mod tests {
                 id: thread_id.to_string(),
                 session_id: ThreadId::new().to_string(),
                 forked_from_id: Some(forked_from_id.to_string()),
+                parent_thread_id: None,
                 preview: "hello".to_string(),
                 ephemeral: false,
                 model_provider: "openai".to_string(),

codex-rs/tui/src/resume_picker.rs

@@ -5726,6 +5726,7 @@ session_picker_view = "dense"
             id: thread_id.to_string(),
             session_id: thread_id.to_string(),
             forked_from_id: None,
+            parent_thread_id: None,
             preview: String::from("remote thread"),
             ephemeral: false,
             model_provider: String::from("openai"),
@@ -5760,6 +5761,7 @@ session_picker_view = "dense"
             id: thread_id.to_string(),
             session_id: thread_id.to_string(),
             forked_from_id: None,
+            parent_thread_id: None,
             preview: String::from("preview"),
             ephemeral: false,
             model_provider: String::from("openai"),
@@ -5828,6 +5830,7 @@ session_picker_view = "dense"
             id: thread_id.to_string(),
             session_id: thread_id.to_string(),
             forked_from_id: None,
+            parent_thread_id: None,
             preview: String::from("preview"),
             ephemeral: false,
             model_provider: String::from("openai"),
@@ -5885,6 +5888,7 @@ session_picker_view = "dense"
             id: thread_id.to_string(),
             session_id: thread_id.to_string(),
             forked_from_id: None,
+            parent_thread_id: None,
             preview: String::from("preview"),
             ephemeral: false,
             model_provider: String::from("openai"),