diff --git a/codex-rs/analytics/src/analytics_client_tests.rs b/codex-rs/analytics/src/analytics_client_tests.rs
index 3a98fbb83..d51112a8b 100644
--- a/codex-rs/analytics/src/analytics_client_tests.rs
+++ b/codex-rs/analytics/src/analytics_client_tests.rs
@@ -58,6 +58,7 @@ use tokio::sync::mpsc;
 fn sample_thread(thread_id: &str, ephemeral: bool) -> Thread {
     Thread {
         id: thread_id.to_string(),
+        forked_from_id: None,
         preview: "first prompt".to_string(),
         ephemeral,
         model_provider: "openai".to_string(),
diff --git a/codex-rs/app-server-protocol/schema/json/ServerNotification.json b/codex-rs/app-server-protocol/schema/json/ServerNotification.json
index fc36adaa5..b8b539a03 100644
--- a/codex-rs/app-server-protocol/schema/json/ServerNotification.json
+++ b/codex-rs/app-server-protocol/schema/json/ServerNotification.json
@@ -2425,6 +2425,13 @@
           "description": "Whether the thread is ephemeral and should not be materialized on disk.",
           "type": "boolean"
         },
+        "forkedFromId": {
+          "description": "Source thread id when this thread was created by forking another thread.",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "gitInfo": {
           "anyOf": [
             {
diff --git a/codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.schemas.json b/codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.schemas.json
index 444c8af7c..a58990303 100644
--- a/codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.schemas.json
+++ b/codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.schemas.json
@@ -12186,6 +12186,13 @@
             "description": "Whether the thread is ephemeral and should not be materialized on disk.",
             "type": "boolean"
           },
+          "forkedFromId": {
+            "description": "Source thread id when this thread was created by forking another thread.",
+            "type": [
+              "string",
+              "null"
+            ]
+          },
           "gitInfo": {
             "anyOf": [
               {
diff --git a/codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.v2.schemas.json b/codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.v2.schemas.json
index e604157ad..f041f8aae 100644
--- a/codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.v2.schemas.json
+++ b/codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.v2.schemas.json
@@ -10041,6 +10041,13 @@
           "description": "Whether the thread is ephemeral and should not be materialized on disk.",
           "type": "boolean"
         },
+        "forkedFromId": {
+          "description": "Source thread id when this thread was created by forking another thread.",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "gitInfo": {
           "anyOf": [
             {
diff --git a/codex-rs/app-server-protocol/schema/json/v2/ThreadForkResponse.json b/codex-rs/app-server-protocol/schema/json/v2/ThreadForkResponse.json
index 5ae0c5a12..88448a165 100644
--- a/codex-rs/app-server-protocol/schema/json/v2/ThreadForkResponse.json
+++ b/codex-rs/app-server-protocol/schema/json/v2/ThreadForkResponse.json
@@ -1042,6 +1042,13 @@
           "description": "Whether the thread is ephemeral and should not be materialized on disk.",
           "type": "boolean"
         },
+        "forkedFromId": {
+          "description": "Source thread id when this thread was created by forking another thread.",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "gitInfo": {
           "anyOf": [
             {
diff --git a/codex-rs/app-server-protocol/schema/json/v2/ThreadListResponse.json b/codex-rs/app-server-protocol/schema/json/v2/ThreadListResponse.json
index 126d78603..f26bd03a3 100644
--- a/codex-rs/app-server-protocol/schema/json/v2/ThreadListResponse.json
+++ b/codex-rs/app-server-protocol/schema/json/v2/ThreadListResponse.json
@@ -800,6 +800,13 @@
           "description": "Whether the thread is ephemeral and should not be materialized on disk.",
           "type": "boolean"
         },
+        "forkedFromId": {
+          "description": "Source thread id when this thread was created by forking another thread.",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "gitInfo": {
           "anyOf": [
             {
diff --git a/codex-rs/app-server-protocol/schema/json/v2/ThreadMetadataUpdateResponse.json b/codex-rs/app-server-protocol/schema/json/v2/ThreadMetadataUpdateResponse.json
index dfdab228d..88c8e688d 100644
--- a/codex-rs/app-server-protocol/schema/json/v2/ThreadMetadataUpdateResponse.json
+++ b/codex-rs/app-server-protocol/schema/json/v2/ThreadMetadataUpdateResponse.json
@@ -800,6 +800,13 @@
           "description": "Whether the thread is ephemeral and should not be materialized on disk.",
           "type": "boolean"
         },
+        "forkedFromId": {
+          "description": "Source thread id when this thread was created by forking another thread.",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "gitInfo": {
           "anyOf": [
             {
diff --git a/codex-rs/app-server-protocol/schema/json/v2/ThreadReadResponse.json b/codex-rs/app-server-protocol/schema/json/v2/ThreadReadResponse.json
index 8f48dee4b..845320738 100644
--- a/codex-rs/app-server-protocol/schema/json/v2/ThreadReadResponse.json
+++ b/codex-rs/app-server-protocol/schema/json/v2/ThreadReadResponse.json
@@ -800,6 +800,13 @@
           "description": "Whether the thread is ephemeral and should not be materialized on disk.",
           "type": "boolean"
         },
+        "forkedFromId": {
+          "description": "Source thread id when this thread was created by forking another thread.",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "gitInfo": {
           "anyOf": [
             {
diff --git a/codex-rs/app-server-protocol/schema/json/v2/ThreadResumeResponse.json b/codex-rs/app-server-protocol/schema/json/v2/ThreadResumeResponse.json
index edccc337d..e21f253b7 100644
--- a/codex-rs/app-server-protocol/schema/json/v2/ThreadResumeResponse.json
+++ b/codex-rs/app-server-protocol/schema/json/v2/ThreadResumeResponse.json
@@ -1042,6 +1042,13 @@
           "description": "Whether the thread is ephemeral and should not be materialized on disk.",
           "type": "boolean"
         },
+        "forkedFromId": {
+          "description": "Source thread id when this thread was created by forking another thread.",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "gitInfo": {
           "anyOf": [
             {
diff --git a/codex-rs/app-server-protocol/schema/json/v2/ThreadRollbackResponse.json b/codex-rs/app-server-protocol/schema/json/v2/ThreadRollbackResponse.json
index cc41aac27..d719ba7d8 100644
--- a/codex-rs/app-server-protocol/schema/json/v2/ThreadRollbackResponse.json
+++ b/codex-rs/app-server-protocol/schema/json/v2/ThreadRollbackResponse.json
@@ -800,6 +800,13 @@
           "description": "Whether the thread is ephemeral and should not be materialized on disk.",
           "type": "boolean"
         },
+        "forkedFromId": {
+          "description": "Source thread id when this thread was created by forking another thread.",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "gitInfo": {
           "anyOf": [
             {
diff --git a/codex-rs/app-server-protocol/schema/json/v2/ThreadStartResponse.json b/codex-rs/app-server-protocol/schema/json/v2/ThreadStartResponse.json
index c3b50fee3..27a8cdd6b 100644
--- a/codex-rs/app-server-protocol/schema/json/v2/ThreadStartResponse.json
+++ b/codex-rs/app-server-protocol/schema/json/v2/ThreadStartResponse.json
@@ -1042,6 +1042,13 @@
           "description": "Whether the thread is ephemeral and should not be materialized on disk.",
           "type": "boolean"
         },
+        "forkedFromId": {
+          "description": "Source thread id when this thread was created by forking another thread.",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "gitInfo": {
           "anyOf": [
             {
diff --git a/codex-rs/app-server-protocol/schema/json/v2/ThreadStartedNotification.json b/codex-rs/app-server-protocol/schema/json/v2/ThreadStartedNotification.json
index 224015039..c202363e3 100644
--- a/codex-rs/app-server-protocol/schema/json/v2/ThreadStartedNotification.json
+++ b/codex-rs/app-server-protocol/schema/json/v2/ThreadStartedNotification.json
@@ -800,6 +800,13 @@
           "description": "Whether the thread is ephemeral and should not be materialized on disk.",
           "type": "boolean"
         },
+        "forkedFromId": {
+          "description": "Source thread id when this thread was created by forking another thread.",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "gitInfo": {
           "anyOf": [
             {
diff --git a/codex-rs/app-server-protocol/schema/json/v2/ThreadUnarchiveResponse.json b/codex-rs/app-server-protocol/schema/json/v2/ThreadUnarchiveResponse.json
index 41b0d2d40..542aea176 100644
--- a/codex-rs/app-server-protocol/schema/json/v2/ThreadUnarchiveResponse.json
+++ b/codex-rs/app-server-protocol/schema/json/v2/ThreadUnarchiveResponse.json
@@ -800,6 +800,13 @@
           "description": "Whether the thread is ephemeral and should not be materialized on disk.",
           "type": "boolean"
         },
+        "forkedFromId": {
+          "description": "Source thread id when this thread was created by forking another thread.",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "gitInfo": {
           "anyOf": [
             {
diff --git a/codex-rs/app-server-protocol/schema/typescript/v2/Thread.ts b/codex-rs/app-server-protocol/schema/typescript/v2/Thread.ts
index 56c568849..7a8bf4aca 100644
--- a/codex-rs/app-server-protocol/schema/typescript/v2/Thread.ts
+++ b/codex-rs/app-server-protocol/schema/typescript/v2/Thread.ts
@@ -7,6 +7,10 @@ import type { ThreadStatus } from "./ThreadStatus";
 import type { Turn } from "./Turn";
 
 export type Thread = { id: string, 
+/**
+ * Source thread id when this thread was created by forking another thread.
+ */
+forkedFromId: string | null, 
 /**
  * Usually the first user message in the thread, if available.
  */
diff --git a/codex-rs/app-server-protocol/src/protocol/common.rs b/codex-rs/app-server-protocol/src/protocol/common.rs
index 30061c716..ebd1ba11c 100644
--- a/codex-rs/app-server-protocol/src/protocol/common.rs
+++ b/codex-rs/app-server-protocol/src/protocol/common.rs
@@ -1307,6 +1307,7 @@ mod tests {
             response: v2::ThreadStartResponse {
                 thread: v2::Thread {
                     id: "67e55044-10b1-426f-9247-bb680e5fe0c8".to_string(),
+                    forked_from_id: None,
                     preview: "first prompt".to_string(),
                     ephemeral: true,
                     model_provider: "openai".to_string(),
@@ -1343,6 +1344,7 @@ mod tests {
                 "response": {
                     "thread": {
                         "id": "67e55044-10b1-426f-9247-bb680e5fe0c8",
+                        "forkedFromId": null,
                         "preview": "first prompt",
                         "ephemeral": true,
                         "modelProvider": "openai",
diff --git a/codex-rs/app-server-protocol/src/protocol/v2.rs b/codex-rs/app-server-protocol/src/protocol/v2.rs
index a6a4aa540..82b568515 100644
--- a/codex-rs/app-server-protocol/src/protocol/v2.rs
+++ b/codex-rs/app-server-protocol/src/protocol/v2.rs
@@ -3574,6 +3574,8 @@ impl From<CoreSkillErrorInfo> for SkillErrorInfo {
 #[ts(export_to = "v2/")]
 pub struct Thread {
     pub id: String,
+    /// Source thread id when this thread was created by forking another thread.
+    pub forked_from_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.
diff --git a/codex-rs/app-server/README.md b/codex-rs/app-server/README.md
index 2f244909f..43cdbf08e 100644
--- a/codex-rs/app-server/README.md
+++ b/codex-rs/app-server/README.md
@@ -134,7 +134,7 @@ Example with notification opt-out:
 
 - `thread/start` — create a new thread; emits `thread/started` (including the current `thread.status`) and auto-subscribes you to turn/item events for that thread.
 - `thread/resume` — reopen an existing thread by id so subsequent `turn/start` calls append to it.
-- `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. 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.
+- `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.
 - `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/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.
diff --git a/codex-rs/app-server/src/codex_message_processor.rs b/codex-rs/app-server/src/codex_message_processor.rs
index 2b9aa12ab..e5d51dd47 100644
--- a/codex-rs/app-server/src/codex_message_processor.rs
+++ b/codex-rs/app-server/src/codex_message_processor.rs
@@ -3523,6 +3523,11 @@ impl CodexMessageProcessor {
             }
             build_thread_from_snapshot(thread_uuid, &config_snapshot, loaded_rollout_path)
         };
+        if thread.forked_from_id.is_none()
+            && let Some(rollout_path) = rollout_path.as_ref()
+        {
+            thread.forked_from_id = forked_from_id_from_rollout(rollout_path).await;
+        }
         self.attach_thread_name(thread_uuid, &mut thread).await;
 
         if include_turns && let Some(rollout_path) = rollout_path.as_ref() {
@@ -4352,7 +4357,12 @@ impl CodexMessageProcessor {
             )
             .await
             {
-                Ok(summary) => summary_to_thread(summary),
+                Ok(summary) => {
+                    let mut thread = summary_to_thread(summary);
+                    thread.forked_from_id =
+                        forked_from_id_from_rollout(fork_rollout_path.as_path()).await;
+                    thread
+                }
                 Err(err) => {
                     self.send_internal_error(
                         request_id,
@@ -4386,6 +4396,14 @@ impl CodexMessageProcessor {
                 }
             };
             thread.preview = preview_from_rollout_items(&history_items);
+            thread.forked_from_id = source_thread_id
+                .or_else(|| {
+                    history_items.iter().find_map(|item| match item {
+                        RolloutItem::SessionMeta(meta_line) => Some(meta_line.meta.id),
+                        _ => None,
+                    })
+                })
+                .map(|id| id.to_string());
             if let Err(message) = populate_thread_turns(
                 &mut thread,
                 ThreadTurnSource::HistoryItems(&history_items),
@@ -8566,6 +8584,7 @@ async fn load_thread_summary_for_rollout(
                 rollout_path.display()
             )
         })?;
+    thread.forked_from_id = forked_from_id_from_rollout(rollout_path).await;
     if let Some(persisted_metadata) = persisted_metadata {
         merge_mutable_thread_metadata(
             &mut thread,
@@ -8577,6 +8596,14 @@ async fn load_thread_summary_for_rollout(
     Ok(thread)
 }
 
+async fn forked_from_id_from_rollout(path: &Path) -> Option<String> {
+    read_session_meta_line(path)
+        .await
+        .ok()
+        .and_then(|meta_line| meta_line.meta.forked_from_id)
+        .map(|thread_id| thread_id.to_string())
+}
+
 fn merge_mutable_thread_metadata(thread: &mut Thread, persisted_thread: Thread) {
     thread.git_info = persisted_thread.git_info;
 }
@@ -8657,6 +8684,7 @@ fn build_thread_from_snapshot(
     let now = time::OffsetDateTime::now_utc().unix_timestamp();
     Thread {
         id: thread_id.to_string(),
+        forked_from_id: None,
         preview: String::new(),
         ephemeral: config_snapshot.ephemeral,
         model_provider: config_snapshot.model_provider_id.clone(),
@@ -8699,6 +8727,7 @@ pub(crate) fn summary_to_thread(summary: ConversationSummary) -> Thread {
 
     Thread {
         id: conversation_id.to_string(),
+        forked_from_id: None,
         preview,
         ephemeral: false,
         model_provider,
@@ -9195,6 +9224,44 @@ mod tests {
         Ok(())
     }
 
+    #[tokio::test]
+    async fn read_summary_from_rollout_preserves_forked_from_id() -> Result<()> {
+        use codex_protocol::protocol::RolloutItem;
+        use codex_protocol::protocol::RolloutLine;
+        use codex_protocol::protocol::SessionMetaLine;
+        use std::fs;
+
+        let temp_dir = TempDir::new()?;
+        let path = temp_dir.path().join("rollout.jsonl");
+
+        let conversation_id = ThreadId::from_string("bfd12a78-5900-467b-9bc5-d3d35df08191")?;
+        let forked_from_id = ThreadId::from_string("ad7f0408-99b8-4f6e-a46f-bd0eec433370")?;
+        let timestamp = "2025-09-05T16:53:11.850Z".to_string();
+
+        let session_meta = SessionMeta {
+            id: conversation_id,
+            forked_from_id: Some(forked_from_id),
+            timestamp: timestamp.clone(),
+            model_provider: Some("test-provider".to_string()),
+            ..SessionMeta::default()
+        };
+
+        let line = RolloutLine {
+            timestamp,
+            item: RolloutItem::SessionMeta(SessionMetaLine {
+                meta: session_meta,
+                git: None,
+            }),
+        };
+        fs::write(&path, format!("{}\n", serde_json::to_string(&line)?))?;
+
+        assert_eq!(
+            forked_from_id_from_rollout(path.as_path()).await,
+            Some(forked_from_id.to_string())
+        );
+        Ok(())
+    }
+
     #[tokio::test]
     async fn aborting_pending_request_clears_pending_state() -> Result<()> {
         let thread_id = ThreadId::from_string("bfd12a78-5900-467b-9bc5-d3d35df08191")?;
diff --git a/codex-rs/app-server/src/thread_status.rs b/codex-rs/app-server/src/thread_status.rs
index 0f0b3eea7..802f7e197 100644
--- a/codex-rs/app-server/src/thread_status.rs
+++ b/codex-rs/app-server/src/thread_status.rs
@@ -792,6 +792,7 @@ mod tests {
     fn test_thread(thread_id: &str, source: codex_app_server_protocol::SessionSource) -> Thread {
         Thread {
             id: thread_id.to_string(),
+            forked_from_id: None,
             preview: String::new(),
             ephemeral: false,
             model_provider: "mock-provider".to_string(),
diff --git a/codex-rs/app-server/tests/suite/v2/thread_fork.rs b/codex-rs/app-server/tests/suite/v2/thread_fork.rs
index 0a62ad7ff..fd65a681a 100644
--- a/codex-rs/app-server/tests/suite/v2/thread_fork.rs
+++ b/codex-rs/app-server/tests/suite/v2/thread_fork.rs
@@ -112,6 +112,7 @@ async fn thread_fork_creates_new_thread_and_emits_started() -> Result<()> {
     );
 
     assert_ne!(thread.id, conversation_id);
+    assert_eq!(thread.forked_from_id, Some(conversation_id.clone()));
     assert_eq!(thread.preview, preview);
     assert_eq!(thread.model_provider, "mock_provider");
     assert_eq!(thread.status, ThreadStatus::Idle);
diff --git a/codex-rs/app-server/tests/suite/v2/thread_read.rs b/codex-rs/app-server/tests/suite/v2/thread_read.rs
index e565cf133..c5fd69985 100644
--- a/codex-rs/app-server/tests/suite/v2/thread_read.rs
+++ b/codex-rs/app-server/tests/suite/v2/thread_read.rs
@@ -7,6 +7,8 @@ use codex_app_server_protocol::JSONRPCError;
 use codex_app_server_protocol::JSONRPCResponse;
 use codex_app_server_protocol::RequestId;
 use codex_app_server_protocol::SessionSource;
+use codex_app_server_protocol::ThreadForkParams;
+use codex_app_server_protocol::ThreadForkResponse;
 use codex_app_server_protocol::ThreadItem;
 use codex_app_server_protocol::ThreadListParams;
 use codex_app_server_protocol::ThreadListResponse;
@@ -152,6 +154,56 @@ async fn thread_read_can_include_turns() -> Result<()> {
     Ok(())
 }
 
+#[tokio::test]
+async fn thread_read_returns_forked_from_id_for_forked_threads() -> Result<()> {
+    let server = create_mock_responses_server_repeating_assistant("Done").await;
+    let codex_home = TempDir::new()?;
+    create_config_toml(codex_home.path(), &server.uri())?;
+
+    let conversation_id = create_fake_rollout_with_text_elements(
+        codex_home.path(),
+        "2025-01-05T12-00-00",
+        "2025-01-05T12:00:00Z",
+        "Saved user message",
+        vec![],
+        Some("mock_provider"),
+        /*git_info*/ None,
+    )?;
+
+    let mut mcp = McpProcess::new(codex_home.path()).await?;
+    timeout(DEFAULT_READ_TIMEOUT, mcp.initialize()).await??;
+
+    let fork_id = mcp
+        .send_thread_fork_request(ThreadForkParams {
+            thread_id: conversation_id.clone(),
+            ..Default::default()
+        })
+        .await?;
+    let fork_resp: JSONRPCResponse = timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_response_message(RequestId::Integer(fork_id)),
+    )
+    .await??;
+    let ThreadForkResponse { thread: forked, .. } = to_response::<ThreadForkResponse>(fork_resp)?;
+
+    let read_id = mcp
+        .send_thread_read_request(ThreadReadParams {
+            thread_id: forked.id,
+            include_turns: false,
+        })
+        .await?;
+    let read_resp: JSONRPCResponse = timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_response_message(RequestId::Integer(read_id)),
+    )
+    .await??;
+    let ThreadReadResponse { thread } = to_response::<ThreadReadResponse>(read_resp)?;
+
+    assert_eq!(thread.forked_from_id, Some(conversation_id));
+
+    Ok(())
+}
+
 #[tokio::test]
 async fn thread_read_loaded_thread_returns_precomputed_path_before_materialization() -> Result<()> {
     let server = create_mock_responses_server_repeating_assistant("Done").await;
diff --git a/codex-rs/exec/src/lib_tests.rs b/codex-rs/exec/src/lib_tests.rs
index b223ba13c..09b43471d 100644
--- a/codex-rs/exec/src/lib_tests.rs
+++ b/codex-rs/exec/src/lib_tests.rs
@@ -242,6 +242,7 @@ async fn resume_lookup_model_providers_filters_only_last_lookup() {
 fn turn_items_for_thread_returns_matching_turn_items() {
     let thread = AppServerThread {
         id: "thread-1".to_string(),
+        forked_from_id: None,
         preview: String::new(),
         ephemeral: false,
         model_provider: "openai".to_string(),
@@ -360,6 +361,7 @@ fn session_configured_from_thread_response_uses_review_policy_from_response() {
     let response = ThreadStartResponse {
         thread: codex_app_server_protocol::Thread {
             id: "67e55044-10b1-426f-9247-bb680e5fe0c8".to_string(),
+            forked_from_id: None,
             preview: String::new(),
             ephemeral: false,
             model_provider: "openai".to_string(),
diff --git a/codex-rs/tui/src/app.rs b/codex-rs/tui/src/app.rs
index 6b8749b3c..e262b55a3 100644
--- a/codex-rs/tui/src/app.rs
+++ b/codex-rs/tui/src/app.rs
@@ -8639,6 +8639,7 @@ guardian_approval = true
             ServerNotification::ThreadStarted(ThreadStartedNotification {
                 thread: Thread {
                     id: agent_thread_id.to_string(),
+                    forked_from_id: None,
                     preview: "agent thread".to_string(),
                     ephemeral: false,
                     model_provider: "agent-provider".to_string(),
@@ -8719,6 +8720,7 @@ guardian_approval = true
             ServerNotification::ThreadStarted(ThreadStartedNotification {
                 thread: Thread {
                     id: agent_thread_id.to_string(),
+                    forked_from_id: None,
                     preview: "agent thread".to_string(),
                     ephemeral: false,
                     model_provider: "agent-provider".to_string(),
@@ -10680,6 +10682,7 @@ guardian_approval = true
             &ThreadRollbackResponse {
                 thread: Thread {
                     id: thread_id.to_string(),
+                    forked_from_id: None,
                     preview: String::new(),
                     ephemeral: false,
                     model_provider: "openai".to_string(),
diff --git a/codex-rs/tui/src/app/app_server_adapter.rs b/codex-rs/tui/src/app/app_server_adapter.rs
index 12b1c669b..ef5a061e3 100644
--- a/codex-rs/tui/src/app/app_server_adapter.rs
+++ b/codex-rs/tui/src/app/app_server_adapter.rs
@@ -1251,6 +1251,7 @@ mod tests {
     fn replays_command_execution_items_from_thread_snapshots() {
         let thread = Thread {
             id: "019cee8c-b993-7e33-88c0-014d4e62612d".to_string(),
+            forked_from_id: None,
             preview: String::new(),
             ephemeral: false,
             model_provider: "openai".to_string(),
@@ -1417,6 +1418,7 @@ mod tests {
         let events = thread_snapshot_events(
             &Thread {
                 id: thread_id.to_string(),
+                forked_from_id: None,
                 preview: "hello".to_string(),
                 ephemeral: false,
                 model_provider: "openai".to_string(),
diff --git a/codex-rs/tui/src/app/loaded_threads.rs b/codex-rs/tui/src/app/loaded_threads.rs
index c6ac09890..6351e3e32 100644
--- a/codex-rs/tui/src/app/loaded_threads.rs
+++ b/codex-rs/tui/src/app/loaded_threads.rs
@@ -111,6 +111,7 @@ mod tests {
     fn test_thread(thread_id: ThreadId, source: SessionSource) -> Thread {
         Thread {
             id: thread_id.to_string(),
+            forked_from_id: None,
             preview: String::new(),
             ephemeral: false,
             model_provider: "openai".to_string(),
diff --git a/codex-rs/tui/src/app_server_session.rs b/codex-rs/tui/src/app_server_session.rs
index 68424de82..c0be4a94f 100644
--- a/codex-rs/tui/src/app_server_session.rs
+++ b/codex-rs/tui/src/app_server_session.rs
@@ -945,6 +945,7 @@ async fn thread_session_state_from_thread_start_response(
 ) -> Result<ThreadSessionState, String> {
     thread_session_state_from_thread_response(
         &response.thread.id,
+        response.thread.forked_from_id.clone(),
         response.thread.name.clone(),
         response.thread.path.clone(),
         response.model.clone(),
@@ -966,6 +967,7 @@ async fn thread_session_state_from_thread_resume_response(
 ) -> Result<ThreadSessionState, String> {
     thread_session_state_from_thread_response(
         &response.thread.id,
+        response.thread.forked_from_id.clone(),
         response.thread.name.clone(),
         response.thread.path.clone(),
         response.model.clone(),
@@ -987,6 +989,7 @@ async fn thread_session_state_from_thread_fork_response(
 ) -> Result<ThreadSessionState, String> {
     thread_session_state_from_thread_response(
         &response.thread.id,
+        response.thread.forked_from_id.clone(),
         response.thread.name.clone(),
         response.thread.path.clone(),
         response.model.clone(),
@@ -1027,6 +1030,7 @@ fn review_target_to_app_server(
 )]
 async fn thread_session_state_from_thread_response(
     thread_id: &str,
+    forked_from_id: Option<String>,
     thread_name: Option<String>,
     rollout_path: Option<PathBuf>,
     model: String,
@@ -1041,12 +1045,17 @@ async fn thread_session_state_from_thread_response(
 ) -> Result<ThreadSessionState, String> {
     let thread_id = ThreadId::from_string(thread_id)
         .map_err(|err| format!("thread id `{thread_id}` is invalid: {err}"))?;
+    let forked_from_id = forked_from_id
+        .as_deref()
+        .map(ThreadId::from_string)
+        .transpose()
+        .map_err(|err| format!("forked_from_id is invalid: {err}"))?;
     let (history_log_id, history_entry_count) = message_history::history_metadata(config).await;
     let history_entry_count = u64::try_from(history_entry_count).unwrap_or(u64::MAX);
 
     Ok(ThreadSessionState {
         thread_id,
-        forked_from_id: None,
+        forked_from_id,
         thread_name,
         model,
         model_provider_id,
@@ -1164,9 +1173,11 @@ mod tests {
         let temp_dir = tempfile::tempdir().expect("tempdir");
         let config = build_config(&temp_dir).await;
         let thread_id = ThreadId::new();
+        let forked_from_id = ThreadId::new();
         let response = ThreadResumeResponse {
             thread: codex_app_server_protocol::Thread {
                 id: thread_id.to_string(),
+                forked_from_id: Some(forked_from_id.to_string()),
                 preview: "hello".to_string(),
                 ephemeral: false,
                 model_provider: "openai".to_string(),
@@ -1215,6 +1226,7 @@ mod tests {
         let started = started_thread_from_resume_response(response.clone(), &config)
             .await
             .expect("resume response should map");
+        assert_eq!(started.session.forked_from_id, Some(forked_from_id));
         assert_eq!(started.turns.len(), 1);
         assert_eq!(started.turns[0], response.thread.turns[0]);
     }
@@ -1234,6 +1246,7 @@ mod tests {
 
         let session = thread_session_state_from_thread_response(
             &thread_id.to_string(),
+            /*forked_from_id*/ None,
             Some("restore".to_string()),
             /*rollout_path*/ None,
             "gpt-5.4".to_string(),
@@ -1253,6 +1266,34 @@ mod tests {
         assert_eq!(session.history_entry_count, 2);
     }
 
+    #[tokio::test]
+    async fn session_configured_preserves_fork_source_thread_id() {
+        let temp_dir = tempfile::tempdir().expect("tempdir");
+        let config = build_config(&temp_dir).await;
+        let thread_id = ThreadId::new();
+        let forked_from_id = ThreadId::new();
+
+        let session = thread_session_state_from_thread_response(
+            &thread_id.to_string(),
+            Some(forked_from_id.to_string()),
+            Some("restore".to_string()),
+            /*rollout_path*/ None,
+            "gpt-5.4".to_string(),
+            "openai".to_string(),
+            /*service_tier*/ None,
+            AskForApproval::Never,
+            codex_protocol::config_types::ApprovalsReviewer::User,
+            SandboxPolicy::new_read_only_policy(),
+            PathBuf::from("/tmp/project"),
+            /*reasoning_effort*/ None,
+            &config,
+        )
+        .await
+        .expect("session should map");
+
+        assert_eq!(session.forked_from_id, Some(forked_from_id));
+    }
+
     #[test]
     fn status_account_display_from_auth_mode_uses_remapped_plan_labels() {
         let business = status_account_display_from_auth_mode(
diff --git a/codex-rs/tui/src/resume_picker.rs b/codex-rs/tui/src/resume_picker.rs
index 0b7c50467..ba399521d 100644
--- a/codex-rs/tui/src/resume_picker.rs
+++ b/codex-rs/tui/src/resume_picker.rs
@@ -2639,6 +2639,7 @@ mod tests {
         let thread_id = ThreadId::new();
         let thread = Thread {
             id: thread_id.to_string(),
+            forked_from_id: None,
             preview: String::from("remote thread"),
             ephemeral: false,
             model_provider: String::from("openai"),