Add SubagentStop hook (#22873)

# What

<img width="1792" height="1024" alt="image"
src="https://github.com/user-attachments/assets/8f81d232-5813-4994-a61d-e42a05a93a3e"
/>

`SubagentStop` runs when a thread-spawned subagent turn is about to
finish. Thread-spawned subagents use `SubagentStop` instead of the
normal root-agent `Stop` hook.

Configured handlers match on `agent_type`. Hook input includes the
normal stop fields plus:

- `agent_id`: the child thread id.
- `agent_type`: the resolved subagent type.
- `agent_transcript_path`: the child subagent transcript path.
- `transcript_path`: the parent thread transcript path.
- `last_assistant_message`: the final assistant message from the child
turn, when available.
- `stop_hook_active`: `true` when the child is already continuing
because an earlier stop-like hook blocked completion.

`SubagentStop` shares the same completion-control semantics as `Stop`,
scoped to the child turn:

- No decision allows the child turn to finish.
- `decision: "block"` with a non-empty `reason` records that reason as
hook feedback and continues the child with that prompt.
- `continue: false` stops the child turn. If `stopReason` is present,
Codex surfaces it as the stop reason.

# Lifecycle Scope

Only thread-spawned subagents run `SubagentStop`.

Internal/system subagents such as Review, Compact, MemoryConsolidation,
and Other do not run normal `Stop` hooks and do not run `SubagentStop`.
This avoids exposing synthetic matcher labels for internal
implementation paths.

# Stack

1. #22782: add `SubagentStart`.
2. This PR: add `SubagentStop`.
3. #22882: add subagent identity to normal hook inputs.

codex-rs/core/src/hook_runtime.rs

@@ -14,8 +14,8 @@ use codex_hooks::PreToolUseOutcome;
 use codex_hooks::PreToolUseRequest;
 use codex_hooks::SessionStartOutcome;
 use codex_hooks::StartHookTarget;
+use codex_hooks::StopHookTarget;
 use codex_hooks::StopOutcome;
-use codex_hooks::StopRequest;
 use codex_hooks::UserPromptSubmitOutcome;
 use codex_hooks::UserPromptSubmitRequest;
 use codex_otel::HOOK_RUN_DURATION_METRIC;
@@ -33,6 +33,7 @@ use codex_protocol::protocol::HookSource;
 use codex_protocol::protocol::HookStartedEvent;
 use codex_protocol::protocol::SessionSource;
 use codex_protocol::protocol::SubAgentSource;
+use codex_thread_store::ReadThreadParams;
 use serde_json::Value;
 
 use crate::context::ContextualUserFragment;
@@ -288,6 +289,78 @@ pub(crate) async fn run_post_tool_use_hooks(
     outcome
 }
 
+pub(crate) async fn run_turn_stop_hooks(
+    sess: &Arc<Session>,
+    turn_context: &Arc<TurnContext>,
+    stop_hook_active: bool,
+    last_assistant_message: Option<String>,
+) -> StopOutcome {
+    // Resolve the stop hook kind from the session source before building the
+    // request. Root turns run Stop; thread-spawned child turns run SubagentStop.
+    let (target, transcript_path) = match &turn_context.session_source {
+        SessionSource::SubAgent(SubAgentSource::ThreadSpawn {
+            agent_role,
+            parent_thread_id,
+            ..
+        }) => {
+            let agent_type = agent_role
+                .clone()
+                .unwrap_or_else(|| crate::agent::role::DEFAULT_ROLE_NAME.to_string());
+            let agent_transcript_path = sess.hook_transcript_path().await;
+            let parent_transcript_path = match sess
+                .services
+                .thread_store
+                .read_thread(ReadThreadParams {
+                    thread_id: *parent_thread_id,
+                    include_archived: true,
+                    include_history: false,
+                })
+                .await
+            {
+                Ok(thread) => thread.rollout_path,
+                Err(error) => {
+                    tracing::warn!(
+                        parent_thread_id = %parent_thread_id,
+                        error = %error,
+                        "failed to resolve parent transcript path for subagent hook"
+                    );
+                    None
+                }
+            };
+            (
+                StopHookTarget::SubagentStop {
+                    agent_id: sess.thread_id().to_string(),
+                    agent_type,
+                    agent_transcript_path,
+                },
+                parent_transcript_path,
+            )
+        }
+        // Internal/synthetic subagents do not expose user-configured lifecycle
+        // hooks, so there is no Stop or SubagentStop request to dispatch.
+        SessionSource::SubAgent(_) => return StopOutcome::default(),
+        _ => (StopHookTarget::Stop, sess.hook_transcript_path().await),
+    };
+    let request = codex_hooks::StopRequest {
+        session_id: sess.session_id().into(),
+        turn_id: turn_context.sub_id.clone(),
+        #[allow(deprecated)]
+        cwd: turn_context.cwd.clone(),
+        transcript_path,
+        model: turn_context.model_info.slug.clone(),
+        permission_mode: hook_permission_mode(turn_context),
+        stop_hook_active,
+        last_assistant_message,
+        target,
+    };
+    let hooks = sess.hooks();
+    emit_hook_started_events(sess, turn_context, hooks.preview_stop(&request)).await;
+
+    let mut outcome = hooks.run_stop(request).await;
+    emit_hook_completed_events(sess, turn_context, std::mem::take(&mut outcome.hook_events)).await;
+    outcome
+}
+
 pub(crate) async fn run_pre_compact_hooks(
     sess: &Arc<Session>,
     turn_context: &Arc<TurnContext>,
@@ -352,30 +425,6 @@ pub(crate) async fn run_post_compact_hooks(
     }
 }
 
-pub(crate) async fn run_stop_hooks(
-    sess: &Arc<Session>,
-    turn_context: &Arc<TurnContext>,
-    stop_hook_active: bool,
-    last_assistant_message: Option<String>,
-) -> StopOutcome {
-    let request = StopRequest {
-        session_id: sess.session_id().into(),
-        turn_id: turn_context.sub_id.clone(),
-        #[allow(deprecated)]
-        cwd: turn_context.cwd.clone(),
-        transcript_path: sess.hook_transcript_path().await,
-        model: turn_context.model_info.slug.clone(),
-        permission_mode: hook_permission_mode(turn_context),
-        stop_hook_active,
-        last_assistant_message,
-    };
-    let hooks = sess.hooks();
-    emit_hook_started_events(sess, turn_context, hooks.preview_stop(&request)).await;
-    let mut outcome = hooks.run_stop(request).await;
-    emit_hook_completed_events(sess, turn_context, std::mem::take(&mut outcome.hook_events)).await;
-    outcome
-}
-
 pub(crate) async fn run_legacy_after_agent_hook(
     sess: &Arc<Session>,
     turn_context: &Arc<TurnContext>,
@@ -638,6 +687,7 @@ fn hook_run_metric_tags(run: &HookRunSummary) -> [(&'static str, &'static str);
         HookEventName::SessionStart => "SessionStart",
         HookEventName::UserPromptSubmit => "UserPromptSubmit",
         HookEventName::SubagentStart => "SubagentStart",
+        HookEventName::SubagentStop => "SubagentStop",
         HookEventName::Stop => "Stop",
     };
     let hook_source = match run.source {

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

@@ -23,7 +23,7 @@ use crate::hook_runtime::record_additional_contexts;
 use crate::hook_runtime::record_pending_input;
 use crate::hook_runtime::run_legacy_after_agent_hook;
 use crate::hook_runtime::run_pending_session_start_hooks;
-use crate::hook_runtime::run_stop_hooks;
+use crate::hook_runtime::run_turn_stop_hooks;
 use crate::injection::ToolMentionKind;
 use crate::injection::app_id_from_path;
 use crate::injection::tool_kind_for_path;
@@ -360,7 +360,7 @@ pub(crate) async fn run_turn(
 
                 if !needs_follow_up {
                     last_agent_message = sampling_request_last_agent_message;
-                    let stop_outcome = run_stop_hooks(
+                    let stop_outcome = run_turn_stop_hooks(
                         &sess,
                         &turn_context,
                         stop_hook_active,