Add SubagentStop hook#22873
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abhinav-oai
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# What `SubagentStart` runs once when Codex creates a thread-spawned subagent, before that child sends its first model request. Thread-spawned subagents use `SubagentStart` instead of the normal root-agent `SessionStart` hook. Configured handlers match on the subagent `agent_type`, using the same value passed to `spawn_agent`. When no agent type is specified, Codex uses the default agent type. Hook input includes the normal session-start fields plus: - `agent_id`: the child thread id. - `agent_type`: the resolved subagent type. `SubagentStart` may return `hookSpecificOutput.additionalContext`. That context is added to the child conversation before the first model request. # Lifecycle Scope Only thread-spawned subagents run `SubagentStart`. Internal/system subagents such as Review, Compact, MemoryConsolidation, and Other do not run normal `SessionStart` hooks and do not run `SubagentStart`. This avoids exposing synthetic matcher labels for internal implementation paths. Also the `SessionStart` hook no longer fires for subagents, this matches behavior with other coding agents' implementation # Stack 1. This PR: add `SubagentStart`. 2. #22873: add `SubagentStop`. 3. #22882: add subagent identity to normal hook inputs.
sayan-oai
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May 20, 2026
eternal-openai
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May 20, 2026
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What
SubagentStopruns when a thread-spawned subagent turn is about to finish. Thread-spawned subagents useSubagentStopinstead of the normal root-agentStophook.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:truewhen the child is already continuing because an earlier stop-like hook blocked completion.SubagentStopshares the same completion-control semantics asStop, scoped to the child turn:decision: "block"with a non-emptyreasonrecords that reason as hook feedback and continues the child with that prompt.continue: falsestops the child turn. IfstopReasonis 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
Stophooks and do not runSubagentStop. This avoids exposing synthetic matcher labels for internal implementation paths.Stack
SubagentStart.SubagentStop.