# Why
Fixes#23993.
Hook command output schemas are published as the contract for hook
authors and schema-driven tooling. The event-specific output schemas
previously described `hookSpecificOutput.hookEventName` as the global
`HookEventNameWire` enum, so a `pre-tool-use.command.output` schema
would validate mismatched values like `PostToolUse`. That made the
schemas less precise than the intended event-specific contract.
# What
Constrain each hook-specific output schema to the matching literal
`hookEventName` value, mirroring the existing input-schema shape.
Also split `SubagentStartHookSpecificOutputWire` from the session-start
output wire so `subagent-start.command.output.schema.json` can emit
`const: "SubagentStart"` instead of sharing the session-start
definition.
# Verification
- `cargo nextest run -p codex-hooks`
- `just fix -p codex-hooks`
- `just argument-comment-lint -p codex-hooks -- --all-targets`
# 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.
# 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.
Abhinav ·