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Add SubagentStart hook (#22782)

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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.
This commit is contained in:
Abhinav
2026-05-19 12:45:08 -07:00
committed by GitHub
Unverified
parent d269aa2af9
commit d661ab70ed
43 changed files with 716 additions and 44 deletions
Download Patch File Download Diff File Expand all files Collapse all files
codex-rs/hooks/src/engine/schema_loader.rs
+12
View File
@@ -16,6 +16,8 @@ pub(crate) struct GeneratedHookSchemas {
pub pre_compact_command_output: Value,
pub session_start_command_input: Value,
pub session_start_command_output: Value,
pub subagent_start_command_input: Value,
pub subagent_start_command_output: Value,
pub user_prompt_submit_command_input: Value,
pub user_prompt_submit_command_output: Value,
pub stop_command_input: Value,
@@ -73,6 +75,14 @@ pub(crate) fn generated_hook_schemas() -> &'static GeneratedHookSchemas {
"session-start.command.output",
include_str!("../../schema/generated/session-start.command.output.schema.json"),
),
subagent_start_command_input: parse_json_schema(
"subagent-start.command.input",
include_str!("../../schema/generated/subagent-start.command.input.schema.json"),
),
subagent_start_command_output: parse_json_schema(
"subagent-start.command.output",
include_str!("../../schema/generated/subagent-start.command.output.schema.json"),
),
user_prompt_submit_command_input: parse_json_schema(
"user-prompt-submit.command.input",
include_str!("../../schema/generated/user-prompt-submit.command.input.schema.json"),
@@ -118,6 +128,8 @@ mod tests {
assert_eq!(schemas.pre_compact_command_output["type"], "object");
assert_eq!(schemas.session_start_command_input["type"], "object");
assert_eq!(schemas.session_start_command_output["type"], "object");
assert_eq!(schemas.subagent_start_command_input["type"], "object");
assert_eq!(schemas.subagent_start_command_output["type"], "object");
assert_eq!(schemas.user_prompt_submit_command_input["type"], "object");
assert_eq!(schemas.user_prompt_submit_command_output["type"], "object");
assert_eq!(schemas.stop_command_input["type"], "object");