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Files

Abhinav d661ab70ed Add SubagentStart hook (#22782 )

# 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.

2026-05-19 12:45:08 -07:00

src

Add SubagentStart hook (#22782 )

2026-05-19 12:45:08 -07:00

BUILD.bazel

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Cargo.toml

Enable --deny-warnings for cargo shear (#21616 )

2026-05-08 20:29:00 +00:00

README.md

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README.md

codex-protocol

This crate defines the "types" for the protocol used by Codex CLI, which includes both "internal types" for communication between codex-core and codex-tui, as well as "external types" used with codex app-server.

This crate should have minimal dependencies.

Ideally, we should avoid "material business logic" in this crate, as we can always introduce Ext-style traits to add functionality to types in other crates.