## Why
Forked threads reported `startup`, causing startup hooks to run again even when their context was inherited from the parent. Resuming with supplied history also reported `startup` instead of `resume`.
## What changed
- Add `fork` as a `SessionStart` source and expose it in the hook input schema.
- Report `fork` for histories with a fork parent and `resume` for supplied histories without one.
- Continue dispatching `SubagentStart` for spawned subagents with either fresh or forked context.
## Testing
Add regression coverage for fork hook matching without duplicate startup context, resume hooks with supplied history, and `SubagentStart` context injection for fresh and forked subagents.
GitOrigin-RevId: 5ac1ae4ab8adcbbbceb8cb0ffb9b32b6402c585b
# Why
Compaction replaces the live conversation history, so hooks that use
`SessionStart` to re-inject durable model context need a way to run
again after that rewrite.
Related - #19905 adds dedicated compact lifecycle hooks
# What
- add `compact` as a supported `SessionStart` source and matcher value
- change pending `SessionStart` state from a single slot to a small FIFO
queue so `resume` / `startup` / `clear` can be preserved alongside a
later `compact`
- drain all queued `SessionStart` sources before the next model request,
preserving their original order
# Testing
The new integration coverage verifies both the basic `compact` matcher
path and the stacked `resume` -> `compact` case where both hooks
contribute `additionalContext` to the next model turn.
(Experimental)
This PR adds a first MVP for hooks, with SessionStart and Stop
The core design is:
- hooks live in a dedicated engine under codex-rs/hooks
- each hook type has its own event-specific file
- hook execution is synchronous and blocks normal turn progression while
running
- matching hooks run in parallel, then their results are aggregated into
a normalized HookRunSummary
On the AppServer side, hooks are exposed as operational metadata rather
than transcript-native items:
- new live notifications: hook/started, hook/completed
- persisted/replayed hook results live on Turn.hookRuns
- we intentionally did not add hook-specific ThreadItem variants
Hooks messages are not persisted, they remain ephemeral. The context
changes they add are (they get appended to the user's prompt)
