Constant COMPACTION_STARTED_TIMEOUT 

const COMPACTION_STARTED_TIMEOUT: Duration;

Fail-fast timeout for publish_compaction_started specifically — much shorter than the 5s client-wide default used for tool_chunk streaming (from_env). That default is fine for exec’s own async runtime, but precompact is invoked as Claude Code’s SYNCHRONOUS PreCompact hook: the CLI blocks compaction on this process’s exit, so inheriting the 5s default means a wedged (but TCP-accepting) backend delays the user’s actual compaction by up to 5 real seconds — directly contradicting this hook’s own “never block/delay the operation for observability” contract (see precompact.rs’s module doc comment). Codex P2, PR #2378 round 4. 500ms comfortably covers a normal same-host round trip while keeping the worst case imperceptible.