Claude Code 2.4.1 changed hook execution order — parallel matchers now run concurrently

Structured summary

Symptoms

After upgrading to 2.4.1, hooks that used to run in a predictable order now interleave. Symptom pattern: a PreToolUse hook that writes a state file (e.g. a lint cache or an audit log line) and a second hook that reads it now intermittently sees a missing or half-written file. You get flaky 'block' decisions, duplicate log lines, or hooks that pass locally and fail in CI. Nothing in the session transcript indicates a problem; you have to diff hook output timestamps to see the overlap.

Likely cause

The 2.4.1 changelog line 'improved hook performance' turns out to mean hooks for matching matchers are now spawned with Promise.all instead of awaited in sequence. Hooks within a single matcher entry still run in order; it's across matcher entries that ordering is gone.

Affected versions

2.4.1

Environments

Any project with two or more PreToolUse/PostToolUse matcher entries that share state

Operating systems

macOS, Linux, Windows

Confidence

High confidence

Reproduction matrix