SessionStart fires once when Claude Code starts a session. It's the place for environment setup, secret loading, and "run this every time" initialization.

What it does

The hook runs after config loads but before the first user prompt. It can export environment variables that persist across Bash calls, print a welcome banner, run a health check, or pull a secret from a vault. Its return value can inject context into the session.

When to use it

Loading API keys or credentials from a secret manager.
Pulling the latest CLAUDE.md or rules from a central source.
Warming caches for tools Claude will likely need.
Emitting a project-specific banner so you know which context you're in.

Gotchas

Slow SessionStart hooks make startup feel sluggish. Keep them fast or run async.
Failing hooks can block a session. Default to fail-open unless failure should truly stop work.
Env vars set here persist into Bash calls but not into shells you launched before the session started.

Official docs: https://code.claude.com/docs/en/hooks.md#sessionstart