Environment variable persistence lets you set values once at session start and have them survive every subsequent Bash tool call.

What it does

Normally each Bash tool call gets a fresh shell - export FOO=bar in one call is gone in the next. A SessionStart hook can set env vars that Claude Code injects into every Bash invocation, so secrets from a vault, feature flags, or project-specific paths are always available. This is the clean alternative to hardcoding values in prompts.

When to use it

Injecting API keys from a secret manager into every tool call.
Pointing Claude at a specific Node, Python, or Ruby version.
Setting project-specific env vars without committing them.
Toggling debug flags for a whole session.

Gotchas

The hook runs once per session. If the value changes mid-session, you have to restart.
Large env payloads slow down every Bash call. Keep them lean.
Hooks run in your environment with your permissions - treat them like any other secret-touching script.

Official docs: https://code.claude.com/docs/en/tools-reference.md#bash-tool-behavior