Permission rules are how you codify "yes, always run pnpm test" and "no, never touch this directory" without answering prompts each session.

What it does

Each rule pairs a tool pattern with an action: allow, ask, or deny. Patterns can match Bash commands, file paths, MCP servers, and more - with wildcards for flexibility. Rules live in settings.json (project-level) or settings.local.json (per-user) and combine with admin-managed rules for teams.

When to use it

Allowlisting common commands (npm run *, git status, ls *).
Denying sensitive paths (production config, secret files).
Locking Claude to a specific MCP server for a given project.
Building up a session-to-session baseline of trusted patterns.

Gotchas

Order matters. More specific rules should come before broad ones.
Wildcards can over-grant. Audit your allow list periodically.
Project rules commit to the repo, so don't put user-specific prefs there. Use settings.local.json.

Official docs: https://code.claude.com/docs/en/permissions.md