mal-mcp: Skill for Claude Code

Mutation testing safety contract

Applies to any mutation tool (add/update/remove/delete/post/follow/favourite/ etc.) once the user has given explicit go-ahead to test it against a real account. For every mutation call:

1. Capture the exact pre-state first (via the matching read tool) — not just an assumption of what it probably is. A target with no existing state has a clear pre-state too: "absent." This applies to every call to a mutation tool, including one you intend only as a validation-only probe — if the schema isn't .strict(), a bogus/typo'd field is silently dropped rather than rejected, and any other real field in the same call still applies for real.
2. Make the smallest possible change that still exercises the behavior (e.g. one field, not a full rewrite). Never combine an unknown/invalid probe field with a real valid field in one call — if the probe field is ignored instead of rejected, the valid field mutates the account for real. Test unknown-param rejection with no other fields set (or on a read-only tool) instead.
3. Verify the change landed by re-fetching via a read tool — a mutation's own echoed response is not always trustworthy (some tools have historically omitted fields they actually changed).
4. Revert to the captured pre-state immediately, in the same turn, and verify the revert too. Don't batch several mutations and revert at the end — revert each one before moving to the next unrelated test.
5. Never leave the target in a different state than you found it, even if a step errors partway through — check and clean up regardless.
6. Never touch an uninvolved third party. Self-targeted mutations (a self-message, a self-created and immediately-deleted test post/ thread/comment) are fine; acting on a random other real user/account is not.