Spec First

Without an external contract, the agent drifts after ~3 iterations — and the failure looks like progress (code written, tests pass, wrong goal solved). Write PROMPT.md BEFORE acting:

• Goal — one sentence.
• Done when — concrete, checkable conditions. "Test suite green: ".
• Never touch — files/areas off-limits.
• Stop if — more than N files outside scope change; a passing test starts failing. The agent re-reads this file every iteration. State (what's done) goes in a separate IMPLEMENTATION_PLAN.md it updates in place. If you can't write "done when" concretely, the task isn't ready — clarify before coding.