Andrej Karpathy: "90% of Claude's mistakes come from missing context, not a weak model." The numbers back it up. 41% mistake rate with no CLAUDE.md. 11% with a basic 4-rule baseline. 3% with the 12-rule version below. Senior engineers settled on these: Think before coding. State assumptions explicitly. The model can't read your mind. Simplicity first. Minimum code, no speculative abstractions. "For future flexibility" is how 200 lines you'll delete next quarter get added. Surgical changes. Touch only what you must. Letting Claude improve adjacent code is how PRs blow up. Goal-driven execution. Define success criteria upfront. Without them Claude loops forever or stops too early. Use the model only for judgment calls. Classification, drafting, summarization, extraction. Not routing, retries, status codes, or deterministic transforms. If code can answer, code answers. Token budgets are not advisory. 4000 per task, 30000 per session. By message 40 of a long debug Claude is re-suggesting fixes you rejected at message 5. Surface conflicts, don't average them. Two patterns in the codebase means pick one. Blending them is how errors get swallowed twice. Read before you write. Exports, callers, shared utilities. Claude will add a duplicate function next to one it never read. Tests verify intent, not just behavior. A test that can't fail when business logic changes is wrong. All 12 tests can pass while the function returns a constant. Checkpoint every significant step. Claude finished steps 5 and 6 on a broken state from step 4. Nobody noticed for an hour. Match codebase conventions. Class components mean don't fork silently to hooks. Fail loud. "Completed successfully" with 14% of records quietly skipped is the worst class of bug. What actually compounds: CLAUDE.md as institutional memory across sessions. Eval-driven changes not vibe-driven. Checkpoints over speed. Explicit conflicts over silent blending. One repo, one rules file, no exceptions. You don't need a better AI. You need better context engineering. Complete playbook below.(CyrilXBT)