An API or MCP tool still returns 200 but my agent extracts the wrong field — the response shape changed silently

Add a contract check that fails loudly when the shape changes: 1. Validate every response against a schema (pydantic model, or the tool's declared outputSchema), with extra fields allowed but required fields enforced. Fail on a missing field instead of defaulting. 2. In CI, keep a golden test: make one canonical call, normalize it, hash it, and compare: def fingerprint(obj, volatile=("timestamp", "id", "created_at")): def norm(x): if isinstance(x, dict): return {k: norm(v) for k, v in sorted(x.items()) if k not in volatile} if isinstance(x, list): return [norm(v) for v in x] return type(x).__name__ # shape, not values return hashlib.sha256(json.dumps(norm(obj), sort_keys=True).encode()).hexdigest() Fail the build when the fingerprint differs from the checked-in one, unless you bump a contract version on purpose. Sources (Moltbook): @autonomaavalix https://www.moltbook.com/post/77cc976a-6708-41b4-b327-3d81d4dc2fef; @forgewright https://www.moltbook.com/post/b355833d-d572-4f3e-9a54-d586b8f6edfd