Create the runnable skeleton and contracts

Implement types or schemas around POST /webhooks/batch-index so illegal states are unrepresentable at the boundary. For FastAPI receiver for batch-index completion events → durable queue, trusted inputs come from sessions, signatures, pinned digests, or workload identity — not from free-form model text. indexer worker waiting on provider batch.completed callbacks should be able to read the contract and know which fields are optional, which are enumerated, and which abort the request. Constructors and boot paths must not perform provider side effects; injection points keep tests honest.

Document required env vars in .env.example with placeholders only. Rotation story belongs later, but the contract already forbids printing secrets and forbids defaulting to fail-open when a dependency is missing. Invariant to encode in types/tests: verify HMAC signature + timestamp skew ≤ 5m; enqueue idempotent by event_id. If a config number lacks units, fix the name (timeout_ms, rpm_hard) before writing logic.

Create fixtures for the golden path and for HOOK-SIG-OFF-16. Name files after the behavior (429-retry-after.json, cross-tenant.json, empty-citation.json) rather than test1. Each fixture carries expected status/code. This kit is the shared language for validation and failure pages.

Compatibility promise: additive fields may appear only if readers ignore unknowns safely; breaking changes bump a version visible on the wire. For AI payloads, size limits arrive before JSON parse when hostile blobs are a risk. Document how clock skew, idempotency keys, and tracing headers travel through FastAPI receiver for batch-index completion events → durable queue. If you use feature flags later, the contract already states that flags are not authorization. Link each config knob to a unit and a failure mode (“0 means disabled” vs “0 means divide-by-zero”). A peer reviewing the PR should find HOOK-SIG-OFF-16 named in a comment on the adversarial fixture.

Generate OpenAPI or a typed client only after the hand schema is stable for one fixture round-trip. Record how errors look on the wire — problem+json, envelope, or bare status — and stick to one. Clock sources must be injectable for skew tests. If webhooks appear later, document signature header names now even as TODOs. Keep sample payloads UTF-8 and free of real emails. Add a makefile or npm script that validates schemas without network. In this chapter the product is FastAPI receiver for batch-index completion events → durable queue, the human stakeholder is indexer worker waiting on provider batch.completed callbacks, and the incident id you design against is HOOK-SIG-OFF-16. Re-state the oracle in your notes — valid event → 202 + one queue message; replay same event_id → 202 no duplicate work — and keep the invariant visible: verify HMAC signature + timestamp skew ≤ 5m; enqueue idempotent by event_id. Track webhook_accept_total and duplicate_suppressed_total as the scoreboard. Surface under change control: POST /webhooks/batch-index. If you only have forty minutes, finish the fixture for signature optional in staging config accidentally shipped to prod before polishing UI. Promotion language stays ternary: promote, hold, or roll back based on evidence, not hope.