Implement one traceable happy path

Code the narrow path that serves release captain flipping rag_index_v2 for cohort canary: accept → authorize/normalize → call dependency → validate → record. Keep stages named so a trace can show which boundary passed. Success must emit evidence useful to flag_exposure_pct and citation_delta_v2_vs_v1, not only a 200 with prose. Predict the observable for POST /v1/rag/answer before running: 5% cohort sees index_v2 citations; kill switch returns everyone to v1 in < 60s.

Drive the path with recording fakes or local stubs. Assert call order and arguments. Idempotency keys or stable ids should keep retries from duplicating costly work where the product requires it. Product under test remains FastAPI RAG endpoint whose v2 index is dark-launched then exposed to 5% — resist adding unrelated features mid-path.

For Feature flags lab, paste the CLI/HTTP transcript beside your prediction for POST /v1/rag/answer. If the oracle is unmet (5% cohort sees index_v2 citations; kill switch returns everyone to v1 in < 60s), stop and debug this page; do not compensate with prompt folktales. Re-run once after a clean process start to catch hidden global state that would invalidate FLAG-CLIENT-TRUST-4.

Performance sketch: measure local p95 for the fake-backed path so later regressions are obvious. Keep concurrency modest until failure-handling proves limits. Log a single structured event per success with request id, revision, and the evidence field behind flag_exposure_pct and citation_delta_v2_vs_v1. Avoid hidden global caches in the happy path unless the lab is about caching — and even then key by tenant. If the path calls a model, pin model id in config and echo it in the response for auditability. Remember release captain flipping rag_index_v2 for cohort canary experiences wall-clock time, not your debugger’s single-step comfort.

Prefer explicit function names over a single god-object handleRequest. Thread a correlation id from ingress to the last log line. When streaming, define what partial failure means before coding. Snapshot one successful response body in fixtures after redaction. If the path writes to a queue, assert message attributes in the fake. Stop adding retries on this page; that is the next concern. In this chapter the product is FastAPI RAG endpoint whose v2 index is dark-launched then exposed to 5%, the human stakeholder is release captain flipping rag_index_v2 for cohort canary, and the incident id you design against is FLAG-CLIENT-TRUST-4. Re-state the oracle in your notes — 5% cohort sees index_v2 citations; kill switch returns everyone to v1 in < 60s — and keep the invariant visible: flag writes require RBAC; client-supplied flag values never authorize access. Track flag_exposure_pct and citation_delta_v2_vs_v1 as the scoreboard. Surface under change control: POST /v1/rag/answer. If you only have forty minutes, finish the fixture for browser-exposed flag lets attackers force v2 (unreviewed) index before polishing UI. Promotion language stays ternary: promote, hold, or roll back based on evidence, not hope.