Prove behavior with deterministic tests

Add executable validation at the trust boundaries of FastAPI RAG endpoint whose v2 index is dark-launched then exposed to 5%. Reject unknown fields where they matter, bound string sizes, and coerce only after auth/signature checks when raw bytes are security-relevant. Invariant under test: flag writes require RBAC; client-supplied flag values never authorize access. A TypeScript type or Python annotation is not runtime validation — pair them with parsers.

Automate the fixtures from setup, including a recreation of FLAG-CLIENT-TRUST-4. Assert both the visible error and the absence of side effects (no provider call, no queue write, no flag flip). Where metrics matter, assert label enums stay bounded.

Document which failures are client mistakes (4xx) versus operator/config mistakes (5xx/503). Oracle still stands: 5% cohort sees index_v2 citations; kill switch returns everyone to v1 in < 60s. Validation should make accidental “success with empty body” impossible for release captain flipping rag_index_v2 for cohort canary.

Property ideas: shuffled field order, Unicode edges, maximum-length strings, and replayed timestamps. Where money, identity, or citations matter, assertion messages should cite the field name. Do not snapshot entire provider payloads in tests; assert semantically. If validation fails open “to keep the demo working,” you have inverted the lab. Tie at least one CI job to the FLAG-CLIENT-TRUST-4 fixture so main cannot regress silently. Re-read flag writes require RBAC; client-supplied flag values never authorize access after each new parser — convenience helpers love to bypass it.

Table-drive status codes and error codes so reviewers see coverage at a glance. Include a Unicode normalization case if user text is accepted. Verify that oversized bodies fail before CPU-heavy work. Where digests or versions are pinned, assert mismatch behavior. Keep golden files small enough to read in review. CI should fail on skipped tests that mark the incident fixture as xfail without a ticket link. 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.