Add observability and tests

Instrument resilient TypeScript fetch wrapper around POST /v1/chat so mobile client waiting on a grounded answer can answer: demand, errors, latency/age, saturation. Emit fields needed by retry_attempts_total{outcome} and p95 wall_ms ≤ 8000 with bounded labels. Sample successful high-volume traces; keep errors and rollout transitions denser within policy.

Define at least one alert that would fire for INC-429-2026-07-11, with a for/pending window that survives deploy blips. Missing scrape or missing revision labels must not look like health. Include a trace/log example id format you will actually search.

Add a unit/integration check that metrics increment on the happy path and on the 429 storm with missing Retry-After that would amplify to 40 calls branch. Store machine-readable output in CI artifacts when practical.

Cardinality discipline: tenant and revision are usually enough; raw question text is not a label. Exemplars or trace links beat screenshots alone when debugging INC-429-2026-07-11. Define who owns alert fatigue review. If you export to a vendor, record retention and access. Synthetic probes should use non-sensitive fixtures and still exercise POST /v1/chat. Practice the query you will type at 2am once, while calm.

Document the exact PromQL or log query in the runbook stub for this service. Verify histograms have buckets around your SLO target. Add a canary synthetic check that exercises the oracle path every few minutes in staging. Confirm that PII redaction happens before export. Track build/version as a label on the golden signals. Delete noisy debug logs before they become accidental product dependencies. In this chapter the product is resilient TypeScript fetch wrapper around POST /v1/chat, the human stakeholder is mobile client waiting on a grounded answer, and the incident id you design against is INC-429-2026-07-11. Re-state the oracle in your notes — 429 + Retry-After:2 → sleep once → 200 with attempts=2 — and keep the invariant visible: attempts ≤ 3, wall budget ≤ 8s, non-idempotent POSTs never replay without Idempotency-Key. Track retry_attempts_total{outcome} and p95 wall_ms ≤ 8000 as the scoreboard. Surface under change control: POST /v1/chat. If you only have forty minutes, finish the fixture for 429 storm with missing Retry-After that would amplify to 40 calls before polishing UI. Promotion language stays ternary: promote, hold, or roll back based on evidence, not hope.