Inject a controlled service-level objectives failure

Add executable validation at the trust boundaries of AI support-answer service with 99.5% availability + TTFT SLO. 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: bad events include malformed upstream + timeout fallbacks; auth 4xx excluded. A TypeScript type or Python annotation is not runtime validation — pair them with parsers.

Automate the fixtures from setup, including a recreation of SLO-VANITY-200-18. 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: 3% provider timeout for 20m trips fast-burn; monthly budget math matches calc-budget.py. Validation should make accidental “success with empty body” impossible for reliability owner gating releases on error budget.

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 SLO-VANITY-200-18 fixture so main cannot regress silently. Re-read bad events include malformed upstream + timeout fallbacks; auth 4xx excluded 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 AI support-answer service with 99.5% availability + TTFT SLO, the human stakeholder is reliability owner gating releases on error budget, and the incident id you design against is SLO-VANITY-200-18. Re-state the oracle in your notes — 3% provider timeout for 20m trips fast-burn; monthly budget math matches calc-budget.py — and keep the invariant visible: bad events include malformed upstream + timeout fallbacks; auth 4xx excluded. Track slo:availability:ratio and burn_rate1h as the scoreboard. Surface under change control: POST /v1/support/answer. If you only have forty minutes, finish the fixture for HTTP 200 fallbacks counted as good while citations empty — vanity availability before polishing UI. Promotion language stays ternary: promote, hold, or roll back based on evidence, not hope.