Run the load testing baseline

Code the narrow path that serves capacity planner before Black Friday traffic 3× baseline: accept → authorize/normalize → call dependency → validate → record. Keep stages named so a trace can show which boundary passed. Success must emit evidence useful to saturation_rps, p95_ttft_ms, test_usd, not only a 200 with prose. Predict the observable for POST /v1/chat/completions (stream) before running: find saturation knee near 180 RPS; beyond that queue depth explodes.

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 k6 streaming load test for chat completion gateway — resist adding unrelated features mid-path.

For Load testing lab, paste the CLI/HTTP transcript beside your prediction for POST /v1/chat/completions (stream). If the oracle is unmet (find saturation knee near 180 RPS; beyond that queue depth explodes), 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 LOAD-FALSE-GREEN-33.

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 saturation_rps, p95_ttft_ms, test_usd. 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 capacity planner before Black Friday traffic 3× baseline 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 k6 streaming load test for chat completion gateway, the human stakeholder is capacity planner before Black Friday traffic 3× baseline, and the incident id you design against is LOAD-FALSE-GREEN-33. Re-state the oracle in your notes — find saturation knee near 180 RPS; beyond that queue depth explodes — and keep the invariant visible: abort if error_rate > 1% or p95_ttft > 1.5s; test cost ≤ $50. Track saturation_rps, p95_ttft_ms, test_usd as the scoreboard. Surface under change control: POST /v1/chat/completions (stream). If you only have forty minutes, finish the fixture for load generator retries hide server shedding — false green before polishing UI. Promotion language stays ternary: promote, hold, or roll back based on evidence, not hope.