API errors & retries
Implement the happy path
One clean transaction through **POST /v1/chat** must match the oracle: 429 + Retry-After:2 → sleep once → 200 with attempts=2.
1Learn the idea
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Order the successful transaction
Code the narrow path that serves mobile client waiting on a grounded answer: accept → authorize/normalize → call dependency → validate → record. Keep stages named so a trace can show which boundary passed. Success must emit evidence useful to retry_attempts_total{outcome} and p95 wall_ms ≤ 8000, not only a 200 with prose. Predict the observable for POST /v1/chat before running: 429 + Retry-After:2 → sleep once → 200 with attempts=2.
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Run with fakes first
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 resilient TypeScript fetch wrapper around POST /v1/chat — resist adding unrelated features mid-path.
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Implementation artifact
async function fetchWithBudget<T>(url: string, init: RequestInit, gate: Gate): Promise<FetchResult<T>> {
const started = Date.now();
const res = await fetch(url, { ...init, signal: AbortSignal.timeout(gate.wallMs) });
if (!res.ok) return { ok: false, code: "http", status: res.status, attempts: 1 };
return { ok: true, data: (await res.json()) as T, attempts: 1, latencyMs: Date.now() - started };
}
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Compare prediction to result
For API errors & retries, paste the CLI/HTTP transcript beside your prediction for POST /v1/chat. If the oracle is unmet (429 + Retry-After:2 → sleep once → 200 with attempts=2), 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 INC-429-2026-07-11.
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Stage depth
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 retry_attempts_total{outcome} and p95 wall_ms ≤ 8000. 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 mobile client waiting on a grounded answer experiences wall-clock time, not your debugger’s single-step comfort.
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Field notes for `api-error-handling` / `happy-path`
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 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.
Go deeper
Before you start
Why this matters
Without calling production, order the steps a single success takes for mobile client waiting on a grounded answer. Circle the first irreversible side effect. Your prediction should mention POST /v1/chat and the evidence field that proves 429 + Retry-After:2 → sleep once → 200 with attempts=2.
In the wild
See how this idea shows up as a product and a company — then come back to the lesson. Skills transfer across vendors.
Related lessons
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Answer from memory. Completion is saved from this evidence, not from opening the next page.
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