Build the first working SSE token relay

Build only what the claim requires: client receives ordered data events and a terminal done event; cancel stops further writes. Prefer boring, deterministic code over frameworks you cannot yet explain. Run the path twice; identical output on this fixture is a feature, not a lack of creativity.

export async function run(input: unknown, deps: Dependencies) {
  const started = performance.now();
  const validated = deps.parseInput(input);
  const raw = await deps.execute(validated); // read the provider response body with a Web ReadableStream and emit framed SSE events
  const value = deps.parseOutput(raw);
  return { value, durationMs: Math.round(performance.now() - started) };
}

Expected evidence: The client renders “Hello 👋” in order, receives done once, and closes cleanly.. Read each printed intermediate as part of the argument that the path works—not as decoration.

Narrate the journey from raw input to result for a single example from fake chunk iterator yielding three tokens then done. If you cannot name an intermediate, the implementation is still too opaque for this lab. Only after this path is solid should you generalize data sources or UI.

While implementing the SSE token relay, print or log at least three intermediates that map to the claim (client receives ordered data events and a terminal done event; cancel stops further writes). Good intermediates are values a teammate could recompute with a calculator or diff. Bad intermediates are framework traces you cannot explain.

Re-run with fake chunk iterator yielding three tokens then done twice. If the second run differs, either the path is nondeterministic (document the seed) or you have hidden global state—both are lab bugs until named.

Stop adding features once the path supports stream tokens to a client while honoring cancel and partial failure. Extra UI, extra tools, or extra models belong in later chapters. The mastery bar for this page is simply: a deterministic end-to-end path with intermediates.

At the implementation stage for streaming-responses, the job is narrower than finishing a product demo. You are creating one progressive evidence piece about fake chunk iterator yielding three tokens then done that later pages inherit without redefining success. Keep that fixture small enough to inspect by hand, keep outputs copy-pasteable as text, and refuse to narrate this baseline as if it were a production SLA: non-streaming JSON endpoint latency for the same prompt.

For this page specifically, success looks like a deterministic path with printed intermediates while still centering the user decision to stream tokens to a client while honoring cancel and partial failure. If you cannot point to a file, command, or assertion that proves that for the SSE token relay, stay on this page instead of advancing.

Glossary: tool · Glossary: structured output · Cheatsheet: production ops signals

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