Define the command-line LLM summarizer data contract

Make malformed input fail before it reaches the interesting algorithm. The accepted contract is a non-empty text string, model name, timeout, and API key loaded from the environment. This boundary matters because missing keys, timeouts, rate limits, malformed JSON, empty choices, and prompt text accidentally appearing in logs; allowing a bad value through makes later debugging look like an algorithm problem. Keep transformation functions separate from scoring or prediction so a test can identify which boundary changed the data.

The artifact's user-facing goal is specific: send one system message and one user message through an HTTP-shaped client and return a bounded one-sentence summary. Its accepted input is a non-empty text string, model name, timeout, and API key loaded from the environment. Those statements are intentionally narrower than “build an AI system.” Narrow scope lets us inspect every input and expected result, and it prevents a toy result from being presented as a production claim. The example implements or probes the input boundary. Copy it into a fresh file and run it without extra packages.

Save this as lesson.js and run node lesson.js. It uses only the language standard library, so the example is reproducible offline.

function request(text){if(!text.trim()) throw new Error('text required'); return {model:'demo',messages:[{role:'system',content:'One sentence.'},{role:'user',content:text}],max_tokens:60};}
console.log(request('Vectors support retrieval.'));

Expected output: a request object with model, two roles, and max_tokens. Exact floating-point formatting may vary slightly, but the asserted behavior must not. Read the output as evidence about this stage, not merely proof that the interpreter started.

Separate request construction, transport, and response parsing. Test each with a fake transport before contacting a provider. For a malformed response, print the status and safe structural fields, never the API key or full prompt. Retry only explicit transient statuses such as 429, cap attempts, and preserve the final error. A missing choices array is a contract failure, not an empty successful summary.

At the data contract stage, save the smallest failing fixture beside the expected result. Change one cause at a time and rerun the exact command printed above; that makes the repair reviewable and keeps this chapter's progressive artifact reproducible.

Run a fixed set of short prompts through the fake contract and, when credentials are intentionally available, through the real adapter. Measure latency, retries, output length, and whether the one-sentence constraint holds. Save expected properties rather than exact prose because model wording can vary. Contract tests must remain deterministic and runnable offline.

For this data contract page, preserve the fixture and result as evidence for the next page. Label observations separately from conclusions: a passing assertion establishes the behavior it names, while broader usefulness requires the chapter's full evaluation set and stated operating limits.