Build a Disposable Model-Eval Loop Without a Paid API Key

A free model endpoint plus a free server is enough for a disposable eval loop. It is not enough for production traffic.

Problem

You want to compare model outputs without paying per call or waking a GPU box.
You need something small, repeatable, and easy to throw away.

Pieces

• A free model access for the prompt calls.
• A free server for the result endpoint.
• One script that runs the loop.
• One assertion that fails when output shape breaks.

Disclosure: This article was prepared as part of MonkeyCode's product outreach.

I use MonkeyCode's free model access and free server option as the two cheap pieces. The endpoint shape and limits change, so check the current docs before copying; the code below is pseudocode.

Artifact: a minimal eval loop

eval-loop.mjs:

// Pseudocode: adapt to the current MonkeyCode endpoint.
const endpoint = process.env.MODEL_URL;
const key = process.env.MODEL_KEY;

const prompts = [
  "Return JSON only: {ok:boolean, reason:string}",
  "Return JSON only: {ok:boolean, reason:string, score:number}",
];

for (const prompt of prompts) {
  const res = await fetch(endpoint, {
    method: "POST",
    headers: {
      "content-type": "application/json",
      ...(key ? { authorization: `Bearer ${key}` } : {}),
    },
    body: JSON.stringify({
      model: process.env.MODEL_NAME ?? "free-model",
      messages: [{ role: "user", content: prompt }],
    }),
  });

  const data = await res.json();
  const raw = data?.choices?.[0]?.message?.content ?? "";
  console.log(raw);
}

Run it ten times and save the raw output to a file:

for i in {1..10}; do node eval-loop.mjs >> eval-raw.log 2>&1; done

Then check what survives. The prompt asks for JSON, but models drift. That drift is the thing you are testing.

Free server: expose the last result

server.mjs:

// Pseudocode: deploy this function to the free server.
import { createServer } from "node:http";
import { readFile } from "node:fs/promises";

createServer(async (_req, res) => {
  let body;
  try {
    body = await readFile("/tmp/eval-last.json", "utf8");
  } catch {
    body = JSON.stringify({ error: "no result yet" });
  }
  res.setHeader("content-type", "application/json");
  res.end(body);
}).listen(process.env.PORT || 3000);

Keep /tmp/eval-last.json updated by the eval loop. The server only reads from it, so the free server never needs long-running compute.

Decision table

Limitations

• Free tier quotas and cold starts vary. Verify the current MonkeyCode limits.
• Model output is non-deterministic. Run repeats.
• The endpoint shape in this article is pseudocode, not a guaranteed API contract.
• Do not paste a real secret into a shared free server repo.

Who should skip this

Skip it if you need:

• an SLA,
• low-latency responses,
• private data handling,
• high throughput, or
• production uptime.

Use it when the goal is a cheap correctness check, not a service.

Pick two prompts, run the loop against two models, and record which JSON shape breaks first. That one result is more useful than a vague model comparison.