aimock/docs/structured-output/index.html at fix/proxy-buffer-cap-pr · CopilotKit/aimock · GitHub

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<!doctype html>
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    <title>Structured Output — aimock</title>
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        <h1>Structured Output / JSON Mode</h1>
        <p class="lead">
          aimock supports matching on <code>response_format</code> so you can return different
          responses for JSON mode requests versus regular text requests. Use
          <code>match.responseFormat</code> in fixtures or the
          <code>onJsonOutput()</code> convenience method.
        </p>

        <h2>How It Works</h2>
        <ul>
          <li>
            When a request includes <code>response_format: { type: "json_object" }</code>, the
            router checks <code>match.responseFormat</code>
          </li>
          <li>
            Fixtures with <code>responseFormat: "json_object"</code> only match JSON mode requests
          </li>
          <li>
            Regular fixtures (without responseFormat) still match JSON mode requests if no specific
            JSON fixture matches first
          </li>
        </ul>

        <h2>Unit Test: Programmatic API</h2>

        <div class="code-block">
          <div class="code-block-header">json-output.test.ts <span class="lang-tag">ts</span></div>
          <pre><code><span class="kw">const</span> <span class="op">mock</span> = <span class="kw">new</span> <span class="type">LLMock</span>();
<span class="kw">await</span> <span class="op">mock</span>.<span class="fn">start</span>();

<span class="cm">// Register JSON output fixture — accepts object or string</span>
<span class="op">mock</span>.<span class="fn">onJsonOutput</span>(<span class="str">"json-output"</span>, { <span class="prop">answer</span>: <span class="num">42</span>, <span class="prop">items</span>: [<span class="str">"a"</span>, <span class="str">"b"</span>] });

<span class="kw">const</span> <span class="op">res</span> = <span class="kw">await</span> <span class="fn">fetch</span>(<span class="str">`${mock.url}/v1/chat/completions`</span>, {
  <span class="prop">method</span>: <span class="str">"POST"</span>,
  <span class="prop">headers</span>: { <span class="str">"Content-Type"</span>: <span class="str">"application/json"</span> },
  <span class="prop">body</span>: <span class="type">JSON</span>.<span class="fn">stringify</span>({
    <span class="prop">model</span>: <span class="str">"gpt-4"</span>,
    <span class="prop">messages</span>: [{ <span class="prop">role</span>: <span class="str">"user"</span>, <span class="prop">content</span>: <span class="str">"json-output"</span> }],
    <span class="prop">stream</span>: <span class="kw">false</span>,
    <span class="prop">response_format</span>: { <span class="prop">type</span>: <span class="str">"json_object"</span> },
  }),
});

<span class="kw">const</span> <span class="op">body</span> = <span class="kw">await</span> <span class="op">res</span>.<span class="fn">json</span>();
<span class="kw">const</span> <span class="op">content</span> = <span class="type">JSON</span>.<span class="fn">parse</span>(<span class="op">body</span>.<span class="prop">choices</span>[<span class="num">0</span>].<span class="prop">message</span>.<span class="prop">content</span>);
<span class="fn">expect</span>(<span class="op">content</span>.<span class="prop">answer</span>).<span class="fn">toBe</span>(<span class="num">42</span>);
<span class="fn">expect</span>(<span class="op">content</span>.<span class="prop">items</span>).<span class="fn">toEqual</span>([<span class="str">"a"</span>, <span class="str">"b"</span>]);</code></pre>
        </div>

        <h2>JSON Fixture</h2>

        <div class="code-block">
          <div class="code-block-header">
            fixtures/json-mode.json <span class="lang-tag">json</span>
          </div>
          <pre><code>{
  <span class="key">"fixtures"</span>: [
    {
      <span class="key">"match"</span>: {
        <span class="key">"userMessage"</span>: <span class="str">"json-output"</span>,
        <span class="key">"responseFormat"</span>: <span class="str">"json_object"</span>
      },
      <span class="key">"response"</span>: {
        <span class="key">"content"</span>: { <span class="key">"answer"</span>: <span class="num">42</span>, <span class="key">"items"</span>: [<span class="str">"a"</span>, <span class="str">"b"</span>] }
      }
    }
  ]
}</code></pre>
        </div>

        <div class="info-box">
          <p>
            <strong>Auto-stringify:</strong> The <code>content</code> field accepts objects in
            fixture files. The loader detects <code>typeof === "object"</code> and calls
            <code>JSON.stringify()</code> automatically. The escaped-string form
            (<code>"{\"answer\":42}"</code>) still works for backward compatibility.
          </p>
        </div>

        <h2>Match Behavior</h2>
        <p>
          Each row shows whether a request matches a fixture based on the combination of their
          <code>responseFormat</code> values. A fixture without <code>responseFormat</code> is a
          wildcard (matches all requests), but a fixture with <code>responseFormat</code> is a
          filter (only matches requests that specify that format).
        </p>
        <table class="endpoint-table">
          <thead>
            <tr>
              <th>Request response_format</th>
              <th>Fixture responseFormat</th>
              <th>Match?</th>
            </tr>
          </thead>
          <tbody>
            <tr>
              <td>{ type: "json_object" }</td>
              <td>"json_object"</td>
              <td>Yes</td>
            </tr>
            <tr>
              <td>{ type: "json_object" }</td>
              <td>(not set)</td>
              <td>Yes (fallthrough)</td>
            </tr>
            <tr>
              <td>(not set)</td>
              <td>"json_object"</td>
              <td>No</td>
            </tr>
            <tr>
              <td>(not set)</td>
              <td>(not set)</td>
              <td>Yes</td>
            </tr>
          </tbody>
        </table>

        <div class="info-box">
          <p>
            The <code>onJsonOutput()</code> method accepts either a plain object (auto-serialized)
            or a string. This makes it easy to return structured data without manual
            <code>JSON.stringify</code>.
          </p>
        </div>
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