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<h1 id="codegen-generate-models-routes-from-your-spec">Codegen — Generate Models &amp; Routes From Your Spec</h1>
<blockquote>
<p><strong>TL;DR</strong> — point <code>openapi-first codegen</code> at your spec and get <code>models</code> + <code>routes</code> scaffolds you can bind with one line. The codegen output is deterministic, and it's a <em>starting point</em> — not a maintained artifact.</p>
</blockquote>
<hr />
<h2 id="1-why-celebrate-codegen">🧭 1. Why Celebrate Codegen?</h2>
<p>Two pain points kill OpenAPI projects:</p>
<ol>
<li><strong>The "write the spec, then write the same thing as Pydantic models" step</strong> — exactly where server/client param types drift (your route says <code>item_id: int</code>, your client sends a string…).</li>
<li><strong>Writing the docs/spec catalog by hand</strong>, once you have more than a handful of operations.</li>
</ol>
<p>Codegen collapses both. One command, one source file, same generated shapes everywhere.</p>
<hr />
<h2 id="2-model-generation">🚀 2. Model Generation</h2>
<div class="language-bash highlight"><table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-0-1">1</a></span></pre></div></td><td class="code"><div><pre><span></span><code><span id="__span-0-1"><a id="__codelineno-0-1" name="__codelineno-0-1"></a>openapi-first<span class="w"> </span>codegen<span class="w"> </span>models<span class="w"> </span>--module<span class="w"> </span>my_project.models<span class="w"> </span>--input<span class="w"> </span>openapi.yaml
</span></code></pre></div></td></tr></table></div>
<p>The generated model module mirrors the spec's <code>components.schemas</code> <strong>by name</strong>:</p>
<table>
<thead>
<tr>
<th>Spec</th>
<th>Generated</th>
</tr>
</thead>
<tbody>
<tr>
<td><code>components.schemas.User</code></td>
<td><code>class User(BaseModel)</code></td>
</tr>
<tr>
<td><code>components.schemas.Item</code></td>
<td><code>class Item(BaseModel)</code></td>
</tr>
<tr>
<td>every <code>$ref</code> (schema)</td>
<td>a <code>type: ClassVar</code> alias ✓</td>
</tr>
<tr>
<td><code>required + type</code> from schema</td>
<td>Pydantic <code>Field(...)</code> / type hints ✓</td>
</tr>
</tbody>
</table>
<p><strong>Never hand-edit those files.</strong> If the spec changes, regenerate — just like you'd re-run <code>cargo build</code> after editing <code>Cargo.toml</code>.</p>
<h3 id="why-constructor-time-validation-still-applies">Why constructor-time validation still applies</h3>
<p>Codegen doesn't change the design: the generated models are plain Pydantic, and the <strong>client/server still validate against the spec</strong> at startup. Codegen is a <em>convenience accelerator</em> on top of the fail-fast guarantees in <a href="../../04_design/">04 Design</a> and <a href="../../06_error_handling/">06 Error Handling</a>.</p>
<hr />
<h2 id="3-route-generation-verification">🔁 3. Route Generation / Verification</h2>
<div class="language-bash highlight"><table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre><span></span><span class="normal"><a href="#__codelineno-1-1">1</a></span>
<span class="normal"><a href="#__codelineno-1-2">2</a></span>
<span class="normal"><a href="#__codelineno-1-3">3</a></span>
<span class="normal"><a href="#__codelineno-1-4">4</a></span>
<span class="normal"><a href="#__codelineno-1-5">5</a></span></pre></div></td><td class="code"><div><pre><span></span><code><span id="__span-1-1"><a id="__codelineno-1-1" name="__codelineno-1-1"></a><span class="c1"># dry-run verification against your routes module</span>
</span><span id="__span-1-2"><a id="__codelineno-1-2" name="__codelineno-1-2"></a>openapi-first<span class="w"> </span>codegen<span class="w"> </span>routes<span class="w"> </span>--module<span class="w"> </span>my_routes<span class="w"> </span>--input<span class="w"> </span>openapi.yaml<span class="w"> </span>--check
</span><span id="__span-1-3"><a id="__codelineno-1-3" name="__codelineno-1-3"></a>
</span><span id="__span-1-4"><a id="__codelineno-1-4" name="__codelineno-1-4"></a><span class="c1"># scaffold an operation skeleton (generates the handler with a TODO)</span>
</span><span id="__span-1-5"><a id="__codelineno-1-5" name="__codelineno-1-5"></a>openapi-first<span class="w"> </span>codegen<span class="w"> </span>routes<span class="w"> </span>--module<span class="w"> </span>my_routes<span class="w"> </span>--input<span class="w"> </span>openapi.yaml
</span></code></pre></div></td></tr></table></div>
<p>Why bother? Because <code>bind_routes</code> (in <a href="../../02_components/">02 Components</a>) needs an <code>operationId</code> → handler <strong>exactly</strong> matching the spec. Codegen guarantees you never type a handler name wrong — it wears the same <code>operationId</code> as the spec says.</p>
<blockquote>
<p><strong>Note:</strong> <code>codegen</code> is build-time tooling. It does <strong>not</strong> run at runtime, and templates (the Bake-your-stuff section of <a href="../02_templates/">02 Templates</a>) make scaffolding a server out-of-the-box even simpler for greenfield projects.</p>
</blockquote>
<hr />
<h2 id="4-idempotent-output">🧪 4. Idempotent Output</h2>
<p>Generated output is deterministic w.r.t. the spec:
- Same spec → byte-identical files (unless you hand-edit — you won't)
- Ordering follows spec declaration order
- No timestamps, no machine names, no hidden randomness</p>
<p>That determinism is what lets you <code>git diff</code> after a spec change and see <strong>exactly</strong> what moved.</p>
<hr />
<h2 id="related">🧩 Related</h2>
<ul>
<li><a href="../../01_overview/">01 Overview</a> · <a href="../02_templates/">02 Templates</a> · <a href="../03_client/">03 Client</a> · <a href="../../04_design/">04 Design</a> · <a href="../../07_testing/">07 Testing</a></li>
</ul>
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