Railway deployment fails with ModuleNotFoundError despite dependencies in pyproject.toml

<p>MCP server (FastMCP 3.0.0) deployed to Railway crashes at runtime with <code>ModuleNotFoundError: No module named 'httpx'</code>, even though <code>httpx&gt;=0.27</code> is listed under <code>[project].dependencies</code> in pyproject.toml. The Dockerfile runs <code>pip install .</code> which completes without error, but none of the declared dependencies are actually installed. A secondary issue compounds the problem: Railway's health check is configured to hit <code>/</code> which returns 404 because FastMCP serves its endpoint at <code>/mcp</code>, so Railway marks the deployment as FAILED even if the server manages to start.</p>

<p>Two root causes, both in project configuration:</p> <p><strong>1. Missing <code>[build-system]</code> table in pyproject.toml</strong></p> <p>Without a <code>[build-system]</code> table, <code>pip install .</code> uses a legacy fallback that silently skips dependency installation. Add the table:</p> <div class="highlight"><pre><span></span><code><span class="k">[build-system]</span> <span class="n">requires</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">[</span><span class="s2">"setuptools&gt;=75.0"</span><span class="p">,</span><span class="w"> </span><span class="s2">"wheel"</span><span class="p">]</span> <span class="n">build-backend</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="s2">"setuptools.backends._legacy:_Backend"</span> </code></pre></div> <p>Or, for belt-and-suspenders safety, also pre-install critical deps explicitly in the Dockerfile before <code>pip install .</code>:</p> <div class="highlight"><pre><span></span><code><span class="k">RUN</span><span class="w"> </span>pip<span class="w"> </span>install<span class="w"> </span>httpx&gt;<span class="o">=</span><span class="m">0</span>.27<span class="w"> </span>fastmcp&gt;<span class="o">=</span><span class="m">2</span>.0 <span class="k">RUN</span><span class="w"> </span>pip<span class="w"> </span>install<span class="w"> </span>. </code></pre></div> <p><strong>2. Health check path mismatch</strong></p> <p>FastMCP serves at <code>/mcp</code>, not <code>/</code>. Configure Railway's health check to match:</p> <ul> <li>Set health check path to <code>/mcp</code> in Railway service settings</li> <li>Or add a root health endpoint in your server code:</li> </ul> <div class="highlight"><pre><span></span><code><span class="kn">from</span><span class="w"> </span><span class="nn">starlette.responses</span><span class="w"> </span><span class="kn">import</span> <span class="n">JSONResponse</span> <span class="kn">from</span><span class="w"> </span><span class="nn">starlette.routing</span><span class="w"> </span><span class="kn">import</span> <span class="n">Route</span> <span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">health</span><span class="p">(</span><span class="n">request</span><span class="p">):</span> <span class="k">return</span> <span class="n">JSONResponse</span><span class="p">({</span><span class="s2">"status"</span><span class="p">:</span> <span class="s2">"ok"</span><span class="p">})</span> <span class="c1"># Add to your ASGI app's routes</span> </code></pre></div> <p>Key points: - <code>pip install .</code> without <code>[build-system]</code> is a silent failure — the build succeeds but deps are missing - Always verify installed packages with <code>pip list</code> in a Dockerfile <code>RUN</code> step during debugging - Railway health checks default to <code>/</code> — confirm the actual serve path of your framework</p>