Wrangler startup profiling: bundle sizes and CPU activity

wrangler check startup now reports your Worker's raw and compressed bundle sizes. It also summarizes local CPU activity during startup directly in your terminal.

Large bundles and costly startup work can introduce cold-start latency, so use this command to find code and large dependencies that slow your Worker before it handles requests.

The summary includes sampled, active, garbage collection, and idle time. Wrangler continues to save a .cpuprofile file for detailed flamegraph analysis in Chrome DevTools or VS Code.

<span class="line"><span class="nb-shiki-1t8gfj">⛅️</span><span class="nb-shiki-mdbnqw"> wrangler</span><span class="nb-shiki-dzsirb"> 4.116.0</span></span>
<span class="line"><span class="nb-shiki-1t8gfj">───────────────────────────────────────────────</span></span>
<span class="line"><span class="nb-shiki-1t8gfj">├</span><span class="nb-shiki-mdbnqw"> Building</span><span class="nb-shiki-mdbnqw"> your</span><span class="nb-shiki-mdbnqw"> Worker</span></span>
<span class="line"><span class="nb-shiki-1t8gfj">│</span><span class="nb-shiki-mdbnqw"> Worker</span><span class="nb-shiki-mdbnqw"> Built!</span><span class="nb-shiki-mdbnqw"> 🎉</span></span>
<span class="line"><span class="nb-shiki-1t8gfj">│</span></span>
<span class="line"><span class="nb-shiki-1t8gfj">├</span><span class="nb-shiki-mdbnqw"> Analysing</span></span>
<span class="line"><span class="nb-shiki-1t8gfj">│</span><span class="nb-shiki-mdbnqw"> Startup</span><span class="nb-shiki-mdbnqw"> phase</span><span class="nb-shiki-mdbnqw"> analysed</span></span>
<span class="line"><span class="nb-shiki-1t8gfj">│</span></span>
<span class="line"><span class="nb-shiki-1t8gfj">│</span><span class="nb-shiki-mdbnqw"> Bundle:</span><span class="nb-shiki-dzsirb"> 7171.25</span><span class="nb-shiki-mdbnqw"> KiB</span><span class="nb-shiki-mdbnqw"> /</span><span class="nb-shiki-mdbnqw"> gzip:</span><span class="nb-shiki-dzsirb"> 2197.00</span><span class="nb-shiki-mdbnqw"> KiB</span></span>
<span class="line"><span class="nb-shiki-1t8gfj">│</span></span>
<span class="line"><span class="nb-shiki-1t8gfj">│</span><span class="nb-shiki-mdbnqw"> Local</span><span class="nb-shiki-mdbnqw"> startup</span><span class="nb-shiki-mdbnqw"> profile:</span></span>
<span class="line"><span class="nb-shiki-1t8gfj">│</span><span class="nb-shiki-mdbnqw">   Profile</span><span class="nb-shiki-mdbnqw"> window:</span><span class="nb-shiki-dzsirb"> 70.3</span><span class="nb-shiki-mdbnqw"> ms</span></span>
<span class="line"><span class="nb-shiki-1t8gfj">│</span><span class="nb-shiki-mdbnqw">   Sampled</span><span class="nb-shiki-mdbnqw"> time:</span><span class="nb-shiki-dzsirb"> 70.3</span><span class="nb-shiki-mdbnqw"> ms</span></span>
<span class="line"><span class="nb-shiki-1t8gfj">│</span><span class="nb-shiki-mdbnqw">   Active:</span><span class="nb-shiki-dzsirb"> 38.5</span><span class="nb-shiki-mdbnqw"> ms</span><span class="nb-shiki-140thh"> (including </span><span class="nb-shiki-dzsirb">3.7</span><span class="nb-shiki-mdbnqw"> ms</span><span class="nb-shiki-mdbnqw"> garbage</span><span class="nb-shiki-mdbnqw"> collection</span><span class="nb-shiki-140thh">)</span></span>
<span class="line"><span class="nb-shiki-1t8gfj">│</span><span class="nb-shiki-mdbnqw">   Idle:</span><span class="nb-shiki-dzsirb"> 31.8</span><span class="nb-shiki-mdbnqw"> ms</span></span>
<span class="line"><span class="nb-shiki-1t8gfj">│</span><span class="nb-shiki-mdbnqw">   Samples:</span><span class="nb-shiki-dzsirb"> 36</span></span>
<span class="line"><span class="nb-shiki-1t8gfj">│</span></span>
<span class="line"><span class="nb-shiki-1t8gfj">│</span><span class="nb-shiki-mdbnqw"> CPU</span><span class="nb-shiki-mdbnqw"> Profile</span><span class="nb-shiki-mdbnqw"> has</span><span class="nb-shiki-mdbnqw"> been</span><span class="nb-shiki-mdbnqw"> written</span><span class="nb-shiki-mdbnqw"> to</span><span class="nb-shiki-mdbnqw"> worker-startup.cpuprofile.</span><span class="nb-shiki-mdbnqw"> Load</span><span class="nb-shiki-mdbnqw"> it</span><span class="nb-shiki-mdbnqw"> into</span><span class="nb-shiki-mdbnqw"> the</span><span class="nb-shiki-mdbnqw"> Chrome</span><span class="nb-shiki-mdbnqw"> DevTools</span><span class="nb-shiki-mdbnqw"> profiler</span><span class="nb-shiki-140thh"> (or </span><span class="nb-shiki-mdbnqw">directly</span><span class="nb-shiki-mdbnqw"> in</span><span class="nb-shiki-mdbnqw"> VSCode</span><span class="nb-shiki-140thh">) to view a flamegraph.</span></span>
<span class="line"><span class="nb-shiki-1t8gfj">│</span></span>
<span class="line"><span class="nb-shiki-1t8gfj">│</span><span class="nb-shiki-mdbnqw"> Note</span><span class="nb-shiki-mdbnqw"> that</span><span class="nb-shiki-mdbnqw"> the</span><span class="nb-shiki-mdbnqw"> CPU</span><span class="nb-shiki-mdbnqw"> Profile</span><span class="nb-shiki-mdbnqw"> was</span><span class="nb-shiki-mdbnqw"> measured</span><span class="nb-shiki-mdbnqw"> on</span><span class="nb-shiki-mdbnqw"> your</span><span class="nb-shiki-mdbnqw"> Worker</span><span class="nb-shiki-mdbnqw"> running</span><span class="nb-shiki-mdbnqw"> locally</span><span class="nb-shiki-mdbnqw"> on</span><span class="nb-shiki-mdbnqw"> your</span><span class="nb-shiki-mdbnqw"> machine,</span><span class="nb-shiki-mdbnqw"> which</span><span class="nb-shiki-mdbnqw"> has</span><span class="nb-shiki-mdbnqw"> a</span><span class="nb-shiki-mdbnqw"> different</span><span class="nb-shiki-mdbnqw"> CPU</span><span class="nb-shiki-mdbnqw"> than</span><span class="nb-shiki-mdbnqw"> when</span><span class="nb-shiki-mdbnqw"> your</span><span class="nb-shiki-mdbnqw"> Worker</span><span class="nb-shiki-mdbnqw"> runs</span><span class="nb-shiki-mdbnqw"> on</span><span class="nb-shiki-mdbnqw"> Cloudflare.</span></span>
<span class="line"><span class="nb-shiki-1t8gfj">│</span></span>
<span class="line"><span class="nb-shiki-1t8gfj">│</span><span class="nb-shiki-mdbnqw"> As</span><span class="nb-shiki-mdbnqw"> such,</span><span class="nb-shiki-mdbnqw"> CPU</span><span class="nb-shiki-mdbnqw"> Profile</span><span class="nb-shiki-mdbnqw"> can</span><span class="nb-shiki-mdbnqw"> be</span><span class="nb-shiki-mdbnqw"> used</span><span class="nb-shiki-mdbnqw"> to</span><span class="nb-shiki-mdbnqw"> understand</span><span class="nb-shiki-mdbnqw"> where</span><span class="nb-shiki-mdbnqw"> time</span><span class="nb-shiki-mdbnqw"> is</span><span class="nb-shiki-mdbnqw"> spent</span><span class="nb-shiki-mdbnqw"> at</span><span class="nb-shiki-mdbnqw"> startup,</span><span class="nb-shiki-mdbnqw"> but</span><span class="nb-shiki-mdbnqw"> the</span><span class="nb-shiki-mdbnqw"> overall</span><span class="nb-shiki-mdbnqw"> startup</span><span class="nb-shiki-mdbnqw"> time</span><span class="nb-shiki-mdbnqw"> in</span><span class="nb-shiki-mdbnqw"> the</span><span class="nb-shiki-mdbnqw"> profile</span><span class="nb-shiki-mdbnqw"> should</span><span class="nb-shiki-mdbnqw"> not</span><span class="nb-shiki-mdbnqw"> be</span><span class="nb-shiki-mdbnqw"> expected</span><span class="nb-shiki-mdbnqw"> to</span><span class="nb-shiki-mdbnqw"> exactly</span><span class="nb-shiki-mdbnqw"> match</span><span class="nb-shiki-mdbnqw"> what</span><span class="nb-shiki-mdbnqw"> your</span><span class="nb-shiki-mdbnqw"> Worker's startup time will be when deploying to Cloudflare.</span></span>

The profile runs locally, so its duration will differ from startup time on Cloudflare. For authoritative startup time, deploy your Worker or upload a version.

Available in Wrangler version 4.116.0 or later. For more information, refer to wrangler check startup.