--- title: What should I do if I receive a 503 error on Vercel? description: Learn about when Serverless Functions return a 503 status code and what can be done about them. url: /kb/guide/what-should-i-do-if-i-receive-a-503-error-on-vercel canonical_url: "https://vercel.com/kb/guide/what-should-i-do-if-i-receive-a-503-error-on-vercel" published: 2025-11-03 last_updated: 2026-08-24 authors: Holden Altaffer related: - /docs/concepts/functions/serverless-functions - /docs/functions/serverless-functions/runtimes - /docs/edge-network/caching - /docs/incremental-static-regeneration - /docs/functions/configuring-functions/duration install_vercel_plugin: npx plugins add vercel/vercel-plugin --- ## Related pages > **For AI agents:** Follow these links to understand how this page connects to the rest of the Vercel ecosystem. For the full cross-link map (inbound, outbound, prerequisites, and semantic neighbors), see the .graph.md link below. - [Legacy Usage & Pricing for Functions](https://vercel.com/docs/functions/usage-and-pricing/legacy-pricing?from=related&source_path=%2Fkb%2Fguide%2Fwhat-should-i-do-if-i-receive-a-503-error-on-vercel&source_site=vercel-kb&relationship=related) — Learn about legacy usage and pricing for Vercel Functions. - [Concurrency scaling](https://vercel.com/docs/functions/concurrency-scaling?from=related&source_path=%2Fkb%2Fguide%2Fwhat-should-i-do-if-i-receive-a-503-error-on-vercel&source_site=vercel-kb&relationship=related) — Learn how Vercel automatically scales your functions to handle traffic surges. - [Vercel Functions](https://vercel.com/docs/functions?from=related&source_path=%2Fkb%2Fguide%2Fwhat-should-i-do-if-i-receive-a-503-error-on-vercel&source_site=vercel-kb&relationship=related) — Run server-side code on Vercel without managing a server. - [Vercel Functions Limits](https://vercel.com/docs/functions/limitations?from=related&source_path=%2Fkb%2Fguide%2Fwhat-should-i-do-if-i-receive-a-503-error-on-vercel&source_site=vercel-kb&relationship=related) — Learn about the limits and restrictions of using Vercel Functions. - [Evolving Vercel Functions](https://vercel.com/blog/evolving-vercel-functions?from=related&source_path=%2Fkb%2Fguide%2Fwhat-should-i-do-if-i-receive-a-503-error-on-vercel&source_site=vercel-kb&relationship=related) - [Debugging slow Vercel Functions](https://vercel.com/docs/functions/debug-slow-functions?from=related&source_path=%2Fkb%2Fguide%2Fwhat-should-i-do-if-i-receive-a-503-error-on-vercel&source_site=vercel-kb&relationship=related) — Diagnose and fix slow Vercel Functions using CLI tools, logs, and timing analysis. - [How do I debug a 502 error from a Vercel Serverless Function?](https://vercel.com/kb/guide/how-to-debug-a-502-error?from=related&source_path=%2Fkb%2Fguide%2Fwhat-should-i-do-if-i-receive-a-503-error-on-vercel&source_site=vercel-kb&relationship=related) — Information on how to debug a 502 error from a Vercel Serverless Function. - [How to stop Vercel Functions from timing out](https://vercel.com/kb/guide/what-can-i-do-about-vercel-serverless-functions-timing-out?from=related&source_path=%2Fkb%2Fguide%2Fwhat-should-i-do-if-i-receive-a-503-error-on-vercel&source_site=vercel-kb&relationship=related) — Vercel Functions that time out usually trace back to a few causes. Learn how Fluid Compute fixes most of them and how to - [How do I lower my Vercel Function execution time?](https://vercel.com/kb/guide/how-do-i-lower-my-serverless-function-execution-time?from=related&source_path=%2Fkb%2Fguide%2Fwhat-should-i-do-if-i-receive-a-503-error-on-vercel&source_site=vercel-kb&relationship=related) — Learn how to lower your Serverless Function execution time. - [What is Vercel's policy regarding load testing deployments?](https://vercel.com/kb/guide/what-s-vercel-s-policy-regarding-load-testing-deployments?from=related&source_path=%2Fkb%2Fguide%2Fwhat-should-i-do-if-i-receive-a-503-error-on-vercel&source_site=vercel-kb&relationship=related) — Learn about Vercel's policies regarding load tests. - [Troubleshooting Build Error: "Serverless Function has exceeded the unzipped maximum size of 250 MB"](https://vercel.com/kb/guide/troubleshooting-function-250mb-limit?from=related&source_path=%2Fkb%2Fguide%2Fwhat-should-i-do-if-i-receive-a-503-error-on-vercel&source_site=vercel-kb&relationship=related) — Learn how to troubleshoot builds failing due to exceeding the maximum function size limit on Vercel. Full cross-link map for this page: [/kb/guide/what-should-i-do-if-i-receive-a-503-error-on-vercel.graph.md](/kb/guide/what-should-i-do-if-i-receive-a-503-error-on-vercel.graph.md?from=related&source_path=%2Fkb%2Fguide%2Fwhat-should-i-do-if-i-receive-a-503-error-on-vercel&source_site=vercel-kb&relationship=graph) A **503: FUNCTION\_THROTTLED** error on your [Vercel Serverless Function](https://vercel.com/docs/concepts/functions/serverless-functions) indicates that the default limit of **concurrent executions** has been reached (30,000 for Pro and 100,000 for Enterprise). Although rare, this guide discusses potential causes and steps to resolve or mitigate this issue. ## Understanding Concurrency Concurrency refers to the ability of your Serverless Function to handle multiple executions simultaneously. ## Identifying Concurrent Issues Exceeding the concurrency limit may indicate a deeper issue. If the serverless execution time per request is increasing, it suggests that the available function capacity is being utilized fully. It's advisable to check your upstream provider (API, Content Management System, Database, etc.) to ensure it scales alongside Vercel, as increasing concurrency may strain your upstream provider further. ## Vercel Function Autoscaling Vercel automatically scales the number of function instances based on demand, up to a maximum of **30,000** concurrent executions for Pro and **100,000** for Enterprise with the option for extended concurrency. This ensures optimal performance during traffic surges without requiring manual intervention. Learn more about [automatic concurrency scaling](https://vercel.com/docs/functions/serverless-functions/runtimes#automatic-concurrency-scaling) for Vercel Functions. ## Burst Concurrency Burst concurrency allows for a temporary increase in the concurrency limit to handle sudden traffic influxes. After the initial burst, Vercel continues to scale your function by a maximum of **500 concurrency per minute**. The scaling process might take several minutes, especially during substantial traffic increases, to ensure a smooth scaling experience that aligns with natural traffic patterns and minimizes impact on performance. Learn more about [burst concurrency](https://vercel.com/docs/functions/serverless-functions/runtimes#automatic-concurrency-scaling) on Vercel. ## Other Solutions ### Caching Headers If your response can be cached, even for a short duration, adding appropriate `cache-control` headers can help reduce the number of requests reaching your function, while also improving response times. Learn more about [caching on Vercel.](https://vercel.com/docs/edge-network/caching) ### Incremental Static Regeneration (ISR) Utilize [Incremental Static Regeneration (ISR)](https://vercel.com/docs/incremental-static-regeneration) to generate static pages on-demand, which can help manage high-traffic events effectively. ### Limiting Function Maximum Duration You can limit the [maximum duration for your functions](https://vercel.com/docs/functions/configuring-functions/duration) to ensure they do not run longer than necessary, which can also help in managing resources better.