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REQUEST_HEADER_TOO_LARGE

The REQUEST_HEADER_TOO_LARGE error occurs when the size of a request header exceeds the allowed limit. Specifically, an individual request header, including the header name, must not exceed 32 KB.

This issue often arises from excessively large headers in a request. On Vercel, applications may have custom headers, which, if overly large, can trigger this error during server request processing.

Agent Prompt

I'm encountering an error and reviewing the docs at https://vercel.com/docs/errors/REQUEST_HEADER_TOO_LARGE.md to understand what's happening. Please help me resolve this by: 1. **Suggest the fix**: Analyze my codebase context and propose what needs to be changed to resolve this error. If you do not have access to my codebase, ask me for the codebase and try to fix the error based on the information you have. 2. **Explain the root cause**: Break down why this error occurred: - What was the code actually doing vs. what it needed to do? - What conditions triggered this specific error? - What misconception or oversight led to this? 3. **Teach the concept**: Help me understand the underlying principle: - Why does this error exist and what is it protecting me from? - What's the correct mental model for this concept? - How does this fit into the broader framework/language design? 4. **Show warning signs**: Help me recognize this pattern in the future: - What should I look out for that might cause this again? - Are there similar mistakes I might make in related scenarios? - What code smells or patterns indicate this issue? 5. **Discuss alternatives**: Explain if there are different valid approaches and their trade-offs My goal is to fix the immediate issue while building lasting understanding so I can avoid and resolve similar errors independently in the future.

To troubleshoot this error, follow these steps:

  1. Limit header size: Ensure that the size of each request header does not exceed 32 KB
  2. Review cookies: A request carries all of its cookies in one Cookie header, so keep the combined size of your cookies within the 32 KB limit
Last updated February 9, 2026

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