●Change8

Migrating to FastAPI 0.136.3

Version 0.136.3 introduces 1 breaking change. This guide details how to update your code.

Released: 5/23/2026

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Breaking Changes
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Migration Steps
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Affected Symbols

⚠️ Check Your Code

If you use any of these symbols, you need to read this guide:

Header conversion logic (when convert_underscores=True)

Breaking Changes

●Issue #1

The default behavior of `convert_underscores=True` now rejects headers containing underscores. If you relied on underscore headers being converted, you must now use `convert_underscores=False` or rename your headers.

Migration Steps

  1. 1
    If using underscore headers with default settings, set `convert_underscores=False` in your route configuration or rename headers to use hyphens instead.

Release Summary

Refactored header processing to strictly enforce that underscore headers are not accepted when `convert_underscores` is enabled by default.

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