Who contributed to FastAPI & Pydantic on May 24, 2026?

1 developer shipped this update, including tiangolo.

What were the notable FastAPI & Pydantic updates?

♻️ Do not accept underscore headers when using `convert_underscores=True` (the default), ♻️ Validate Server Sent Event fields to avoid applications from sending broken data, and 0.136.3.

FastAPI & Pydantic

FastAPI and the Pydantic + SQLModel async-Python stack

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FASTAPI TIGHTENS HEADER VALIDATION AND SSE SAFETY IN RAPID RELEASES

By RepoJournal · Filed 06:03 UTC on May 24, 2026 · About FastAPI & Pydantic

1 person shipped this

tiangolo @tiangolo 3 cited

FastAPI shipped two consecutive patches that enforce stricter HTTP header handling and validate Server Sent Event data to prevent malformed responses.

FastAPI 0.136.3 enforces a breaking change on header validation [3]: the framework now rejects underscores in header names by default when `convert_underscores=True`, accepting only hyphenated headers per HTTP spec [1]. This catches a subtle bug where applications could silently accept malformed headers. The preceding 0.136.2 release added validation to Server Sent Event fields [4], preventing applications from shipping broken SSE data that clients can't parse [2]. Both changes close gaps where FastAPI was too permissive, accepting invalid data that HTTP specs explicitly forbid. Documentation also landed for the `--entrypoint` CLI option in 0.136.2 [4]. A test cleanup in the pipeline removes duplicate engine disposal that was causing noise in the test suite [5].

Action items

→ Review header names in your API definitions before upgrading to 0.136.3 - replace underscores with hyphens fastapi/fastapi [plan]
→ Validate your SSE implementations if you're using Server Sent Events - ensure field data is properly formatted fastapi/fastapi [plan]
→ Upgrade to 0.136.3 as part of your next release cycle fastapi/fastapi [monitor]

References

[1] ♻️ Do not accept underscore headers when using `convert_underscores=True` (the default) ↗ fastapi/fastapi
[2] ♻️ Validate Server Sent Event fields to avoid applications from sending broken data ↗ fastapi/fastapi
[3] 0.136.3 ↗ fastapi/fastapi
[4] 0.136.2 ↗ fastapi/fastapi
[5] ✅ Update tests, don't double dispose the engine ↗ fastapi/fastapi

Quick answers

What shipped in FastAPI & Pydantic on May 24, 2026?: FastAPI shipped two consecutive patches that enforce stricter HTTP header handling and validate Server Sent Event data to prevent malformed responses. In total, 7 commits, 5 pull requests, and 2 releases landed.
Who contributed to FastAPI & Pydantic on May 24, 2026?: 1 developer shipped this update, including tiangolo.
What were the notable FastAPI & Pydantic updates?: ♻️ Do not accept underscore headers when using `convert_underscores=True` (the default), ♻️ Validate Server Sent Event fields to avoid applications from sending broken data, and 0.136.3.

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