Who contributed to FastAPI & Pydantic on August 12, 2026?

4 developers shipped this update, including Yurii Motov, latest-changes[bot], Viicos, and remi-fongaufier.

What were the notable FastAPI & Pydantic updates?

Fix missing GC traversal on some `pydantic-core` struct fields, Fix missing GC traversal in `pydantic-core` for `GeneralFieldsSerializer` (#13629), and Fix JSON schema discriminator mapping keys for `bool` discriminators.

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PYDANTIC PATCHES GARBAGE COLLECTION LEAKS IN CORE STRUCTS

By RepoJournal · Filed 07:00 UTC on August 12, 2026 · About FastAPI & Pydantic

4 people shipped this

Viicos @Viicos 2 cited

Yurii Motov @YuriiMotov 1 cited

remi-fongaufier @remi-fongaufier 1 cited

latest-changes[bot] @latest-changes[bot]

Pydantic shipped fixes for missing GC traversal on pydantic-core struct fields that could cause memory leaks in long-running applications.

The core issue surfaces in two places: missing GC traversal on some pydantic-core struct fields [1] and a separate traversal gap in GeneralFieldsSerializer [2]. Both fixes are shipping in the same window, so upgrade together. On the FastAPI side, Yurii Motov corrected a workflow configuration error where the zizmor.yml file referenced the wrong default branch [4], a small fix but the kind that breaks CI silently if missed. Pydantic also landed a JSON schema fix for bool discriminators [3] that normalizes mapping keys to proper JSON representation (lowercase true/false instead of Python-style True/False), ensuring OpenAPI compatibility. The Pydantic fixes address both memory safety and spec compliance in a single push.

Action items

→ Check if your Pydantic models use discriminated unions with bool fields and regenerate JSON schemas pydantic/pydantic [plan]
→ Monitor for GC traversal fixes in next Pydantic release and test with long-running services pydantic/pydantic [monitor]
→ Verify FastAPI workflows use correct default branch name if you've forked the repo fastapi/fastapi [plan]

References

[1] Fix missing GC traversal on some `pydantic-core` struct fields ↗ pydantic/pydantic
[2] Fix missing GC traversal in `pydantic-core` for `GeneralFieldsSerializer` (#13629) pydantic/pydantic
[3] Fix JSON schema discriminator mapping keys for `bool` discriminators ↗ pydantic/pydantic
[4] 👷 Fix branch name in `zizmor.yml` workflow (`main` -> `master`) (#16178) fastapi/fastapi

Quick answers

What shipped in FastAPI & Pydantic on August 12, 2026?: Pydantic shipped fixes for missing GC traversal on pydantic-core struct fields that could cause memory leaks in long-running applications. In total, 5 commits and 4 pull requests landed.
Who contributed to FastAPI & Pydantic on August 12, 2026?: 4 developers shipped this update, including Yurii Motov, latest-changes[bot], Viicos, and remi-fongaufier.
What were the notable FastAPI & Pydantic updates?: Fix missing GC traversal on some `pydantic-core` struct fields, Fix missing GC traversal in `pydantic-core` for `GeneralFieldsSerializer` (#13629), and Fix JSON schema discriminator mapping keys for `bool` discriminators.

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