Who contributed to FastAPI & Pydantic on August 17, 2026?

3 developers shipped this update, including Viicos, Kropiunig, and shashiKundur1.

What were the notable FastAPI & Pydantic updates?

Support bare `None` annotation for discriminated unions, Fix application of other constraints in the pipeline API, and Return `NotImplemented` from URL equality comparisons with foreign types.

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PYDANTIC FIXES DISCRIMINATED UNIONS AND VALIDATOR PIPELINE BUGS

By RepoJournal · Filed 07:09 UTC on August 17, 2026 · About FastAPI & Pydantic

3 people shipped this

Viicos @Viicos 3 cited

Kropiunig @Kropiunig 1 cited

shashiKundur1 @shashiKundur1 1 cited

Viicos landed back-to-back fixes for discriminated union annotation handling and constraint application in the validation pipeline, closing two separate issues that broke real validation workflows.

The core fix targets bare `None` annotations in discriminated unions [1], a feature gap that forced developers into verbose workarounds. That same PR also corrects JSON Schema generation for mapping keys and modernizes internal logic with match statements. In parallel, a second critical fix addresses constraint application in the pipeline API [2], catching a regression missed in an earlier commit that left certain field constraints silently unapplied. On the types side, Pydantic's URL equality comparisons now return `NotImplemented` for foreign types [3], fixing an asymmetry that broke comparison contracts with mock objects and custom equality implementations. Two more PRs in flight: one enforcing runtime alias validation config when wrap validators are present [4], and another teaching JSON Schema generation to respect the `ser_json_temporal` config flag [5], which prevents the schema from rejecting payloads that `model_dump_json()` itself produces.

Action items

→ Review and merge the discriminated union fix [ref:1] and pipeline constraint fix [ref:2] before your next release pydantic/pydantic [immediate]
→ Monitor the wrap validator and JSON Schema temporal config PRs [ref:4] [ref:5] for merge readiness pydantic/pydantic [plan]

References

[1] Support bare `None` annotation for discriminated unions ↗ pydantic/pydantic
[2] Fix application of other constraints in the pipeline API ↗ pydantic/pydantic
[3] Return `NotImplemented` from URL equality comparisons with foreign types ↗ pydantic/pydantic
[4] Respect runtime alias validation configuration when wrap validators are present ↗ pydantic/pydantic
[5] Take `ser_json_temporal` into account when generating JSON Schema ↗ pydantic/pydantic

Quick answers

What shipped in FastAPI & Pydantic on August 17, 2026?: Viicos landed back-to-back fixes for discriminated union annotation handling and constraint application in the validation pipeline, closing two separate issues that broke real validation workflows. In total, 6 commits and 6 pull requests landed.
Who contributed to FastAPI & Pydantic on August 17, 2026?: 3 developers shipped this update, including Viicos, Kropiunig, and shashiKundur1.
What were the notable FastAPI & Pydantic updates?: Support bare `None` annotation for discriminated unions, Fix application of other constraints in the pipeline API, and Return `NotImplemented` from URL equality comparisons with foreign types.

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