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wire 2026-08-17
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PYDANTIC FIXES DISCRIMINATED UNIONS AND VALIDATOR PIPELINE BUGS

By RepoJournal · Filed · About FastAPI & Pydantic · Composed from the cited sources · methodology

Viicos Kropiunig shashiKundur1
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.

$ ls fastapi/reviews/ # the step-back reads

the week in review Pydantic drops Python 3.10, then layers serialization fast paths the month in review Five Pydantic validation bypasses and a quadratic URL check
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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.
♥

$ log FastAPI & Pydantic --recent

Pydantic fixes GC tracking in pydantic-core and caches UTC TzInfo 50 shipped Pydantic drops Python 3.10, accepts list and DecimalTuple for Decimal 37 shipped Pydantic reworks `TypeAdapter` call arguments, tightens `Decimal` schemas 13 shipped FastAPI bumps pillow and dnspython, Pydantic corrects arguments JSON Schema 6 shipped

$ log --all # elsewhere on the wire

Agent Security Quiet day in agent security as guard coverage reaches four clients Alignment & Post-training verl defaults Ascend RL mode, open-instruct unblocks SFT packing

$ ls showcases/

Safety Evals AI Red Teaming Agent Security Interpretability Alignment & Post-training

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