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wire 2026-08-27
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Pydantic's JSON schema lies to you

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

MLuc24 YuriiMotov strawgate latest-changes[bot]
4 people shipped this
MLuc24 @MLuc24 3 cited YuriiMotov @YuriiMotov 1 cited strawgate @strawgate 1 cited latest-changes[bot] @latest-changes[bot]

Pydantic is merging three PRs that fix silent mismatches between validation and generated JSON schemas, so clients stop approving data the model rejects.

Three Pydantic PRs [1][2][3] fix cases where the JSON schema does not reflect what the validator actually accepts. When you set `str_min_length` or `str_max_length` in config, the schema leaves out `minLength` and `maxLength`, so consumers accept input that later fails validation [1]. Likewise, serialization temporal formats like `ser_json_temporal='seconds'` leak into validation schemas, declaring a `number` where ISO date strings are expected [2]. And a docs update turns the Logfire integration page into a practical reference for capturing failed validations and connecting them to traces [4]. FastAPI gets an unglamorous but necessary fix: Typer's minimum is bumped to 0.26.1 to unstick failing CI tests under `lowest-direct` resolution [5]. With just 5 commits and 4 PRs across both repos, the stack is quiet, but these schema fixes are worth pulling into your next upgrade, especially if you expose JSON schema to clients. The Logfire docs now "turn the Logfire integration page into a practical setup reference," per the PR summary [4].

Action items

References

  1. [1] Reflect `str_min_length` and `str_max_length` config in the JSON schema ↗ pydantic/pydantic
  2. [2] Don't apply serialization temporal formats to validation JSON schemas ↗ pydantic/pydantic
  3. [3] Reflect `str_min_length` and `str_max_length` config in the JSON schema (#13714) ↗ pydantic/pydantic
  4. [4] Improve Logfire guidance for validation errors ↗ pydantic/pydantic
  5. [5] ⬆️ Bump Typer min version to 0.26.1 ↗ fastapi/fastapi

$ 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
Full archive → Weekly reviews →

Quick answers

What shipped in FastAPI & Pydantic on August 27, 2026?
Pydantic is merging three PRs that fix silent mismatches between validation and generated JSON schemas, so clients stop approving data the model rejects. In total, 5 commits and 4 pull requests landed.
Who contributed to FastAPI & Pydantic on August 27, 2026?
4 developers shipped this update, including YuriiMotov, latest-changes[bot], strawgate, and MLuc24.
What were the notable FastAPI & Pydantic updates?
Reflect `str_min_length` and `str_max_length` config in the JSON schema, Don't apply serialization temporal formats to validation JSON schemas, and Reflect `str_min_length` and `str_max_length` config in the JSON schema (#13714).
♥

$ 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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