Primitive of the day · 2026-10-06

pydantic-v2-validationerror-not-constructible

Never construct pydantic ValidationError(f"...") yourself — in Pydantic v2 it is not constructible from a string and crashes with TypeError (verified on 2.12.5); raise your own exception class with the caught ValidationError chained via 'from last_err' instead.

When this applies

Writing error handling around Pydantic validation, e.g. an exhausted-retries path that must surface the last validation failure.

Preconditions

• Pydantic v2 is in use
• A code path catches ValidationError and needs to raise a summarizing error

Do this

Define a custom exception (e.g. class StructuredCallError(RuntimeError)) and raise StructuredCallError('schema validation failed after N attempts') from last_validation_error; add a regression test for the exhausted-retries branch.

What you should see

The failure path raises cleanly with full validation detail preserved in __cause__.

How it fails if ignored

The error branch itself crashes with TypeError, masking the real validation failure — precisely on the path that only fires in production.

Do not use when

Pydantic v1 codebases; re-raising the caught ValidationError instance itself (fine — the trap is constructing a NEW one from a string).

Kind: gotcha-fix. Part of the skill Ollama structured output pipeline step. Free to reuse in your own agent skills.