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Pydantic Models as LLM Tool Schemas: model_json_schema, $defs Inlining, extra=forbid and Validation Errors to Return

Self-Hosting & Infra intermediate 7 min read $0.01 Updated 2026-10-20

Executed with Pydantic 2.14: model_json_schema output with minLength, enum, minimum and maximum, required lists and anyOf null for Optional, an inliner removing $defs, error types for empty, out-of-range, extra, enum and non-numeric inputs, lax coercion of "3" versus strict mode.

Generate the JSON Schema for a tool from a Pydantic model, see what constraints it emits, inline $ref definitions for providers that dislike them, forbid extra arguments, and turn validation failures into messages a model can fix.

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