How should Pydantic request models be used at a FastAPI boundary?

Uses Pydantic models as explicit transport contracts without making them persistence or domain models automatically.

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A Pydantic request model should describe the data a specific operation accepts from a client. FastAPI validates and converts the incoming representation into that model before the handler runs. The handler can then map the transport values into a command or domain operation instead of passing arbitrary JSON or a persistence entity through the system. Separate create, replace, and patch models keep writable fields explicit and prevent clients from setting server-owned values such as IDs, roles, tenant ownership, or audit fields.

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Request-contract ownership, validation boundaries, overposting control, and separation from internal state.

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A validated request model is safe to parse, not automatically safe to apply to every internal object.
A Pydantic request model should describe the data a specific operation accepts from a client. FastAPI validates and converts the incoming representation into that model before the handler runs. The handler can then map the transport values into a command or domain operation instead of passing arbitrary JSON or a persistence entity through the system. Separate create, replace, and patch models keep writable fields explicit and prevent clients from setting server-owned values such as IDs, roles, tenant ownership, or audit fields.
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I put structural rules close to the request shape: required fields, bounded strings, supported enum values, and relationships between fields that can be decided from the payload. Authorization, uniqueness under concurrency, inventory state, and other changing facts belong in the application or database boundary. Mapping is deliberate because Pydantic may normalize or coerce input, and the domain may use richer value objects. That separation also lets the external API evolve without forcing the storage schema to match it.

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Using one Pydantic model for create, update, persistence, and response.
Assuming validated input is authorized input.
Blindly unpacking a request model into an ORM entity.
Putting database lookups inside model construction.

How should Pydantic request models be used at a FastAPI boundary?

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