How does FastAPI decide whether a parameter comes from the path, query, header, cookie, body, or a dependency?

Explains FastAPI parameter-source inference and when explicit markers make an HTTP contract clearer.

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FastAPI inspects the route template, function signature, annotations, defaults, and explicit parameter markers. A matching route-template name is a path parameter; simple annotated values are normally query parameters; and a Pydantic model is normally read from the request body. Header, Cookie, Body, Query, Path, and related markers make the source and constraints explicit. Depends marks a value that FastAPI obtains by resolving another callable rather than reading one field directly from the request.

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Whether the candidate understands signature inspection, explicit source markers, Pydantic bodies, and ambiguous boundary design.

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Overview
FastAPI derives much of an endpoint contract from its signature, so the signature must remain readable and unambiguous.
FastAPI inspects the route template, function signature, annotations, defaults, and explicit parameter markers. A matching route-template name is a path parameter; simple annotated values are normally query parameters; and a Pydantic model is normally read from the request body. Header, Cookie, Body, Query, Path, and related markers make the source and constraints explicit. Depends marks a value that FastAPI obtains by resolving another callable rather than reading one field directly from the request.

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Details

Assuming every scalar parameter comes from JSON.
Using a body field and route field as competing sources of truth.
Treating Depends as another client-supplied parameter source.
Relying on inference when the public contract is ambiguous.

How does FastAPI decide whether a parameter comes from the path, query, header, cookie, body, or a dependency?

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