How would you implement a consistent API error contract with FastAPI exception handlers?

Maps validation, HTTP, domain, and unexpected failures into a stable client contract while preserving diagnostics.

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I define a small public error envelope and register FastAPI exception handlers for the failure categories that cross the HTTP boundary. Request-validation failures become client errors with safe field locations; deliberate HTTPException values preserve their status and required headers; domain outcomes map to documented application error identities; and unexpected exceptions return a generic server error. Internal logs and traces keep the original exception and correlation data without exposing stack traces, SQL, or secrets to the client.

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Error taxonomy, FastAPI handler boundaries, safe disclosure, protocol correctness, and observability.

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Overview
Consistency comes from classifying failures, not from turning every exception into the same JSON object.
I define a small public error envelope and register FastAPI exception handlers for the failure categories that cross the HTTP boundary. Request-validation failures become client errors with safe field locations; deliberate HTTPException values preserve their status and required headers; domain outcomes map to documented application error identities; and unexpected exceptions return a generic server error. Internal logs and traces keep the original exception and correlation data without exposing stack traces, SQL, or secrets to the client.
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The handler layer translates rather than invents business meaning. A not-found domain outcome may map differently depending on disclosure rules, while an authorization failure must preserve authentication challenge behavior when applicable. I keep human-readable detail separate from a stable machine-readable type or code so clients do not branch on prose. Response schemas document common errors, and integration tests cover validation, expected domain failures, unexpected exceptions, and protocol headers.

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Returning stack traces or database details to clients.
Using exception message text as the stable client code.
Converting every exception into one status.
Dropping headers such as Retry-After or WWW-Authenticate.

How would you implement a consistent API error contract with FastAPI exception handlers?

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