How do you diagnose a request that never reaches the FastAPI path operation?

Locates pre-handler failures across proxies, routing, parsing, validation, dependencies, and middleware using structured evidence.

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TL;DR

I first confirm the exact request and response: method, path, query, headers, content type, body, status, and any correlation ID. Then I determine whether the request reached the process and whether routing selected the expected operation. FastAPI can reject a request during route matching, body parsing, parameter validation, or dependency execution before entering the handler. I inspect access logs and the standardized error body and reproduce the smallest request rather than adding a try block inside code that never executes.

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Systematic FastAPI debugging rather than adding handler try/catch blocks that cannot observe the failure.

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Core answer
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Overview
If the handler never runs, its logging and exception handling cannot explain the request outcome.
I first confirm the exact request and response: method, path, query, headers, content type, body, status, and any correlation ID. Then I determine whether the request reached the process and whether routing selected the expected operation. FastAPI can reject a request during route matching, body parsing, parameter validation, or dependency execution before entering the handler. I inspect access logs and the standardized error body and reproduce the smallest request rather than adding a try block inside code that never executes.
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Stronger answer
Optional upgrade with examples, trade-offs, and sharper phrasing.
The status and evidence narrow the boundary: a proxy may reject size or media type, routing may produce not-found or method behavior, validation may produce structured field errors, and an authentication dependency may return its own failure. I compare the generated OpenAPI contract with the client's actual payload and inspect recent annotation, alias, default, middleware, and dependency changes. A temporary trace at the outer middleware and dependency boundaries shows the last stage reached without logging secrets.

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Details

Adding exception handling only inside the path operation.
Testing by calling the handler function directly.
Logging raw request bodies to find validation problems.
Assuming every 4xx response came from application logic.

How do you diagnose a request that never reaches the FastAPI path operation?

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