Your API Is Replying… But Authentication Is Already Broken (FastAPI JWT)

🔥 Opening

/token works.

You receive a JWT.

Swagger shows “Authorized”.

Everything looks correct.

And then —

/me returns 401.

You check the endpoint.

You rewrite the dependency.

You add print logs.

Nothing changes.

The API is replying.

But authentication is already broken.

🧩 Why This Happens

Most of the time, the endpoint is not the problem.

The token is valid.

The route is correct.

The dependency is wired properly.

The failure happens one layer deeper.

Common structural causes:

• Different SECRET_KEY between environments
• Docker using a different .env file
• Multiple instances running with inconsistent configs
• Clock drift causing token validation issues
• Algorithm mismatch (HS256 vs RS256)

Nothing is “wrong” in the controller.

The structure is inconsistent.

🧠 The Real Problem

When authentication fails, most developers try to fix code.

They debug the endpoint.

They rewrite dependencies.

They patch logic.

But authentication is not just code.

It is configuration.

It is environment.

It is instance consistency.

It is key management.

If those layers are misaligned,

no endpoint fix will solve it.

🛠 My Approach

I specialize in structural FastAPI/JWT authentication incidents.

I don’t start by modifying code.

I map the layers first:

• Environment variables
• Key consistency
• Instance configuration
• Token structure
• Deployment differences

Playground (reproducible setup): 👉 https://github.com/hiro-kuroe/fastapi-auth-crud-docker

Only after the structure is clarified, modification makes sense.

🏁 Closing

If your API keeps replying

but authentication keeps failing,

you may be debugging the wrong layer.

I built a reproducible playground for this exact type of incident:

👉 https://github.com/hiro-kuroe/fastapi-auth-crud-docker

Analysis-first. Structure before patching.

This is not a patch guide.
It’s a structural diagnosis.