Why Authentication Fails Despite Setting Environment Variables: Understanding SECRET_KEY as None in FastAPI

Introduction

• "The application is running, but for some reason, only authentication is broken."
• "It works locally, but it dies only in production (Docker)."
  (Signature verification failed / 401 Unauthorized)

This is not a bug in FastAPI or the JWT library.

In a FastAPI × Docker setup,

even a single misconfiguration can easily create a state where "it's broken but silently running."

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Common Symptoms

• SECRET_KEY is set in .env but becomes None.
• It works in local execution, but fails only in Docker.
• You print(SECRET_KEY) and despair.

If you are using JWT, this state is almost certainly

the entrance to 401 / 403 hell.

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Why it's tricky

At this point, a JWTError is actually occurring internally,
and FastAPI is exposing it as a 401 Unauthorized.

• The application itself starts up.
• The logs do not output any specific errors.
• When combined with JWT issues, the cause is difficult to pinpoint.

"It's a configuration error, but no exception is thrown."

This is the most troublesome part of this accident.

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Cause Patterns

1. Forgetting to specify --env-file

In the case of docker run, .env is not loaded automatically.

docker run --env-file .env -p 8000:8000 app

Without this,

SECRET_KEY does not exist inside the container.

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2. Difference in specification between docker run and docker compose

• docker run: Requires explicit --env-file.
• docker compose: Handling of env_file: and .env differs.

This becomes a cause for behavior changing even though the same .env exists.

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3. settings.py import order is too early

SECRET_KEY = os.getenv("SECRET_KEY")

If you are evaluating this at module import time,

• .env has not been loaded yet.
• As a result, None is retrieved.
• None is cached as the correct value.

This kind of accident occurs.

The setting is not "non-existent," it was just "too early."

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What can be confirmed with this setup

• State where environment variables are loaded correctly.
• Reproduction when .env or import order is broken.
• Perspective on isolating why None occurs.

It is not for "running it," but rather a "setup for diagnosing when things break."

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Conclusion

• It is not FastAPI that is broken.
• It is the flow of configuration that is broken.
• FastAPI is working surprisingly honestly.

If you read this, even if the same accident happens next time,

you will not get lost in the isolation process.

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A parting thought

I have published other accident memos around FastAPI / Docker / JWT.
If you have similar symptoms, please try to create "normal" with this setup first.

*I perform cause isolation for similar authentication troubles. A consultation contact is listed at the end of the article.

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🛠️ FastAPI Incident Analysis Series

I am publishing cases where cause isolation is difficult, such as "401 / JWT / Breaking only in Docker," by pattern.

🔐 Authentication & JWT Troubles

Why /me returns 401 even though /token passes
[https://zenn.dev/fastapier/articles/0022f125547300]

Mistaking 401 / 403 makes the cause invisible
[https://zenn.dev/fastapier/articles/41f9e2e10e7c19]

Why Depends breaks silently resulting in 401 / 403
[https://zenn.dev/fastapier/articles/efba40f5bcbbda]

🐳 Docker & Production Environment Troubles

Why it works locally but crashes in production
[https://zenn.dev/fastapier/articles/c90e5199e0bafc]

Why logging stopped working after switching to Docker
[https://zenn.dev/fastapier/articles/cd530c54b6e47c]

🏗️ Design & Operations Troubles

Why collapse occurs when main.py exceeds 1000 lines
[https://zenn.dev/fastapier/articles/a2a9a5209dedac]

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🩺 FastAPI Incident Consultation (Cause Identification & Structural Analysis)

For authentication troubles around FastAPI / JWT / Docker, such as:

• /token passes but /me returns 401
• InvalidSignatureError occurs only in the production environment
• Authentication fails only after containerizing with Docker

I provide isolation and correction strategies for these "accidents where the layer of the cause is hard to see."

If you have been investigating for over 3 hours without identifying the cause,
it is highly likely that there is a problem with the "structure" rather than the code.

📩 How to Consult

Please contact the email address listed in my GitHub profile.
[https://github.com/hiro-kuroe]

Please include only the following 3 points in the body of the email (short text is fine):

① Symptoms
(e.g., /me returns 401 / Signature error only in production / Authentication failure after Dockerization)

② Environment
(e.g., Local is OK, Production NG / Using Docker / Using Gunicorn, etc.)

③ Recent changes
(e.g., Changed SECRET_KEY / Added .env / Updated dependency packages, etc.)

*I will proceed with the analysis phase (cause identification and structural organization) first.
I will propose correction work after the cause is clarified.