I’m deploying a development branch as preview with a FastAPI backend located at src/api/index.py. When hitting the api/auth/me route, I’m getting a ModuleNotFoundError: No module named 'pydantic_core._pydantic_core The code in src/api/index.py has changed very little compared to the current produc…

Hi, Do you have .python-version in your project? Could you temporarily enable source code visibility so we can take a look? https://vercel.com/kb/guide/how-to-allow-the-vercel-support-team-to-access-your-deployment-source-code

I’m experiencing the exact same issue when running FastAPI with pydantic on Vercel Functions. I think it’s because pydantic_core is actually built with native code (Rust in this case) ….see this issue from pydantic No module named 'pydantic_core._pydantic_core' using AWS Lambdas · Issue #6557 · pyd…

Hi @alexkan , Could you enable source code visibility https://vercel.com/kb/guide/how-to-allow-the-vercel-support-team-to-access-your-deployment-source-code and share us the project?

:white_check_mark: This is the solution. It’s because of a version conflict

FastAPI on Vercel Function: ModuleNotFoundError: No module named 'pydantic_core._pydantic_core'

Help

nfomis (Nfomis) January 9, 2026, 8:59pm 8

I can confirm that removing `.python-version` from the folder where the backend lives solves the problem.

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FastAPI on Vercel Function: ModuleNotFoundError: No module named 'pydantic_core._pydantic_core'

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