WIP: Add Pyrefly support

[yangdanny97]

📝 Summary

Similar to recent PRs for ty & basedpyright, this PR adds support for Pyrefly, another fast language server and type checker

I read through the discussion in #1163, and I think that Pyrefly would be a good addition (perhaps falling somewhere between basedpyright & ty on the spectrum of maturity).

It's being used in similar tools like Positron, and is significantly more complete in terms of type system coverage than ty

Diagnostics

🔍 Description of Changes

📋 Checklist

• I have read the contributor guidelines.
• For large changes, or changes that affect the public API: this change was discussed or approved through an issue, on Discord, or the community discussions (Please provide a link if applicable).
• I have added tests for the changes made.
• I have run the code and verified that it works as expected.

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Project	Deployment	Review	Updated (UTC)
marimo-docs	Ready	Preview, Comment	Dec 21, 2025 6:35pm

[github-actions]

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[akshayka]

Thanks @yangdanny97! We're happy to review this. If this gets merged, we can document its support in our own docs, but also on Pyrefly docs (here: https://pyrefly.org/en/docs/IDE-features/#notebook-support) — does that sound good to you?

[yangdanny97]

I have read the CLA Document and I hereby sign the CLA

[yangdanny97]

@akshayka hi, thanks! Yep, I'll update Pyrefly's docs once this is merged.

I'm still working on debugging the diagnostics. Pyrefly is sending them (I can see it in the lsp logs) but somehow the listener does not pick them up, even if diagnostics box is checked in the user config. All other LSP functionalities work.

[yangdanny97]

I wonder if there's an issue with my local setup. I can't get diagnostics to appear for basedpyright or ty in my local dev build either, even without the changes in this PR.

@winstxnhdw do you remember having to do any sort of special dev setup to test your PRs for bpr/ty, or were the steps in https://github.com/marimo-team/marimo/blob/main/CONTRIBUTING.md#setup sufficient?

[winstxnhdw]

Hey @yangdanny97

I didn't have to do anything special. I did have to restart my setup every time I switch LSPs though. It might be worth looking at the console logs to see if an exception is being thrown.