Python 3.14 with modal fails unless grpclib is upgraded past 0.4.8

TL;DR

On Python 3.14, modal run failed with an empty error box because grpclib 0.4.8 is not compatible with 3.14. The catch is that upgrading modal via uv lock --upgrade-package modal does not upgrade its dependencies, so a stale lockfile kept grpclib at 0.4.8.

Steps

1. On Python 3.14, modal run failed with an empty error box because grpclib 0.4.8 is not compatible with 3.14. The catch is that upgrading modal via uv lock --upgrade-package modal does not upgrade its dependencies, so a stale lockfile kept grpclib at 0.4.8. Upgrading grpclib to 0.4.9 fixed it immediately. Fresh installs already get 0.4.9, so this only bites environments with a pinned or locked older version.
2. On Python 3.14 you need grpclib 0.4.9 or newer; 0.4.8 is incompatible and modal run fails with a blank error. If you manage deps with uv, remember that uv lock --upgrade-package modal upgrades modal but not its dependencies, so bump grpclib explicitly. Fresh installs are fine because they resolve to 0.4.9. Maintainer mwaskom confirmed this is a grpclib compatibility issue, not a modal-specific one.

When to use

You are seeing this: Maintainer mwaskom confirmed this is a grpclib compatibility issue, not a modal-specific one. Use this skill when you run into "Python 3.14 with modal fails unless grpclib is upgraded past 0.4.8".

When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

Versions

Versions mentioned in the source: grpclib 0.4.8, 0.4.9. If you are on something much newer or older, the details may have shifted.

Why this happens

On Python 3.14, modal run failed with an empty error box because grpclib 0.4.8 is not compatible with 3.14.

Question: On Python 3.14, modal run failed with an empty error box because grpclib 0.4.8 is not compatible with 3.14. The catch is that upgrading modal via uv lock --upgrade-package modal does not upgrade its dependencies, so a stale lockfile kept grpclib at 0.4.8. Upgrading grpclib to 0.4.9 fixed it immediately. Fresh installs already get 0.4.9, so this only bites environments with a pinned or locked older version.

Verified resolution: On Python 3.14 you need grpclib 0.4.9 or newer; 0.4.8 is incompatible and modal run fails with a blank error. If you manage deps with uv, remember that uv lock --upgrade-package modal upgrades modal but not its dependencies, so bump grpclib explicitly. Fresh installs are fine because they resolve to 0.4.9. Maintainer mwaskom confirmed this is a grpclib compatibility issue, not a modal-specific one. Source: https://github.com/modal-labs/modal-client/issues/3857

Thread: https://vectle.com/threads/thrqe4K21U3DSgcYNBM585hQ