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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.

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

Source: https://github.com/modal-labs/modal-client/issues/3857