Summary

pydantic-core versions pulled in by pydantic < 2.13 have no prebuilt wheel for Python 3.14 and fail to build from source under it, breaking install/CI for langchain-mongodb and langchain-mongodb-deepagents-vfs on 3.14. Pin pydantic>=2.13.0 in both packages.

Background

Initial symptom looked like a warnings-related test failure: langchain-core emits a warning when its pydantic_v1 compatibility shim is imported, and libs/langchain-mongodb/pyproject.toml sets filterwarnings = ["error", ...] in [tool.pytest.ini_options], which turns any uncaught warning into a test failure. That mechanism is real, but it is not what breaks Python 3.14 support.

Root cause (verified)

• pydantic-core is a Rust extension built via PyO3.
• Reproduced directly: installing pydantic==2.9.2 (pulling pydantic-core==2.23.4) into a clean Python 3.14 venv fails at build time, not at import/warning time:

  error: the configured Python interpreter version (3.14) is newer than
  PyO3's maximum supported version (3.13)
  💥 maturin failed
  

• PyO3 0.22.2 (the version pydantic-core 2.23.4 was built against) has no support for the Python 3.14 ABI at all, and there is no prebuilt wheel to fall back to — the failure is a hard build error, unrelated to any filterwarnings setting.
• pydantic>=2.13.0 resolves to a pydantic-core build that ships an actual Python 3.14 wheel — confirmed it installs instantly under 3.14 with no compilation step.
• Checked libs/langchain-mongodb-deepagents-vfs/pyproject.toml specifically: it has no filterwarnings override in its own [tool.pytest.ini_options] at all, so the "warnings as errors" explanation cannot apply there regardless — the underlying problem in both packages is the same native-extension incompatibility, not test configuration.

Fix

• Add "pydantic>=2.13.0" to dependencies in libs/langchain-mongodb/pyproject.toml and libs/langchain-mongodb-deepagents-vfs/pyproject.toml.
• Regenerate the affected uv.lock files (top-level and both packages) so the resolved dependency graph reflects the new floor.

Verification

• Clean-venv reproduction on Python 3.14: pydantic==2.9.2 fails to build (PyO3/maturin error above); pydantic>=2.13.0 installs from a prebuilt wheel with no compilation.
• langchain-core's own declared constraint is pydantic<3.0.0,>=2.7.4 — well below what's actually required for 3.14, confirming the floor needed tightening explicitly rather than relying on the transitive constraint.

Notes

• No code changes were required — this is a dependency floor bump plus lockfile regeneration in both affected packages.
• The filterwarnings = ["error"] behavior in langchain-mongodb's pytest config is a legitimate, separate mechanism worth knowing about (a genuine langchain-core deprecation warning would fail that package's tests), but it was a red herring for this specific Python 3.14 build failure.