mypy "Cannot find implementation or library stub" in CI

mypy could not find type information for an imported module - no inline types, no bundled py.typed, and no installed stub package. It reports the module as unresolved for typing and fails the run.

What this error means

mypy fails with error: Cannot find implementation or library stub for module named "X" (or [import-untyped]/[import-not-found]), often suggesting a types-X stub package to install.

mypy output

app/client.py:3: error: Cannot find implementation or library stub for
module named "requests"  [import-untyped]
app/client.py:3: note: Hint: "python3 -m pip install types-requests"

Common causes

The library ships no types and no stub is installed

The package has no inline annotations or py.typed, and the corresponding types-* stub package is not installed, so mypy has nothing to check against.

A first-party module not on mypy’s path

For your own modules, import-not-found means the package is not installed/discoverable, so mypy cannot locate the implementation.

How to fix it

Install the stub package

Add the types-* stub mypy suggests (or let mypy install them).

Terminal

pip install types-requests
# or let mypy fetch missing stubs
mypy --install-types --non-interactive .

Configure handling for genuinely untyped deps

For a dependency with no stubs, ignore missing imports for that module rather than globally.

pyproject.toml

[[tool.mypy.overrides]]
module = ["some_untyped_lib.*"]
ignore_missing_imports = true

How to prevent it

Install needed types-* stub packages in the typecheck environment.
Scope ignore_missing_imports to specific untyped modules, not globally.
Install your own project so first-party modules resolve for mypy.