## TL;DR
mypy cannot find type information for a third-party module in CI. Locally the stubs exist; in CI they were never installed. Install the matching types package in the CI environment, or tell mypy to ignore that specific import. The code is fine - the environment is incomplete.

## The error
```text
mypy error: cannot find implementation or library stub for module in CI
```

## Steps

### 1. Identify the module
Read the full error line. It names the module mypy cannot resolve.
Expected output: you know exactly which import is missing stubs.

### 2. Check what is installed locally versus CI
```bash
pip show types-requests
```
Use the types package matching the module. Run it locally and in CI.
Expected output: installed locally, missing in CI. That gap is the bug.

### 3. Add the types package to CI dependencies
Add the matching `types-` package (for example types-requests for the requests library) to the dev or test requirements file that CI installs.
Expected output: CI installs the stubs alongside the library.

### 4. Or ignore that specific import
If no stubs exist for the module, silence just that import in the mypy config:
```ini
[mypy-requests.*]
ignore_missing_imports = True
```
Expected output: mypy skips the unresolvable import without hiding other errors.

### 5. Pin the mypy version in CI
Version drift changes stub resolution behavior. Pin the same mypy version locally and in CI.
Expected output: both environments resolve imports identically.

### 6. Re-run the type check
Expected output: mypy exits 0 with no missing-stub errors.

## When to use
- The exact missing-stub error in CI
- Type checking passes locally but fails in CI
- A new third-party dependency was just added

## When not to use
- Genuine type errors in your own code
- mypy config file errors
- Import errors at runtime (different problem)

## Tool compatibility
- mypy 1.x
- typeshed and the types stub packages on PyPI
- pip, poetry, and other Python installers

## Variant phrasings
### mypy cannot find stub for module in GitHub Actions
Same fix. The Actions environment is missing the types package.

### mypy library stubs missing after adding dependency
Every new third-party import may need its types package. Add both together.

### ignore_missing_imports versus installing stubs
Installing stubs is better: it type-checks the library usage. Ignoring is the fallback when no stubs exist.

## Why it happens
mypy needs type stubs for every imported module. Typeshed ships stubs for the standard library, but third-party stubs live in separate types packages that must be installed. Dev machines accumulate them over time; fresh CI environments do not.

## Edge cases
- Namespace packages and plugins sometimes need explicit mypy plugin config beyond the stubs.
- A library that ships its own inline types needs no types package. Check before adding one.
- ignore_missing_imports on a broad pattern hides real problems. Scope it to the single module.
- Stub version must match the library version loosely. A stale types package can report phantom errors.

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_3fSUGcUtXTNYjE-aMY68EA