Complete library typing and enforce mypy strict in CI

[HardMax71]

Hi! I use LLMs to help write code, and passing strict type checks such as mypy --strict is one of my guardrails. A project I'm working on depends on this library, so more complete type information would save me a lot of troubleshooting and help catch mistakes earlier.

Is your feature request related to a problem? Please describe.

The library already has substantial type annotations and runs mypy in CI. However, strict checking still exposes gaps that affect consumers. On the checkout described below, ordinary mypy passes, while mypy --strict reports 298 errors across the eight library packages.

For example, checking this consumer code with mypy --strict example.py reports implicit-export errors for both imports, an untyped call to get_key(), and an Any return:

from kiota_abstractions.serialization import Parsable
from kiota_http.middleware.options import RetryHandlerOption


def retry_option_key() -> str:
    return RetryHandlerOption().get_key()

I also built the microsoft-kiota-bundle wheel locally and confirmed that it lacks py.typed. Mypy treats an installed import of kiota_bundle.default_request_adapter as untyped, despite the annotations in its source.

Describe the solution you'd like

Complete the existing typing support and enforce strict mypy checks for the library packages.

Suggested definition of done:

• All eight library source packages pass mypy --strict in CI with a pinned mypy version across the supported Python versions.
• Consumer checks against installed wheels verify public imports and representative inferred return types; all intended typed packages ship py.typed.
• Public types remain useful, with any necessary suppressions narrowly scoped and explained, rather than broadly replacing types with Any or disabling checks.
• Existing runtime tests pass and public behavior remains compatible.

I suggest keeping unit-test typing and adopting another checker outside the initial scope. Recurring generated-code annotation changes could be tracked separately with the generator work they require.

Describe alternatives you've considered

Local stubs or per-project mypy overrides could work around individual problems, but would leave consumers maintaining those workarounds. Completing the shared library annotations would benefit dependent projects directly.

Additional context

• Checkout: eef36d7e72197063f234b24407edad15ad873bea; local packages report version 1.12.1.
• Environment: Linux, Python 3.13.5, mypy 2.3.1; dependencies installed using uv, with all eight packages installed in editable mode.
• The library-only baseline is 298 errors in 101 checked source files. The generated validation client adds 345 errors in 86 files; those are separate from the library count.
• The largest recurring library categories are missing explicit exports, incomplete function annotations, untyped calls, and missing generic arguments.
• Related earlier typing discussions: Typing questions #355 and Inconsistent typing used in generated code and the libraries. #333, both closed.

Would you be open to using strict mypy compliance as a tracked enhancement for the library packages?

[HardMax71]

I do know that typing improvements may be P2/P3 or even lower priority, and bringing a whole library up to strict checking is difficult to do in one pass. I'm happy to help with the implementation in small batches.

If the scope warrants it, I can split this into linked sub-issues and work through them gradually. My idea is to aim for roughly one PR per week, with each diff under 300 lines where practical, so the review workload stays manageable.