Unfragile

The request names mypy, so mypy itself comes first: it is an optional static type checker for Python 1. Alternatives that check Python types are Pyrefly, a fast type checker and language server 4, and ty, Astral's extremely fast Python type checker 313. Mypy's own extensions package is listed on the Python developer guide page 2 but its source does not describe type checking, so it is left out.

1
Mypymypy-lang.orgwebsite read, tools not yet checked
Mypy is an optional static type checker for Python that adds type annotations and checks them without changing runtime behavior 1.
Website
2
Pyreflypyrefly.orgverified and signed
Pyrefly is a fast Python type checker and language server that catches type errors in editors and in AI-written Python code 4.
Website Docs
+1 community tools built for it
for agents: typechecker · typechecking
3
tyastral.shverified and signed
ty is the extremely fast Python type checker and language server from Astral, with a guide for type checking Python code and setting up type checks 313.
Website Docs
+3 community tools built for it
for agents: ty · check · pylint

Sources

1mypy - Optional Static Typing for Pythonmypy-lang.org · read 2026-10-29
2Python Developer’s Guidedevguide.python.org · read 2026-10-29
3Astral: High-performance Python toolingastral.sh · read 2026-10-29
4Pyrefly: A Fast Python Type Checker and Language Server | Pyreflypyrefly.org · read 2026-10-29
13ty: declares "ty" (the record we read and signed)unfragile.ai · read 2026-10-24

For agents: the 10 actions behind this answer

· Mypy-lang website read, tools not yet checked
Mypy is an optional static type checker for Python.
· Python Developer’s Guide Mypy Extensions website read, tools not yet checked
This guide is a comprehensive resource for contributing to Python – for both new and experienced contributors. It is maintained by the same community that maintains Python. We welcome your contribu...
ty · ty verified and signed
Guide for using ty, the extremely fast Python type checker and language server. Use this when type checking Python code or setting up type checking in Python projects.
typechecker · pyrefly verified and signed
mypy · typedload verified and signed
typechecking · typing-extensions verified and signed
· Pydantic Typing Inspection website read, tools not yet checked
Pydantic is the AI engineering stack for teams building with AI. Observe with Logfire, evaluate with Pydantic Evals, build type-safe agents with Pydantic AI, and track LLM costs with AI Gateway.
pydantic · pydantic verified and signed
Pydantic is a Python data validation and serialization library, based on type hints. Use this skill whenever you need to do relatively complex data modeling using Pydantic, e.g. when adding constraints, defining a model hierarchy with subclasses, etc.
doublecheck · doublecheck verified and signed
Three-layer verification pipeline for AI output. Extracts verifiable claims, finds supporting or contradicting sources via web search, runs adversarial review for hallucination patterns, and produces a structured verification report with source links for human review.
python · fastcore verified and signed

Labels are the instruments' verdict and are never changed by ranking. Agents and robots get this same answer as JSON at /api/v1/find?q=…&answer=1 and through the MCP server.