Migrate from mypy to ty

Migrate from mypy to ty (AUTHORITATIVE)

This skill replaces mypy with ty as the static type checker across all project surfaces: dependencies, configuration, Makefile targets, pre-commit hooks, CI pipelines, and documentation.

ty is the Astral type checker (uv add --group dev ty). The canonical invocation in a uv-managed project is uv run ty check.

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Scope

This skill applies to all surfaces where mypy is referenced:

• pyproject.toml — dependency groups and [tool.mypy] configuration
• Makefile — mypy targets and phony declarations
• .pre-commit-config.yaml — mypy hooks
• CI pipeline files (.github/workflows/, tox.ini, etc.)
• Documentation and README files

This skill MUST NOT modify:

• Source code type annotations
• Any file outside the surfaces listed above

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Relationship with migrate-to-uv

If the migrate-to-uv skill was previously applied, the following artifacts reference mypy and MUST be updated by this skill:

• Verification commands: uv run mypy {{ mypy_paths }} → uv run ty check
• Makefile targets: make mypy → make ty (or equivalent)
• pyproject.toml template: [tool.mypy] → [tool.ty]

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Phase 1 — Survey (READ-ONLY)

Before making any change, identify all mypy references:

• pyproject.toml: mypy in [dependency-groups] or [project.optional-dependencies]; [tool.mypy] section
• Makefile: targets, .PHONY, variables, and help output mentioning mypy
• .pre-commit-config.yaml: any hook with mypy in id, name, or repo
• CI files: steps or commands referencing mypy
• README.*, docs/: command examples referencing mypy
• tox.ini or tox.toml: any mypy commands in [testenv] sections

Record every file and line before editing. The survey output drives the migration; do not skip files that appear to be low-risk.

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Phase 2 — Dependency migration

MUST remove mypy and all mypy plugins from every dependency group:

uv remove mypy
uv remove mypy-extensions
uv remove types-*      # remove all stub packages added for mypy

MUST add ty:

uv add --group dev ty

Rules:

• ty goes in the dev dependency group unless the project uses a dedicated lint or typecheck group, in which case it goes there.
• Third-party stub packages (types-requests, types-PyYAML, etc.) MUST be evaluated individually — ty bundles many stubs; remove only those no longer needed.
• Do not add ty as a runtime ([project].dependencies) dependency.

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Phase 3 — Configuration migration

Remove mypy configuration

Remove the [tool.mypy] section and any [[tool.mypy.overrides]] sections from pyproject.toml in their entirety.

MUST NOT leave a [tool.mypy] section with no entries as a "placeholder."

Add ty configuration

Add a minimal [tool.ty.environment] section. Encode only settings that are explicitly required by the project:

[tool.ty.environment]
# Required when the package lives under src/ — tells ty where to resolve imports
root = ["src"]

For multi-root projects (examples, benchmarks, etc.) list every directory that must be importable:

[tool.ty.environment]
root = ["src", "benchmarks", "examples/gae"]

Valid top-level keys under [tool.ty] are: environment, src, rules, terminal, analysis, overrides. Attempting to add any other key (e.g. exclude) causes a hard TOML parse error.

Rules:

• Do not mechanically translate every mypy flag to a ty equivalent; ty has different defaults and a different flag surface.
• Strictness settings from mypy (disallow_untyped_defs, etc.) MUST NOT be blindly copied; verify whether ty supports the equivalent before adding it.
• If ty does not support a mypy flag that the project relied on, document the gap in a comment and raise it in the completion report rather than silently dropping it.

Ignore comment migration

# type: ignore (PEP 484 / mypy syntax) is a silent no-op in ty. ty uses its own syntax:

some_expression  # ty: ignore[rule-name]

The rule name matches exactly what ty prints in brackets, e.g. error[unresolved-reference] → # ty: ignore[unresolved-reference].

Running uv run ty check --add-ignore inserts # ty: ignore comments automatically for every current diagnostic — useful as a starting point.

MUST audit every existing # type: ignore comment in the codebase:

1. Run uv run ty check and note which locations ty flags.
2. For each # type: ignore location:
   • If ty still flags it → replace # type: ignore[...] with # ty: ignore[rule-name].
   • If ty does not flag it → remove the comment (it is dead).
3. Do not leave # type: ignore comments as migration artefacts — they provide no suppression and create false confidence.

Jupyter notebook awareness

ty checks .ipynb files by default when it scans the project root. Projects with Jupyter notebooks may receive unexpected unresolved-attribute errors because notebook cells call methods on union return types without type narrowing.

For each failing notebook cell that accesses attributes on a typed result:

• Preferred fix: add assert isinstance(res, ExpectedType) in the cell where the variable is assigned. ty narrows the type for all subsequent cells.
• Alternative: add # ty: ignore[unresolved-attribute] to the specific line inside the cell source.

Do not attempt to suppress notebook errors via exclude under [tool.ty] — that key is not valid there and will cause a parse error.

Common ty error patterns

Patterns that ty catches which mypy often accepted silently:

Pattern	ty error code	Fix
mod.attr = val on a types.ModuleType	unresolved-attribute	Use setattr(mod, "attr", val) or # ty: ignore[unresolved-attribute]
Referencing a runtime-injected global (e.g. kernprof's profile)	unresolved-reference	Use # ty: ignore[unresolved-reference]
Calling a method that doesn't exist on the actual type (e.g. .close() on list, hidden by contextlib.suppress)	unresolved-attribute	Remove the dead call
Accessing attributes on a union type without narrowing (e.g. Optional[Union[str, Result]])	unresolved-attribute	Add assert isinstance(...) or cast before attribute access

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Phase 4 — Makefile migration

MUST replace every mypy-related target:

• Rename the target: mypy: → ty: (or typecheck:)
• Replace the command: uv run mypy $(MYPY_PATHS) → uv run ty check
• Update .PHONY to reference the new target name
• Update the help output if the project uses a help target
• Remove or rename any variable that was dedicated to mypy paths (e.g. MYPY_PATHS) if it is no longer needed

MUST NOT leave a dead mypy: target alongside a new ty: target.

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Phase 5 — Pre-commit migration

Inspect .pre-commit-config.yaml for any mypy hook:

• If a mypy hook from https://github.com/pre-commit/mirrors-mypy is present, remove the entire repo block.
• If mypy appears as a local hook, replace the command with ty check.
• If ty does not yet have a pre-commit mirror available, use a local hook:

- repo: local
  hooks:
    - id: ty
      name: ty
      entry: uv run ty check
      language: system
      types: [python]
      pass_filenames: false

Critical — extras and import resolution: uv run ty check creates a fresh .venv containing only the project's base (non-optional) dependencies. ty uses that environment for import resolution. If any source files import packages that live in optional dependency groups (e.g. pytest in test, ty itself in lint), ty will emit unresolved-import errors for those modules even though they are installed elsewhere.

Fix: pass --extra <group> for every optional group whose packages appear in source files that ty analyses:

entry: uv run --extra lint --extra test ty check

Include every group whose imports appear in files scanned by ty — typically test (for pytest, test helpers) and lint (for ty itself if it is not a base dependency). The --extra flags cause uv to install those groups into the managed .venv before ty runs.

Run uv run pre-commit run --all-files after updating the file.

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Phase 6 — CI migration

In every CI pipeline file, replace:

• mypy install steps with ty (usually handled by uv sync)
• mypy or uv run mypy commands with uv run ty check
• Any caching configuration keyed on mypy (.mypy_cache) with the ty equivalent (.ty_cache) if caching is enabled

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Phase 7 — Documentation migration

Update all documentation referencing mypy:

• README setup or usage instructions
• Developer guides
• docs/ pages with type checking instructions
• AGENTS.md or SKILL.md files that name mypy explicitly (e.g. verification sequences in migrate-to-uv or dev-workflow)

MUST NOT leave user-facing documentation that instructs users to run mypy after this migration is complete.

If any code examples in Markdown reference mypy, they MUST be updated to reference ty. Code examples in Markdown MUST comply with the doc-codeblock-tests skill (Python code blocks must be named test_*).

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Phase 8 — Verification

MUST run these checks in order after completing all changes:

uv lock
uv sync --all-groups --all-extras
uv run ty check
uv run pre-commit run --all-files

If a Makefile target was created:

make ty          # or make typecheck — match the new target name

A migration is NOT complete until uv run ty check exits zero.

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Prohibited practices

• MUST NOT leave mypy in any dependency group after migration
• MUST NOT leave [tool.mypy] in pyproject.toml after migration
• MUST NOT blindly copy mypy configuration flags that ty does not support
• MUST NOT silently drop type errors that mypy caught but ty does not yet catch — document any known regressions in the completion report
• MUST NOT leave # type: ignore comments in the codebase after migration — ty does not honour them; they suppress nothing and create false confidence. Convert to # ty: ignore[rule-name] where ty still flags the location, or remove them where ty does not.

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Completion report (MANDATORY)

Report after migration:

• All files modified and the nature of each change
• Mypy flags that were dropped because ty does not support them
• Third-party stub packages removed and any retained with justification
• Known type errors suppressed by ty that mypy would have flagged
• Any surfaces left unmodified (e.g. tox skipped because the project uses it for legacy CI only) with explicit justification