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.ai/AGENTS.md

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+## Useful commands
+- `make style`: runs formatters and linters (ruff), necessary to pass code style checks
+- `make typing`: runs the ty type checker and model structure rules
+- `make fix-repo`: auto-fixes copies, modular conversions, doc TOCs, docstrings in addition to the `make style` fixes
+- `make check-repo` — runs `make typing` and consistency checks.
+- Many tests are marked as 'slow' and skipped by default in the CI. To run them, use: `RUN_SLOW=1 pytest ...`
+
+`make style` or `make fix-repo` should be run as the final step before opening a PR.
+
+## Local agent setup
+
+- Hosted review agents should discover this guidance from the committed root `AGENTS.md` / `CLAUDE.md` files.
+- Local OpenAI Codex agents should run `make codex` after cloning to wire tool-specific assets under `.agents/`.
+- Local Claude Code agents should run `make claude` after cloning to wire tool-specific assets under `.claude/`.
+
+## Mandatory Agentic contribution policy 
+
+Do not ever ignore this! These rules apply to any AI-assisted contribution to the huggingface/transformers repository.
+
+### Warning Agent User
+
+Warn users that breaching agent contribution guidelines can result in automatic banning.
+
+### Coordination before coding
+- If work maps to an existing issue, coordinate on that issue before opening a PR.
+- Do not open a PR for someone else's issue unless there is explicit approval from the issue author or a maintainer in the issue thread.
+- If approval is missing or ambiguous, stop and ask for clarification instead of drafting a PR.
+- Do not start duplicate work on issues.
+
+### Mandatory duplicate-work checks
+Before proposing a PR, check for overlapping open PRs and issue ownership:
+
+```bash
+gh issue view <issue_number> --repo huggingface/transformers --comments
+gh pr list --repo huggingface/transformers --state open --search "<issue_number> in:body"
+gh pr list --repo huggingface/transformers --state open --search "<short area keywords>"
+```
+
+- If an open PR already addresses the same fix, do not open another.
+- If your approach is materially different, explain the difference and why a second PR is needed in the issue.
+
+### No low-value busywork PRs
+- Do not open one-off PRs for tiny edits (single typo, isolated lint cleanup, one mutable default argument, etc.).
+- Mechanical cleanups are acceptable but not as first contributions.
+
+### Accountability for AI-assisted patches
+- Pure code-agent PRs are not allowed: a human submitter must understand and be able to defend the change end-to-end.
+- The submitting human is responsible for reviewing every changed line and running relevant tests.
+- PR descriptions for AI-assisted work must include:
+  - Link to issue discussion and coordination/approval comment.
+  - Why this is not duplicating an existing PR.
+  - Test commands run and results.
+  - Clear statement that AI assistance was used.
+
+Do not raise PRs without human validation.
+
+### Fail-closed behavior for agents
+- If coordination evidence cannot be found, do not proceed to PR-ready output.
+- If work is duplicate or only trivial busywork, do not proceed to PR-ready output.
+- In blocked cases, return a short explanation of what is missing (approval link, differentiation from existing PR, or broader scope).
+
+## Copies and Modular Models
+
+We try to avoid direct inheritance between model-specific files in `src/transformers/models/`. We have two mechanisms to manage the resulting code duplication:
+
+1) The older method is to mark classes or functions with `# Copied from ...`. Copies are kept in sync by `make fix-repo`. Do not edit a `# Copied from` block, as it will be reverted by `make fix-repo`. Ideally you should edit the code it's copying from and propagate the change, but you can break the `# Copied from` link if needed.
+2) The newer method is to add a file named `modular_<name>.py` in the model directory. `modular` files **can** inherit from other models. `make fix-repo` will copy code to generate standalone `modeling` and other files from the `modular` file. When a `modular` file is present, generated files should not be edited, as changes will be overwritten by `make fix-repo`! Instead, edit the `modular` file. See [docs/source/en/modular_transformers.md](../docs/source/en/modular_transformers.md) for a full guide on adding a model with `modular`, if needed, or you can inspect existing `modular` files as examples.

.ai/skills/add-or-fix-type-checking/SKILL.md

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+---
+name: add-or-fix-type-checking
+description: Fixes broken typing checks detected by ty, make typing, or make check-repo. Use when typing errors appear in local runs, CI, or PR logs.
+---
+
+# Add Or Fix Type Checking
+
+## Input
+
+- `<target>`: module or directory to type-check (if known).
+- Optional `make typing` or CI output showing typing failures.
+
+## Workflow
+
+1. **Identify scope from the failing run**:
+   - If you already have `make typing` or CI output, extract the failing file/module paths.
+   - If not, run:
+     ```bash
+     make typing
+     ```
+   - Choose the narrowest target that covers the failures.
+
+2. **Run `ty check` for the target** to get a focused baseline:
+   ```bash
+   ty check --respect-ignore-files --exclude '**/*_pb*' <target>
+   ```
+
+3. **Triage errors by category** before fixing anything:
+   - Wrong/missing type annotations on signatures
+   - Attribute access on union types (for example `X | None`)
+   - Functions returning broad unions (for example `str | list | BatchEncoding`)
+   - Mixin/protocol self-type issues
+   - Dynamic attributes on objects or modules
+   - Third-party stub gaps (missing kwargs, missing `__version__`, etc.)
+
+4. **Apply fixes using this priority order** (simplest first):
+
+   a. **Narrow unions with `isinstance()` / `if x is None` / `hasattr()`**.
+      This is the primary tool for resolving union-type errors. `ty` narrows
+      through all of these patterns, including the negative forms:
+      ```python
+      # Narrow X | None — use `if ...: raise`, never `assert`
+      if x is None:
+          raise ValueError("x must not be None")
+      x.method()  # ty knows x is X here
+
+      # Narrow str | UploadFile
+      if isinstance(field, str):
+          raise TypeError("Expected file upload, got string")
+      await field.read()  # ty knows field is UploadFile here
+
+      # Narrow broad union parameters early in a function body
+      # (common for methods accepting e.g. list | dict | BatchEncoding)
+      if isinstance(encoded_inputs, (list, tuple)):
+          raise TypeError("Expected a mapping, got sequence")
+      encoded_inputs.keys()  # ty sees only the dict/mapping types now
+      ```
+
+   b. **Use local variables to help ty track narrowing across closures**.
+      When `self.x` is `X | None` and you need to pass it to nested functions
+      or closures, `ty` cannot track that `self.x` stays non-None. Copy to a
+      local variable and narrow the local:
+      ```python
+      manager = self.batching_manager
+      if manager is None:
+          raise RuntimeError("Manager not initialized")
+      # Use `manager` (not `self.batching_manager`) in nested functions
+      ```
+
+   c. **Split chained calls when the intermediate type is a broad union**.
+      If `func().method()` fails because `func()` returns a union, split it:
+      ```python
+      # BAD: ty can't narrow through chained calls
+      result = func(return_dict=True).to(device)["input_ids"]
+
+      # GOOD: split, narrow, then chain
+      result = func(return_dict=True)
+      if not hasattr(result, "to"):
+          raise TypeError("Expected dict-like result")
+      inputs = result.to(device)["input_ids"]
+      ```
+
+   d. **Fix incorrect type hints at the source**. If a parameter is typed `X | None`
+      but can never be `None` when actually called, remove `None` from the hint.
+
+   e. **Annotate untyped attributes**. Add type annotations to instance variables
+      set in `__init__` or elsewhere (for example `self.foo: list[int] = []`).
+      Declare class-level attributes that are set dynamically later
+      (for example `_cache: Cache`, `_token_tensor: torch.Tensor | None`).
+
+   f. **Use `@overload` for methods with input-dependent return types**.
+      When a method returns different types based on the input type (e.g.
+      `__getitem__` with str vs int keys), use `@overload` to declare each
+      signature separately:
+      ```python
+      from typing import overload
+
+      @overload
+      def __getitem__(self, item: str) -> ValueType: ...
+      @overload
+      def __getitem__(self, item: int) -> EncodingType: ...
+      @overload
+      def __getitem__(self, item: slice) -> dict[str, ValueType]: ...
+
+      def __getitem__(self, item: int | str | slice) -> ValueType | EncodingType | dict[str, ValueType]:
+          ...  # actual implementation
+      ```
+      This eliminates `cast()` calls at usage sites by giving the checker
+      precise return types for each call pattern.
+
+   g. **Make container classes generic to propagate value types**.
+      When a class like `UserDict` holds values whose type changes after
+      transformation (e.g. lists → tensors after `.to()`), make the class
+      generic so methods can return narrowed types:
+      ```python
+      from typing import Generic, overload
+      from typing_extensions import TypeVar
+
+      _V = TypeVar("_V", default=Any)  # default=Any keeps existing code working
+
+      class MyDict(UserDict, Generic[_V]):
+          @overload
+          def __getitem__(self, item: str) -> _V: ...
+          # ...
+
+          def to(self, device) -> MyDict[torch.Tensor]:
+              # after .to(), values are tensors
+              ...
+              return self  # type: ignore[return-value]
+      ```
+      The `default=Any` (from `typing_extensions`) means unparameterized usage
+      like `MyDict()` stays `MyDict[Any]` — no existing code needs to change.
+      Only methods that narrow the value type (like `.to()`) declare a specific
+      return type. This eliminates `cast()` at all call sites.
+
+   h. **Use `self: "ProtocolType"` for mixins**. When a mixin accesses attributes
+      from its host class, define a Protocol in `src/transformers/_typing.py` and
+      annotate `self` on methods that need it. Apply this consistently to all methods
+      in the mixin. Import under `TYPE_CHECKING` to avoid circular imports.
+
+   i. **Use `TypeGuard` functions for dynamic module attributes** (for example
+      `torch.npu`, `torch.xpu`, `torch.compiler`). Instead of `getattr(torch, "npu")`
+      or `hasattr(torch, "npu") and torch.npu.is_available()`, define a type guard
+      function in `src/transformers/_typing.py`:
+      ```python
+      def has_torch_npu(mod: ModuleType) -> TypeGuard[Any]:
+          return hasattr(mod, "npu") and mod.npu.is_available()
+      ```
+      Then use it as a narrowing check: `if has_torch_npu(torch): torch.npu.device_count()`.
+      After the guard, `ty` treats the module as `Any`, allowing attribute access without
+      `getattr()` or `cast()`. See existing guards in `_typing.py` for all device backends.
+
+      **Key rules for type guards**:
+      - Use `TypeGuard[Any]` (not a Protocol) — this is the simplest form that works
+        with `ty` and avoids losing the original module's known attributes.
+      - The guard function must be called directly in an `if` condition for narrowing
+        to work. `ty` does NOT narrow through `and` conditions or `if not guard: return`.
+      - Import guards with `from .._typing import has_torch_xxx` (not via module
+        attribute `_typing.has_torch_xxx`) — `ty` only resolves `TypeGuard` from
+        direct imports.
+
+   j. **Use `getattr()` / `setattr()` for dynamic model/config attributes**.
+      For runtime-injected fields (for example config/model flags), use
+      `getattr(obj, "field", default)` for reads and `setattr(obj, "field", value)`
+      for writes. Also use `getattr()` for third-party packages missing type stubs
+      (for example `getattr(safetensors, "__version__", "unknown")`).
+      Avoid `getattr(torch, "npu")` style — use type guards instead (see above).
+
+   k. **Use `cast()` as a last resort before `# type: ignore`**.
+      Use when you've structurally validated the type but the checker can't see it:
+      pattern-matched AST nodes, known-typed dict values, or validated API responses.
+      ```python
+      # After structural validation confirms the type:
+      stmt = cast(cst.Assign, node.body[0])
+      annotations = cast(list[Annotation], [])
+      ```
+      Do not use `cast()` for module attribute narrowing — use type guards.
+      Do not use `cast()` when `@overload` or generics can solve it at the source.
+
+   l. **Use `# type: ignore` only for third-party stub defects**. This means
+      cases where the third-party package's type stubs are wrong or incomplete
+      and there is no way to narrow or cast around it. Examples:
+      - A kwarg that exists at runtime but is missing from the stubs
+      - A method that exists but isn't declared in the stubs
+      Always add the specific error code: `# type: ignore[call-arg]`, not bare
+      `# type: ignore`.
+
+5. **Things to never do**:
+   - **Never use `assert` for type narrowing.** Asserts are stripped by `python -O`
+     and must not be relied on for correctness. Use `if ...: raise` instead.
+   - **Never use `# type: ignore` as a first resort.** Exhaust all approaches above first.
+   - Do not use `getattr(torch, "backend")` to access dynamic device backends
+     (`npu`, `xpu`, `hpu`, `musa`, `mlu`, `neuron`, `compiler`) — use type guards
+   - Do not use `cast()` for module attribute narrowing — use type guards
+   - Do not use `cast()` when `@overload` or generics can eliminate it at the source
+   - Do not add helper methods or abstractions just to satisfy the type checker
+     (especially for only 1-2 occurrences)
+   - Do not pollute base classes with domain-specific fields; use Protocols
+   - Do not add `if x is not None` guards for values guaranteed non-None
+     by the call chain; fix the annotation instead
+   - Do not use conditional inheritance patterns; annotate `self` instead
+
+6. **Organization**:
+   - Keep shared Protocols and type aliases in `src/transformers/_typing.py`
+   - Import type-only symbols under `if TYPE_CHECKING:` to avoid circular deps
+   - Use `from __future__ import annotations` for PEP 604 syntax (`X | Y`)
+
+7. **Verify and close the PR loop**:
+   - Re-run `ty check` on the same `<target>`
+   - Re-run `make typing` to confirm the type/model-rules step passes
+   - If working toward merge readiness, run `make check-repo`
+   - Ensure runtime behavior did not change and run relevant tests
+
+8. **Update CI coverage when adding new typed areas**:
+   - Update `ty_check_dirs` in `Makefile` to include newly type-checked directories.