Diagnostic Rules

Every Basilisk diagnostic has a unique code in the format BSK-EXXXX (error) or BSK-WXXXX (warning).

Rules are enabled by default. You can dial individual rules down per-file or per-path from your editor or pyproject.toml — strict is the default, not a cage.

Basilisk ships 151 diagnostic codes (146 errors, 5 warnings) spanning the full Python typing surface — generics, protocols, dataclasses, TypedDicts, overloads, literals, enums, and more — and is validated against the official Python typing conformance suite (currently 92.5%, 135 / 146). The two foundational groups have worked examples:

Group	Codes	Description
Missing Annotations	E0001–E0009	Unannotated parameters, return types, variables, and attributes
Type Safety	E0010–E0029	Type mismatches, incorrect annotations, unsound type usage

Roadmap: Mojo-inspired ownership and immutability analysis is planned for a future release. It is not yet part of the shipping rule set.

Complete diagnostic reference

Every code currently emitted by the checker. This table is generated from the checker source (scripts/gen_rules_reference.py) — it is the authoritative list.

Code	Description
`BSK-E0001`	Missing parameter type annotation
`BSK-E0002`	Missing return type annotation
`BSK-E0003`	Missing variable type annotation
`BSK-E0004`	Missing `args` / `*kwargs` type annotation
`BSK-E0005`	Missing class attribute type annotation
`BSK-E0010`	Unresolved import
`BSK-E0011`	Explicit `Any` annotation / return type mismatch
`BSK-E0012`	Argument type mismatch at a call site
`BSK-E0013`	Return type mismatch — inferred return type incompatible with annotation
`BSK-E0014`	Assignment type incompatibility (literal mismatches)
`BSK-E0015`	Invalid type argument count or form
`BSK-E0016`	Incompatible method override
`BSK-E0017`	Incompatible class attribute override
`BSK-E0018`	Undefined variable used in a return statement
`BSK-E0019`	Unbound variable on some code paths
`BSK-E0020`	Missing `@overload` implementation
`BSK-E0021`	Overlapping `@overload` signatures
`BSK-E0022`	Unhashable type used as a dict key
`BSK-E0023`	Non-exhaustive `match` statement
`BSK-E0024`	Invalid type form — numeric literal used as type annotation
`BSK-E0025`	Missing `@override` decorator
`BSK-E0026`	`TypeVar` declared with exactly one constraint
`BSK-E0027`	Duplicate `TypeVar` in a `Generic[...]` base
`BSK-E0029`	Method defined inside a `TypedDict` class
`BSK-E0030`	Non-default `TypeVar` follows a default `TypeVar` in `Generic[...]`
`BSK-E0031`	Invalid `cast()` call
`BSK-E0032`	Invalid keyword argument in `TypedDict` class definition
`BSK-E0033`	Invalid `reveal_type()` call
`BSK-E0034`	`@final` decorator violations
`BSK-E0035`	`Required` / `NotRequired` used in an invalid context
`BSK-E0036`	`ClassVar` used in an invalid context
`BSK-E0037`	Invalid `TypedDict(...)` functional-syntax call
`BSK-E0038`	Invalid `TypedDict` inheritance
`BSK-E0039`	Invalid `assert_type()` call
`BSK-E0040`	Invalid Enum subclassing
`BSK-E0041`	Too few arguments in a function call
`BSK-E0042`	PEP 695 type parameter syntax mixed with traditional `TypeVars`
`BSK-E0043`	Non-TypeVar argument in `Generic[...]` or `Protocol[...]`
`BSK-E0044`	`Final` used in an invalid position
`BSK-E0045`	Invalid first argument to `Annotated[...]`
`BSK-E0046`	Enum member annotated with an explicit type
`BSK-E0047`	Invalid type expression in annotation
`BSK-E0048`	Invalid right-hand side for a `TypeAlias` annotation
`BSK-E0049`	Multiple unbounded tuple components in a single tuple type
`BSK-E0050`	Invalid `NewType(...)` call
`BSK-E0051`	Invalid `Literal` parameterization
`BSK-E0052`	Assignment to attribute of a frozen dataclass instance, or invalid frozen/non-frozen dataclass inheritance
`BSK-E0053`	`assert_type()` type mismatch
`BSK-E0054`	`Final` type qualifier annotation violations
`BSK-E0055`	Invalid `TypeVar` / `TypeVarTuple` / `ParamSpec` keyword argument combination
`BSK-E0056`	Mutation of `ReadOnly` `TypedDict` fields
`BSK-E0057`	Invalid RHS in a PEP 695 `type X = rhs` statement
`BSK-E0058`	`Annotated[...]` requires at least two arguments
`BSK-E0059`	Access to `__match_args__` on a dataclass with `match_args=False`
`BSK-E0060`	Invalid ordering comparison of dataclass instances
`BSK-E0061`	`assert_type` with `Literal[Enum.MEMBER]` on enum-typed param
`BSK-E0062`	`-> NoReturn` / `-> Never` function can fall through
`BSK-E0063`	Non-hashable dataclass assigned to a `Hashable`-annotated variable
`BSK-E0064`	Invalid argument in a `NamedTuple` constructor call
`BSK-E0065`	Access to an `int`-only attribute on a `float`-typed parameter
`BSK-E0066`	Enum member value incompatible with `_value_` type annotation
`BSK-E0067`	Non-member referenced in `Literal[EnumClass.X]` annotation
`BSK-E0068`	`Literal["EnumClass.MEMBER"]` (string) used where `Literal[EnumClass.MEMBER]` (enum member reference) is required
`BSK-E0069`	Dataclass constructor argument violations
`BSK-E0070`	`Never` type compatibility violations
`BSK-E0071`	Historical positional-only parameter violations
`BSK-E0072`	No matching overload for subscript indexing
`BSK-E0073`	`NamedTuple`-to-tuple type incompatibility
`BSK-E0074`	Constructor call type mismatch with specialized generic class
`BSK-E0075`	Incompatible type for `Self`-typed attribute
`BSK-E0076`	Overload union expansion failure
`BSK-E0077`	Protocol `Self`-return conformance violation
`BSK-E0078`	`Self` type violations in generics
`BSK-E0079`	Module assigned to incompatible protocol type
`BSK-E0080`	`TypeVar` upper bound violation at call site
`BSK-E0081`	`TypeVarTuple` unpack minimum type argument violation
`BSK-E0082`	`TypeVarTuple` callable/tuple argument mismatch
`BSK-E0083`	`TypeVarTuple` must be unpacked with `*` operator
`BSK-E0084`	`TypeVarTuple` variance/bounds/constraints violation
`BSK-E0085`	`TypeVarTuple` argument count mismatch
`BSK-E0086`	Multiple `TypeVarTuple` unpacks in generic or tuple type
`BSK-E0087`	Reserved for future PEP 695 type parameter checks
`BSK-E0088`	`TypedDict` runtime violation
`BSK-E0089`	Invalid PEP 695 type parameter bound or constraint
`BSK-E0090`	Invalid tuple type syntax
`BSK-E0091`	Incompatible `TypeVar` bound or constraint with its default
`BSK-E0092`	Wrong number of type arguments to a generic class or type alias
`BSK-E0093`	Invalid key or value type in `TypedDict` assignment
`BSK-E0094`	`Self` type used in an invalid location
`BSK-E0095`	`InitVar` field validation in dataclasses
`BSK-E0096`	Type mismatch between a dataclass `field(default_factory=…)` and the field's declared type annotation
`BSK-E0097`	Protocol `__new__`/`__init__` sets self-attributes not declared in Protocol
`BSK-E0098`	Non-Protocol base class in a Protocol definition
`BSK-E0099`	Direct instantiation of a Protocol class
`BSK-E0100`	Augmented assignment widens `Literal` type
`BSK-E0101`	`TypeGuard` or `TypeIs` on method with no narrowing parameter
`BSK-E0102`	Invalid `TypeVar` default referencing another `TypeVar`
`BSK-E0103`	Tuple index out of bounds
`BSK-E0104`	Cyclical type alias reference
`BSK-E0105`	Invalid attribute access on bounded type variable
`BSK-E0106`	Protocol class used where `type[Proto]` is expected
`BSK-E0107`	Variance incompatibility in base class parameterisation
`BSK-E0108`	Dataclass slots violations
`BSK-E0109`	`TypeVar` bound violation at call site
`BSK-E0110`	Protocol variance violation
`BSK-E0111`	Constructor call errors via `__init__` method
`BSK-E0112`	TypeGuard/TypeIs return type incompatibility in callable arguments
`BSK-E0113`	`TypeIs` narrows to a type inconsistent with the input type
`BSK-E0114`	Protocol `isinstance`/`issubclass` violations
`BSK-E0115`	Use of deprecated class, function, or method
`BSK-E0116`	`NamedTuple` class definition errors
`BSK-E0117`	Unbound type variable in scope
`BSK-E0118`	Calling `super().method()` on an abstract method with no default implementation
`BSK-E0119`	Protocol `isinstance`/`issubclass` violations
`BSK-E0120`	Generator return type and yield type violations
`BSK-E0121`	Protocol conformance violation in annotated assignment
`BSK-E0122`	Callable call-site arity and argument validation
`BSK-E0123`	`super()` call on abstract protocol method with no default implementation
`BSK-E0124`	Protocol attribute tuple element type mismatch
`BSK-E0125`	Access to instance attribute on a class object
`BSK-E0126`	`LiteralString` and `Literal` assignment incompatibilities
`BSK-E0127`	Tuple index out of range
`BSK-E0128`	`TypeVar` default referential violations
`BSK-E0129`	Literal value assignment incompatibility
`BSK-E0130`	`TypeVar` scoping violation
`BSK-E0131`	Generator yield/send/return type mismatch
`BSK-E0132`	Inconsistent `TypeVar` ordering across base classes
`BSK-E0133`	Protocol `TypeVar` variance mismatch
`BSK-E0134`	Invariant generic type mismatch at call site
`BSK-E0136`	Callable subtyping violations (covariance / contravariance)
`BSK-E0137`	Generic protocol violations
`BSK-E0138`	`dataclass_transform` metaclass violations
`BSK-E0139`	Invalid `TypeVarTuple` specialization of generic alias
`BSK-E0140`	Callable and Protocol assignment compatibility
`BSK-E0141`	Unpack[`TypedDict`] kwargs violations
`BSK-E0142`	`dataclass_transform` violations when the transform is applied via a base class
`BSK-E0143`	`NamedTuple` usage violations
`BSK-E0144`	Invalid constructor call via `type[T]` parameter
`BSK-E0145`	Invalid `type[X]` usage violations
`BSK-E0146`	Protocol class object violations
`BSK-E0147`	Tuple starred-unpack type compatibility violation
`BSK-E0148`	Generic type argument violations
`BSK-E0149`	PEP 695 generic type parameter scoping violations
`BSK-E0150`	Variable defined only in dead version/platform branch
`BSK-E0151`	Invalid `TypeAliasType(...)` call
`BSK-E0152`	Missing type stubs for installed package
`BSK-E0153`	Invalid call to a constructor-derived callable
`BSK-E0154`	Access to a module attribute the local stub does not declare
`BSK-E0155`	PEP 695 syntax used below the configured target Python version
`BSK-W0011`	Undeclared dependency import
`BSK-W0012`	Unused dependency
`BSK-W0013`	Stale uv lock file
`BSK-W0040`	Lambda function missing type annotations
`BSK-W0050`	Redundant type annotation warning