Python: Improve error message when TypeVar is used in handler registration

python/packages/core/agent_framework/_workflows/_executor.py

@@ -21,7 +21,7 @@
 from ._request_info_mixin import RequestInfoMixin
 from ._runner_context import MessageType, RunnerContext, WorkflowMessage
 from ._state import State
-from ._typing_utils import is_instance_of, normalize_type_to_list, resolve_type_annotation
+from ._typing_utils import contains_typevar, is_instance_of, normalize_type_to_list, resolve_type_annotation
 from ._workflow_context import WorkflowContext, validate_workflow_context_annotation
 
 logger = logging.getLogger(__name__)
@@ -650,6 +650,20 @@ def decorator(
                 resolve_type_annotation(workflow_output, func.__globals__) if workflow_output is not None else None
             )
 
+            # Check for unresolved TypeVars in explicit type parameters
+            for param_name, param_type in [
+                ("input", resolved_input_type),
+                ("output", resolved_output_type),
+                ("workflow_output", resolved_workflow_output_type),
+            ]:
+                if param_type is not None and contains_typevar(param_type):
+                    raise ValueError(
+                        f"Handler '{func.__name__}' has an unresolved TypeVar '{param_type}' "
+                        f"as its {param_name} type. "
+                        f"Use @handler(input=ConcreteType, output=ConcreteType) with concrete types "
+                        f"for parameterized executors."
+                    )
+
             # Validate signature structure (correct number of params, ctx is WorkflowContext)
             # but skip type extraction since we're using explicit types
             _validate_handler_signature(func, skip_message_annotation=True)
@@ -680,6 +694,15 @@ def decorator(
                     "or explicit type parameters (input, output, workflow_output)"
                 )
 
+            # Check for unresolved TypeVar in introspected message type
+            if contains_typevar(message_type):
+                raise ValueError(
+                    f"Handler '{func.__name__}' has an unresolved TypeVar '{message_type}' "
+                    f"as its message type. "
+                    f"Use @handler(input=ConcreteType, output=ConcreteType) with concrete types "
+                    f"for parameterized executors."
+                )
+
             final_output_types = inferred_output_types
             final_workflow_output_types = inferred_workflow_output_types
 
@@ -765,7 +788,7 @@ def _validate_handler_signature(
 
     # Reject unresolved TypeVar in message annotation -- these are not supported
     # for workflow type validation and must be replaced with concrete types.
-    if not skip_message_annotation and isinstance(message_type, TypeVar):
+    if not skip_message_annotation and contains_typevar(message_type):
         raise ValueError(
             f"Handler {func.__name__} has an unresolved TypeVar '{message_type}' as its message type annotation. "
             "Generic TypeVar annotations are not supported for workflow type validation. "

python/packages/core/agent_framework/_workflows/_function_executor.py

@@ -24,7 +24,7 @@
 from typing import Any
 
 from ._executor import Executor
-from ._typing_utils import normalize_type_to_list, resolve_type_annotation
+from ._typing_utils import contains_typevar, normalize_type_to_list, resolve_type_annotation
 from ._workflow_context import WorkflowContext, validate_workflow_context_annotation
 
 if sys.version_info >= (3, 11):
@@ -94,6 +94,19 @@ def __init__(
             _validate_function_signature(func, skip_message_annotation=resolved_input_type is not None)
         )
 
+        # Check for unresolved TypeVars in explicit type parameters
+        for param_name, param_type in [
+            ("input", resolved_input_type),
+            ("output", resolved_output_type),
+            ("workflow_output", resolved_workflow_output_type),
+        ]:
+            if param_type is not None and contains_typevar(param_type):
+                raise ValueError(
+                    f"Executor '{func.__name__}' has an unresolved TypeVar '{param_type}' "
+                    f"as its {param_name} type. "
+                    f"Use @executor(input=ConcreteType, output=ConcreteType) with concrete types."
+                )
+
         # Use explicit types if provided, otherwise fall back to introspection
         message_type = resolved_input_type if resolved_input_type is not None else introspected_message_type
         output_types: list[type[Any] | types.UnionType] = (
@@ -114,6 +127,14 @@ def __init__(
                 "or an explicit input_type parameter"
             )
 
+        # Check for unresolved TypeVar in introspected message type
+        if contains_typevar(message_type):
+            raise ValueError(
+                f"Executor '{func.__name__}' has an unresolved TypeVar '{message_type}' "
+                f"as its message type. "
+                f"Use @executor(input=ConcreteType, output=ConcreteType) with concrete types."
+            )
+
         # Store the original function
         self._original_func = func
         # Determine if function has WorkflowContext parameter
@@ -351,7 +372,7 @@ def _validate_function_signature(
 
     # Reject unresolved TypeVar in message annotation -- these are not supported
     # for workflow type validation and must be replaced with concrete types.
-    if not skip_message_annotation and isinstance(message_type, typing.TypeVar):
+    if not skip_message_annotation and contains_typevar(message_type):
         raise ValueError(
             f"Function instance {func.__name__} has an unresolved TypeVar '{message_type}' as its message type "
             "annotation. Generic TypeVar annotations are not supported for workflow type validation. "

python/packages/core/agent_framework/_workflows/_typing_utils.py

@@ -1,10 +1,45 @@
 # Copyright (c) Microsoft. All rights reserved.
 
+import typing
 from types import UnionType
 from typing import Any, TypeGuard, Union, cast, get_args, get_origin
 
+import typing_extensions
+
 from .._agents import Agent
 
+# Pre-compute the TypeVar types for runtime-safe detection.
+# isinstance(x, TypeVar) can fail if TypeVar is a factory/callable
+# on some Python versions, so we compare against the actual runtime type.
+_TYPEVAR_TYPES: tuple[type, ...] = (type(typing.TypeVar("_T")), type(typing_extensions.TypeVar("_T")))  # pyright: ignore[reportUnknownVariableType]
+
+
+def is_typevar(x: Any) -> bool:
+    """Check if x is an unresolved TypeVar instance (from typing or typing_extensions).
+
+    Args:
+        x: The value to check.
+
+    Returns:
+        True if x is a TypeVar instance, False otherwise.
+    """
+    return isinstance(x, _TYPEVAR_TYPES)
+
+
+def contains_typevar(annotation: Any) -> bool:
+    """Check if an annotation contains an unresolved TypeVar at any nesting level.
+
+    Args:
+        annotation: The annotation to inspect.
+
+    Returns:
+        True if the annotation or any nested type argument is a TypeVar, False otherwise.
+    """
+    if is_typevar(annotation):
+        return True
+
+    return any(contains_typevar(arg) for arg in get_args(annotation))
+
 
 def is_chat_agent(agent: Any) -> TypeGuard[Agent]:
     """Check if the given agent is a Agent.

python/packages/core/agent_framework/_workflows/_workflow_context.py

@@ -21,6 +21,7 @@
 )
 from ._runner_context import RunnerContext, WorkflowMessage
 from ._state import State
+from ._typing_utils import contains_typevar
 
 if TYPE_CHECKING:
     from ._executor import Executor
@@ -176,6 +177,15 @@ def _is_type_like(x: Any) -> bool:
             if type_arg is Any:
                 continue
 
+            # Check for unresolved TypeVar early with an actionable error message
+            if contains_typevar(type_arg):
+                raise ValueError(
+                    f"{context_description} {parameter_name} {param_description} "
+                    f"contains an unresolved TypeVar in '{type_arg}'. "
+                    f"Use @handler(input=ConcreteType, output=ConcreteType) with concrete types "
+                    f"for parameterized executors."
+                )
+
             # Check if it's a union type and validate each member
             union_origin = get_origin(type_arg)
             if union_origin in (Union, UnionType):