Python: Fix Pydantic error when using Literal type for tool params

python/packages/core/agent_framework/_tools.py

@@ -886,6 +886,8 @@ def _parse_annotation(annotation: Any) -> Any:
     If the second annotation (after the type) is a string, then we convert that to a Pydantic Field description.
     The rest are returned as-is, allowing for multiple annotations.
 
+    Literal types are returned as-is to preserve their enum-like values.
+
     Args:
         annotation: The type annotation to parse.
 
@@ -894,6 +896,12 @@ def _parse_annotation(annotation: Any) -> Any:
     """
     origin = get_origin(annotation)
     if origin is not None:
+        # Literal types should be returned as-is - their args are the allowed values,
+        # not type annotations to be parsed. For example, Literal["Data", "Security"]
+        # has args ("Data", "Security") which are the valid string values.
+        if origin is Literal:
+            return annotation
+
         args = get_args(annotation)
         # For other generics, return the origin type (e.g., list for List[int])
         if len(args) > 1 and isinstance(args[1], str):

python/packages/core/tests/core/test_tools.py

@@ -1,5 +1,5 @@
 # Copyright (c) Microsoft. All rights reserved.
-from typing import Any
+from typing import Annotated, Any, Literal
 from unittest.mock import Mock
 
 import pytest
@@ -14,7 +14,7 @@
     ToolProtocol,
     ai_function,
 )
-from agent_framework._tools import _parse_inputs
+from agent_framework._tools import _parse_annotation, _parse_inputs
 from agent_framework.exceptions import ToolException
 from agent_framework.observability import OtelAttr
 
@@ -128,6 +128,95 @@ def test_tool(self, x: int, y: int) -> int:
     assert test_tool(1, 2) == 3
 
 
+def test_ai_function_with_literal_type_parameter():
+    """Test ai_function decorator with Literal type parameter (issue #2891)."""
+
+    @ai_function
+    def search_flows(category: Literal["Data", "Security", "Network"], issue: str) -> str:
+        """Search flows by category."""
+        return f"{category}: {issue}"
+
+    assert isinstance(search_flows, AIFunction)
+    schema = search_flows.parameters()
+    assert schema == {
+        "properties": {
+            "category": {"enum": ["Data", "Security", "Network"], "title": "Category", "type": "string"},
+            "issue": {"title": "Issue", "type": "string"},
+        },
+        "required": ["category", "issue"],
+        "title": "search_flows_input",
+        "type": "object",
+    }
+    # Verify invocation works
+    assert search_flows("Data", "test issue") == "Data: test issue"
+
+
+def test_ai_function_with_literal_type_in_class_method():
+    """Test ai_function decorator with Literal type parameter in a class method (issue #2891)."""
+
+    class MyTools:
+        @ai_function
+        def search_flows(self, category: Literal["Data", "Security", "Network"], issue: str) -> str:
+            """Search flows by category."""
+            return f"{category}: {issue}"
+
+    tools = MyTools()
+    search_tool = tools.search_flows
+    assert isinstance(search_tool, AIFunction)
+    schema = search_tool.parameters()
+    assert schema == {
+        "properties": {
+            "category": {"enum": ["Data", "Security", "Network"], "title": "Category", "type": "string"},
+            "issue": {"title": "Issue", "type": "string"},
+        },
+        "required": ["category", "issue"],
+        "title": "search_flows_input",
+        "type": "object",
+    }
+    # Verify invocation works
+    assert search_tool("Security", "test issue") == "Security: test issue"
+
+
+def test_ai_function_with_literal_int_type():
+    """Test ai_function decorator with Literal int type parameter."""
+
+    @ai_function
+    def set_priority(priority: Literal[1, 2, 3], task: str) -> str:
+        """Set priority for a task."""
+        return f"Priority {priority}: {task}"
+
+    assert isinstance(set_priority, AIFunction)
+    schema = set_priority.parameters()
+    assert schema == {
+        "properties": {
+            "priority": {"enum": [1, 2, 3], "title": "Priority", "type": "integer"},
+            "task": {"title": "Task", "type": "string"},
+        },
+        "required": ["priority", "task"],
+        "title": "set_priority_input",
+        "type": "object",
+    }
+    assert set_priority(1, "important task") == "Priority 1: important task"
+
+
+def test_ai_function_with_literal_and_annotated():
+    """Test ai_function decorator with Literal type combined with Annotated for description."""
+
+    @ai_function
+    def categorize(
+        category: Annotated[Literal["A", "B", "C"], "The category to assign"],
+        name: str,
+    ) -> str:
+        """Categorize an item."""
+        return f"{category}: {name}"
+
+    assert isinstance(categorize, AIFunction)
+    schema = categorize.parameters()
+    # Literal type inside Annotated should preserve enum values
+    assert schema["properties"]["category"]["enum"] == ["A", "B", "C"]
+    assert categorize("A", "test") == "A: test"
+
+
 async def test_ai_function_decorator_shared_state():
     """Test that decorated methods maintain shared state across multiple calls and tool usage."""
 
@@ -1368,3 +1457,70 @@ def tool_with_kwargs(x: int, **kwargs: Any) -> str:
         arguments=tool_with_kwargs.input_model(x=10),
     )
     assert result_default == "x=10, user=unknown"
+
+
+# region _parse_annotation tests
+
+
+def test_parse_annotation_with_literal_type():
+    """Test that _parse_annotation returns Literal types unchanged (issue #2891)."""
+    from typing import get_args, get_origin
+
+    # Literal with string values
+    literal_annotation = Literal["Data", "Security", "Network"]
+    result = _parse_annotation(literal_annotation)
+    assert result is literal_annotation
+    assert get_origin(result) is Literal
+    assert get_args(result) == ("Data", "Security", "Network")
+
+
+def test_parse_annotation_with_literal_int_type():
+    """Test that _parse_annotation returns Literal int types unchanged."""
+    from typing import get_args, get_origin
+
+    literal_annotation = Literal[1, 2, 3]
+    result = _parse_annotation(literal_annotation)
+    assert result is literal_annotation
+    assert get_origin(result) is Literal
+    assert get_args(result) == (1, 2, 3)
+
+
+def test_parse_annotation_with_literal_bool_type():
+    """Test that _parse_annotation returns Literal bool types unchanged."""
+    from typing import get_args, get_origin
+
+    literal_annotation = Literal[True, False]
+    result = _parse_annotation(literal_annotation)
+    assert result is literal_annotation
+    assert get_origin(result) is Literal
+    assert get_args(result) == (True, False)
+
+
+def test_parse_annotation_with_simple_types():
+    """Test that _parse_annotation returns simple types unchanged."""
+    assert _parse_annotation(str) is str
+    assert _parse_annotation(int) is int
+    assert _parse_annotation(float) is float
+    assert _parse_annotation(bool) is bool
+
+
+def test_parse_annotation_with_annotated_and_literal():
+    """Test that Annotated[Literal[...], description] works correctly."""
+    from typing import get_args, get_origin
+
+    # When Literal is inside Annotated, it should still be preserved
+    annotated_literal = Annotated[Literal["A", "B", "C"], "The category"]
+    result = _parse_annotation(annotated_literal)
+
+    # The Annotated type should be preserved
+    origin = get_origin(result)
+    assert origin is Annotated
+
+    args = get_args(result)
+    # First arg is the Literal type
+    literal_type = args[0]
+    assert get_origin(literal_type) is Literal
+    assert get_args(literal_type) == ("A", "B", "C")
+
+
+# endregion