fix: Support string enum for elicitation

Author: vincent0426

src/mcp/server/elicitation.py

@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 import types
+from enum import Enum, StrEnum
 from typing import Generic, Literal, TypeVar, Union, get_args, get_origin
 
 from pydantic import BaseModel
@@ -37,7 +38,7 @@ class CancelledElicitation(BaseModel):
 
 
 # Primitive types allowed in elicitation schemas
-_ELICITATION_PRIMITIVE_TYPES = (str, int, float, bool)
+_ELICITATION_PRIMITIVE_TYPES = (str, int, float, bool, StrEnum)
 
 
 def _validate_elicitation_schema(schema: type[BaseModel]) -> None:
@@ -70,6 +71,10 @@ def _is_primitive_field(field_info: FieldInfo) -> bool:
         # All args must be primitive types or None
         return all(arg is types.NoneType or arg in _ELICITATION_PRIMITIVE_TYPES for arg in args)
 
+    # Handle Enum types
+    if isinstance(annotation, type) and issubclass(annotation, str) and issubclass(annotation, Enum):
+        return True
+
     return False
 
 

tests/server/fastmcp/test_elicitation.py

@@ -2,6 +2,7 @@
 Test the elicitation feature using stdio transport.
 """
 
+from enum import StrEnum
 from typing import Any
 
 import pytest
@@ -142,6 +143,39 @@ async def elicitation_callback(context: RequestContext[ClientSession, None], par
             assert "Validation failed as expected" in result.content[0].text
             assert field_name in result.content[0].text
 
+    # Test valid Enum types (should not fail validation)
+    class Status(StrEnum):
+        ACTIVE = "active"
+        INACTIVE = "inactive"
+
+    class ValidStrEnumSchema(BaseModel):
+        status: Status = Field(description="Status using StrEnum")
+
+    def create_valid_validation_tool(name: str, schema_class: type[BaseModel]):
+        @mcp.tool(name=name, description=f"Tool testing {name}")
+        async def tool(ctx: Context[ServerSession, None]) -> str:
+            # This should succeed without validation error
+            result = await ctx.elicit(message="Test valid schema", schema=schema_class)
+            return f"Success: {result.action}"
+
+        return tool
+
+    create_valid_validation_tool("valid_strenum", ValidStrEnumSchema)
+
+    async def enum_callback(context: RequestContext[ClientSession, None], params: ElicitRequestParams):
+        # Return the required status field
+        return ElicitResult(action="accept", content={"status": "active"})
+
+    async with create_connected_server_and_client_session(
+        mcp._mcp_server, elicitation_callback=enum_callback
+    ) as client_session:
+        await client_session.initialize()
+
+        result = await client_session.call_tool("valid_strenum", {})
+        assert len(result.content) == 1
+        assert isinstance(result.content[0], TextContent)
+        assert "Success: accept" == result.content[0].text
+
 
 @pytest.mark.anyio
 async def test_elicitation_with_optional_fields():