SEP: Elicitation Enum Schema Improvements and Standards Compliance

Author: Tapan Chugh

src/mcp/server/elicitation.py

@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 import types
+from collections.abc import Sequence
 from typing import Generic, Literal, TypeVar, Union, get_args, get_origin
 
 from pydantic import BaseModel
@@ -46,11 +47,22 @@ def _validate_elicitation_schema(schema: type[BaseModel]) -> None:
         if not _is_primitive_field(field_info):
             raise TypeError(
                 f"Elicitation schema field '{field_name}' must be a primitive type "
-                f"{_ELICITATION_PRIMITIVE_TYPES} or Optional of these types. "
-                f"Complex types like lists, dicts, or nested models are not allowed."
+                f"{_ELICITATION_PRIMITIVE_TYPES}, a sequence of strings (list[str], etc.), "
+                f"or Optional of these types. Nested models and complex types are not allowed."
             )
 
 
+def _is_string_sequence(annotation: type) -> bool:
+    """Check if annotation is a sequence of strings (list[str], Sequence[str], etc)."""
+    origin = get_origin(annotation)
+    # Check if it's a sequence-like type with str elements
+    if origin and issubclass(origin, Sequence):
+        args = get_args(annotation)
+        # Should have single str type arg
+        return len(args) == 1 and args[0] is str
+    return False
+
+
 def _is_primitive_field(field_info: FieldInfo) -> bool:
     """Check if a field is a primitive type allowed in elicitation schemas."""
     annotation = field_info.annotation
@@ -63,12 +75,21 @@ def _is_primitive_field(field_info: FieldInfo) -> bool:
     if annotation in _ELICITATION_PRIMITIVE_TYPES:
         return True
 
+    # Handle string sequences for multi-select enums
+    if annotation is not None and _is_string_sequence(annotation):
+        return True
+
     # Handle Union types
     origin = get_origin(annotation)
     if origin is Union or origin is types.UnionType:
         args = get_args(annotation)
-        # All args must be primitive types or None
-        return all(arg is types.NoneType or arg in _ELICITATION_PRIMITIVE_TYPES for arg in args)
+        # All args must be primitive types, None, or string sequences
+        return all(
+            arg is types.NoneType
+            or arg in _ELICITATION_PRIMITIVE_TYPES
+            or (arg is not None and _is_string_sequence(arg))
+            for arg in args
+        )
 
     return False
 

src/mcp/types.py

@@ -1304,7 +1304,7 @@ class ElicitResult(Result):
     - "cancel": User dismissed without making an explicit choice
     """
 
-    content: dict[str, str | int | float | bool | None] | None = None
+    content: dict[str, str | int | float | bool | list[str] | None] | None = None
     """
     The submitted form data, only present when action is "accept".
     Contains values matching the requested schema.

tests/server/fastmcp/test_elicitation.py

@@ -116,7 +116,7 @@ async def tool(ctx: Context[ServerSession, None]) -> str:  # pragma: no cover
 
     # Test cases for invalid schemas
     class InvalidListSchema(BaseModel):
-        names: list[str] = Field(description="List of names")
+        numbers: list[int] = Field(description="List of numbers")
 
     class NestedModel(BaseModel):
         value: str
@@ -139,7 +139,7 @@ async def elicitation_callback(
         await client_session.initialize()
 
         # Test both invalid schemas
-        for tool_name, field_name in [("invalid_list", "names"), ("nested_model", "nested")]:
+        for tool_name, field_name in [("invalid_list", "numbers"), ("nested_model", "nested")]:
             result = await client_session.call_tool(tool_name, {})
             assert len(result.content) == 1
             assert isinstance(result.content[0], TextContent)
@@ -197,7 +197,7 @@ async def callback(context: RequestContext[ClientSession, None], params: ElicitR
     # Test invalid optional field
     class InvalidOptionalSchema(BaseModel):
         name: str = Field(description="Name")
-        optional_list: list[str] | None = Field(default=None, description="Invalid optional list")
+        optional_list: list[int] | None = Field(default=None, description="Invalid optional list")
 
     @mcp.tool(description="Tool with invalid optional field")
     async def invalid_optional_tool(ctx: Context[ServerSession, None]) -> str:  # pragma: no cover
@@ -220,6 +220,25 @@ async def elicitation_callback(
         text_contains=["Validation failed:", "optional_list"],
     )
 
+    # Test valid list[str] for multi-select enum
+    class ValidMultiSelectSchema(BaseModel):
+        name: str = Field(description="Name")
+        tags: list[str] = Field(description="Tags")
+
+    @mcp.tool(description="Tool with valid list[str] field")
+    async def valid_multiselect_tool(ctx: Context[ServerSession, None]) -> str:
+        result = await ctx.elicit(message="Please provide tags", schema=ValidMultiSelectSchema)
+        if result.action == "accept" and result.data:
+            return f"Name: {result.data.name}, Tags: {', '.join(result.data.tags)}"
+        return f"User {result.action}"
+
+    async def multiselect_callback(context: RequestContext[ClientSession, Any], params: ElicitRequestParams):
+        if "Please provide tags" in params.message:
+            return ElicitResult(action="accept", content={"name": "Test", "tags": ["tag1", "tag2"]})
+        return ElicitResult(action="decline")
+
+    await call_tool_and_assert(mcp, multiselect_callback, "valid_multiselect_tool", {}, "Name: Test, Tags: tag1, tag2")
+
 
 @pytest.mark.anyio
 async def test_elicitation_with_default_values():
@@ -274,3 +293,89 @@ async def callback_override(context: RequestContext[ClientSession, None], params
     await call_tool_and_assert(
         mcp, callback_override, "defaults_tool", {}, "Name: John, Age: 25, Subscribe: False, Email: john@example.com"
     )
+
+
+@pytest.mark.anyio
+async def test_elicitation_with_enum_titles():
+    """Test elicitation with enum schemas using oneOf/anyOf for titles."""
+    mcp = FastMCP(name="ColorPreferencesApp")
+
+    # Test single-select with titles using oneOf
+    class FavoriteColorSchema(BaseModel):
+        user_name: str = Field(description="Your name")
+        favorite_color: str = Field(
+            description="Select your favorite color",
+            json_schema_extra={
+                "oneOf": [
+                    {"const": "red", "title": "Red"},
+                    {"const": "green", "title": "Green"},
+                    {"const": "blue", "title": "Blue"},
+                    {"const": "yellow", "title": "Yellow"},
+                ]
+            },
+        )
+
+    @mcp.tool(description="Single color selection")
+    async def select_favorite_color(ctx: Context) -> str:
+        result = await ctx.elicit(message="Select your favorite color", schema=FavoriteColorSchema)
+        if result.action == "accept" and result.data:
+            return f"User: {result.data.user_name}, Favorite: {result.data.favorite_color}"
+        return f"User {result.action}"
+
+    # Test multi-select with titles using anyOf
+    class FavoriteColorsSchema(BaseModel):
+        user_name: str = Field(description="Your name")
+        favorite_colors: list[str] = Field(
+            description="Select your favorite colors",
+            json_schema_extra={
+                "items": {
+                    "anyOf": [
+                        {"const": "red", "title": "Red"},
+                        {"const": "green", "title": "Green"},
+                        {"const": "blue", "title": "Blue"},
+                        {"const": "yellow", "title": "Yellow"},
+                    ]
+                }
+            },
+        )
+
+    @mcp.tool(description="Multiple color selection")
+    async def select_favorite_colors(ctx: Context) -> str:
+        result = await ctx.elicit(message="Select your favorite colors", schema=FavoriteColorsSchema)
+        if result.action == "accept" and result.data:
+            return f"User: {result.data.user_name}, Colors: {', '.join(result.data.favorite_colors)}"
+        return f"User {result.action}"
+
+    # Test deprecated enumNames format
+    class DeprecatedColorSchema(BaseModel):
+        user_name: str = Field(description="Your name")
+        color: str = Field(
+            description="Select a color",
+            json_schema_extra={"enum": ["red", "green", "blue"], "enumNames": ["Red", "Green", "Blue"]},
+        )
+
+    @mcp.tool(description="Deprecated enum format")
+    async def select_color_deprecated(ctx: Context) -> str:
+        result = await ctx.elicit(message="Select a color (deprecated format)", schema=DeprecatedColorSchema)
+        if result.action == "accept" and result.data:
+            return f"User: {result.data.user_name}, Color: {result.data.color}"
+        return f"User {result.action}"
+
+    async def enum_callback(context, params):
+        if "colors" in params.message and "deprecated" not in params.message:
+            return ElicitResult(action="accept", content={"user_name": "Bob", "favorite_colors": ["red", "green"]})
+        elif "color" in params.message:
+            if "deprecated" in params.message:
+                return ElicitResult(action="accept", content={"user_name": "Charlie", "color": "green"})
+            else:
+                return ElicitResult(action="accept", content={"user_name": "Alice", "favorite_color": "blue"})
+        return ElicitResult(action="decline")
+
+    # Test single-select with titles
+    await call_tool_and_assert(mcp, enum_callback, "select_favorite_color", {}, "User: Alice, Favorite: blue")
+
+    # Test multi-select with titles
+    await call_tool_and_assert(mcp, enum_callback, "select_favorite_colors", {}, "User: Bob, Colors: red, green")
+
+    # Test deprecated enumNames format
+    await call_tool_and_assert(mcp, enum_callback, "select_color_deprecated", {}, "User: Charlie, Color: green")