Fix titled enum elicitation schema to comply with MCP spec (#2774)

src/fastmcp/server/elicitation.py

@@ -304,15 +304,19 @@ def _dict_to_enum_schema(
         multi_select: If True, use anyOf pattern; if False, use oneOf pattern
 
     Returns:
-        {"oneOf": [{"const": "low", "title": "Low Priority"}, ...]} for single-select
-        {"anyOf": [{"const": "low", "title": "Low Priority"}, ...]} for multi-select
+        {"type": "string", "oneOf": [...]} for single-select
+        {"anyOf": [...]} for multi-select (used as array items)
     """
     pattern_key = "anyOf" if multi_select else "oneOf"
     pattern = []
     for value, metadata in enum_dict.items():
         title = metadata.get("title", value)
         pattern.append({"const": value, "title": title})
-    return {pattern_key: pattern}
+
+    result: dict[str, Any] = {pattern_key: pattern}
+    if not multi_select:
+        result["type"] = "string"
+    return result
 
 
 def get_elicitation_schema(response_type: type[T]) -> dict[str, Any]:

tests/client/test_elicitation.py

@@ -742,6 +742,17 @@ async def test_dict_based_titled_single_select():
         return "declined"
 
     async def elicitation_handler(message, response_type, params, ctx):
+        # Verify schema follows SEP-1330 pattern with type: "string"
+        schema = params.requestedSchema
+        assert schema["type"] == "object"
+        assert "value" in schema["properties"]
+        value_schema = schema["properties"]["value"]
+        assert value_schema["type"] == "string"
+        assert "oneOf" in value_schema
+        one_of = value_schema["oneOf"]
+        assert {"const": "low", "title": "Low Priority"} in one_of
+        assert {"const": "high", "title": "High Priority"} in one_of
+
         return ElicitResult(action="accept", content={"value": "low"})
 
     async with Client(mcp, elicitation_handler=elicitation_handler) as client:
@@ -800,14 +811,15 @@ async def test_list_dict_multi_select_titled():
         return "declined"
 
     async def elicitation_handler(message, response_type, params, ctx):
-        # Verify schema has array with anyOf pattern
+        # Verify schema has array with SEP-1330 compliant items (anyOf pattern)
         schema = params.requestedSchema
         assert schema["type"] == "object"
         assert "value" in schema["properties"]
         value_schema = schema["properties"]["value"]
         assert value_schema["type"] == "array"
-        assert "anyOf" in value_schema["items"]
-        any_of = value_schema["items"]["anyOf"]
+        items_schema = value_schema["items"]
+        assert "anyOf" in items_schema
+        any_of = items_schema["anyOf"]
         assert {"const": "low", "title": "Low Priority"} in any_of
         assert {"const": "high", "title": "High Priority"} in any_of
 
@@ -895,7 +907,7 @@ async def test_validation_allows_enum_arrays():
 
 
 async def test_validation_allows_enum_arrays_with_anyof():
-    """Test validation accepts arrays with anyOf enum pattern."""
+    """Test validation accepts arrays with anyOf enum pattern (SEP-1330 compliant)."""
     schema = {
         "type": "object",
         "properties": {