SEP-1330: Elicitation Enum Schema Improvements and Standards Compliance (#1246)

Co-authored-by: Tapan Chugh <tapanc@cs.washington.edu>
Co-authored-by: Felix Weinberger <fweinberger@anthropic.com>
Co-authored-by: Felix Weinberger <3823880+felixweinberger@users.noreply.github.com>

src/mcp/server/elicitation.py

@@ -3,10 +3,10 @@
 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
-from pydantic.fields import FieldInfo
 
 from mcp.server.session import ServerSession
 from mcp.types import RequestId
@@ -43,22 +43,40 @@ _ELICITATION_PRIMITIVE_TYPES = (str, int, float, bool)
 def _validate_elicitation_schema(schema: type[BaseModel]) -> None:
     """Validate that a Pydantic model only contains primitive field types."""
     for field_name, field_info in schema.model_fields.items():
-        if not _is_primitive_field(field_info):
+        annotation = field_info.annotation
+
+        if annotation is None or annotation is types.NoneType:  # pragma: no cover
+            continue
+        elif _is_primitive_field(annotation):
+            continue
+        elif _is_string_sequence(annotation):
+            continue
+        else:
             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_primitive_field(field_info: FieldInfo) -> bool:
+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:
+        try:
+            if issubclass(origin, Sequence):
+                args = get_args(annotation)
+                # Should have single str type arg
+                return len(args) == 1 and args[0] is str
+        except TypeError:  # pragma: no cover
+            # origin is not a class, so it can't be a subclass of Sequence
+            pass
+    return False
+
+
+def _is_primitive_field(annotation: type) -> bool:
     """Check if a field is a primitive type allowed in elicitation schemas."""
-    annotation = field_info.annotation
-
-    # Handle None type
-    if annotation is types.NoneType:  # pragma: no cover
-        return True
-
     # Handle basic primitive types
     if annotation in _ELICITATION_PRIMITIVE_TYPES:
         return True
@@ -67,8 +85,10 @@ def _is_primitive_field(field_info: FieldInfo) -> bool:
     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 _is_string_sequence(arg) for arg in args
+        )
 
     return False