fix: coerce non-string enum values to strings on string-typed Gemini schemas

Some MCP servers declare a string-typed field whose enum lists integer
values. Gemini requires enum members to match the declared string type, so
the tool declaration was rejected and the integration failed. Normalize enum
values to their string form when the effective (non-null) type is string,
leaving numeric enums on numeric types untouched.

Close #3401

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 963579568

Author: GWeale

src/google/adk/tools/_gemini_schema_util.py

@@ -14,6 +14,7 @@
 
 from __future__ import annotations
 
+import json
 import re
 from typing import Any
 from typing import Optional
@@ -100,6 +101,21 @@ def _sanitize_schema_type(
   if is_array:
     schema.setdefault("items", {"type": "string"})
 
+  effective_type = schema_type
+  if isinstance(schema_type, list):
+    non_null = [t for t in schema_type if t != "null"]
+    effective_type = non_null[0] if non_null else None
+  if effective_type == "string" and isinstance(schema.get("enum"), list):
+    # Gemini rejects non-string enum values on a string-typed field; some
+    # servers emit integer enums on string fields, so render them in their
+    # JSON form. A null member is dropped: nullability is carried by the
+    # schema type, not by an enum entry.
+    schema["enum"] = [
+        v if isinstance(v, str) else json.dumps(v)
+        for v in schema["enum"]
+        if v is not None
+    ]
+
   return schema
 
 

tests/unittests/tools/test_gemini_schema_util.py

@@ -217,6 +217,49 @@ def test_to_gemini_schema_enum(self):
     gemini_schema = _to_gemini_schema(openapi_schema)
     assert gemini_schema.enum == ["a", "b", "c"]
 
+  def test_to_gemini_schema_stringifies_int_enum_on_string_type(self):
+    openapi_schema = {"type": "string", "enum": [256, 512, 1024]}
+    gemini_schema = _to_gemini_schema(openapi_schema)
+    assert gemini_schema.type == Type.STRING
+    assert gemini_schema.enum == ["256", "512", "1024"]
+
+  def test_to_gemini_schema_stringifies_nested_int_enum(self):
+    openapi_schema = {
+        "type": "object",
+        "properties": {
+            "p": {"type": "string", "enum": [256, 512]},
+        },
+    }
+    gemini_schema = _to_gemini_schema(openapi_schema)
+    assert gemini_schema.properties["p"].type == Type.STRING
+    assert gemini_schema.properties["p"].enum == ["256", "512"]
+
+  def test_to_gemini_schema_int_enum_on_integer_type_unchanged(self):
+    openapi_schema = {"type": "integer", "enum": [1, 2]}
+    gemini_schema = _to_gemini_schema(openapi_schema)
+    assert gemini_schema.type == Type.INTEGER
+    assert gemini_schema.enum == [1, 2]
+
+  def test_to_gemini_schema_nullable_string_enum(self):
+    openapi_schema = {"type": ["string", "null"], "enum": [256]}
+    gemini_schema = _to_gemini_schema(openapi_schema)
+    assert gemini_schema.type == Type.STRING
+    assert gemini_schema.nullable
+    assert gemini_schema.enum == ["256"]
+
+  def test_to_gemini_schema_drops_null_enum_member(self):
+    openapi_schema = {"type": ["string", "null"], "enum": ["a", None]}
+    gemini_schema = _to_gemini_schema(openapi_schema)
+    assert gemini_schema.type == Type.STRING
+    assert gemini_schema.nullable
+    assert gemini_schema.enum == ["a"]
+
+  def test_to_gemini_schema_stringifies_bool_enum_as_json(self):
+    openapi_schema = {"type": "string", "enum": [True, False]}
+    gemini_schema = _to_gemini_schema(openapi_schema)
+    assert gemini_schema.type == Type.STRING
+    assert gemini_schema.enum == ["true", "false"]
+
   def test_to_gemini_schema_required(self):
     openapi_schema = {
         "type": "object",