fix(tools): preserve const schemas for Gemini

Author: he-yufeng

src/google/adk/tools/_gemini_schema_util.py

@@ -26,6 +26,10 @@
 
 
 class _ExtendedJSONSchema(JSONSchema):
+  const: Optional[Any] = Field(
+      default=None,
+      description="""Optional. Restricts the value to a single constant.""",
+  )
   property_ordering: Optional[list[str]] = Field(
       default=None,
       description="""Optional. The order of the properties. Not a standard field in open api spec. Only used to support the order of the properties.""",
@@ -183,6 +187,8 @@ def _sanitize_schema_formats_for_gemini(
       "any_of",  # 'one_of', 'all_of', 'not' to come
   }
   snake_case_schema: dict[str, Any] = {}
+  has_const = False
+  const_value = None
   dict_schema_field_names: tuple[str, ...] = (
       "properties",
       "defs",
@@ -218,9 +224,15 @@ def _sanitize_schema_formats_for_gemini(
           (current_type == "string" and field_value in ("date-time", "enum"))
       ):
         snake_case_schema[field_name] = field_value
+    elif field_name == "const":
+      has_const = True
+      const_value = field_value
     elif field_name in supported_fields and field_value is not None:
       snake_case_schema[field_name] = field_value
 
+  if has_const and const_value is not None:
+    snake_case_schema["enum"] = [str(const_value)]
+
   return _sanitize_schema_type(snake_case_schema, preserve_null_type)
 
 

tests/unittests/tools/test_gemini_schema_util.py

@@ -217,6 +217,11 @@ 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_const(self):
+    openapi_schema = {"type": "string", "const": "BaseAgent"}
+    gemini_schema = _to_gemini_schema(openapi_schema)
+    assert gemini_schema.enum == ["BaseAgent"]
+
   def test_to_gemini_schema_required(self):
     openapi_schema = {
         "type": "object",