Python: Exclude ClassVar annotations from generated JSON schemas

KernelJsonSchemaBuilder.build_model_schema builds the instance schema from get_type_hints, which includes ClassVar annotations, so a class constant became a required object-valued property. Pydantic excludes ClassVars from model fields and serialized instances, so the generated schema rejected valid payloads (a serialized {'value': 1} failed validation because 'marker' was required). The builder now skips ClassVar-annotated hints (both the parameterized and bare forms, including inherited ones), for KernelBaseModel subclasses, plain annotated classes and instances, covering KernelParameterMetadata, generated tool definitions and structured_output schemas. Fixes #14527
Pushpak Siva Sai (Pushpak731) · Oct 2, 2026 · Oct 2, 2026 · Oct 9, 2026 · Oct 14, 2026 · Oct 20, 2026
commit 4007fd3509342c18a8c459975652bad31585e3dd
@@ -3,7 +3,7 @@
 import sys
 import types
 from enum import Enum
-from typing import Any, Union, get_args, get_origin, get_type_hints
+from typing import Any, ClassVar, Union, get_args, get_origin, get_type_hints
 
 from semantic_kernel.const import PARSED_ANNOTATION_UNION_DELIMITER
 from semantic_kernel.exceptions.function_exceptions import FunctionInvalidParameterConfiguration
@@ -86,6 +86,11 @@ def build_model_schema(
         hints = get_type_hints(model, globalns=model_module_globals, localns={})
 
         for field_name, field_type in hints.items():
+            if field_type is ClassVar or get_origin(field_type) is ClassVar:
+                # ClassVar annotations are class-level constants: pydantic
+                # excludes them from model fields and serialized instances,
+                # so they must not become (required) schema properties.
+                continue
             field_description = None
             if hasattr(model, "model_fields") and field_name in model.model_fields:
                 field_info = model.model_fields[field_name]

@@ -2,7 +2,7 @@
 
 import json
 from enum import Enum
-from typing import Annotated, Any, Optional, Union
+from typing import Annotated, Any, ClassVar, Optional, Union
 from unittest.mock import Mock
 
 import pytest
@@ -455,3 +455,104 @@ def test_build_schema_with_nonpydantic_structured_output():
     }
 
     assert structured_output_schema == expected_schema
+
+
+class ModelWithClassVar(KernelBaseModel):
+    marker: ClassVar[str] = "constant"
+    value: int
+
+
+class ModelWithBareClassVar(KernelBaseModel):
+    marker: ClassVar = 0
+    value: str
+
+
+class ModelInheritingClassVar(ModelWithClassVar):
+    extra: float
+
+
+class PlainClassWithClassVar:
+    marker: ClassVar[dict] = {}
+    name: str
+
+
+class ModelWithClassVarAndOptional(KernelBaseModel):
+    counter: ClassVar[int] = 5
+    keep: str
+    drop: Optional[str] = None
+
+
+def test_build_model_schema_excludes_class_var():
+    schema = KernelJsonSchemaBuilder.build(ModelWithClassVar)
+
+    assert schema == {"type": "object", "properties": {"value": {"type": "integer"}}, "required": ["value"]}
+
+
+def test_build_model_schema_excludes_bare_class_var():
+    schema = KernelJsonSchemaBuilder.build(ModelWithBareClassVar)
+
+    assert schema == {"type": "object", "properties": {"value": {"type": "string"}}, "required": ["value"]}
+
+
+def test_build_model_schema_excludes_inherited_class_var():
+    schema = KernelJsonSchemaBuilder.build(ModelInheritingClassVar)
+
+    assert schema == {
+        "type": "object",
+        "properties": {"value": {"type": "integer"}, "extra": {"type": "number"}},
+        "required": ["value", "extra"],
+    }
+
+
+def test_build_model_schema_excludes_class_var_on_plain_class():
+    schema = KernelJsonSchemaBuilder.build(PlainClassWithClassVar)
+
+    assert schema == {"type": "object", "properties": {"name": {"type": "string"}}, "required": ["name"]}
+
+
+def test_build_model_schema_class_var_with_optional_fields():
+    schema = KernelJsonSchemaBuilder.build(ModelWithClassVarAndOptional)
+
+    assert schema == {
+        "type": "object",
+        "properties": {"keep": {"type": "string"}, "drop": {"type": ["string", "null"]}},
+        "required": ["keep"],
+    }
+
+
+def test_build_model_schema_class_var_structured_output():
+    schema = KernelJsonSchemaBuilder.build(ModelWithClassVar, structured_output=True)
+
+    assert schema == {
+        "type": "object",
+        "properties": {"value": {"type": "integer"}},
+        "required": ["value"],
+        "additionalProperties": False,
+    }
+
+
+def test_build_model_schema_class_var_with_description():
+    schema = KernelJsonSchemaBuilder.build(ModelWithClassVar, description="A payload")
+
+    assert schema == {
+        "type": "object",
+        "properties": {"value": {"type": "integer"}},
+        "required": ["value"],
+        "description": "A payload",
+    }
+
+
+def test_build_model_schema_class_var_via_instance():
+    instance = ModelWithClassVar(value=1)
+
+    schema = KernelJsonSchemaBuilder.build(instance)
+
+    assert schema == {"type": "object", "properties": {"value": {"type": "integer"}}, "required": ["value"]}
+
+
+def test_build_model_schema_allows_serialized_payload():
+    schema = KernelJsonSchemaBuilder.build(ModelWithClassVar)
+
+    serialized = ModelWithClassVar(value=1).model_dump()
+    for field in schema.get("required", []):
+        assert field in serialized