diff --git a/tests/test_types.py b/tests/test_types.py
index 415eba66a7..bb2042c809 100644
--- a/tests/test_types.py
+++ b/tests/test_types.py
@@ -9,6 +9,8 @@
     InitializeRequestParams,
     JSONRPCMessage,
     JSONRPCRequest,
+    ListToolsResult,
+    Tool,
 )
 
 
@@ -56,3 +58,66 @@ async def test_method_initialization():
     assert initialize_request.method == "initialize", "method should be set to 'initialize'"
     assert initialize_request.params is not None
     assert initialize_request.params.protocolVersion == LATEST_PROTOCOL_VERSION
+
+
+def test_tool_preserves_json_schema_2020_12_fields():
+    """Verify that JSON Schema 2020-12 keywords are preserved in Tool.inputSchema.
+
+    SEP-1613 establishes JSON Schema 2020-12 as the default dialect for MCP.
+    This test ensures the SDK doesn't strip $schema, $defs, or additionalProperties.
+    """
+    input_schema = {
+        "$schema": "https://json-schema.org/draft/2020-12/schema",
+        "type": "object",
+        "$defs": {
+            "address": {
+                "type": "object",
+                "properties": {"street": {"type": "string"}, "city": {"type": "string"}},
+            }
+        },
+        "properties": {
+            "name": {"type": "string"},
+            "address": {"$ref": "#/$defs/address"},
+        },
+        "additionalProperties": False,
+    }
+
+    tool = Tool(name="test_tool", description="A test tool", inputSchema=input_schema)
+
+    # Verify fields are preserved in the model
+    assert tool.inputSchema["$schema"] == "https://json-schema.org/draft/2020-12/schema"
+    assert "$defs" in tool.inputSchema
+    assert "address" in tool.inputSchema["$defs"]
+    assert tool.inputSchema["additionalProperties"] is False
+
+    # Verify fields survive serialization round-trip
+    serialized = tool.model_dump(mode="json", by_alias=True)
+    assert serialized["inputSchema"]["$schema"] == "https://json-schema.org/draft/2020-12/schema"
+    assert "$defs" in serialized["inputSchema"]
+    assert serialized["inputSchema"]["additionalProperties"] is False
+
+
+def test_list_tools_result_preserves_json_schema_2020_12_fields():
+    """Verify JSON Schema 2020-12 fields survive ListToolsResult deserialization."""
+    raw_response = {
+        "tools": [
+            {
+                "name": "json_schema_tool",
+                "description": "Tool with JSON Schema 2020-12 features",
+                "inputSchema": {
+                    "$schema": "https://json-schema.org/draft/2020-12/schema",
+                    "type": "object",
+                    "$defs": {"item": {"type": "string"}},
+                    "properties": {"items": {"type": "array", "items": {"$ref": "#/$defs/item"}}},
+                    "additionalProperties": False,
+                },
+            }
+        ]
+    }
+
+    result = ListToolsResult.model_validate(raw_response)
+    tool = result.tools[0]
+
+    assert tool.inputSchema["$schema"] == "https://json-schema.org/draft/2020-12/schema"
+    assert "$defs" in tool.inputSchema
+    assert tool.inputSchema["additionalProperties"] is False