test: add regression test for content type literal defaults

Github-Issue: #1731

Author: felixweinberger

tests/test_types.py

@@ -9,12 +9,14 @@
     CreateMessageRequestParams,
     CreateMessageResult,
     CreateMessageResultWithTools,
+    ImageContent,
     Implementation,
     InitializeRequest,
     InitializeRequestParams,
     JSONRPCMessage,
     JSONRPCRequest,
     ListToolsResult,
+    PromptReference,
     SamplingCapability,
     SamplingMessage,
     TextContent,
@@ -71,6 +73,17 @@ async def test_method_initialization():
     assert initialize_request.params.protocolVersion == LATEST_PROTOCOL_VERSION
 
 
+def test_content_type_literal_defaults():
+    """Content types should default their Literal type field.
+
+    This allows instantiation without explicitly passing the type discriminator,
+    e.g., TextContent(text="hello") instead of TextContent(type="text", text="hello").
+    """
+    assert TextContent(text="hello").type == "text"
+    assert ImageContent(data="base64", mimeType="image/png").type == "image"
+    assert PromptReference(name="test").type == "ref/prompt"
+
+
 @pytest.mark.anyio
 async def test_tool_use_content():
     """Test ToolUseContent type for SEP-1577."""