18. Testing MCP

Chapter 18 of 22 · 20 min

KEY INSIGHT

Thorough MCP testing covers protocol compliance, tool behavior, and integration scenariosΓÇöeach layer reveals different defects that tests must catch. Unit tests verify individual tool behavior in isolation: ```python import pytest @pytest.fixture def mcp_server(): return create_test_server() @pytest.mark.asyncio async def test_file_tool_read(mcp_server): # Arrange create_test_file("/tmp/test.txt", "hello world") # Act result = await mcp_server.call_tool("read_file", {"path": "/tmp/test.txt"}) # Assert assert result == "hello world" @pytest.mark.asyncio async def test_file_tool_not_found(mcp_server): with pytest.raises(ToolError) as exc_info: await mcp_server.call_tool("read_file", {"path": "/nonexistent"}) assert "not found" in str(exc_info.value) ``` Protocol tests verify JSON-RPC message handling: ```python @pytest.mark.asyncio async def test_protocol_invalid_json(): response = await test_client.post( "/mcp", data="not valid json", headers={"Content-Type": "application/json"} ) assert response.status_code == 400 @pytest.mark.asyncio async def test_protocol_missing_method(): response = await test_client.post( "/mcp", json={"jsonrpc": "2.0", "params": {}, "id": 1} ) assert response.status_code == 400 assert "method" in response.json()["error"]["message"] ``` Integration tests verify the full request-response cycle: ```python @pytest.mark.asyncio async def test_full_tool_invocation(): # Start server in test mode async with TestServer() as server: client = MCPClient(transport=stdio()) # Initialize await client.initialize() # List tools tools = await client.list_tools() assert any(t.name == "read_file" for t in tools) # Call tool result = await client.call_tool("read_file", {"path": "/etc/hostname"}) assert isinstance(result, str) # Cleanup await client.close() ``` Property-based testing validates edge cases: ```python from hypothesis import given, strategies as st @given(path=st.text(min_size=1, max_size=1000)) @pytest.mark.asyncio async def test_path_handling(path): server = create_server() # Should either succeed or raise proper error try: result = await server.call_tool("read_file", {"path": path}) assert isinstance(result, str) except ToolError as e: # Errors should be informative assert len(str(e)) > 0 ``` Contract tests ensure compatibility with MCP protocol versions: ```python @pytest.mark.asyncio async def test_protocol_version_handshake(): client = TestClient() init_response = await client.send_request({ "jsonrpc": "2.0", "method": "initialize", "params": { "protocolVersion": "2024-11-05", "capabilities": {}, "clientInfo": {"name": "test", "version": "1.0"} }, "id": 1 }) assert "protocolVersion" in init_response assert "capabilities" in init_response ```

EXERCISE

Write a test suite covering three scenarios: successful tool execution, validation errors, and protocol-level malformed requests. Run tests with coverage reporting and identify untested code paths.