feat(client): store InitializeResult on ClientSession, decompose on Client

docs/migration.md

@@ -169,6 +169,30 @@ result = await session.list_resources(params=PaginatedRequestParams(cursor="next
 result = await session.list_tools(params=PaginatedRequestParams(cursor="next_page_token"))
 ```
 
+### `ClientSession.get_server_capabilities()` replaced by `initialize_result` property
+
+`ClientSession` now stores the full `InitializeResult` via an `initialize_result` property. This provides access to `server_info`, `capabilities`, `instructions`, and the negotiated `protocol_version` through a single property. The `get_server_capabilities()` method has been removed.
+
+**Before (v1):**
+
+```python
+capabilities = session.get_server_capabilities()
+# server_info, instructions, protocol_version were not stored — had to capture initialize() return value
+```
+
+**After (v2):**
+
+```python
+result = session.initialize_result
+if result is not None:
+    capabilities = result.capabilities
+    server_info = result.server_info
+    instructions = result.instructions
+    version = result.protocol_version
+```
+
+The high-level `Client` exposes these directly as non-nullable properties (initialization is guaranteed inside the context manager): `client.server_capabilities`, `client.server_info`, and `client.server_instructions`.
+
 ### `McpError` renamed to `MCPError`
 
 The `McpError` exception class has been renamed to `MCPError` for consistent naming with the MCP acronym style used throughout the SDK.

src/mcp/client/client.py

@@ -19,6 +19,7 @@
     EmptyResult,
     GetPromptResult,
     Implementation,
+    InitializeResult,
     ListPromptsResult,
     ListResourcesResult,
     ListResourceTemplatesResult,
@@ -96,6 +97,7 @@ async def main():
     """Callback for handling elicitation requests."""
 
     _session: ClientSession | None = field(init=False, default=None)
+    _initialize_result: InitializeResult | None = field(init=False, default=None)
     _exit_stack: AsyncExitStack | None = field(init=False, default=None)
     _transport: Transport = field(init=False)
 
@@ -129,7 +131,7 @@ async def __aenter__(self) -> Client:
                 )
             )
 
-            await self._session.initialize()
+            self._initialize_result = await self._session.initialize()
 
             # Transfer ownership to self for __aexit__ to handle
             self._exit_stack = exit_stack.pop_all()
@@ -140,6 +142,7 @@ async def __aexit__(self, exc_type: type[BaseException] | None, exc_val: BaseExc
         if self._exit_stack:  # pragma: no branch
             await self._exit_stack.__aexit__(exc_type, exc_val, exc_tb)
         self._session = None
+        self._initialize_result = None
 
     @property
     def session(self) -> ClientSession:
@@ -155,9 +158,25 @@ def session(self) -> ClientSession:
         return self._session
 
     @property
-    def server_capabilities(self) -> ServerCapabilities | None:
-        """The server capabilities received during initialization, or None if not yet initialized."""
-        return self.session.get_server_capabilities()
+    def server_capabilities(self) -> ServerCapabilities:
+        """Capabilities the server advertised during initialization."""
+        if self._initialize_result is None:
+            raise RuntimeError("Client must be used within an async context manager")
+        return self._initialize_result.capabilities
+
+    @property
+    def server_info(self) -> Implementation:
+        """The server's name, version, and other implementation details."""
+        if self._initialize_result is None:
+            raise RuntimeError("Client must be used within an async context manager")
+        return self._initialize_result.server_info
+
+    @property
+    def server_instructions(self) -> str | None:
+        """Instructions describing how to use the server and its features, if provided."""
+        if self._initialize_result is None:
+            raise RuntimeError("Client must be used within an async context manager")
+        return self._initialize_result.instructions
 
     async def send_ping(self, *, meta: RequestParamsMeta | None = None) -> EmptyResult:
         """Send a ping request to the server."""

src/mcp/client/session.py

@@ -131,7 +131,7 @@ def __init__(
         self._logging_callback = logging_callback or _default_logging_callback
         self._message_handler = message_handler or _default_message_handler
         self._tool_output_schemas: dict[str, dict[str, Any] | None] = {}
-        self._server_capabilities: types.ServerCapabilities | None = None
+        self._initialize_result: types.InitializeResult | None = None
         self._experimental_features: ExperimentalClientFeatures | None = None
 
         # Experimental: Task handlers (use defaults if not provided)
@@ -185,18 +185,19 @@ async def initialize(self) -> types.InitializeResult:
         if result.protocol_version not in SUPPORTED_PROTOCOL_VERSIONS:
             raise RuntimeError(f"Unsupported protocol version from the server: {result.protocol_version}")
 
-        self._server_capabilities = result.capabilities
+        self._initialize_result = result
 
         await self.send_notification(types.InitializedNotification())
 
         return result
 
-    def get_server_capabilities(self) -> types.ServerCapabilities | None:
-        """Return the server capabilities received during initialization.
+    @property
+    def initialize_result(self) -> types.InitializeResult | None:
+        """The server's InitializeResult. None until initialize() has been called.
 
-        Returns None if the session has not been initialized yet.
+        Contains server_info, capabilities, instructions, and the negotiated protocol_version.
         """
-        return self._server_capabilities
+        return self._initialize_result
 
     @property
     def experimental(self) -> ExperimentalClientFeatures:

tests/client/test_client.py

@@ -107,6 +107,8 @@ async def test_client_is_initialized(app: MCPServer):
                 tools=ToolsCapability(list_changed=False),
             )
         )
+        assert client.server_info.name == "test"
+        assert client.server_instructions is None
 
 
 async def test_client_with_simple_server(simple_server: Server):
@@ -194,6 +196,17 @@ def test_client_session_property_before_enter(app: MCPServer):
         client.session
 
 
+def test_client_server_properties_before_enter(app: MCPServer):
+    """Test that server_* properties raise RuntimeError outside the context manager."""
+    client = Client(app)
+    with pytest.raises(RuntimeError, match="Client must be used within an async context manager"):
+        client.server_capabilities
+    with pytest.raises(RuntimeError, match="Client must be used within an async context manager"):
+        client.server_info
+    with pytest.raises(RuntimeError, match="Client must be used within an async context manager"):
+        client.server_instructions
+
+
 async def test_client_reentry_raises_runtime_error(app: MCPServer):
     """Test that reentering a client raises RuntimeError."""
     async with Client(app) as client:

tests/client/test_session.py

@@ -540,8 +540,8 @@ async def mock_server():
 
 
 @pytest.mark.anyio
-async def test_get_server_capabilities():
-    """Test that get_server_capabilities returns None before init and capabilities after"""
+async def test_initialize_result():
+    """Test that initialize_result is None before init and contains the full result after."""
     client_to_server_send, client_to_server_receive = anyio.create_memory_object_stream[SessionMessage](1)
     server_to_client_send, server_to_client_receive = anyio.create_memory_object_stream[SessionMessage](1)
 
@@ -551,6 +551,8 @@ async def test_get_server_capabilities():
         resources=types.ResourcesCapability(subscribe=True, list_changed=True),
         tools=types.ToolsCapability(list_changed=False),
     )
+    expected_server_info = Implementation(name="mock-server", version="0.1.0")
+    expected_instructions = "Use the tools wisely."
 
     async def mock_server():
         session_message = await client_to_server_receive.receive()
@@ -564,7 +566,8 @@ async def mock_server():
         result = InitializeResult(
             protocol_version=LATEST_PROTOCOL_VERSION,
             capabilities=expected_capabilities,
-            server_info=Implementation(name="mock-server", version="0.1.0"),
+            server_info=expected_server_info,
+            instructions=expected_instructions,
         )
 
         async with server_to_client_send:
@@ -590,21 +593,17 @@ async def mock_server():
         server_to_client_send,
         server_to_client_receive,
     ):
-        assert session.get_server_capabilities() is None
+        assert session.initialize_result is None
 
         tg.start_soon(mock_server)
         await session.initialize()
 
-        capabilities = session.get_server_capabilities()
-        assert capabilities is not None
-        assert capabilities == expected_capabilities
-        assert capabilities.logging is not None
-        assert capabilities.prompts is not None
-        assert capabilities.prompts.list_changed is True
-        assert capabilities.resources is not None
-        assert capabilities.resources.subscribe is True
-        assert capabilities.tools is not None
-        assert capabilities.tools.list_changed is False
+        result = session.initialize_result
+        assert result is not None
+        assert result.server_info == expected_server_info
+        assert result.capabilities == expected_capabilities
+        assert result.instructions == expected_instructions
+        assert result.protocol_version == LATEST_PROTOCOL_VERSION
 
 
 @pytest.mark.anyio

tests/client/transports/test_memory.py

@@ -69,7 +69,7 @@ async def test_with_mcpserver(mcpserver_server: MCPServer):
 async def test_server_is_running(mcpserver_server: MCPServer):
     """Test that the server is running and responding to requests."""
     async with Client(mcpserver_server) as client:
-        assert client.server_capabilities is not None
+        assert client.server_capabilities.tools is not None
 
 
 async def test_list_tools(mcpserver_server: MCPServer):