Set 2025-03-26 as the latest protocol version

src/mcp/shared/version.py

@@ -1,3 +1,3 @@
 from mcp.types import LATEST_PROTOCOL_VERSION
 
-SUPPORTED_PROTOCOL_VERSIONS: tuple[int, str] = (1, LATEST_PROTOCOL_VERSION)
+SUPPORTED_PROTOCOL_VERSIONS: list[str] = ["2024-11-05", LATEST_PROTOCOL_VERSION]

src/mcp/types.py

@@ -29,7 +29,7 @@
   not separate types in the schema.
 """
 
-LATEST_PROTOCOL_VERSION = "2024-11-05"
+LATEST_PROTOCOL_VERSION = "2025-03-26"
 
 ProgressToken = str | int
 Cursor = str

tests/client/test_session.py

@@ -5,6 +5,7 @@
 from mcp.client.session import DEFAULT_CLIENT_INFO, ClientSession
 from mcp.shared.message import SessionMessage
 from mcp.shared.session import RequestResponder
+from mcp.shared.version import SUPPORTED_PROTOCOL_VERSIONS
 from mcp.types import (
     LATEST_PROTOCOL_VERSION,
     ClientNotification,
@@ -250,3 +251,132 @@ async def mock_server():
 
     # Assert that the default client info was sent
     assert received_client_info == DEFAULT_CLIENT_INFO
+
+
+@pytest.mark.anyio
+async def test_client_session_version_negotiation_success():
+    """Test successful version negotiation with supported version"""
+    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)
+
+    async def mock_server():
+        session_message = await client_to_server_receive.receive()
+        jsonrpc_request = session_message.message
+        assert isinstance(jsonrpc_request.root, JSONRPCRequest)
+        request = ClientRequest.model_validate(
+            jsonrpc_request.model_dump(by_alias=True, mode="json", exclude_none=True)
+        )
+        assert isinstance(request.root, InitializeRequest)
+
+        # Verify client sent the latest protocol version
+        assert request.root.params.protocolVersion == LATEST_PROTOCOL_VERSION
+
+        # Server responds with a supported older version
+        result = ServerResult(
+            InitializeResult(
+                protocolVersion="2024-11-05",
+                capabilities=ServerCapabilities(),
+                serverInfo=Implementation(name="mock-server", version="0.1.0"),
+            )
+        )
+
+        async with server_to_client_send:
+            await server_to_client_send.send(
+                SessionMessage(
+                    JSONRPCMessage(
+                        JSONRPCResponse(
+                            jsonrpc="2.0",
+                            id=jsonrpc_request.root.id,
+                            result=result.model_dump(
+                                by_alias=True, mode="json", exclude_none=True
+                            ),
+                        )
+                    )
+                )
+            )
+            # Receive initialized notification
+            await client_to_server_receive.receive()
+
+    async with (
+        ClientSession(
+            server_to_client_receive,
+            client_to_server_send,
+        ) as session,
+        anyio.create_task_group() as tg,
+        client_to_server_send,
+        client_to_server_receive,
+        server_to_client_send,
+        server_to_client_receive,
+    ):
+        tg.start_soon(mock_server)
+        result = await session.initialize()
+
+    # Assert the result with negotiated version
+    assert isinstance(result, InitializeResult)
+    assert result.protocolVersion == "2024-11-05"
+    assert result.protocolVersion in SUPPORTED_PROTOCOL_VERSIONS
+
+
+@pytest.mark.anyio
+async def test_client_session_version_negotiation_failure():
+    """Test version negotiation failure with unsupported version"""
+    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)
+
+    async def mock_server():
+        session_message = await client_to_server_receive.receive()
+        jsonrpc_request = session_message.message
+        assert isinstance(jsonrpc_request.root, JSONRPCRequest)
+        request = ClientRequest.model_validate(
+            jsonrpc_request.model_dump(by_alias=True, mode="json", exclude_none=True)
+        )
+        assert isinstance(request.root, InitializeRequest)
+
+        # Server responds with an unsupported version
+        result = ServerResult(
+            InitializeResult(
+                protocolVersion="2020-01-01",  # Unsupported old version
+                capabilities=ServerCapabilities(),
+                serverInfo=Implementation(name="mock-server", version="0.1.0"),
+            )
+        )
+
+        async with server_to_client_send:
+            await server_to_client_send.send(
+                SessionMessage(
+                    JSONRPCMessage(
+                        JSONRPCResponse(
+                            jsonrpc="2.0",
+                            id=jsonrpc_request.root.id,
+                            result=result.model_dump(
+                                by_alias=True, mode="json", exclude_none=True
+                            ),
+                        )
+                    )
+                )
+            )
+
+    async with (
+        ClientSession(
+            server_to_client_receive,
+            client_to_server_send,
+        ) as session,
+        anyio.create_task_group() as tg,
+        client_to_server_send,
+        client_to_server_receive,
+        server_to_client_send,
+        server_to_client_receive,
+    ):
+        tg.start_soon(mock_server)
+
+        # Should raise RuntimeError for unsupported version
+        with pytest.raises(RuntimeError, match="Unsupported protocol version"):
+            await session.initialize()