fix: clean up failed HTTP connect handshakes

Author: atesgoral

lib/mcp/client/http.rb

@@ -65,6 +65,7 @@ def connect(client_info: nil, protocol_version: nil, capabilities: {})
         })
 
         if response.is_a?(Hash) && response.key?("error")
+          clear_session
           error = response["error"]
           raise RequestHandlerError.new(
             "Server initialization failed: #{error["message"]}",
@@ -74,6 +75,7 @@ def connect(client_info: nil, protocol_version: nil, capabilities: {})
         end
 
         unless response.is_a?(Hash) && response["result"].is_a?(Hash)
+          clear_session
           raise RequestHandlerError.new(
             "Server initialization failed: missing result in response",
             { method: MCP::Methods::INITIALIZE },
@@ -82,11 +84,25 @@ def connect(client_info: nil, protocol_version: nil, capabilities: {})
         end
 
         @server_info = response["result"]
+        negotiated_protocol_version = @server_info["protocolVersion"]
+        unless MCP::Configuration::SUPPORTED_STABLE_PROTOCOL_VERSIONS.include?(negotiated_protocol_version)
+          clear_session
+          raise RequestHandlerError.new(
+            "Server initialization failed: unsupported protocol version #{negotiated_protocol_version.inspect}",
+            { method: MCP::Methods::INITIALIZE },
+            error_type: :internal_error,
+          )
+        end
 
-        send_request(request: {
-          jsonrpc: JsonRpcHandler::Version::V2_0,
-          method: MCP::Methods::NOTIFICATIONS_INITIALIZED,
-        })
+        begin
+          send_request(request: {
+            jsonrpc: JsonRpcHandler::Version::V2_0,
+            method: MCP::Methods::NOTIFICATIONS_INITIALIZED,
+          })
+        rescue StandardError
+          clear_session
+          raise
+        end
 
         @connected = true
         @server_info

test/mcp/client/http_test.rb

@@ -797,7 +797,7 @@ def test_connect_is_idempotent
       def test_connect_raises_on_jsonrpc_error_response
         stub_request(:post, url).to_return(
           status: 200,
-          headers: { "Content-Type" => "application/json" },
+          headers: { "Content-Type" => "application/json", "Mcp-Session-Id" => "session-abc" },
           body: { error: { code: -32602, message: "Unsupported protocol version" } }.to_json,
         )
 
@@ -807,12 +807,15 @@ def test_connect_raises_on_jsonrpc_error_response
 
         assert_includes(error.message, "Unsupported protocol version")
         refute_predicate(client, :connected?)
+        assert_nil(client.session_id)
+        assert_nil(client.protocol_version)
+        assert_nil(client.server_info)
       end
 
       def test_connect_raises_on_missing_result
         stub_request(:post, url).to_return(
           status: 200,
-          headers: { "Content-Type" => "application/json" },
+          headers: { "Content-Type" => "application/json", "Mcp-Session-Id" => "session-abc" },
           body: { jsonrpc: "2.0", id: "x" }.to_json,
         )
 
@@ -822,6 +825,45 @@ def test_connect_raises_on_missing_result
 
         assert_includes(error.message, "missing result in response")
         refute_predicate(client, :connected?)
+        assert_nil(client.session_id)
+        assert_nil(client.protocol_version)
+        assert_nil(client.server_info)
+      end
+
+      def test_connect_raises_on_unsupported_negotiated_protocol_version
+        stub_request(:post, url)
+          .with { |req| JSON.parse(req.body)["method"] == "initialize" }
+          .to_return(
+            status: 200,
+            headers: { "Content-Type" => "application/json", "Mcp-Session-Id" => "session-abc" },
+            body: { result: { protocolVersion: "2099-01-01" } }.to_json,
+          )
+
+        error = assert_raises(RequestHandlerError) do
+          client.connect
+        end
+
+        assert_includes(error.message, 'unsupported protocol version "2099-01-01"')
+        refute_predicate(client, :connected?)
+        assert_nil(client.session_id)
+        assert_nil(client.protocol_version)
+        assert_nil(client.server_info)
+      end
+
+      def test_connect_clears_session_when_initialized_notification_fails
+        stub_initialize
+        stub_request(:post, url)
+          .with { |req| JSON.parse(req.body)["method"] == "notifications/initialized" }
+          .to_return(status: 500)
+
+        assert_raises(RequestHandlerError) do
+          client.connect
+        end
+
+        refute_predicate(client, :connected?)
+        assert_nil(client.session_id)
+        assert_nil(client.protocol_version)
+        assert_nil(client.server_info)
       end
 
       def test_connected_lifecycle