fix(mcpproxy): return clear error when initialize SSE ends before a response

Address review feedback: the initialize reader broke on EOF/deadline with
rawMsg still nil, producing the misleading "MCP message is not a response:
<nil>". Track the read error and surface a dedicated error instead, and add
a regression test for an initialize stream that closes before any JSON-RPC
response arrives.

Signed-off-by: pjdurden <prajjwalchittori1@gmail.com>

Author: pjdurden

internal/mcpproxy/mcpproxy.go

@@ -352,6 +352,7 @@ func (m *mcpRequestContext) initializeSession(ctx context.Context, routeName fil
 		}
 		if rawMsg == nil {
 			parser := newSSEEventParser(sseReader, backend.Name)
+			var readErr error
 			for rawMsg == nil {
 				event, parseErr := parser.next()
 				// TODO: handle reconnect. We need to re-arrange the event ID so that it will also contain the backend name and the original session ID.
@@ -366,14 +367,27 @@ func (m *mcpRequestContext) initializeSession(ctx context.Context, routeName fil
 						}
 					}
 				}
+				if rawMsg != nil {
+					// Found the response; a trailing EOF on this same event is not a failure.
+					break
+				}
 				if parseErr != nil {
-					if errors.Is(parseErr, io.EOF) || strings.Contains(parseErr.Error(), "context deadline exceeded") {
-						break
+					readErr = parseErr
+					if !errors.Is(parseErr, io.EOF) && !strings.Contains(parseErr.Error(), "context deadline exceeded") {
+						m.l.Error("failed to read MCP GET response body", slog.String("error", parseErr.Error()))
 					}
-					m.l.Error("failed to read MCP GET response body", slog.String("error", parseErr.Error()))
 					break
 				}
 			}
+			if rawMsg == nil {
+				// The SSE stream ended (EOF/deadline) or errored before any JSON-RPC
+				// response arrived. Surface a clear error instead of falling through to
+				// the misleading "MCP message is not a response: <nil>".
+				if readErr != nil && !errors.Is(readErr, io.EOF) && !strings.Contains(readErr.Error(), "context deadline exceeded") {
+					return nil, fmt.Errorf("failed to read MCP initialize response from backend %q: %w", backend.Name, readErr)
+				}
+				return nil, fmt.Errorf("MCP initialize stream from backend %q ended before a JSON-RPC response was received", backend.Name)
+			}
 		}
 
 		msg, ok := rawMsg.(*jsonrpc.Response)

internal/mcpproxy/mcpproxy_test.go

@@ -399,6 +399,33 @@ func TestInitializeSession_InitializeFailure(t *testing.T) {
 	require.Contains(t, err.Error(), "failed with status code")
 }
 
+func TestInitializeSession_SSEEndsBeforeResponse(t *testing.T) {
+	// Backend returns a 200 text/event-stream whose initialize response contains
+	// only non-response events (a keep-alive) and then closes before ever sending
+	// the JSON-RPC response. This must produce a clear error rather than the
+	// misleading "MCP message is not a response: <nil>".
+	backendServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.Header().Set(sessionIDHeader, "test-session-123")
+		w.Header().Set("Content-Type", "text/event-stream")
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write([]byte(`id: keepalive_0000
+data:
+
+`))
+	}))
+	defer backendServer.Close()
+
+	proxy := newTestMCPProxy()
+	proxy.backendListenerAddr = backendServer.URL
+
+	sessionID, err := proxy.initializeSession(t.Context(), "route1", filterapi.MCPBackend{Name: "test-backend"}, &mcp.InitializeParams{}, time.Now())
+
+	require.Error(t, err)
+	require.Empty(t, sessionID)
+	require.Contains(t, err.Error(), "ended before a JSON-RPC response was received")
+	require.NotContains(t, err.Error(), "is not a response")
+}
+
 func TestInitializeSession_NotificationsInitializedFailure(t *testing.T) {
 	// Mock backend server.
 	var callCount perBackendCallCount