diff --git a/pkg/remote/trans/ttstream/stream_client.go b/pkg/remote/trans/ttstream/stream_client.go
index e4127d6ed7..9775ac5758 100644
--- a/pkg/remote/trans/ttstream/stream_client.go
+++ b/pkg/remote/trans/ttstream/stream_client.go
@@ -302,6 +302,13 @@ func (s *clientStream) onReadHeaderFrame(fr *Frame) error {
 	select {
 	case s.headerSig <- streamSigActive:
 	default:
+		// close() marks the stream inactive before parking a terminal signal in
+		// headerSig. A header that arrives after that point is an in-flight frame
+		// for an already-closed stream and must not tear down the mux connection.
+		if atomic.LoadInt32(&s.state) == streamStateInactive {
+			s.header = nil
+			return nil
+		}
 		return errUnexpectedHeader.newBuilder().withSide(clientSide).withCause(fmt.Errorf("stream[%d] already set header", s.sid))
 	}
 	s.handleStreamRecvHeaderEvent(rpcinfo.StreamRecvHeaderEvent{TTStreamHeader: s.header})
diff --git a/pkg/remote/trans/ttstream/stream_client_lateheader_test.go b/pkg/remote/trans/ttstream/stream_client_lateheader_test.go
new file mode 100644
index 0000000000..a570402fb2
--- /dev/null
+++ b/pkg/remote/trans/ttstream/stream_client_lateheader_test.go
@@ -0,0 +1,75 @@
+package ttstream
+
+import (
+	"context"
+	"errors"
+	"sync"
+	"testing"
+
+	"github.com/cloudwego/kitex/internal/test"
+)
+
+// A late header frame that arrives after close() must be ignored. close() parks
+// a terminal signal (streamSigNone/streamSigInactive) in the buffered headerSig
+// channel; without the fix, onReadHeaderFrame falls into the non-blocking
+// send's default branch and surfaces errUnexpectedHeader, which the read loop
+// treats as fatal and tears down the whole mux transport connection.
+func Test_onReadHeaderFrameAfterCloseIsIgnored(t *testing.T) {
+	cs := newTestClientStream(context.Background())
+	cs.close(nil, false, "", nil)
+
+	late := &Frame{
+		streamFrame: streamFrame{
+			sid:    cs.sid,
+			method: cs.method,
+			header: map[string]string{"late": "header"},
+		},
+		typ: headerFrameType,
+	}
+	err := cs.onReadHeaderFrame(late)
+	test.Assert(t, err == nil, "late header after close must be ignored, got err=%v", err)
+	// the dropped late header must not leak into s.header
+	test.Assert(t, cs.header == nil, "dropped header must not be stored")
+}
+
+// A genuine duplicate header (two header frames, no close in between) must
+// still be reported as a protocol error. This guards against over-loosening:
+// only the close-parked terminal signal path may be tolerated.
+func Test_onReadDuplicateHeaderStillErrors(t *testing.T) {
+	cs := newTestClientStream(context.Background())
+	first := &Frame{streamFrame: streamFrame{sid: cs.sid, header: map[string]string{"a": "1"}}, typ: headerFrameType}
+	test.Assert(t, cs.onReadHeaderFrame(first) == nil, "first header must be accepted")
+
+	second := &Frame{streamFrame: streamFrame{sid: cs.sid, header: map[string]string{"b": "2"}}, typ: headerFrameType}
+	err := cs.onReadHeaderFrame(second)
+	test.Assert(t, err != nil, "duplicate header without close must still error")
+	test.Assert(t, errors.Is(err, errUnexpectedHeader), "expected errUnexpectedHeader, got %v", err)
+}
+
+// Race oracle: hammer close() concurrently with onReadHeaderFrame. With the
+// fix, onReadHeaderFrame must NEVER return errUnexpectedHeader once the stream
+// has been closed (it either accepts the header before close, or ignores it
+// after); without the fix the race detector run reproduces the spurious fatal.
+func Test_onReadHeaderFrameRaceWithClose(t *testing.T) {
+	const iterations = 200
+	for i := 0; i < iterations; i++ {
+		cs := newTestClientStream(context.Background())
+		var wg sync.WaitGroup
+		wg.Add(2)
+		go func() {
+			defer wg.Done()
+			cs.close(nil, false, "", nil)
+		}()
+		go func() {
+			defer wg.Done()
+			fr := &Frame{streamFrame: streamFrame{sid: cs.sid, header: map[string]string{"k": "v"}}, typ: headerFrameType}
+			err := cs.onReadHeaderFrame(fr)
+			// If the header wins the race it is accepted; if close wins, the
+			// in-flight header is ignored. Neither ordering is a protocol error.
+			if err != nil {
+				t.Errorf("header/close race returned a fatal error: %v", err)
+			}
+		}()
+		wg.Wait()
+	}
+}