diff --git a/backend/internal/service/openai_ws_v2/passthrough_relay.go b/backend/internal/service/openai_ws_v2/passthrough_relay.go
index e28223e091..fdb539a073 100644
--- a/backend/internal/service/openai_ws_v2/passthrough_relay.go
+++ b/backend/internal/service/openai_ws_v2/passthrough_relay.go
@@ -533,17 +533,26 @@ func runUpstreamToClient(
 		msgType, payload, err := upstreamConn.ReadFrame(ctx)
 		if err != nil {
 			emitTurnComplete(onTurnComplete, state, finalizePendingBareError(state, nowFn()))
+			graceful := isDisconnectError(err)
+			// A clean WebSocket close only describes the transport handshake. Once
+			// the upstream has started a Responses turn, success still requires a
+			// terminal protocol event. Treat an early 1000/EOF as a relay failure so
+			// the adapter does not report relay_completed with an active turn.
+			if graceful && openAIWSRelayActiveTurnID(state) != "" {
+				graceful = false
+				err = errors.New("upstream websocket closed before terminal event: " + err.Error())
+			}
 			emitRelayTrace(onTrace, RelayTraceEvent{
 				Stage:           "read_upstream_failed",
 				Direction:       "upstream_to_client",
 				Error:           err.Error(),
-				Graceful:        isDisconnectError(err),
+				Graceful:        graceful,
 				WroteDownstream: wroteDownstream,
 			})
 			exitCh <- relayExitSignal{
 				stage:           "read_upstream",
 				err:             err,
-				graceful:        isDisconnectError(err),
+				graceful:        graceful,
 				wroteDownstream: wroteDownstream,
 			}
 			return
diff --git a/backend/internal/service/openai_ws_v2/passthrough_relay_test.go b/backend/internal/service/openai_ws_v2/passthrough_relay_test.go
index 28b896f248..a5505b4981 100644
--- a/backend/internal/service/openai_ws_v2/passthrough_relay_test.go
+++ b/backend/internal/service/openai_ws_v2/passthrough_relay_test.go
@@ -41,6 +41,11 @@ type closeSpyFrameConn struct {
 	closeCalls atomic.Int32
 }
 
+type eofReplacementFrameConn struct {
+	FrameConn
+	err error
+}
+
 type cancelJoinProbeFrameConn struct {
 	readStarted  chan struct{}
 	readCanceled chan struct{}
@@ -65,6 +70,14 @@ func newPassthroughTestFrameConn(frames []passthroughTestFrame, autoClose bool)
 	return c
 }
 
+func (c *eofReplacementFrameConn) ReadFrame(ctx context.Context) (coderws.MessageType, []byte, error) {
+	msgType, payload, err := c.FrameConn.ReadFrame(ctx)
+	if errors.Is(err, io.EOF) {
+		return msgType, payload, c.err
+	}
+	return msgType, payload, err
+}
+
 func (c *passthroughTestFrameConn) ReadFrame(ctx context.Context) (coderws.MessageType, []byte, error) {
 	if ctx == nil {
 		ctx = context.Background()
@@ -301,6 +314,38 @@ func TestRelay_UpstreamDisconnect(t *testing.T) {
 	require.Equal(t, "gpt-4o", result.RequestModel)
 }
 
+func TestRelay_UpstreamNormalCloseBeforeTerminalIsFailure(t *testing.T) {
+	t.Parallel()
+
+	clientConn := newPassthroughTestFrameConn(nil, false)
+	upstreamConn := &eofReplacementFrameConn{
+		FrameConn: newPassthroughTestFrameConn([]passthroughTestFrame{
+			{
+				msgType: coderws.MessageText,
+				payload: []byte(`{"type":"response.created","response":{"id":"resp_incomplete","status":"in_progress"}}`),
+			},
+			{
+				msgType: coderws.MessageText,
+				payload: []byte(`{"type":"response.in_progress","response":{"id":"resp_incomplete","status":"in_progress"}}`),
+			},
+		}, true),
+		err: coderws.CloseError{Code: coderws.StatusNormalClosure},
+	}
+
+	firstPayload := []byte(`{"type":"response.create","model":"gpt-5.6-sol","input":[]}`)
+	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
+	defer cancel()
+
+	result, relayExit := Relay(ctx, clientConn, upstreamConn, firstPayload, RelayOptions{})
+	require.NotNil(t, relayExit)
+	require.Equal(t, "read_upstream", relayExit.Stage)
+	require.False(t, relayExit.Graceful)
+	require.True(t, relayExit.WroteDownstream)
+	require.ErrorContains(t, relayExit.Err, "upstream websocket closed before terminal event")
+	require.Empty(t, result.TerminalEventType)
+	require.Len(t, clientConn.Writes(), 2)
+}
+
 func TestRelay_ClientDisconnect(t *testing.T) {
 	t.Parallel()