diff --git a/sdk/api/handlers/openai/openai_responses_handlers.go b/sdk/api/handlers/openai/openai_responses_handlers.go
index 578977d62..8dd1a0a7b 100644
--- a/sdk/api/handlers/openai/openai_responses_handlers.go
+++ b/sdk/api/handlers/openai/openai_responses_handlers.go
@@ -148,9 +148,12 @@ func responsesSSEFrameWithData(frame, payload []byte) []byte {
 		out.Write(line)
 		out.WriteByte('\n')
 	}
-	out.WriteString("data: ")
-	out.Write(payload)
-	out.WriteString("\n\n")
+	for _, line := range bytes.Split(payload, []byte("\n")) {
+		out.WriteString("data: ")
+		out.Write(line)
+		out.WriteByte('\n')
+	}
+	out.WriteByte('\n')
 	return out.Bytes()
 }
 
diff --git a/sdk/api/handlers/openai/openai_responses_handlers_stream_test.go b/sdk/api/handlers/openai/openai_responses_handlers_stream_test.go
index 3851278fb..151da9a79 100644
--- a/sdk/api/handlers/openai/openai_responses_handlers_stream_test.go
+++ b/sdk/api/handlers/openai/openai_responses_handlers_stream_test.go
@@ -122,6 +122,40 @@ func TestForwardResponsesStreamRepairsMixedIndexedAndUnindexedDoneItems(t *testi
 	}
 }
 
+func TestForwardResponsesStreamRepairsMultilineCompletedOutputAsSSEDataLines(t *testing.T) {
+	h, recorder, c, flusher := newResponsesStreamTestHandler(t)
+
+	data := make(chan []byte, 2)
+	errs := make(chan *interfaces.ErrorMessage)
+	data <- []byte(`data: {"type":"response.output_item.done","item":{"type":"function_call","arguments":"{}"}}`)
+	data <- []byte("data: {\"type\":\"response.completed\",\ndata: \"response\":{\"id\":\"resp-1\",\"output\":[]}}\n\n")
+	close(data)
+	close(errs)
+
+	h.forwardResponsesStream(c, flusher, func(error) {}, data, errs, nil)
+
+	parts := strings.Split(strings.TrimSpace(recorder.Body.String()), "\n\n")
+	if len(parts) != 2 {
+		t.Fatalf("expected 2 SSE events, got %d. Body: %q", len(parts), recorder.Body.String())
+	}
+
+	completedFrame := []byte(parts[1])
+	for _, line := range strings.Split(parts[1], "\n") {
+		if line != "" && !strings.HasPrefix(line, "data: ") {
+			t.Fatalf("expected every completed payload line to be an SSE data line, got %q in %q", line, parts[1])
+		}
+	}
+
+	payload, ok := responsesSSEDataPayload(completedFrame)
+	if !ok {
+		t.Fatalf("expected completed frame to contain data payload: %q", parts[1])
+	}
+	output := gjson.GetBytes(payload, "response.output")
+	if !output.IsArray() || len(output.Array()) != 1 {
+		t.Fatalf("expected repaired completed output with 1 item, got %s from %q", output.Raw, payload)
+	}
+}
+
 func TestForwardResponsesStreamReassemblesSplitSSEEventChunks(t *testing.T) {
 	h, recorder, c, flusher := newResponsesStreamTestHandler(t)