diff --git a/sdk/api/handlers/openai/openai_responses_websocket.go b/sdk/api/handlers/openai/openai_responses_websocket.go
index 6457cd3ee..273547d83 100644
--- a/sdk/api/handlers/openai/openai_responses_websocket.go
+++ b/sdk/api/handlers/openai/openai_responses_websocket.go
@@ -703,29 +703,20 @@ func mergeJSONArrayRaw(existingRaw, appendRaw string) (string, error) {
 	return string(out), nil
 }
 
-// inputContainsFullTranscript returns true when the input array looks like a
-// complete conversation history rather than an incremental append.  After a
-// client-side compact the input already carries the full (compacted) transcript
-// which may include assistant messages or compaction items.  Merging that with
-// the stale lastRequest / lastResponseOutput would duplicate or break
-// function_call / function_call_output pairings, so the caller should use the
-// input as-is.
+// inputContainsFullTranscript returns true when the input array carries compact
+// replay markers that indicate the client already sent the full conversation
+// transcript. Merging that input with stale lastRequest/lastResponseOutput
+// would duplicate or break function_call/function_call_output pairings, so the
+// caller should use the input as-is.
 //
-// Heuristic: the array is a full transcript when it contains either
-//   - a message with role="assistant", or
-//   - a compaction item (type="compaction" or "compaction_summary").
-//
-// Normal incremental turns only contain user messages or function_call_output
-// items and never carry either of these signals.
+// Assistant messages alone are not enough to classify the payload as a replay:
+// incremental websocket requests may legitimately append assistant items.
 func inputContainsFullTranscript(input gjson.Result) bool {
 	if !input.IsArray() {
 		return false
 	}
 	for _, item := range input.Array() {
 		t := item.Get("type").String()
-		if t == "message" && item.Get("role").String() == "assistant" {
-			return true
-		}
 		if t == "compaction" || t == "compaction_summary" {
 			return true
 		}
diff --git a/sdk/api/handlers/openai/openai_responses_websocket_test.go b/sdk/api/handlers/openai/openai_responses_websocket_test.go
index 2c5ef579b..82b96f141 100644
--- a/sdk/api/handlers/openai/openai_responses_websocket_test.go
+++ b/sdk/api/handlers/openai/openai_responses_websocket_test.go
@@ -1401,13 +1401,13 @@ func TestResponsesWebsocketCompactionResetsTurnStateOnTranscriptReplacement(t *t
 	}
 }
 
-func TestInputContainsFullTranscriptDetectsAssistantMessage(t *testing.T) {
+func TestInputContainsFullTranscriptFalseForAssistantMessageOnly(t *testing.T) {
 	input := gjson.Parse(`[
 		{"type":"message","role":"user","content":"hello"},
 		{"type":"message","role":"assistant","content":"hi there"}
 	]`)
-	if !inputContainsFullTranscript(input) {
-		t.Fatal("expected full transcript when assistant message is present")
+	if inputContainsFullTranscript(input) {
+		t.Fatal("assistant message alone must not be treated as full transcript")
 	}
 }
 
@@ -1501,3 +1501,33 @@ func TestNormalizeSubsequentRequestIncrementalInputStillMerges(t *testing.T) {
 		}
 	}
 }
+
+func TestNormalizeSubsequentRequestAssistantIncrementalInputStillMerges(t *testing.T) {
+	lastRequest := []byte(`{"model":"gpt-5.4","stream":true,"input":[
+		{"type":"message","role":"user","id":"msg-1","content":"hello"}
+	]}`)
+	lastResponseOutput := []byte(`[
+		{"type":"message","role":"assistant","id":"msg-2","content":"prior assistant"},
+		{"type":"function_call","id":"fc-1","call_id":"call-1","name":"bash","arguments":"{}"}
+	]`)
+	raw := []byte(`{"type":"response.append","input":[
+		{"type":"message","role":"assistant","id":"msg-3","content":"patched assistant turn"}
+	]}`)
+
+	normalized, _, errMsg := normalizeResponsesWebsocketRequest(raw, lastRequest, lastResponseOutput)
+	if errMsg != nil {
+		t.Fatalf("unexpected error: %v", errMsg.Error)
+	}
+
+	input := gjson.GetBytes(normalized, "input").Array()
+	if len(input) != 4 {
+		t.Fatalf("input len = %d, want 4 (merged)", len(input))
+	}
+	wantIDs := []string{"msg-1", "msg-2", "fc-1", "msg-3"}
+	for i, want := range wantIDs {
+		got := input[i].Get("id").String()
+		if got != want {
+			t.Fatalf("input[%d].id = %q, want %q", i, got, want)
+		}
+	}
+}