diff --git a/internal/proxy/transform/stream.go b/internal/proxy/transform/stream.go
index e71a17c..4052b75 100644
--- a/internal/proxy/transform/stream.go
+++ b/internal/proxy/transform/stream.go
@@ -1114,6 +1114,9 @@ func (st *StreamTransformer) transformResponsesAPIToOpenAIChat(r io.Reader, w io
 		headerSent bool
 	}
 	toolCalls := make(map[int]*toolMeta)
+	// Guard against duplicate role chunks when both response.created and
+	// response.in_progress arrive for the same stream.
+	roleSent := false
 
 	emitChunk := func(delta map[string]interface{}, finishReason interface{}) {
 		choice := map[string]interface{}{
@@ -1194,7 +1197,10 @@ func (st *StreamTransformer) transformResponsesAPIToOpenAIChat(r io.Reader, w io
 					model = m
 				}
 			}
-			emitChunk(map[string]interface{}{"role": "assistant", "content": ""}, nil)
+			if !roleSent {
+				roleSent = true
+				emitChunk(map[string]interface{}{"role": "assistant", "content": ""}, nil)
+			}
 
 		case "response.output_item.added":
 			// Capture function_call metadata so we can emit proper headers later.
diff --git a/internal/proxy/transform/stream_test.go b/internal/proxy/transform/stream_test.go
index c5613f2..dd17274 100644
--- a/internal/proxy/transform/stream_test.go
+++ b/internal/proxy/transform/stream_test.go
@@ -1279,3 +1279,38 @@ func TestTransformResponsesAPIToOpenAIChat_ToolCall(t *testing.T) {
 		t.Error("should emit [DONE] sentinel")
 	}
 }
+
+func TestTransformResponsesAPIToOpenAIChat_NoDuplicateRoleChunk(t *testing.T) {
+	// When both response.created and response.in_progress arrive, only one
+	// assistant role chunk should be emitted.
+	input := strings.Join([]string{
+		`event: response.created`,
+		`data: {"type":"response.created","response":{"id":"resp_dup1","status":"in_progress","model":"gpt-5","output":[]}}`,
+		``,
+		`event: response.in_progress`,
+		`data: {"type":"response.in_progress","response":{"id":"resp_dup1","status":"in_progress","model":"gpt-5","output":[]}}`,
+		``,
+		`event: response.output_text.delta`,
+		`data: {"type":"response.output_text.delta","output_index":0,"content_index":0,"delta":"Hi"}`,
+		``,
+		`event: response.completed`,
+		`data: {"type":"response.completed","response":{"id":"resp_dup1","status":"completed","model":"gpt-5","output":[]}}`,
+		``,
+	}, "\n")
+
+	st := &StreamTransformer{
+		ClientFormat:   FormatOpenAIChat,
+		ProviderFormat: FormatOpenAIResponses,
+	}
+	reader := st.TransformSSEStream(strings.NewReader(input))
+	output, err := io.ReadAll(reader)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	// Count how many times a role:assistant delta is emitted
+	roleCount := strings.Count(string(output), `"role":"assistant"`)
+	if roleCount != 1 {
+		t.Errorf("expected exactly 1 role:assistant chunk, got %d; output:\n%s", roleCount, string(output))
+	}
+}