diff --git a/backend/internal/service/openai_gateway_messages.go b/backend/internal/service/openai_gateway_messages.go
index 7a4862d3..a72b9bbf 100644
--- a/backend/internal/service/openai_gateway_messages.go
+++ b/backend/internal/service/openai_gateway_messages.go
@@ -270,6 +270,7 @@ func (s *OpenAIGatewayService) handleAnthropicBufferedStreamingResponse(
 
 	var finalResponse *apicompat.ResponsesResponse
 	var usage OpenAIUsage
+	acc := apicompat.NewBufferedResponseAccumulator()
 
 	for scanner.Scan() {
 		line := scanner.Text()
@@ -288,8 +289,12 @@ func (s *OpenAIGatewayService) handleAnthropicBufferedStreamingResponse(
 			continue
 		}
 
+		// Accumulate delta content for fallback when terminal output is empty.
+		acc.ProcessEvent(&event)
+
 		// Terminal events carry the complete ResponsesResponse with output + usage.
-		if (event.Type == "response.completed" || event.Type == "response.incomplete" || event.Type == "response.failed") &&
+		if (event.Type == "response.completed" || event.Type == "response.done" ||
+			event.Type == "response.incomplete" || event.Type == "response.failed") &&
 			event.Response != nil {
 			finalResponse = event.Response
 			if event.Response.Usage != nil {
@@ -318,6 +323,10 @@ func (s *OpenAIGatewayService) handleAnthropicBufferedStreamingResponse(
 		return nil, fmt.Errorf("upstream stream ended without terminal event")
 	}
 
+	// When the terminal event has an empty output array, reconstruct from
+	// accumulated delta events so the client receives the full content.
+	acc.SupplementResponseOutput(finalResponse)
+
 	anthropicResp := apicompat.ResponsesToAnthropic(finalResponse, originalModel)
 
 	if s.responseHeaderFilter != nil {