fix: prevent duplicate tool calls in Responses-to-Chat streaming (#6225)

When a function call was already registered under its output_index key
via response.output_item.added, the synthetic events built from the
terminal response.completed output carry no output_index and resolve to
a different item-based key. ensureToolForEvent then created a second
tool index and resent the full arguments, so Chat Completions clients
received the same tool call twice.

Reuse the tool registered under itemIDToKey/callIDToKey before creating
a new one, and alias the new key to the existing tool.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

service/relayconvert/internal/oai_responses/to_oai_chat_resp_test.go

@@ -281,6 +281,64 @@ func TestResponsesStreamEventToChatChunksUsesTerminalDoneOutput(t *testing.T) {
 	assert.Equal(t, "tool_calls", *chunks[3].Choices[0].FinishReason)
 }
 
+func TestResponsesStreamEventToChatChunksDoesNotResendToolOnTerminalOutput(t *testing.T) {
+	state := newTestResponsesStreamState()
+	outputIndex := 0
+
+	var chunks []dto.ChatCompletionsStreamResponse
+	chunks = append(chunks, mustStreamChunks(t, state, &dto.ResponsesStreamResponse{Type: responsesEventCreated})...)
+	chunks = append(chunks, mustStreamChunks(t, state, &dto.ResponsesStreamResponse{
+		Type:        responsesEventOutputItemAdded,
+		OutputIndex: &outputIndex,
+		Item: &dto.ResponsesOutput{
+			Type:   responsesOutputTypeFunctionCall,
+			ID:     "fc_1",
+			CallId: "call_1",
+			Name:   "lookup",
+		},
+	})...)
+	chunks = append(chunks, mustStreamChunks(t, state, &dto.ResponsesStreamResponse{
+		Type:        responsesEventFunctionArgsDelta,
+		OutputIndex: &outputIndex,
+		Delta:       `{"q":"x"}`,
+	})...)
+	chunks = append(chunks, mustStreamChunks(t, state, &dto.ResponsesStreamResponse{
+		Type: responsesEventCompleted,
+		Response: &dto.OpenAIResponsesResponse{
+			Status: []byte(`"completed"`),
+			Output: []dto.ResponsesOutput{
+				{
+					Type:      responsesOutputTypeFunctionCall,
+					ID:        "fc_1",
+					CallId:    "call_1",
+					Name:      "lookup",
+					Arguments: []byte(`{"q":"x"}`),
+				},
+			},
+		},
+	})...)
+
+	totalArgs := ""
+	toolIndexes := map[int]bool{}
+	var finishReason string
+	for _, chunk := range chunks {
+		for _, choice := range chunk.Choices {
+			for _, tc := range choice.Delta.ToolCalls {
+				require.NotNil(t, tc.Index)
+				toolIndexes[*tc.Index] = true
+				totalArgs += tc.Function.Arguments
+			}
+			if choice.FinishReason != nil {
+				finishReason = *choice.FinishReason
+			}
+		}
+	}
+
+	assert.Equal(t, map[int]bool{0: true}, toolIndexes)
+	assert.Equal(t, `{"q":"x"}`, totalArgs)
+	assert.Equal(t, "tool_calls", finishReason)
+}
+
 func TestFinalizeResponsesToChatStreamFlushesPendingDeltaOnlyArguments(t *testing.T) {
 	state := newTestResponsesStreamState()
 	outputIndex := 2

service/relayconvert/internal/oai_responses/to_oai_chat_stream_resp.go

@@ -269,6 +269,23 @@ func (s *ResponsesToChatStreamState) ensureToolForEvent(event *dto.ResponsesStre
 	}
 
 	tool := s.toolByKey[key]
+	if tool == nil {
+		if itemID := responseStreamEventItemID(event); itemID != "" {
+			if existingKey := s.itemIDToKey[itemID]; existingKey != "" {
+				tool = s.toolByKey[existingKey]
+			}
+		}
+		if tool == nil {
+			if callID := strings.TrimSpace(event.Item.CallId); callID != "" {
+				if existingKey := s.callIDToKey[callID]; existingKey != "" {
+					tool = s.toolByKey[existingKey]
+				}
+			}
+		}
+		if tool != nil {
+			s.toolByKey[key] = tool
+		}
+	}
 	if tool == nil {
 		tool = &responsesStreamTool{Key: key, Index: s.nextToolIndex}
 		s.nextToolIndex++