fix: handle chatcompletions streaming tool calls with no text preamble and non-zero indices

bridge_integration_test.go

@@ -437,6 +437,114 @@ func TestOpenAIChatCompletions(t *testing.T) {
 			})
 		}
 	})
+
+	t.Run("streaming injected tool call edge cases", func(t *testing.T) {
+		t.Parallel()
+
+		cases := []struct {
+			name         string
+			fixture      []byte
+			expectedArgs map[string]any
+		}{
+			{
+				name:         "tool call no preamble",
+				fixture:      fixtures.OaiChatStreamingInjectedToolNoPreamble,
+				expectedArgs: map[string]any{"owner": "me"},
+			},
+			{
+				name:         "tool call with non-zero index",
+				fixture:      fixtures.OaiChatStreamingInjectedToolNonzeroIndex,
+				expectedArgs: nil, // No arguments in this fixture
+			},
+		}
+
+		for _, tc := range cases {
+			t.Run(tc.name, func(t *testing.T) {
+				t.Parallel()
+
+				arc := txtar.Parse(tc.fixture)
+				t.Logf("%s: %s", t.Name(), arc.Comment)
+
+				files := filesMap(arc)
+				require.Len(t, files, 3)
+				require.Contains(t, files, fixtureRequest)
+				require.Contains(t, files, fixtureStreamingResponse)
+				require.Contains(t, files, fixtureStreamingToolResponse)
+
+				reqBody := files[fixtureRequest]
+
+				// Add the stream param to the request.
+				newBody, err := setJSON(reqBody, "stream", true)
+				require.NoError(t, err)
+				reqBody = newBody
+
+				ctx, cancel := context.WithTimeout(t.Context(), time.Second*30)
+				t.Cleanup(cancel)
+
+				// Setup mock server with response mutator for multi-turn interaction.
+				srv := newMockServer(ctx, t, files, func(reqCount uint32, resp []byte) []byte {
+					if reqCount == 1 {
+						// First request gets the tool call response
+						return resp
+					}
+					// Second request gets final response
+					return files[fixtureStreamingToolResponse]
+				})
+				t.Cleanup(srv.Close)
+
+				recorderClient := &testutil.MockRecorder{}
+
+				// Setup MCP proxies with the tool from the fixture
+				mcpProxiers, mcpCalls := setupMCPServerProxiesForTest(t, testTracer)
+				mcpMgr := mcp.NewServerProxyManager(mcpProxiers, testTracer)
+				require.NoError(t, mcpMgr.Init(ctx))
+
+				logger := slogtest.Make(t, &slogtest.Options{IgnoreErrors: false}).Leveled(slog.LevelDebug)
+				providers := []aibridge.Provider{provider.NewOpenAI(openaiCfg(srv.URL, apiKey))}
+				b, err := aibridge.NewRequestBridge(t.Context(), providers, recorderClient, mcpMgr, logger, nil, testTracer)
+				require.NoError(t, err)
+
+				mockSrv := httptest.NewUnstartedServer(b)
+				t.Cleanup(mockSrv.Close)
+				mockSrv.Config.BaseContext = func(_ net.Listener) context.Context {
+					return aibcontext.AsActor(ctx, userID, nil)
+				}
+				mockSrv.Start()
+
+				req := createOpenAIChatCompletionsReq(t, mockSrv.URL, reqBody)
+
+				client := &http.Client{}
+				resp, err := client.Do(req)
+				require.NoError(t, err)
+				require.Equal(t, http.StatusOK, resp.StatusCode)
+
+				// Consume the full response body to ensure the interception completes
+				_, err = io.ReadAll(resp.Body)
+				require.NoError(t, err)
+				resp.Body.Close()
+
+				// Verify the MCP tool was actually invoked
+				invocations := mcpCalls.getCallsByTool(mockToolName)
+				require.Len(t, invocations, 1, "expected MCP tool to be invoked")
+
+				// Verify tool was invoked with the expected args (if specified)
+				if tc.expectedArgs != nil {
+					expected, err := json.Marshal(tc.expectedArgs)
+					require.NoError(t, err)
+					actual, err := json.Marshal(invocations[0])
+					require.NoError(t, err)
+					require.EqualValues(t, expected, actual)
+				}
+
+				// Verify tool usage was recorded
+				toolUsages := recorderClient.RecordedToolUsages()
+				require.Len(t, toolUsages, 1)
+				assert.Equal(t, mockToolName, toolUsages[0].Tool)
+
+				recorderClient.VerifyAllInterceptionsEnded(t)
+			})
+		}
+	})
 }
 
 func TestSimple(t *testing.T) {

fixtures/fixtures.go

@@ -45,6 +45,12 @@ var (
 
 	//go:embed openai/chatcompletions/non_stream_error.txtar
 	OaiChatNonStreamError []byte
+
+	//go:embed openai/chatcompletions/streaming_injected_tool_no_preamble.txtar
+	OaiChatStreamingInjectedToolNoPreamble []byte
+
+	//go:embed openai/chatcompletions/streaming_injected_tool_nonzero_index.txtar
+	OaiChatStreamingInjectedToolNonzeroIndex []byte
 )
 
 var (

fixtures/openai/chatcompletions/streaming_injected_tool_no_preamble.txtar

@@ -0,0 +1,70 @@
+Streaming response where the provider returns an injected tool call as the first chunk with no text preamble.
+This test ensures tool invocation continues even when no chunks are relayed to the client.
+
+-- request --
+{
+    "messages": [
+      {
+        "content": "<current_datetime>2026-01-22T18:35:17.612Z</current_datetime>\n\nlist all my coder workspaces",
+        "role": "user"
+      }
+    ],
+    "model": "claude-haiku-4.5",
+    "n": 1,
+    "temperature": 1,
+    "parallel_tool_calls": false,
+    "stream_options": {
+        "include_usage": true
+    },
+    "stream": true
+}
+
+-- streaming --
+data: {"choices":[{"index":0,"delta":{"content":null,"tool_calls":[{"function":{"name":"bmcp_coder_coder_list_workspaces"},"id":"toolu_vrtx_01CvBi1d4qpKTG2PCuc9wDbZ","index":0,"type":"function"}]}}],"created":1769106921,"id":"msg_vrtx_01UoiRJwj3JXcwNYAh3z7ARs","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"index":0,"delta":{"content":null,"tool_calls":[{"function":{"arguments":""},"index":0}]}}],"created":1769106921,"id":"msg_vrtx_01UoiRJwj3JXcwNYAh3z7ARs","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"index":0,"delta":{"content":null,"tool_calls":[{"function":{"arguments":"{\"own"},"index":0}]}}],"created":1769106921,"id":"msg_vrtx_01UoiRJwj3JXcwNYAh3z7ARs","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"index":0,"delta":{"content":null,"tool_calls":[{"function":{"arguments":"er\": \"me\"}"},"index":0}]}}],"created":1769106921,"id":"msg_vrtx_01UoiRJwj3JXcwNYAh3z7ARs","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"finish_reason":"tool_calls","index":0,"delta":{"content":null}}],"created":1769106921,"id":"msg_vrtx_01UoiRJwj3JXcwNYAh3z7ARs","usage":{"completion_tokens":65,"prompt_tokens":25716,"prompt_tokens_details":{"cached_tokens":20470},"total_tokens":25781},"model":"claude-haiku-4.5"}
+
+data: [DONE]
+
+-- streaming/tool-call --
+data: {"choices":[{"index":0,"delta":{"content":"You","role":"assistant"}}],"created":1769198061,"id":"msg_vrtx_015B1npskreQgEjMrfsdjH1m","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"index":0,"delta":{"content":" have one","role":"assistant"}}],"created":1769198061,"id":"msg_vrtx_015B1npskreQgEjMrfsdjH1m","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"index":0,"delta":{"content":" Coder workspace:","role":"assistant"}}],"created":1769198062,"id":"msg_vrtx_015B1npskreQgEjMrfsdjH1m","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"index":0,"delta":{"content":"\n\n**test-scf** (","role":"assistant"}}],"created":1769198062,"id":"msg_vrtx_015B1npskreQgEjMrfsdjH1m","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"index":0,"delta":{"content":"ID: a174a2e5","role":"assistant"}}],"created":1769198062,"id":"msg_vrtx_015B1npskreQgEjMrfsdjH1m","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"index":0,"delta":{"content":"-5050-445d-89","role":"assistant"}}],"created":1769198062,"id":"msg_vrtx_015B1npskreQgEjMrfsdjH1m","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"index":0,"delta":{"content":"ff-dd720e5b442","role":"assistant"}}],"created":1769198062,"id":"msg_vrtx_015B1npskreQgEjMrfsdjH1m","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"index":0,"delta":{"content":"e)\n- Template: docker","role":"assistant"}}],"created":1769198062,"id":"msg_vrtx_015B1npskreQgEjMrfsdjH1m","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"index":0,"delta":{"content":"\n- Template Version","role":"assistant"}}],"created":1769198062,"id":"msg_vrtx_015B1npskreQgEjMrfsdjH1m","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"index":0,"delta":{"content":" ID","role":"assistant"}}],"created":1769198062,"id":"msg_vrtx_015B1npskreQgEjMrfsdjH1m","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"index":0,"delta":{"content":": ad1b5ab1-","role":"assistant"}}],"created":1769198062,"id":"msg_vrtx_015B1npskreQgEjMrfsdjH1m","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"index":0,"delta":{"content":"fc18-4792-84f","role":"assistant"}}],"created":1769198062,"id":"msg_vrtx_015B1npskreQgEjMrfsdjH1m","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"index":0,"delta":{"content":"7-797787607d30","role":"assistant"}}],"created":1769198062,"id":"msg_vrtx_015B1npskreQgEjMrfsdjH1m","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"index":0,"delta":{"content":"\n- Status","role":"assistant"}}],"created":1769198062,"id":"msg_vrtx_015B1npskreQgEjMrfsdjH1m","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"index":0,"delta":{"content":": Up","role":"assistant"}}],"created":1769198062,"id":"msg_vrtx_015B1npskreQgEjMrfsdjH1m","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"index":0,"delta":{"content":" to date","role":"assistant"}}],"created":1769198062,"id":"msg_vrtx_015B1npskreQgEjMrfsdjH1m","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"finish_reason":"stop","index":0,"delta":{"content":null}}],"created":1769198062,"id":"msg_vrtx_015B1npskreQgEjMrfsdjH1m","usage":{"completion_tokens":85,"prompt_tokens":25989,"prompt_tokens_details":{"cached_tokens":0},"total_tokens":26074},"model":"claude-haiku-4.5"}
+
+data: [DONE]

fixtures/openai/chatcompletions/streaming_injected_tool_nonzero_index.txtar

@@ -0,0 +1,69 @@
+Streaming response where the provider returns text content followed by an injected tool call at index 1 (instead of index 0).
+This can happen when the provider incorrectly continues indexing from a previous response.
+This tests that nil entries are removed from the tool calls array caused by non-zero starting indices.
+
+-- request --
+{
+    "messages": [
+      {
+        "content": "<current_datetime>2026-01-23T20:22:43.781Z</current_datetime>\n\nI want you to do to this in order:\n1) create a file in my current directory with name \"test.txt\"\n2) list all my coder workspaces",
+        "role": "user"
+      }
+    ],
+    "model": "claude-haiku-4.5",
+    "n": 1,
+    "temperature": 1,
+    "parallel_tool_calls": false,
+    "stream_options": {
+    "include_usage": true
+    },
+    "stream": true
+}
+
+-- streaming --
+data: {"choices":[{"index":0,"delta":{"content":"Now","role":"assistant"}}],"created":1769199774,"id":"msg_vrtx_01Fiieb5Z3kqJf9a3FwvLkky","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"index":0,"delta":{"content":" listing","role":"assistant"}}],"created":1769199774,"id":"msg_vrtx_01Fiieb5Z3kqJf9a3FwvLkky","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"index":0,"delta":{"content":" your","role":"assistant"}}],"created":1769199774,"id":"msg_vrtx_01Fiieb5Z3kqJf9a3FwvLkky","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"index":0,"delta":{"content":" C","role":"assistant"}}],"created":1769199774,"id":"msg_vrtx_01Fiieb5Z3kqJf9a3FwvLkky","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"index":0,"delta":{"content":"oder workspaces:","role":"assistant"}}],"created":1769199774,"id":"msg_vrtx_01Fiieb5Z3kqJf9a3FwvLkky","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"index":0,"delta":{"content":null,"tool_calls":[{"function":{"name":"bmcp_coder_coder_list_workspaces"},"id":"toolu_vrtx_01DbFqUgk6aAtJ4nDBqzFWDF","index":1,"type":"function"}]}}],"created":1769199774,"id":"msg_vrtx_01Fiieb5Z3kqJf9a3FwvLkky","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"index":0,"delta":{"content":null,"tool_calls":[{"function":{"arguments":""},"index":1}]}}],"created":1769199774,"id":"msg_vrtx_01Fiieb5Z3kqJf9a3FwvLkky","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"finish_reason":"tool_calls","index":0,"delta":{"content":null}}],"created":1769199774,"id":"msg_vrtx_01Fiieb5Z3kqJf9a3FwvLkky","usage":{"completion_tokens":58,"prompt_tokens":25939,"prompt_tokens_details":{"cached_tokens":25429},"total_tokens":25997},"model":"claude-haiku-4.5"}
+
+data: [DONE]
+
+-- streaming/tool-call --
+data: {"choices":[{"index":0,"delta":{"content":"Done","role":"assistant"}}],"created":1769199776,"id":"msg_vrtx_01RVxamMyw1DBtpoENDpmnQK","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"index":0,"delta":{"content":"! I create","role":"assistant"}}],"created":1769199776,"id":"msg_vrtx_01RVxamMyw1DBtpoENDpmnQK","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"index":0,"delta":{"content":"d `","role":"assistant"}}],"created":1769199776,"id":"msg_vrtx_01RVxamMyw1DBtpoENDpmnQK","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"index":0,"delta":{"content":"test.txt` in","role":"assistant"}}],"created":1769199776,"id":"msg_vrtx_01RVxamMyw1DBtpoENDpmnQK","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"index":0,"delta":{"content":" your current directory.","role":"assistant"}}],"created":1769199776,"id":"msg_vrtx_01RVxamMyw1DBtpoENDpmnQK","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"index":0,"delta":{"content":" You","role":"assistant"}}],"created":1769199776,"id":"msg_vrtx_01RVxamMyw1DBtpoENDpmnQK","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"index":0,"delta":{"content":" have","role":"assistant"}}],"created":1769199776,"id":"msg_vrtx_01RVxamMyw1DBtpoENDpmnQK","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"index":0,"delta":{"content":" 1","role":"assistant"}}],"created":1769199776,"id":"msg_vrtx_01RVxamMyw1DBtpoENDpmnQK","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"index":0,"delta":{"content":" ","role":"assistant"}}],"created":1769199776,"id":"msg_vrtx_01RVxamMyw1DBtpoENDpmnQK","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"index":0,"delta":{"content":"Coder workspace:\n\n-","role":"assistant"}}],"created":1769199776,"id":"msg_vrtx_01RVxamMyw1DBtpoENDpmnQK","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"index":0,"delta":{"content":" **test-scf** (docker","role":"assistant"}}],"created":1769199776,"id":"msg_vrtx_01RVxamMyw1DBtpoENDpmnQK","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"index":0,"delta":{"content":" template)","role":"assistant"}}],"created":1769199776,"id":"msg_vrtx_01RVxamMyw1DBtpoENDpmnQK","model":"claude-haiku-4.5"}
+
+data: {"choices":[{"finish_reason":"stop","index":0,"delta":{"content":null}}],"created":1769199776,"id":"msg_vrtx_01RVxamMyw1DBtpoENDpmnQK","usage":{"completion_tokens":39,"prompt_tokens":26166,"prompt_tokens_details":{"cached_tokens":25934},"total_tokens":26205},"model":"claude-haiku-4.5"}
+
+data: [DONE]

intercept/chatcompletions/streaming.go

@@ -7,6 +7,7 @@ import (
 	"errors"
 	"fmt"
 	"net/http"
+	"slices"
 	"strings"
 	"time"
 
@@ -148,16 +149,24 @@ func (i *StreamingInterception) ProcessRequest(w http.ResponseWriter, r *http.Re
 			}
 		}
 
-		// Builtin tools are not intercepted.
-		if toolCall != nil && i.getInjectedToolByName(toolCall.Name) == nil {
-			_ = i.recorder.RecordToolUsage(streamCtx, &recorder.ToolUsageRecord{
-				InterceptionID: i.ID().String(),
-				MsgID:          processor.getMsgID(),
-				Tool:           toolCall.Name,
-				Args:           i.unmarshalArgs(toolCall.Arguments),
-				Injected:       false,
-			})
-			toolCall = nil
+		if toolCall != nil {
+			// Builtin tools are not intercepted.
+			if i.getInjectedToolByName(toolCall.Name) == nil {
+				_ = i.recorder.RecordToolUsage(streamCtx, &recorder.ToolUsageRecord{
+					InterceptionID: i.ID().String(),
+					MsgID:          processor.getMsgID(),
+					Tool:           toolCall.Name,
+					Args:           i.unmarshalArgs(toolCall.Arguments),
+					Injected:       false,
+				})
+				toolCall = nil
+			} else {
+				// Injected tools mark the stream as initiated so we continue to tool invocation.
+				// When the provider responds with a tool call as the first chunk (no text
+				// preamble), no chunks are relayed to the client. Marking as initiated
+				// ensures we continue to tool invocation instead of returning early.
+				events.MarkInitiated()
+			}
 		}
 
 		if prompt != nil {
@@ -239,7 +248,13 @@ func (i *StreamingInterception) ProcessRequest(w http.ResponseWriter, r *http.Re
 
 		// Invoke the injected tool, and use the tool result to make a subsequent request to the upstream.
 		// Append the completion from this stream as context.
-		i.req.Messages = append(i.req.Messages, processor.getLastCompletion().ToParam())
+		// Some providers may return tool calls with non-zero starting indices,
+		// resulting in nil entries in the array that must be removed.
+		completion := processor.getLastCompletion()
+		if completion != nil {
+			compactToolCalls(completion)
+			i.req.Messages = append(i.req.Messages, completion.ToParam())
+		}
 
 		id := toolCall.ID
 		args := i.unmarshalArgs(toolCall.Arguments)
@@ -486,3 +501,13 @@ func (s *streamProcessor) getLastUsage() openai.CompletionUsage {
 func (s *streamProcessor) getCumulativeUsage() openai.CompletionUsage {
 	return s.cumulativeUsage
 }
+
+// compactToolCalls removes nil/empty tool call entries (without an ID).
+func compactToolCalls(msg *openai.ChatCompletionMessage) {
+	if msg == nil || len(msg.ToolCalls) == 0 {
+		return
+	}
+	msg.ToolCalls = slices.DeleteFunc(msg.ToolCalls, func(tc openai.ChatCompletionMessageToolCallUnion) bool {
+		return tc.ID == ""
+	})
+}

intercept/eventstream/eventstream.go

@@ -199,6 +199,15 @@ func (s *EventStream) IsStreaming() bool {
 	return s.initiated.Load() || len(s.eventsCh) > 0
 }
 
+// MarkInitiated marks the stream as initiated, even if no events have been
+// sent to the client yet. A stream is considered initiated when processing
+// injected tool calls that don't relay chunks to the client.
+func (s *EventStream) MarkInitiated() {
+	s.initiateOnce.Do(func() {
+		s.initiated.Store(true)
+	})
+}
+
 // IsConnError checks if an error is related to client disconnection or context cancellation.
 func IsConnError(err error) bool {
 	if err == nil {