runtime/llm/sse.go

@@ -195,6 +195,10 @@ func emitParsed(ctx context.Context, r io.Reader, out chan<- Event) {
 	// A finish reason alone does not prove the tool arguments arrived intact.
 	// Preserve returned usage when rejecting a partial terminal, so the host
 	// can charge this call and stop rather than treating it as a free retry.
+	if finished == "tool_calls" && len(order) == 0 {
+		sendTerminalFailure("tool call finish reason without a tool call")
+		return
+	}
 	if len(order) > 0 {
 		problem := ""
 		switch {

runtime/llm/sse_test.go

@@ -93,6 +93,24 @@ func TestParseSSEToolCallIncrements(t *testing.T) {
 	}
 }
 
+func TestParseSSEToolFinishWithoutCallFails(t *testing.T) {
+	body := strings.Join([]string{
+		`data: {"choices":[{"delta":{},"finish_reason":"tool_calls"}],"usage":{"prompt_tokens":12,"completion_tokens":5,"total_tokens":17}}`,
+		`data: [DONE]`,
+		"",
+	}, "\n")
+	var end Event
+	for ev := range ParseSSE(context.Background(), strings.NewReader(body)) {
+		if ev.Kind == KindMessageEnd {
+			end = ev
+		}
+	}
+	if end.Err == nil || end.Message.StopReason != StopError || end.Usage.TotalTokens != 17 ||
+		!end.StreamComplete || len(end.Message.ToolCalls) != 0 {
+		t.Fatalf("empty tool finish must fail and retain Usage: %#v", end)
+	}
+}
+
 func TestParseSSELengthFailsUnfinishedToolCall(t *testing.T) {
 	body := strings.Join([]string{
 		`data: {"choices":[{"delta":{"tool_calls":[{"index":0,"id":"call_1","function":{"name":"read_file","arguments":"{\"path\":"}}]}}]}`,