Fix infinite loop in fix_message_list with multiple tool_calls

src/cai/util.py

@@ -1244,9 +1244,19 @@ def fix_message_list(messages):  # pylint: disable=R0914,R0915,R0912
 
             # If this isn't the first message, check if the previous message is a matching assistant message
             if i > 0:
-                prev_msg = processed_messages[i - 1]
-
-                # Check if the previous message is an assistant message with matching tool_call_id
+                # Walk backward past sibling tool messages to find the nearest
+                # assistant. This avoids an infinite loop when an assistant has
+                # multiple tool_calls and their responses arrive out of order:
+                # the previous message may be a sibling tool response rather
+                # than the parent assistant message, which is still valid.
+                k = i - 1
+                while k >= 0 and processed_messages[k].get("role") == "tool":
+                    k -= 1
+
+                prev_msg = processed_messages[k] if k >= 0 else {}
+
+                # Check if the nearest non-tool ancestor is an assistant message
+                # with a matching tool_call_id
                 is_valid_sequence = (
                     prev_msg.get("role") == "assistant"
                     and prev_msg.get("tool_calls")

tests/test_fix_message_list.py

@@ -0,0 +1,45 @@
+import signal
+
+import pytest
+
+from cai.util import fix_message_list
+
+
+def test_fix_message_list_handles_multiple_tool_results_without_loop():
+    if not hasattr(signal, "SIGALRM"):
+        pytest.skip("SIGALRM is required for this infinite-loop regression test")
+
+    messages = [
+        {"role": "user", "content": "Run both tools"},
+        {
+            "role": "assistant",
+            "content": None,
+            "tool_calls": [
+                {
+                    "id": "call_one",
+                    "type": "function",
+                    "function": {"name": "first_tool", "arguments": "{}"},
+                },
+                {
+                    "id": "call_two",
+                    "type": "function",
+                    "function": {"name": "second_tool", "arguments": "{}"},
+                },
+            ],
+        },
+        {"role": "tool", "tool_call_id": "call_one", "content": "first result"},
+        {"role": "tool", "tool_call_id": "call_two", "content": "second result"},
+    ]
+
+    def fail_on_timeout(_signum, _frame):
+        raise TimeoutError("fix_message_list did not terminate")
+
+    previous_handler = signal.signal(signal.SIGALRM, fail_on_timeout)
+    signal.alarm(2)
+    try:
+        fixed_messages = fix_message_list(messages)
+    finally:
+        signal.alarm(0)
+        signal.signal(signal.SIGALRM, previous_handler)
+
+    assert fixed_messages == messages