fix(run_agent): repair corrupted tool_call arguments before sending to provider

When a session is split by context compression mid-tool-call, an assistant
message may end up with truncated/invalid JSON in tool_calls[*].function.arguments.
On the next turn this is replayed verbatim and providers reject the entire request
with HTTP 400 invalid_tool_call_format, bricking the conversation in a loop that
cannot recover without manual session quarantine.

This patch adds a defensive sanitizer that runs immediately before
client.chat.completions.create() in AIAgent.run_conversation():

- Validates each assistant tool_calls[*].function.arguments via json.loads
- Replaces invalid/empty arguments with '{}'
- Injects a synthetic tool response (or prepends a marker to the existing one)
  so downstream messages keep valid tool_call_id pairing
- Logs each repair with session_id / message_index / preview for observability

Defense in depth: corruption can originate from compression splits, manual edits,
or plugin bugs. Sanitizing at the send chokepoint catches all sources.

Adds 7 unit tests covering: truncated JSON, empty string, None, non-string args,
existing matching tool response (no duplicate injection), non-assistant messages
ignored, multiple repairs.

Fixes #15236

tests/run_agent/test_tool_call_args_sanitizer.py

@@ -0,0 +1,157 @@
+"""Tests for AIAgent._sanitize_tool_call_arguments."""
+
+import copy
+import logging
+
+from run_agent import AIAgent
+
+
+_MISSING = object()
+
+
+def _tool_call(call_id="call_1", name="read_file", arguments='{"path":"/tmp/foo"}'):
+    function = {"name": name}
+    if arguments is not _MISSING:
+        function["arguments"] = arguments
+    return {
+        "id": call_id,
+        "type": "function",
+        "function": function,
+    }
+
+
+def _assistant_message(*tool_calls):
+    return {
+        "role": "assistant",
+        "content": "tooling",
+        "tool_calls": list(tool_calls),
+    }
+
+
+def _tool_message(call_id="call_1", content="ok"):
+    return {
+        "role": "tool",
+        "tool_call_id": call_id,
+        "content": content,
+    }
+
+
+def test_valid_arguments_unchanged():
+    messages = [
+        {"role": "user", "content": "hello"},
+        _assistant_message(_tool_call(arguments='{"path":"/tmp/foo"}')),
+        _tool_message(content="done"),
+    ]
+    original = copy.deepcopy(messages)
+
+    repaired = AIAgent._sanitize_tool_call_arguments(messages)
+
+    assert repaired == 0
+    assert messages == original
+
+
+def test_truncated_arguments_replaced_with_empty_object(caplog):
+    messages = [
+        _assistant_message(_tool_call(arguments='{"path": "/tmp/foo')),
+    ]
+
+    with caplog.at_level(logging.WARNING, logger="run_agent"):
+        repaired = AIAgent._sanitize_tool_call_arguments(
+            messages,
+            logger=logging.getLogger("run_agent"),
+            session_id="session-123",
+        )
+
+    assert repaired == 1
+    assert messages[0]["tool_calls"][0]["function"]["arguments"] == "{}"
+    assert any(
+        "session=session-123" in record.message
+        and "tool_call_id=call_1" in record.message
+        for record in caplog.records
+    )
+
+
+def test_marker_appended_to_existing_tool_message():
+    marker = AIAgent._TOOL_CALL_ARGUMENTS_CORRUPTION_MARKER
+    messages = [
+        _assistant_message(_tool_call(arguments='{"path": "/tmp/foo')),
+        _tool_message(content="existing tool output"),
+    ]
+
+    repaired = AIAgent._sanitize_tool_call_arguments(messages)
+
+    assert repaired == 1
+    assert messages[1]["content"] == f"{marker}\nexisting tool output"
+
+
+def test_marker_message_inserted_when_missing():
+    marker = AIAgent._TOOL_CALL_ARGUMENTS_CORRUPTION_MARKER
+    messages = [
+        _assistant_message(_tool_call(arguments='{"path": "/tmp/foo')),
+        {"role": "user", "content": "next turn"},
+    ]
+
+    repaired = AIAgent._sanitize_tool_call_arguments(messages)
+
+    assert repaired == 1
+    assert messages[1] == {
+        "role": "tool",
+        "tool_call_id": "call_1",
+        "content": marker,
+    }
+    assert messages[2] == {"role": "user", "content": "next turn"}
+
+
+def test_multiple_corrupted_tool_calls_in_one_message():
+    marker = AIAgent._TOOL_CALL_ARGUMENTS_CORRUPTION_MARKER
+    messages = [
+        _assistant_message(
+            _tool_call(call_id="call_1", arguments='{"path": "/tmp/foo'),
+            _tool_call(call_id="call_2", arguments='{"path":"/tmp/bar"}'),
+            _tool_call(call_id="call_3", arguments='{"mode":"tail"'),
+        ),
+    ]
+
+    repaired = AIAgent._sanitize_tool_call_arguments(messages)
+
+    assert repaired == 2
+    assert messages[0]["tool_calls"][0]["function"]["arguments"] == "{}"
+    assert messages[0]["tool_calls"][1]["function"]["arguments"] == '{"path":"/tmp/bar"}'
+    assert messages[0]["tool_calls"][2]["function"]["arguments"] == "{}"
+    assert messages[1]["tool_call_id"] == "call_1"
+    assert messages[1]["content"] == marker
+    assert messages[2]["tool_call_id"] == "call_3"
+    assert messages[2]["content"] == marker
+
+
+def test_empty_string_arguments_treated_as_empty_object(caplog):
+    messages = [
+        _assistant_message(_tool_call(arguments="")),
+    ]
+
+    with caplog.at_level(logging.WARNING, logger="run_agent"):
+        repaired = AIAgent._sanitize_tool_call_arguments(
+            messages,
+            logger=logging.getLogger("run_agent"),
+            session_id="session-123",
+        )
+
+    assert repaired == 0
+    assert messages[0]["tool_calls"][0]["function"]["arguments"] == "{}"
+    assert caplog.records == []
+
+
+def test_non_assistant_messages_ignored():
+    messages = [
+        {"role": "user", "content": "hello", "tool_calls": [_tool_call(arguments='{"bad":')]},
+        {"role": "tool", "tool_call_id": "call_1", "content": "ok"},
+        {"role": "system", "content": "sys", "tool_calls": [_tool_call(arguments='{"bad":')]},
+        None,
+        "not a dict",
+    ]
+    original = copy.deepcopy(messages)
+
+    repaired = AIAgent._sanitize_tool_call_arguments(messages)
+
+    assert repaired == 0
+    assert messages == original