fix(agents): tell the model to re-issue tool calls whose arguments are not valid JSON

A streamed turn that ends mid-tool-call leaves the client with a JSON prefix
as the arguments. Instead of the SDK's generic parse-error result, the tool
wrapper now short-circuits with a clear 'not executed, re-issue with complete
arguments' message so the model recovers in one turn.

strix/agents/factory.py

@@ -18,6 +18,7 @@ from pydantic import ValidationError
 
 from strix.agents.prompt import render_system_prompt
 from strix.config import load_settings
+from strix.config.tool_call_arguments import describe_malformed_arguments
 from strix.tools.agents_graph.tools import (
     agent_finish,
     create_agent,
@@ -260,6 +261,10 @@ def _with_coerced_arguments(tool: FunctionTool) -> FunctionTool:
     nullish = tool.name.startswith(_QUERY_TOOL_PREFIXES)
 
     async def invoke(ctx: Any, raw_input: str) -> Any:
+        malformed = describe_malformed_arguments(tool.name, raw_input)
+        if malformed is not None:
+            logger.debug("Tool %s got malformed arguments; asking the model to re-issue", tool.name)
+            return malformed
         return await invoke_tool(ctx, _coerce_arguments(raw_input, schema, nullish=nullish))
 
     tool.on_invoke_tool = invoke

strix/config/tool_call_arguments.py

@@ -22,6 +22,31 @@ from openai.types.responses import ResponseFunctionToolCall
 MALFORMED_ARGUMENTS_KEY = "malformed_arguments"
 
 
+def describe_malformed_arguments(tool_name: str, arguments: str) -> str | None:
+    """Return a model-facing recovery message if ``arguments`` is not a JSON object.
+
+    A tool call whose arguments do not parse is almost always one the stream cut
+    off (the server flushed a prefix of the JSON, or the client closed early),
+    so instead of the SDK's generic parse-error result the model is told the
+    call never ran and must be re-issued whole.
+    """
+    if not arguments.strip():
+        return None
+    try:
+        parsed = json.loads(arguments)
+    except ValueError as exc:
+        detail = str(exc)
+    else:
+        if isinstance(parsed, dict):
+            return None
+        detail = f"expected a JSON object, got {type(parsed).__name__}"
+    return (
+        f"{tool_name}: the tool call was not executed because its arguments were "
+        f"truncated or otherwise not valid JSON ({detail}). The response was likely "
+        "cut off mid-call. Re-issue the call with complete, valid JSON arguments."
+    )
+
+
 def repair_arguments(arguments: object) -> str | None:
     """Return replacement arguments a strict server accepts, or ``None`` if already valid."""
     if not isinstance(arguments, str) or not arguments.strip():

tests/test_agent_factory_tool_arguments.py

@@ -38,6 +38,20 @@ async def _roundtrip(
 
 
 _STRING = {"todos": {"type": "string"}}
+
+
+@pytest.mark.asyncio
+async def test_truncated_arguments_short_circuit_with_a_reissue_message() -> None:
+    captured: dict[str, str] = {}
+    wrapped = factory._with_coerced_arguments(_capturing_tool(captured, _STRING))
+
+    result = await wrapped.on_invoke_tool(cast("Any", None), '{"todos": "a, b')
+
+    assert "not executed" in result
+    assert "Re-issue the call" in result
+    assert "raw_input" not in captured
+
+
 _ARRAY = {"tags": {"type": "array", "items": {"type": "string"}}}
 _NULLABLE_ARRAY = {
     "tags": {"anyOf": [{"type": "array", "items": {"type": "string"}}, {"type": "null"}]}
@@ -136,12 +150,14 @@ async def test_unknown_and_null_arguments_are_untouched() -> None:
 
 
 @pytest.mark.asyncio
-async def test_non_object_payloads_pass_through_unchanged() -> None:
+async def test_non_object_payloads_are_reported_instead_of_invoked() -> None:
     captured: dict[str, str] = {}
     wrapped = factory._with_coerced_arguments(_capturing_tool(captured, _ARRAY))
 
-    assert await wrapped.on_invoke_tool(cast("Any", None), "not json") == "ok"
+    result = await wrapped.on_invoke_tool(cast("Any", None), "not json")
-    assert captured["raw_input"] == "not json"
+
+    assert result.startswith("probe: the tool call was not executed")
+    assert "raw_input" not in captured
 
 
 @pytest.mark.asyncio

tests/test_tool_call_arguments.py

@@ -25,6 +25,7 @@ from openai.types.responses import ResponseFunctionToolCall
 from strix.config.models import _NonStreamingModel, _TurnGuardModel
 from strix.config.tool_call_arguments import (
     MALFORMED_ARGUMENTS_KEY,
+    describe_malformed_arguments,
     repair_arguments,
     repair_history_arguments,
     repair_input,
@@ -244,3 +245,17 @@ def test_repair_input_returns_same_object_when_nothing_changes() -> None:
 
     assert repair_input(items) is items
     assert repair_input("plain prompt") == "plain prompt"
+
+
+@pytest.mark.parametrize("arguments", ['{"cmd": "ls -la', "[1, 2]", "null", "not json"])
+def test_describe_malformed_arguments_tells_the_model_to_reissue(arguments: str) -> None:
+    message = describe_malformed_arguments("exec_command", arguments)
+
+    assert message is not None
+    assert message.startswith("exec_command: the tool call was not executed")
+    assert "Re-issue the call" in message
+
+
+@pytest.mark.parametrize("arguments", ["", "   ", "{}", '{"cmd": "ls"}'])
+def test_describe_malformed_arguments_accepts_objects_and_empty_input(arguments: str) -> None:
+    assert describe_malformed_arguments("exec_command", arguments) is None