fix(fireworks): preserve malformed tool arguments as diagnostic JSON (#40818)

`ChatFireworks` can replay historical tool calls with malformed or
non-object JSON arguments without forwarding invalid argument strings to
Fireworks.

---

Fireworks rejects conversation history containing malformed or
non-object tool-call arguments, preventing an agent from recovering on
its next turn.

Wrap these arguments in a JSON object under
`__invalid_tool_call_arguments` when serializing history, preserving the
original payload, call IDs, and tool-result pairing. Apply this to
parsed and raw tool-call history without mutating messages or executing
repaired arguments; valid object strings stay unchanged.

Made by [Open SWE](https://github.com/langchain-ai/open-swe) · [view
thread](https://openswe.vercel.app/agents/0da06f8d-3ea1-5f01-aa55-953acb9a71fa)
· openai:gpt-6-astra (medium)

Co-authored-by: Mason Daugherty <mdrxy@users.noreply.github.com>
Co-authored-by: open-swe[bot] <open-swe@users.noreply.github.com>

libs/partners/fireworks/langchain_fireworks/chat_models.py

@@ -343,6 +343,21 @@ def _format_message_content(content: Any) -> Any:
     return formatted
 
 
+def _format_tool_call_arguments(arguments: str | dict | None) -> str | dict:
+    """Preserve invalid historical arguments inside a JSON object for replay."""
+    if isinstance(arguments, dict):
+        return arguments
+    try:
+        parsed = json.loads(arguments) if arguments is not None else None
+        if isinstance(parsed, dict) and arguments is not None:
+            json.dumps(parsed, allow_nan=False)
+            return arguments
+    except ValueError:
+        logger.debug("Invalid JSON in historical Fireworks tool call arguments")
+    logger.warning("Wrapping invalid historical Fireworks tool call arguments")
+    return json.dumps({"__invalid_tool_call_arguments": arguments}, ensure_ascii=False)
+
+
 def _convert_message_to_dict(message: BaseMessage) -> dict:
     """Convert a LangChain message to a dictionary.
 
@@ -392,6 +407,19 @@ def _convert_message_to_dict(message: BaseMessage) -> dict:
             ]
         elif "tool_calls" in message.additional_kwargs:
             message_dict["tool_calls"] = message.additional_kwargs["tool_calls"]
+        if "tool_calls" in message_dict:
+            message_dict["tool_calls"] = [
+                {
+                    **tool_call,
+                    "function": {
+                        **tool_call["function"],
+                        "arguments": _format_tool_call_arguments(
+                            tool_call["function"]["arguments"]
+                        ),
+                    },
+                }
+                for tool_call in message_dict["tool_calls"]
+            ]
         # If tool calls only, content is None not empty string
         if "tool_calls" in message_dict and message_dict["content"] == "":
             message_dict["content"] = None

libs/partners/fireworks/tests/unit_tests/test_chat_models.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import json
 import logging
 import os
 from typing import Any
@@ -205,12 +206,71 @@ def test_convert_v1_message_filters_invalid_tool_call_content() -> None:
             {
                 "type": "function",
                 "id": "call_invalid",
-                "function": {"name": "get_weather", "arguments": '{"city":'},
+                "function": {
+                    "name": "get_weather",
+                    "arguments": json.dumps(
+                        {"__invalid_tool_call_arguments": '{"city":'}
+                    ),
+                },
             }
         ],
     }
 
 
+@pytest.mark.parametrize("raw_history", [False, True])
+@pytest.mark.parametrize(
+    ("arguments", "wrapped"),
+    [
+        ('{"city":', True),
+        ("[]", True),
+        ("null", True),
+        (None, True),
+        ('{"x": NaN}', True),
+        ('{"city": "Paris"}', False),
+    ],
+)
+def test_replay_invalid_tool_call_arguments(
+    arguments: str | None, *, wrapped: bool, raw_history: bool
+) -> None:
+    message = AIMessage(
+        content="",
+        tool_calls=[{"name": "get_weather", "args": {"city": "Paris"}, "id": "valid"}],
+        invalid_tool_calls=[
+            {"name": "get_weather", "args": arguments, "id": "invalid", "error": "bad"}
+        ],
+    )
+    if raw_history:
+        message.additional_kwargs["tool_calls"] = [
+            {
+                "type": "function",
+                "id": "invalid",
+                "function": {"name": "get_weather", "arguments": arguments},
+            }
+        ]
+        message.tool_calls = []
+        message.invalid_tool_calls = []
+    original = message.model_dump()
+
+    result = _convert_message_to_dict(message)
+
+    assert message.model_dump() == original
+    invalid = result["tool_calls"][-1]
+    assert invalid["id"] == "invalid"
+    assert invalid["function"]["name"] == "get_weather"
+    if wrapped:
+        assert json.loads(invalid["function"]["arguments"]) == {
+            "__invalid_tool_call_arguments": arguments
+        }
+    else:
+        assert invalid["function"]["arguments"] == arguments
+    if not raw_history:
+        assert json.loads(result["tool_calls"][0]["function"]["arguments"]) == {
+            "city": "Paris"
+        }
+    tool_result = ToolMessage(content="Invalid JSON", tool_call_id="invalid")
+    assert _convert_message_to_dict(tool_result)["tool_call_id"] == invalid["id"]
+
+
 def test_sanitize_chat_completions_content_passthrough_non_text_block() -> None:
     blocks = [{"type": "image_url", "image_url": {"url": "https://x/y.png"}}]
     assert _sanitize_chat_completions_content(blocks) == blocks