fix(ui): stabilize issue chat message identities

ui/src/components/IssueChatThread.tsx

@@ -36,8 +36,10 @@ import { usePaperclipIssueRuntime, type PaperclipIssueRuntimeReassignment } from
 import {
   buildIssueChatMessages,
   formatDurationWords,
+  stabilizeThreadMessages,
   type IssueChatComment,
   type IssueChatLinkedRun,
+  type StableThreadMessageCacheEntry,
   type IssueChatTranscriptEntry,
   type SegmentTiming,
 } from "../lib/issue-chat-messages";
@@ -1925,8 +1927,7 @@ export function IssueChatThread({
   });
   const resolvedTranscriptByRun = transcriptsByRunId ?? transcriptByRun;
   const resolvedHasOutputForRun = hasOutputForRunOverride ?? hasOutputForRun;
-
-  const messages = useMemo(
+  const rawMessages = useMemo(
     () =>
       buildIssueChatMessages({
         comments,
@@ -1957,6 +1958,18 @@ export function IssueChatThread({
       currentUserId,
     ],
   );
+  const stableMessagesRef = useRef<readonly import("@assistant-ui/react").ThreadMessage[]>([]);
+  const stableMessageCacheRef = useRef<Map<string, StableThreadMessageCacheEntry>>(new Map());
+  const messages = useMemo(() => {
+    const stabilized = stabilizeThreadMessages(
+      rawMessages,
+      stableMessagesRef.current,
+      stableMessageCacheRef.current,
+    );
+    stableMessagesRef.current = stabilized.messages;
+    stableMessageCacheRef.current = stabilized.cache;
+    return stabilized.messages;
+  }, [rawMessages]);
 
   const isRunning = displayLiveRuns.some((run) => run.status === "queued" || run.status === "running");
   const feedbackVoteByTargetId = useMemo(() => {

ui/src/lib/issue-chat-messages.test.ts

@@ -3,6 +3,7 @@ import type { Agent } from "@paperclipai/shared";
 import {
   buildAssistantPartsFromTranscript,
   buildIssueChatMessages,
+  stabilizeThreadMessages,
   type IssueChatComment,
   type IssueChatLinkedRun,
 } from "./issue-chat-messages";
@@ -527,3 +528,69 @@ describe("buildIssueChatMessages", () => {
     });
   });
 });
+
+describe("stabilizeThreadMessages", () => {
+  it("reuses unchanged message objects across rebuilds", () => {
+    const firstPass = buildIssueChatMessages({
+      comments: [createComment()],
+      timelineEvents: [],
+      linkedRuns: [],
+      liveRuns: [],
+      currentUserId: "user-1",
+    });
+
+    const firstStable = stabilizeThreadMessages(firstPass, [], new Map());
+    const secondPass = buildIssueChatMessages({
+      comments: [
+        createComment(),
+        createComment({
+          id: "comment-2",
+          body: "New message",
+          createdAt: new Date("2026-04-06T12:01:00.000Z"),
+          updatedAt: new Date("2026-04-06T12:01:00.000Z"),
+        }),
+      ],
+      timelineEvents: [],
+      linkedRuns: [],
+      liveRuns: [],
+      currentUserId: "user-1",
+    });
+
+    const secondStable = stabilizeThreadMessages(
+      secondPass,
+      firstStable.messages,
+      firstStable.cache,
+    );
+
+    expect(secondStable.messages).toHaveLength(2);
+    expect(secondStable.messages[0]).toBe(firstStable.messages[0]);
+    expect(secondStable.messages[1]?.id).toBe("comment-2");
+  });
+
+  it("reuses the previous array when nothing semantically changed", () => {
+    const firstPass = buildIssueChatMessages({
+      comments: [createComment()],
+      timelineEvents: [],
+      linkedRuns: [],
+      liveRuns: [],
+      currentUserId: "user-1",
+    });
+
+    const firstStable = stabilizeThreadMessages(firstPass, [], new Map());
+    const secondPass = buildIssueChatMessages({
+      comments: [createComment()],
+      timelineEvents: [],
+      linkedRuns: [],
+      liveRuns: [],
+      currentUserId: "user-1",
+    });
+
+    const secondStable = stabilizeThreadMessages(
+      secondPass,
+      firstStable.messages,
+      firstStable.cache,
+    );
+
+    expect(secondStable.messages).toBe(firstStable.messages);
+  });
+});

ui/src/lib/issue-chat-messages.ts

@@ -81,6 +81,11 @@ type MessageWithOrder = {
   message: ThreadMessage;
 };
 
+export interface StableThreadMessageCacheEntry {
+  fingerprint: string;
+  message: ThreadMessage;
+}
+
 function toDate(value: Date | string | null | undefined) {
   return value instanceof Date ? value : new Date(value ?? Date.now());
 }
@@ -89,6 +94,41 @@ function toTimestamp(value: Date | string | null | undefined) {
   return toDate(value).getTime();
 }
 
+function fingerprintThreadMessage(message: ThreadMessage) {
+  return JSON.stringify(message);
+}
+
+export function stabilizeThreadMessages(
+  messages: readonly ThreadMessage[],
+  previousMessages: readonly ThreadMessage[],
+  previousById: ReadonlyMap<string, StableThreadMessageCacheEntry>,
+) {
+  const nextById = new Map<string, StableThreadMessageCacheEntry>();
+  let sameSequence = previousMessages.length === messages.length;
+
+  const stabilizedMessages = messages.map((message, index) => {
+    const fingerprint = fingerprintThreadMessage(message);
+    const cached = previousById.get(message.id);
+    const stableMessage =
+      cached && cached.fingerprint === fingerprint
+        ? cached.message
+        : message;
+    nextById.set(message.id, {
+      fingerprint,
+      message: stableMessage,
+    });
+    if (sameSequence && previousMessages[index] !== stableMessage) {
+      sameSequence = false;
+    }
+    return stableMessage;
+  });
+
+  return {
+    messages: sameSequence ? previousMessages : stabilizedMessages,
+    cache: nextById,
+  };
+}
+
 function sortByCreated<T extends { createdAt: Date | string; id: string }>(items: readonly T[]) {
   return [...items].sort((a, b) => {
     const diff = toTimestamp(a.createdAt) - toTimestamp(b.createdAt);