fix(chat): stabilize message identities

src/stores/chat/chat-history-sync.ts

@@ -15,6 +15,8 @@ const log = new Logger("StoresChatChatHistorySync");
 export type ReadAndSyncHistoryOutput = {
   /** Network-authoritative messages. Empty network history includes a UI greeting. */
   messages: UiMessage[];
+  /** Display identities present in the local snapshot before the request. */
+  localDisplayIds: readonly string[];
   /** True when network history was written back to local storage. */
   localOverwritten: boolean;
   localCount: number;
@@ -31,8 +33,12 @@ export type LocalHistorySnapshotOutput = {
 
 export type NetworkHistorySyncOutput = ReadAndSyncHistoryOutput;
 
-export function createGreetingMessage(content: string): UiMessage {
+export function createGreetingMessage(
+  characterId: string,
+  content: string,
+): UiMessage {
   return {
+    displayId: `greeting:${encodeURIComponent(characterId)}`,
     content,
     isFromAI: true,
     date: todayString(),
@@ -49,10 +55,14 @@ export async function resolveHistoryCacheIdentity(
 
 export async function readLocalHistorySnapshot(
   cacheIdentity: string | null,
+  characterId: string,
   emptyChatGreeting: string,
 ): Promise<LocalHistorySnapshotOutput> {
   const chatRepo = await loadChatRepository();
-  const greetingMessage = createGreetingMessage(emptyChatGreeting);
+  const greetingMessage = createGreetingMessage(
+    characterId,
+    emptyChatGreeting,
+  );
 
   const localResult = cacheIdentity
     ? await chatRepo.getLocalMessages(cacheIdentity)
@@ -80,12 +90,16 @@ export async function readLocalHistorySnapshot(
 export async function syncNetworkHistory(
   characterId: string,
   localCount: number,
+  localDisplayIds: readonly string[],
   cacheIdentity: string | null,
   emptyChatGreeting: string,
   signal?: AbortSignal,
 ): Promise<NetworkHistorySyncOutput | null> {
   const chatRepo = await loadChatRepository();
-  const greetingMessage = createGreetingMessage(emptyChatGreeting);
+  const greetingMessage = createGreetingMessage(
+    characterId,
+    emptyChatGreeting,
+  );
 
   const networkResult = await chatRepo.getHistory(
     characterId,
@@ -134,6 +148,7 @@ export async function syncNetworkHistory(
 
   return {
     messages: finalMessages,
+    localDisplayIds,
     localOverwritten,
     localCount,
     networkCount: networkUi.length,
@@ -156,11 +171,13 @@ export async function readAndSyncHistory(
   const cacheIdentity = await resolveHistoryCacheIdentity(characterId);
   const localSnapshot = await readLocalHistorySnapshot(
     cacheIdentity,
+    characterId,
     emptyChatGreeting,
   );
   const networkSnapshot = await syncNetworkHistory(
     characterId,
     localSnapshot.localCount,
+    localSnapshot.messages.map((message) => message.displayId),
     cacheIdentity,
     emptyChatGreeting,
     signal,
@@ -169,6 +186,9 @@ export async function readAndSyncHistory(
 
   return {
     messages: localSnapshot.messages,
+    localDisplayIds: localSnapshot.messages.map(
+      (message) => message.displayId,
+    ),
     localOverwritten: false,
     localCount: localSnapshot.localCount,
     networkCount: 0,