diff --git a/src/stores/useSessionStore.ts b/src/stores/useSessionStore.ts
index 60ace484f..c2c170250 100644
--- a/src/stores/useSessionStore.ts
+++ b/src/stores/useSessionStore.ts
@@ -103,19 +103,85 @@ function createEmptySlot(): SessionSlot {
 }
 
 /**
- * Compute merged messages: server + realtime, deduped by id.
+ * Compute merged messages: server + realtime, deduped by stable message identity.
  * Server messages take priority (they're the persisted source of truth).
  * Realtime messages that aren't yet in server stay (in-flight streaming).
  */
 function computeMerged(server: NormalizedMessage[], realtime: NormalizedMessage[]): NormalizedMessage[] {
   if (realtime.length === 0) return server;
   if (server.length === 0) return realtime;
-  const serverIds = new Set(server.map(m => m.id));
-  const extra = realtime.filter(m => !serverIds.has(m.id));
+  const extra = realtime.filter(m => !server.some(existing => isEquivalentNormalizedMessage(existing, m)));
   if (extra.length === 0) return server;
   return [...server, ...extra];
 }
 
+function isEquivalentNormalizedMessage(a: NormalizedMessage, b: NormalizedMessage): boolean {
+  if (a.id === b.id) return true;
+
+  if (
+    a.rowid !== undefined &&
+    b.rowid !== undefined &&
+    a.rowid === b.rowid &&
+    a.kind === b.kind
+  ) {
+    return a.sequence === undefined || b.sequence === undefined || a.sequence === b.sequence;
+  }
+
+  if (
+    a.sequence !== undefined &&
+    b.sequence !== undefined &&
+    a.sequence === b.sequence &&
+    a.kind === b.kind
+  ) {
+    return true;
+  }
+
+  if (a.requestId && b.requestId && a.requestId === b.requestId && a.kind === b.kind) {
+    return true;
+  }
+
+  if (a.toolId && b.toolId && a.toolId === b.toolId && a.kind === b.kind) {
+    return true;
+  }
+
+  if (
+    a.kind === 'session_created' &&
+    b.kind === 'session_created' &&
+    a.newSessionId &&
+    a.newSessionId === b.newSessionId
+  ) {
+    return true;
+  }
+
+  if (a.role === 'user' || b.role === 'user') {
+    return false;
+  }
+
+  return (
+    a.kind === b.kind &&
+    a.timestamp === b.timestamp &&
+    a.role === b.role &&
+    a.content === b.content &&
+    a.text === b.text &&
+    a.toolName === b.toolName
+  );
+}
+
+function upsertRealtimeMessage(messages: NormalizedMessage[], msg: NormalizedMessage): NormalizedMessage[] {
+  const existingIndex = messages.findIndex(existing => isEquivalentNormalizedMessage(existing, msg));
+
+  if (existingIndex === -1) {
+    return [...messages, msg];
+  }
+
+  const next = [...messages];
+  next[existingIndex] = {
+    ...messages[existingIndex],
+    ...msg,
+  };
+  return next;
+}
+
 /**
  * Recompute slot.merged only when the input arrays have actually changed
  * (by reference). Returns true if merged was recomputed.
@@ -273,7 +339,7 @@ export function useSessionStore() {
    */
   const appendRealtime = useCallback((sessionId: string, msg: NormalizedMessage) => {
     const slot = getSlot(sessionId);
-    let updated = [...slot.realtimeMessages, msg];
+    let updated = upsertRealtimeMessage(slot.realtimeMessages, msg);
     if (updated.length > MAX_REALTIME_MESSAGES) {
       updated = updated.slice(-MAX_REALTIME_MESSAGES);
     }
@@ -288,7 +354,10 @@ export function useSessionStore() {
   const appendRealtimeBatch = useCallback((sessionId: string, msgs: NormalizedMessage[]) => {
     if (msgs.length === 0) return;
     const slot = getSlot(sessionId);
-    let updated = [...slot.realtimeMessages, ...msgs];
+    let updated = slot.realtimeMessages;
+    for (const msg of msgs) {
+      updated = upsertRealtimeMessage(updated, msg);
+    }
     if (updated.length > MAX_REALTIME_MESSAGES) {
       updated = updated.slice(-MAX_REALTIME_MESSAGES);
     }