diff --git a/apps/web/src/routes/threads/controller.tsx b/apps/web/src/routes/threads/controller.tsx
index 20594057..2dab7006 100644
--- a/apps/web/src/routes/threads/controller.tsx
+++ b/apps/web/src/routes/threads/controller.tsx
@@ -77,10 +77,10 @@ function ThreadsWorkspace({
   const actions = useThreadActions({
     activeAppId,
     activeThreadId: route.activeThreadId,
-    allThreads: threads.allThreads,
     closeComposeDialog: route.closeComposeDialog,
     markThreadReadLocal: ui.markThreadRead,
     navigateToList: route.backToList,
+    threadsById: threads.threadsById,
     togglePinnedThreadLocal: ui.togglePinnedThread,
   });
   const handleReadSyncError = useCallback(
diff --git a/apps/web/src/routes/threads/model/use-actions.ts b/apps/web/src/routes/threads/model/use-actions.ts
index 568e8722..0effcfc7 100644
--- a/apps/web/src/routes/threads/model/use-actions.ts
+++ b/apps/web/src/routes/threads/model/use-actions.ts
@@ -20,18 +20,18 @@ import type { ThreadListItem } from "./thread";
 export function useThreadActions({
   activeAppId,
   activeThreadId,
-  allThreads,
   closeComposeDialog,
   markThreadReadLocal,
   navigateToList,
+  threadsById,
   togglePinnedThreadLocal,
 }: {
   activeAppId: string | null;
   activeThreadId: string | null;
-  allThreads: readonly ThreadListItem[];
   closeComposeDialog: () => void;
   markThreadReadLocal: (input: { readAt: string; threadId: string }) => void;
   navigateToList: () => void;
+  threadsById: ReadonlyMap<string, ThreadListItem>;
   togglePinnedThreadLocal: (threadId: string) => void;
 }) {
   const queryClient = useQueryClient();
@@ -156,7 +156,7 @@ export function useThreadActions({
 
   const togglePinnedThread = useCallback(
     async (threadId: string): Promise<void> => {
-      const thread = allThreads.find((candidate) => candidate.id === threadId) ?? null;
+      const thread = threadsById.get(threadId) ?? null;
 
       if (thread === null) {
         return;
@@ -169,12 +169,12 @@ export function useThreadActions({
         setActionError(getMutationErrorMessage(error, "Failed to update pinned state."));
       }
     },
-    [allThreads, togglePinnedThreadLocal],
+    [threadsById, togglePinnedThreadLocal],
   );
 
   const archiveThread = useCallback(
     async (threadId: string): Promise<void> => {
-      const thread = allThreads.find((candidate) => candidate.id === threadId) ?? null;
+      const thread = threadsById.get(threadId) ?? null;
 
       try {
         if (thread === null) {
@@ -190,7 +190,7 @@ export function useThreadActions({
         setActionError(getMutationErrorMessage(error, "Failed to archive thread."));
       }
     },
-    [allThreads, archiveMutation],
+    [threadsById, archiveMutation],
   );
 
   const deleteThread = useCallback(
@@ -201,7 +201,7 @@ export function useThreadActions({
       }
 
       try {
-        const thread = allThreads.find((candidate) => candidate.id === threadId) ?? null;
+        const thread = threadsById.get(threadId) ?? null;
 
         if (thread === null) {
           throw new Error("Thread not found.");
@@ -220,7 +220,7 @@ export function useThreadActions({
         setActionError(getMutationErrorMessage(error, "Failed to delete thread."));
       }
     },
-    [activeThreadId, allThreads, deleteMutation, navigateToList],
+    [activeThreadId, threadsById, deleteMutation, navigateToList],
   );
 
   const sendFollowUp = useCallback(
diff --git a/apps/web/src/routes/threads/model/use-queries.ts b/apps/web/src/routes/threads/model/use-queries.ts
index f15b626d..016cf555 100644
--- a/apps/web/src/routes/threads/model/use-queries.ts
+++ b/apps/web/src/routes/threads/model/use-queries.ts
@@ -78,7 +78,11 @@ export function useThreadQueries({
       )
       .toSorted(compareThreads);
   }, [activeSessionsQuery.data, agentsById, archivedSessionsQuery.data, ui]);
-  const selectedThread = allThreads.find((thread) => thread.id === activeThreadId) ?? null;
+  const threadsById = useMemo(
+    () => new Map<string, ThreadListItem>(allThreads.map((thread) => [thread.id, thread])),
+    [allThreads],
+  );
+  const selectedThread = activeThreadId === null ? null : (threadsById.get(activeThreadId) ?? null);
   const messagesQuery = useQuery({
     enabled: selectedThread !== null,
     queryFn: async () => {
@@ -182,5 +186,6 @@ export function useThreadQueries({
     retrieveQuery,
     selectedThread,
     threadsBySection,
+    threadsById,
   };
 }