diff --git a/apps/server/src/orchestration/projector.ts b/apps/server/src/orchestration/projector.ts
index 85d9db3fdfed..dd418b95b9bc 100644
--- a/apps/server/src/orchestration/projector.ts
+++ b/apps/server/src/orchestration/projector.ts
@@ -117,12 +117,26 @@ function settledTurnStateForSessionStatus(
   }
 }
 
+// Runs for every thread event (including streaming deltas) against every
+// thread the server has ever seen, so copy the array rather than map it.
 function updateThread(
   threads: ReadonlyArray<OrchestrationThread>,
   threadId: ThreadId,
   patch: ThreadPatch,
-): OrchestrationThread[] {
-  return threads.map((thread) => (thread.id === threadId ? { ...thread, ...patch } : thread));
+): ReadonlyArray<OrchestrationThread> {
+  const index = threads.findIndex((thread) => thread.id === threadId);
+  return index === -1 ? threads : patchThreadAt(threads, index, patch);
+}
+
+/** For callers that already located the thread and must not scan again. */
+function patchThreadAt(
+  threads: ReadonlyArray<OrchestrationThread>,
+  index: number,
+  patch: ThreadPatch,
+): ReadonlyArray<OrchestrationThread> {
+  const next = threads.slice();
+  next[index] = { ...threads[index]!, ...patch };
+  return next;
 }
 
 /** Patch that swaps a thread's links and re-derives the legacy single-PR field from them. */
@@ -767,7 +781,8 @@ export function projectEvent(
           event.type,
           "payload",
         );
-        const thread = nextBase.threads.find((entry) => entry.id === payload.threadId);
+        const threadIndex = nextBase.threads.findIndex((entry) => entry.id === payload.threadId);
+        const thread = nextBase.threads[threadIndex];
         if (!thread) {
           return nextBase;
         }
@@ -815,7 +830,7 @@ export function projectEvent(
 
         return {
           ...nextBase,
-          threads: updateThread(nextBase.threads, payload.threadId, {
+          threads: patchThreadAt(nextBase.threads, threadIndex, {
             messages: cappedMessages,
             updatedAt: event.occurredAt,
           }),
@@ -1055,7 +1070,8 @@ export function projectEvent(
         "payload",
       ).pipe(
         Effect.map((payload) => {
-          const thread = nextBase.threads.find((entry) => entry.id === payload.threadId);
+          const threadIndex = nextBase.threads.findIndex((entry) => entry.id === payload.threadId);
+          const thread = nextBase.threads[threadIndex];
           if (!thread) {
             return nextBase;
           }
@@ -1069,7 +1085,7 @@ export function projectEvent(
 
           return {
             ...nextBase,
-            threads: updateThread(nextBase.threads, payload.threadId, {
+            threads: patchThreadAt(nextBase.threads, threadIndex, {
               activities,
               updatedAt: event.occurredAt,
             }),