perf(web): reuse work log rows during streaming

apps/web/src/session-logic.test.ts

@@ -2205,3 +2205,64 @@ describe("rerun workflows", () => {
     expect(spawnRows.map((row) => row.turnId)).toEqual(["turn-1", "turn-2"]);
   });
 });
+
+describe("session activity performance", () => {
+  it("reuses entries for unchanged activities", () => {
+    const activities = ["status", "diff", "log"].map((command, index) =>
+      makeActivity({
+        id: `stable-tool-${index}`,
+        kind: "tool.completed",
+        sequence: index,
+        payload: {
+          itemType: "command_execution",
+          data: { toolCallId: `stable-tool-${index}`, item: { command: ["git", command] } },
+        },
+      }),
+    );
+
+    const initialEntries = deriveWorkLogEntries(activities.slice(0, 2));
+    const appendedEntries = deriveWorkLogEntries(activities);
+    expect(appendedEntries[0]).toBe(initialEntries[0]);
+    expect(appendedEntries[1]).toBe(initialEntries[1]);
+  });
+
+  it("updates 20,000 ordered tool activities within 100 ms", () => {
+    const activities = Array.from({ length: 20_000 }, (_, index) =>
+      makeActivity({
+        id: `benchmark-tool-${index}`,
+        createdAt: new Date(1_700_000_000_000 + index).toISOString(),
+        kind: "tool.completed",
+        summary: "Ran command",
+        sequence: index,
+        payload: {
+          itemType: "command_execution",
+          title: "Ran command",
+          data: {
+            toolCallId: `benchmark-tool-${index}`,
+            item: { command: ["git", "status"] },
+          },
+        },
+      }),
+    );
+    deriveWorkLogEntries(activities);
+    const updatedActivities = [
+      ...activities,
+      makeActivity({
+        id: "benchmark-tool-appended",
+        createdAt: new Date(1_700_000_000_000 + activities.length).toISOString(),
+        kind: "tool.completed",
+        summary: "Ran command",
+        sequence: activities.length,
+        payload: {
+          itemType: "command_execution",
+          title: "Ran command",
+          data: { toolCallId: "benchmark-tool-appended", item: { command: ["git", "diff"] } },
+        },
+      }),
+    ];
+
+    const startedAt = performance.now();
+    expect(deriveWorkLogEntries(updatedActivities)).toHaveLength(20_001);
+    expect(performance.now() - startedAt).toBeLessThan(100);
+  });
+});

apps/web/src/session-logic.ts

@@ -98,15 +98,22 @@ export interface WorkLogEntry {
   };
 }
 
+const workLogCollapseKey = Symbol();
+
 interface DerivedWorkLogEntry extends WorkLogEntry {
-  activityKind: OrchestrationThreadActivity["kind"];
-  collapseKey?: string;
+  sourceActivityKind: OrchestrationThreadActivity["kind"];
+  [workLogCollapseKey]?: string;
   toolCallId?: string;
   isWorkflowCoordinator?: boolean;
   /** Shell/monitor/plan tasks: ordinary work-log rows, never spawn CTAs. */
   isBackgroundTask?: boolean;
 }
 
+const derivedWorkLogEntryByActivity = new WeakMap<
+  OrchestrationThreadActivity,
+  DerivedWorkLogEntry
+>();
+
 export interface PendingApproval {
   requestId: ApprovalRequestId;
   requestKind: "command" | "file-read" | "file-change";
@@ -854,10 +861,7 @@ export function deriveWorkLogEntries(
     if (isAgentInternalActivity(activity)) continue;
     entries.push(toDerivedWorkLogEntry(activity));
   }
-  return collapseDerivedWorkLogEntries(entries).map((entry) => {
-    const { activityKind, collapseKey: _collapseKey, ...rest } = entry;
-    return Object.assign(rest, { sourceActivityKind: activityKind });
-  });
+  return collapseDerivedWorkLogEntries(entries);
 }
 
 function isPlanBoundaryToolActivity(activity: OrchestrationThreadActivity): boolean {
@@ -892,6 +896,10 @@ function extractWorkLogToolLifecycleStatus(
 }
 
 function toDerivedWorkLogEntry(activity: OrchestrationThreadActivity): DerivedWorkLogEntry {
+  const cachedEntry = derivedWorkLogEntryByActivity.get(activity);
+  if (cachedEntry) {
+    return cachedEntry;
+  }
   const payload =
     activity.payload && typeof activity.payload === "object"
       ? (activity.payload as Record<string, unknown>)
@@ -935,7 +943,7 @@ function toDerivedWorkLogEntry(activity: OrchestrationThreadActivity): DerivedWo
         : activity.tone === "approval"
           ? "info"
           : activity.tone,
-    activityKind: activity.kind,
+    sourceActivityKind: activity.kind,
   };
   const itemType = extractWorkLogItemType(payload);
   const requestKind = extractWorkLogRequestKind(payload);
@@ -994,8 +1002,9 @@ function toDerivedWorkLogEntry(activity: OrchestrationThreadActivity): DerivedWo
   }
   const collapseKey = deriveToolLifecycleCollapseKey(entry);
   if (collapseKey) {
-    entry.collapseKey = collapseKey;
+    entry[workLogCollapseKey] = collapseKey;
   }
+  derivedWorkLogEntryByActivity.set(activity, entry);
   return entry;
 }
 
@@ -1026,7 +1035,10 @@ function agentSpawnGroupKey(entry: DerivedWorkLogEntry): string {
 }
 
 function toolLifecycleCollapseMapKey(entry: DerivedWorkLogEntry): string | undefined {
-  if (entry.activityKind !== "tool.updated" && entry.activityKind !== "tool.completed") {
+  if (
+    entry.sourceActivityKind !== "tool.updated" &&
+    entry.sourceActivityKind !== "tool.completed"
+  ) {
     return undefined;
   }
   return entry.toolCallId ? `tool:${entry.turnId ?? "no-turn"}:${entry.toolCallId}` : undefined;
@@ -1053,9 +1065,9 @@ function collapseDerivedWorkLogEntries(
     const isTaskRow =
       entry.taskId !== undefined &&
       !entry.isBackgroundTask &&
-      (entry.activityKind === "task.started" ||
-        entry.activityKind === "task.progress" ||
-        entry.activityKind === "task.completed");
+      (entry.sourceActivityKind === "task.started" ||
+        entry.sourceActivityKind === "task.progress" ||
+        entry.sourceActivityKind === "task.completed");
     if (isTaskRow && entry.taskId !== undefined) {
       const rememberedKey = groupKeyByTaskId.get(entry.taskId);
       const groupKey = rememberedKey ?? agentSpawnGroupKey(entry);
@@ -1131,19 +1143,25 @@ function shouldCollapseToolLifecycleEntries(
   previous: DerivedWorkLogEntry,
   next: DerivedWorkLogEntry,
 ): boolean {
-  if (previous.activityKind !== "tool.updated" && previous.activityKind !== "tool.completed") {
+  if (
+    previous.sourceActivityKind !== "tool.updated" &&
+    previous.sourceActivityKind !== "tool.completed"
+  ) {
     return false;
   }
-  if (next.activityKind !== "tool.updated" && next.activityKind !== "tool.completed") {
+  if (next.sourceActivityKind !== "tool.updated" && next.sourceActivityKind !== "tool.completed") {
     return false;
   }
   if (previous.turnId !== next.turnId) {
     return false;
   }
-  if (previous.activityKind === "tool.completed") {
+  if (previous.sourceActivityKind === "tool.completed") {
     return false;
   }
-  if (previous.collapseKey !== undefined && previous.collapseKey === next.collapseKey) {
+  if (
+    previous[workLogCollapseKey] !== undefined &&
+    previous[workLogCollapseKey] === next[workLogCollapseKey]
+  ) {
     return true;
   }
   return (
@@ -1166,7 +1184,7 @@ function mergeDerivedWorkLogEntries(
   const toolTitle = next.toolTitle ?? previous.toolTitle;
   const itemType = next.itemType ?? previous.itemType;
   const requestKind = next.requestKind ?? previous.requestKind;
-  const collapseKey = next.collapseKey ?? previous.collapseKey;
+  const collapseKey = next[workLogCollapseKey] ?? previous[workLogCollapseKey];
   const toolCallId = next.toolCallId ?? previous.toolCallId;
   const toolLifecycleStatus = next.toolLifecycleStatus ?? previous.toolLifecycleStatus;
   const toolData = next.toolData ?? previous.toolData;
@@ -1180,7 +1198,7 @@ function mergeDerivedWorkLogEntries(
     ...(toolTitle ? { toolTitle } : {}),
     ...(itemType ? { itemType } : {}),
     ...(requestKind ? { requestKind } : {}),
-    ...(collapseKey ? { collapseKey } : {}),
+    ...(collapseKey ? { [workLogCollapseKey]: collapseKey } : {}),
     ...(toolCallId ? { toolCallId } : {}),
     ...(toolLifecycleStatus !== undefined ? { toolLifecycleStatus } : {}),
     ...(toolData !== undefined ? { toolData } : {}),
@@ -1203,11 +1221,14 @@ function deriveToolLifecycleCollapseKey(entry: DerivedWorkLogEntry): string | un
   // progress ticks fold into it, the terminal row wins the label.
   if (
     entry.taskId &&
-    (entry.activityKind === "task.progress" || entry.activityKind === "task.completed")
+    (entry.sourceActivityKind === "task.progress" || entry.sourceActivityKind === "task.completed")
   ) {
     return `task${entry.taskId}`;
   }
-  if (entry.activityKind !== "tool.updated" && entry.activityKind !== "tool.completed") {
+  if (
+    entry.sourceActivityKind !== "tool.updated" &&
+    entry.sourceActivityKind !== "tool.completed"
+  ) {
     return undefined;
   }
   if (entry.toolCallId) {