fix(claude): replay large child histories without spread

Streaming removes whole-transcript duplication, but folding many visible child events still passed the complete aggregate as arguments to Array.push. Large histories exceed the V8 argument limit, the loader catches a RangeError, and history remains empty. Append replay observations iteratively and cover 160,000 aggregate child events.

Adapted from #7511, independently reproduced with the public resumeSession and streamHistory interfaces.

Co-authored-by: Alin Jiang <zonglinj@amazon.com>

packages/server/src/server/agent/providers/claude/agent.sub-agent-replay.test.ts

@@ -466,6 +466,46 @@ describe("ClaudeAgentSession persisted subagent replay", () => {
     );
   });
 
+  test("replays more child events than V8 permits in one spread call", async () => {
+    const childCount = 16;
+    const entriesPerChild = 10_000;
+    const children = Array.from({ length: childCount }, (_, index) => ({
+      agentId: `large-child-${index}`,
+      toolUseId: `large-task-${index}`,
+    }));
+    const parentLines = children.flatMap(({ agentId, toolUseId }) => [
+      parentEntry([{ type: "tool_use", id: toolUseId, name: "Task", input: {} }]),
+      JSON.stringify({
+        type: "user",
+        message: {
+          content: [
+            {
+              type: "tool_result",
+              tool_use_id: toolUseId,
+              content: `agentId: ${agentId}\ndone`,
+            },
+          ],
+        },
+      }),
+    ]);
+    const subagentDir = writeParentSession(parentLines);
+    for (const { agentId, toolUseId } of children) {
+      writeSubagent({
+        subagentDir,
+        agentId,
+        meta: JSON.stringify({ toolUseId }),
+        sidechainLines: Array.from({ length: entriesPerChild }, () => sidechainEntry({ agentId })),
+      });
+    }
+
+    const events = await replayDescriptors();
+    const timelineCount = events.filter((event) => event.event.type === "timeline").length;
+    expect(timelineCount).toBe(childCount * entriesPerChild);
+    expect(upserts(events).filter((event) => event.status === "completed")).toHaveLength(
+      childCount,
+    );
+  }, 120_000);
+
   test("limits retained child shell output to the timeline's existing content limit", async () => {
     const output = "x".repeat(128 * 1024);
     writeSession({

packages/server/src/server/agent/providers/claude/agent.ts

@@ -5261,15 +5261,13 @@ class ClaudeAgentSession implements AgentSession {
     };
     if (observations.length === 0) return replay;
 
-    this.persistedProviderSubagentEvents.push(
-      ...foldSubagentObservations(observations).map(
-        (event): Extract<AgentStreamEvent, { type: "provider_subagent" }> => ({
-          type: "provider_subagent",
-          provider: "claude",
-          event,
-        }),
-      ),
-    );
+    for (const event of foldSubagentObservations(observations)) {
+      this.persistedProviderSubagentEvents.push({
+        type: "provider_subagent",
+        provider: "claude",
+        event,
+      });
+    }
     this.historyPending = true;
     return replay;
   }
@@ -5327,12 +5325,12 @@ class ClaudeAgentSession implements AgentSession {
     }
 
     if (items.length > 0) {
-      timeline.push(
-        ...items.map((item) => ({
+      for (const item of items) {
+        timeline.push({
           item,
           timestamp: historyTimestamp ?? undefined,
-        })),
-      );
+        });
+      }
     }
   }
 

packages/server/src/server/agent/providers/claude/subagents/replay-source.ts

@@ -295,7 +295,7 @@ function observeSubagent(
     lastTimestamp,
   );
   if (subtitle) observations.push(subtitle);
-  observations.push(...timeline);
+  for (const observation of timeline) observations.push(observation);
   const terminalStatus = link.status ?? childTerminalStatus;
   if (terminalStatus) {
     observations.push({
@@ -346,9 +346,15 @@ export function observeReplaySubagents(input: {
       const { ownerId, parent, link } = resolved;
 
       // Only proven descendants may own notifications; ambient sidecars cannot claim them.
-      observations.push(
-        ...observeSubagent(subagent, parent, input.convertEntry, toolOwners, ownerId),
-      );
+      for (const observation of observeSubagent(
+        subagent,
+        parent,
+        input.convertEntry,
+        toolOwners,
+        ownerId,
+      )) {
+        observations.push(observation);
+      }
       if (subagent.parentFacts) resolvedParents.set(link.id, subagent.parentFacts);
       unresolved.splice(index, 1);
       madeProgress = true;

packages/server/src/server/agent/providers/claude/subagents/workflow-replay-source.ts

@@ -79,15 +79,15 @@ export function observeReplayWorkflows(input: {
       });
     }
 
-    observations.push(
-      ...observeWorkflowTimeline({
-        id,
-        result: workflow.result,
-        entries: input.entriesByRunId?.get(workflow.runId) ?? [],
-        convertEntry: input.convertEntry,
-        finishedAt,
-      }),
-    );
+    for (const observation of observeWorkflowTimeline({
+      id,
+      result: workflow.result,
+      entries: input.entriesByRunId?.get(workflow.runId) ?? [],
+      convertEntry: input.convertEntry,
+      finishedAt,
+    })) {
+      observations.push(observation);
+    }
 
     const subtitle = buildClaudeSubagentSubtitle({
       title: "Workflow",