diff --git a/packages/views/common/task-transcript/agent-transcript-dialog.test.tsx b/packages/views/common/task-transcript/agent-transcript-dialog.test.tsx
index e8cf14f2e..17020185c 100644
--- a/packages/views/common/task-transcript/agent-transcript-dialog.test.tsx
+++ b/packages/views/common/task-transcript/agent-transcript-dialog.test.tsx
@@ -408,11 +408,13 @@ describe("AgentTranscriptDialog", () => {
 
     fireEvent.click(screen.getByRole("button", { name: "Copy all" }));
 
+    // Full body (not the truncated summary) with the RFC 3339 prefix, events
+    // separated by a blank line.
     expect(copyTextMock).toHaveBeenCalledWith(
       [
-        "[2026-06-08T00:00:00.000Z] [Agent] Agent summary",
+        "[2026-06-08T00:00:00.000Z] [Agent] Agent summary\nAgent hidden detail",
         "[2026-06-08T08:00:05.123Z] [Thinking] Thinking summary",
-      ].join("\n"),
+      ].join("\n\n"),
     );
   });
 
@@ -434,7 +436,7 @@ describe("AgentTranscriptDialog", () => {
     fireEvent.click(screen.getByRole("button", { name: "Copy all" }));
 
     expect(copyTextMock).toHaveBeenCalledWith(
-      ["[Agent] Missing timestamp", "[Error] Invalid timestamp"].join("\n"),
+      ["[Agent] Missing timestamp", "[Error] Invalid timestamp"].join("\n\n"),
     );
   });
 });
diff --git a/packages/views/common/task-transcript/trace-event-presenter.ts b/packages/views/common/task-transcript/trace-event-presenter.ts
index fc04601ab..6ccddda69 100644
--- a/packages/views/common/task-transcript/trace-event-presenter.ts
+++ b/packages/views/common/task-transcript/trace-event-presenter.ts
@@ -147,7 +147,8 @@ export function traceEventSummary(event: TraceEvent): string {
 /**
  * Full, untruncated text for "copy all" — the complete body, not the one-line
  * summary. Tool calls copy their full input JSON; results and prose copy their
- * whole content. Callers apply secret redaction on the result.
+ * whole content. An RFC 3339 timestamp prefixes the line when the event has a
+ * valid `created_at` (#5873). Callers apply secret redaction on the result.
  */
 export function traceEventCopyText(event: TraceEvent): string {
   const label = traceEventLabel(event);
@@ -162,7 +163,9 @@ export function traceEventCopyText(event: TraceEvent): string {
     default:
       body = event.content ?? "";
   }
-  return body ? `[${label}] ${body}` : `[${label}]`;
+  const date = event.created_at ? new Date(event.created_at) : null;
+  const timestamp = date && !Number.isNaN(date.getTime()) ? `[${date.toISOString()}] ` : "";
+  return body ? `${timestamp}[${label}] ${body}` : `${timestamp}[${label}]`;
 }
 
 export function traceEventHasDetail(event: TraceEvent): boolean {