diff --git a/src/agents/compaction.identifier-preservation.test.ts b/src/agents/compaction.identifier-preservation.test.ts
index 3ef66f3b5cf3..81ccfa13e93c 100644
--- a/src/agents/compaction.identifier-preservation.test.ts
+++ b/src/agents/compaction.identifier-preservation.test.ts
@@ -15,6 +15,7 @@ vi.mock("./sessions/index.js", async () => {
 
 const mockGenerateSummary = vi.mocked(agentSessions.generateSummary);
 type SummarizeInStagesInput = Parameters<typeof import("./compaction.js").summarizeInStages>[0];
+const MESSAGE_TIME_BASE_MS = Date.UTC(2026, 0, 1);
 
 const { buildCompactionSummarizationInstructions, summarizeInStages } =
   await import("./compaction.js");
@@ -23,7 +24,7 @@ function makeMessage(index: number, size = 1200): AgentMessage {
   return {
     role: "user",
     content: `m${index}-${"x".repeat(size)}`,
-    timestamp: index,
+    timestamp: MESSAGE_TIME_BASE_MS + index * 60_000,
   };
 }
 
@@ -113,6 +114,14 @@ describe("compaction identifier-preservation instructions", () => {
         "Preserve all opaque identifiers exactly as written",
       );
     }
+
+    type SyntheticMergeMessage = { role: "user"; content: string; timestamp: number };
+    const mergeMessages = mockGenerateSummary.mock.calls[2]![0] as SyntheticMergeMessage[];
+    expect(mergeMessages.map((message) => message.content)).toEqual([
+      "[Chunk 1 — oldest messages [2026-01-01 00:01 — 2026-01-01 00:02 UTC]]\nsummary",
+      "[Chunk 2 — most recent messages [2026-01-01 00:03 — 2026-01-01 00:04 UTC]]\nsummary",
+    ]);
+    expect(mergeMessages[1]!.timestamp).toBe(mergeMessages[0]!.timestamp + 1);
   });
 
   it("avoids duplicate additional-focus headers in split+merge path", async () => {
diff --git a/src/agents/compaction.ts b/src/agents/compaction.ts
index e672ad9941ad..eb70a2277d7e 100644
--- a/src/agents/compaction.ts
+++ b/src/agents/compaction.ts
@@ -2,9 +2,9 @@
  * Summarization and fallback helpers for transcript compaction.
  */
 import type { AgentCompactionIdentifierPolicy } from "../config/types.agent-defaults.js";
+import { isAbortError } from "../infra/abort-signal.js";
 import { formatErrorMessage } from "../infra/errors.js";
 import { retryAsync } from "../infra/retry.js";
-import { isAbortError } from "../infra/abort-signal.js";
 import { createSubsystemLogger } from "../logging/subsystem.js";
 import {
   buildOversizedFallbackPlanWithWorker,
@@ -338,6 +338,31 @@ export async function summarizeWithFallback(params: {
   );
 }
 
+/** Extracts a compact timestamp range from a chunk of messages for merge metadata. */
+function extractChunkTimeRange(chunk: AgentMessage[]): string {
+  let earliest: number | undefined;
+  let latest: number | undefined;
+  for (const message of chunk) {
+    const timestamp = message.timestamp;
+    if (
+      typeof timestamp !== "number" ||
+      timestamp <= 0 ||
+      !Number.isFinite(new Date(timestamp).getTime())
+    ) {
+      continue;
+    }
+    earliest = earliest === undefined ? timestamp : Math.min(earliest, timestamp);
+    latest = latest === undefined ? timestamp : Math.max(latest, timestamp);
+  }
+  if (earliest === undefined || latest === undefined) {
+    return "";
+  }
+  const format = (timestamp: number) =>
+    new Date(timestamp).toISOString().replace("T", " ").slice(0, 16);
+  const range = earliest === latest ? format(earliest) : `${format(earliest)} — ${format(latest)}`;
+  return ` [${range} UTC]`;
+}
+
 /** Summarizes history in multiple stages when a single pass would be too large. */
 export async function summarizeInStages(params: {
   messages: AgentMessage[];
@@ -386,11 +411,28 @@ export async function summarizeInStages(params: {
     return partialSummaries[0];
   }
 
-  const summaryMessages: AgentMessage[] = partialSummaries.map((summary) => ({
-    role: "user",
-    content: summary,
-    timestamp: Date.now(),
-  }));
+  // Capture once so timestamps are strictly monotonic across
+  // all synthetic messages regardless of how long the map iteration takes.
+  const now = Date.now();
+  const summaryMessages: AgentMessage[] = partialSummaries.map((summary, index) => {
+    // serializeConversation preserves content but not timestamps, so chronology
+    // must be explicit in the text consumed by the merge model.
+    const chunk = plan.chunks[index];
+    const timeRange = extractChunkTimeRange(chunk);
+    const label =
+      index === 0
+        ? `[Chunk 1 — oldest messages${timeRange}]`
+        : index === partialSummaries.length - 1
+          ? `[Chunk ${partialSummaries.length} — most recent messages${timeRange}]`
+          : `[Chunk ${index + 1}/${partialSummaries.length}${timeRange}]`;
+    return {
+      role: "user",
+      content: `${label}\n${summary}`,
+      // Ascending timestamps preserve chronological order for any code
+      // path that reads the AgentMessage timestamp field directly.
+      timestamp: now - (partialSummaries.length - 1 - index),
+    };
+  });
 
   const custom = params.customInstructions?.trim();
   const mergeInstructions = custom