diff --git a/src/auto-reply/reply/memory-flush.test.ts b/src/auto-reply/reply/memory-flush.test.ts
index e444b9e7a80..0e04e7e0ea3 100644
--- a/src/auto-reply/reply/memory-flush.test.ts
+++ b/src/auto-reply/reply/memory-flush.test.ts
@@ -1,6 +1,6 @@
 import { describe, expect, it } from "vitest";
 import type { OpenClawConfig } from "../../config/config.js";
-import { resolveMemoryFlushPromptForRun } from "./memory-flush.js";
+import { DEFAULT_MEMORY_FLUSH_PROMPT, resolveMemoryFlushPromptForRun } from "./memory-flush.js";
 
 describe("resolveMemoryFlushPromptForRun", () => {
   const cfg = {
@@ -36,3 +36,16 @@ describe("resolveMemoryFlushPromptForRun", () => {
     expect((prompt.match(/Current time:/g) ?? []).length).toBe(1);
   });
 });
+
+describe("DEFAULT_MEMORY_FLUSH_PROMPT", () => {
+  it("includes append-only instruction to prevent overwrites (#6877)", () => {
+    expect(DEFAULT_MEMORY_FLUSH_PROMPT).toMatch(/APPEND/i);
+    expect(DEFAULT_MEMORY_FLUSH_PROMPT).toContain("do not overwrite");
+  });
+
+  it("includes anti-fragmentation instruction to prevent timestamped variant files (#34919)", () => {
+    // Agents must not create YYYY-MM-DD-HHMM.md variants alongside the canonical file
+    expect(DEFAULT_MEMORY_FLUSH_PROMPT).toContain("timestamped variant");
+    expect(DEFAULT_MEMORY_FLUSH_PROMPT).toContain("YYYY-MM-DD.md");
+  });
+});
diff --git a/src/auto-reply/reply/memory-flush.ts b/src/auto-reply/reply/memory-flush.ts
index e23703c7b6c..c02fad5eca0 100644
--- a/src/auto-reply/reply/memory-flush.ts
+++ b/src/auto-reply/reply/memory-flush.ts
@@ -13,7 +13,8 @@ export const DEFAULT_MEMORY_FLUSH_FORCE_TRANSCRIPT_BYTES = 2 * 1024 * 1024;
 export const DEFAULT_MEMORY_FLUSH_PROMPT = [
   "Pre-compaction memory flush.",
   "Store durable memories now (use memory/YYYY-MM-DD.md; create memory/ if needed).",
-  "IMPORTANT: If the file already exists, APPEND new content only and do not overwrite existing entries.",
+  "IMPORTANT: If the file already exists, APPEND new content only — do not overwrite existing entries.",
+  "Do NOT create timestamped variant files (e.g., YYYY-MM-DD-HHMM.md); always use the canonical YYYY-MM-DD.md filename.",
   `If nothing to store, reply with ${SILENT_REPLY_TOKEN}.`,
 ].join(" ");