diff --git a/extensions/memory-core/src/short-term-promotion.test.ts b/extensions/memory-core/src/short-term-promotion.test.ts
index 3422425ee042..082c21dd8112 100644
--- a/extensions/memory-core/src/short-term-promotion.test.ts
+++ b/extensions/memory-core/src/short-term-promotion.test.ts
@@ -3339,4 +3339,73 @@ describe("short-term promotion", () => {
       });
     });
   });
+
+  describe("MEMORY.md atomic promotion write", () => {
+    it("preserves the existing MEMORY.md when the promotion write fails", async () => {
+      await withTempWorkspace(async (workspaceDir) => {
+        await writeDailyMemoryNote(workspaceDir, "2026-04-29", [
+          "Notes",
+          "",
+          "Rotate the staging Postgres credentials before next deploy.",
+        ]);
+
+        const memoryPath = path.join(workspaceDir, "MEMORY.md");
+        const sentinel = "FINAL-USER-MEMORY-SENTINEL-do-not-lose";
+        const seeded = `# Long-Term Memory\n\n${"pad line filler content ".repeat(9_000)}\n- ${sentinel}\n`;
+        await fs.writeFile(memoryPath, seeded, "utf-8");
+        await recordShortTermRecalls({
+          workspaceDir,
+          query: "rotate creds",
+          nowMs: Date.parse("2026-04-29T10:00:00.000Z"),
+          results: [
+            {
+              path: "memory/2026-04-29.md",
+              startLine: 3,
+              endLine: 3,
+              score: 0.96,
+              snippet: "Rotate the staging Postgres credentials before next deploy.",
+              source: "memory",
+            },
+          ],
+        });
+        const ranked = await rankShortTermPromotionCandidates({
+          workspaceDir,
+          minScore: 0,
+          minRecallCount: 0,
+          minUniqueQueries: 0,
+        });
+
+        const originalWriteFile = fs.writeFile.bind(fs);
+        vi.spyOn(fs, "writeFile").mockImplementation((async (target, data, options) => {
+          const targetPath =
+            typeof target === "string" ? target : target instanceof URL ? target.pathname : "";
+          if (targetPath && path.basename(targetPath).startsWith("MEMORY.md")) {
+            const text = typeof data === "string" ? data : Buffer.from(data).toString();
+            await originalWriteFile(target, text.slice(0, 51_200), options);
+            throw Object.assign(new Error("EFBIG: file too large, write"), { code: "EFBIG" });
+          }
+          return await originalWriteFile(target, data, options);
+        }) as typeof fs.writeFile);
+
+        await expect(
+          applyShortTermPromotions({
+            workspaceDir,
+            candidates: ranked,
+            minScore: 0,
+            minRecallCount: 0,
+            minUniqueQueries: 0,
+            nowMs: Date.parse("2026-04-29T10:00:00.000Z"),
+            memoryFileMaxChars: 5_000_000,
+          }),
+        ).rejects.toMatchObject({ code: "EFBIG" });
+
+        expect(await fs.readFile(memoryPath, "utf-8")).toBe(seeded);
+        expect(
+          (await fs.readdir(workspaceDir)).filter((entry) =>
+            entry.startsWith("MEMORY.md.promotion"),
+          ),
+        ).toEqual([]);
+      });
+    });
+  });
 });
diff --git a/extensions/memory-core/src/short-term-promotion.ts b/extensions/memory-core/src/short-term-promotion.ts
index e4e9f4a1a3c2..e63a25345fbc 100644
--- a/extensions/memory-core/src/short-term-promotion.ts
+++ b/extensions/memory-core/src/short-term-promotion.ts
@@ -9,6 +9,7 @@ import {
   isSameMemoryDreamingDay,
 } from "openclaw/plugin-sdk/memory-core-host-status";
 import { appendMemoryHostEvent } from "openclaw/plugin-sdk/memory-host-events";
+import { replaceFileAtomic } from "openclaw/plugin-sdk/security-runtime";
 import {
   normalizeLowercaseStringOrEmpty,
   normalizeStringEntries,
@@ -2468,11 +2469,18 @@ export async function applyShortTermPromotions(
       compactedDates = compaction.droppedDates;
       const baseMemory = compaction.compacted;
       const header = baseMemory.trim().length > 0 ? "" : "# Long-Term Memory\n\n";
-      await fs.writeFile(
-        memoryPath,
-        `${header}${withTrailingNewline(baseMemory)}${section}`,
-        "utf-8",
-      );
+      const workspaceMode = (await fs.stat(workspaceDir)).mode & 0o7777;
+      await replaceFileAtomic({
+        filePath: memoryPath,
+        content: `${header}${withTrailingNewline(baseMemory)}${section}`,
+        dirMode: workspaceMode,
+        mode: 0o600,
+        preserveExistingMode: true,
+        tempPrefix: `${path.basename(memoryPath)}.promotion`,
+        syncTempFile: true,
+        syncParentDir: true,
+        throwOnCleanupError: true,
+      });
     }
 
     for (const candidate of rehydratedSelected) {