diff --git a/src/gateway/session-archive-cleanup.test.ts b/src/gateway/session-archive-cleanup.test.ts
index 86c51156d76..1b4f6282732 100644
--- a/src/gateway/session-archive-cleanup.test.ts
+++ b/src/gateway/session-archive-cleanup.test.ts
@@ -122,6 +122,20 @@ describe("sweepSessionArchiveFiles", () => {
     ]);
   });
 
+  it("trims legacy numeric .bak files from rotateSessionFile", async () => {
+    // rotateSessionFile() creates backups as sessions.json.bak.${Date.now()}.
+    const baseTs = Date.now();
+    for (let i = 0; i < 5; i++) {
+      fs.writeFileSync(path.join(sessionsDir, `sessions.json.bak.${baseTs + i * 1000}`), "");
+    }
+
+    const result = await sweepSessionArchiveFiles({ stateDir: tempDir });
+
+    const remaining = fs.readdirSync(sessionsDir).filter((f) => f.startsWith("sessions.json.bak."));
+    expect(remaining).toHaveLength(3);
+    expect(result.removed).toBe(2);
+  });
+
   it("returns zeros when no agent session directories exist", async () => {
     mocks.resolveAgentSessionDirs.mockResolvedValue([]);
     const result = await sweepSessionArchiveFiles({ stateDir: tempDir });
diff --git a/src/gateway/session-archive-cleanup.ts b/src/gateway/session-archive-cleanup.ts
index f5ad95005eb..e575199a1d8 100644
--- a/src/gateway/session-archive-cleanup.ts
+++ b/src/gateway/session-archive-cleanup.ts
@@ -70,10 +70,13 @@ export async function sweepSessionArchiveFiles(params: {
     // name. This mirrors the rotation logic in rotateSessionFile().
     const bakByBase = new Map<string, string[]>();
     for (const entry of entries) {
-      // Use parseSessionArchiveTimestamp to validate the file is a genuine
-      // .bak.* archive — plain indexOf(".bak.") could false-match session
-      // IDs that contain ".bak." (e.g. "foo.bak.bar.jsonl.deleted.<ts>").
-      if (parseSessionArchiveTimestamp(entry, "bak") == null) {
+      // Match genuine .bak.* archives: either ISO-timestamped (from archive
+      // operations) or numeric (from rotateSessionFile's Date.now() suffix).
+      // Plain indexOf(".bak.") would false-match session IDs that contain
+      // ".bak." as a substring (e.g. "foo.bak.bar.jsonl.deleted.<ts>").
+      const isIsoBak = parseSessionArchiveTimestamp(entry, "bak") != null;
+      const isNumericBak = /\.bak\.\d+$/.test(entry);
+      if (!isIsoBak && !isNumericBak) {
         continue;
       }
       const bakIdx = entry.lastIndexOf(".bak.");