diff --git a/packages/cultcache-rs/src/lib.rs b/packages/cultcache-rs/src/lib.rs
index ecb69145..6c2c9030 100644
--- a/packages/cultcache-rs/src/lib.rs
+++ b/packages/cultcache-rs/src/lib.rs
@@ -5437,6 +5437,41 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn push_all_refuses_a_store_in_a_format_this_runtime_does_not_read_and_leaves_it() -> Result<()> {
+        let vector = fs::read(
+            Path::new(env!("CARGO_MANIFEST_DIR")).join("../../tests/vectors/document-variants-c2a/v3-base.msgpack"),
+        )?;
+        let entry = CultCacheEnvelope {
+            key: "probe".into(),
+            r#type: "probe".into(),
+            payload: vec![0x80],
+            stored_at: "2026-10-22T00:00:00Z".into(),
+            schema_id: None,
+        };
+        // A newer store whole, and cut short after its header: the header alone is judged.
+        for bytes in [&vector[..], &vector[..vector.len() / 2]] {
+            let temp = tempfile::tempdir()?;
+            let path = temp.path().join("store.msgpack");
+            fs::write(&path, bytes)?;
+            let error = SingleFileMessagePackBackingStore::new(&path)
+                .push_all(std::slice::from_ref(&entry), PushAllOptions::default())
+                .unwrap_err();
+            assert_eq!(unreadable_kind(&error), Some(CultCacheStoreUnreadableKind::UnsupportedFormat), "{error:#}");
+            assert!(format!("{error:#}").contains("cultcache.store.v3"), "{error:#}");
+            assert_eq!(fs::read(&path)?, bytes);
+        }
+        // A file with no readable stamp is not a newer store: a whole write still heals it.
+        let temp = tempfile::tempdir()?;
+        let path = temp.path().join("store.msgpack");
+        fs::write(&path, [0x93u8, 0xa0])?;
+        let mut store = SingleFileMessagePackBackingStore::new(&path);
+        store.push_all(std::slice::from_ref(&entry), PushAllOptions::default())?;
+        assert_eq!(store.pull_all()?.len(), 1);
+        Ok(())
+    }
+
+
     #[test]
     fn a_store_that_cannot_be_read_is_an_io_error_and_never_unreadable() -> Result<()> {
         let temp = tempfile::tempdir()?;
diff --git a/packages/cultcache-ts/test/cult-cache.test.ts b/packages/cultcache-ts/test/cult-cache.test.ts
index 86d4a090..605c68d9 100644
--- a/packages/cultcache-ts/test/cult-cache.test.ts
+++ b/packages/cultcache-ts/test/cult-cache.test.ts
@@ -1481,6 +1481,22 @@ test("SingleFileMessagePackBackingStore refusals say what was found without blam
   );
 });
 
+test("pushAll refuses a store in a format this runtime does not read and leaves it byte-identical", async () => {
+  const vector = await readFile(resolve(cultLibRoot, "tests", "vectors", "document-variants-c2a", "v3-base.msgpack"));
+  const envelope = { key: "probe", type: "probe", payload: documentPayloadFromBytes(new Uint8Array([0x80])), storedAt: "2026-10-22T00:00:00Z" };
+  for (const bytes of [vector, vector.subarray(0, Math.floor(vector.length / 2))]) {
+    const file = join(await mkdtemp(join(tmpdir(), "cultcache-pushall-v3-")), "store.msgpack");
+    await writeFile(file, bytes);
+    await assert.rejects(() => new SingleFileMessagePackBackingStore(file).pushAll([envelope]), /cultcache\.store\.v3/u);
+    assert.equal(Buffer.compare(await readFile(file), bytes), 0);
+  }
+  const healed = join(await mkdtemp(join(tmpdir(), "cultcache-pushall-heal-")), "store.msgpack");
+  await writeFile(healed, new Uint8Array([0x93, 0xa0]));
+  const store = new SingleFileMessagePackBackingStore(healed);
+  await store.pushAll([envelope]);
+  assert.equal((await store.pullAll()).length, 1);
+});
+
 test("SingleFileMessagePackBackingStore reads only a missing store as empty", async () => {
   const dir = await mkdtemp(join(tmpdir(), "cultcache-absent-"));
   assert.deepEqual(await new SingleFileMessagePackBackingStore(join(dir, "store.cc")).pullAll(), []);
diff --git a/tests/vectors/document-variants-c2a/v3-base.msgpack b/tests/vectors/document-variants-c2a/v3-base.msgpack
new file mode 100644
index 00000000..9fc9b64f
--- /dev/null
+++ b/tests/vectors/document-variants-c2a/v3-base.msgpack
@@ -0,0 +1 @@
+“²cultcache.store.v3‘—¬vectors.item¬vectors.item¯vectors.item.v1¬vectors.itemÙL{"schemaName":"vectors.item","schemaVersion":"vectors.item.v1","members":[]}‘¬vectors.item’”¥alpha¬vectors.item¹2026-09-29T00:00:00+00:00Ä’¥alpha
”€beta¬vectors.item¹2026-09-29T00:00:01+00:00Ä’€beta
\ No newline at end of file