diff --git a/openviking/storage/viking_fs.py b/openviking/storage/viking_fs.py
index 2d2360dcd..83e564baf 100644
--- a/openviking/storage/viking_fs.py
+++ b/openviking/storage/viking_fs.py
@@ -17,7 +17,7 @@
 import hashlib
 import json
 import re
-from contextlib import contextmanager
+from contextlib import asynccontextmanager, contextmanager
 from dataclasses import dataclass, field
 from datetime import datetime
 from pathlib import PurePath
@@ -40,6 +40,17 @@
 logger = get_logger(__name__)
 
 
+@asynccontextmanager
+async def _noop_async_lock() -> Any:
+    yield
+
+
+@asynccontextmanager
+async def _asyncio_lock_context(lock: asyncio.Lock) -> Any:
+    async with lock:
+        yield
+
+
 # ========== Dataclass ==========
 
 
@@ -179,6 +190,19 @@ def __init__(
         self._bound_ctx: contextvars.ContextVar[Optional[RequestContext]] = contextvars.ContextVar(
             "vikingfs_bound_ctx", default=None
         )
+        self._append_fallback_locks: Dict[str, asyncio.Lock] = {}
+
+    def _append_lock_context(self, path: str):
+        try:
+            from openviking.storage.transaction import LockContext, get_lock_manager
+
+            return LockContext(get_lock_manager(), [path], lock_mode="point")
+        except RuntimeError:
+            lock = self._append_fallback_locks.get(path)
+            if lock is None:
+                lock = asyncio.Lock()
+                self._append_fallback_locks[path] = lock
+            return _asyncio_lock_context(lock)
 
     @staticmethod
     def _default_ctx() -> RequestContext:
@@ -1716,26 +1740,37 @@ async def append_file(
         ctx: Optional[RequestContext] = None,
     ) -> None:
         """Append content to file."""
+        from openviking.storage.errors import LockAcquisitionError, ResourceBusyError
+
         self._ensure_access(uri, ctx)
         path = self._uri_to_path(uri, ctx=ctx)
+        lock_path = path.rsplit("/", 1)[0] or path
 
         try:
-            existing = ""
-            try:
-                existing_bytes = self._handle_agfs_read(self.agfs.read(path))
-                existing_bytes = await self._decrypt_content(existing_bytes, ctx=ctx)
-                existing = self._decode_bytes(existing_bytes)
-            except AGFSHTTPError as e:
-                if e.status_code != 404:
+            async with self._append_lock_context(lock_path):
+                existing = ""
+                try:
+                    existing_bytes = self._handle_agfs_read(self.agfs.read(path))
+                    existing_bytes = await self._decrypt_content(existing_bytes, ctx=ctx)
+                    existing = self._decode_bytes(existing_bytes)
+                except AGFSHTTPError as e:
+                    if e.status_code != 404:
+                        raise
+                except (FileNotFoundError, RuntimeError) as e:
+                    if not any(
+                        msg in str(e).lower() for msg in ["not found", "no such file or directory"]
+                    ):
+                        raise
+                except AGFSClientError:
                     raise
-            except AGFSClientError:
-                raise
 
-            await self._ensure_parent_dirs(path)
-            final_content = (existing + content).encode("utf-8")
-            final_content = await self._encrypt_content(final_content, ctx=ctx)
-            self.agfs.write(path, final_content)
+                await self._ensure_parent_dirs(path)
+                final_content = (existing + content).encode("utf-8")
+                final_content = await self._encrypt_content(final_content, ctx=ctx)
+                self.agfs.write(path, final_content)
 
+        except LockAcquisitionError as e:
+            raise ResourceBusyError(f"Resource is being processed: {uri}") from e
         except Exception as e:
             logger.error(f"[VikingFS] Failed to append to file {uri}: {e}")
             raise IOError(f"Failed to append to file {uri}: {e}")
diff --git a/tests/storage/test_viking_fs_append.py b/tests/storage/test_viking_fs_append.py
new file mode 100644
index 000000000..fe94c58b5
--- /dev/null
+++ b/tests/storage/test_viking_fs_append.py
@@ -0,0 +1,74 @@
+import asyncio
+from unittest.mock import AsyncMock, MagicMock
+
+import pytest
+
+from openviking.storage.viking_fs import VikingFS
+
+
+class _RaceAGFS:
+    def __init__(self) -> None:
+        self.contents: dict[str, bytes] = {}
+
+    def read(self, path: str) -> bytes:
+        if path not in self.contents:
+            raise RuntimeError(f"not found: {path}")
+        return self.contents[path]
+
+    def write(self, path: str, data: bytes) -> None:
+        self.contents[path] = data
+
+
+@pytest.mark.asyncio
+async def test_append_file_treats_runtime_not_found_as_missing() -> None:
+    agfs = MagicMock()
+    agfs.read.side_effect = RuntimeError(
+        "not found: /local/default/session/default/session-1/messages.jsonl"
+    )
+    fs = VikingFS(agfs=agfs)
+    fs._ensure_parent_dirs = AsyncMock()  # type: ignore[method-assign]
+
+    await fs.append_file(
+        "viking://session/default/session-1/messages.jsonl",
+        '{"role":"user"}\n',
+    )
+
+    agfs.write.assert_called_once_with(
+        "/local/default/session/default/session-1/messages.jsonl",
+        b'{"role":"user"}\n',
+    )
+
+
+@pytest.mark.asyncio
+async def test_append_file_still_raises_unrelated_runtime_errors() -> None:
+    agfs = MagicMock()
+    agfs.read.side_effect = RuntimeError("backend unavailable")
+    fs = VikingFS(agfs=agfs)
+    fs._ensure_parent_dirs = AsyncMock()  # type: ignore[method-assign]
+
+    with pytest.raises(IOError, match="backend unavailable"):
+        await fs.append_file(
+            "viking://session/default/session-1/messages.jsonl",
+            '{"role":"user"}\n',
+        )
+
+
+@pytest.mark.asyncio
+async def test_append_file_serializes_concurrent_first_writes() -> None:
+    agfs = _RaceAGFS()
+    fs = VikingFS(agfs=agfs)
+
+    async def slow_ensure_parent_dirs(_path: str) -> None:
+        await asyncio.sleep(0.01)
+
+    fs._ensure_parent_dirs = slow_ensure_parent_dirs  # type: ignore[method-assign]
+
+    await asyncio.gather(
+        fs.append_file("viking://session/default/session-1/messages.jsonl", "first\n"),
+        fs.append_file("viking://session/default/session-1/messages.jsonl", "second\n"),
+    )
+
+    assert agfs.contents["/local/default/session/default/session-1/messages.jsonl"] in {
+        b"first\nsecond\n",
+        b"second\nfirst\n",
+    }