diff --git a/backend/packages/harness/deerflow/community/aio_sandbox/quarantine.py b/backend/packages/harness/deerflow/community/aio_sandbox/quarantine.py
index 202c7a5c8f..9908bce300 100644
--- a/backend/packages/harness/deerflow/community/aio_sandbox/quarantine.py
+++ b/backend/packages/harness/deerflow/community/aio_sandbox/quarantine.py
@@ -21,7 +21,13 @@ class SandboxQuarantine:
 
     @staticmethod
     def _key(value: str) -> str:
-        return hashlib.sha256(value.encode("utf-8")).hexdigest()
+        # Truncate to 16 hex chars (64 bits). The quarantine nests three hashed
+        # components (scope, sandbox id, container id); three full 64-char
+        # SHA-256 hexdigests exceed Windows' 260-char MAX_PATH when the root
+        # lives under a long path, failing record creation with ENOENT. 64 bits
+        # is far beyond what non-adversarial sandbox/container ids need, and
+        # matches the sandbox-naming convention elsewhere in this module.
+        return hashlib.sha256(value.encode("utf-8")).hexdigest()[:16]
 
     def _directory(self, info: SandboxInfo) -> Path:
         return self._root / self._key(info.sandbox_id)
diff --git a/backend/tests/test_sandbox_quarantine.py b/backend/tests/test_sandbox_quarantine.py
index dfe9f087c5..eda32e1e21 100644
--- a/backend/tests/test_sandbox_quarantine.py
+++ b/backend/tests/test_sandbox_quarantine.py
@@ -90,3 +90,14 @@ def test_retire_replaced_generations_keeps_unversioned_crash_fence(tmp_path):
     store.retire_replaced_generations(SandboxInfo("thread-sandbox", "http://sandbox", container_id="new-generation"))
 
     assert (directory / "unknown").exists()
+
+
+def test_quarantine_keys_are_truncated_for_windows_max_path():
+    # The quarantine nests three hashed path components (scope, sandbox id,
+    # container id). Three full 64-char SHA-256 hexdigests exceed Windows'
+    # 260-char MAX_PATH under a long root, so each key is truncated to 16 hex
+    # chars (64 bits). Pin that bound so a revert can't silently re-break
+    # Windows record creation while Linux CI still passes.
+    assert len(SandboxQuarantine._key("thread-sandbox")) == 16
+    assert len(SandboxQuarantine._key("provisioner-a")) == 16
+    assert len(SandboxQuarantine._key("old-generation")) == 16