diff --git a/backend/packages/harness/deerflow/community/aio_sandbox/aio_sandbox_provider.py b/backend/packages/harness/deerflow/community/aio_sandbox/aio_sandbox_provider.py
index f3f7df088..39f75d212 100644
--- a/backend/packages/harness/deerflow/community/aio_sandbox/aio_sandbox_provider.py
+++ b/backend/packages/harness/deerflow/community/aio_sandbox/aio_sandbox_provider.py
@@ -87,6 +87,19 @@ class SandboxBeingDestroyedError(RuntimeError):
         self.sandbox_id = sandbox_id
 
 
+class SandboxIdentityCollisionError(RuntimeError):
+    """A deterministic ID is already tracked for a different user/thread."""
+
+    def __init__(
+        self,
+        sandbox_id: str,
+        stored_key: tuple[str, str] | None,
+        requested_key: tuple[str, str],
+    ) -> None:
+        super().__init__(f"sandbox ID collision for {sandbox_id}: tracked identity is {stored_key!r}, requested identity is {requested_key!r}")
+        self.sandbox_id = sandbox_id
+
+
 def _lock_file_exclusive(lock_file) -> None:
     if fcntl is not None:
         fcntl.flock(lock_file, fcntl.LOCK_EX)
@@ -183,6 +196,8 @@ class AioSandboxProvider(WarmPoolLifecycleMixin[SandboxInfo], SandboxProvider):
         # Containers here can be reclaimed quickly (no cold-start) or destroyed
         # when replicas capacity is exhausted.
         self._warm_pool: dict[str, tuple[SandboxInfo, float]] = {}
+        self._active_sandbox_identity: dict[str, tuple[str, str] | None] = {}
+        self._warm_pool_identity: dict[str, tuple[str, str] | None] = {}
         # sandbox_id -> when reconciliation first saw it running with no lease.
         # Gates adoption behind a recovery grace (see _adoptable_after_grace).
         self._unowned_since: dict[str, float] = {}
@@ -700,6 +715,7 @@ class AioSandboxProvider(WarmPoolLifecycleMixin[SandboxInfo], SandboxProvider):
                     logger.debug("Deferring container %s during reconciliation: this instance is tearing it down", info.sandbox_id)
                     continue
                 self._warm_pool[info.sandbox_id] = (info, current_time)
+                self._warm_pool_identity[info.sandbox_id] = None
             self._unowned_since.pop(info.sandbox_id, None)
             adopted += 1
             logger.info(f"Adopted container {info.sandbox_id} into warm pool (age: {age:.0f}s)")
@@ -728,8 +744,42 @@ class AioSandboxProvider(WarmPoolLifecycleMixin[SandboxInfo], SandboxProvider):
 
         Includes user_id so a previously-created default-bucket sandbox cannot be
         reused for an auth/channel run that should mount a user-scoped bucket.
+
+        During a mixed-version rollout, older 8-character containers are not
+        reused under the new 16-character identity. They remain eligible for
+        normal orphan cleanup while the first new-version acquire cold-starts.
         """
-        return hashlib.sha256(f"{user_id}:{thread_id}".encode()).hexdigest()[:8]
+        return hashlib.sha256(f"{user_id}:{thread_id}".encode()).hexdigest()[:16]
+
+    def _assert_active_identity_available_locked(
+        self,
+        sandbox_id: str,
+        requested_key: tuple[str, str],
+    ) -> None:
+        """Fail closed if an active truncated ID belongs to another identity."""
+        if sandbox_id not in self._sandboxes and sandbox_id not in self._sandbox_infos:
+            return
+
+        stored_key = self._active_sandbox_identity.get(sandbox_id)
+        if stored_key is None:
+            matching_keys = [key for key, mapped_id in self._thread_sandboxes.items() if mapped_id == sandbox_id]
+            if len(matching_keys) == 1:
+                stored_key = matching_keys[0]
+        if stored_key != requested_key:
+            raise SandboxIdentityCollisionError(sandbox_id, stored_key, requested_key)
+
+    def _assert_warm_identity_available_locked(
+        self,
+        sandbox_id: str,
+        requested_key: tuple[str, str],
+    ) -> None:
+        """Fail closed if a warm ID changed tenants during an acquire."""
+        if sandbox_id not in self._warm_pool:
+            return
+        # Startup-adopted entries have unknown identity until their first reclaim.
+        stored_key = self._warm_pool_identity.get(sandbox_id)
+        if stored_key is not None and stored_key != requested_key:
+            raise SandboxIdentityCollisionError(sandbox_id, stored_key, requested_key)
 
     # ── Mount helpers ────────────────────────────────────────────────────
 
@@ -1069,8 +1119,10 @@ class AioSandboxProvider(WarmPoolLifecycleMixin[SandboxInfo], SandboxProvider):
 
             sandbox = self._sandboxes.pop(sandbox_id, None)
             self._sandbox_infos.pop(sandbox_id, None)
+            self._active_sandbox_identity.pop(sandbox_id, None)
             self._last_activity.pop(sandbox_id, None)
             self._warm_pool.pop(sandbox_id, None)
+            self._warm_pool_identity.pop(sandbox_id, None)
             self._acquire_epoch.pop(sandbox_id, None)
             for key, mapped_id in list(self._thread_sandboxes.items()):
                 if mapped_id == sandbox_id:
@@ -1310,6 +1362,7 @@ class AioSandboxProvider(WarmPoolLifecycleMixin[SandboxInfo], SandboxProvider):
         with self._lock:
             if sandbox_id not in self._warm_pool:
                 return None
+            self._assert_warm_identity_available_locked(sandbox_id, key)
             if self._being_torn_down_locally(sandbox_id):
                 # The entry deliberately stays in `_warm_pool` for the whole stop
                 # (so a refused claim does not lose the container), so pool
@@ -1350,13 +1403,16 @@ class AioSandboxProvider(WarmPoolLifecycleMixin[SandboxInfo], SandboxProvider):
                 # that is already stopped.
                 logger.info("Warm-pool sandbox %s was claimed for teardown while publishing ownership; not reclaiming it", sandbox_id)
                 return None
+            self._assert_warm_identity_available_locked(sandbox_id, key)
             warm_item = self._warm_pool.pop(sandbox_id, None)
             if warm_item is None:
                 return None
+            self._warm_pool_identity.pop(sandbox_id, None)
             info, _ = warm_item
             sandbox = AioSandbox(id=sandbox_id, base_url=info.sandbox_url)
             self._sandboxes[sandbox_id] = sandbox
             self._sandbox_infos[sandbox_id] = info
+            self._active_sandbox_identity[sandbox_id] = key
             self._last_activity[sandbox_id] = time.time()
             self._thread_sandboxes[key] = sandbox_id
 
@@ -1385,6 +1441,7 @@ class AioSandboxProvider(WarmPoolLifecycleMixin[SandboxInfo], SandboxProvider):
                 prevent. The window is a peer's in-flight container stop, so the
                 thread's next turn discovers nothing and cold-starts cleanly.
         """
+        key = self._thread_key(thread_id, user_id)
         with self._lock:
             if self._being_torn_down_locally(info.sandbox_id):
                 # Discovery is the fall-through once the caches miss, so it is
@@ -1392,9 +1449,10 @@ class AioSandboxProvider(WarmPoolLifecycleMixin[SandboxInfo], SandboxProvider):
                 # only refuse this once the reaper's `del:` claim has landed;
                 # until then it succeeds against our own lease.
                 raise SandboxBeingDestroyedError(info.sandbox_id)
+            self._assert_active_identity_available_locked(info.sandbox_id, key)
+            self._assert_warm_identity_available_locked(info.sandbox_id, key)
 
         sandbox = AioSandbox(id=info.sandbox_id, base_url=info.sandbox_url)
-        key = self._thread_key(thread_id, user_id)
         # Ownership first, so a failure cannot leave a tracked-but-unowned sandbox.
         # There is no container to roll back (we did not create it), but the
         # host-side HTTP client constructed above is ours and must not leak —
@@ -1408,6 +1466,8 @@ class AioSandboxProvider(WarmPoolLifecycleMixin[SandboxInfo], SandboxProvider):
                     # trip. Do not install a client for a container that reaper
                     # has already committed to stopping.
                     raise SandboxBeingDestroyedError(info.sandbox_id)
+                self._assert_active_identity_available_locked(info.sandbox_id, key)
+                self._assert_warm_identity_available_locked(info.sandbox_id, key)
                 # Active and warm are exclusive states, and only this insert can
                 # violate that: a warm entry for the same id is stale the moment
                 # the id becomes active. Leaving it there gives the container two
@@ -1416,11 +1476,17 @@ class AioSandboxProvider(WarmPoolLifecycleMixin[SandboxInfo], SandboxProvider):
                 # agent is actively using while `_sandboxes` still hands out its
                 # client.
                 self._warm_pool.pop(info.sandbox_id, None)
+                self._warm_pool_identity.pop(info.sandbox_id, None)
                 self._sandboxes[info.sandbox_id] = sandbox
                 self._sandbox_infos[info.sandbox_id] = info
+                self._active_sandbox_identity[info.sandbox_id] = key
                 self._last_activity[info.sandbox_id] = time.time()
                 self._thread_sandboxes[key] = info.sandbox_id
-        except (OwnershipBackendError, SandboxBeingDestroyedError):
+        except (
+            OwnershipBackendError,
+            SandboxBeingDestroyedError,
+            SandboxIdentityCollisionError,
+        ):
             try:
                 sandbox.close()
             except Exception as e:
@@ -1433,6 +1499,14 @@ class AioSandboxProvider(WarmPoolLifecycleMixin[SandboxInfo], SandboxProvider):
     def _register_created_sandbox(self, thread_id: str | None, sandbox_id: str, info: SandboxInfo, *, user_id: str | None = None) -> str:
         """Track a newly-created sandbox in the active maps."""
         sandbox = AioSandbox(id=sandbox_id, base_url=info.sandbox_url)
+        key = (
+            self._thread_key(
+                thread_id,
+                self._effective_acquire_user_id(user_id),
+            )
+            if thread_id
+            else None
+        )
         # Ownership first. Unlike the discover path there IS something to roll
         # back: we just started this container, and an unowned running container
         # is exactly what a peer's reconciliation adopts. Leaking it would hand a
@@ -1441,9 +1515,34 @@ class AioSandboxProvider(WarmPoolLifecycleMixin[SandboxInfo], SandboxProvider):
         # mid-stop leaves a teardown marker until its TTL lapses. Roll back on
         # both, or the container we just started is leaked.
         try:
+            if key is not None:
+                with self._lock:
+                    self._assert_active_identity_available_locked(sandbox_id, key)
+                    self._assert_warm_identity_available_locked(sandbox_id, key)
             self._publish_ownership(sandbox_id)
-        except (OwnershipBackendError, SandboxBeingDestroyedError):
-            logger.error("Could not publish ownership for new sandbox %s; destroying it rather than leaking an unowned container", sandbox_id)
+
+            with self._lock:
+                if key is not None:
+                    self._assert_active_identity_available_locked(sandbox_id, key)
+                    self._assert_warm_identity_available_locked(sandbox_id, key)
+                # Same exclusivity rule as the discover path.
+                self._warm_pool.pop(sandbox_id, None)
+                self._warm_pool_identity.pop(sandbox_id, None)
+                self._sandboxes[sandbox_id] = sandbox
+                self._sandbox_infos[sandbox_id] = info
+                self._active_sandbox_identity[sandbox_id] = key
+                self._last_activity[sandbox_id] = time.time()
+                if key is not None:
+                    self._thread_sandboxes[key] = sandbox_id
+        except (
+            OwnershipBackendError,
+            SandboxBeingDestroyedError,
+            SandboxIdentityCollisionError,
+        ):
+            logger.error(
+                "Could not register new sandbox %s; destroying it rather than leaking an untracked container",
+                sandbox_id,
+            )
             try:
                 sandbox.close()
             except Exception as e:
@@ -1451,18 +1550,13 @@ class AioSandboxProvider(WarmPoolLifecycleMixin[SandboxInfo], SandboxProvider):
             try:
                 self._backend.destroy(info)
             except Exception as e:
-                logger.error("Failed to destroy unowned sandbox %s after ownership failure: %s", sandbox_id, e)
+                logger.error(
+                    "Failed to destroy sandbox %s after registration failure: %s",
+                    sandbox_id,
+                    e,
+                )
             raise
 
-        with self._lock:
-            # Same exclusivity rule as the discover path.
-            self._warm_pool.pop(sandbox_id, None)
-            self._sandboxes[sandbox_id] = sandbox
-            self._sandbox_infos[sandbox_id] = info
-            self._last_activity[sandbox_id] = time.time()
-            if thread_id:
-                self._thread_sandboxes[self._thread_key(thread_id, self._effective_acquire_user_id(user_id))] = sandbox_id
-
         logger.info(f"Created sandbox {sandbox_id} for thread {thread_id} at {info.sandbox_url}")
         return sandbox_id
 
@@ -1498,6 +1592,7 @@ class AioSandboxProvider(WarmPoolLifecycleMixin[SandboxInfo], SandboxProvider):
 
             sandbox = self._sandboxes.pop(sandbox_id, None)
             info = self._sandbox_infos.pop(sandbox_id, None)
+            self._active_sandbox_identity.pop(sandbox_id, None)
             thread_keys_to_remove = [key for key, sid in self._thread_sandboxes.items() if sid == sandbox_id]
             for key in thread_keys_to_remove:
                 del self._thread_sandboxes[key]
@@ -1507,6 +1602,7 @@ class AioSandboxProvider(WarmPoolLifecycleMixin[SandboxInfo], SandboxProvider):
                 info, _ = self._warm_pool.pop(sandbox_id)
             else:
                 self._warm_pool.pop(sandbox_id, None)
+            self._warm_pool_identity.pop(sandbox_id, None)
 
         return sandbox, info, True
 
@@ -1623,7 +1719,10 @@ class AioSandboxProvider(WarmPoolLifecycleMixin[SandboxInfo], SandboxProvider):
             # The pop stays deferred relative to the *stop* (a refused or failed
             # stop keeps the entry), just no longer relative to the reservation.
             with self._lock:
-                self._warm_pool.pop(sandbox_id, None)
+                current = self._warm_pool.get(sandbox_id)
+                if current is not None and current[0] is entry:
+                    self._warm_pool.pop(sandbox_id, None)
+                    self._warm_pool_identity.pop(sandbox_id, None)
         finally:
             self._finish_local_teardown(sandbox_id)
 
@@ -1692,6 +1791,10 @@ class AioSandboxProvider(WarmPoolLifecycleMixin[SandboxInfo], SandboxProvider):
 
         # Deterministic ID for thread-specific, random for anonymous
         sandbox_id = self._sandbox_id_for_thread(thread_id, user_id)
+        if thread_id:
+            key = self._thread_key(thread_id, user_id)
+            with self._lock:
+                self._assert_active_identity_available_locked(sandbox_id, key)
 
         # ── Layer 1.5: Warm pool (container still running, no cold-start) ──
         reclaimed_id = self._reclaim_warm_pool_sandbox(thread_id, sandbox_id, user_id=user_id)
@@ -1715,6 +1818,10 @@ class AioSandboxProvider(WarmPoolLifecycleMixin[SandboxInfo], SandboxProvider):
 
         # Deterministic ID for thread-specific, random for anonymous
         sandbox_id = self._sandbox_id_for_thread(thread_id, user_id)
+        if thread_id:
+            key = self._thread_key(thread_id, user_id)
+            with self._lock:
+                self._assert_active_identity_available_locked(sandbox_id, key)
 
         # ── Layer 1.5: Warm pool (container still running, no cold-start) ──
         reclaimed_id = await asyncio.to_thread(self._reclaim_warm_pool_sandbox, thread_id, sandbox_id, user_id=user_id)
@@ -1909,10 +2016,12 @@ class AioSandboxProvider(WarmPoolLifecycleMixin[SandboxInfo], SandboxProvider):
             thread_keys_to_remove = [key for key, sid in self._thread_sandboxes.items() if sid == sandbox_id]
             for key in thread_keys_to_remove:
                 del self._thread_sandboxes[key]
+            active_identity = self._active_sandbox_identity.pop(sandbox_id, None)
             self._last_activity.pop(sandbox_id, None)
             # Park in warm pool — container keeps running
             if info and sandbox_id not in self._warm_pool:
                 self._warm_pool[sandbox_id] = (info, time.time())
+                self._warm_pool_identity[sandbox_id] = thread_keys_to_remove[0] if thread_keys_to_remove else active_identity
 
         if sandbox is not None:
             # Defense-in-depth: close() already swallows its own errors; this
@@ -2015,6 +2124,7 @@ class AioSandboxProvider(WarmPoolLifecycleMixin[SandboxInfo], SandboxProvider):
             sandbox_ids = list(self._sandboxes.keys())
             warm_items = list(self._warm_pool.items())
             self._warm_pool.clear()
+            self._warm_pool_identity.clear()
 
         self._stop_idle_checker()
         # Stop renewing before destroying: the destroy paths claim ownership
diff --git a/backend/packages/harness/deerflow/community/boxlite/provider.py b/backend/packages/harness/deerflow/community/boxlite/provider.py
index fa0981be4..e49fecdb1 100644
--- a/backend/packages/harness/deerflow/community/boxlite/provider.py
+++ b/backend/packages/harness/deerflow/community/boxlite/provider.py
@@ -41,6 +41,7 @@ T = TypeVar("T")
 
 DEFAULT_IMAGE = "python:3.12-slim"
 _BOX_NAME_PREFIX = "deer-flow-boxlite-"
+_NO_ACTIVE_IDENTITY = object()
 # DeerFlow's virtual prefixes, materialised on the box rootfs at start so the
 # Sandbox file APIs (which address /mnt/user-data/...) resolve natively.
 _VIRTUAL_DIRS = (
@@ -51,6 +52,18 @@ _VIRTUAL_DIRS = (
 )
 
 
+class SandboxIdentityCollisionError(RuntimeError):
+    """A deterministic ID is already tracked for a different user/thread."""
+
+    def __init__(
+        self,
+        sandbox_id: str,
+        stored_key: tuple[str, str] | None,
+        requested_key: tuple[str, str],
+    ) -> None:
+        super().__init__(f"Sandbox ID collision for {sandbox_id}: active box belongs to {stored_key!r}, not {requested_key!r}")
+
+
 def _import_simplebox() -> type[SimpleBox]:
     """Import BoxLite's async ``SimpleBox`` lazily.
 
@@ -166,8 +179,13 @@ class BoxliteProvider(WarmPoolLifecycleMixin[BoxliteBox], SandboxProvider):
 
         Includes user_id so a box created for one user's bucket cannot be
         reclaimed by another user's thread with the same thread_id.
+
+        The 8-to-16-character change intentionally causes one cold start during
+        a mixed-version rollout. Older 8-character boxes remain in the warm pool
+        until the normal orphan cleanup removes them; they are never reused
+        under a new 16-character identity.
         """
-        return hashlib.sha256(f"{user_id}:{thread_id}".encode()).hexdigest()[:8]
+        return hashlib.sha256(f"{user_id}:{thread_id}".encode()).hexdigest()[:16]
 
     # ── Provider ────────────────────────────────────────────────────────
 
@@ -176,6 +194,8 @@ class BoxliteProvider(WarmPoolLifecycleMixin[BoxliteBox], SandboxProvider):
         self._boxes: dict[str, BoxliteBox] = {}
         self._thread_boxes: dict[tuple[str, str], str] = {}
         self._warm_pool: dict[str, tuple[BoxliteBox, float]] = {}
+        self._active_box_identity: dict[str, tuple[str, str] | None] = {}
+        self._warm_pool_identity: dict[str, tuple[str, str] | None] = {}
         self._skip_health_check_warm_ids: set[str] = set()
         self._acquire_locks: dict[str, threading.Lock] = {}
         self._idle_checker_stop = threading.Event()
@@ -245,7 +265,9 @@ class BoxliteProvider(WarmPoolLifecycleMixin[BoxliteBox], SandboxProvider):
     def _destroy_warm_entry(self, sandbox_id: str, entry: BoxliteBox, *, reason: str) -> None:
         """Close a removed warm-pool entry and log with context."""
         with self._lock:
-            self._skip_health_check_warm_ids.discard(sandbox_id)
+            if sandbox_id not in self._warm_pool:
+                self._skip_health_check_warm_ids.discard(sandbox_id)
+                self._warm_pool_identity.pop(sandbox_id, None)
         try:
             entry.close()
             if reason == "idle_timeout":
@@ -268,6 +290,8 @@ class BoxliteProvider(WarmPoolLifecycleMixin[BoxliteBox], SandboxProvider):
         with self._lock:
             active_box = self._boxes.pop(sandbox_id, None)
             warm_entry = self._warm_pool.pop(sandbox_id, None)
+            self._active_box_identity.pop(sandbox_id, None)
+            self._warm_pool_identity.pop(sandbox_id, None)
             self._skip_health_check_warm_ids.discard(sandbox_id)
             for key in [k for k, sid in self._thread_boxes.items() if sid == sandbox_id]:
                 self._thread_boxes.pop(key, None)
@@ -335,6 +359,7 @@ class BoxliteProvider(WarmPoolLifecycleMixin[BoxliteBox], SandboxProvider):
                     box_runtime.stop()
                     continue
                 self._warm_pool[sandbox_id] = (wrapped, now)
+                self._warm_pool_identity[sandbox_id] = None
                 adopted += 1
             logger.info("Adopted existing BoxLite box %s (%s) into warm pool", sandbox_id, name)
 
@@ -348,6 +373,7 @@ class BoxliteProvider(WarmPoolLifecycleMixin[BoxliteBox], SandboxProvider):
             box = self._create_box(sandbox_id)
             with self._lock:
                 self._boxes[box.id] = box
+                self._active_box_identity[box.id] = None
             return box.id
 
         key = self._thread_key(thread_id, user_id)
@@ -358,17 +384,42 @@ class BoxliteProvider(WarmPoolLifecycleMixin[BoxliteBox], SandboxProvider):
                 existing = self._thread_boxes.get(key)
                 if existing is not None and existing in self._boxes:
                     return existing
+                if sandbox_id in self._boxes:
+                    owner = self._active_box_identity.get(sandbox_id)
+                    if owner != key:
+                        raise SandboxIdentityCollisionError(
+                            sandbox_id,
+                            owner,
+                            key,
+                        )
+                    self._thread_boxes[key] = sandbox_id
+                    return sandbox_id
 
-            reclaimed = self._reclaim_warm_pool(sandbox_id)
+            reclaimed = self._reclaim_warm_pool(sandbox_id, key)
             if reclaimed is not None:
                 with self._lock:
                     self._thread_boxes[key] = reclaimed
                 return reclaimed
 
             box = self._create_box(sandbox_id)
+            conflict: tuple[str, str] | None | object = _NO_ACTIVE_IDENTITY
             with self._lock:
-                self._boxes[box.id] = box
-                self._thread_boxes[key] = box.id
+                if box.id in self._boxes:
+                    conflict = self._active_box_identity.get(box.id)
+                    if conflict == key:
+                        self._thread_boxes[key] = box.id
+                else:
+                    self._boxes[box.id] = box
+                    self._active_box_identity[box.id] = key
+                    self._thread_boxes[key] = box.id
+            if conflict is not _NO_ACTIVE_IDENTITY:
+                box.close()
+                if conflict != key:
+                    raise SandboxIdentityCollisionError(
+                        sandbox_id,
+                        conflict,
+                        key,
+                    )
             return box.id
 
     def _create_box(self, sandbox_id: str) -> BoxliteBox:
@@ -410,15 +461,19 @@ class BoxliteProvider(WarmPoolLifecycleMixin[BoxliteBox], SandboxProvider):
         close_box: BoxliteBox | None = None
         with self._lock:
             box = self._boxes.pop(sandbox_id, None)
-            for key in [k for k, sid in self._thread_boxes.items() if sid == sandbox_id]:
+            released_keys = [k for k, sid in self._thread_boxes.items() if sid == sandbox_id]
+            for key in released_keys:
                 self._thread_boxes.pop(key, None)
+            active_identity = self._active_box_identity.pop(sandbox_id, None)
             if box is None:
                 return
             if self._shutdown_called:
                 close_box = box
                 self._skip_health_check_warm_ids.discard(sandbox_id)
+                self._warm_pool_identity.pop(sandbox_id, None)
             else:
                 self._warm_pool[sandbox_id] = (box, time.time())
+                self._warm_pool_identity[sandbox_id] = released_keys[0] if released_keys else active_identity
                 self._skip_health_check_warm_ids.add(sandbox_id)
 
         if close_box is not None:
@@ -427,7 +482,7 @@ class BoxliteProvider(WarmPoolLifecycleMixin[BoxliteBox], SandboxProvider):
         else:
             logger.info("Released sandbox %s to warm pool (VM still running)", sandbox_id)
 
-    def _reclaim_warm_pool(self, sandbox_id: str) -> str | None:
+    def _reclaim_warm_pool(self, sandbox_id: str, expected_key: tuple[str, str]) -> str | None:
         """Try to reclaim a warm-pool box by sandbox_id.
 
         Returns sandbox_id on success, None if not found or dead.
@@ -440,6 +495,14 @@ class BoxliteProvider(WarmPoolLifecycleMixin[BoxliteBox], SandboxProvider):
         with self._lock:
             if sandbox_id not in self._warm_pool:
                 return None
+            # Startup-adopted entries have unknown identity until their first reclaim.
+            stored_key = self._warm_pool_identity.get(sandbox_id)
+            if stored_key is not None and stored_key != expected_key:
+                raise SandboxIdentityCollisionError(
+                    sandbox_id,
+                    stored_key,
+                    expected_key,
+                )
             box, released_at = self._warm_pool[sandbox_id]
             skip_eligible = sandbox_id in self._skip_health_check_warm_ids
 
@@ -449,10 +512,21 @@ class BoxliteProvider(WarmPoolLifecycleMixin[BoxliteBox], SandboxProvider):
             # health-check round trip, but never return an adapter that this
             # process already knows is closed.
             with self._lock:
+                current = self._warm_pool.get(sandbox_id)
+                stored_key = self._warm_pool_identity.get(sandbox_id)
+                if stored_key is not None and stored_key != expected_key:
+                    raise SandboxIdentityCollisionError(
+                        sandbox_id,
+                        stored_key,
+                        expected_key,
+                    )
+                if current is None or current[0] is not box:
+                    return None
                 warm_entry = self._warm_pool.pop(sandbox_id, None)
                 if warm_entry is None:
                     return None  # Raced with another thread
                 self._skip_health_check_warm_ids.discard(sandbox_id)
+                self._warm_pool_identity.pop(sandbox_id, None)
                 box, _ = warm_entry
                 if box.is_closed:
                     logger.warning("Warm-pool box %s was closed before skipped health check reclaim", sandbox_id)
@@ -460,6 +534,7 @@ class BoxliteProvider(WarmPoolLifecycleMixin[BoxliteBox], SandboxProvider):
                 else:
                     close_box = None
                     self._boxes[sandbox_id] = box
+                    self._active_box_identity[sandbox_id] = expected_key
             if close_box is not None:
                 close_box.close()
                 return None
@@ -476,26 +551,60 @@ class BoxliteProvider(WarmPoolLifecycleMixin[BoxliteBox], SandboxProvider):
             if "ok" not in result:
                 logger.warning("Warm pool box %s health check failed: %s", sandbox_id, result)
                 with self._lock:
-                    warm_entry = self._warm_pool.pop(sandbox_id, None)
+                    current = self._warm_pool.get(sandbox_id)
+                    stored_key = self._warm_pool_identity.get(sandbox_id)
+                    if current is not None and current[0] is not box and stored_key is not None and stored_key != expected_key:
+                        raise SandboxIdentityCollisionError(
+                            sandbox_id,
+                            stored_key,
+                            expected_key,
+                        )
+                    warm_entry = self._warm_pool.pop(sandbox_id, None) if current is not None and current[0] is box else None
+                    if warm_entry is not None:
+                        self._warm_pool_identity.pop(sandbox_id, None)
                 if warm_entry is not None:
                     self._destroy_warm_entry(sandbox_id, warm_entry[0], reason="health_check_failed")
                 return None
+        except SandboxIdentityCollisionError:
+            raise
         except Exception as e:
             logger.warning("Warm pool box %s health check error: %s", sandbox_id, e)
             with self._lock:
-                warm_entry = self._warm_pool.pop(sandbox_id, None)
+                current = self._warm_pool.get(sandbox_id)
+                stored_key = self._warm_pool_identity.get(sandbox_id)
+                if current is not None and current[0] is not box and stored_key is not None and stored_key != expected_key:
+                    raise SandboxIdentityCollisionError(
+                        sandbox_id,
+                        stored_key,
+                        expected_key,
+                    )
+                warm_entry = self._warm_pool.pop(sandbox_id, None) if current is not None and current[0] is box else None
+                if warm_entry is not None:
+                    self._warm_pool_identity.pop(sandbox_id, None)
             if warm_entry is not None:
                 self._destroy_warm_entry(sandbox_id, warm_entry[0], reason="health_check_failed")
             return None
 
         # Promote from warm pool to active
         with self._lock:
+            current = self._warm_pool.get(sandbox_id)
+            stored_key = self._warm_pool_identity.get(sandbox_id)
+            if stored_key is not None and stored_key != expected_key:
+                raise SandboxIdentityCollisionError(
+                    sandbox_id,
+                    stored_key,
+                    expected_key,
+                )
+            if current is None or current[0] is not box:
+                return None
             warm_entry = self._warm_pool.pop(sandbox_id, None)
             if warm_entry is None:
                 return None  # Raced with another thread
             self._skip_health_check_warm_ids.discard(sandbox_id)
+            self._warm_pool_identity.pop(sandbox_id, None)
             box, _ = warm_entry
             self._boxes[sandbox_id] = box
+            self._active_box_identity[sandbox_id] = expected_key
 
         logger.info("Reclaimed warm-pool box %s", sandbox_id)
         return sandbox_id
@@ -513,8 +622,10 @@ class BoxliteProvider(WarmPoolLifecycleMixin[BoxliteBox], SandboxProvider):
             now = time.time()
             for sandbox_id, box in self._boxes.items():
                 self._warm_pool.setdefault(sandbox_id, (box, now))
+                self._warm_pool_identity.setdefault(sandbox_id, self._active_box_identity.get(sandbox_id))
                 self._skip_health_check_warm_ids.discard(sandbox_id)
             self._boxes.clear()
+            self._active_box_identity.clear()
             self._thread_boxes.clear()
             self._acquire_locks.clear()
 
@@ -531,6 +642,8 @@ class BoxliteProvider(WarmPoolLifecycleMixin[BoxliteBox], SandboxProvider):
             warm = [box for box, _ in self._warm_pool.values()]
             self._boxes.clear()
             self._warm_pool.clear()
+            self._active_box_identity.clear()
+            self._warm_pool_identity.clear()
             self._thread_boxes.clear()
             self._acquire_locks.clear()
             self._skip_health_check_warm_ids.clear()
diff --git a/backend/tests/blocking_io/test_aio_sandbox_get.py b/backend/tests/blocking_io/test_aio_sandbox_get.py
index 4f6e11761..5ad2f1313 100644
--- a/backend/tests/blocking_io/test_aio_sandbox_get.py
+++ b/backend/tests/blocking_io/test_aio_sandbox_get.py
@@ -82,6 +82,8 @@ def _make_provider(tmp_path: Path):
     provider._thread_locks = {}
     provider._last_activity = {}
     provider._warm_pool = {}
+    provider._active_sandbox_identity = {}
+    provider._warm_pool_identity = {}
     provider._local_teardown = set()
     provider._acquire_epoch = {}
     provider._acquire_epoch_counter = 0
diff --git a/backend/tests/blocking_io/test_sandbox_release.py b/backend/tests/blocking_io/test_sandbox_release.py
index 2dcd4b6b7..226c8e284 100644
--- a/backend/tests/blocking_io/test_sandbox_release.py
+++ b/backend/tests/blocking_io/test_sandbox_release.py
@@ -75,6 +75,7 @@ def _make_provider_with_active_sandbox(tmp_path: Path, sandbox_id: str):
     provider = AioSandboxProvider.__new__(AioSandboxProvider)
     provider._lock = threading.Lock()
     provider._sandboxes = {sandbox_id: MagicMock()}
+    provider._active_sandbox_identity = {sandbox_id: ("default", "thread-1")}
     provider._sandbox_infos = {
         sandbox_id: SandboxInfo(
             sandbox_id=sandbox_id,
@@ -87,6 +88,7 @@ def _make_provider_with_active_sandbox(tmp_path: Path, sandbox_id: str):
     provider._thread_locks = {}
     provider._last_activity = {sandbox_id: 1.0}
     provider._warm_pool = {}
+    provider._warm_pool_identity = {}
     provider._unowned_since = {}
     provider._local_teardown = set()
     provider._acquire_epoch = {}
diff --git a/backend/tests/test_aio_sandbox_provider.py b/backend/tests/test_aio_sandbox_provider.py
index 4da31b581..6e08d5146 100644
--- a/backend/tests/test_aio_sandbox_provider.py
+++ b/backend/tests/test_aio_sandbox_provider.py
@@ -1,6 +1,8 @@
 """Tests for AioSandboxProvider mount helpers."""
 
 import asyncio
+import contextlib
+import hashlib
 import importlib
 import stat
 from types import SimpleNamespace
@@ -11,6 +13,11 @@ import pytest
 from deerflow.config.paths import Paths, join_host_path
 from deerflow.runtime.user_context import reset_current_user, set_current_user
 
+_LEGACY_COLLIDING_IDENTITIES = (
+    ("user-9721", "thread-9721"),
+    ("user-94361", "thread-94361"),
+)
+
 # ── ensure_thread_dirs ───────────────────────────────────────────────────────
 
 
@@ -62,6 +69,8 @@ def _make_provider(tmp_path):
         provider = aio_mod.AioSandboxProvider.__new__(aio_mod.AioSandboxProvider)
         provider._config = {"idle_timeout": 600, "replicas": 3}
         provider._sandboxes = {}
+        provider._active_sandbox_identity = {}
+        provider._warm_pool_identity = {}
         provider._local_teardown = set()
         provider._acquire_epoch = {}
         provider._acquire_epoch_counter = 0
@@ -883,3 +892,101 @@ def test_create_sandbox_evicts_oldest_warm_replica_via_shared_lifecycle(tmp_path
     assert "warm-oldest" not in provider._warm_pool
     assert provider._warm_pool == {"warm-newest": (newest_info, 20.0)}
     assert provider._sandbox_infos["created"] is created_info
+
+
+def _make_tenant_isolation_provider(tmp_path, monkeypatch):
+    aio_mod = importlib.import_module("deerflow.community.aio_sandbox.aio_sandbox_provider")
+    provider = _make_provider(tmp_path)
+    provider._lock = aio_mod.threading.Lock()
+    provider._sandboxes = {}
+    provider._sandbox_infos = {}
+    provider._thread_sandboxes = {}
+    provider._thread_locks = {}
+    provider._last_activity = {}
+    provider._warm_pool = {}
+    provider._active_sandbox_identity = {}
+    provider._warm_pool_identity = {}
+    provider._shutdown_called = False
+    provider._config = {"replicas": 3, "idle_timeout": 0}
+
+    create_calls = []
+
+    def _create(thread_id, sandbox_id, **kwargs):
+        create_calls.append((thread_id, sandbox_id, kwargs.get("user_id")))
+        return aio_mod.SandboxInfo(
+            sandbox_id=sandbox_id,
+            sandbox_url=f"http://sandbox-{len(create_calls)}.local",
+            container_name=f"deer-flow-sandbox-{sandbox_id}",
+        )
+
+    provider._backend = SimpleNamespace(
+        create=MagicMock(side_effect=_create),
+        destroy=MagicMock(),
+        discover=MagicMock(return_value=None),
+        is_alive=MagicMock(return_value=True),
+        list_running=MagicMock(return_value=[]),
+    )
+    provider._claim_ownership = MagicMock(return_value=True)
+    provider._held_teardown_lease = lambda _sandbox_id: contextlib.nullcontext()
+
+    monkeypatch.setattr(aio_mod, "get_paths", lambda: Paths(base_dir=tmp_path))
+    monkeypatch.setattr(
+        aio_mod.AioSandboxProvider,
+        "_get_extra_mounts",
+        lambda self, thread_id, *, user_id=None: [],
+    )
+    monkeypatch.setattr(
+        aio_mod,
+        "wait_for_sandbox_ready",
+        lambda _url, timeout=60: True,
+    )
+    return provider, create_calls, aio_mod
+
+
+def test_aio_wider_id_separates_known_legacy_collision():
+    aio_mod = importlib.import_module("deerflow.community.aio_sandbox.aio_sandbox_provider")
+    identity_a, identity_b = _LEGACY_COLLIDING_IDENTITIES
+    user_a, thread_a = identity_a
+    user_b, thread_b = identity_b
+
+    old_a = hashlib.sha256(f"{user_a}:{thread_a}".encode()).hexdigest()[:8]
+    old_b = hashlib.sha256(f"{user_b}:{thread_b}".encode()).hexdigest()[:8]
+
+    assert old_a == old_b
+    assert aio_mod.AioSandboxProvider._deterministic_sandbox_id(
+        thread_a,
+        user_a,
+    ) != aio_mod.AioSandboxProvider._deterministic_sandbox_id(
+        thread_b,
+        user_b,
+    )
+
+
+def test_aio_forced_collision_never_overwrites_active_tenant(
+    tmp_path,
+    monkeypatch,
+):
+    provider, create_calls, aio_mod = _make_tenant_isolation_provider(
+        tmp_path,
+        monkeypatch,
+    )
+    monkeypatch.setattr(
+        aio_mod.AioSandboxProvider,
+        "_deterministic_sandbox_id",
+        staticmethod(lambda thread_id, user_id: "deadbeefdeadbeef"),
+    )
+
+    sandbox_id = provider.acquire("thread-a", user_id="user-a")
+    info_a = provider._sandbox_infos[sandbox_id]
+    provider.release(sandbox_id)
+
+    assert sandbox_id in provider._warm_pool
+
+    with pytest.raises(aio_mod.SandboxIdentityCollisionError):
+        provider.acquire("thread-b", user_id="user-b")
+
+    assert provider._warm_pool[sandbox_id][0] is info_a
+    provider._backend.destroy.assert_not_called()
+    assert len(create_calls) == 1
+    assert provider.acquire("thread-a", user_id="user-a") == sandbox_id
+    assert provider._sandbox_infos[sandbox_id] is info_a
diff --git a/backend/tests/test_boxlite_provider.py b/backend/tests/test_boxlite_provider.py
index dff07e888..f163285dd 100644
--- a/backend/tests/test_boxlite_provider.py
+++ b/backend/tests/test_boxlite_provider.py
@@ -7,6 +7,7 @@ which need a live box.
 from __future__ import annotations
 
 import asyncio
+import hashlib
 import logging
 import sys
 import threading
@@ -18,6 +19,11 @@ import pytest
 from deerflow.community.boxlite.box import BoxliteBox
 from deerflow.community.boxlite.provider import BoxliteProvider, _import_simplebox
 
+_LEGACY_COLLIDING_IDENTITIES = (
+    ("user-9721", "thread-9721"),
+    ("user-94361", "thread-94361"),
+)
+
 # ── Fake BoxLite SDK ──────────────────────────────────────────────────
 
 
@@ -262,7 +268,7 @@ def test_sandbox_id_deterministic(monkeypatch):
     id1 = provider._sandbox_id("thread-1", "user-a")
     id2 = provider._sandbox_id("thread-1", "user-a")
     assert id1 == id2
-    assert len(id1) == 8
+    assert len(id1) == 16
 
 
 def test_sandbox_id_different_users(monkeypatch):
@@ -460,7 +466,7 @@ def test_explicit_recent_reclaim_skip_avoids_health_check(monkeypatch):
 
     monkeypatch.setattr(box, "execute_command", _fail_if_called)
 
-    reclaimed = provider._reclaim_warm_pool(sid)
+    reclaimed = provider._reclaim_warm_pool(sid, ("u1", "thread-1"))
     assert reclaimed == sid
     assert sid in provider._boxes
     assert sid not in provider._warm_pool
@@ -494,7 +500,7 @@ def test_recent_reclaim_validates_by_default(monkeypatch):
 
     monkeypatch.setattr(box, "execute_command", _record_health_check)
 
-    reclaimed = provider._reclaim_warm_pool(sid)
+    reclaimed = provider._reclaim_warm_pool(sid, ("u1", "thread-1"))
     assert reclaimed == sid
     assert calls == 1
     assert sid in provider._boxes
@@ -526,7 +532,7 @@ def test_default_recent_reclaim_drops_dead_warm_box(monkeypatch):
 
     monkeypatch.setattr(box, "execute_command", _dead_health_check)
 
-    reclaimed = provider._reclaim_warm_pool(sid)
+    reclaimed = provider._reclaim_warm_pool(sid, ("u1", "thread-1"))
     assert reclaimed is None
     assert sid not in provider._boxes
     assert sid not in provider._warm_pool
@@ -592,7 +598,10 @@ def test_adopted_warm_pool_box_still_health_checks(monkeypatch):
 
     monkeypatch.setattr(adopted, "execute_command", _record_health_check)
 
-    reclaimed = provider._reclaim_warm_pool("adopted")
+    reclaimed = provider._reclaim_warm_pool(
+        "adopted",
+        ("some-user", "some-thread"),
+    )
     assert reclaimed == "adopted"
     assert calls == 1
     assert "adopted" in provider._boxes
@@ -1044,3 +1053,143 @@ def test_shutdown_stops_idle_reaper_and_destroys_all_boxes(monkeypatch):
     assert len(provider._boxes) == 0
     assert len(provider._warm_pool) == 0
     assert len(provider._thread_boxes) == 0
+
+
+def test_wider_id_separates_known_legacy_collision():
+    identity_a, identity_b = _LEGACY_COLLIDING_IDENTITIES
+    user_a, thread_a = identity_a
+    user_b, thread_b = identity_b
+
+    old_a = hashlib.sha256(f"{user_a}:{thread_a}".encode()).hexdigest()[:8]
+    old_b = hashlib.sha256(f"{user_b}:{thread_b}".encode()).hexdigest()[:8]
+
+    assert old_a == old_b
+    assert BoxliteProvider._sandbox_id(thread_a, user_a) != BoxliteProvider._sandbox_id(
+        thread_b,
+        user_b,
+    )
+
+
+def test_forced_collision_never_overwrites_active_tenant(monkeypatch):
+    monkeypatch.setattr(
+        "deerflow.community.boxlite.provider.get_app_config",
+        lambda: _stub_config(),
+    )
+    monkeypatch.setattr(
+        "deerflow.community.boxlite.provider._import_simplebox",
+        lambda: _FakeBox,
+    )
+    monkeypatch.setattr(
+        BoxliteProvider,
+        "_sandbox_id",
+        staticmethod(lambda thread_id, user_id: "deadbeefdeadbeef"),
+    )
+
+    provider = BoxliteProvider()
+    provider._loop.run = _fake_run
+
+    sandbox_id = provider.acquire("thread-a", user_id="user-a")
+    box_a = provider.get(sandbox_id)
+    provider.release(sandbox_id)
+
+    assert box_a is not None
+    assert sandbox_id in provider._warm_pool
+
+    with pytest.raises(RuntimeError, match="Sandbox ID collision"):
+        provider.acquire("thread-b", user_id="user-b")
+
+    assert provider._warm_pool[sandbox_id][0] is box_a
+    assert not box_a.is_closed
+    assert provider.acquire("thread-a", user_id="user-a") == sandbox_id
+    assert provider.get(sandbox_id) is box_a
+    provider.shutdown()
+
+
+def test_late_same_tenant_collision_reuses_active_box(monkeypatch):
+    monkeypatch.setattr(
+        "deerflow.community.boxlite.provider.get_app_config",
+        lambda: _stub_config(),
+    )
+
+    provider = BoxliteProvider()
+    sandbox_id = "deadbeefdeadbeef"
+    key = ("user-a", "thread-a")
+    active = BoxliteBox(
+        sandbox_id,
+        _FakeBox(name=f"deer-flow-boxlite-{sandbox_id}"),
+        _fake_run,
+        default_env={},
+    )
+    duplicate = BoxliteBox(
+        sandbox_id,
+        _FakeBox(name=f"deer-flow-boxlite-{sandbox_id}"),
+        _fake_run,
+        default_env={},
+    )
+
+    monkeypatch.setattr(
+        BoxliteProvider,
+        "_sandbox_id",
+        staticmethod(lambda thread_id, user_id: sandbox_id),
+    )
+
+    def _create_box_with_late_registration(_sandbox_id):
+        with provider._lock:
+            provider._boxes[sandbox_id] = active
+            provider._active_box_identity[sandbox_id] = key
+        return duplicate
+
+    monkeypatch.setattr(provider, "_create_box", _create_box_with_late_registration)
+
+    assert provider.acquire("thread-a", user_id="user-a") == sandbox_id
+    assert duplicate.is_closed
+    assert provider.get(sandbox_id) is active
+    assert provider._thread_boxes[key] == sandbox_id
+    provider.shutdown()
+
+
+def test_failed_health_check_does_not_remove_swapped_warm_entry(monkeypatch):
+    monkeypatch.setattr(
+        "deerflow.community.boxlite.provider.get_app_config",
+        lambda: _stub_config(),
+    )
+
+    provider = BoxliteProvider()
+    stale = BoxliteBox(
+        "shared-id",
+        _FakeBox(name="deer-flow-boxlite-shared-id"),
+        _fake_run,
+        default_env={},
+    )
+    replacement = BoxliteBox(
+        "shared-id",
+        _FakeBox(name="deer-flow-boxlite-shared-id"),
+        _fake_run,
+        default_env={},
+    )
+    provider._warm_pool["shared-id"] = (stale, time.time())
+    provider._warm_pool_identity["shared-id"] = ("user-a", "thread-a")
+
+    def _swap_during_health_check(*args, **kwargs):
+        with provider._lock:
+            provider._warm_pool["shared-id"] = (replacement, time.time())
+            provider._warm_pool_identity["shared-id"] = (
+                "user-b",
+                "thread-b",
+            )
+        return "not healthy"
+
+    monkeypatch.setattr(stale, "execute_command", _swap_during_health_check)
+
+    with pytest.raises(RuntimeError, match="Sandbox ID collision"):
+        provider._reclaim_warm_pool(
+            "shared-id",
+            ("user-a", "thread-a"),
+        )
+    assert provider._warm_pool["shared-id"][0] is replacement
+    assert provider._warm_pool_identity["shared-id"] == (
+        "user-b",
+        "thread-b",
+    )
+    assert not replacement.is_closed
+    provider.shutdown()
diff --git a/backend/tests/test_sandbox_orphan_reconciliation.py b/backend/tests/test_sandbox_orphan_reconciliation.py
index a0a2aa7b8..a68de083c 100644
--- a/backend/tests/test_sandbox_orphan_reconciliation.py
+++ b/backend/tests/test_sandbox_orphan_reconciliation.py
@@ -431,6 +431,8 @@ def _make_provider_for_reconciliation(tmp_path=None, *, worker_id: str = "worker
     provider._thread_locks = {}
     provider._last_activity = {}
     provider._warm_pool = {}
+    provider._active_sandbox_identity = {}
+    provider._warm_pool_identity = {}
     provider._unowned_since = {}
     provider._local_teardown = set()
     provider._acquire_epoch = {}