diff --git a/api/core/virtual_environment/sandbox_manager.py b/api/core/virtual_environment/sandbox_manager.py
index b29f4e25c9..8443208632 100644
--- a/api/core/virtual_environment/sandbox_manager.py
+++ b/api/core/virtual_environment/sandbox_manager.py
@@ -8,56 +8,93 @@ logger = logging.getLogger(__name__)
 
 
 class SandboxManager:
-    _lock: Final[threading.Lock] = threading.Lock()
-    _sandboxes: dict[str, VirtualEnvironment] = {}
+    """Process-local registry for workflow sandboxes.
+
+    Stores `VirtualEnvironment` references keyed by `workflow_execution_id`.
+
+    Concurrency: the registry is split into hash shards and each shard is updated with
+    copy-on-write under a shard lock. Reads are lock-free (snapshot dict) to reduce
+    contention in hot paths like `get()`.
+    """
+
+    # FIXME: Prefer a workflow-level context on GraphRuntimeState to store workflow-scoped shared objects.
+
+    _NUM_SHARDS: Final[int] = 1024
+    _SHARD_MASK: Final[int] = _NUM_SHARDS - 1
+
+    _shard_locks: Final[tuple[threading.Lock, ...]] = tuple(threading.Lock() for _ in range(_NUM_SHARDS))
+    _shards: list[dict[str, VirtualEnvironment]] = [{} for _ in range(_NUM_SHARDS)]
+
+    @classmethod
+    def _shard_index(cls, workflow_execution_id: str) -> int:
+        return hash(workflow_execution_id) & cls._SHARD_MASK
 
     @classmethod
     def register(cls, workflow_execution_id: str, sandbox: VirtualEnvironment) -> None:
         if not workflow_execution_id:
             raise ValueError("workflow_execution_id cannot be empty")
 
-        with cls._lock:
-            if workflow_execution_id in cls._sandboxes:
+        shard_index = cls._shard_index(workflow_execution_id)
+        with cls._shard_locks[shard_index]:
+            shard = cls._shards[shard_index]
+            if workflow_execution_id in shard:
                 raise RuntimeError(
                     f"Sandbox already registered for workflow_execution_id={workflow_execution_id}. "
                     "Call unregister() first if you need to replace it."
                 )
-            cls._sandboxes[workflow_execution_id] = sandbox
-            logger.debug(
-                "Registered sandbox for workflow_execution_id=%s, sandbox_id=%s",
-                workflow_execution_id,
-                sandbox.metadata.id,
-            )
+
+            new_shard = dict(shard)
+            new_shard[workflow_execution_id] = sandbox
+            cls._shards[shard_index] = new_shard
+
+        logger.debug(
+            "Registered sandbox for workflow_execution_id=%s, sandbox_id=%s",
+            workflow_execution_id,
+            sandbox.metadata.id,
+        )
 
     @classmethod
     def get(cls, workflow_execution_id: str) -> VirtualEnvironment | None:
-        with cls._lock:
-            return cls._sandboxes.get(workflow_execution_id)
+        shard_index = cls._shard_index(workflow_execution_id)
+        return cls._shards[shard_index].get(workflow_execution_id)
 
     @classmethod
     def unregister(cls, workflow_execution_id: str) -> VirtualEnvironment | None:
-        with cls._lock:
-            sandbox = cls._sandboxes.pop(workflow_execution_id, None)
-            if sandbox:
-                logger.debug(
-                    "Unregistered sandbox for workflow_execution_id=%s, sandbox_id=%s",
-                    workflow_execution_id,
-                    sandbox.metadata.id,
-                )
-            return sandbox
+        shard_index = cls._shard_index(workflow_execution_id)
+        with cls._shard_locks[shard_index]:
+            shard = cls._shards[shard_index]
+            sandbox = shard.get(workflow_execution_id)
+            if sandbox is None:
+                return None
+
+            new_shard = dict(shard)
+            new_shard.pop(workflow_execution_id, None)
+            cls._shards[shard_index] = new_shard
+
+        logger.debug(
+            "Unregistered sandbox for workflow_execution_id=%s, sandbox_id=%s",
+            workflow_execution_id,
+            sandbox.metadata.id,
+        )
+        return sandbox
 
     @classmethod
     def has(cls, workflow_execution_id: str) -> bool:
-        with cls._lock:
-            return workflow_execution_id in cls._sandboxes
+        shard_index = cls._shard_index(workflow_execution_id)
+        return workflow_execution_id in cls._shards[shard_index]
 
     @classmethod
     def clear(cls) -> None:
-        with cls._lock:
-            cls._sandboxes.clear()
+        for lock in cls._shard_locks:
+            lock.acquire()
+        try:
+            for i in range(cls._NUM_SHARDS):
+                cls._shards[i] = {}
             logger.debug("Cleared all registered sandboxes")
+        finally:
+            for lock in reversed(cls._shard_locks):
+                lock.release()
 
     @classmethod
     def count(cls) -> int:
-        with cls._lock:
-            return len(cls._sandboxes)
+        return sum(len(shard) for shard in cls._shards)