diff --git a/api/core/sandbox/builder.py b/api/core/sandbox/builder.py
index d403f4da5b..9edf54005b 100644
--- a/api/core/sandbox/builder.py
+++ b/api/core/sandbox/builder.py
@@ -113,10 +113,21 @@ class SandboxBuilder:
             assets_id=self._assets_id,
         )
 
+        """
+        # Run synchronous initializers before marking sandbox as ready.
+        """
+        for init in self._initializers:
+            if init.async_initialize():
+                continue
+            init.initialize(sandbox)
+
         # Run sandbox setup asynchronously so workflow execution can proceed.
         def initialize() -> None:
             try:
                 for init in self._initializers:
+                    if not init.async_initialize():
+                        continue
+
                     if sandbox.is_cancelled():
                         return
                     init.initialize(sandbox)
diff --git a/api/core/sandbox/initializer/base.py b/api/core/sandbox/initializer/base.py
index c2213d9714..ba358e7371 100644
--- a/api/core/sandbox/initializer/base.py
+++ b/api/core/sandbox/initializer/base.py
@@ -6,3 +6,9 @@ from core.sandbox.sandbox import Sandbox
 class SandboxInitializer(ABC):
     @abstractmethod
     def initialize(self, env: Sandbox) -> None: ...
+
+    def async_initialize(self) -> bool:
+        """
+        Whether the initializer needs to run asynchronously.
+        """
+        return False