Forward sandbox into agent builder backend

agent_builder/builder.py

@@ -27,6 +27,7 @@ class BuilderContext:
     settings: Settings | None = None
     project_prefix: str | None = None
     workspace: Any | None = None
+    sandbox: Any | None = None
     grant_token: str | None = None
     # LLM creds forwarded by the orchestrator according to this agent's Card.
     # The builder accepts caller-selected creds and falls back to platform
@@ -368,7 +369,12 @@ def _build_project_backend(ctx: BuilderContext, *, settings: Settings) -> Any |
         except Exception:  # noqa: BLE001
             return _with_builder_skills(StateBackend())
         return _with_builder_skills(
-            _workspace_backend_with_grep_compat(WorkspaceBackend, ctx.workspace),
+            _workspace_backend_with_grep_compat(
+                WorkspaceBackend,
+                ctx.workspace,
+                sandbox=ctx.sandbox,
+                default_image=settings.image,
+            ),
             artifacts_root=f"/{prefix}/{_BUILDER_ARTIFACT_DIR}",
         )
     try:
@@ -404,12 +410,21 @@ def _build_project_backend(ctx: BuilderContext, *, settings: Settings) -> Any |
         )
     )
     return _with_builder_skills(
-        _workspace_backend_with_grep_compat(WorkspaceBackend, workspace),
+        _workspace_backend_with_grep_compat(
+            WorkspaceBackend,
+            workspace,
+            sandbox=ctx.sandbox,
+            default_image=settings.image,
+        ),
         artifacts_root=f"/{prefix}/{_BUILDER_ARTIFACT_DIR}",
     )
 
 
-def _workspace_backend_with_grep_compat(workspace_backend_cls: Any, workspace: Any) -> Any:
+def _workspace_backend_with_grep_compat(
+    workspace_backend_cls: Any,
+    workspace: Any,
+    **backend_kwargs: Any,
+) -> Any:
     """Patch older a2a-pack WorkspaceBackend grep glob behavior.
 
     Older ``WorkspaceBackend.grep`` only matched ``glob`` against the full
@@ -436,7 +451,7 @@ def _workspace_backend_with_grep_compat(workspace_backend_cls: Any, workspace: A
                 return result
             return super().grep(pattern, path=path, glob=scoped_glob)
 
-    return BuilderWorkspaceBackend(workspace)
+    return BuilderWorkspaceBackend(workspace, **backend_kwargs)
 
 
 def _with_builder_skills(default_backend: Any, *, artifacts_root: str = "/") -> Any: