Forward sandbox into agent builder backend

tests/test_builder_prompt.py

@@ -27,6 +27,8 @@ class BuilderPromptTests(unittest.TestCase):
             "config={\"recursion_limit\": DEEPAGENTS_RECURSION_LIMIT}",
             agent_source,
         )
+        self.assertIn("sandbox = ctx.sandbox", agent_source)
+        self.assertIn("sandbox=sandbox", agent_source)
         self.assertIn("stream=True", agent_source)
         self.assertIn('grant_write_prefixes=("agents/{name}/",)', agent_source)
 
@@ -256,3 +258,37 @@ inline = Pricing(price_per_call_usd=0.01, compute={"runtime_seconds": 600}, tota
             result.matches,
             [{"path": "/agents/github-repo-inspector/agent.py"}],
         )
+
+    def test_project_backend_grep_compat_preserves_sandbox_kwargs(self) -> None:
+        class _Backend:
+            def __init__(
+                self,
+                workspace: object,
+                *,
+                sandbox: object | None = None,
+                default_image: str = "python:3.11-slim",
+            ) -> None:
+                self.workspace = workspace
+                self.sandbox = sandbox
+                self.default_image = default_image
+
+            def grep(
+                self,
+                pattern: str,
+                path: str | None = None,
+                glob: str | None = None,
+            ):
+                return type("GrepResult", (), {"error": None, "matches": []})()
+
+        workspace = object()
+        sandbox = object()
+        backend = _workspace_backend_with_grep_compat(
+            _Backend,
+            workspace,
+            sandbox=sandbox,
+            default_image="python:3.12-slim",
+        )
+
+        self.assertIs(backend.workspace, workspace)
+        self.assertIs(backend.sandbox, sandbox)
+        self.assertEqual(backend.default_image, "python:3.12-slim")