diff --git a/packages/code-runtime/code-runtime-python/py/bootstrap.py b/packages/code-runtime/code-runtime-python/py/bootstrap.py
index df0a4bb42b..93638a50fb 100644
--- a/packages/code-runtime/code-runtime-python/py/bootstrap.py
+++ b/packages/code-runtime/code-runtime-python/py/bootstrap.py
@@ -753,6 +753,10 @@ async def _run(channel: ProtocolChannel) -> None:
     # assignment RHS resolves the module global, not an unbound local.
     _RuntimeError_cls = RuntimeError
     _BindingRejection_cls = _BindingRejection
+    # `str` for dispatch's rejection conversion is likewise bound: a program
+    # rebinding `__main__.str` would otherwise run a hostile callable when the
+    # binding-rejection message is formatted.
+    _str = str
     # 1. Boot handshake.
     boot = channel.read_frame()
     if boot is None or boot.get("type") != "boot":
@@ -917,7 +921,7 @@ async def _run(channel: ProtocolChannel) -> None:
         try:
             return await fut
         except _BindingRejection_cls as exc:
-            raise call_failure(str(exc)) from None
+            raise call_failure(_str(exc)) from None
 
     namespaces: dict[str, Any] = {}
     for entry in boot["namespaces"]:
diff --git a/packages/code-runtime/code-runtime-python/tests/runtime.spec.ts b/packages/code-runtime/code-runtime-python/tests/runtime.spec.ts
index 692ed9d51b..c4c3f3e934 100644
--- a/packages/code-runtime/code-runtime-python/tests/runtime.spec.ts
+++ b/packages/code-runtime/code-runtime-python/tests/runtime.spec.ts
@@ -690,7 +690,7 @@ describe('PythonCodeRuntime — programs and bindings', () => {
         'try:',
         '    await tools.fail({})',
         'except RuntimeError as e:',
-        '    caught = str(e)',
+        '    caught = e.args[0] if e.args else ""',
         'except Exception as e:',
         '    caught = "WRONG TYPE: " + type(e).__name__',
         'return caught',