diff --git a/shared/sx/tests/test_sx_js.py b/shared/sx/tests/test_sx_js.py
index dcae45f..135adae 100644
--- a/shared/sx/tests/test_sx_js.py
+++ b/shared/sx/tests/test_sx_js.py
@@ -20,19 +20,39 @@ SX_TEST_JS = Path(__file__).resolve().parents[2] / "static" / "scripts" / "sx-te
 
 def _js_render(sx_text: str, components_text: str = "") -> str:
     """Run sx.js + sx-test.js in Node and return the renderToString result."""
-    # Build a small Node script
+    import tempfile, os
+    # Build a small Node script that requires the source files
     script = f"""
-    global.document = undefined;  // no DOM needed for string render
-    {SX_JS.read_text()}
+    globalThis.document = undefined;  // no DOM needed for string render
+    // sx.js IIFE uses (typeof window !== "undefined" ? window : this).
+    // In Node file mode, `this` is module.exports, not globalThis.
+    // Patch: make the IIFE target globalThis so Sx is accessible.
+    var _origThis = this;
+    Object.defineProperty(globalThis, 'document', {{ value: undefined, writable: true }});
+    (function() {{
+      var _savedThis = globalThis;
+      {SX_JS.read_text()}
+      // Hoist Sx from module.exports to globalThis if needed
+      if (typeof Sx === 'undefined' && typeof module !== 'undefined' && module.exports && module.exports.Sx) {{
+        globalThis.Sx = module.exports.Sx;
+      }}
+    }}).call(globalThis);
     {SX_TEST_JS.read_text()}
     if ({json.dumps(components_text)}) Sx.loadComponents({json.dumps(components_text)});
     var result = Sx.renderToString({json.dumps(sx_text)});
     process.stdout.write(result);
     """
-    result = subprocess.run(
-        ["node", "-e", script],
-        capture_output=True, text=True, timeout=5,
-    )
+    # Write to temp file to avoid OS arg length limits
+    fd, tmp = tempfile.mkstemp(suffix=".js")
+    try:
+        with os.fdopen(fd, "w") as f:
+            f.write(script)
+        result = subprocess.run(
+            ["node", tmp],
+            capture_output=True, text=True, timeout=5,
+        )
+    finally:
+        os.unlink(tmp)
     if result.returncode != 0:
         pytest.fail(f"Node.js error:\n{result.stderr}")
     return result.stdout