diff --git a/.changeset/bright-donkeys-yawn.md b/.changeset/bright-donkeys-yawn.md
new file mode 100644
index 00000000..fde91847
--- /dev/null
+++ b/.changeset/bright-donkeys-yawn.md
@@ -0,0 +1,7 @@
+---
+"@open-pioneer/runtime": major
+---
+
+Remove support for closed shadow roots.
+Shadow roots used by trails applications are now always `"open"`.
+See #19.
diff --git a/src/packages/runtime/CustomElement.test.ts b/src/packages/runtime/CustomElement.test.ts
index 6f794c26..4f667b42 100644
--- a/src/packages/runtime/CustomElement.test.ts
+++ b/src/packages/runtime/CustomElement.test.ts
@@ -75,17 +75,16 @@ describe("simple rendering", function () {
     });
 });
 
-it("explicitly setting the shadow dom mode hides the shadow root", async () => {
+it("uses mode 'open' for the internal shadow root", async () => {
     function TestComponent() {
         return createElement("span", undefined, "Hello World");
     }
 
     const elem = createCustomElement({
-        component: TestComponent,
-        openShadowRoot: false
+        component: TestComponent
     });
     const { node } = await renderComponent(elem);
-    expect(node.shadowRoot).toBeNull();
+    expect(node.shadowRoot).toBeTruthy();
 });
 
 it("allows customization of package properties", async () => {
diff --git a/src/packages/runtime/CustomElement.ts b/src/packages/runtime/CustomElement.ts
index e5431693..7762a8bd 100644
--- a/src/packages/runtime/CustomElement.ts
+++ b/src/packages/runtime/CustomElement.ts
@@ -64,12 +64,6 @@ export interface CustomElementOptions {
      */
     resolveConfig?(ctx: ConfigContext): Promise<ApplicationConfig | undefined>;
 
-    /**
-     * Whether the shadow root element is accessible from the outside.
-     * Defaults to `false` in production mode and `true` during development to make testing easier.
-     */
-    openShadowRoot?: boolean;
-
     /**
      * Chakra theming object.
      */
@@ -161,9 +155,8 @@ export function createCustomElement(options: CustomElementOptions): ApplicationE
         constructor() {
             super();
 
-            const mode = options.openShadowRoot ?? import.meta.env.DEV ? "open" : "closed";
             this.#shadowRoot = this.attachShadow({
-                mode: mode
+                mode: "open"
             });
 
             if (import.meta.env.DEV) {