diff --git a/packages/host/app/components/operator-mode/preview-panel/index.gts b/packages/host/app/components/operator-mode/preview-panel/index.gts
index 5ad553f2113..c9b280b8222 100644
--- a/packages/host/app/components/operator-mode/preview-panel/index.gts
+++ b/packages/host/app/components/operator-mode/preview-panel/index.gts
@@ -26,6 +26,7 @@ import {
   isCardInstance,
   isFileDefInstance,
   isResolvedCodeRef,
+  rri,
   cardDefFormats,
   fileDefFormats,
   fieldDefFormats,
@@ -133,10 +134,13 @@ export default class PreviewPanel extends Component<Signature> {
   private editTemplate = () => {
     const type = identifyCard(this.args.card.constructor as any);
     if (type && isResolvedCodeRef(type)) {
-      const gtsFileUrl = type.module.endsWith('.gts')
+      // `identifyCard` reports a module in canonical form, which for a
+      // mapped realm is a prefix identifier rather than a URL — parsing it
+      // here would throw. `updateCodePath` resolves an identifier itself.
+      const gtsFile = type.module.endsWith('.gts')
         ? type.module
         : `${type.module}.gts`;
-      this.operatorModeStateService.updateCodePath(new URL(gtsFileUrl));
+      this.operatorModeStateService.updateCodePath(rri(gtsFile));
     }
   };
 
diff --git a/packages/host/tests/acceptance/code-submode-test.ts b/packages/host/tests/acceptance/code-submode-test.ts
index 809e5266aa7..a9096d1d224 100644
--- a/packages/host/tests/acceptance/code-submode-test.ts
+++ b/packages/host/tests/acceptance/code-submode-test.ts
@@ -1,5 +1,6 @@
 import {
   click,
+  find,
   waitFor,
   fillIn,
   triggerEvent,
@@ -2911,5 +2912,40 @@ module('Acceptance | code submode tests', function (_hooks) {
         .dom('[data-test-card-url-bar-input]')
         .hasValue(`${testRealmURL}person.gts`);
     });
+
+    test('Edit Template opens the source of a card whose type lives in a mapped realm', async function (assert) {
+      // `person-entry` is a Spec, so its type resolves to the base realm — a
+      // prefix identifier rather than a URL. The button used to parse that
+      // module as a URL, which throws for any mapped realm, so the source
+      // never opened.
+      await visitOperatorMode({
+        stacks: [
+          [
+            {
+              id: `${testRealmURL}person-entry`,
+              format: 'isolated',
+            },
+          ],
+        ],
+        submode: 'code',
+        codePath: `${testRealmURL}person-entry.json`,
+      });
+
+      await waitFor('[data-test-card-resource-loaded]');
+      await click('[data-test-edit-template-button]');
+
+      await waitFor('[data-test-card-url-bar-input]');
+      let codePath = (
+        find('[data-test-card-url-bar-input]') as HTMLInputElement
+      ).value;
+      assert.true(
+        codePath.endsWith('spec.gts'),
+        `the base realm's spec source opened (got ${codePath})`,
+      );
+      assert.false(
+        codePath.startsWith('@'),
+        'the identifier was resolved to a URL rather than passed through raw',
+      );
+    });
   });
 });