2 files changed, 23 insertions, 0 deletions
diff --git a/src/app/App.test.ts b/src/app/App.test.ts
index 94b9c0c..026c5c7 100644
--- a/src/app/App.test.ts
+++ b/src/app/App.test.ts
@@ -87,6 +87,23 @@ function fakeFetchImpl(): typeof fetch {
         status: 200,
       });
     }
+    if (/\/plugins\/[^/]+\/frontend/.test(url)) {
+      // plan 03 frontend module (no fixtures declare one) — the plain 404
+      // the route returns for unknown/unloaded/frontend-less plugins.
+      return new Response(JSON.stringify({ error: "No frontend module for this plugin" }), {
+        status: 404,
+      });
+    }
+    if (/\/plugins(\?.*)?$/.test(url)) {
+      // Valid EMPTY list (plan 03): the plugin host's boot
+      // `extensions-changed`/workspace syncs read this — a shapeless
+      // fallback body would be cast to PluginListResponse and crash the
+      // diff (`list.loaded` not iterable).
+      return new Response(
+        JSON.stringify({ loaded: [], disabledWithReason: [], discovered: [], missing: [] }),
+        { status: 200 },
+      );
+    }
     return new Response(JSON.stringify({ chunks: [], latestSeq: 0 }), { status: 200 });
   };
 }
diff --git a/src/app/plugin-host.svelte.ts b/src/app/plugin-host.svelte.ts
index 9e53bc1..729a8aa 100644
--- a/src/app/plugin-host.svelte.ts
+++ b/src/app/plugin-host.svelte.ts
@@ -122,6 +122,12 @@ export function createPluginHost(opts: PluginHostOptions): PluginHost {
         opts.http.workspacePlugins(workspaceId),
       ]);
       if (list === null) return;
+      // Shape guard at the outermost edge (asymmetric testing: the shell
+      // guards, the pure diff stays trusting): a malformed-but-200
+      // `/plugins` response (wrong shape, contract drift, a test fake that
+      // doesn't model the route) must fail SOFT exactly like `null` — keep
+      // the current imported set, never throw from the fire-and-forget sync.
+      if (!Array.isArray(list.loaded)) return;
       const candidates = frontendCandidates(list, {
         workspaceId,
         usableIds: usablePluginIds(workspace?.plugins ?? []),