diff --git a/.changeset/five-hornets-provide.md b/.changeset/five-hornets-provide.md
new file mode 100644
index 0000000000..a422c5fbc8
--- /dev/null
+++ b/.changeset/five-hornets-provide.md
@@ -0,0 +1,5 @@
+---
+'@backstage/frontend-app-api': patch
+---
+
+The app will now reject any extensions that attach to nonexistent inputs.
diff --git a/packages/frontend-app-api/src/extensions/Core.tsx b/packages/frontend-app-api/src/extensions/Core.tsx
index 4a66afe38d..ec00059bf8 100644
--- a/packages/frontend-app-api/src/extensions/Core.tsx
+++ b/packages/frontend-app-api/src/extensions/Core.tsx
@@ -27,6 +27,9 @@ export const Core = createExtension({
     apis: createExtensionInput({
       api: coreExtensionData.apiFactory,
     }),
+    themes: createExtensionInput({
+      theme: coreExtensionData.theme,
+    }),
   },
   output: {},
   factory() {},
diff --git a/packages/frontend-app-api/src/routing/extractRouteInfoFromInstanceTree.test.ts b/packages/frontend-app-api/src/routing/extractRouteInfoFromInstanceTree.test.ts
index a510adf1b7..b833bda1b5 100644
--- a/packages/frontend-app-api/src/routing/extractRouteInfoFromInstanceTree.test.ts
+++ b/packages/frontend-app-api/src/routing/extractRouteInfoFromInstanceTree.test.ts
@@ -48,7 +48,7 @@ function createTestExtension(options: {
     id: options.id,
     attachTo: options.parent
       ? { id: options.parent, input: 'children' }
-      : { id: 'core.routes', input: 'children' },
+      : { id: 'core.routes', input: 'routes' },
     output: {
       element: coreExtensionData.reactElement,
       path: coreExtensionData.routePath.optional(),
@@ -305,6 +305,7 @@ describe('discovery', () => {
       }),
       createTestExtension({
         id: 'fooEmpty',
+        parent: 'foo',
       }),
       createTestExtension({
         id: 'page3',
diff --git a/packages/frontend-app-api/src/wiring/createApp.test.tsx b/packages/frontend-app-api/src/wiring/createApp.test.tsx
index abd0fe9b00..600901f96e 100644
--- a/packages/frontend-app-api/src/wiring/createApp.test.tsx
+++ b/packages/frontend-app-api/src/wiring/createApp.test.tsx
@@ -16,6 +16,7 @@
 
 import {
   createExtension,
+  createExtensionInput,
   createPageExtension,
   createPlugin,
   createThemeExtension,
@@ -143,6 +144,9 @@ describe('createApp', () => {
       extension: createExtension({
         id: 'root',
         attachTo: { id: '', input: '' },
+        inputs: {
+          children: createExtensionInput({}),
+        },
         output: {},
         factory() {},
       }),
@@ -181,6 +185,9 @@ describe('createApp', () => {
       extension: createExtension({
         id: 'root',
         attachTo: { id: '', input: '' },
+        inputs: {
+          children: createExtensionInput({}),
+        },
         output: {},
         factory() {},
       }),
diff --git a/packages/frontend-app-api/src/wiring/createExtensionInstance.test.ts b/packages/frontend-app-api/src/wiring/createExtensionInstance.test.ts
index 6550d76608..6e69e99cb7 100644
--- a/packages/frontend-app-api/src/wiring/createExtensionInstance.test.ts
+++ b/packages/frontend-app-api/src/wiring/createExtensionInstance.test.ts
@@ -250,6 +250,92 @@ describe('createExtensionInstance', () => {
     );
   });
 
+  it('should refuse to create an instance with undeclared inputs', () => {
+    expect(() =>
+      createExtensionInstance({
+        attachments: new Map([
+          [
+            'declared',
+            [
+              createExtensionInstance({
+                attachments: new Map(),
+                config: { output: 'many1' },
+                extension: simpleExtension,
+              }),
+            ],
+          ],
+          [
+            'undeclared',
+            [
+              createExtensionInstance({
+                attachments: new Map(),
+                config: { output: 'many1' },
+                extension: simpleExtension,
+              }),
+            ],
+          ],
+        ]),
+        config: undefined,
+        extension: createExtension({
+          id: 'core.test',
+          attachTo: { id: 'ignored', input: 'ignored' },
+          inputs: {
+            declared: createExtensionInput({
+              test: testDataRef,
+            }),
+          },
+          output: {},
+          factory() {},
+        }),
+      }),
+    ).toThrow(
+      "Failed to instantiate extension 'core.test', received undeclared input 'undeclared' from extension 'core.test'",
+    );
+  });
+
+  it('should refuse to create an instance with multiple undeclared inputs', () => {
+    expect(() =>
+      createExtensionInstance({
+        attachments: new Map([
+          [
+            'undeclared1',
+            [
+              createExtensionInstance({
+                attachments: new Map(),
+                config: { output: 'many1' },
+                extension: simpleExtension,
+              }),
+            ],
+          ],
+          [
+            'undeclared2',
+            [
+              createExtensionInstance({
+                attachments: new Map(),
+                config: { output: 'many1' },
+                extension: simpleExtension,
+              }),
+              createExtensionInstance({
+                attachments: new Map(),
+                config: { output: 'many1' },
+                extension: simpleExtension,
+              }),
+            ],
+          ],
+        ]),
+        config: undefined,
+        extension: createExtension({
+          id: 'core.test',
+          attachTo: { id: 'ignored', input: 'ignored' },
+          output: {},
+          factory() {},
+        }),
+      }),
+    ).toThrow(
+      "Failed to instantiate extension 'core.test', received undeclared inputs 'undeclared1' from extension 'core.test' and 'undeclared2' from extensions 'core.test', 'core.test'",
+    );
+  });
+
   it('should refuse to create an instance with multiple inputs for required singleton', () => {
     expect(() =>
       createExtensionInstance({
diff --git a/packages/frontend-app-api/src/wiring/createExtensionInstance.ts b/packages/frontend-app-api/src/wiring/createExtensionInstance.ts
index 9aec3078bc..5a325dc0a0 100644
--- a/packages/frontend-app-api/src/wiring/createExtensionInstance.ts
+++ b/packages/frontend-app-api/src/wiring/createExtensionInstance.ts
@@ -60,6 +60,24 @@ function resolveInputs(
   inputMap: AnyExtensionInputMap,
   attachments: Map<string, ExtensionInstance[]>,
 ) {
+  const undeclaredAttachments = Array.from(attachments.entries()).filter(
+    ([inputName]) => inputMap[inputName] === undefined,
+  );
+  if (undeclaredAttachments.length > 0) {
+    throw new Error(
+      `received undeclared input${
+        undeclaredAttachments.length > 1 ? 's' : ''
+      } ${undeclaredAttachments
+        .map(
+          ([k, exts]) =>
+            `'${k}' from extension${exts.length > 1 ? 's' : ''} '${exts
+              .map(e => e.id)
+              .join("', '")}'`,
+        )
+        .join(' and ')}`,
+    );
+  }
+
   return mapValues(inputMap, (input, inputName) => {
     const attachedInstances = attachments.get(inputName) ?? [];
     if (input.config.singleton) {