diff --git a/.changeset/smart-beers-give.md b/.changeset/smart-beers-give.md
new file mode 100644
index 0000000000..5b9d9888ba
--- /dev/null
+++ b/.changeset/smart-beers-give.md
@@ -0,0 +1,6 @@
+---
+'@backstage/frontend-plugin-api': patch
+'@backstage/frontend-test-utils': patch
+---
+
+Internal refactor
diff --git a/packages/frontend-internal/.eslintrc.js b/packages/frontend-internal/.eslintrc.js
new file mode 100644
index 0000000000..e487f765b2
--- /dev/null
+++ b/packages/frontend-internal/.eslintrc.js
@@ -0,0 +1,5 @@
+module.exports = require('@backstage/cli/config/eslint-factory')(__dirname, {
+  rules: {
+    '@backstage/no-top-level-material-ui-4-imports': 'error',
+  },
+});
diff --git a/packages/frontend-internal/README.md b/packages/frontend-internal/README.md
new file mode 100644
index 0000000000..72e54f21cf
--- /dev/null
+++ b/packages/frontend-internal/README.md
@@ -0,0 +1,3 @@
+# @internal/frontend
+
+This is an internal package used by the other frontend packages. It does not get published to NPM, but instead inlined into consuming packages due to the `backstage.inline` flag in `package.json`.
diff --git a/packages/frontend-internal/catalog-info.yaml b/packages/frontend-internal/catalog-info.yaml
new file mode 100644
index 0000000000..630f53275a
--- /dev/null
+++ b/packages/frontend-internal/catalog-info.yaml
@@ -0,0 +1,9 @@
+apiVersion: backstage.io/v1alpha1
+kind: Component
+metadata:
+  name: internal-frontend
+  title: '@internal/frontend'
+spec:
+  lifecycle: experimental
+  type: backstage-web-library
+  owner: maintainers
diff --git a/packages/frontend-internal/package.json b/packages/frontend-internal/package.json
new file mode 100644
index 0000000000..1e637b9d78
--- /dev/null
+++ b/packages/frontend-internal/package.json
@@ -0,0 +1,39 @@
+{
+  "name": "@internal/frontend",
+  "version": "0.0.0",
+  "backstage": {
+    "role": "web-library",
+    "inline": true
+  },
+  "private": true,
+  "repository": {
+    "type": "git",
+    "url": "https://github.com/backstage/backstage",
+    "directory": "packages/frontend-internal"
+  },
+  "license": "Apache-2.0",
+  "sideEffects": false,
+  "main": "src/index.ts",
+  "types": "src/index.ts",
+  "files": [
+    "dist"
+  ],
+  "scripts": {
+    "lint": "backstage-cli package lint",
+    "test": "backstage-cli package test"
+  },
+  "dependencies": {
+    "@backstage/frontend-plugin-api": "workspace:^",
+    "@backstage/types": "workspace:^",
+    "@backstage/version-bridge": "workspace:^",
+    "zod": "^3.22.4"
+  },
+  "devDependencies": {
+    "@backstage/cli": "workspace:^",
+    "@backstage/frontend-app-api": "workspace:^",
+    "@backstage/frontend-test-utils": "workspace:^",
+    "@backstage/test-utils": "workspace:^",
+    "@testing-library/jest-dom": "^6.0.0",
+    "@testing-library/react": "^15.0.0"
+  }
+}
diff --git a/packages/frontend-internal/src/index.ts b/packages/frontend-internal/src/index.ts
new file mode 100644
index 0000000000..a5728f2ff6
--- /dev/null
+++ b/packages/frontend-internal/src/index.ts
@@ -0,0 +1,17 @@
+/*
+ * Copyright 2024 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+export * from './wiring';
diff --git a/packages/frontend-internal/src/wiring/InternalExtensionDefinition.ts b/packages/frontend-internal/src/wiring/InternalExtensionDefinition.ts
new file mode 100644
index 0000000000..840e45ff4a
--- /dev/null
+++ b/packages/frontend-internal/src/wiring/InternalExtensionDefinition.ts
@@ -0,0 +1,104 @@
+/*
+ * Copyright 2024 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import {
+  AnyExtensionDataRef,
+  ApiHolder,
+  AppNode,
+  ExtensionDataValue,
+  ExtensionDefinition,
+  ExtensionDefinitionParameters,
+  ExtensionInput,
+  PortableSchema,
+  ResolvedExtensionInputs,
+} from '@backstage/frontend-plugin-api';
+
+export type InternalExtensionDefinition<
+  T extends ExtensionDefinitionParameters = ExtensionDefinitionParameters,
+> = ExtensionDefinition<T> & {
+  readonly kind?: string;
+  readonly namespace?: string;
+  readonly name?: string;
+  readonly attachTo: { id: string; input: string };
+  readonly disabled: boolean;
+  readonly configSchema?: PortableSchema<T['config'], T['configInput']>;
+} & (
+    | {
+        readonly version: 'v1';
+        readonly inputs: {
+          [inputName in string]: {
+            $$type: '@backstage/ExtensionInput';
+            extensionData: {
+              [name in string]: AnyExtensionDataRef;
+            };
+            config: { optional: boolean; singleton: boolean };
+          };
+        };
+        readonly output: {
+          [name in string]: AnyExtensionDataRef;
+        };
+        factory(context: {
+          node: AppNode;
+          apis: ApiHolder;
+          config: object;
+          inputs: {
+            [inputName in string]: unknown;
+          };
+        }): {
+          [inputName in string]: unknown;
+        };
+      }
+    | {
+        readonly version: 'v2';
+        readonly inputs: {
+          [inputName in string]: ExtensionInput<
+            AnyExtensionDataRef,
+            { optional: boolean; singleton: boolean }
+          >;
+        };
+        readonly output: Array<AnyExtensionDataRef>;
+        factory(context: {
+          node: AppNode;
+          apis: ApiHolder;
+          config: object;
+          inputs: ResolvedExtensionInputs<{
+            [inputName in string]: ExtensionInput<
+              AnyExtensionDataRef,
+              { optional: boolean; singleton: boolean }
+            >;
+          }>;
+        }): Iterable<ExtensionDataValue<any, any>>;
+      }
+  );
+
+/** @internal */
+export function toInternalExtensionDefinition<
+  T extends ExtensionDefinitionParameters,
+>(overrides: ExtensionDefinition<T>): InternalExtensionDefinition<T> {
+  const internal = overrides as InternalExtensionDefinition<T>;
+  if (internal.$$type !== '@backstage/ExtensionDefinition') {
+    throw new Error(
+      `Invalid extension definition instance, bad type '${internal.$$type}'`,
+    );
+  }
+  const version = internal.version;
+  if (version !== 'v1' && version !== 'v2') {
+    throw new Error(
+      `Invalid extension definition instance, bad version '${version}'`,
+    );
+  }
+  return internal;
+}
diff --git a/packages/frontend-internal/src/wiring/index.ts b/packages/frontend-internal/src/wiring/index.ts
new file mode 100644
index 0000000000..d0aefbed64
--- /dev/null
+++ b/packages/frontend-internal/src/wiring/index.ts
@@ -0,0 +1,20 @@
+/*
+ * Copyright 2024 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+export {
+  toInternalExtensionDefinition,
+  type InternalExtensionDefinition,
+} from './InternalExtensionDefinition';
diff --git a/packages/frontend-plugin-api/src/wiring/createExtension.ts b/packages/frontend-plugin-api/src/wiring/createExtension.ts
index 445bbd8a68..4d7ebee521 100644
--- a/packages/frontend-plugin-api/src/wiring/createExtension.ts
+++ b/packages/frontend-plugin-api/src/wiring/createExtension.ts
@@ -15,7 +15,6 @@
  */
 
 import { ApiHolder, AppNode } from '../apis';
-import { PortableSchema } from '../schema';
 import { Expand } from '../types';
 import {
   ResolveInputValueOverrides,
@@ -32,6 +31,7 @@ import {
 import { ExtensionInput } from './createExtensionInput';
 import { z } from 'zod';
 import { createSchemaFromZod } from '../schema/createSchemaFromZod';
+import { InternalExtensionDefinition } from '@internal/frontend';
 
 /**
  * Convert a single extension input into a matching resolved input.
@@ -235,84 +235,6 @@ export type ExtensionDefinition<
   }>;
 };
 
-/** @internal */
-export type InternalExtensionDefinition<
-  T extends ExtensionDefinitionParameters = ExtensionDefinitionParameters,
-> = ExtensionDefinition<T> & {
-  readonly kind?: string;
-  readonly namespace?: string;
-  readonly name?: string;
-  readonly attachTo: { id: string; input: string };
-  readonly disabled: boolean;
-  readonly configSchema?: PortableSchema<T['config'], T['configInput']>;
-} & (
-    | {
-        readonly version: 'v1';
-        readonly inputs: {
-          [inputName in string]: {
-            $$type: '@backstage/ExtensionInput';
-            extensionData: {
-              [name in string]: AnyExtensionDataRef;
-            };
-            config: { optional: boolean; singleton: boolean };
-          };
-        };
-        readonly output: {
-          [name in string]: AnyExtensionDataRef;
-        };
-        factory(context: {
-          node: AppNode;
-          apis: ApiHolder;
-          config: object;
-          inputs: {
-            [inputName in string]: unknown;
-          };
-        }): {
-          [inputName in string]: unknown;
-        };
-      }
-    | {
-        readonly version: 'v2';
-        readonly inputs: {
-          [inputName in string]: ExtensionInput<
-            AnyExtensionDataRef,
-            { optional: boolean; singleton: boolean }
-          >;
-        };
-        readonly output: Array<AnyExtensionDataRef>;
-        factory(context: {
-          node: AppNode;
-          apis: ApiHolder;
-          config: object;
-          inputs: ResolvedExtensionInputs<{
-            [inputName in string]: ExtensionInput<
-              AnyExtensionDataRef,
-              { optional: boolean; singleton: boolean }
-            >;
-          }>;
-        }): Iterable<ExtensionDataValue<any, any>>;
-      }
-  );
-
-/** @internal */
-export function toInternalExtensionDefinition<
-  T extends ExtensionDefinitionParameters,
->(overrides: ExtensionDefinition<T>): InternalExtensionDefinition<T> {
-  const internal = overrides as InternalExtensionDefinition<T>;
-  if (internal.$$type !== '@backstage/ExtensionDefinition') {
-    throw new Error(
-      `Invalid extension definition instance, bad type '${internal.$$type}'`,
-    );
-  }
-  const version = internal.version;
-  if (version !== 'v1' && version !== 'v2') {
-    throw new Error(
-      `Invalid extension definition instance, bad version '${version}'`,
-    );
-  }
-  return internal;
-}
-
 /** @public */
 export function createExtension<
   UOutput extends AnyExtensionDataRef,
diff --git a/packages/frontend-plugin-api/src/wiring/createExtensionBlueprint.test.tsx b/packages/frontend-plugin-api/src/wiring/createExtensionBlueprint.test.tsx
index fc77c63e97..47ef7a75b4 100644
--- a/packages/frontend-plugin-api/src/wiring/createExtensionBlueprint.test.tsx
+++ b/packages/frontend-plugin-api/src/wiring/createExtensionBlueprint.test.tsx
@@ -27,11 +27,9 @@ import {
 } from './createExtensionDataRef';
 import { createExtensionInput } from './createExtensionInput';
 import { RouteRef } from '../routing';
-import {
-  ExtensionDefinition,
-  toInternalExtensionDefinition,
-} from './createExtension';
+import { ExtensionDefinition } from './createExtension';
 import { createExtensionDataContainer } from './createExtensionDataContainer';
+import { toInternalExtensionDefinition } from '@internal/frontend';
 
 function unused(..._any: any[]) {}
 
diff --git a/packages/frontend-plugin-api/src/wiring/createFrontendModule.ts b/packages/frontend-plugin-api/src/wiring/createFrontendModule.ts
index aacc34ccf0..e3fa8b6682 100644
--- a/packages/frontend-plugin-api/src/wiring/createFrontendModule.ts
+++ b/packages/frontend-plugin-api/src/wiring/createFrontendModule.ts
@@ -15,10 +15,10 @@
  */
 
 import {
-  ExtensionDefinition,
   InternalExtensionDefinition,
   toInternalExtensionDefinition,
-} from './createExtension';
+} from '@internal/frontend';
+import { ExtensionDefinition } from './createExtension';
 import {
   Extension,
   resolveExtensionDefinition,
diff --git a/packages/frontend-plugin-api/src/wiring/createFrontendPlugin.ts b/packages/frontend-plugin-api/src/wiring/createFrontendPlugin.ts
index 03c843543d..adcd2b0867 100644
--- a/packages/frontend-plugin-api/src/wiring/createFrontendPlugin.ts
+++ b/packages/frontend-plugin-api/src/wiring/createFrontendPlugin.ts
@@ -15,10 +15,10 @@
  */
 
 import {
-  ExtensionDefinition,
   InternalExtensionDefinition,
   toInternalExtensionDefinition,
-} from './createExtension';
+} from '@internal/frontend';
+import { ExtensionDefinition } from './createExtension';
 import {
   Extension,
   ResolveExtensionId,
diff --git a/packages/frontend-plugin-api/src/wiring/resolveExtensionDefinition.ts b/packages/frontend-plugin-api/src/wiring/resolveExtensionDefinition.ts
index 870fb258f5..ce275406e7 100644
--- a/packages/frontend-plugin-api/src/wiring/resolveExtensionDefinition.ts
+++ b/packages/frontend-plugin-api/src/wiring/resolveExtensionDefinition.ts
@@ -19,7 +19,6 @@ import {
   ExtensionDefinition,
   ExtensionDefinitionParameters,
   ResolvedExtensionInputs,
-  toInternalExtensionDefinition,
 } from './createExtension';
 import { PortableSchema } from '../schema';
 import { ExtensionInput } from './createExtensionInput';
@@ -27,6 +26,7 @@ import {
   AnyExtensionDataRef,
   ExtensionDataValue,
 } from './createExtensionDataRef';
+import { toInternalExtensionDefinition } from '@internal/frontend';
 
 /** @public */
 export interface Extension<TConfig, TConfigInput = TConfig> {
diff --git a/packages/frontend-test-utils/package.json b/packages/frontend-test-utils/package.json
index a84d296c55..8ada3151bf 100644
--- a/packages/frontend-test-utils/package.json
+++ b/packages/frontend-test-utils/package.json
@@ -36,7 +36,9 @@
     "@backstage/frontend-plugin-api": "workspace:^",
     "@backstage/plugin-app": "workspace:^",
     "@backstage/test-utils": "workspace:^",
-    "@backstage/types": "workspace:^"
+    "@backstage/types": "workspace:^",
+    "@backstage/version-bridge": "workspace:^",
+    "zod": "^3.22.4"
   },
   "devDependencies": {
     "@backstage/cli": "workspace:^",
diff --git a/packages/frontend-test-utils/src/app/createExtensionTester.tsx b/packages/frontend-test-utils/src/app/createExtensionTester.tsx
index f9c1b3cb89..31ca9c86ce 100644
--- a/packages/frontend-test-utils/src/app/createExtensionTester.tsx
+++ b/packages/frontend-test-utils/src/app/createExtensionTester.tsx
@@ -29,8 +29,6 @@ import { JsonArray, JsonObject, JsonValue } from '@backstage/types';
 // eslint-disable-next-line @backstage/no-relative-monorepo-imports
 import { resolveExtensionDefinition } from '../../../frontend-plugin-api/src/wiring/resolveExtensionDefinition';
 // eslint-disable-next-line @backstage/no-relative-monorepo-imports
-import { toInternalExtensionDefinition } from '../../../frontend-plugin-api/src/wiring/createExtension';
-// eslint-disable-next-line @backstage/no-relative-monorepo-imports
 import { resolveAppTree } from '../../../frontend-app-api/src/tree/resolveAppTree';
 // eslint-disable-next-line @backstage/no-relative-monorepo-imports
 import { resolveAppNodeSpecs } from '../../../frontend-app-api/src/tree/resolveAppNodeSpecs';
@@ -39,6 +37,7 @@ import { instantiateAppNodeTree } from '../../../frontend-app-api/src/tree/insta
 // eslint-disable-next-line @backstage/no-relative-monorepo-imports
 import { readAppExtensionsConfig } from '../../../frontend-app-api/src/tree/readAppExtensionsConfig';
 import { TestApiRegistry } from '@backstage/test-utils';
+import { toInternalExtensionDefinition } from '@internal/frontend';
 
 /** @public */
 export class ExtensionQuery<UOutput extends AnyExtensionDataRef> {
diff --git a/yarn.lock b/yarn.lock
index dfc8cc25cd..bba0d759b4 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -4522,8 +4522,10 @@ __metadata:
     "@backstage/plugin-app": "workspace:^"
     "@backstage/test-utils": "workspace:^"
     "@backstage/types": "workspace:^"
+    "@backstage/version-bridge": "workspace:^"
     "@testing-library/jest-dom": ^6.0.0
     "@types/react": "*"
+    zod: ^3.22.4
   peerDependencies:
     "@testing-library/react": ^15.0.0
     react: ^16.13.1 || ^17.0.0 || ^18.0.0
@@ -10002,6 +10004,23 @@ __metadata:
   languageName: node
   linkType: hard
 
+"@internal/frontend@workspace:packages/frontend-internal":
+  version: 0.0.0-use.local
+  resolution: "@internal/frontend@workspace:packages/frontend-internal"
+  dependencies:
+    "@backstage/cli": "workspace:^"
+    "@backstage/frontend-app-api": "workspace:^"
+    "@backstage/frontend-plugin-api": "workspace:^"
+    "@backstage/frontend-test-utils": "workspace:^"
+    "@backstage/test-utils": "workspace:^"
+    "@backstage/types": "workspace:^"
+    "@backstage/version-bridge": "workspace:^"
+    "@testing-library/jest-dom": ^6.0.0
+    "@testing-library/react": ^15.0.0
+    zod: ^3.22.4
+  languageName: unknown
+  linkType: soft
+
 "@internal/plugin-todo-list-backend@workspace:plugins/example-todo-list-backend":
   version: 0.0.0-use.local
   resolution: "@internal/plugin-todo-list-backend@workspace:plugins/example-todo-list-backend"