diff --git a/.changeset/funny-rocks-train.md b/.changeset/funny-rocks-train.md
new file mode 100644
index 0000000000..f5b2fbba63
--- /dev/null
+++ b/.changeset/funny-rocks-train.md
@@ -0,0 +1,6 @@
+---
+'@backstage/frontend-plugin-api': patch
+'@backstage/frontend-test-utils': patch
+---
+
+Internal refactor of usage of opaque types.
diff --git a/packages/frontend-internal/src/wiring/InternalExtensionDefinition.ts b/packages/frontend-internal/src/wiring/InternalExtensionDefinition.ts
index 840e45ff4a..a77ffd485b 100644
--- a/packages/frontend-internal/src/wiring/InternalExtensionDefinition.ts
+++ b/packages/frontend-internal/src/wiring/InternalExtensionDefinition.ts
@@ -25,19 +25,19 @@ import {
   PortableSchema,
   ResolvedExtensionInputs,
 } from '@backstage/frontend-plugin-api';
+import { OpaqueType } from './OpaqueType';
 
-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']>;
-} & (
+export const OpaqueExtensionDefinition = OpaqueType.create<{
+  public: ExtensionDefinition<ExtensionDefinitionParameters>;
+  versions:
     | {
         readonly version: 'v1';
+        readonly kind?: string;
+        readonly namespace?: string;
+        readonly name?: string;
+        readonly attachTo: { id: string; input: string };
+        readonly disabled: boolean;
+        readonly configSchema?: PortableSchema<any, any>;
         readonly inputs: {
           [inputName in string]: {
             $$type: '@backstage/ExtensionInput';
@@ -63,6 +63,12 @@ export type InternalExtensionDefinition<
       }
     | {
         readonly version: 'v2';
+        readonly kind?: string;
+        readonly namespace?: string;
+        readonly name?: string;
+        readonly attachTo: { id: string; input: string };
+        readonly disabled: boolean;
+        readonly configSchema?: PortableSchema<any, any>;
         readonly inputs: {
           [inputName in string]: ExtensionInput<
             AnyExtensionDataRef,
@@ -81,24 +87,8 @@ export type InternalExtensionDefinition<
             >;
           }>;
         }): Iterable<ExtensionDataValue<any, any>>;
-      }
-  );
-
-/** @internal */
-export function toInternalExtensionDefinition<
-  T extends ExtensionDefinitionParameters,
->(overrides: ExtensionDefinition<T>): InternalExtensionDefinition<T> {
-  const internal = overrides as InternalExtensionDefinition<T>;
-  if (internal.$$type !== '@backstage/ExtensionDefinition') {
-    throw new Error(
-      `Invalid extension definition instance, bad type '${internal.$$type}'`,
-    );
-  }
-  const version = internal.version;
-  if (version !== 'v1' && version !== 'v2') {
-    throw new Error(
-      `Invalid extension definition instance, bad version '${version}'`,
-    );
-  }
-  return internal;
-}
+      };
+}>({
+  type: '@backstage/ExtensionDefinition',
+  versions: ['v1', 'v2'],
+});
diff --git a/packages/frontend-internal/src/wiring/OpaqueType.test.ts b/packages/frontend-internal/src/wiring/OpaqueType.test.ts
new file mode 100644
index 0000000000..2e49ca9029
--- /dev/null
+++ b/packages/frontend-internal/src/wiring/OpaqueType.test.ts
@@ -0,0 +1,416 @@
+/*
+ * Copyright 2024 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { OpaqueType } from './OpaqueType';
+
+describe('OpaqueType', () => {
+  it('should create a basic opaque type with a single version', () => {
+    type MyType = {
+      $$type: 'my-type';
+    };
+
+    const OpaqueMyType = OpaqueType.create<{
+      public: MyType;
+      versions: {
+        version: 'v1';
+        foo: string;
+      };
+    }>({
+      type: 'my-type',
+      versions: ['v1'],
+    });
+
+    // @ts-expect-error - unsupported version
+    OpaqueMyType.createInstance('v2', {
+      foo: 'bar',
+    });
+
+    // @ts-expect-error - missing internal field
+    OpaqueMyType.createInstance('v1', {});
+
+    OpaqueMyType.createInstance('v1', {
+      // @ts-expect-error - invalid internal field
+      foo: 3,
+    });
+
+    const myInstance = OpaqueMyType.createInstance('v1', {
+      foo: 'bar',
+    });
+
+    expect(myInstance.$$type).toBe('my-type');
+    // @ts-expect-error - version field not accessible
+    expect(myInstance.version).toBe('v1');
+    // @ts-expect-error - internal field not accessible
+    expect(myInstance.foo).toBe('bar');
+
+    expect(OpaqueMyType.isType(myInstance)).toBe(true);
+    expect(OpaqueMyType.isType('hello')).toBe(false);
+    expect(OpaqueMyType.isType({ $$type: 'some-other' })).toBe(false);
+    expect(OpaqueMyType.isType({ $$type: 'my-type' })).toBe(true);
+
+    const myInternal = OpaqueMyType.toInternal(myInstance);
+    expect(myInternal).toBe(myInstance);
+    // All fields accessible
+    expect(myInternal.$$type).toBe('my-type');
+    expect(myInternal.version).toBe('v1');
+    expect(myInternal.foo).toBe('bar');
+
+    expect(() =>
+      OpaqueMyType.toInternal('hello'),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `"Invalid opaque type, expected 'my-type', but got '<string>'"`,
+    );
+    expect(() => OpaqueMyType.toInternal(3)).toThrowErrorMatchingInlineSnapshot(
+      `"Invalid opaque type, expected 'my-type', but got '<number>'"`,
+    );
+    expect(() =>
+      OpaqueMyType.toInternal(undefined),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `"Invalid opaque type, expected 'my-type', but got '<undefined>'"`,
+    );
+    expect(() =>
+      OpaqueMyType.toInternal(Symbol('wat')),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `"Invalid opaque type, expected 'my-type', but got '<symbol>'"`,
+    );
+    expect(() =>
+      OpaqueMyType.toInternal(null),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `"Invalid opaque type, expected 'my-type', but got '<null>'"`,
+    );
+    expect(() =>
+      OpaqueMyType.toInternal(() => {}),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `"Invalid opaque type, expected 'my-type', but got '<function>'"`,
+    );
+    expect(() =>
+      OpaqueMyType.toInternal({ $$type: 'some-other-type' }),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `"Invalid opaque type, expected 'my-type', but got 'some-other-type'"`,
+    );
+    expect(() =>
+      OpaqueMyType.toInternal({ an: 'object' }),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `"Invalid opaque type, expected 'my-type', but got '[object Object]'"`,
+    );
+    expect(() =>
+      OpaqueMyType.toInternal({ $$type: 'my-type' }),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `"Invalid opaque type instance, got version undefined, expected 'v1'"`,
+    );
+    expect(() =>
+      OpaqueMyType.toInternal({ $$type: 'my-type', version: 'v0' }),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `"Invalid opaque type instance, got version 'v0', expected 'v1'"`,
+    );
+    expect(() =>
+      OpaqueMyType.toInternal({ $$type: 'my-type', version: { foo: 'bar' } }),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `"Invalid opaque type instance, got version '[object Object]', expected 'v1'"`,
+    );
+  });
+
+  it('should create a basic opaque type with multiple versions', () => {
+    type MyType = {
+      $$type: 'my-type';
+    };
+
+    const OpaqueMyType = OpaqueType.create<{
+      public: MyType;
+      versions:
+        | {
+            version: 'v1';
+            foo: string;
+          }
+        | {
+            version: 'v2';
+            bar: string;
+          }
+        | {
+            version: 'v3';
+            baz: string;
+          };
+    }>({
+      type: 'my-type',
+      versions: ['v1', 'v2', 'v3'],
+    });
+
+    // @ts-expect-error - unsupported version
+    OpaqueMyType.createInstance('v0', {
+      foo: 'bar',
+    });
+
+    // @ts-expect-error - missing internal field
+    OpaqueMyType.createInstance('v1', {});
+
+    OpaqueMyType.createInstance('v1', {
+      // @ts-expect-error - invalid internal field
+      foo: 3,
+    });
+
+    OpaqueMyType.createInstance('v2', {
+      // @ts-expect-error - version mismatch
+      foo: 'bar',
+    });
+
+    OpaqueMyType.createInstance('v1', {
+      // @ts-expect-error - version mismatch
+      bar: 'foo',
+    });
+
+    const myInstanceV1 = OpaqueMyType.createInstance('v1', {
+      foo: 'bar',
+    });
+
+    const myInstanceV2 = OpaqueMyType.createInstance('v2', {
+      bar: 'foo',
+    });
+
+    expect(myInstanceV1.$$type).toBe('my-type');
+    // @ts-expect-error - version field not accessible
+    expect(myInstanceV1.version).toBe('v1');
+    // @ts-expect-error - internal field not accessible
+    expect(myInstanceV1.foo).toBe('bar');
+
+    expect(myInstanceV2.$$type).toBe('my-type');
+    // @ts-expect-error - version field not accessible
+    expect(myInstanceV2.version).toBe('v2');
+    // @ts-expect-error - internal field not accessible
+    expect(myInstanceV2.bar).toBe('foo');
+
+    expect(OpaqueMyType.isType(myInstanceV1)).toBe(true);
+    expect(OpaqueMyType.isType(myInstanceV2)).toBe(true);
+    expect(OpaqueMyType.isType('hello')).toBe(false);
+
+    const myInternalV1 = OpaqueMyType.toInternal(myInstanceV1);
+    expect(myInternalV1).toBe(myInstanceV1);
+    // All fields accessible
+    expect(myInternalV1.$$type).toBe('my-type');
+    expect(myInternalV1.version).toBe('v1');
+    // @ts-expect-error - version has not been narrowed down
+    expect(myInternalV1.foo).toBe('bar');
+
+    const myInternalV2 = OpaqueMyType.toInternal(myInstanceV2);
+    expect(myInternalV2).toBe(myInstanceV2);
+    // All fields accessible
+    expect(myInternalV2.$$type).toBe('my-type');
+    expect(myInternalV2.version).toBe('v2');
+    // @ts-expect-error - version has not been narrowed down
+    expect(myInternalV2.bar).toBe('foo');
+
+    // Narrowing the version allows access to internal fields
+    expect(myInternalV1.version === 'v1' && myInternalV1.foo).toBe('bar');
+    expect(myInternalV2.version === 'v2' && myInternalV2.bar).toBe('foo');
+
+    expect(() =>
+      OpaqueMyType.toInternal({ $$type: 'my-type' }),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `"Invalid opaque type instance, got version undefined, expected 'v1', 'v2', or 'v3'"`,
+    );
+    expect(() =>
+      OpaqueMyType.toInternal({ $$type: 'my-type', version: 'v0' }),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `"Invalid opaque type instance, got version 'v0', expected 'v1', 'v2', or 'v3'"`,
+    );
+  });
+
+  it('should support undefined version for backwards compatibility', () => {
+    type MyType = {
+      $$type: 'my-type';
+    };
+
+    const OpaqueMyType = OpaqueType.create<{
+      public: MyType;
+      versions: {
+        version: undefined;
+        foo: string;
+      };
+    }>({
+      type: 'my-type',
+      versions: [undefined],
+    });
+
+    // @ts-expect-error - unsupported version
+    OpaqueMyType.createInstance('v1', {
+      foo: 'bar',
+    });
+
+    // @ts-expect-error - missing internal field
+    OpaqueMyType.createInstance(undefined, {});
+
+    OpaqueMyType.createInstance(undefined, {
+      // @ts-expect-error - invalid internal field
+      foo: 3,
+    });
+
+    const myInstance = OpaqueMyType.createInstance(undefined, {
+      foo: 'bar',
+    });
+
+    expect(myInstance.$$type).toBe('my-type');
+    // @ts-expect-error - version field not accessible
+    expect(myInstance.version).toBe(undefined);
+    // @ts-expect-error - internal field not accessible
+    expect(myInstance.foo).toBe('bar');
+
+    expect(OpaqueMyType.isType(myInstance)).toBe(true);
+    expect(OpaqueMyType.isType('hello')).toBe(false);
+
+    const myInternal = OpaqueMyType.toInternal(myInstance);
+    expect(myInternal).toBe(myInstance);
+    // All fields accessible
+    expect(myInternal.$$type).toBe('my-type');
+    expect(myInternal.version).toBe(undefined);
+    expect(myInternal.foo).toBe('bar');
+
+    expect(OpaqueMyType.toInternal({ $$type: 'my-type' })).toBeDefined();
+    expect(() =>
+      OpaqueMyType.toInternal({ $$type: 'my-type', version: 'v0' }),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `"Invalid opaque type instance, got version 'v0', expected undefined"`,
+    );
+  });
+
+  it('should support undefined version mixed with defined versions', () => {
+    type MyType = {
+      $$type: 'my-type';
+    };
+
+    const OpaqueMyType = OpaqueType.create<{
+      public: MyType;
+      versions:
+        | {
+            version: 'v1';
+            foo: string;
+          }
+        | {
+            version: undefined;
+            bar: string;
+          };
+    }>({
+      type: 'my-type',
+      versions: [undefined, 'v1'],
+    });
+
+    // @ts-expect-error - unsupported version
+    OpaqueMyType.createInstance('v0', {
+      foo: 'bar',
+    });
+
+    // @ts-expect-error - missing internal field
+    OpaqueMyType.createInstance('v1', {});
+
+    OpaqueMyType.createInstance('v1', {
+      // @ts-expect-error - invalid internal field
+      foo: 3,
+    });
+
+    OpaqueMyType.createInstance(undefined, {
+      // @ts-expect-error - version mismatch
+      foo: 'bar',
+    });
+
+    OpaqueMyType.createInstance('v1', {
+      // @ts-expect-error - version mismatch
+      bar: 'foo',
+    });
+
+    const myInstanceV1 = OpaqueMyType.createInstance('v1', {
+      foo: 'bar',
+    });
+
+    const myInstanceV2 = OpaqueMyType.createInstance(undefined, {
+      bar: 'foo',
+    });
+
+    expect(myInstanceV1.$$type).toBe('my-type');
+    // @ts-expect-error - version field not accessible
+    expect(myInstanceV1.version).toBe('v1');
+    // @ts-expect-error - internal field not accessible
+    expect(myInstanceV1.foo).toBe('bar');
+
+    expect(myInstanceV2.$$type).toBe('my-type');
+    // @ts-expect-error - version field not accessible
+    expect(myInstanceV2.version).toBe(undefined);
+    // @ts-expect-error - internal field not accessible
+    expect(myInstanceV2.bar).toBe('foo');
+
+    expect(OpaqueMyType.isType(myInstanceV1)).toBe(true);
+    expect(OpaqueMyType.isType(myInstanceV2)).toBe(true);
+    expect(OpaqueMyType.isType('hello')).toBe(false);
+
+    const myInternalV1 = OpaqueMyType.toInternal(myInstanceV1);
+    expect(myInternalV1).toBe(myInstanceV1);
+    // All fields accessible
+    expect(myInternalV1.$$type).toBe('my-type');
+    expect(myInternalV1.version).toBe('v1');
+    // @ts-expect-error - version has not been narrowed down
+    expect(myInternalV1.foo).toBe('bar');
+
+    const myInternalV2 = OpaqueMyType.toInternal(myInstanceV2);
+    expect(myInternalV2).toBe(myInstanceV2);
+    // All fields accessible
+    expect(myInternalV2.$$type).toBe('my-type');
+    expect(myInternalV2.version).toBe(undefined);
+    // @ts-expect-error - version has not been narrowed down
+    expect(myInternalV2.bar).toBe('foo');
+
+    // Narrowing the version allows access to internal fields
+    expect(myInternalV1.version === 'v1' && myInternalV1.foo).toBe('bar');
+    expect(myInternalV2.version === undefined && myInternalV2.bar).toBe('foo');
+
+    expect(OpaqueMyType.toInternal({ $$type: 'my-type' })).toBeDefined();
+    expect(() =>
+      OpaqueMyType.toInternal({ $$type: 'my-type', version: 'v3' }),
+    ).toThrowErrorMatchingInlineSnapshot(
+      `"Invalid opaque type instance, got version 'v3', expected undefined or 'v1'"`,
+    );
+  });
+
+  it('should create an empty opaque type with no versions', () => {
+    type MyType = {
+      $$type: 'my-type';
+    };
+
+    const OpaqueMyType = OpaqueType.create<{
+      public: MyType;
+      versions: {
+        version: undefined;
+      };
+    }>({
+      type: 'my-type',
+      versions: [undefined],
+    });
+
+    // @ts-expect-error - unsupported version
+    OpaqueMyType.createInstance('v0', {
+      foo: 'bar',
+    });
+
+    const myInstance = OpaqueMyType.createInstance(undefined, {});
+
+    expect(myInstance.$$type).toBe('my-type');
+
+    expect(OpaqueMyType.isType(myInstance)).toBe(true);
+    expect(OpaqueMyType.isType('hello')).toBe(false);
+
+    const myInternal = OpaqueMyType.toInternal(myInstance);
+    expect(myInternal).toBe(myInstance);
+    // All fields accessible
+    expect(myInternal.$$type).toBe('my-type');
+    expect(myInternal.version).toBe(undefined);
+  });
+});
diff --git a/packages/frontend-internal/src/wiring/OpaqueType.ts b/packages/frontend-internal/src/wiring/OpaqueType.ts
new file mode 100644
index 0000000000..3d05b619bc
--- /dev/null
+++ b/packages/frontend-internal/src/wiring/OpaqueType.ts
@@ -0,0 +1,168 @@
+/*
+ * 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.
+ */
+
+// TODO(Rugvip): This lives here temporarily, but should be moved to a more
+// central location. It's useful for backend packages too so we'll need to have
+// it in a common package, but it might also be that we want to make it
+// available publicly too in which case it would make sense to have this be part
+// of @backstage/version-bridge. The problem with exporting it from there is
+// that it would need to be very stable at that point, so it might be a bit
+// early to put it there already.
+
+/**
+ * A helper for working with opaque types.
+ */
+export class OpaqueType<
+  T extends {
+    public: { $$type: string };
+    versions: { version: string | undefined };
+  },
+> {
+  /**
+   * Creates a new opaque type.
+   *
+   * @param options.type The type identifier of the opaque type
+   * @param options.versions The available versions of the opaque type
+   * @returns A new opaque type helper
+   */
+  static create<
+    T extends {
+      public: { $$type: string };
+      versions: { version: string | undefined };
+    },
+  >(options: {
+    type: T['public']['$$type'];
+    versions: Array<T['versions']['version']>;
+  }) {
+    return new OpaqueType<T>(options.type, new Set(options.versions));
+  }
+
+  #type: string;
+  #versions: Set<string | undefined>;
+
+  private constructor(type: string, versions: Set<string | undefined>) {
+    this.#type = type;
+    this.#versions = versions;
+  }
+
+  /**
+   * The internal version of the opaque type, used like this: `typeof MyOpaqueType.TPublic`
+   *
+   * @remarks
+   *
+   * This property is only useful for type checking, its runtime value is `undefined`.
+   */
+  TPublic: T['public'] = undefined as any;
+
+  /**
+   * The internal version of the opaque type, used like this: `typeof MyOpaqueType.TInternal`
+   *
+   * @remarks
+   *
+   * This property is only useful for type checking, its runtime value is `undefined`.
+   */
+  TInternal: T['public'] & T['versions'] = undefined as any;
+
+  /**
+   * @param value Input value expected to be an instance of this opaque type
+   * @returns True if the value matches this opaque type
+   */
+  isType(value: unknown): value is T['public'] {
+    return this.#isThisInternalType(value);
+  }
+
+  /**
+   * @param value Input value expected to be an instance of this opaque type
+   * @throws If the value is not an instance of this opaque type or is of an unsupported version
+   * @returns The internal version of the opaque type
+   */
+  toInternal(value: unknown): T['public'] & T['versions'] {
+    if (!this.#isThisInternalType(value)) {
+      throw new TypeError(
+        `Invalid opaque type, expected '${
+          this.#type
+        }', but got '${this.#stringifyUnknown(value)}'`,
+      );
+    }
+
+    if (!this.#versions.has(value.version)) {
+      const versions = Array.from(this.#versions).map(this.#stringifyVersion);
+      if (versions.length > 1) {
+        versions[versions.length - 1] = `or ${versions[versions.length - 1]}`;
+      }
+      const expected =
+        versions.length > 2 ? versions.join(', ') : versions.join(' ');
+      throw new TypeError(
+        `Invalid opaque type instance, got version ${this.#stringifyVersion(
+          value.version,
+        )}, expected ${expected}`,
+      );
+    }
+
+    return value;
+  }
+
+  /**
+   * Creates an instance of the opaque type, returning the public type.
+   *
+   * @param version The version of the instance to create
+   * @param value The remaining public and internal properties of the instance
+   * @returns An instance of the opaque type
+   */
+  createInstance<
+    TVersion extends T['versions']['version'],
+    TPublic extends T['public'],
+  >(
+    version: TVersion,
+    props: Omit<T['public'], '$$type'> &
+      (T['versions'] extends infer UVersion
+        ? UVersion extends { version: TVersion }
+          ? Omit<UVersion, 'version'>
+          : never
+        : never) &
+      Object, // & Object to allow for object properties too, e.g. toString()
+  ): TPublic {
+    return {
+      ...(props as object),
+      $$type: this.#type,
+      ...(version && { version }),
+    } as unknown as TPublic;
+  }
+
+  #isThisInternalType(value: unknown): value is T['public'] & T['versions'] {
+    if (value === null || typeof value !== 'object') {
+      return false;
+    }
+    return (value as T['public']).$$type === this.#type;
+  }
+
+  #stringifyUnknown(value: unknown) {
+    if (typeof value !== 'object') {
+      return `<${typeof value}>`;
+    }
+    if (value === null) {
+      return '<null>';
+    }
+    if ('$$type' in value) {
+      return String(value.$$type);
+    }
+    return String(value);
+  }
+
+  #stringifyVersion = (version: string | undefined) => {
+    return version ? `'${version}'` : 'undefined';
+  };
+}
diff --git a/packages/frontend-internal/src/wiring/index.ts b/packages/frontend-internal/src/wiring/index.ts
index d0aefbed64..86f1ed9ab7 100644
--- a/packages/frontend-internal/src/wiring/index.ts
+++ b/packages/frontend-internal/src/wiring/index.ts
@@ -14,7 +14,4 @@
  * limitations under the License.
  */
 
-export {
-  toInternalExtensionDefinition,
-  type InternalExtensionDefinition,
-} from './InternalExtensionDefinition';
+export { OpaqueExtensionDefinition } from './InternalExtensionDefinition';
diff --git a/packages/frontend-plugin-api/src/wiring/createExtension.ts b/packages/frontend-plugin-api/src/wiring/createExtension.ts
index 7aef3fc1b5..ac7b1de432 100644
--- a/packages/frontend-plugin-api/src/wiring/createExtension.ts
+++ b/packages/frontend-plugin-api/src/wiring/createExtension.ts
@@ -31,7 +31,7 @@ import {
 import { ExtensionInput } from './createExtensionInput';
 import { z } from 'zod';
 import { createSchemaFromZod } from '../schema/createSchemaFromZod';
-import { InternalExtensionDefinition } from '@internal/frontend';
+import { OpaqueExtensionDefinition } from '@internal/frontend';
 
 /**
  * Convert a single extension input into a matching resolved input.
@@ -366,26 +366,6 @@ export function createExtension<
   namespace: string | undefined extends TNamespace ? undefined : TNamespace;
   name: string | undefined extends TName ? undefined : TName;
 }> {
-  type T = {
-    config: string extends keyof TConfigSchema
-      ? {}
-      : {
-          [key in keyof TConfigSchema]: z.infer<ReturnType<TConfigSchema[key]>>;
-        };
-    configInput: string extends keyof TConfigSchema
-      ? {}
-      : z.input<
-          z.ZodObject<{
-            [key in keyof TConfigSchema]: ReturnType<TConfigSchema[key]>;
-          }>
-        >;
-    output: UOutput;
-    inputs: TInputs;
-    kind: string | undefined extends TKind ? undefined : TKind;
-    namespace: string | undefined extends TNamespace ? undefined : TNamespace;
-    name: string | undefined extends TName ? undefined : TName;
-  };
-
   const schemaDeclaration = options.config?.schema;
   const configSchema =
     schemaDeclaration &&
@@ -397,10 +377,28 @@ export function createExtension<
       ),
     );
 
-  return {
-    $$type: '@backstage/ExtensionDefinition',
-    version: 'v2',
-    T: undefined as unknown as T,
+  return OpaqueExtensionDefinition.createInstance('v2', {
+    T: undefined as unknown as {
+      config: string extends keyof TConfigSchema
+        ? {}
+        : {
+            [key in keyof TConfigSchema]: z.infer<
+              ReturnType<TConfigSchema[key]>
+            >;
+          };
+      configInput: string extends keyof TConfigSchema
+        ? {}
+        : z.input<
+            z.ZodObject<{
+              [key in keyof TConfigSchema]: ReturnType<TConfigSchema[key]>;
+            }>
+          >;
+      output: UOutput;
+      inputs: TInputs;
+      kind: string | undefined extends TKind ? undefined : TKind;
+      namespace: string | undefined extends TNamespace ? undefined : TNamespace;
+      name: string | undefined extends TName ? undefined : TName;
+    },
     kind: options.kind,
     namespace: options.namespace,
     name: options.name,
@@ -512,5 +510,5 @@ export function createExtension<
         },
       }) as ExtensionDefinition<any>;
     },
-  } as InternalExtensionDefinition<T>;
+  });
 }
diff --git a/packages/frontend-plugin-api/src/wiring/createExtensionBlueprint.test.tsx b/packages/frontend-plugin-api/src/wiring/createExtensionBlueprint.test.tsx
index 47ef7a75b4..16c763d725 100644
--- a/packages/frontend-plugin-api/src/wiring/createExtensionBlueprint.test.tsx
+++ b/packages/frontend-plugin-api/src/wiring/createExtensionBlueprint.test.tsx
@@ -29,12 +29,12 @@ import { createExtensionInput } from './createExtensionInput';
 import { RouteRef } from '../routing';
 import { ExtensionDefinition } from './createExtension';
 import { createExtensionDataContainer } from './createExtensionDataContainer';
-import { toInternalExtensionDefinition } from '@internal/frontend';
+import { OpaqueExtensionDefinition } from '@internal/frontend';
 
 function unused(..._any: any[]) {}
 
 function factoryOutput(ext: ExtensionDefinition, inputs: unknown = undefined) {
-  const int = toInternalExtensionDefinition(ext);
+  const int = OpaqueExtensionDefinition.toInternal(ext);
   if (int.version !== 'v2') {
     throw new Error('Expected v2 extension');
   }
@@ -680,7 +680,7 @@ describe('createExtensionBlueprint', () => {
       },
     });
 
-    const ext = toInternalExtensionDefinition(
+    const ext = OpaqueExtensionDefinition.toInternal(
       blueprint.makeWithOverrides({
         output: [testDataRef2],
         factory(origFactory) {
diff --git a/packages/frontend-plugin-api/src/wiring/createFrontendModule.ts b/packages/frontend-plugin-api/src/wiring/createFrontendModule.ts
index e3fa8b6682..bece1539fa 100644
--- a/packages/frontend-plugin-api/src/wiring/createFrontendModule.ts
+++ b/packages/frontend-plugin-api/src/wiring/createFrontendModule.ts
@@ -14,10 +14,7 @@
  * limitations under the License.
  */
 
-import {
-  InternalExtensionDefinition,
-  toInternalExtensionDefinition,
-} from '@internal/frontend';
+import { OpaqueExtensionDefinition } from '@internal/frontend';
 import { ExtensionDefinition } from './createExtension';
 import {
   Extension,
@@ -58,11 +55,11 @@ export function createFrontendModule<
   const extensions = new Array<Extension<any>>();
   const extensionDefinitionsById = new Map<
     string,
-    InternalExtensionDefinition
+    typeof OpaqueExtensionDefinition.TInternal
   >();
 
   for (const def of options.extensions ?? []) {
-    const internal = toInternalExtensionDefinition(def);
+    const internal = OpaqueExtensionDefinition.toInternal(def);
     const ext = resolveExtensionDefinition(def, { namespace: pluginId });
     extensions.push(ext);
     extensionDefinitionsById.set(ext.id, {
diff --git a/packages/frontend-plugin-api/src/wiring/createFrontendPlugin.ts b/packages/frontend-plugin-api/src/wiring/createFrontendPlugin.ts
index adcd2b0867..865eae6444 100644
--- a/packages/frontend-plugin-api/src/wiring/createFrontendPlugin.ts
+++ b/packages/frontend-plugin-api/src/wiring/createFrontendPlugin.ts
@@ -14,10 +14,7 @@
  * limitations under the License.
  */
 
-import {
-  InternalExtensionDefinition,
-  toInternalExtensionDefinition,
-} from '@internal/frontend';
+import { OpaqueExtensionDefinition } from '@internal/frontend';
 import { ExtensionDefinition } from './createExtension';
 import {
   Extension,
@@ -96,11 +93,11 @@ export function createFrontendPlugin<
   const extensions = new Array<Extension<any>>();
   const extensionDefinitionsById = new Map<
     string,
-    InternalExtensionDefinition
+    typeof OpaqueExtensionDefinition.TInternal
   >();
 
   for (const def of options.extensions ?? []) {
-    const internal = toInternalExtensionDefinition(def);
+    const internal = OpaqueExtensionDefinition.toInternal(def);
     const ext = resolveExtensionDefinition(def, { namespace: options.id });
     extensions.push(ext);
     extensionDefinitionsById.set(ext.id, {
diff --git a/packages/frontend-plugin-api/src/wiring/resolveExtensionDefinition.ts b/packages/frontend-plugin-api/src/wiring/resolveExtensionDefinition.ts
index a377cde2b9..53de20bff2 100644
--- a/packages/frontend-plugin-api/src/wiring/resolveExtensionDefinition.ts
+++ b/packages/frontend-plugin-api/src/wiring/resolveExtensionDefinition.ts
@@ -26,7 +26,7 @@ import {
   AnyExtensionDataRef,
   ExtensionDataValue,
 } from './createExtensionDataRef';
-import { toInternalExtensionDefinition } from '@internal/frontend';
+import { OpaqueExtensionDefinition } from '@internal/frontend';
 
 /** @public */
 export interface Extension<TConfig, TConfigInput = TConfig> {
@@ -141,7 +141,7 @@ export function resolveExtensionDefinition<
   definition: ExtensionDefinition<T>,
   context?: { namespace?: string },
 ): Extension<T['config'], T['configInput']> {
-  const internalDefinition = toInternalExtensionDefinition(definition);
+  const internalDefinition = OpaqueExtensionDefinition.toInternal(definition);
   const {
     name,
     kind,
diff --git a/packages/frontend-test-utils/src/app/createExtensionTester.tsx b/packages/frontend-test-utils/src/app/createExtensionTester.tsx
index 5eda2d6e11..37ec1d8fd9 100644
--- a/packages/frontend-test-utils/src/app/createExtensionTester.tsx
+++ b/packages/frontend-test-utils/src/app/createExtensionTester.tsx
@@ -37,7 +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';
+import { OpaqueExtensionDefinition } from '@internal/frontend';
 
 /** @public */
 export class ExtensionQuery<UOutput extends AnyExtensionDataRef> {
@@ -105,7 +105,7 @@ export class ExtensionTester<UOutput extends AnyExtensionDataRef> {
       );
     }
 
-    const { name, namespace } = toInternalExtensionDefinition(extension);
+    const { name, namespace } = OpaqueExtensionDefinition.toInternal(extension);
 
     const definition = {
       ...extension,
@@ -143,7 +143,7 @@ export class ExtensionTester<UOutput extends AnyExtensionDataRef> {
     const tree = this.#resolveTree();
 
     // Same fallback logic as in .add
-    const { name, namespace } = toInternalExtensionDefinition(extension);
+    const { name, namespace } = OpaqueExtensionDefinition.toInternal(extension);
     const definition = {
       ...extension,
       name: !namespace && !name ? 'test' : name,