diff --git a/examples/lb3-application/src/__tests__/acceptance/lb3app.acceptance.ts b/examples/lb3-application/src/__tests__/acceptance/lb3app.acceptance.ts
index 447d7353326f..784629bfad08 100644
--- a/examples/lb3-application/src/__tests__/acceptance/lb3app.acceptance.ts
+++ b/examples/lb3-application/src/__tests__/acceptance/lb3app.acceptance.ts
@@ -138,7 +138,7 @@ describe('CoffeeShopApplication', () => {
     let apiSpec: OpenApiSpec;
 
     before(async () => {
-      apiSpec = app.lbApp.restServer.getApiSpec();
+      apiSpec = await app.lbApp.restServer.getApiSpec();
     });
 
     it('has the same properties in both the LB3 and LB4 specs', () => {
diff --git a/packages/booter-lb3app/src/__tests__/acceptance/booter-lb3app.acceptance.ts b/packages/booter-lb3app/src/__tests__/acceptance/booter-lb3app.acceptance.ts
index d632a1141f4f..7ad9bf303e6d 100644
--- a/packages/booter-lb3app/src/__tests__/acceptance/booter-lb3app.acceptance.ts
+++ b/packages/booter-lb3app/src/__tests__/acceptance/booter-lb3app.acceptance.ts
@@ -41,8 +41,8 @@ describe('booter-lb3app', () => {
   });
 
   context('generated OpenAPI spec', () => {
-    it('uses different request-body schema for "create" operation', () => {
-      const spec = app.restServer.getApiSpec();
+    it('uses different request-body schema for "create" operation', async () => {
+      const spec = await app.restServer.getApiSpec();
       const createOp: OperationObject = spec.paths['/api/CoffeeShops'].post;
       expect(createOp.requestBody).to.containDeep({
         content: {
@@ -66,8 +66,8 @@ describe('booter-lb3app', () => {
         });
     });
 
-    it('includes the target model as a property of the source model in a relation', () => {
-      const spec = app.restServer.getApiSpec();
+    it('includes the target model as a property of the source model in a relation', async () => {
+      const spec = await app.restServer.getApiSpec();
       const schemas = (spec.components ?? {}).schemas ?? {};
 
       expect(schemas.CoffeeShop)
@@ -118,8 +118,8 @@ describe('booter-lb3app', () => {
       }
     });
 
-    it('includes LoopBack 3 endpoints with `/api` base in OpenApiSpec', () => {
-      const apiSpec = app.restServer.getApiSpec();
+    it('includes LoopBack 3 endpoints with `/api` base in OpenApiSpec', async () => {
+      const apiSpec = await app.restServer.getApiSpec();
       const paths = Object.keys(apiSpec.paths);
       expect(paths).to.containDeep([
         '/api/CoffeeShops/{id}',
@@ -219,8 +219,8 @@ describe('booter-lb3app', () => {
       }));
     });
 
-    it('does apply the spec modification', () => {
-      const spec = app.restServer.getApiSpec();
+    it('does apply the spec modification', async () => {
+      const spec = await app.restServer.getApiSpec();
       const createOp: OperationObject = spec.paths['/api/CoffeeShops'].post;
       expect(createOp.summary).to.eql('just a very simple modification');
     });
diff --git a/packages/rest-crud/src/__tests__/acceptance/default-model-crud-rest.acceptance.ts b/packages/rest-crud/src/__tests__/acceptance/default-model-crud-rest.acceptance.ts
index 6e6caa438f63..c10b4f92bbbb 100644
--- a/packages/rest-crud/src/__tests__/acceptance/default-model-crud-rest.acceptance.ts
+++ b/packages/rest-crud/src/__tests__/acceptance/default-model-crud-rest.acceptance.ts
@@ -126,7 +126,7 @@ describe('CrudRestController for a simple Product model', () => {
     // a new test suite that will configure a PK with a different name
     // and type, e.g. `pk: string` instead of `id: number`.
     it('uses correct schema for the id parameter', async () => {
-      const spec = app.restServer.getApiSpec();
+      const spec = await app.restServer.getApiSpec();
       const findByIdOp = spec.paths['/products/{id}'].get;
       expect(findByIdOp).to.containDeep({
         parameters: [
@@ -210,7 +210,7 @@ describe('CrudRestController for a simple Product model', () => {
     // a new test suite that will configure a PK with a different name
     // and type, e.g. `pk: string` instead of `id: number`.
     it('uses correct schema for the id parameter', async () => {
-      const spec = app.restServer.getApiSpec();
+      const spec = await app.restServer.getApiSpec();
       const findByIdOp = spec.paths['/products/{id}'].patch;
       expect(findByIdOp).to.containDeep({
         parameters: [
@@ -245,7 +245,7 @@ describe('CrudRestController for a simple Product model', () => {
     // a new test suite that will configure a PK with a different name
     // and type, e.g. `pk: string` instead of `id: number`.
     it('uses correct schema for the id parameter', async () => {
-      const spec = app.restServer.getApiSpec();
+      const spec = await app.restServer.getApiSpec();
       const findByIdOp = spec.paths['/products/{id}']['patch'];
       expect(findByIdOp).to.containDeep({
         parameters: [
@@ -278,7 +278,7 @@ describe('CrudRestController for a simple Product model', () => {
     // a new test suite that will configure a PK with a different name
     // and type, e.g. `pk: string` instead of `id: number`.
     it('uses correct schema for the id parameter', async () => {
-      const spec = app.restServer.getApiSpec();
+      const spec = await app.restServer.getApiSpec();
       const findByIdOp = spec.paths['/products/{id}']['delete'];
       expect(findByIdOp).to.containDeep({
         parameters: [
diff --git a/packages/rest/src/__tests__/integration/rest.application.integration.ts b/packages/rest/src/__tests__/integration/rest.application.integration.ts
index 79d67b9a22d7..808f50afe958 100644
--- a/packages/rest/src/__tests__/integration/rest.application.integration.ts
+++ b/packages/rest/src/__tests__/integration/rest.application.integration.ts
@@ -139,7 +139,7 @@ describe('RestApplication (integration)', () => {
       'x-foo': 'bar',
     });
 
-    const spec = restApp.restServer.getApiSpec();
+    const spec = await restApp.restServer.getApiSpec();
     expect(spec).to.deepEqual({
       openapi: '3.0.0',
       info: {
@@ -231,7 +231,7 @@ describe('RestApplication (integration)', () => {
       restApp.mountExpressRouter('/dogs', router, spec);
       await client.get('/dogs/hello').expect(200, 'Hello dogs!');
 
-      const openApiSpec = restApp.restServer.getApiSpec();
+      const openApiSpec = await restApp.restServer.getApiSpec();
       expect(openApiSpec.paths).to.deepEqual({
         '/dogs/hello': {
           get: {
diff --git a/packages/rest/src/__tests__/unit/rest.server/rest.server.open-api-spec.unit.ts b/packages/rest/src/__tests__/unit/rest.server/rest.server.open-api-spec.unit.ts
index 43dea9dd4a92..8bbbd1cd178f 100644
--- a/packages/rest/src/__tests__/unit/rest.server/rest.server.open-api-spec.unit.ts
+++ b/packages/rest/src/__tests__/unit/rest.server/rest.server.open-api-spec.unit.ts
@@ -20,10 +20,10 @@ describe('RestServer.getApiSpec()', () => {
   beforeEach(givenApplication);
 
   it('comes with a valid default spec', async () => {
-    await validateApiSpec(server.getApiSpec());
+    await validateApiSpec(await server.getApiSpec());
   });
 
-  it('honours API defined via app.api()', () => {
+  it('honours API defined via app.api()', async () => {
     server.api({
       openapi: '3.0.0',
       info: {
@@ -35,7 +35,7 @@ describe('RestServer.getApiSpec()', () => {
       'x-foo': 'bar',
     });
 
-    const spec = server.getApiSpec();
+    const spec = await server.getApiSpec();
     expect(spec).to.deepEqual({
       openapi: '3.0.0',
       info: {
@@ -72,11 +72,11 @@ describe('RestServer.getApiSpec()', () => {
     expect(binding.tagNames).containEql('route');
   });
 
-  it('returns routes registered via app.route(route)', () => {
+  it('returns routes registered via app.route(route)', async () => {
     function greet() {}
     server.route('get', '/greet', {responses: {}}, greet);
 
-    const spec = server.getApiSpec();
+    const spec = await server.getApiSpec();
     expect(spec.paths).to.eql({
       '/greet': {
         get: {
@@ -86,7 +86,7 @@ describe('RestServer.getApiSpec()', () => {
     });
   });
 
-  it('ignores routes marked as "x-visibility" via app.route(route)', () => {
+  it('ignores routes marked as "x-visibility" via app.route(route)', async () => {
     function greet() {}
     function meet() {}
     server.route(
@@ -96,7 +96,7 @@ describe('RestServer.getApiSpec()', () => {
       greet,
     );
     server.route('get', '/meet', {responses: {}, spec: {}}, meet);
-    const spec = server.getApiSpec();
+    const spec = await server.getApiSpec();
     expect(spec.paths).to.eql({
       '/meet': {
         get: {
@@ -107,7 +107,7 @@ describe('RestServer.getApiSpec()', () => {
     });
   });
 
-  it('returns routes registered via app.route(..., Controller, method)', () => {
+  it('returns routes registered via app.route(..., Controller, method)', async () => {
     class MyController {
       greet() {}
     }
@@ -121,7 +121,7 @@ describe('RestServer.getApiSpec()', () => {
       'greet',
     );
 
-    const spec = server.getApiSpec();
+    const spec = await server.getApiSpec();
     expect(spec.paths).to.eql({
       '/greet': {
         get: {
@@ -134,7 +134,7 @@ describe('RestServer.getApiSpec()', () => {
     });
   });
 
-  it('ignores routes marked as "x-visibility" via app.route(..., Controller, method)', () => {
+  it('ignores routes marked as "x-visibility" via app.route(..., Controller, method)', async () => {
     class GreetController {
       greet() {}
     }
@@ -161,7 +161,7 @@ describe('RestServer.getApiSpec()', () => {
       'meet',
     );
 
-    const spec = server.getApiSpec();
+    const spec = await server.getApiSpec();
     expect(spec.paths).to.eql({
       '/meet': {
         get: {
@@ -174,14 +174,14 @@ describe('RestServer.getApiSpec()', () => {
     });
   });
 
-  it('honors tags in the operation spec', () => {
+  it('honors tags in the operation spec', async () => {
     class MyController {
       @get('/greet', {responses: {'200': {description: ''}}, tags: ['MyTag']})
       greet() {}
     }
     app.controller(MyController);
 
-    const spec = server.getApiSpec();
+    const spec = await server.getApiSpec();
     expect(spec.paths).to.eql({
       '/greet': {
         get: {
@@ -195,7 +195,7 @@ describe('RestServer.getApiSpec()', () => {
     });
   });
 
-  it('emits all media types for request body', () => {
+  it('emits all media types for request body', async () => {
     const expectedOpSpec = anOperationSpec()
       .withRequestBody({
         description: 'Any object value.',
@@ -229,18 +229,18 @@ describe('RestServer.getApiSpec()', () => {
     }
     app.controller(MyController);
 
-    const spec = server.getApiSpec();
+    const spec = await server.getApiSpec();
     expect(spec.paths['/show-body'].post).to.containDeep(expectedOpSpec);
   });
 
-  it('returns routes registered via app.controller()', () => {
+  it('returns routes registered via app.controller()', async () => {
     class MyController {
       @get('/greet')
       greet() {}
     }
     app.controller(MyController);
 
-    const spec = server.getApiSpec();
+    const spec = await server.getApiSpec();
     expect(spec.paths).to.eql({
       '/greet': {
         get: {
@@ -256,7 +256,7 @@ describe('RestServer.getApiSpec()', () => {
     });
   });
 
-  it('returns definitions inferred via app.controller()', () => {
+  it('returns definitions inferred via app.controller()', async () => {
     @model()
     class MyModel {
       @property()
@@ -268,7 +268,7 @@ describe('RestServer.getApiSpec()', () => {
     }
     app.controller(MyController);
 
-    const spec = server.getApiSpec();
+    const spec = await server.getApiSpec();
     expect(spec.components && spec.components.schemas).to.deepEqual({
       MyModel: {
         title: 'MyModel',
@@ -282,7 +282,7 @@ describe('RestServer.getApiSpec()', () => {
     });
   });
 
-  it('preserves routes specified in app.api()', () => {
+  it('preserves routes specified in app.api()', async () => {
     function status() {}
     server.api(
       anOpenApiSpec()
@@ -296,7 +296,7 @@ describe('RestServer.getApiSpec()', () => {
     function greet() {}
     server.route('get', '/greet', {responses: {}}, greet);
 
-    const spec = server.getApiSpec();
+    const spec = await server.getApiSpec();
     expect(spec.paths).to.eql({
       '/greet': {
         get: {
diff --git a/packages/rest/src/rest.server.ts b/packages/rest/src/rest.server.ts
index 24f5dde5d5a4..bb6e2387aedc 100644
--- a/packages/rest/src/rest.server.ts
+++ b/packages/rest/src/rest.server.ts
@@ -458,7 +458,7 @@ export class RestServer extends Context implements Server, HttpServerLike {
     );
 
     specForm = specForm ?? {version: '3.0.0', format: 'json'};
-    const specObj = this.getApiSpec(requestContext);
+    const specObj = await this.getApiSpec(requestContext);
 
     if (specForm.format === 'json') {
       const spec = JSON.stringify(specObj, null, 2);
@@ -736,8 +736,8 @@ export class RestServer extends Context implements Server, HttpServerLike {
    * @param requestContext - Optional context to update the `servers` list
    * in the returned spec
    */
-  getApiSpec(requestContext?: RequestContext): OpenApiSpec {
-    let spec = this.getSync<OpenApiSpec>(RestBindings.API_SPEC);
+  async getApiSpec(requestContext?: RequestContext): Promise<OpenApiSpec> {
+    let spec = await this.get<OpenApiSpec>(RestBindings.API_SPEC);
     const defs = this.httpHandler.getApiDefinitions();
 
     // Apply deep clone to prevent getApiSpec() callers from