diff --git a/src/v1/applications_client.ts b/src/v1/applications_client.ts
index 1a39606..e133a5c 100644
--- a/src/v1/applications_client.ts
+++ b/src/v1/applications_client.ts
@@ -23,6 +23,7 @@ import {
   CallOptions,
   Descriptors,
   ClientOptions,
+  GrpcClientOptions,
   LROperation,
 } from 'google-gax';
 
@@ -68,7 +69,7 @@ export class ApplicationsClient {
    *
    * @param {object} [options] - The configuration object.
    * The options accepted by the constructor are described in detail
-   * in [this document](https://github.com/googleapis/gax-nodejs/blob/master/client-libraries.md#creating-the-client-instance).
+   * in [this document](https://github.com/googleapis/gax-nodejs/blob/main/client-libraries.md#creating-the-client-instance).
    * The common options are:
    * @param {object} [options.credentials] - Credentials object.
    * @param {string} [options.credentials.client_email]
@@ -91,11 +92,10 @@ export class ApplicationsClient {
    *     API remote host.
    * @param {gax.ClientConfig} [options.clientConfig] - Client configuration override.
    *     Follows the structure of {@link gapicConfig}.
-   * @param {boolean} [options.fallback] - Use HTTP fallback mode.
-   *     In fallback mode, a special browser-compatible transport implementation is used
-   *     instead of gRPC transport. In browser context (if the `window` object is defined)
-   *     the fallback mode is enabled automatically; set `options.fallback` to `false`
-   *     if you need to override this behavior.
+   * @param {boolean | "rest"} [options.fallback] - Use HTTP fallback mode.
+   *     Pass "rest" to use HTTP/1.1 REST API instead of gRPC.
+   *     For more information, please check the
+   *     {@link https://github.com/googleapis/gax-nodejs/blob/main/client-libraries.md#http11-rest-api-mode documentation}.
    */
   constructor(opts?: ClientOptions) {
     // Ensure that options include all the required fields.
@@ -168,16 +168,36 @@ export class ApplicationsClient {
     };
 
     const protoFilesRoot = this._gaxModule.protobuf.Root.fromJSON(jsonProtos);
-
     // This API contains "long-running operations", which return a
     // an Operation object that allows for tracking of the operation,
     // rather than holding a request open.
-
+    const lroOptions: GrpcClientOptions = {
+      auth: this.auth,
+      grpc: 'grpc' in this._gaxGrpc ? this._gaxGrpc.grpc : undefined,
+    };
+    if (opts.fallback === 'rest') {
+      lroOptions.protoJson = protoFilesRoot;
+      lroOptions.httpRules = [
+        {
+          selector: 'google.cloud.location.Locations.GetLocation',
+          get: '/v1/{name=apps/*/locations/*}',
+        },
+        {
+          selector: 'google.cloud.location.Locations.ListLocations',
+          get: '/v1/{name=apps/*}/locations',
+        },
+        {
+          selector: 'google.longrunning.Operations.GetOperation',
+          get: '/v1/{name=apps/*/operations/*}',
+        },
+        {
+          selector: 'google.longrunning.Operations.ListOperations',
+          get: '/v1/{name=apps/*}/operations',
+        },
+      ];
+    }
     this.operationsClient = this._gaxModule
-      .lro({
-        auth: this.auth,
-        grpc: 'grpc' in this._gaxGrpc ? this._gaxGrpc.grpc : undefined,
-      })
+      .lro(lroOptions)
       .operationsClient(opts);
     const createApplicationResponse = protoFilesRoot.lookup(
       '.google.appengine.v1.Application'
diff --git a/src/v1/authorized_certificates_client.ts b/src/v1/authorized_certificates_client.ts
index d392974..4ce007f 100644
--- a/src/v1/authorized_certificates_client.ts
+++ b/src/v1/authorized_certificates_client.ts
@@ -71,7 +71,7 @@ export class AuthorizedCertificatesClient {
    *
    * @param {object} [options] - The configuration object.
    * The options accepted by the constructor are described in detail
-   * in [this document](https://github.com/googleapis/gax-nodejs/blob/master/client-libraries.md#creating-the-client-instance).
+   * in [this document](https://github.com/googleapis/gax-nodejs/blob/main/client-libraries.md#creating-the-client-instance).
    * The common options are:
    * @param {object} [options.credentials] - Credentials object.
    * @param {string} [options.credentials.client_email]
@@ -94,11 +94,10 @@ export class AuthorizedCertificatesClient {
    *     API remote host.
    * @param {gax.ClientConfig} [options.clientConfig] - Client configuration override.
    *     Follows the structure of {@link gapicConfig}.
-   * @param {boolean} [options.fallback] - Use HTTP fallback mode.
-   *     In fallback mode, a special browser-compatible transport implementation is used
-   *     instead of gRPC transport. In browser context (if the `window` object is defined)
-   *     the fallback mode is enabled automatically; set `options.fallback` to `false`
-   *     if you need to override this behavior.
+   * @param {boolean | "rest"} [options.fallback] - Use HTTP fallback mode.
+   *     Pass "rest" to use HTTP/1.1 REST API instead of gRPC.
+   *     For more information, please check the
+   *     {@link https://github.com/googleapis/gax-nodejs/blob/main/client-libraries.md#http11-rest-api-mode documentation}.
    */
   constructor(opts?: ClientOptions) {
     // Ensure that options include all the required fields.
diff --git a/src/v1/authorized_domains_client.ts b/src/v1/authorized_domains_client.ts
index 6df3cd0..4eea17a 100644
--- a/src/v1/authorized_domains_client.ts
+++ b/src/v1/authorized_domains_client.ts
@@ -72,7 +72,7 @@ export class AuthorizedDomainsClient {
    *
    * @param {object} [options] - The configuration object.
    * The options accepted by the constructor are described in detail
-   * in [this document](https://github.com/googleapis/gax-nodejs/blob/master/client-libraries.md#creating-the-client-instance).
+   * in [this document](https://github.com/googleapis/gax-nodejs/blob/main/client-libraries.md#creating-the-client-instance).
    * The common options are:
    * @param {object} [options.credentials] - Credentials object.
    * @param {string} [options.credentials.client_email]
@@ -95,11 +95,10 @@ export class AuthorizedDomainsClient {
    *     API remote host.
    * @param {gax.ClientConfig} [options.clientConfig] - Client configuration override.
    *     Follows the structure of {@link gapicConfig}.
-   * @param {boolean} [options.fallback] - Use HTTP fallback mode.
-   *     In fallback mode, a special browser-compatible transport implementation is used
-   *     instead of gRPC transport. In browser context (if the `window` object is defined)
-   *     the fallback mode is enabled automatically; set `options.fallback` to `false`
-   *     if you need to override this behavior.
+   * @param {boolean | "rest"} [options.fallback] - Use HTTP fallback mode.
+   *     Pass "rest" to use HTTP/1.1 REST API instead of gRPC.
+   *     For more information, please check the
+   *     {@link https://github.com/googleapis/gax-nodejs/blob/main/client-libraries.md#http11-rest-api-mode documentation}.
    */
   constructor(opts?: ClientOptions) {
     // Ensure that options include all the required fields.
diff --git a/src/v1/domain_mappings_client.ts b/src/v1/domain_mappings_client.ts
index e420ec8..79c1b6b 100644
--- a/src/v1/domain_mappings_client.ts
+++ b/src/v1/domain_mappings_client.ts
@@ -23,6 +23,7 @@ import {
   CallOptions,
   Descriptors,
   ClientOptions,
+  GrpcClientOptions,
   LROperation,
   PaginationCallback,
   GaxCall,
@@ -72,7 +73,7 @@ export class DomainMappingsClient {
    *
    * @param {object} [options] - The configuration object.
    * The options accepted by the constructor are described in detail
-   * in [this document](https://github.com/googleapis/gax-nodejs/blob/master/client-libraries.md#creating-the-client-instance).
+   * in [this document](https://github.com/googleapis/gax-nodejs/blob/main/client-libraries.md#creating-the-client-instance).
    * The common options are:
    * @param {object} [options.credentials] - Credentials object.
    * @param {string} [options.credentials.client_email]
@@ -95,11 +96,10 @@ export class DomainMappingsClient {
    *     API remote host.
    * @param {gax.ClientConfig} [options.clientConfig] - Client configuration override.
    *     Follows the structure of {@link gapicConfig}.
-   * @param {boolean} [options.fallback] - Use HTTP fallback mode.
-   *     In fallback mode, a special browser-compatible transport implementation is used
-   *     instead of gRPC transport. In browser context (if the `window` object is defined)
-   *     the fallback mode is enabled automatically; set `options.fallback` to `false`
-   *     if you need to override this behavior.
+   * @param {boolean | "rest"} [options.fallback] - Use HTTP fallback mode.
+   *     Pass "rest" to use HTTP/1.1 REST API instead of gRPC.
+   *     For more information, please check the
+   *     {@link https://github.com/googleapis/gax-nodejs/blob/main/client-libraries.md#http11-rest-api-mode documentation}.
    */
   constructor(opts?: ClientOptions) {
     // Ensure that options include all the required fields.
@@ -183,16 +183,36 @@ export class DomainMappingsClient {
     };
 
     const protoFilesRoot = this._gaxModule.protobuf.Root.fromJSON(jsonProtos);
-
     // This API contains "long-running operations", which return a
     // an Operation object that allows for tracking of the operation,
     // rather than holding a request open.
-
+    const lroOptions: GrpcClientOptions = {
+      auth: this.auth,
+      grpc: 'grpc' in this._gaxGrpc ? this._gaxGrpc.grpc : undefined,
+    };
+    if (opts.fallback === 'rest') {
+      lroOptions.protoJson = protoFilesRoot;
+      lroOptions.httpRules = [
+        {
+          selector: 'google.cloud.location.Locations.GetLocation',
+          get: '/v1/{name=apps/*/locations/*}',
+        },
+        {
+          selector: 'google.cloud.location.Locations.ListLocations',
+          get: '/v1/{name=apps/*}/locations',
+        },
+        {
+          selector: 'google.longrunning.Operations.GetOperation',
+          get: '/v1/{name=apps/*/operations/*}',
+        },
+        {
+          selector: 'google.longrunning.Operations.ListOperations',
+          get: '/v1/{name=apps/*}/operations',
+        },
+      ];
+    }
     this.operationsClient = this._gaxModule
-      .lro({
-        auth: this.auth,
-        grpc: 'grpc' in this._gaxGrpc ? this._gaxGrpc.grpc : undefined,
-      })
+      .lro(lroOptions)
       .operationsClient(opts);
     const createDomainMappingResponse = protoFilesRoot.lookup(
       '.google.appengine.v1.DomainMapping'
diff --git a/src/v1/firewall_client.ts b/src/v1/firewall_client.ts
index f07b8a7..fee78bb 100644
--- a/src/v1/firewall_client.ts
+++ b/src/v1/firewall_client.ts
@@ -79,7 +79,7 @@ export class FirewallClient {
    *
    * @param {object} [options] - The configuration object.
    * The options accepted by the constructor are described in detail
-   * in [this document](https://github.com/googleapis/gax-nodejs/blob/master/client-libraries.md#creating-the-client-instance).
+   * in [this document](https://github.com/googleapis/gax-nodejs/blob/main/client-libraries.md#creating-the-client-instance).
    * The common options are:
    * @param {object} [options.credentials] - Credentials object.
    * @param {string} [options.credentials.client_email]
@@ -102,11 +102,10 @@ export class FirewallClient {
    *     API remote host.
    * @param {gax.ClientConfig} [options.clientConfig] - Client configuration override.
    *     Follows the structure of {@link gapicConfig}.
-   * @param {boolean} [options.fallback] - Use HTTP fallback mode.
-   *     In fallback mode, a special browser-compatible transport implementation is used
-   *     instead of gRPC transport. In browser context (if the `window` object is defined)
-   *     the fallback mode is enabled automatically; set `options.fallback` to `false`
-   *     if you need to override this behavior.
+   * @param {boolean | "rest"} [options.fallback] - Use HTTP fallback mode.
+   *     Pass "rest" to use HTTP/1.1 REST API instead of gRPC.
+   *     For more information, please check the
+   *     {@link https://github.com/googleapis/gax-nodejs/blob/main/client-libraries.md#http11-rest-api-mode documentation}.
    */
   constructor(opts?: ClientOptions) {
     // Ensure that options include all the required fields.
diff --git a/src/v1/instances_client.ts b/src/v1/instances_client.ts
index c7d0863..9cb72c6 100644
--- a/src/v1/instances_client.ts
+++ b/src/v1/instances_client.ts
@@ -23,6 +23,7 @@ import {
   CallOptions,
   Descriptors,
   ClientOptions,
+  GrpcClientOptions,
   LROperation,
   PaginationCallback,
   GaxCall,
@@ -72,7 +73,7 @@ export class InstancesClient {
    *
    * @param {object} [options] - The configuration object.
    * The options accepted by the constructor are described in detail
-   * in [this document](https://github.com/googleapis/gax-nodejs/blob/master/client-libraries.md#creating-the-client-instance).
+   * in [this document](https://github.com/googleapis/gax-nodejs/blob/main/client-libraries.md#creating-the-client-instance).
    * The common options are:
    * @param {object} [options.credentials] - Credentials object.
    * @param {string} [options.credentials.client_email]
@@ -95,11 +96,10 @@ export class InstancesClient {
    *     API remote host.
    * @param {gax.ClientConfig} [options.clientConfig] - Client configuration override.
    *     Follows the structure of {@link gapicConfig}.
-   * @param {boolean} [options.fallback] - Use HTTP fallback mode.
-   *     In fallback mode, a special browser-compatible transport implementation is used
-   *     instead of gRPC transport. In browser context (if the `window` object is defined)
-   *     the fallback mode is enabled automatically; set `options.fallback` to `false`
-   *     if you need to override this behavior.
+   * @param {boolean | "rest"} [options.fallback] - Use HTTP fallback mode.
+   *     Pass "rest" to use HTTP/1.1 REST API instead of gRPC.
+   *     For more information, please check the
+   *     {@link https://github.com/googleapis/gax-nodejs/blob/main/client-libraries.md#http11-rest-api-mode documentation}.
    */
   constructor(opts?: ClientOptions) {
     // Ensure that options include all the required fields.
@@ -183,16 +183,36 @@ export class InstancesClient {
     };
 
     const protoFilesRoot = this._gaxModule.protobuf.Root.fromJSON(jsonProtos);
-
     // This API contains "long-running operations", which return a
     // an Operation object that allows for tracking of the operation,
     // rather than holding a request open.
-
+    const lroOptions: GrpcClientOptions = {
+      auth: this.auth,
+      grpc: 'grpc' in this._gaxGrpc ? this._gaxGrpc.grpc : undefined,
+    };
+    if (opts.fallback === 'rest') {
+      lroOptions.protoJson = protoFilesRoot;
+      lroOptions.httpRules = [
+        {
+          selector: 'google.cloud.location.Locations.GetLocation',
+          get: '/v1/{name=apps/*/locations/*}',
+        },
+        {
+          selector: 'google.cloud.location.Locations.ListLocations',
+          get: '/v1/{name=apps/*}/locations',
+        },
+        {
+          selector: 'google.longrunning.Operations.GetOperation',
+          get: '/v1/{name=apps/*/operations/*}',
+        },
+        {
+          selector: 'google.longrunning.Operations.ListOperations',
+          get: '/v1/{name=apps/*}/operations',
+        },
+      ];
+    }
     this.operationsClient = this._gaxModule
-      .lro({
-        auth: this.auth,
-        grpc: 'grpc' in this._gaxGrpc ? this._gaxGrpc.grpc : undefined,
-      })
+      .lro(lroOptions)
       .operationsClient(opts);
     const deleteInstanceResponse = protoFilesRoot.lookup(
       '.google.protobuf.Empty'
diff --git a/src/v1/services_client.ts b/src/v1/services_client.ts
index 3b7f8ea..991e9f3 100644
--- a/src/v1/services_client.ts
+++ b/src/v1/services_client.ts
@@ -23,6 +23,7 @@ import {
   CallOptions,
   Descriptors,
   ClientOptions,
+  GrpcClientOptions,
   LROperation,
   PaginationCallback,
   GaxCall,
@@ -72,7 +73,7 @@ export class ServicesClient {
    *
    * @param {object} [options] - The configuration object.
    * The options accepted by the constructor are described in detail
-   * in [this document](https://github.com/googleapis/gax-nodejs/blob/master/client-libraries.md#creating-the-client-instance).
+   * in [this document](https://github.com/googleapis/gax-nodejs/blob/main/client-libraries.md#creating-the-client-instance).
    * The common options are:
    * @param {object} [options.credentials] - Credentials object.
    * @param {string} [options.credentials.client_email]
@@ -95,11 +96,10 @@ export class ServicesClient {
    *     API remote host.
    * @param {gax.ClientConfig} [options.clientConfig] - Client configuration override.
    *     Follows the structure of {@link gapicConfig}.
-   * @param {boolean} [options.fallback] - Use HTTP fallback mode.
-   *     In fallback mode, a special browser-compatible transport implementation is used
-   *     instead of gRPC transport. In browser context (if the `window` object is defined)
-   *     the fallback mode is enabled automatically; set `options.fallback` to `false`
-   *     if you need to override this behavior.
+   * @param {boolean | "rest"} [options.fallback] - Use HTTP fallback mode.
+   *     Pass "rest" to use HTTP/1.1 REST API instead of gRPC.
+   *     For more information, please check the
+   *     {@link https://github.com/googleapis/gax-nodejs/blob/main/client-libraries.md#http11-rest-api-mode documentation}.
    */
   constructor(opts?: ClientOptions) {
     // Ensure that options include all the required fields.
@@ -183,16 +183,36 @@ export class ServicesClient {
     };
 
     const protoFilesRoot = this._gaxModule.protobuf.Root.fromJSON(jsonProtos);
-
     // This API contains "long-running operations", which return a
     // an Operation object that allows for tracking of the operation,
     // rather than holding a request open.
-
+    const lroOptions: GrpcClientOptions = {
+      auth: this.auth,
+      grpc: 'grpc' in this._gaxGrpc ? this._gaxGrpc.grpc : undefined,
+    };
+    if (opts.fallback === 'rest') {
+      lroOptions.protoJson = protoFilesRoot;
+      lroOptions.httpRules = [
+        {
+          selector: 'google.cloud.location.Locations.GetLocation',
+          get: '/v1/{name=apps/*/locations/*}',
+        },
+        {
+          selector: 'google.cloud.location.Locations.ListLocations',
+          get: '/v1/{name=apps/*}/locations',
+        },
+        {
+          selector: 'google.longrunning.Operations.GetOperation',
+          get: '/v1/{name=apps/*/operations/*}',
+        },
+        {
+          selector: 'google.longrunning.Operations.ListOperations',
+          get: '/v1/{name=apps/*}/operations',
+        },
+      ];
+    }
     this.operationsClient = this._gaxModule
-      .lro({
-        auth: this.auth,
-        grpc: 'grpc' in this._gaxGrpc ? this._gaxGrpc.grpc : undefined,
-      })
+      .lro(lroOptions)
       .operationsClient(opts);
     const updateServiceResponse = protoFilesRoot.lookup(
       '.google.appengine.v1.Service'
diff --git a/src/v1/versions_client.ts b/src/v1/versions_client.ts
index 9b08355..5e0b54e 100644
--- a/src/v1/versions_client.ts
+++ b/src/v1/versions_client.ts
@@ -23,6 +23,7 @@ import {
   CallOptions,
   Descriptors,
   ClientOptions,
+  GrpcClientOptions,
   LROperation,
   PaginationCallback,
   GaxCall,
@@ -72,7 +73,7 @@ export class VersionsClient {
    *
    * @param {object} [options] - The configuration object.
    * The options accepted by the constructor are described in detail
-   * in [this document](https://github.com/googleapis/gax-nodejs/blob/master/client-libraries.md#creating-the-client-instance).
+   * in [this document](https://github.com/googleapis/gax-nodejs/blob/main/client-libraries.md#creating-the-client-instance).
    * The common options are:
    * @param {object} [options.credentials] - Credentials object.
    * @param {string} [options.credentials.client_email]
@@ -95,11 +96,10 @@ export class VersionsClient {
    *     API remote host.
    * @param {gax.ClientConfig} [options.clientConfig] - Client configuration override.
    *     Follows the structure of {@link gapicConfig}.
-   * @param {boolean} [options.fallback] - Use HTTP fallback mode.
-   *     In fallback mode, a special browser-compatible transport implementation is used
-   *     instead of gRPC transport. In browser context (if the `window` object is defined)
-   *     the fallback mode is enabled automatically; set `options.fallback` to `false`
-   *     if you need to override this behavior.
+   * @param {boolean | "rest"} [options.fallback] - Use HTTP fallback mode.
+   *     Pass "rest" to use HTTP/1.1 REST API instead of gRPC.
+   *     For more information, please check the
+   *     {@link https://github.com/googleapis/gax-nodejs/blob/main/client-libraries.md#http11-rest-api-mode documentation}.
    */
   constructor(opts?: ClientOptions) {
     // Ensure that options include all the required fields.
@@ -183,16 +183,36 @@ export class VersionsClient {
     };
 
     const protoFilesRoot = this._gaxModule.protobuf.Root.fromJSON(jsonProtos);
-
     // This API contains "long-running operations", which return a
     // an Operation object that allows for tracking of the operation,
     // rather than holding a request open.
-
+    const lroOptions: GrpcClientOptions = {
+      auth: this.auth,
+      grpc: 'grpc' in this._gaxGrpc ? this._gaxGrpc.grpc : undefined,
+    };
+    if (opts.fallback === 'rest') {
+      lroOptions.protoJson = protoFilesRoot;
+      lroOptions.httpRules = [
+        {
+          selector: 'google.cloud.location.Locations.GetLocation',
+          get: '/v1/{name=apps/*/locations/*}',
+        },
+        {
+          selector: 'google.cloud.location.Locations.ListLocations',
+          get: '/v1/{name=apps/*}/locations',
+        },
+        {
+          selector: 'google.longrunning.Operations.GetOperation',
+          get: '/v1/{name=apps/*/operations/*}',
+        },
+        {
+          selector: 'google.longrunning.Operations.ListOperations',
+          get: '/v1/{name=apps/*}/operations',
+        },
+      ];
+    }
     this.operationsClient = this._gaxModule
-      .lro({
-        auth: this.auth,
-        grpc: 'grpc' in this._gaxGrpc ? this._gaxGrpc.grpc : undefined,
-      })
+      .lro(lroOptions)
       .operationsClient(opts);
     const createVersionResponse = protoFilesRoot.lookup(
       '.google.appengine.v1.Version'