diff --git a/dev/src/index.ts b/dev/src/index.ts
index f43428878..854ef7477 100644
--- a/dev/src/index.ts
+++ b/dev/src/index.ts
@@ -527,6 +527,13 @@ export class Firestore implements firestore.Firestore {
    * to `true`, these properties are skipped and not written to Firestore. If
    * set `false` or omitted, the SDK throws an exception when it encounters
    * properties of type `undefined`.
+   * @param {boolean=} settings.preferRest Whether to force the use of HTTP/1.1 REST
+   * transport until a method that requires gRPC is called. When a method requires gRPC,
+   * this Firestore client will load dependent gRPC libraries and then use gRPC transport
+   * for communication from that point forward. Currently the only operation
+   * that requires gRPC is creating a snapshot listener with the method
+   * `DocumentReference<T>.onSnapshot()`, `CollectionReference<T>.onSnapshot()`, or
+   * `Query<T>.onSnapshot()`.
    */
   constructor(settings?: firestore.Settings) {
     const libraryHeader = {
diff --git a/dev/src/pool.ts b/dev/src/pool.ts
index 0110f2798..35578a135 100644
--- a/dev/src/pool.ts
+++ b/dev/src/pool.ts
@@ -93,6 +93,12 @@ export class ClientPool<T> {
     let selectedClient: T | null = null;
     let selectedClientRequestCount = -1;
 
+    // Transition to grpc when we see the first operation that requires grpc.
+    this.grpcEnabled = this.grpcEnabled || requiresGrpc;
+
+    // Require a grpc client for this operation if we have transitioned to grpc.
+    requiresGrpc = requiresGrpc || this.grpcEnabled;
+
     for (const [client, metadata] of this.activeClients) {
       // Use the "most-full" client that can still accommodate the request
       // in order to maximize the number of idle clients as operations start to
@@ -101,7 +107,7 @@ export class ClientPool<T> {
         !this.failedClients.has(client) &&
         metadata.activeRequestCount > selectedClientRequestCount &&
         metadata.activeRequestCount < this.concurrentOperationLimit &&
-        (!requiresGrpc || metadata.grpcEnabled)
+        (metadata.grpcEnabled || !requiresGrpc)
       ) {
         selectedClient = client;
         selectedClientRequestCount = metadata.activeRequestCount;
@@ -223,6 +229,21 @@ export class ClientPool<T> {
     return activeOperationCount;
   }
 
+  /**
+   * The currently active clients.
+   *
+   * @return The currently active clients.
+   * @private
+   * @internal
+   */
+  // Visible for testing.
+  get _activeClients(): Map<
+    T,
+    {activeRequestCount: number; grpcEnabled: boolean}
+  > {
+    return this.activeClients;
+  }
+
   /**
    * Runs the provided operation in this pool. This function may create an
    * additional client if all existing clients already operate at the concurrent
diff --git a/dev/test/pool.ts b/dev/test/pool.ts
index 3c1466f6f..f79e6b8f5 100644
--- a/dev/test/pool.ts
+++ b/dev/test/pool.ts
@@ -34,6 +34,26 @@ function deferredPromises(count: number): Array<Deferred<void>> {
   return deferred;
 }
 
+function assertOpCount<T>(
+  pool: ClientPool<T>,
+  grpcClientOpCount: number,
+  restClientOpCount: number
+): void {
+  let actualGrpcClientOpCount = 0;
+  let actualRestClientOpCount = 0;
+
+  pool._activeClients.forEach(clientConfig => {
+    if (clientConfig.grpcEnabled) {
+      actualGrpcClientOpCount += clientConfig.activeRequestCount;
+    } else {
+      actualRestClientOpCount += clientConfig.activeRequestCount;
+    }
+  });
+
+  expect(actualGrpcClientOpCount).to.equal(grpcClientOpCount);
+  expect(actualRestClientOpCount).to.equal(restClientOpCount);
+}
+
 describe('Client pool', () => {
   it('creates new instances as needed', () => {
     const clientPool = new ClientPool<{}>(3, 0, () => {
@@ -133,6 +153,7 @@ describe('Client pool', () => {
       () => operationPromises[1].promise
     );
     expect(clientPool.size).to.equal(2);
+    assertOpCount(clientPool, 1, 1);
 
     operationPromises[0].resolve();
     operationPromises[1].resolve();
@@ -156,9 +177,166 @@ describe('Client pool', () => {
       () => operationPromises[1].promise
     );
     expect(clientPool.size).to.equal(1);
+    assertOpCount(clientPool, 2, 0);
+
+    operationPromises[0].resolve();
+    operationPromises[1].resolve();
+  });
+
+  it('does not re-use rest instance after beginning the transition to grpc', async () => {
+    const clientPool = new ClientPool<{}>(10, 1, () => {
+      return {};
+    });
+
+    const operationPromises = deferredPromises(3);
+
+    void clientPool.run(
+      REQUEST_TAG,
+      USE_REST,
+      () => operationPromises[0].promise
+    );
+    void clientPool.run(
+      REQUEST_TAG,
+      USE_GRPC,
+      () => operationPromises[1].promise
+    );
+    void clientPool.run(
+      REQUEST_TAG,
+      USE_REST,
+      () => operationPromises[2].promise
+    );
+
+    expect(clientPool.size).to.equal(2);
+    assertOpCount(clientPool, 2, 1);
 
     operationPromises[0].resolve();
     operationPromises[1].resolve();
+    operationPromises[2].resolve();
+  });
+
+  it('does not re-use rest instance after beginning the transition to grpc - rest operation resolved', async () => {
+    const clientPool = new ClientPool<{}>(10, 1, () => {
+      return {};
+    });
+
+    const operationPromises = deferredPromises(3);
+
+    const restOperation = clientPool.run(
+      REQUEST_TAG,
+      USE_REST,
+      () => operationPromises[0].promise
+    );
+    void clientPool.run(
+      REQUEST_TAG,
+      USE_GRPC,
+      () => operationPromises[1].promise
+    );
+
+    // resolve rest operation
+    operationPromises[0].resolve();
+    await restOperation;
+    expect(clientPool.opCount).to.equal(1);
+
+    // Run new rest operation
+    void clientPool.run(
+      REQUEST_TAG,
+      USE_REST,
+      () => operationPromises[2].promise
+    );
+
+    // Assert client pool status
+    expect(clientPool.size).to.equal(1);
+    assertOpCount(clientPool, 2, 0);
+
+    operationPromises[1].resolve();
+    operationPromises[2].resolve();
+  });
+
+  it('does not re-use rest instance after beginning the transition to grpc - grpc client full', async () => {
+    const operationLimit = 10;
+    const clientPool = new ClientPool<{}>(operationLimit, 1, () => {
+      return {};
+    });
+
+    const restPromises = deferredPromises(operationLimit);
+    const grpcPromises = deferredPromises(1);
+
+    // First operation use GRPC
+    void clientPool.run(REQUEST_TAG, USE_GRPC, () => grpcPromises[0].promise);
+
+    // Next X operations can use rest, this will fill the first
+    // client and create a new client.
+    // The new client should use GRPC since we have transitioned.
+    restPromises.forEach(restPromise => {
+      void clientPool.run(REQUEST_TAG, USE_REST, () => restPromise.promise);
+    });
+    expect(clientPool.opCount).to.equal(11);
+    expect(clientPool.size).to.equal(2);
+    assertOpCount(clientPool, 11, 0);
+
+    grpcPromises.forEach(grpcPromise => grpcPromise.resolve());
+    restPromises.forEach(restPromise => restPromise.resolve());
+  });
+
+  it('does not re-use rest instance after beginning the transition to grpc - multiple rest clients', async () => {
+    const operationLimit = 10;
+    const clientPool = new ClientPool<{}>(operationLimit, 1, () => {
+      return {};
+    });
+
+    const restPromises = deferredPromises(15);
+    const grpcPromises = deferredPromises(5);
+
+    // First 15 operations can use rest, this will fill the first
+    // client and create a new client.
+    restPromises.forEach(restPromise => {
+      void clientPool.run(REQUEST_TAG, USE_REST, () => restPromise.promise);
+    });
+    expect(clientPool.opCount).to.equal(15);
+    expect(clientPool.size).to.equal(2);
+    assertOpCount(clientPool, 0, 15);
+
+    // Next 5 operations alternate between gRPC and REST, this will create a new client using gRPC
+    let transport = USE_GRPC;
+    grpcPromises.forEach(grpcPromise => {
+      void clientPool.run(REQUEST_TAG, transport, () => grpcPromise.promise);
+      transport = !transport;
+    });
+    expect(clientPool.opCount).to.equal(20);
+    expect(clientPool.size).to.equal(3);
+    assertOpCount(clientPool, 5, 15);
+
+    grpcPromises.forEach(grpcPromise => grpcPromise.resolve());
+    restPromises.forEach(restPromise => restPromise.resolve());
+  });
+
+  it('does not re-use rest instance after beginning the transition to grpc - grpc client RST_STREAM', async () => {
+    const clientPool = new ClientPool<{}>(10, 1, () => {
+      return {};
+    });
+
+    const operationPromises = deferredPromises(1);
+
+    const grpcOperation = clientPool.run(REQUEST_TAG, USE_GRPC, () =>
+      Promise.reject(
+        new GoogleError('13 INTERNAL: Received RST_STREAM with code 2')
+      )
+    );
+
+    await grpcOperation.catch(e => e);
+
+    // Run new rest operation
+    void clientPool.run(
+      REQUEST_TAG,
+      USE_REST,
+      () => operationPromises[0].promise
+    );
+
+    // Assert client pool status
+    expect(clientPool.size).to.equal(1);
+    assertOpCount(clientPool, 1, 0);
+
+    operationPromises[0].resolve();
   });
 
   it('bin packs operations', async () => {
diff --git a/types/firestore.d.ts b/types/firestore.d.ts
index 1e33ce92a..bf08f17e8 100644
--- a/types/firestore.d.ts
+++ b/types/firestore.d.ts
@@ -290,12 +290,12 @@ declare namespace FirebaseFirestore {
     ignoreUndefinedProperties?: boolean;
 
     /**
-     * Use HTTP for requests that can be served over HTTP and JSON. This reduces
-     * the amount of networking code that is loaded to serve requests within
-     * Firestore.
-     *
-     * This setting does not apply to `onSnapshot` APIs as they cannot be served
-     * over native HTTP.
+     * Whether to force the use of HTTP/1.1 REST transport until a method that requires gRPC
+     * is called. When a method requires gRPC, this Firestore client will load dependent gRPC
+     * libraries and then use gRPC transport for communication from that point forward.
+     * Currently the only operation that requires gRPC is creating a snapshot listener with
+     * the method `DocumentReference<T>.onSnapshot()`, `CollectionReference<T>.onSnapshot()`,
+     * or `Query<T>.onSnapshot()`.
      */
     preferRest?: boolean;