diff --git a/src/@types/partials.ts b/src/@types/partials.ts
index e4700b3c1..a729d80dc 100644
--- a/src/@types/partials.ts
+++ b/src/@types/partials.ts
@@ -82,3 +82,12 @@ export enum HistoryVisibility {
     Shared = "shared",
     WorldReadable = "world_readable",
 }
+
+export interface IUsageLimit {
+    // "hs_disabled" is NOT a specced string, but is used in Synapse
+    // This is tracked over at https://github.com/matrix-org/synapse/issues/9237
+    // eslint-disable-next-line camelcase
+    limit_type: "monthly_active_user" | "hs_disabled" | string;
+    // eslint-disable-next-line camelcase
+    admin_contact?: string;
+}
diff --git a/src/client.ts b/src/client.ts
index a54161d6d..bd3a631fe 100644
--- a/src/client.ts
+++ b/src/client.ts
@@ -97,7 +97,7 @@ import {
     IRecoveryKey,
     ISecretStorageKeyInfo,
 } from "./crypto/api";
-import { SyncState } from "./sync.api";
+import { SyncState } from "./sync";
 import { EventTimelineSet } from "./models/event-timeline-set";
 import { VerificationRequest } from "./crypto/verification/request/VerificationRequest";
 import { VerificationBase as Verification } from "./crypto/verification/Base";
diff --git a/src/http-api.ts b/src/http-api.ts
index ac98c35ae..077a254e9 100644
--- a/src/http-api.ts
+++ b/src/http-api.ts
@@ -30,7 +30,7 @@ import type { Request as _Request, CoreOptions } from "request";
 // waiting for the delay to elapse.
 import * as callbacks from "./realtime-callbacks";
 import { IUploadOpts } from "./@types/requests";
-import { IAbortablePromise } from "./@types/partials";
+import { IAbortablePromise, IUsageLimit } from "./@types/partials";
 import { IDeferred } from "./utils";
 import { Callback } from "./client";
 import * as utils from "./utils";
@@ -1015,7 +1015,7 @@ function getResponseContentType(response: XMLHttpRequest | IncomingMessage): Par
     }
 }
 
-interface IErrorJson {
+interface IErrorJson extends Partial<IUsageLimit> {
     [key: string]: any; // extensible
     errcode?: string;
     error?: string;
diff --git a/src/sync.api.ts b/src/sync.api.ts
deleted file mode 100644
index 384e027f6..000000000
--- a/src/sync.api.ts
+++ /dev/null
@@ -1,26 +0,0 @@
-/*
-Copyright 2021 The Matrix.org Foundation C.I.C.
-
-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: Merge this with sync.js once converted
-
-export enum SyncState {
-    Error = "ERROR",
-    Prepared = "PREPARED",
-    Stopped = "STOPPED",
-    Syncing = "SYNCING",
-    Catchup = "CATCHUP",
-    Reconnecting = "RECONNECTING",
-}
diff --git a/src/sync.ts b/src/sync.ts
index f870284f6..88d12f9ef 100644
--- a/src/sync.ts
+++ b/src/sync.ts
@@ -34,7 +34,6 @@ import { PushProcessor } from "./pushprocessor";
 import { logger } from './logger';
 import { InvalidStoreError } from './errors';
 import { IStoredClientOpts, MatrixClient, PendingEventOrdering } from "./client";
-import { SyncState } from "./sync.api";
 import {
     Category,
     IEphemeral,
@@ -68,6 +67,15 @@ const BUFFER_PERIOD_MS = 80 * 1000;
 // keepAlive is successful but the server /sync fails.
 const FAILED_SYNC_ERROR_THRESHOLD = 3;
 
+export enum SyncState {
+    Error = "ERROR",
+    Prepared = "PREPARED",
+    Stopped = "STOPPED",
+    Syncing = "SYNCING",
+    Catchup = "CATCHUP",
+    Reconnecting = "RECONNECTING",
+}
+
 function getFilterName(userId: string, suffix?: string): string {
     // scope this on the user ID because people may login on many accounts
     // and they all need to be stored!
@@ -87,7 +95,7 @@ interface ISyncOptions {
 }
 
 export interface ISyncStateData {
-    error?: Error;
+    error?: MatrixError;
     oldSyncToken?: string;
     nextSyncToken?: string;
     catchingUp?: boolean;
@@ -477,7 +485,7 @@ export class SyncApi {
         return this.syncStateData;
     }
 
-    public async recoverFromSyncStartupError(savedSyncPromise: Promise<void>, err: Error): Promise<void> {
+    public async recoverFromSyncStartupError(savedSyncPromise: Promise<void>, err: MatrixError): Promise<void> {
         // Wait for the saved sync to complete - we send the pushrules and filter requests
         // before the saved sync has finished so they can run in parallel, but only process
         // the results after the saved sync is done. Equivalently, we wait for it to finish