Add basic skeleton of log fowarding.

src/WidgetApi.ts

@@ -95,6 +95,7 @@ import {
     IUpdateDelayedEventFromWidgetResponseData,
     UpdateDelayedEventAction,
 } from "./interfaces/UpdateDelayedEventAction";
+import { IForwardLogLineFromWidgetRequestData } from "./interfaces/ForwardLogLineAction";
 
 export class WidgetApiResponseError extends Error {
     static {
@@ -834,6 +835,22 @@ export class WidgetApi extends EventEmitter {
         );
     }
 
+    /**
+     * Forward a log line to the parent window.
+     * Note: This should only be called if the client has indicated they would like logging, 
+     * as per `org.matrix.mscXXXX.log_forwarding` in the URL params.
+     * 
+     * @param level A string log level.
+     * @param parts Parts of the log line, may be a string or object based.
+     * @returns When the line has been acknowledged.
+     */
+    public async forwardLogLine(level: string, ...parts: unknown[]): Promise<IWidgetApiAcknowledgeResponseData> {
+        return this.transport.send<IForwardLogLineFromWidgetRequestData>(
+            WidgetApiFromWidgetAction.MSCXXXXForwardLogEvent,
+            { level, parts },
+        );
+    }
+
     /**
      * Starts the communication channel. This should be done early to ensure
      * that messages are not missed. Communication can only be stopped by the client.

src/interfaces/ForwardLogLineAction.ts

@@ -0,0 +1,28 @@
+/*
+ * Copyright 2026 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.
+ */
+
+import { IWidgetApiRequest, IWidgetApiRequestData } from "./IWidgetApiRequest";
+import { WidgetApiFromWidgetAction } from "./WidgetApiAction";
+
+export interface IForwardLogLineFromWidgetRequestData extends IWidgetApiRequestData {
+    level: string;
+    parts: unknown[];
+}
+
+export interface IDownloadFileActionFromWidgetActionRequest extends IWidgetApiRequest {
+    action: WidgetApiFromWidgetAction.MSCXXXXForwardLogEvent;
+    data: IForwardLogLineFromWidgetRequestData;
+}

src/interfaces/WidgetApiAction.ts

@@ -92,6 +92,11 @@ export enum WidgetApiFromWidgetAction {
      * @experimental It is not recommended to rely on this existing - it can be removed without notice.
      */
     MSC4157UpdateDelayedEvent = "org.matrix.msc4157.update_delayed_event",
+
+    /**
+     * @experimental It is not recommended to rely on this existing - it can be removed without notice.
+     */
+    MSCXXXXForwardLogEvent = "org.matrix.mscXXXX.forwarded_log_line"
 }
 
 export type WidgetApiAction = WidgetApiToWidgetAction | WidgetApiFromWidgetAction | string;

src/templating/url-template.ts

@@ -26,6 +26,7 @@ export interface ITemplateParams {
     clientLanguage?: string;
     deviceId?: string;
     baseUrl?: string;
+    logging?: boolean;
 }
 
 export function runTemplate(url: string, widget: IWidget, params: ITemplateParams): string {
@@ -47,6 +48,9 @@ export function runTemplate(url: string, widget: IWidget, params: ITemplateParam
 
         // TODO: Convert to stable (https://github.com/matrix-org/matrix-spec-proposals/pull/4039)
         "org.matrix.msc4039.matrix_base_url": params.baseUrl || "",
+
+        // TODO: Convert to stable (https://github.com/matrix-org/matrix-spec-proposals/pull/4039)
+        "org.matrix.mscXXXX.log_forwarding": params.logging
     });
     let result = url;
     for (const key of Object.keys(variables)) {