feat(coding-agent): allow message_end replacements

closes #3982

packages/coding-agent/src/core/extensions/runner.ts

@@ -34,6 +34,8 @@ import type {
 	InputEvent,
 	InputEventResult,
 	InputSource,
+	MessageEndEvent,
+	MessageEndEventResult,
 	MessageRenderer,
 	ProviderConfig,
 	RegisteredCommand,
@@ -118,6 +120,7 @@ type RunnerEmitEvent = Exclude<
 	| ContextEvent
 	| BeforeProviderRequestEvent
 	| BeforeAgentStartEvent
+	| MessageEndEvent
 	| ResourcesDiscoverEvent
 	| InputEvent
 >;
@@ -708,6 +711,48 @@ export class ExtensionRunner {
 		return result as RunnerEmitResult<TEvent>;
 	}
 
+	async emitMessageEnd(event: MessageEndEvent): Promise<AgentMessage | undefined> {
+		const ctx = this.createContext();
+		let currentMessage = event.message;
+		let modified = false;
+
+		for (const ext of this.extensions) {
+			const handlers = ext.handlers.get("message_end");
+			if (!handlers || handlers.length === 0) continue;
+
+			for (const handler of handlers) {
+				try {
+					const currentEvent: MessageEndEvent = { ...event, message: currentMessage };
+					const handlerResult = (await handler(currentEvent, ctx)) as MessageEndEventResult | undefined;
+					if (!handlerResult?.message) continue;
+
+					if (handlerResult.message.role !== currentMessage.role) {
+						this.emitError({
+							extensionPath: ext.path,
+							event: "message_end",
+							error: "message_end handlers must return a message with the same role",
+						});
+						continue;
+					}
+
+					currentMessage = handlerResult.message;
+					modified = true;
+				} catch (err) {
+					const message = err instanceof Error ? err.message : String(err);
+					const stack = err instanceof Error ? err.stack : undefined;
+					this.emitError({
+						extensionPath: ext.path,
+						event: "message_end",
+						error: message,
+						stack,
+					});
+				}
+			}
+		}
+
+		return modified ? currentMessage : undefined;
+	}
+
 	async emitToolResult(event: ToolResultEvent): Promise<ToolResultEventResult | undefined> {
 		const ctx = this.createContext();
 		const currentEvent: ToolResultEvent = { ...event };