feat(coding-agent): allow message_end replacements

closes #3982

packages/coding-agent/src/core/agent-session.ts

@@ -610,6 +610,22 @@ export class AgentSession {
 		return undefined;
 	}
 
+	private _replaceMessageInPlace(target: AgentMessage, replacement: AgentMessage): void {
+		// Agent-core stores the finalized message object in its state before emitting message_end.
+		// SessionManager persistence happens later in _processAgentEvent() with event.message.
+		// Mutating this object in place keeps agent state, later turn/agent events, listeners,
+		// and the eventual SessionManager.appendMessage(event.message) persistence in sync.
+		if (target === replacement) {
+			return;
+		}
+
+		const targetRecord = target as unknown as Record<string, unknown>;
+		for (const key of Object.keys(targetRecord)) {
+			delete targetRecord[key];
+		}
+		Object.assign(targetRecord, replacement);
+	}
+
 	/** Emit extension events based on agent events */
 	private async _emitExtensionEvent(event: AgentEvent): Promise<void> {
 		if (event.type === "agent_start") {
@@ -651,7 +667,10 @@ export class AgentSession {
 				type: "message_end",
 				message: event.message,
 			};
-			await this._extensionRunner.emit(extensionEvent);
+			const replacement = await this._extensionRunner.emitMessageEnd(extensionEvent);
+			if (replacement) {
+				this._replaceMessageInPlace(event.message, replacement);
+			}
 		} else if (event.type === "tool_execution_start") {
 			const extensionEvent: ToolExecutionStartEvent = {
 				type: "tool_execution_start",