/**
 * Agent Message Context Usage Extension
 *
 * Automatically appends the agent's current context usage to messages sent
 * to its parent, so the orchestrator/parent is always aware of subagent context load.
 *
 * Usage:
 *   Copy to ~/.prime/agent/extensions/agent-message-context-usage.ts
 *   or run with `prime-agent -e ./examples/extensions/agent-message-context-usage.ts`
 */
import type { ExtensionAPI } from "@earendil-works/pi-coding-agent";

export default function (pi: ExtensionAPI) {
	pi.on("before_agent_message_send", async (event, ctx) => {
		// Only attach context usage when sending to the parent
		if (event.receiverRole !== "parent") {
			return;
		}

		const usage = ctx.getContextUsage();
		if (!usage) {
			return;
		}

		const tokensStr = usage.tokens !== null ? usage.tokens.toLocaleString() : "unknown";
		const windowStr = usage.contextWindow.toLocaleString();
		const percentStr = usage.percent !== null ? `${usage.percent.toFixed(1)}%` : "unknown";

		const footer = `\n\n[Context usage: ${tokensStr} / ${windowStr} tokens (${percentStr})]`;
		return {
			message: event.message + footer,
		};
	});
}