learnharness/src/agent/teacher-agent.ts

import * as fs from "node:fs";
import * as path from "node:path";
import { ToolLoopAgent, stepCountIs, type LanguageModel, type ToolSet } from "ai";
import { openrouter } from "@openrouter/ai-sdk-provider";
import { PATHS } from "../../config.ts";
import { createTeacherTools } from "./tools.ts";

/**
 * The teacher agent. Everything pi used to provide (agent loop, tool calling,
 * streaming, multi-step) is these few lines now.
 *
 * The model is injectable so tests can pass a MockLanguageModelV4 and drive
 * deterministic scripted lessons (see tests/chat-flow.test.ts).
 *
 * The system prompt is COMPOSED: system-prompt.md is the base, then every
 * src/agent/prompts/*.md fragment in sorted filename order (workers add
 * fragments — nobody edits shared sections of the base prompt). Prefix
 * fragments with a numeric order key: 10-plan-tree.md, 20-jobs.md, …
 */

const DEFAULT_MODEL = process.env.LH_MODEL ?? "anthropic/claude-sonnet-4.5";

const PROMPTS_DIR = path.join(PATHS.root, "src", "agent", "prompts");

function composeInstructions(): string {
	const base = fs.readFileSync(PATHS.teacherPrompt, "utf-8");
	if (!fs.existsSync(PROMPTS_DIR)) return base;
	const fragments = fs
		.readdirSync(PROMPTS_DIR)
		.filter((f) => f.endsWith(".md"))
		.sort()
		.map((f) => fs.readFileSync(path.join(PROMPTS_DIR, f), "utf-8").trim())
		.filter(Boolean);
	return [base.trim(), ...fragments].join("\n\n---\n\n");
}

const instructions = composeInstructions();

export function createTeacherAgent(model?: LanguageModel, tools?: ToolSet) {
	return new ToolLoopAgent({
		model:
			model ??
			openrouter(DEFAULT_MODEL, {
				// Ask OpenRouter for reasoning tokens when the model supports them;
				// they stream as reasoning parts into the UI's collapsible block.
				reasoning: { enabled: true, effort: "medium" },
			}),
		instructions,
		// Tools are per-request (plan tools close over chatId); chat-handler
		// passes createTeacherTools({ chatId }). Tests pass a mock model and get
		// the default question-only set.
		tools: tools ?? createTeacherTools({ chatId: "" }),
		// Question and job tools are client-side — a call ends the server turn
		// automatically. stepCountIs is just the safety net.
		stopWhen: stepCountIs(20),
	});
}

export type TeacherAgent = ReturnType<typeof createTeacherAgent>;