co-mono/packages/ai/src/agent/agent-loop.ts

import { streamSimple } from "../stream.js";
import type { AssistantMessage, Context, Message, ToolResultMessage, UserMessage } from "../types.js";
import { EventStream } from "../utils/event-stream.js";
import { validateToolArguments } from "../utils/validation.js";
import type { AgentContext, AgentEvent, AgentLoopConfig, AgentTool, AgentToolResult, QueuedMessage } from "./types.js";

/**
 * Start an agent loop with a new user message.
 * The prompt is added to the context and events are emitted for it.
 */
export function agentLoop(
	prompt: UserMessage,
	context: AgentContext,
	config: AgentLoopConfig,
	signal?: AbortSignal,
	streamFn?: typeof streamSimple,
): EventStream<AgentEvent, AgentContext["messages"]> {
	const stream = createAgentStream();

	(async () => {
		const newMessages: AgentContext["messages"] = [prompt];
		const currentContext: AgentContext = {
			...context,
			messages: [...context.messages, prompt],
		};

		stream.push({ type: "agent_start" });
		stream.push({ type: "turn_start" });
		stream.push({ type: "message_start", message: prompt });
		stream.push({ type: "message_end", message: prompt });

		await runLoop(currentContext, newMessages, config, signal, stream, streamFn);
	})();

	return stream;
}

/**
 * Continue an agent loop from the current context without adding a new message.
 * Used for retry after overflow - context already has user message or tool results.
 * Throws if the last message is not a user message or tool result.
 */
export function agentLoopContinue(
	context: AgentContext,
	config: AgentLoopConfig,
	signal?: AbortSignal,
	streamFn?: typeof streamSimple,
): EventStream<AgentEvent, AgentContext["messages"]> {
	// Validate that we can continue from this context
	const lastMessage = context.messages[context.messages.length - 1];
	if (!lastMessage) {
		throw new Error("Cannot continue: no messages in context");
	}
	if (lastMessage.role !== "user" && lastMessage.role !== "toolResult") {
		throw new Error(`Cannot continue from message role: ${lastMessage.role}. Expected 'user' or 'toolResult'.`);
	}

	const stream = createAgentStream();

	(async () => {
		const newMessages: AgentContext["messages"] = [];
		const currentContext: AgentContext = { ...context };

		stream.push({ type: "agent_start" });
		stream.push({ type: "turn_start" });
		// No user message events - we're continuing from existing context

		await runLoop(currentContext, newMessages, config, signal, stream, streamFn);
	})();

	return stream;
}

function createAgentStream(): EventStream<AgentEvent, AgentContext["messages"]> {
	return new EventStream<AgentEvent, AgentContext["messages"]>(
		(event: AgentEvent) => event.type === "agent_end",
		(event: AgentEvent) => (event.type === "agent_end" ? event.messages : []),
	);
}

/**
 * Shared loop logic for both agentLoop and agentLoopContinue.
 */
async function runLoop(
	currentContext: AgentContext,
	newMessages: AgentContext["messages"],
	config: AgentLoopConfig,
	signal: AbortSignal | undefined,
	stream: EventStream<AgentEvent, AgentContext["messages"]>,
	streamFn?: typeof streamSimple,
): Promise<void> {
	let hasMoreToolCalls = true;
	let firstTurn = true;
	let queuedMessages: QueuedMessage<any>[] = (await config.getQueuedMessages?.()) || [];

	while (hasMoreToolCalls || queuedMessages.length > 0) {
		if (!firstTurn) {
			stream.push({ type: "turn_start" });
		} else {
			firstTurn = false;
		}

		// Process queued messages first (inject before next assistant response)
		if (queuedMessages.length > 0) {
			for (const { original, llm } of queuedMessages) {
				stream.push({ type: "message_start", message: original });
				stream.push({ type: "message_end", message: original });
				if (llm) {
					currentContext.messages.push(llm);
					newMessages.push(llm);
				}
			}
			queuedMessages = [];
		}

		// Stream assistant response
		const message = await streamAssistantResponse(currentContext, config, signal, stream, streamFn);
		newMessages.push(message);

		if (message.stopReason === "error" || message.stopReason === "aborted") {
			// Stop the loop on error or abort
			stream.push({ type: "turn_end", message, toolResults: [] });
			stream.push({ type: "agent_end", messages: newMessages });
			stream.end(newMessages);
			return;
		}

		// Check for tool calls
		const toolCalls = message.content.filter((c) => c.type === "toolCall");
		hasMoreToolCalls = toolCalls.length > 0;

		const toolResults: ToolResultMessage[] = [];
		if (hasMoreToolCalls) {
			// Execute tool calls
			toolResults.push(...(await executeToolCalls(currentContext.tools, message, signal, stream)));
			currentContext.messages.push(...toolResults);
			newMessages.push(...toolResults);
		}
		stream.push({ type: "turn_end", message, toolResults: toolResults });

		// Get queued messages after turn completes
		queuedMessages = (await config.getQueuedMessages?.()) || [];
	}

	stream.push({ type: "agent_end", messages: newMessages });
	stream.end(newMessages);
}

// Helper functions
async function streamAssistantResponse(
	context: AgentContext,
	config: AgentLoopConfig,
	signal: AbortSignal | undefined,
	stream: EventStream<AgentEvent, AgentContext["messages"]>,
	streamFn?: typeof streamSimple,
): Promise<AssistantMessage> {
	// Convert AgentContext to Context for streamSimple
	// Use a copy of messages to avoid mutating the original context
	const processedMessages = config.preprocessor
		? await config.preprocessor(context.messages, signal)
		: [...context.messages];
	const processedContext: Context = {
		systemPrompt: context.systemPrompt,
		messages: [...processedMessages].map((m) => {
			if (m.role === "toolResult") {
				const { details, ...rest } = m;
				return rest;
			} else {
				return m;
			}
		}),
		tools: context.tools, // AgentTool extends Tool, so this works
	};

	// Use custom stream function if provided, otherwise use default streamSimple
	const streamFunction = streamFn || streamSimple;
	const response = await streamFunction(config.model, processedContext, { ...config, signal });

	let partialMessage: AssistantMessage | null = null;
	let addedPartial = false;

	for await (const event of response) {
		switch (event.type) {
			case "start":
				partialMessage = event.partial;
				context.messages.push(partialMessage);
				addedPartial = true;
				stream.push({ type: "message_start", message: { ...partialMessage } });
				break;

			case "text_start":
			case "text_delta":
			case "text_end":
			case "thinking_start":
			case "thinking_delta":
			case "thinking_end":
			case "toolcall_start":
			case "toolcall_delta":
			case "toolcall_end":
				if (partialMessage) {
					partialMessage = event.partial;
					context.messages[context.messages.length - 1] = partialMessage;
					stream.push({ type: "message_update", assistantMessageEvent: event, message: { ...partialMessage } });
				}
				break;

			case "done":
			case "error": {
				const finalMessage = await response.result();
				if (addedPartial) {
					context.messages[context.messages.length - 1] = finalMessage;
				} else {
					context.messages.push(finalMessage);
				}
				if (!addedPartial) {
					stream.push({ type: "message_start", message: { ...finalMessage } });
				}
				stream.push({ type: "message_end", message: finalMessage });
				return finalMessage;
			}
		}
	}

	return await response.result();
}

async function executeToolCalls<T>(
	tools: AgentTool<any, T>[] | undefined,
	assistantMessage: AssistantMessage,
	signal: AbortSignal | undefined,
	stream: EventStream<AgentEvent, Message[]>,
): Promise<ToolResultMessage<T>[]> {
	const toolCalls = assistantMessage.content.filter((c) => c.type === "toolCall");
	const results: ToolResultMessage<any>[] = [];

	for (const toolCall of toolCalls) {
		const tool = tools?.find((t) => t.name === toolCall.name);

		stream.push({
			type: "tool_execution_start",
			toolCallId: toolCall.id,
			toolName: toolCall.name,
			args: toolCall.arguments,
		});

		let resultOrError: AgentToolResult<T> | string;
		let isError = false;

		try {
			if (!tool) throw new Error(`Tool ${toolCall.name} not found`);

			// Validate arguments using shared validation function
			const validatedArgs = validateToolArguments(tool, toolCall);

			// Execute with validated, typed arguments
			resultOrError = await tool.execute(toolCall.id, validatedArgs, signal);
		} catch (e) {
			resultOrError = e instanceof Error ? e.message : String(e);
			isError = true;
		}

		stream.push({
			type: "tool_execution_end",
			toolCallId: toolCall.id,
			toolName: toolCall.name,
			result: resultOrError,
			isError,
		});

		// Convert result to content blocks
		const content: ToolResultMessage<T>["content"] =
			typeof resultOrError === "string" ? [{ type: "text", text: resultOrError }] : resultOrError.content;

		const toolResultMessage: ToolResultMessage<T> = {
			role: "toolResult",
			toolCallId: toolCall.id,
			toolName: toolCall.name,
			content,
			details: typeof resultOrError === "string" ? ({} as T) : resultOrError.details,
			isError,
			timestamp: Date.now(),
		};

		results.push(toolResultMessage);
		stream.push({ type: "message_start", message: toolResultMessage });
		stream.push({ type: "message_end", message: toolResultMessage });
	}

	return results;
}