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";

// Main prompt function - returns a stream of events
export function agentLoop(
	prompt: UserMessage,
	context: AgentContext,
	config: AgentLoopConfig,
	signal?: AbortSignal,
	streamFn?: typeof streamSimple,
): EventStream<AgentEvent, AgentContext["messages"]> {
	const stream = new EventStream<AgentEvent, AgentContext["messages"]>(
		(event: AgentEvent) => event.type === "agent_end",
		(event: AgentEvent) => (event.type === "agent_end" ? event.messages : []),
	);

	// Run the prompt async
	(async () => {
		// Track new messages generated during this prompt
		const newMessages: AgentContext["messages"] = [];
		// Create user message for the prompt
		const messages = [...context.messages, prompt];
		newMessages.push(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 });

		// Update context with new messages
		const currentContext: AgentContext = {
			...context,
			messages,
		};

		// Keep looping while we have tool calls or queued messages
		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 = [];
			}

			// console.log("agent-loop: ", [...currentContext.messages]);

			// 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);
	})();

	return stream;
}

// 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);
				}
				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;
}