How to Build an AI Agent with the Vercel AI SDK

1npm install ai zod
2export ANTHROPIC_API_KEY="sk-ant-..."

1import { tool } from 'ai';
2import { z } from 'zod';
3
4const ACCOUNTS: Record<string, { plan: string; usage: number; nextInvoice: number }> = {
5  'acct_8831': { plan: 'Team', usage: 92, nextInvoice: 240 },
6};
7
8const lookupSubscription = tool({
9  description: 'Look up a customer account by ID. Returns plan, usage percent, and next invoice amount.',
10  inputSchema: z.object({ accountId: z.string() }),
11  execute: async ({ accountId }) => {
12    const account = ACCOUNTS[accountId];
13    if (!account) return { error: `No account found for ${accountId}` };
14    return account;
15  },
16});

1const issueAccountCredit = tool({
2  description: 'Issue an account credit in dollars for billing errors or outages.',
3  inputSchema: z.object({
4    accountId: z.string(),
5    amount: z.number().positive(),
6    reason: z.string(),
7  }),
8  execute: async ({ accountId, amount, reason }) => {
9    // In production this calls your billing API, e.g. Stripe customer balance transactions
10    return { credited: amount, accountId, reason, postedAt: new Date().toISOString() };
11  },
12});

1import { ToolLoopAgent, isStepCount } from 'ai';
2
3const agent = new ToolLoopAgent({
4  model: 'anthropic/claude-sonnet-5.5',
5  instructions:
6    'You are a billing support agent. Always call lookupSubscription before making any claim ' +
7    'about an account. Never tell a customer you issued a credit without actually calling issueAccountCredit.',
8  tools: { lookupSubscription, issueAccountCredit },
9  toolApproval: {
10    issueAccountCredit: async ({ amount }) => (amount > 50 ? 'user-approval' : undefined),
11  },
12  stopWhen: isStepCount(10),
13});

1import type { ModelMessage, ToolApprovalResponse } from 'ai';
2
3const messages: ModelMessage[] = [
4  { role: 'user', content: 'Account acct_8831 had a 6-hour outage last week, credit them $75.' },
5];
6
7const result = await agent.generate({ messages });
8messages.push(...result.responseMessages);
9
10const approvals: ToolApprovalResponse[] = [];
11for (const part of result.content) {
12  if (part.type === 'tool-approval-request' && !part.isAutomatic) {
13    console.log(`[APPROVAL NEEDED] $${part.input.amount} on ${part.input.accountId}: ${part.input.reason}`);
14    approvals.push({
15      type: 'tool-approval-response',
16      approvalId: part.approvalId,
17      approved: true,
18      reason: 'Confirmed outage in status page history',
19    });
20  }
21}
22
23if (approvals.length) {
24  messages.push({ role: 'tool', content: approvals });
25  const final = await agent.generate({ messages });
26  console.log(final.text);
27}

1const issueAccountCredit = tool({
2  description: 'Issue an account credit in dollars for billing errors or outages.',
3  inputSchema: z.object({ accountId: z.string(), amount: z.number().positive(), reason: z.string() }),
4  execute: async ({ accountId, amount, reason }) => {
5    try {
6      const result = await billingClient.createCreditBalanceTransaction({ accountId, amount, reason });
7      return { credited: amount, accountId, transactionId: result.id };
8    } catch (err) {
9      return { error: `Credit failed: ${(err as Error).message}` };
10    }
11  },
12});