# Vercel AI SDK 3.x & Next.js 15: Streaming Email Tool Calling Guide

_By Tayyab Mughal, Founder & AI Chief · 22 August 2026 · 2 min read_

> How to implement real-time streaming email tool calling in Next.js 15 App Router using the Vercel AI SDK (ai package) and Zod schema validation.

Build modern AI chat interfaces in Next.js 15 that can draft, review, and dispatch transactional emails using the Vercel AI SDK.

## Unified tool calling in modern Next.js applications

The Vercel AI SDK provides a standard interface for connecting LLMs to external APIs across React Server Components, Server Actions, and Route Handlers.

When giving a chat assistant the ability to send emails, using tool() with a Zod schema guarantees type safety and enables client-side tool call rendering.

## Defining the email tool with Zod schema validation

In your Next.js route handler (/api/chat/route.ts), define the email tool using the tool helper from the ai package.

```typescript
import { openai } from '@ai-sdk/openai';
import { streamText, tool } from 'ai';
import { z } from 'zod';

export async function POST(req: Request) {
  const { messages } = await req.json();

  const result = streamText({
    model: openai('gpt-4o'),
    messages,
    tools: {
      sendEmail: tool({
        description: 'Send a transactional email notification to a team member or customer.',
        parameters: z.object({
          to: z.string().email().describe('The destination email address'),
          subject: z.string().min(3).describe('Subject line of the email'),
          text: z.string().min(5).describe('Plain-text body content'),
        }),
        execute: async ({ to, subject, text }) => {
          const res = await fetch('https://api.sadasend.com/v1/emails', {
            method: 'POST',
            headers: {
              'Authorization': `Bearer ${process.env.SADASEND_API_KEY}`,
              'Content-Type': 'application/json',
            },
            body: JSON.stringify({ to, subject, text }),
          });
          const data = await res.json();
          return { success: res.ok, messageId: data.id, status: data.status };
        },
      }),
    },
  });

  return result.toDataStreamResponse();
}
```

## Handling confirmation UI on the frontend

Because sending an email is a state-changing side effect, the frontend can inspect toolInvocations to display a rich preview card with a "Confirm Send" button before triggering the API.

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_Tags: Vercel AI SDK, Next.js, TypeScript, React Email_