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Tools

Tool Approval Flow

The tool approval flow allows you to require user approval before executing sensitive tools, giving users control over actions like sending emails, making purchases, or deleting data. A tool call moves through the ToolCallState lifecycle:

  1. awaiting-input — Tool call started, no arguments yet
  2. input-streaming — Arguments arriving incrementally
  3. input-complete — All arguments received
  4. approval-requested — Waiting for user approval (only if needsApproval: true)
  5. approval-responded — User approved or denied

After approval-responded the call executes (if approved). Although complete exists in the ToolCallState union, the runtime never transitions the tool-call part to it — the result surfaces as a populated part.output plus a sibling tool-result part whose own state is complete or error.

When a tool requires approval, the typical flow is:

  1. Model calls the tool
  2. Tool execution is paused
  3. User is prompted to approve or deny
  4. Tool executes (if approved) or is cancelled (if denied)
  5. Conversation continues with the result

Enabling Approval

Tools can be marked as requiring approval by setting needsApproval: true in the definition:

ts
import { toolDefinition } from '@tanstack/ai'
import { z } from 'zod'
import { emailService } from './email-service'

// Step 1: Define tool with approval requirement
const sendEmailDef = toolDefinition({
  name: 'send_email',
  description: 'Send an email to a recipient',
  inputSchema: z.object({
    to: z.string().email(),
    subject: z.string(),
    body: z.string(),
  }),
  outputSchema: z.object({
    success: z.boolean(),
    messageId: z.string(),
  }),
  needsApproval: true, // This tool requires approval
})

// Step 2: Create server implementation
const sendEmail = sendEmailDef.server(async ({ to, subject, body }) => {
  // Only executes if approved
  await emailService.send({ to, subject, body })
  return { success: true, messageId: '...' }
})

Server-Side Approval

On the server, tools with needsApproval: true will pause execution and wait for approval:

ts
import { chat, toServerSentEventsResponse } from '@tanstack/ai'
import { openaiText } from '@tanstack/ai-openai'
import { sendEmail } from './tools'

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

  const stream = chat({
    adapter: openaiText('gpt-5.5'),
    messages,
    tools: [sendEmail],
  })

  return toServerSentEventsResponse(stream)
}

Client-Side Approval Handling

The client receives approval requests and can respond:

tsx
import { useChat, fetchServerSentEvents } from '@tanstack/ai-react'

function ChatComponent() {
  const { messages, sendMessage, addToolApprovalResponse } = useChat({
    connection: fetchServerSentEvents('/api/chat'),
  })

  return (
    <div>
      {messages.map((message) => (
        <div key={message.id}>
          {message.parts.map((part) => {
            // Check for approval requests
            if (
              part.type === 'tool-call' &&
              part.state === 'approval-requested' &&
              part.approval
            ) {
              return (
                <div key={part.id} className="approval-prompt">
                  <p>Approve: {part.name}</p>
                  <pre>{JSON.stringify(part.input, null, 2)}</pre>
                  <button
                    onClick={() =>
                      addToolApprovalResponse({
                        id: part.approval!.id,
                        approved: true,
                      })
                    }
                  >
                    Approve
                  </button>
                  <button
                    onClick={() =>
                      addToolApprovalResponse({
                        id: part.approval!.id,
                        approved: false,
                      })
                    }
                  >
                    Deny
                  </button>
                </div>
              )
            }
            // ... render other parts
            return null
          })}
        </div>
      ))}
    </div>
  )
}

Type safety: When you pass typed tools to useChat, the approval field exists only on tool-call parts for tools declared with needsApproval: true — tools without approval have no approval field at all, so reading it is a compile error that catches a real footgun (checking for approval on a tool that can never request it). See Generic approval handlers for how to write a tool-agnostic handler under this constraint.

Generic Approval Handlers

A handler that renders an approval prompt for any tool (not one specific tool) is still fully supported — you just can't read part.approval off a typed mixed tool union without first establishing that the field exists. Pick whichever of these fits:

1. Narrow with 'approval' in part. This narrows the tool-call union to exactly the members that can carry approval, so one loop handles every approval tool with full type safety:

tsx
import { toolDefinition } from '@tanstack/ai'
import { z } from 'zod'
import { useChat, fetchServerSentEvents } from '@tanstack/ai-react'

const deleteData = toolDefinition({
  name: 'delete_data',
  description: 'Delete data (requires approval)',
  inputSchema: z.object({ key: z.string() }),
  needsApproval: true,
}).client(async ({ key }) => ({ deleted: key }))

const listData = toolDefinition({
  name: 'list_data',
  description: 'List available keys',
  inputSchema: z.object({}),
}).client(async () => ({ keys: [] as Array<string> }))

function ApprovalHandler() {
  const { messages, addToolApprovalResponse } = useChat({
    connection: fetchServerSentEvents('/api/chat'),
    tools: [deleteData, listData],
  })

  return (
    <div>
      {messages.flatMap((message) =>
        message.parts.map((part, i) => {
          // `'approval' in part` narrows the union to `needsApproval` tools,
          // so this single handler covers every approval tool — no per-tool
          // `part.name` branch needed.
          if (
            part.type === 'tool-call' &&
            part.state === 'approval-requested' &&
            'approval' in part &&
            part.approval
          ) {
            return (
              <button
                key={i}
                onClick={() =>
                  addToolApprovalResponse({
                    id: part.approval!.id,
                    approved: true,
                  })
                }
              >
                Approve {part.name}
              </button>
            )
          }
          return null
        }),
      )}
    </div>
  )
}

2. Type a shared component against the base ToolCallPart. The base type (from @tanstack/ai-client, untyped tools) always carries approval?, so a reusable component works across every tool regardless of the caller's tool union — this is the Approval UI Example below.

3. Use an untyped useChat(). With no tools generic, every tool-call part keeps approval? exactly as before — no narrowing needed.

Approval UI Example

Here's a more complete approval UI component:

tsx
import type { ToolCallPart } from '@tanstack/ai-client'

function ApprovalPrompt({
  part,
  onApprove,
  onDeny,
}: {
  part: ToolCallPart
  onApprove: () => void
  onDeny: () => void
}) {
  // `part.input` is the parsed, fully-typed argument object — always populated
  // by approval time (the arguments are complete). The raw `part.arguments`
  // string is still available if you need it.
  const args = part.input

  return (
    <div className="border border-yellow-500 rounded-lg p-4 bg-yellow-50">
      <div className="font-semibold mb-2">
        🔒 Approval Required: {part.name}
      </div>
      <div className="text-sm text-gray-600 mb-4">
        <pre className="bg-gray-100 p-2 rounded text-xs overflow-x-auto">
          {JSON.stringify(args, null, 2)}
        </pre>
      </div>
      <div className="flex gap-2">
        <button
          onClick={onApprove}
          className="px-4 py-2 bg-green-600 text-white rounded-lg"
        >
          ✓ Approve
        </button>
        <button
          onClick={onDeny}
          className="px-4 py-2 bg-red-600 text-white rounded-lg"
        >
          ✗ Deny
        </button>
      </div>
    </div>
  )
}

Wire it up from your message renderer. Note the id you pass is the approval id (part.approval.id), not the tool call id:

tsx
{
  part.type === 'tool-call' &&
    part.state === 'approval-requested' &&
    part.approval && (
      <ApprovalPrompt
        part={part}
        onApprove={() =>
          addToolApprovalResponse({ id: part.approval!.id, approved: true })
        }
        onDeny={() =>
          addToolApprovalResponse({ id: part.approval!.id, approved: false })
        }
      />
    )
}

Client Tools with Approval

Client tools can also require approval:

ts
import { toolDefinition } from '@tanstack/ai'
import { z } from 'zod'
import { useChat, fetchServerSentEvents } from '@tanstack/ai-react'

// tools/definitions.ts
const deleteLocalDataDef = toolDefinition({
  name: 'delete_local_data',
  description: 'Delete data from local storage',
  inputSchema: z.object({
    key: z.string(),
  }),
  outputSchema: z.object({
    deleted: z.boolean(),
  }),
  needsApproval: true, // Requires approval even on client
})

// Client: Create implementation
const deleteLocalData = deleteLocalDataDef.client((input) => {
  // This will only execute after approval
  localStorage.removeItem(input.key)
  return { deleted: true }
})

const { messages, addToolApprovalResponse } = useChat({
  connection: fetchServerSentEvents('/api/chat'),
  // Pass client tools as a plain array — literal tool-name inference works
  // without a wrapper, so `part.name === "delete_local_data"` still narrows
  // `part.input` / `part.output` to this tool's types.
  tools: [deleteLocalData], // Automatic execution after approval
})

Example: E-commerce Purchase

ts
import { toolDefinition } from '@tanstack/ai'
import { z } from 'zod'
import { createOrder } from './orders'

// Define tool with approval requirement
const purchaseItemDef = toolDefinition({
  name: 'purchase_item',
  description: 'Purchase an item from the store',
  inputSchema: z.object({
    itemId: z.string(),
    quantity: z.number(),
    price: z.number(),
  }),
  outputSchema: z.object({
    orderId: z.string(),
    total: z.number(),
  }),
  needsApproval: true,
})

// Create server implementation
const purchaseItem = purchaseItemDef.server(
  async ({ itemId, quantity, price }) => {
    const order = await createOrder({ itemId, quantity, price })
    return { orderId: order.id, total: price * quantity }
  },
)

The user will see an approval prompt showing the item, quantity, and price before the purchase is made. The tool will only execute after the user approves.

Best Practices

  • Use approval for sensitive operations - Sending emails, making payments, deleting data
  • Show clear information - Display what the tool will do before approval
  • Provide context - Show tool arguments in a readable format
  • Handle denial gracefully - Don't break the conversation if a tool is denied
  • Timeout handling - Consider timeouts for approval requests

Next Steps

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