I opened the editor at 16:42 and it was running in production at 17:09.
Here's the full code for implementing Claude API tool use — 27 minutes of actual work.
I expected it to be complicated. It wasn't.

What I built

Added a natural language interface to a SaaS admin dashboard. Type "show me last month's revenue" or "how many active users do we have?" and Claude queries the DB and returns the answer.

The mechanic is simple: you tell Claude "you're allowed to call these functions." Claude decides when to call them and constructs the arguments. Your code actually executes the function and sends the result back to Claude. Claude generates the final text answer.

Stack:

Next.js 14 (App Router)
@anthropic-ai/sdk 0.20+
TypeScript
Supabase (DB)

Step 1: Define the tools

Tools are defined with JSON Schema. The description field is the most important part — write it so Claude clearly understands when to call the tool and what it returns. Vague descriptions cause misfires or missed calls.

// lib/claude-tools.ts
import Anthropic from '@anthropic-ai/sdk'

export const tools: Anthropic.Tool[] = [
  {
    name: 'get_revenue',
    description:
      'Fetches total revenue and transaction count for a given period. ' +
      'Supports daily, weekly, and monthly aggregation. ' +
      'Returns the latest 12 entries if no date range is specified.',
    input_schema: {
      type: 'object',
      properties: {
        period: {
          type: 'string',
          enum: ['daily', 'weekly', 'monthly'],
          description: 'Aggregation unit',
        },
        date_from: {
          type: 'string',
          description: 'Start date (YYYY-MM-DD)',
        },
        date_to: {
          type: 'string',
          description: 'End date (YYYY-MM-DD)',
        },
      },
      required: ['period'],
    },
  },
  {
    name: 'list_users',
    description:
      'Returns a list of users. Filterable by status (active/canceled/trial). Max 100 results.',
    input_schema: {
      type: 'object',
      properties: {
        status: {
          type: 'string',
          enum: ['active', 'canceled', 'trial'],
        },
        limit: {
          type: 'number',
          description: 'Number of results (max 100, default 20)',
        },
      },
      required: [],
    },
  },
]