Claude API Tool Use: Building Reliable Agentic Workflows in Production

Claude's tool use (function calling) API is what separates toy chatbots from actual agents. I've built production agents with it — here's what reliable tool use looks like when the stakes are real.

How tool use works

You define tools as JSON schemas. Claude decides when to call them and with what arguments. Your code executes the actual function and returns the result. Claude incorporates the result and continues.

import Anthropic from '@anthropic-ai/sdk';

const client = new Anthropic();

// Define your tools
const tools: Anthropic.Tool[] = [
  {
    name: 'get_user',
    description: 'Retrieve a user by their ID or email address',
    input_schema: {
      type: 'object' as const,
      properties: {
        identifier: {
          type: 'string',
          description: 'User ID (uuid) or email address',
        },
        identifier_type: {
          type: 'string',
          enum: ['id', 'email'],
          description: 'Whether the identifier is an ID or email',
        },
      },
      required: ['identifier', 'identifier_type'],
    },
  },
];

const response = await client.messages.create({
  model: 'claude-opus-4-6',
  max_tokens: 1024,
  tools,
  messages: [
    { role: 'user', content: 'Get the user with email [e-mail]' },
  ],
});

If Claude wants to call the tool, response.stop_reason is 'tool_use' and the response contains a tool_use block.

The complete agentic loop

Tool use is not a single API call — it's a loop:

async function runAgent(userMessage: string): Promise<string> {
  const messages: Anthropic.MessageParam[] = [
    { role: 'user', content: userMessage },
  ];

  while (true) {
    const response = await client.messages.create({
      model: 'claude-opus-4-6',
      max_tokens: 4096,
      tools,
      messages,
    });

    // Add Claude's response to message history
    messages.push({ role: 'assistant', content: response.content });

    // If Claude is done, return the text response
    if (response.stop_reason === 'end_turn') {
      const textBlock = response.content.find(b => b.type === 'text');
      return textBlock?.text ?? '';
    }

    // Process tool calls
    if (response.stop_reason === 'tool_use') {
      const toolResults: Anthropic.ToolResultBlockParam[] = [];

      for (const block of response.content) {
        if (block.type !== 'tool_use') continue;

        const result = await executeTool(block.name, block.input);

        toolResults.push({
          type: 'tool_result',
          tool_use_id: block.id,
          content: JSON.stringify(result),
        });
      }

      // Feed results back to Claude
      messages.push({ role: 'user', content: toolResults });
    }
  }
}

Executing tools safely

async function executeTool(
  name: string,
  input: Record<string, unknown>
): Promise<unknown> {
  console.log(`[tool] ${name}`, input);

  switch (name) {
    case 'get_user':
      return getUser(input as { identifier: string; identifier_type: 'id' | 'email' });

    case 'update_subscription':
      return updateSubscription(input as UpdateSubscriptionInput);

    case 'send_email':
      return sendEmail(input as SendEmailInput);

    default:
      // Return an error result — don't throw. Claude will handle it.
      return { error: `Unknown tool: ${name}` };
  }
}

async function getUser({ identifier, identifier_type }: {
  identifier: string;
  identifier_type: 'id' | 'email';
}) {
  const user = identifier_type === 'email'
    ? await db.query.users.findFirst({ where: eq(users.email, identifier) })
    : await db.query.users.findFirst({ where: eq(users.id, identifier) });

  if (!user) return { error: 'User not found' };

  // Don't return sensitive fields
  const { passwordHash, ...safeUser } = user;
  return safeUser;
}

Parallel tool calls

Claude can request multiple tool calls in a single response. Always handle them:

if (response.stop_reason === 'tool_use') {
  const toolUseBlocks = response.content.filter(
    (b): b is Anthropic.ToolUseBlock => b.type === 'tool_use'
  );

  // Execute all tool calls in parallel
  const results = await Promise.allSettled(
    toolUseBlocks.map(block => executeTool(block.name, block.input))
  );

  const toolResults: Anthropic.ToolResultBlockParam[] = toolUseBlocks.map(
    (block, i) => {
      const result = results[i];
      return {
        type: 'tool_result' as const,
        tool_use_id: block.id,
        content: result.status === 'fulfilled'
          ? JSON.stringify(result.value)
          : JSON.stringify({ error: result.reason?.message ?? 'Tool failed' }),
        is_error: result.status === 'rejected',
      };
    }
  );

  messages.push({ role: 'user', content: toolResults });
}

Promise.allSettled — not Promise.all. One failed tool call shouldn't crash the agent when other calls succeeded.

Tool definitions that actually work

Bad tool definition → Claude calls it wrong. Here's what makes the difference:

// Bad: vague description, no examples
{
  name: 'update_user',
  description: 'Update a user',
  input_schema: {
    type: 'object',
    properties: {
      data: { type: 'object' },
    },
  },
}

// Good: precise description, typed fields, examples in description
{
  name: 'update_user_subscription',
  description: 'Update a user\'s subscription plan. Use when the user needs to upgrade, downgrade, or cancel. Do NOT use for payment method changes — use update_payment_method instead.',
  input_schema: {
    type: 'object',
    properties: {
      user_id: {
        type: 'string',
        description: 'UUID of the user to update',
      },
      plan: {
        type: 'string',
        enum: ['free', 'pro', 'enterprise'],
        description: 'New plan to switch the user to',
      },
      reason: {
        type: 'string',
        description: 'Why the plan is being changed (for audit log). Example: "User requested downgrade via support ticket #1234"',
      },
    },
    required: ['user_id', 'plan', 'reason'],
  },
}

Key rules:

• Name describes the action, not the object (update_user_subscription not user)
• Description says what NOT to use it for — Claude reads this
• Examples in field descriptions dramatically improve accuracy
• Use enum when the set of valid values is known

Limiting runaway agents

Without guards, an agent can loop forever or rack up massive API costs:

async function runAgent(
  userMessage: string,
  options: { maxTurns?: number; maxTokens?: number } = {}
): Promise<string> {
  const { maxTurns = 10, maxTokens = 50_000 } = options;

  const messages: Anthropic.MessageParam[] = [
    { role: 'user', content: userMessage },
  ];

  let totalInputTokens = 0;
  let totalOutputTokens = 0;
  let turns = 0;

  while (turns < maxTurns) {
    turns++;

    const response = await client.messages.create({
      model: 'claude-opus-4-6',
      max_tokens: 4096,
      tools,
      messages,
    });

    totalInputTokens += response.usage.input_tokens;
    totalOutputTokens += response.usage.output_tokens;

    console.log(`[agent] Turn ${turns} | Tokens: ${totalInputTokens}in ${totalOutputTokens}out`);

    if (totalInputTokens + totalOutputTokens > maxTokens) {
      return 'Agent stopped: token budget exceeded. Please narrow your request.';
    }

    messages.push({ role: 'assistant', content: response.content });

    if (response.stop_reason === 'end_turn') {
      const textBlock = response.content.find(b => b.type === 'text');
      return textBlock?.text ?? '';
    }

    // ... handle tool_use
  }

  return `Agent stopped after ${maxTurns} turns. Task may be too complex for a single run.`;
}

Tool results that help Claude reason better

The quality of your tool results affects the quality of Claude's next action:

// Bad: raw database row
return user;  // { id: '...', email: '...', created_at: Date, metadata: {...} }

// Good: contextual result with inferred facts
return {
  user: {
    id: user.id,
    email: user.email,
    plan: user.plan,
    accountAge: `${daysSince(user.createdAt)} days`,
  },
  context: {
    isActive: user.status === 'active',
    hasPaymentMethod: !!user.stripeCustomerId,
    recentActivity: `Last login ${daysSince(user.lastLoginAt)} days ago`,
  },
};

You're not just returning data — you're giving Claude the facts it needs to decide what to do next. A user with no payment method who's asking to upgrade needs a different response than one with valid billing.

Error handling that doesn't crash the agent

async function executeToolSafe(
  name: string,
  input: Record<string, unknown>
): Promise<{ result?: unknown; error?: string }> {
  try {
    const result = await executeTool(name, input);
    return { result };
  } catch (error) {
    const message = error instanceof Error ? error.message : String(error);
    console.error(`[tool error] ${name}:`, message);
    // Return error as result — let Claude decide how to proceed
    return { error: `Tool ${name} failed: ${message}` };
  }
}

Return errors as results, not exceptions. Claude can read the error and either retry with different arguments, fall back to a different tool, or tell the user what went wrong — but only if it knows about the error.

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