TinyFn MCP vs Custom Tool Implementation

When building AI agents, you need deterministic tools for calculations, validations, and conversions. Should you build these tools yourself or use TinyFn's ready-made MCP server? This guide breaks down the costs, tradeoffs, and best use cases for each approach.

The Real Cost of Building Custom MCP Tools

Building an MCP tool seems simple at first. Write a function, expose it via the MCP protocol, done. But the real complexity emerges when you account for everything production-ready tools need.

Development Time Per Tool

At 4-8 hours per tool, building 50 tools requires 200-400 hours of development time. At an average developer cost of $100/hour, that's $20,000-$40,000 just for initial development.

Example: Building a Percentage Calculator

// This looks simple...
function calculatePercentage(value, percentage, operation) {
  if (operation === 'increase') {
    return value * (1 + percentage / 100);
  }
  return value * (1 - percentage / 100);
}

// But production-ready requires:
// - Decimal precision handling (floating point errors)
// - Input validation (negative percentages? null values?)
// - Rounding options (banker's rounding, ceiling, floor)
// - Currency formatting
// - Error messages that help the LLM understand what went wrong
// - Schema definition for MCP
// - Tests for all edge cases

Infrastructure Costs

Custom MCP tools need hosting:

• Server costs: $20-100/month minimum for reliable hosting
• Monitoring: $15-50/month for alerting and logging
• SSL certificates: Free to $100/year depending on setup
• CI/CD pipeline: Developer time for setup and maintenance

Maintenance Burden Over Time

The initial build is just the beginning. Custom tools require ongoing maintenance that compounds over time.

Ongoing Maintenance Tasks

• Bug fixes: Edge cases you didn't anticipate
• Security updates: Dependencies need regular updates
• Scaling: As usage grows, you need better infrastructure
• New features: LLMs work better with enhanced tool descriptions
• MCP protocol updates: The protocol evolves

Example: Date Calculation Complexity

Consider building a "add business days" tool. Seems simple, right?

// You need to handle:
// - Different country weekends (Sat/Sun vs Fri/Sat)
// - Regional holidays (US federal vs state vs corporate)
// - Moving holidays (Easter, Thanksgiving)
// - Timezone edge cases (date changes at midnight)
// - Leap years and leap seconds
// - Historical calendar changes
// - Date formats from different locales

// TinyFn handles all of this. Do you want to?

TinyFn Advantages: 500+ Ready Tools

TinyFn provides over 500 deterministic tools, all production-tested and maintained. Here's what you get:

Comprehensive Tool Categories

• Math: Percentages, arithmetic, statistics, financial calculations
• String: Character counting, text manipulation, encoding
• Convert: Temperature, length, weight, currency, timezones
• Validate: Email, phone, credit card, UUID, URL
• DateTime: Business days, formatting, relative time, age
• Hash: SHA256, MD5, HMAC, bcrypt
• Encode: Base64, URL encoding, HTML entities
• And 22 more categories...

Zero Setup Time

{
  "mcpServers": {
    "tinyfn": {
      "url": "https://api.tinyfn.io/mcp/all/",
      "headers": {
        "X-API-Key": "your-api-key"
      }
    }
  }
}

// That's it. 500+ tools, ready to use.

Production-Grade Quality

• 99.9% uptime SLA on paid plans
• Edge-deployed for low latency worldwide
• Comprehensive error messages that help LLMs recover
• Consistent API design across all tools
• Regular updates without any work from you

Cost Comparison

When to Build Custom Tools

TinyFn covers generic utilities, but some tools need to be custom-built:

Build Custom When You Need:

• Proprietary business logic: Your specific pricing algorithms, custom workflows
• Internal system integration: Connecting to your CRM, ERP, or databases
• Confidential data access: Tools that query sensitive internal information
• Unique calculations: Industry-specific formulas that don't exist elsewhere

Example: Custom vs TinyFn Tools

// USE TINYFN for generic operations:
- Calculate percentage discount     -> TinyFn math/percentage
- Validate customer email           -> TinyFn validate/email
- Convert Celsius to Fahrenheit     -> TinyFn convert/temperature
- Add 5 business days to date       -> TinyFn datetime/add-business-days
- Generate UUID for order           -> TinyFn generate/uuid

// BUILD CUSTOM for business-specific:
- Look up customer tier discount    -> Custom (needs your DB)
- Check inventory levels            -> Custom (needs your systems)
- Calculate shipping based on zone  -> Custom (your pricing logic)
- Verify loyalty points balance     -> Custom (needs your API)

The Hybrid Approach

The best architecture uses both TinyFn and custom tools. MCP supports multiple tool servers, so you can combine them seamlessly.

{
  "mcpServers": {
    "tinyfn": {
      "url": "https://api.tinyfn.io/mcp/all/",
      "headers": {
        "X-API-Key": "your-tinyfn-key"
      }
    },
    "internal": {
      "url": "https://mcp.yourcompany.com/tools",
      "headers": {
        "Authorization": "Bearer your-internal-token"
      }
    }
  }
}

With this setup, your AI agent can:

• Use TinyFn for all standard calculations and validations
• Use your custom tools for business-specific operations
• The LLM automatically chooses the right tool for each task

Side-by-Side Comparison