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The Art of Building Great MCPs

Building good MCPs is an art. You want to minimize wrong tool calls and maximize AI success.

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Core Principles

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1. Less is More

Don’t expose every API endpoint as an MCP tool. Pick the most important ones. ❌ Bad: 50 similar tools confuse AI 
["create_user", "add_user", "new_user", "register_user", "signup_user"]
✅ Good: One clear tool per action 
["create_user"]

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2. Crystal Clear Descriptions

AI depends on your descriptions to understand when to use tools. ❌ Bad: Vague description 
{
  "name": "process_data",
  "description": "Processes data"
}
✅ Good: Specific description 
{
  "name": "calculate_tax",
  "description": "Calculate tax amount for a purchase based on item price and location"
}

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3. Simple Schemas

Complex input schemas lead to AI errors. ❌ Bad: Nested complexity 
{
  "properties": {
    "user": {
      "type": "object",
      "properties": {
        "profile": {
          "type": "object", 
          "properties": {
            "settings": { /* more nesting... */ }
          }
        }
      }
    }
  }
}
✅ Good: Flat and simple 
{
  "properties": {
    "user_id": {"type": "string"},
    "email": {"type": "string"},
    "name": {"type": "string"}
  }
}

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Tool Design Patterns

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Action-Oriented Naming

Use verbs that clearly indicate what happens: 
{
  "send_email": "✅ Clear action",
  "email": "❌ Unclear if reading or sending",
  "get_email": "✅ Clear action", 
  "email_data": "❌ What happens to the data?"
}

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Consistent Naming Conventions

Pick a pattern and stick to it: 
{
  "get_user": "✅ Consistent pattern",
  "create_user": "✅ Consistent pattern",
  "fetchUserData": "❌ Different pattern",
  "user_delete": "❌ Different pattern"
}

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Error-Resistant Parameters

Design parameters that are hard to mess up: ❌ Error-prone: Free text that needs parsing 
{
  "date": {"type": "string", "description": "Date in any format"}
}
✅ Error-resistant: Structured input 
{
  "year": {"type": "integer", "minimum": 2020, "maximum": 2030},
  "month": {"type": "integer", "minimum": 1, "maximum": 12},
  "day": {"type": "integer", "minimum": 1, "maximum": 31}
}

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Resource Best Practices

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Meaningful URIs

Use URIs that make sense to AI: ✅ Good URIs: 
user://profile          # User's profile data
contacts://list         # List of contacts  
calendar://events       # Calendar events
tasks://pending         # Pending tasks
❌ Bad URIs: 
resource://1            # What is resource 1?
data://x                # What is x?
api://endpoint          # Too generic

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Clear Resource Descriptions

Explain what data the resource contains: ❌ Vague: 
{
  "uri": "user://data",
  "description": "User data"
}
✅ Specific: 
{
  "uri": "user://profile", 
  "description": "User profile including name, email, preferences, and account settings"
}

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Prompt Templates

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Use Variables Wisely

Make templates flexible but not complex: ✅ Good template: 
Hi {name}, 

Your meeting with {attendee} is scheduled for {date} at {time}.

Best regards,
{sender}
❌ Complex template: 
{greeting_type} {name_with_title}, 

{conditional_text_based_on_time_of_day} {meeting_type} with {attendee_list_formatted} is {scheduling_verb} for {date_formatted_long} at {time_with_timezone}.

{closing_based_on_relationship},
{sender_with_signature}

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Testing Your Designs

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Test with Realistic Prompts

Don’t just test if tools work - test if AI chooses the right tools: 
Test prompts:
❌ "Call the send_email function"
✅ "Email John about tomorrow's meeting"
✅ "Let Sarah know I'll be late"  
✅ "Send a thank you note to the client"

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Watch for Confusion Patterns

Common signs AI is confused:
  • Wrong tool selection: AI picks similar but wrong tool
  • Missing parameters: AI doesn’t provide required fields
  • Repeated failures: AI keeps trying the same broken approach
  • No tool usage: AI gives up and doesn’t use any tools

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Common Mistakes

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Mistake #1: Too Many Similar Tools

// ❌ Confusing for AI
["email_send", "send_email", "mail_send", "dispatch_email"]

// ✅ Pick one clear name
["send_email"]

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Mistake #2: Boolean Parameters for Actions

// ❌ AI gets confused by booleans
{
  "name": "manage_user",
  "parameters": {
    "create": {"type": "boolean"},
    "delete": {"type": "boolean"}
  }
}

// ✅ Separate tools for separate actions
{
  "create_user": { /* parameters */ },
  "delete_user": { /* parameters */ }
}

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Mistake #3: Generic Error Messages

// ❌ Unhelpful for AI
"An error occurred"

// ✅ Specific guidance
"Email address must be valid format (example@domain.com)"

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Performance Tips

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Optimize for Common Use Cases

Design tools around what users actually ask for: Common request: “Schedule a meeting with John tomorrow at 2pm” ❌ Poor design: Separate tools for each step 
1. find_user → get_calendar → check_availability → create_event → send_invite
✅ Good design: One tool for the complete workflow 
schedule_meeting(attendee, date, time) → handles all steps internally

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Minimize Tool Chains

Long chains of tool calls often fail: ❌ Fragile: 5-step process where any step can fail ✅ Robust: 2-step process with error handling

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Quality Checklist

Before deploying your MCP:
  • Tool names clearly indicate actions
  • Descriptions are specific and helpful
  • Parameters use simple types when possible
  • Similar tools have distinct purposes
  • Error messages guide AI toward correct usage
  • Resources have meaningful URIs
  • Prompts use clear variable names
  • Test results show AI picks correct tools
  • Success rate is above 80% in testing

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Next Steps

Testing Guide

Learn how to test your MCPs effectively

API Reference

Explore LeanMCP API features
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