Tool Design

Purpose

Design individual MCP tools with clear input/output schemas, proper error handling, and token-efficient responses following best practices.

When to Use

• Designing new MCP tools
• Refining existing tool schemas
• Implementing error handling
• Optimizing tool responses
• Validating tool design

Input Schema Design

Clear, Minimal Parameters

{
  "name": "search",
  "input": {
    "pattern": {
      "type": "string",
      "required": true,
      "description": "Search pattern (regex supported)"
    },
    "filter": {
      "type": "string",
      "required": false,
      "description": "File filter (*.ts, src/**)"
    },
    "max": {
      "type": "integer",
      "required": false,
      "default": 50,
      "description": "Maximum results"
    }
  }
}

Key principles:

• Required vs optional clearly marked
• Defaults documented
• Types explicit
• Descriptions concise

Accept Extra Parameters

// Pseudocode - Always permissive
function search(params) {
  const {pattern, filter, max, ...extra} = params

  const warnings = []
  if (Object.keys(extra).length > 0) {
    warnings.push(`Unknown params ignored: ${Object.keys(extra).join(', ')}`)
  }

  return {results: search(pattern, filter, max), warnings}
}

Output Schema Design

Standard Structure

{
  "data": {},           // Primary response data
  "metadata": {},       // Automation flags (has_more, total, etc.)
  "errors": [],         // Error list (empty if none)
  "warnings": [],       // Warning list (unknown params, etc.)
  "next_steps": ""      // Guidance for user
}

Token-Efficient Responses

Don't repeat inputs:

// Bad
{
  "query": "authenticate",  // User just sent this
  "filter": "*.ts",         // User just sent this
  "results": [...]
}

// Good
{
  "results": [...],
  "total": 127
}

Progressive Detail

{
  "results": [
    {"id": "a1", "name": "...", "confidence": 0.95, "full_details": {...}},  // High confidence
    {"id": "b2", "name": "...", "confidence": 0.70, "summary": "..."},       // Medium
    {"id": "c3", "name": "...", "confidence": 0.40}                           // Low - just ID
  ]
}

Error Handling

See also: Client Guidance for comprehensive error-as-progressive-enhancement patterns including similar tool suggestions, parameter corrections, and schema hints.

Error Response Structure

{
  "error": {
    "code": "INVALID_PATTERN",
    "message": "Regex pattern is malformed",
    "details": {
      "pattern": "([unclosed",
      "position": 2,
      "expected": "]"
    },
    "suggestion": "Check regex syntax. Example: \"function.*User\"",
    "schema_hint": {
      "pattern": {"type": "string", "required": true}
    }
  }
}

Standard Error Codes

INVALID_INPUT       - Bad input parameters
NOT_FOUND           - Resource not found
PERMISSION_DENIED   - Access denied
TIMEOUT             - Operation timed out
INTERNAL_ERROR      - Server error
RATE_LIMITED        - Too many requests
UNKNOWN_TOOL        - Tool name not found (suggest similar)
MISSING_REQUIRED    - Required parameter missing (include schema)

Graceful Degradation

{
  "results": [...],  // Partial results
  "warnings": ["Timeout after 5s, showing partial results"],
  "complete": false,
  "partial": true
}

Client Guidance in Errors

Errors should guide clients toward success:

{
  "error": {
    "code": "UNKNOWN_TOOL",
    "message": "Tool 'serach' not found",
    "suggestions": {
      "did_you_mean": "search",
      "similar_tools": ["search", "search_code"]
    }
  }
}

{
  "error": {
    "code": "MISSING_REQUIRED",
    "message": "Required parameter 'pattern' is missing",
    "schema_hint": {
      "required": ["pattern"],
      "optional": ["filter", "max"],
      "example": {"pattern": "User", "filter": "*.ts"}
    }
  }
}

Tool Types and Patterns

Query Tools (Fast, <100 tokens)

{
  "name": "search",
  "purpose": "Fast discovery",
  "output": {
    "results": [{"id": "...", "preview": "..."}],
    "has_more": true,
    "total": 127
  },
  "avg_tokens": 80
}

Lookup Tools (Medium, ~200 tokens)

{
  "name": "get_definition",
  "purpose": "Detailed information",
  "input": {"id": "from_search"},
  "output": {
    "id": "...",
    "full_details": {...}
  },
  "avg_tokens": 200
}

Analysis Tools (Slow, ~500 tokens)

{
  "name": "trace_callers",
  "purpose": "Deep analysis",
  "input": {"symbol_id": "..."},
  "output": {
    "call_graph": {...},
    "analysis": {...}
  },
  "avg_tokens": 500,
  "warning": "Expensive operation"
}

Validation Patterns

Input Validation

// Pseudocode
function validateInput(params) {
  if (!params.pattern) {
    return error("INVALID_INPUT", "pattern is required")
  }

  if (params.max && (params.max < 1 || params.max > 1000)) {
    return error("INVALID_INPUT", "max must be 1-1000")
  }

  // Validate but don't reject extra params
  const known = ['pattern', 'filter', 'max']
  const extra = Object.keys(params).filter(k => !known.includes(k))

  return {valid: true, warnings: extra.length > 0 ? [`Unknown: ${extra}`] : []}
}

Quick Reference

Tool design checklist:

• Clear input schema with required/optional marked
• Output schema documented
• Error codes and messages defined
• Accept extra parameters with warnings
• Progressive detail by relevance
• Token budget appropriate for tool type
• Automation flags included
• Next steps provided to user

Client guidance checklist:

• Unknown tool errors suggest similar tools
• Unknown params suggest correct names
• Missing required errors include schema hints
• Type errors show expected vs received
• Errors include actionable next_steps
• High-confidence typos can be auto-corrected

Token budgets by tool type:

• Query: 50-100 tokens
• Lookup: 150-250 tokens
• Analysis: 400-600 tokens
• Management: 50-150 tokens