WebMCP Tool Schema Design

Before writing code

Fetch live docs:

1. Fetch https://webmachinelearning.github.io/webmcp/ for the inputSchema specification and any constraints
2. Web-search webmcp inputSchema JSON Schema requirements for schema format details
3. Web-search JSON Schema draft specification for the JSON Schema version used by WebMCP
4. Web-search webmcp tool schema best practices for community guidance

Conceptual Architecture

Role of Schemas in WebMCP

The inputSchema in a tool definition serves two purposes:

1. Agent guidance — The agent reads the schema to understand what parameters to provide
2. Input validation — The browser validates agent-provided input against the schema before calling execute

Schema Structure

WebMCP uses standard JSON Schema. Each tool's inputSchema is a JSON Schema object:

inputSchema: {
  type: "object",
  properties: {
    paramName: { type: "string", description: "What this parameter is" },
    // ...more properties
  },
  required: ["paramName"]
}

Property Types

Description Annotations

Always add description to properties — agents use these to decide what values to provide:

properties: {
  query: {
    type: "string",
    description: "Search keywords for product catalog"
  },
  maxPrice: {
    type: "number",
    description: "Maximum price in USD. Omit to search all prices."
  }
}

Constraints

Use JSON Schema constraints to guide agent input:

• minLength / maxLength for strings
• minimum / maximum for numbers
• enum for fixed value sets
• pattern for regex validation
• minItems / maxItems for arrays
• default for optional parameters with defaults

Commerce Schema Patterns

Product search:

{
  type: "object",
  properties: {
    query: { type: "string", description: "Search keywords" },
    category: { type: "string", enum: ["electronics", "clothing", "home"] },
    maxPrice: { type: "number", minimum: 0 },
    sortBy: { type: "string", enum: ["relevance", "price_asc", "price_desc", "rating"] }
  },
  required: ["query"]
}

Add to cart:

{
  type: "object",
  properties: {
    productId: { type: "string", description: "Product identifier" },
    quantity: { type: "integer", minimum: 1, maximum: 99, default: 1 },
    variant: { type: "string", description: "Size, color, or variant ID" }
  },
  required: ["productId"]
}

Shipping address:

{
  type: "object",
  properties: {
    street: { type: "string" },
    city: { type: "string" },
    state: { type: "string" },
    zipCode: { type: "string", pattern: "^[0-9]{5}(-[0-9]{4})?$" },
    country: { type: "string", enum: ["US", "CA", "UK"] }
  },
  required: ["street", "city", "state", "zipCode", "country"]
}

Output Conventions

While WebMCP doesn't formally specify an output schema, tools should return predictable JSON:

• Always include a status or success field
• Return structured data, not HTML or plain text
• Include pagination info for list results (total, page, pageSize)
• Return error objects with code and message for failures

Schema Design Principles

1. Minimal surface — Only require parameters the tool actually needs
2. Descriptive — Every property should have a description
3. Constrained — Use enum, minimum, maximum to prevent invalid inputs
4. Flat when possible — Avoid deep nesting; agents handle flat schemas better
5. Consistent — Use the same naming conventions across all tools (camelCase)
6. No sensitive data — Never ask for passwords, SSNs, or full card numbers in schemas

Fetch the WebMCP spec for the exact JSON Schema version supported and any WebMCP-specific constraints before designing schemas.