Schemas & Structured Data

JSON-RPC 2.0 message types

All MCP data-layer messages are JSON-RPC 2.0:

Type	Has `id`?	Expects response?	Example
Request	Yes	Yes	`tools/call`
Response	Yes (matches request)	—	result or error
Notification	No	No	`notifications/tools/list_changed`

sequenceDiagram
participant C as Client
participant S as Server
C->>S: Request tools/call (id 1)
S-->>C: Response (id 1, result or error)
S-)C: Notification tools/list_changed (no id, no reply)

Message types in action

Tool definitions use JSON Schema

Each tool declares a required inputSchema (and optionally an outputSchema) using JSON Schema — the 2020-12 dialect by default — so the model knows exactly what arguments are valid:

{
  "name": "get_weather",
  "title": "Weather Information Provider",
  "description": "Get current weather for a location",
  "inputSchema": {
    "type": "object",
    "properties": {
      "location": {
        "type": "string",
        "description": "City name or zip code"
      }
    },
    "required": ["location"]
  }
}

Other tool definition fields include optional icons and annotations (behavior hints such as readOnlyHint — clients must treat these as untrusted unless they come from a trusted server).

Why schemas matter

The model uses the schema + description to decide when and how to call a tool
The client/host can validate arguments before sending them
The server validates inputs again on receipt (never trust the caller)
Structured output (structuredContent conforming to the tool’s outputSchema) lets tools return machine-readable results

Resources and structured data

Resources are identified by URIs following RFC 3986 (e.g. file:///path/to/doc.md; custom schemes allowed)
Each resource has metadata: uri, name, title, description, mimeType, size
Resource contents can be text (text field) or binary (blob, base64-encoded)

Key exam points

MCP uses JSON-RPC 2.0 as its message format and JSON Schema for tool parameters
Notifications have no id and never receive a response
Tool descriptions are part of the contract — the model relies on them to select tools

Check your knowledge

📝 Check your knowledge

1. What field distinguishes a JSON-RPC request from a notification?

2. Which tool definition field is required and describes valid arguments?

3. What JSON Schema dialect do MCP tool schemas use by default?

4. How are MCP resources uniquely identified?

5. How should a client treat tool annotations like readOnlyHint?