Tools

Protocol Revision: draft

The Model Context Protocol (MCP) allows servers to expose tools that can be invoked by language models. Tools enable models to interact with external systems, such as querying databases, calling APIs, or performing computations. Each tool is uniquely identified by a name and includes metadata describing its schema.

User Interaction Model

Tools in MCP are designed to be model-controlled, meaning that the language model can discover and invoke tools automatically based on its contextual understanding and the user’s prompts. However, implementations are free to expose tools through any interface pattern that suits their needs—the protocol itself does not mandate any specific user interaction model.

For trust & safety and security, there SHOULD always be a human in the loop with the ability to deny tool invocations.Applications SHOULD:

Provide UI that makes clear which tools are being exposed to the AI model
Insert clear visual indicators when tools are invoked
Present confirmation prompts to the user for operations, to ensure a human is in the loop

Capabilities

Servers that support tools MUST declare the tools capability:

{
  "capabilities": {
    "tools": {
      "listChanged": true
    }
  }
}

listChanged indicates whether the server will emit notifications when the list of available tools changes.

Protocol Messages

Listing Tools

To discover available tools, clients send a tools/list request. This operation supports pagination. Request:

{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "tools/list",
  "params": {
    "cursor": "optional-cursor-value"
  }
}

Response:

{
  "jsonrpc": "2.0",
  "id": 1,
  "result": {
    "tools": [
      {
        "name": "get_weather",
        "title": "Weather Information Provider",
        "description": "Get current weather information for a location",
        "inputSchema": {
          "type": "object",
          "properties": {
            "location": {
              "type": "string",
              "description": "City name or zip code"
            }
          },
          "required": ["location"]
        },
        "icons": [
          {
            "src": "https://example.com/weather-icon.png",
            "mimeType": "image/png",
            "sizes": ["48x48"]
          }
        ]
      }
    ],
    "nextCursor": "next-page-cursor"
  }
}

Calling Tools

To invoke a tool, clients send a tools/call request: Request:

{
  "jsonrpc": "2.0",
  "id": 2,
  "method": "tools/call",
  "params": {
    "name": "get_weather",
    "arguments": {
      "location": "New York"
    }
  }
}

Response:

{
  "jsonrpc": "2.0",
  "id": 2,
  "result": {
    "content": [
      {
        "type": "text",
        "text": "Current weather in New York:\nTemperature: 72°F\nConditions: Partly cloudy"
      }
    ],
    "isError": false
  }
}

List Changed Notification

When the list of available tools changes, servers that declared the listChanged capability SHOULD send a notification:

{
  "jsonrpc": "2.0",
  "method": "notifications/tools/list_changed"
}

Message Flow

Data Types

Tool

A tool definition includes:

name: Unique identifier for the tool
title: Optional human-readable name of the tool for display purposes.
description: Human-readable description of functionality
inputSchema: JSON Schema defining expected parameters
- Follows the JSON Schema usage guidelines
- Defaults to 2020-12 if no $schema field is present
- MUST be a valid JSON Schema object (not null)
- For tools with no parameters, use one of these valid approaches:
  - { "type": "object", "additionalProperties": false } - Recommended: explicitly accepts only empty objects
  - { "type": "object" } - accepts any object (including with properties)
outputSchema: Optional JSON Schema defining expected output structure
- Follows the JSON Schema usage guidelines
- Defaults to 2020-12 if no $schema field is present
annotations: optional properties describing tool behavior

For trust & safety and security, clients MUST consider tool annotations to be untrusted unless they come from trusted servers.

Tool Names

Tool names SHOULD be between 1 and 128 characters in length (inclusive).
Tool names SHOULD be considered case-sensitive.
The following SHOULD be the only allowed characters: uppercase and lowercase ASCII letters (A-Z, a-z), digits (0-9), underscore (_), dash (-), and dot (.)
Tool names SHOULD NOT contain spaces, commas, or other special characters.
Tool names SHOULD be unique within a server.
Example valid tool names:
- getUser
- DATA_EXPORT_v2
- admin.tools.list

Tool Result

Tool results may contain structured or unstructured content. Unstructured content is returned in the content field of a result, and can contain multiple content items of different types:

All content types (text, image, audio, resource links, and embedded resources) support optional annotations that provide metadata about audience, priority, and modification times. This is the same annotation format used by resources and prompts.

Text Content

{
  "type": "text",
  "text": "Tool result text"
}

Image Content

{
  "type": "image",
  "data": "base64-encoded-data",
  "mimeType": "image/png",
  "annotations": {
    "audience": ["user"],
    "priority": 0.9
  }
}

Audio Content

{
  "type": "audio",
  "data": "base64-encoded-audio-data",
  "mimeType": "audio/wav"
}

Resource Links

A tool MAY return links to Resources, to provide additional context or data. In this case, the tool will return a URI that can be subscribed to or fetched by the client:

{
  "type": "resource_link",
  "uri": "file:///project/src/main.rs",
  "name": "main.rs",
  "description": "Primary application entry point",
  "mimeType": "text/x-rust"
}

Resource links support the same Resource annotations as regular resources to help clients understand how to use them.

Resource links returned by tools are not guaranteed to appear in the results of a resources/list request.

Embedded Resources

Resources MAY be embedded to provide additional context or data using a suitable URI scheme. Servers that use embedded resources SHOULD implement the resources capability:

{
  "type": "resource",
  "resource": {
    "uri": "file:///project/src/main.rs",
    "mimeType": "text/x-rust",
    "text": "fn main() {\n    println!(\"Hello world!\");\n}",
    "annotations": {
      "audience": ["user", "assistant"],
      "priority": 0.7,
      "lastModified": "2025-05-03T14:30:00Z"
    }
  }
}

Embedded resources support the same Resource annotations as regular resources to help clients understand how to use them.

Structured Content

Structured content is returned as a JSON object in the structuredContent field of a result. For backwards compatibility, a tool that returns structured content SHOULD also return the serialized JSON in a TextContent block.

Output Schema

Tools may also provide an output schema for validation of structured results. If an output schema is provided:

Servers MUST provide structured results that conform to this schema.
Clients SHOULD validate structured results against this schema.

Example tool with output schema:

{
  "name": "get_weather_data",
  "title": "Weather Data Retriever",
  "description": "Get current weather data for a location",
  "inputSchema": {
    "type": "object",
    "properties": {
      "location": {
        "type": "string",
        "description": "City name or zip code"
      }
    },
    "required": ["location"]
  },
  "outputSchema": {
    "type": "object",
    "properties": {
      "temperature": {
        "type": "number",
        "description": "Temperature in celsius"
      },
      "conditions": {
        "type": "string",
        "description": "Weather conditions description"
      },
      "humidity": {
        "type": "number",
        "description": "Humidity percentage"
      }
    },
    "required": ["temperature", "conditions", "humidity"]
  }
}

Example valid response for this tool:

{
  "jsonrpc": "2.0",
  "id": 5,
  "result": {
    "content": [
      {
        "type": "text",
        "text": "{\"temperature\": 22.5, \"conditions\": \"Partly cloudy\", \"humidity\": 65}"
      }
    ],
    "structuredContent": {
      "temperature": 22.5,
      "conditions": "Partly cloudy",
      "humidity": 65
    }
  }
}

Providing an output schema helps clients and LLMs understand and properly handle structured tool outputs by:

Enabling strict schema validation of responses
Providing type information for better integration with programming languages
Guiding clients and LLMs to properly parse and utilize the returned data
Supporting better documentation and developer experience

Schema Examples

Tool with default 2020-12 schema:

{
  "name": "calculate_sum",
  "description": "Add two numbers",
  "inputSchema": {
    "type": "object",
    "properties": {
      "a": { "type": "number" },
      "b": { "type": "number" }
    },
    "required": ["a", "b"]
  }
}

Tool with explicit draft-07 schema:

{
  "name": "calculate_sum",
  "description": "Add two numbers",
  "inputSchema": {
    "$schema": "http://json-schema.org/draft-07/schema#",
    "type": "object",
    "properties": {
      "a": { "type": "number" },
      "b": { "type": "number" }
    },
    "required": ["a", "b"]
  }
}

Tool with no parameters:

{
  "name": "get_current_time",
  "description": "Returns the current server time",
  "inputSchema": {
    "type": "object",
    "additionalProperties": false
  }
}

Error Handling

Tools use two error reporting mechanisms:

Protocol Errors: Standard JSON-RPC errors for issues like:
- Unknown tools
- Malformed requests (requests that fail to satisfy CallToolRequest schema)
- Server errors
Tool Execution Errors: Reported in tool results with isError: true:
- API failures
- Input validation errors (e.g., date in wrong format, value out of range)
- Business logic errors

Tool Execution Errors contain actionable feedback that language models can use to self-correct and retry with adjusted parameters. Protocol Errors indicate issues with the request structure itself that models are less likely to be able to fix. Clients SHOULD provide tool execution errors to language models to enable self-correction. Clients MAY provide protocol errors to language models, though these are less likely to result in successful recovery. Example protocol error:

{
  "jsonrpc": "2.0",
  "id": 3,
  "error": {
    "code": -32602,
    "message": "Unknown tool: invalid_tool_name"
  }
}

Example tool execution error (input validation):

{
  "jsonrpc": "2.0",
  "id": 4,
  "result": {
    "content": [
      {
        "type": "text",
        "text": "Invalid departure date: must be in the future. Current date is 08/08/2025."
      }
    ],
    "isError": true
  }
}

Security Considerations

Servers MUST:
- Validate all tool inputs
- Implement proper access controls
- Rate limit tool invocations
- Sanitize tool outputs
Clients SHOULD:
- Prompt for user confirmation on sensitive operations
- Show tool inputs to the user before calling the server, to avoid malicious or accidental data exfiltration
- Validate tool results before passing to LLM
- Implement timeouts for tool calls
- Log tool usage for audit purposes

Base Protocol

Client Features

Server Features

User Interaction Model

Capabilities

Protocol Messages

Listing Tools

Calling Tools

List Changed Notification

Message Flow

Data Types

Tool

Tool Names

Tool Result

Text Content

Image Content

Audio Content

Resource Links

Embedded Resources

Structured Content

Output Schema

Schema Examples

Tool with default 2020-12 schema:

Tool with explicit draft-07 schema:

Tool with no parameters:

Error Handling

Security Considerations

Base Protocol

Client Features

Server Features

​User Interaction Model

​Capabilities

​Protocol Messages

​Listing Tools

​Calling Tools

​List Changed Notification

​Message Flow

​Data Types

​Tool

​Tool Names

​Tool Result

​Text Content

​Image Content

​Audio Content

​Resource Links

​Embedded Resources

​Structured Content

​Output Schema

​Schema Examples

​Tool with default 2020-12 schema:

​Tool with explicit draft-07 schema:

​Tool with no parameters:

​Error Handling

​Security Considerations

User Interaction Model

Capabilities

Protocol Messages

Listing Tools

Calling Tools

List Changed Notification

Message Flow

Data Types

Tool

Tool Names

Tool Result

Text Content

Image Content

Audio Content

Resource Links

Embedded Resources

Structured Content

Output Schema

Schema Examples

Tool with default 2020-12 schema:

Tool with explicit draft-07 schema:

Tool with no parameters:

Error Handling

Security Considerations