Tools

specification/2025-06-18/server/tools New page · 436 lines, new page

# Tools ## User Interaction Model ## Capabilities ## Protocol Messages ### Listing Tools ### Calling Tools ### List Changed Notification ## Message Flow ## Data Types ### Tool ### Tool Result #### Text Content #### Image Content #### Audio Content #### Resource Links #### Embedded Resources #### Structured Content #### Output Schema ## Error Handling ## Security Considerations

A whole new page. There's nothing to diff it against, so here is what it says.

# Tools

<div id="enable-section-numbers" />

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.

<Warning>
  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
</Warning>

## Capabilities

Servers that support tools **MUST** declare the `tools` capability:

```json theme={null}
{
  "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](/specification/2025-06-18/server/utilities/pagination).

**Request:**

```json theme={null}
{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "tools/list",
  "params": {
    "cursor": "optional-cursor-value"
  }
}
```

**Response:**

```json theme={null}
{
  "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"]
        }
      }
    ],
    "nextCursor": "next-page-cursor"
  }
}
```

### Calling Tools

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

**Request:**

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

**Response:**

```json theme={null}
{
  "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:

```json theme={null}
{
  "jsonrpc": "2.0",
  "method": "notifications/tools/list_changed"
}
```

## Message Flow

```mermaid theme={null}
sequenceDiagram
    participant LLM
    participant Client
    participant Server

    Note over Client,Server: Discovery
    Client->>Server: tools/list
    Server-->>Client: List of tools

    Note over Client,LLM: Tool Selection
    LLM->>Client: Select tool to use

    Note over Client,Server: Invocation
    Client->>Server: tools/call
    Server-->>Client: Tool result
    Client->>LLM: Process result

    Note over Client,Server: Updates
    Server--)Client: tools/list_changed
    Client->>Server: tools/list
    Server-->>Client: Updated tools
```

## 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
* `outputSchema`: Optional JSON Schema defining expected output structure
* `annotations`: optional properties describing tool behavior

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

### Tool Result

Tool results may contain [**structured**](#structured-content) or **unstructured** content.

**Unstructured** content is returned in the `content` field of a result, and can contain multiple content items of different types:

<Note>
  All content types (text, image, audio, resource links, and embedded resources)
  support optional
  [annotations](/specification/2025-06-18/server/resources#annotations) that
  provide metadata about audience, priority, and modification times. This is the
  same annotation format used by resources and prompts.
</Note>

#### Text Content

```json theme={null}
{
  "type": "text",
  "text": "Tool result text"
}
```

#### Image Content

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

}
```

This example demonstrates the use of an optional Annotation.

#### Audio Content

```json theme={null}
{
  "type": "audio",
  "data": "base64-encoded-audio-data",
  "mimeType": "audio/wav"
}
```

#### Resource Links

A tool **MAY** return links to [Resources](/specification/2025-06-18/server/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:

```json theme={null}
{
  "type": "resource_link",
  "uri": "file:///project/src/main.rs",
  "name": "main.rs",
  "description": "Primary application entry point",
  "mimeType": "text/x-rust",
  "annotations": {
    "audience": ["assistant"],
    "priority": 0.9
  }
}
```

Resource links support the same [Resource annotations](/specification/2025-06-18/server/resources#annotations) as regular resources to help clients understand how to use them.

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

#### Embedded Resources

[Resources](/specification/2025-06-18/server/resources) **MAY** be embedded to provide additional context
or data using a suitable [URI scheme](./resources#common-uri-schemes). Servers that use embedded resources **SHOULD** implement the `resources` capability:

```json theme={null}
{
  "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](/specification/2025-06-18/server/resources#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.

<Note>
  `structuredContent` is server-produced result data and is unrelated to LLM
  "structured outputs" (schema-constrained model generation).
</Note>

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