Resources

ℹ️

Protocol Revision: 2024-11-05

The Model Context Protocol (MCP) provides a standardized way for servers to expose resources to clients. Resources allow servers to share data that provides context to language models, such as files, database schemas, or application-specific information. Each resource is uniquely identified by a URI.

User Interaction Model

Resources in MCP are designed to be application-driven, with host applications determining how to incorporate context based on their needs.

For example, applications could:

Expose resources through UI elements for explicit selection, in a tree or list view
Allow the user to search through and filter available resources
Implement automatic context inclusion, based on heuristics or the AI model’s selection

Example of resource context picker

However, implementations are free to expose resources through any interface pattern that suits their needs—the protocol itself does not mandate any specific user interaction model.

Capabilities

Servers that support resources MUST declare the resources capability:

{
  "capabilities": {
    "resources": {
      "subscribe": true,
      "listChanged": true
    }
  }
}

The capability supports two optional features:

subscribe: whether the client can subscribe to be notified of changes to individual resources.
listChanged: whether the server will emit notifications when the list of available resources changes.

Both subscribe and listChanged are optional—servers can support neither, either, or both:

{
  "capabilities": {
    "resources": {}  // Neither feature supported
  }
}

{
  "capabilities": {
    "resources": {
      "subscribe": true  // Only subscriptions supported
    }
  }
}

{
  "capabilities": {
    "resources": {
      "listChanged": true  // Only list change notifications supported
    }
  }
}

Protocol Messages

Listing Resources

To discover available resources, clients send a resources/list request. This operation supports pagination.

Request:

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

Response:

{
  "jsonrpc": "2.0",
  "id": 1,
  "result": {
    "resources": [
      {
        "uri": "file:///project/src/main.rs",
        "name": "main.rs",
        "description": "Primary application entry point",
        "mimeType": "text/x-rust"
      }
    ],
    "nextCursor": "next-page-cursor"
  }
}

Reading Resources

To retrieve resource contents, clients send a resources/read request:

Request:

{
  "jsonrpc": "2.0",
  "id": 2,
  "method": "resources/read",
  "params": {
    "uri": "file:///project/src/main.rs"
  }
}

Response:

{
  "jsonrpc": "2.0",
  "id": 2,
  "result": {
    "contents": [
      {
        "uri": "file:///project/src/main.rs",
        "mimeType": "text/x-rust",
        "text": "fn main() {\n    println!(\"Hello world!\");\n}"
      }
    ]
  }
}

Resource Templates

Resource templates allow servers to expose parameterized resources using URI templates. Arguments may be auto-completed through the completion API.

Request:

{
  "jsonrpc": "2.0",
  "id": 3,
  "method": "resources/templates/list"
}

Response:

{
  "jsonrpc": "2.0",
  "id": 3,
  "result": {
    "resourceTemplates": [
      {
        "uriTemplate": "file:///{path}",
        "name": "Project Files",
        "description": "Access files in the project directory",
        "mimeType": "application/octet-stream"
      }
    ]
  }
}

List Changed Notification

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

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

Subscriptions

The protocol supports optional subscriptions to resource changes. Clients can subscribe to specific resources and receive notifications when they change:

Subscribe Request:

{
  "jsonrpc": "2.0",
  "id": 4,
  "method": "resources/subscribe",
  "params": {
    "uri": "file:///project/src/main.rs"
  }
}

Update Notification:

{
  "jsonrpc": "2.0",
  "method": "notifications/resources/updated",
  "params": {
    "uri": "file:///project/src/main.rs"
  }
}

Message Flow

sequenceDiagram
    participant Client
    participant Server

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

    Note over Client,Server: Resource Access
    Client->>Server: resources/read
    Server-->>Client: Resource contents

    Note over Client,Server: Subscriptions
    Client->>Server: resources/subscribe
    Server-->>Client: Subscription confirmed

    Note over Client,Server: Updates
    Server--)Client: notifications/resources/updated
    Client->>Server: resources/read
    Server-->>Client: Updated contents

Data Types

Resource

A resource definition includes:

uri: Unique identifier for the resource
name: Human-readable name
description: Optional description
mimeType: Optional MIME type

Resource Contents

Resources can contain either text or binary data:

Text Content

{
  "uri": "file:///example.txt",
  "mimeType": "text/plain",
  "text": "Resource content"
}

Binary Content

{
  "uri": "file:///example.png",
  "mimeType": "image/png",
  "blob": "base64-encoded-data"
}

Common URI Schemes

The protocol defines several standard URI schemes. This list not exhaustive—implementations are always free to use additional, custom URI schemes.

https://

Used to represent a resource available on the web.

Servers SHOULD use this scheme only when the client is able to fetch and load the resource directly from the web on its own—that is, it doesn’t need to read the resource via the MCP server.

For other use cases, servers SHOULD prefer to use another URI scheme, or define a custom one, even if the server will itself be downloading resource contents over the internet.

file://

Used to identify resources that behave like a filesystem. However, the resources do not need to map to an actual physical filesystem.

MCP servers MAY identify file:// resources with an XDG MIME type, like inode/directory, to represent non-regular files (such as directories) that don’t otherwise have a standard MIME type.

git://

Git version control integration.

Error Handling

Servers SHOULD return standard JSON-RPC errors for common failure cases:

Resource not found: -32002
Internal errors: -32603

Example error:

{
  "jsonrpc": "2.0",
  "id": 5,
  "error": {
    "code": -32002,
    "message": "Resource not found",
    "data": {
      "uri": "file:///nonexistent.txt"
    }
  }
}

Security Considerations

Servers MUST validate all resource URIs
Access controls SHOULD be implemented for sensitive resources
Binary data MUST be properly encoded
Resource permissions SHOULD be checked before operations

Prompts Tools