The Agent Client Protocol provides built-in extension mechanisms that allow implementations to add custom functionality while maintaining compatibility with the core protocol. These mechanisms ensure that Agents and Clients can innovate without breaking interoperability.

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The _meta Field

All types in the protocol include a _meta field that implementations can use to attach custom information. This includes requests, responses, notifications, and even nested types like content blocks, tool calls, plan entries, and capability objects. 
{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "session/prompt",
  "params": {
    "sessionId": "sess_abc123def456",
    "prompt": [
      {
        "type": "text",
        "text": "Hello, world!"
      }
    ],
    "_meta": {
      "zed.dev/debugMode": true
    }
  }
}
Implementations MUST NOT add any custom fields at the root of a type that’s part of the specification. All possible names are reserved for future protocol versions.

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Extension Methods

The protocol reserves any method name starting with an underscore (_) for custom extensions. This allows implementations to add new functionality without the risk of conflicting with future protocol versions. Extension methods follow standard JSON-RPC 2.0 semantics:

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Custom Requests

In addition to the requests specified by the protocol, implementations MAY expose and call custom JSON-RPC requests as long as their name starts with an underscore (_). 
{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "_zed.dev/workspace/buffers",
  "params": {
    "language": "rust"
  }
}
Upon receiving a custom request, implementations MUST respond accordingly with the provided id: 
{
  "jsonrpc": "2.0",
  "id": 1,
  "result": {
    "buffers": [
      { "id": 0, "path": "/home/user/project/src/main.rs" },
      { "id": 1, "path": "/home/user/project/src/editor.rs" }
    ]
  }
}
If the receiving end doesn’t recognize the custom method name, it should respond with the standard “Method not found” error: 
{
  "jsonrpc": "2.0",
  "id": 1,
  "error": {
    "code": -32601,
    "message": "Method not found"
  }
}
To avoid such cases, extensions SHOULD advertise their custom capabilities so that callers can check their availability first and adapt their behavior or interface accordingly.

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Custom Notifications

Custom notifications are regular JSON-RPC notifications that start with an underscore (_). Like all notifications, they omit the id field: 
{
  "jsonrpc": "2.0",
  "method": "_zed.dev/file_opened",
  "params": {
    "path": "/home/user/project/src/editor.rs"
  }
}
Unlike with custom requests, implementations SHOULD ignore unrecognized notifications.

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Advertising Custom Capabilities

Implementations SHOULD use the _meta field in capability objects to advertise support for extensions and their methods: 
{
  "jsonrpc": "2.0",
  "id": 0,
  "result": {
    "protocolVersion": 1,
    "agentCapabilities": {
      "loadSession": true,
      "_meta": {
        "zed.dev": {
          "workspace": true,
          "fileNotifications": true
        }
      }
    }
  }
}
This allows implementations to negotiate custom features during initialization without breaking compatibility with standard Clients and Agents.