# MCP protocol types

> Reference for the Go AI SDK MCP protocol types: JSON-RPC messages, capabilities, initialize and discover, tool, resource and prompt payloads, and logging.

Canonical URL: https://goaisdk.com/docs/reference/mcp/protocol-types
Documentation index: https://goaisdk.com/llms.txt

Types in `pkg/mcp` that model the protocol. Most code uses the [client](https://goaisdk.com/docs/reference/mcp/client.md) and never builds these directly. They matter when you write a transport or a server-side bridge.

## JSON-RPC messages

`mcp.MCPMessage` is a generic JSON-RPC 2.0 message.

{/* gen:fields mcp.MCPMessage */}

| Field | Type | Description |
| --- | --- | --- |
| `JSONRpc` | `string` |  |
| `ID` | `interface{}` |  |
| `Method` | `string` |  |
| `Params` | `json.RawMessage` |  |
| `Result` | `json.RawMessage` |  |
| `Error` | `*MCPError` |  |

{/* /gen:fields */}

| Function | Description |
| --- | --- |
| `mcp.CreateRequest(id, method, params) (*MCPMessage, error)` | Builds a request. |
| `mcp.CreateNotification(method, params) (*MCPMessage, error)` | Builds a notification (no ID). |
| `mcp.CreateResponse(id, result) (*MCPMessage, error)` | Builds a success response. |
| `mcp.CreateErrorResponse(id, code, message, data) *MCPMessage` | Builds an error response. |
| `mcp.ValidateJSONRPCMessage(raw []byte) (*MCPMessage, error)` | Checks that `raw` is a well-formed request, notification or response. |
| `mcp.IsRequest(msg)`, `mcp.IsNotification(msg)`, `mcp.IsResponse(msg)`, `mcp.IsError(msg)` | Classify a message. |
| `mcp.GetError(msg) error` | Returns the error from an error response. |
| `mcp.ParseParams(msg, target)`, `mcp.ParseResult(msg, target)` | Decode `params` or `result` into `target`. |
| `mcp.NewIDGenerator() *IDGenerator` | Creates a generator. `Next()` returns a new request ID. |

## Initialize and discover

| Type | Description |
| --- | --- |
| `mcp.InitializeParams` | Parameters of the `initialize` request. |
| `mcp.InitializeResult` | Result of `initialize`. |
| `mcp.ClientInfo`, `mcp.ServerInfo` | Name and version of each side. |
| `mcp.ClientCapabilities` | What the client supports. |
| `mcp.ServerCapabilities` | What the server supports. |
| `mcp.DiscoverResult` | Result of `server/discover`. |

{/* gen:fields mcp.ClientCapabilities */}

| Field | Type | Description |
| --- | --- | --- |
| `Experimental` | `map[string]interface{}` |  |
| `Extensions` | `map[string]interface{}` |  |
| `Roots` | `*RootsCapability` |  |
| `Sampling` | `*SamplingCapability` |  |
| `Elicitation` | `map[string]interface{}` |  |

{/* /gen:fields */}

{/* gen:fields mcp.ServerCapabilities */}

| Field | Type | Description |
| --- | --- | --- |
| `Experimental` | `map[string]interface{}` |  |
| `Logging` | `*LoggingCapability` |  |
| `Completions` | `*CompletionsCapability` | Completions advertises support for the completion/complete method (hash 68a739a). See (\*MCPClient).Complete. |
| `Prompts` | `*PromptsCapability` |  |
| `Resources` | `*ResourcesCapability` |  |
| `Tools` | `*ToolsCapability` |  |

{/* /gen:fields */}

The capability types are `mcp.RootsCapability`, `mcp.SamplingCapability`, `mcp.ToolsCapability`, `mcp.ResourcesCapability`, `mcp.PromptsCapability`, `mcp.LoggingCapability` and `mcp.CompletionsCapability`.

## Tools

{/* gen:fields mcp.MCPTool */}

| Field | Type | Description |
| --- | --- | --- |
| `Name` | `string` |  |
| `Title` | `string` |  |
| `Description` | `string` |  |
| `InputSchema` | `map[string]interface{}` |  |
| `OutputSchema` | `map[string]interface{}` |  |
| `Annotations` | `map[string]interface{}` |  |
| `Meta` | `map[string]interface{}` |  |

{/* /gen:fields */}

| Type | Description |
| --- | --- |
| `mcp.ListToolsParams`, `mcp.ListToolsResult` | Parameters and result of `tools/list`, including pagination. |
| `mcp.CallToolParams`, `mcp.CallToolResult` | Parameters and result of `tools/call`. |
| `mcp.ToolResultContent` | One content item in a tool result. |

{/* gen:fields mcp.CallToolResult */}

| Field | Type | Description |
| --- | --- | --- |
| `Content` | `[]ToolResultContent` |  |
| `StructuredContent` | `interface{}` |  |
| `ToolResult` | `interface{}` |  |
| `IsError` | `bool` |  |
| `Metadata` | `map[string]interface{}` |  |

{/* /gen:fields */}

## Resources, prompts and completions

See [MCP client](https://goaisdk.com/docs/reference/mcp/client.md#resources) for `MCPResource`, `MCPResourceTemplate`, `ReadResourceParams`, `ReadResourceResult`, `ResourceContent`, `MCPPrompt`, `PromptMessage`, `CompleteRequestParams` and related types.

## Logging

A client can set the server's log level and receive log messages.

{/* gen:consts mcp.LoggingLevel */}

| Constant | Value | Description |
| --- | --- | --- |
| `LoggingLevelDebug` | `"debug"` |  |
| `LoggingLevelInfo` | `"info"` |  |
| `LoggingLevelNotice` | `"notice"` |  |
| `LoggingLevelWarning` | `"warning"` |  |
| `LoggingLevelError` | `"error"` |  |
| `LoggingLevelCritical` | `"critical"` |  |
| `LoggingLevelAlert` | `"alert"` |  |
| `LoggingLevelEmergency` | `"emergency"` |  |

{/* /gen:consts */}

{/* gen:fields mcp.SetLevelParams */}

| Field | Type | Description |
| --- | --- | --- |
| `Level` | `LoggingLevel` |  |

{/* /gen:fields */}

{/* gen:fields mcp.LoggingMessageNotification */}

| Field | Type | Description |
| --- | --- | --- |
| `Level` | `LoggingLevel` |  |
| `Logger` | `string` |  |
| `Data` | `interface{}` |  |

{/* /gen:fields */}