MCP protocol types

Types in pkg/mcp that model the protocol. Most code uses the client 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.

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

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.

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

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

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

Tools

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{}

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.

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

Resources, prompts and completions

See MCP client 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.

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

Field	Type	Description
Level	LoggingLevel

Field	Type	Description
Level	LoggingLevel
Logger	string
Data	interface{}