Method CreateMessage

mcp/server.go:1186–1217

CreateMessage sends a sampling request to the client. If the client returns multiple content blocks (e.g. parallel tool calls), CreateMessage returns an error. Use [ServerSession.CreateMessageWithTools] for tool-enabled sampling.

(ctx context.Context, params *CreateMessageParams)

// CreateMessage returns an error. Use [ServerSession.CreateMessageWithTools]
// for tool-enabled sampling.
func (ss *ServerSession) CreateMessage(ctx context.Context, params *CreateMessageParams) (*CreateMessageResult, error) {
	if err := ss.checkInitialized(methodCreateMessage); err != nil {
		return nil, err
	}
	if params == nil {
		params = &CreateMessageParams{Messages: []*SamplingMessage{}}
	}
	if params.Messages == nil {
		p2 := *params
		p2.Messages = []*SamplingMessage{} // avoid JSON "null"
		params = &p2
	}
	res, err := handleSend[*CreateMessageWithToolsResult](ctx, methodCreateMessage, newServerRequest(ss, orZero[Params](params)))
	if err != nil {
		return nil, err
	}
	// Downconvert to singular content.
	if len(res.Content) > 1 {
		return nil, fmt.Errorf("CreateMessage result has %d content blocks; use CreateMessageWithTools for multiple content", len(res.Content))
	}
	var content Content
	if len(res.Content) > 0 {
		content = res.Content[0]
	}
	return &CreateMessageResult{
		Meta:       res.Meta,
		Content:    content,
		Model:      res.Model,
		Role:       res.Role,
		StopReason: res.StopReason,
	}, nil
}

// CreateMessageWithTools sends a sampling request with tools to the client,
// returning a [CreateMessageWithToolsResult] that supports array content