Serialization

Tool and resource methods can return virtually any Delphi type. MCPConnect picks the most appropriate serialization strategy automatically based on the return type.

How Return Values Are Serialized

Return type	Strategy
string, Integer, Boolean, Double, …	Wrapped directly into a text content item — no further conversion.
TStream, TBytes	Encoded as base64 and returned as a binary blob.
TPicture	Encoded as a PNG image content item (requires TMCPPictureWriter).
Any Delphi class or generic list	Serialized to JSON via Neon and returned as a text content item.
TContentList	Passed through unchanged — gives you full control over the output.

Primitive types

Simple scalar values are converted to their string representation and returned as a single text content item:

[McpTool('double_or_nothing', 'Doubles or zeroes the input')]
function TestParam(
  [McpParam('value', 'The value')] AValue: Int64;
  [McpParam('double', 'Whether to double')] ADouble: Boolean
): Integer;

Delphi objects and collections

Any class or generic object list is serialized to JSON by Neon. No extra setup is needed — just return the object:

type
  TTicket = class
  private
    FId: Integer;
    FTitle: string;
    FPrice: Currency;
  public
    property Id: Integer read FId write FId;
    property Title: string read FTitle write FTitle;
    property Price: Currency read FPrice write FPrice;
  end;

  TTickets = class(TObjectList<TTicket>);

// Tool method — MCPConnect serializes TTickets to a JSON array
[McpTool('get_tickets', 'Get available tickets')]
function GetTickets: TTickets;

The LLM receives a JSON array with one object per ticket.

Objects returned from tool methods are owned and freed by MCPConnect. If you need the object to outlive the call (e.g., it is owned by a session), use the [Context] IGarbageCollector helper to transfer ownership. See the Tools chapter for details.

TContentList — Full Control

When you need to include multiple content items, mix content types (text + image, text + audio, …), or produce binary output, return a TContentList built with TToolResultBuilder:

uses
  MCPConnect.MCP.Types;

[McpTool('buy_ticket', 'Purchase a ticket and return confirmation')]
function BuyTicket(
  [McpParam('id', 'Ticket ID')] AId: Integer
): TContentList;

function TMyTool.BuyTicket(AId: Integer): TContentList;
var
  LBuilder: IToolResultBuilder;
  LStream: TFileStream;
begin
  LBuilder := TToolResultBuilder.CreateInstance;

  LBuilder.AddText('Purchase completed successfully.');

  LStream := TFileStream.Create('ticket.png', fmOpenRead or fmShareDenyWrite);
  try
    LBuilder.AddImage('image/png', LStream);
  finally
    LStream.Free;
  end;

  Result := LBuilder.Build;
end;

TToolResultBuilder methods

Method	Description
AddText(text)	Adds a plain-text content item
AddImage(mime, stream)	Encodes a stream as base64 and adds an image content item
AddAudio(mime, stream)	Encodes a stream as base64 and adds an audio content item
AddBlob(mime, stream)	Encodes a stream as base64 and adds an embedded binary resource
AddLink(mime, uri, description)	Adds a resource link (external URI reference)
Build	Returns the assembled TContentList

Configuring Neon

Neon is the JSON serialization library used internally by MCPConnect. By default it serializes public fields using camelCase names. You can override this behavior through the IJRPCNeonConfig plugin.

Add MCPConnect.Configuration.Neon and Neon.Core.Persistence to the uses clause:

uses
  Neon.Core.Persistence,
  MCPConnect.Configuration.Neon;

Then build an INeonConfiguration and pass it to the plugin:

var
  LNeonConfig: INeonConfiguration;
begin
  LNeonConfig := TNeonConfiguration.Default;
  LNeonConfig
    .SetMemberCase(TNeonCase.CamelCase)
    .SetVisibility([mvPublic, mvPublished]);

  FJRPCServer
    .Plugin.Configure<IJRPCNeonConfig>
      .SetNeonConfig(LNeonConfig)
    .ApplyConfig;
end;

Common Neon options

Option	Example	Effect
SetMemberCase	TNeonCase.CamelCase	FirstName → "firstName"
SetMemberCase	TNeonCase.PascalCase	FirstName → "FirstName"
SetMemberCase	TNeonCase.SnakeCase	FirstName → "first_name"
SetVisibility	[mvPublic, mvPublished]	Which member visibility levels are serialized
GetSerializers.RegisterSerializer	TMySerializer	Plug in a custom type serializer

Refer to the Neon documentation for the full list of configuration options and how to write custom serializers.