ruby-sdk/lib/mcp/resource.rb
Yuki Minamiya 91ff179d78 Add size field to MCP::Resource per MCP specification
## Motivation and Context

The MCP specification defines an optional `size` field on the `Resource`
type, added in spec revision 2025-06-18 and retained in 2025-11-25:
https://modelcontextprotocol.io/specification/2025-11-25/server/resources

> The size of the raw resource content, in bytes (i.e., before base64
> encoding or any tokenization), if known. This can be used by Hosts to
> display file sizes and estimate context window usage.

`MCP::Resource` was missing this field, so servers had no way to declare a
resource's byte size. Without it, hosts cannot show file sizes or estimate
context window usage before reading the resource contents.

This aligns the Ruby SDK with the spec, matching the TypeScript SDK
(modelcontextprotocol/typescript-sdk#1574).

## How Has This Been Tested?

Added tests in `test/mcp/resource_test.rb`:

- `#to_h` omits `size` when nil
- `#to_h` includes `size` when present
- `#to_h` includes `size` when zero (distinguishes an unset size from a
  0-byte resource)

The full unit suite and RuboCop pass locally.

## Breaking Changes

None. `size:` is a new optional keyword argument defaulting to `nil`, and it
is omitted from `#to_h` output when not set, so existing callers are unaffected.
2026-06-07 21:10:41 +09:00

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Ruby

# frozen_string_literal: true
require_relative "resource/contents"
require_relative "resource/embedded"
module MCP
class Resource
attr_reader :uri, :name, :title, :description, :icons, :mime_type, :size, :meta
def initialize(uri:, name:, title: nil, description: nil, icons: [], mime_type: nil, size: nil, meta: nil)
@uri = uri
@name = name
@title = title
@description = description
@icons = icons
@mime_type = mime_type
@size = size
@meta = meta
end
def to_h
{
uri: uri,
name: name,
title: title,
description: description,
icons: icons&.then { |icons| icons.empty? ? nil : icons.map(&:to_h) },
mimeType: mime_type,
size: size,
_meta: meta,
}.compact
end
end
end