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Validate the src and sizes of an icon against the specification

Browse source 
## Motivation and Context

`MCP::Icon` validated `theme` but accepted anything for `src` and `sizes`: `src:` defaulted to `nil`,
so `MCP::Icon.new.to_h` was `{}`, an icon with no source, and a `sizes` given as a String reached `to_h` unchanged
and serialized as a JSON string where the specification's `Icon` type requires an array of strings.
Nothing on the server side reported either; the server answered every request with 200 and the failure surfaced
in a client, with no reference to the field responsible. The TypeScript SDK types `sizes` as `string[]`
and `src` as required, and the Python SDK's model refuses such an icon at construction, so this SDK was
the one that let it through.

`src` is now a required keyword and must be a non-empty String, `sizes` must be `nil` or an Array of Strings,
and `mime_type` must be `nil` or a String, each refused with `ArgumentError` at construction the way `theme` already was.
The checks stop at the schema's types, as the reference SDKs do: the scheme of a URL and the `WxH` form of a size stay
the caller's to get right. A message names the class of the rejected value, never the value,
since `src` may carry a `data:` URI of any length. Coercing a String into a one-element Array was considered and rejected:
it would hide the caller's mistake that the class exists to surface. The `theme` check let `false` through its truthiness test
and serialized it; it now refuses everything but `nil`, `"light"`, and `"dark"`.

Fixes #562.

## How Has This Been Tested?

New tests in `test/mcp/icon_test.rb`. Against the previous library, an icon without `src` and an icon whose `sizes` is
a String are both constructed and serialized.

## Breaking Changes

`MCP::Icon.new` now raises `ArgumentError` where earlier releases produced an icon the specification's schema rejects:
`src:` is required and must be a non-empty String, so a call without it, or with `nil`, an empty String, or a non-String,
fails; `sizes:` must be `nil` or an Array of Strings, so a String, or an Array holding `nil` or a non-String, fails;
`mime_type:` must be `nil` or a String; `theme:` must be `nil`, `"light"`, or `"dark"`, so `false` fails where it used
to be serialized. Leaving the optional keywords out, or passing `nil` for them, is unchanged.
This commit is contained in:
Koichi ITO 2026-09-19 17:08:22 +09:00
commit 8173a38280
2 changed files with 122 additions and 9 deletions
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37
lib/mcp/icon.rb
View file

@ -1,13 +1,35 @@
# frozen_string_literal: true
module MCP
# An icon attached to a server, tool, prompt, or resource, per the specification's `Icon` type:
# https://modelcontextprotocol.io/specification/2026-07-28/basic#icons
#
# Each argument is checked against the schema's type, as the TypeScript and Python SDKs check theirs,
# so an icon that would serialize into something a client rejects fails here instead. What a value means
# is not judged: `src` may be any non-empty `String` (the schema types it as a URI, which an empty `String`
# is not, and the specification allows an HTTP/HTTPS URL or a `data:` URI), and a size may be any `String`
# (the specification expects `WxH` or `"any"`).
class Icon
SUPPORTED_THEMES = ["light", "dark"].freeze
attr_reader :mime_type, :sizes, :src, :theme
def initialize(mime_type: nil, sizes: nil, src: nil, theme: nil)
raise ArgumentError, 'The value of theme must specify "light" or "dark".' if theme && !SUPPORTED_THEMES.include?(theme)
def initialize(mime_type: nil, sizes: nil, src:, theme: nil)
unless src.is_a?(String) && !src.empty?
raise ArgumentError, "The value of src must be a non-empty String (got #{src.class})."
end
unless mime_type.nil? || mime_type.is_a?(String)
raise ArgumentError, "The value of mime_type must be a String (got #{mime_type.class})."
end
if (problem = sizes_problem(sizes))
raise ArgumentError, "The value of sizes must be an Array of Strings such as [\"48x48\"] or [\"any\"] (#{problem})."
end
unless theme.nil? || SUPPORTED_THEMES.include?(theme)
raise ArgumentError, 'The value of theme must specify "light" or "dark".'
end
@mime_type = mime_type
@sizes = sizes
@ -18,5 +40,16 @@ module MCP
def to_h
{ mimeType: mime_type, sizes: sizes, src: src, theme: theme }.compact
end
private
def sizes_problem(sizes)
return if sizes.nil?
return "got #{sizes.class}" unless sizes.is_a?(Array)
index = sizes.index { |size| !size.is_a?(String) }
"got #{sizes[index].class} inside the Array" if index
end
end
end

94
test/mcp/icon_test.rb
View file

@ -15,32 +15,112 @@ module MCP
assert_equal({ mimeType: "image/png", sizes: ["48x48", "96x96"], src: "https://example.com", theme: "light" }, icon.to_h)
end
def test_initialization_by_default
icon = Icon.new
def test_initialization_with_only_src
icon = Icon.new(src: "https://example.com/icon.png")
assert_nil(icon.mime_type)
assert_nil(icon.sizes)
assert_nil(icon.src)
assert_equal("https://example.com/icon.png", icon.src)
assert_nil(icon.theme)
assert_equal({}, icon.to_h)
assert_equal({ src: "https://example.com/icon.png" }, icon.to_h)
end
def test_src_is_required
exception = assert_raises(ArgumentError) do
Icon.new
end
assert_equal("missing keyword: :src", exception.message)
end
def test_src_rejects_nil_and_an_empty_string
[nil, ""].each do |src|
exception = assert_raises(ArgumentError) do
Icon.new(src: src)
end
assert_equal("The value of src must be a non-empty String (got #{src.class}).", exception.message)
end
end
def test_src_rejects_a_non_string
exception = assert_raises(ArgumentError) do
Icon.new(src: :icon)
end
assert_equal("The value of src must be a non-empty String (got Symbol).", exception.message)
end
def test_sizes_accepts_an_array_of_strings
[["48x48", "96x96"], ["any"], []].each do |sizes|
icon = Icon.new(sizes: sizes, src: "https://example.com/icon.png")
assert_equal(sizes, icon.to_h[:sizes])
end
end
# https://github.com/modelcontextprotocol/ruby-sdk/issues/562
def test_sizes_rejects_a_string
exception = assert_raises(ArgumentError) do
Icon.new(mime_type: "image/png", sizes: "51x51", src: "https://example.com/icon.png")
end
assert_equal(
'The value of sizes must be an Array of Strings such as ["48x48"] or ["any"] (got String).',
exception.message,
)
end
def test_sizes_rejects_an_array_holding_a_non_string
{
["48x48", 96] => "Integer",
[nil] => "NilClass",
["48x48", nil] => "NilClass",
[nil, 96] => "NilClass",
}.each do |sizes, offender|
exception = assert_raises(ArgumentError) do
Icon.new(sizes: sizes, src: "https://example.com/icon.png")
end
assert_equal(
"The value of sizes must be an Array of Strings such as [\"48x48\"] or [\"any\"] (got #{offender} inside the Array).",
exception.message,
)
end
end
def test_src_accepts_any_scheme_and_sizes_accept_any_string
string_subclass = Class.new(String)
icon = Icon.new(sizes: ["unconventional", +"48x48", string_subclass.new("any")], src: "custom:icon")
assert_equal({ sizes: ["unconventional", "48x48", "any"], src: "custom:icon" }, icon.to_h)
end
def test_mime_type_rejects_a_non_string
exception = assert_raises(ArgumentError) do
Icon.new(mime_type: :png, src: "https://example.com/icon.png")
end
assert_equal("The value of mime_type must be a String (got Symbol).", exception.message)
end
def test_valid_theme_for_light
assert_nothing_raised do
Icon.new(theme: "light")
Icon.new(src: "https://example.com/icon.png", theme: "light")
end
end
def test_valid_theme_for_dark
assert_nothing_raised do
Icon.new(theme: "dark")
Icon.new(src: "https://example.com/icon.png", theme: "dark")
end
end
def test_invalid_theme
exception = assert_raises(ArgumentError) do
Icon.new(theme: "unexpected")
Icon.new(src: "https://example.com/icon.png", theme: "unexpected")
end
assert_equal('The value of theme must specify "light" or "dark".', exception.message)
end
def test_theme_rejects_false
exception = assert_raises(ArgumentError) do
Icon.new(src: "https://example.com/icon.png", theme: false)
end
assert_equal('The value of theme must specify "light" or "dark".', exception.message)
end