Add support for _meta field on tool schema (#124)

* Add support for _meta field on tool schema

* use `meta` naming for `_meta` field on tool schema

* apply suggestions to fix `_meta` tests

Co-authored-by: Ateş Göral <ates@magnetiq.com>

* Update lib/mcp/tool.rb

Co-authored-by: Ateş Göral <ates@magnetiq.com>

* Update lib/mcp/tool.rb

Co-authored-by: Ateş Göral <ates@magnetiq.com>

---------

Co-authored-by: Topher Bullock <topherbullock@users.noreply.github.com>
Co-authored-by: Ateş Göral <ates@magnetiq.com>

lib/mcp/server.rb

@@ -96,8 +96,8 @@ module MCP
       end
     end
 
-    def define_tool(name: nil, title: nil, description: nil, input_schema: nil, annotations: nil, &block)
+    def define_tool(name: nil, title: nil, description: nil, input_schema: nil, annotations: nil, meta: nil, &block)
-      tool = Tool.define(name:, title:, description:, input_schema:, annotations:, &block)
+      tool = Tool.define(name:, title:, description:, input_schema:, annotations:, meta:, &block)
       @tools[tool.name_value] = tool
 
       validate!

lib/mcp/tool.rb

@@ -8,6 +8,7 @@ module MCP
       attr_reader :title_value
       attr_reader :description_value
       attr_reader :annotations_value
+      attr_reader :meta_value
 
       def call(*args, server_context: nil)
         raise NotImplementedError, "Subclasses must implement call"
@@ -21,6 +22,7 @@ module MCP
           inputSchema: input_schema_value.to_h,
           outputSchema: @output_schema_value&.to_h,
           annotations: annotations_value&.to_h,
+          _meta: meta_value,
         }.compact
       end
 
@@ -32,6 +34,7 @@ module MCP
         subclass.instance_variable_set(:@input_schema_value, nil)
         subclass.instance_variable_set(:@output_schema_value, nil)
         subclass.instance_variable_set(:@annotations_value, nil)
+        subclass.instance_variable_set(:@meta_value, nil)
       end
 
       def tool_name(value = NOT_SET)
@@ -90,6 +93,14 @@ module MCP
         end
       end
 
+      def meta(value = NOT_SET)
+        if value == NOT_SET
+          @meta_value
+        else
+          @meta_value = value
+        end
+      end
+
       def annotations(hash = NOT_SET)
         if hash == NOT_SET
           @annotations_value
@@ -98,12 +109,13 @@ module MCP
         end
       end
 
-      def define(name: nil, title: nil, description: nil, input_schema: nil, output_schema: nil, annotations: nil, &block)
+      def define(name: nil, title: nil, description: nil, input_schema: nil, output_schema: nil, meta: nil, annotations: nil, &block)
         Class.new(self) do
           tool_name name
           title title
           description description
           input_schema input_schema
+          meta meta
           output_schema output_schema
           self.annotations(annotations) if annotations
           define_singleton_method(:call, &block) if block

test/mcp/server_test.rb

@@ -10,6 +10,7 @@ module MCP
         name: "test_tool",
         title: "Test tool",
         description: "A test tool",
+        meta: { foo: "bar" },
       )
 
       @tool_that_raises = Tool.define(
@@ -195,6 +196,7 @@ module MCP
       assert_equal "Test tool", result[:tools][0][:title]
       assert_equal "A test tool", result[:tools][0][:description]
       assert_equal({ type: "object" }, result[:tools][0][:inputSchema])
+      assert_equal({ foo: "bar" }, result[:tools][0][:_meta])
       assert_instrumentation_data({ method: "tools/list" })
     end
 
@@ -211,6 +213,7 @@ module MCP
       assert_equal "test_tool", result[:tools][0][:name]
       assert_equal "Test tool", result[:tools][0][:title]
       assert_equal "A test tool", result[:tools][0][:description]
+      assert_equal({ foo: "bar" }, result[:tools][0][:_meta])
     end
 
     test "#tools_list_handler sets the tools/list handler" do
@@ -848,6 +851,7 @@ module MCP
         name: "defined_tool",
         description: "Defined tool",
         input_schema: { type: "object", properties: { message: { type: "string" } }, required: ["message"] },
+        meta: { foo: "bar" },
       ) do |message:|
         Tool::Response.new(message)
       end

test/mcp/tool_test.rb

@@ -15,6 +15,9 @@ module MCP
         read_only_hint: true,
         title: "Test Tool",
       )
+      meta(
+        foo: "bar",
+      )
 
       class << self
         def call(message:, server_context: nil)
@@ -52,6 +55,14 @@ module MCP
       assert_equal expected_annotations, tool.to_h[:annotations]
     end
 
+    test "#to_h includes meta when present" do
+      tool = TestTool
+      expected_meta = {
+        foo: "bar",
+      }
+      assert_equal expected_meta, tool.to_h[:_meta]
+    end
+
     test "#call invokes the tool block and returns the response" do
       tool = TestTool
       response = tool.call(message: "test")
@@ -152,6 +163,23 @@ module MCP
       assert_equal({ destructiveHint: true, idempotentHint: false, openWorldHint: true, readOnlyHint: true, title: "Mock Tool" }, tool.annotations_value.to_h)
     end
 
+    test ".define allows definition of tools with meta" do
+      tool = Tool.define(
+        name: "mock_tool",
+        title: "Mock Tool",
+        description: "a mock tool for testing",
+        meta: { foo: "bar" },
+      ) do |_|
+        Tool::Response.new([{ type: "text", content: "OK" }])
+      end
+
+      assert_equal "mock_tool", tool.name_value
+      assert_equal "Mock Tool", tool.title
+      assert_equal "a mock tool for testing", tool.description
+      assert_equal tool.input_schema, Tool::InputSchema.new
+      assert_equal({ foo: "bar" }, tool.meta_value)
+    end
+
     test "Tool class method annotations can be set and retrieved" do
       class AnnotationsTestTool < Tool
         tool_name "annotations_test"
@@ -180,6 +208,30 @@ module MCP
       assert_equal "Updated", tool.annotations_value.title
     end
 
+    test "Tool class method meta can be set and retrieved" do
+      class MetaTestTool < Tool
+        tool_name "meta_test"
+        meta(foo: "bar")
+      end
+
+      tool = MetaTestTool
+      assert_instance_of Hash, tool.meta_value
+      assert_equal "bar", tool.meta_value[:foo]
+    end
+
+    test "Tool class method meta can be updated" do
+      class UpdatableMetaTool < Tool
+        tool_name "updatable_meta"
+      end
+
+      tool = UpdatableMetaTool
+      tool.meta(foo: "baz")
+      assert_equal({ foo: "baz" }, tool.meta_value)
+
+      tool.meta(foo: "qux")
+      assert_equal({ foo: "qux" }, tool.meta_value)
+    end
+
     test "#call with Sorbet typed tools invokes the tool block and returns the response" do
       class TypedTestTool < Tool
         tool_name "test_tool"