diff --git a/lib/mcp/server.rb b/lib/mcp/server.rb
index e37a785f..df6c4c6a 100644
--- a/lib/mcp/server.rb
+++ b/lib/mcp/server.rb
@@ -5,6 +5,15 @@
 require_relative "methods"
 
 module MCP
+  class ToolNotUnique < StandardError
+    def initialize(duplicated_tool_names)
+      super(<<~MESSAGE)
+        Tool names should be unique. Use `tool_name` to assign unique names to:
+        #{duplicated_tool_names.join(", ")}
+      MESSAGE
+    end
+  end
+
   class Server
     DEFAULT_VERSION = "0.1.0"
 
@@ -53,6 +62,7 @@ def initialize(
       @title = title
       @version = version
       @instructions = instructions
+      @tool_names = tools.map(&:name_value)
       @tools = tools.to_h { |t| [t.name_value, t] }
       @prompts = prompts.to_h { |p| [p.name_value, p] }
       @resources = resources
@@ -101,7 +111,10 @@ def handle_json(request)
 
     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:, meta:, &block)
-      @tools[tool.name_value] = tool
+      tool_name = tool.name_value
+
+      @tool_names << tool_name
+      @tools[tool_name] = tool
 
       validate!
     end
@@ -176,6 +189,8 @@ def prompts_get_handler(&block)
     private
 
     def validate!
+      validate_tool_name!
+
       # NOTE: The draft protocol version is the next version after 2025-11-25.
       if @configuration.protocol_version <= "2025-06-18"
         if server_info.key?(:description)
@@ -216,6 +231,12 @@ def validate!
       end
     end
 
+    def validate_tool_name!
+      duplicated_tool_names = @tool_names.tally.filter_map { |name, count| name if count >= 2 }
+
+      raise ToolNotUnique, duplicated_tool_names unless duplicated_tool_names.empty?
+    end
+
     def handle_request(request, method)
       handler = @handlers[method]
       unless handler
diff --git a/test/mcp/server_test.rb b/test/mcp/server_test.rb
index 91d4435d..e37afee8 100644
--- a/test/mcp/server_test.rb
+++ b/test/mcp/server_test.rb
@@ -374,6 +374,44 @@ def call(message:, server_context: nil)
       assert_instrumentation_data({ method: "tools/call", tool_name: "tool_that_raises" })
     end
 
+    test "registers tools with the same class name in different namespaces" do
+      module Foo
+        class Example < Tool
+        end
+      end
+
+      module Bar
+        class Example < Tool
+        end
+      end
+
+      error = assert_raises(MCP::ToolNotUnique) { Server.new(tools: [Foo::Example, Bar::Example]) }
+      assert_equal(<<~MESSAGE, error.message)
+        Tool names should be unique. Use `tool_name` to assign unique names to:
+        example
+      MESSAGE
+    end
+
+    test "registers tools with the same tool name" do
+      module Baz
+        class Example < Tool
+          tool_name "foo"
+        end
+      end
+
+      module Qux
+        class Example < Tool
+          tool_name "foo"
+        end
+      end
+
+      error = assert_raises(MCP::ToolNotUnique) { Server.new(tools: [Baz::Example, Qux::Example]) }
+      assert_equal(<<~MESSAGE, error.message)
+        Tool names should be unique. Use `tool_name` to assign unique names to:
+        foo
+      MESSAGE
+    end
+
     test "#handle_json returns error response with isError true if the tool raises an error" do
       request = JSON.generate({
         jsonrpc: "2.0",
@@ -950,6 +988,20 @@ def call(message:, server_context: nil)
       assert_equal({ content: "success", isError: false }, response[:result])
     end
 
+    test "#define_tool adds a tool with duplicated tool name to the server" do
+      error = assert_raises(MCP::ToolNotUnique) do
+        @server.define_tool(
+          name: "test_tool", # NOTE: Already registered tool name
+          description: "Defined tool",
+          input_schema: { type: "object", properties: { message: { type: "string" } }, required: ["message"] },
+          meta: { foo: "bar" },
+        ) do |message:|
+          Tool::Response.new(message)
+        end
+      end
+      assert_match(/\ATool names should be unique. Use `tool_name` to assign unique names to/, error.message)
+    end
+
     test "#define_tool call definition allows tool arguments and server context" do
       @server.server_context = { user_id: "123" }