diff --git a/source/client/Dockerfile b/source/client/Dockerfile
index 868d4943..aa788e27 100644
--- a/source/client/Dockerfile
+++ b/source/client/Dockerfile
@@ -1,4 +1,4 @@
-FROM cyberdojo/sinatra-base:759c4e9@sha256:d5f87f343a9f88a598b810c0f02b81db0bb67319701a956aec3577cbd51c1c24
+FROM ghcr.io/cyber-dojo/sinatra-base:5ab6a10@sha256:c096154011cc1cef9cc69e8be948fb4329543f9670d4fb4fd3851a8aa016630d
 LABEL maintainer=jon@jaggersoft.com
 
 ARG COMMIT_SHA
diff --git a/source/server/differ.rb b/source/server/differ.rb
index 240d7191..b5f82246 100644
--- a/source/server/differ.rb
+++ b/source/server/differ.rb
@@ -14,17 +14,17 @@ def diff_summary(was_files:, now_files:)
     diff_plus(was_files, now_files, lines: false)
   end
 
-  # :nocov:
+  # simplecov:disable
   def diff_lines_files(was_files:, now_files:)
     diff_plus(was_files, now_files, lines: true)
   end
-  # :nocov:
+  # simplecov:enable
 
-  # :nocov:
+  # simplecov:disable
   def diff_summary_files(was_files:, now_files:)
     diff_plus(was_files, now_files, lines: false)
   end
-  # :nocov:
+  # simplecov:enable
 
   private
 
diff --git a/test/client/lib/coverage.rb b/test/client/lib/coverage.rb
index c5bbba57..e899276a 100644
--- a/test/client/lib/coverage.rb
+++ b/test/client/lib/coverage.rb
@@ -6,19 +6,19 @@
 SimpleCov.start do
   enable_coverage :branch
   filters.clear
-  add_filter('test/lib/id58_test_base.rb')
+  skip('test/lib/id58_test_base.rb')
   coverage_dir(ENV.fetch('COVERAGE_ROOT', nil))
   root(APP_DIR)
 
   test_tab = ENV.fetch('COVERAGE_TEST_TAB_NAME')
   code_tab = ENV.fetch('COVERAGE_CODE_TAB_NAME')
-  # add_group('debug') { |the| puts the.filename; false }
-  add_group(test_tab) { |the| the.filename.start_with?("#{APP_DIR}/test/") }
-  add_group(code_tab) { |the| the.filename.start_with?("#{APP_DIR}/source/") }
+  # group('debug') { |path| puts path.filename; false }
+  group(test_tab) { |path| path.filename.start_with?("#{APP_DIR}/test/") }
+  group(code_tab) { |path| path.filename.start_with?("#{APP_DIR}/source/") }
 end
 
 formatters = [
   SimpleCov::Formatter::HTMLFormatter,
-  SimpleCov::Formatter::JSONFormatter
+  CoverageMetricsFormatter
 ]
 SimpleCov.formatters = SimpleCov::Formatter::MultiFormatter.new(formatters)
diff --git a/test/client/lib/simplecov_formatter_json.rb b/test/client/lib/simplecov_formatter_json.rb
index 215dc51e..4507c71c 100644
--- a/test/client/lib/simplecov_formatter_json.rb
+++ b/test/client/lib/simplecov_formatter_json.rb
@@ -1,55 +1,56 @@
 require 'simplecov'
 require 'json'
 
-module SimpleCov
-  module Formatter
-    class JSONFormatter
-      # based on https://github.com/vicentllongo/simplecov-json
-
-      def format(result)
-        data = {
-          timestamp: result.created_at.to_i,
-          command_name: result.command_name
+# A SimpleCov formatter writing coverage_metrics.json, which check_metrics.rb
+# reads. SimpleCov ships its own JSON formatter, shaped per file rather than
+# per group, so this one carries its own name rather than reopening that class
+# and redefining its format method. Redefining it makes ruby -w report the
+# redefinition.
+#
+# based on https://github.com/vicentllongo/simplecov-json
+class CoverageMetricsFormatter
+  def format(result)
+    data = {
+      timestamp: result.created_at.to_i,
+      command_name: result.command_name
+    }
+    result.groups.each do |name, file_list|
+      # name == 'code' or name == 'test'
+      data[name] = {
+        lines: {
+          total: file_list.lines_of_code,
+          covered: file_list.covered_lines,
+          missed: file_list.missed_lines
+        },
+        branches: {
+          total: file_list.total_branches,
+          covered: file_list.covered_branches,
+          missed: file_list.missed_branches
         }
-        result.groups.each do |name, file_list|
-          # name == 'code' or name == 'test'
-          data[name] = {
-            lines: {
-              total: file_list.lines_of_code,
-              covered: file_list.covered_lines,
-              missed: file_list.missed_lines
-            },
-            branches: {
-              total: file_list.total_branches,
-              covered: file_list.covered_branches,
-              missed: file_list.missed_branches
-            }
-          }
-        end
-        File.open(output_filepath, 'w+') do |file|
-          file.print(JSON.pretty_generate(data))
-        end
-        puts output_message(result)
-        data.to_json
-      end
+      }
+    end
+    File.open(output_filepath, 'w+') do |file|
+      file.print(JSON.pretty_generate(data))
+    end
+    puts output_message(result)
+    data.to_json
+  end
 
-      def output_filepath
-        File.join(output_path, output_filename)
-      end
+  def output_filepath
+    File.join(output_path, output_filename)
+  end
 
-      def output_filename
-        'coverage_metrics.json'
-      end
+  def output_filename
+    'coverage_metrics.json'
+  end
 
-      def output_message(result)
-        "Coverage report generated for #{result.command_name} to #{output_filepath}. #{result.covered_lines} / #{result.total_lines} LOC (#{result.covered_percent.round(2)}%) covered."
-      end
+  def output_message(result)
+    "Coverage report generated for #{result.command_name} to #{output_filepath}. #{result.covered_lines} / #{result.total_lines} LOC (#{result.covered_percent.round(2)}%) covered."
+  end
 
-      private
+  private
 
-      def output_path
-        SimpleCov.coverage_path
-      end
-    end
+  def output_path
+    SimpleCov.coverage_path
   end
 end
diff --git a/test/server/lib/coverage.rb b/test/server/lib/coverage.rb
index 68ab75e5..87d119ee 100644
--- a/test/server/lib/coverage.rb
+++ b/test/server/lib/coverage.rb
@@ -6,19 +6,19 @@
 SimpleCov.start do
   enable_coverage :branch
   filters.clear
-  add_filter('test/lib/id58_test_base.rb')
+  skip('test/lib/id58_test_base.rb')
   coverage_dir(ENV.fetch('COVERAGE_ROOT'))
   root(APP_DIR)
 
   test_tab = ENV.fetch('COVERAGE_TEST_TAB_NAME')
   code_tab = ENV.fetch('COVERAGE_CODE_TAB_NAME')
-  # add_group('debug') { |the| puts the.filename; false }
-  add_group(test_tab) { |the| the.filename.start_with?("#{APP_DIR}/test/") }
-  add_group(code_tab) { |the| the.filename.start_with?("#{APP_DIR}/source/") }
+  # group('debug') { |path| puts path.filename; false }
+  group(test_tab) { |path| path.filename.start_with?("#{APP_DIR}/test/") }
+  group(code_tab) { |path| path.filename.start_with?("#{APP_DIR}/source/") }
 end
 
 formatters = [
   SimpleCov::Formatter::HTMLFormatter,
-  SimpleCov::Formatter::JSONFormatter
+  CoverageMetricsFormatter
 ]
 SimpleCov.formatters = SimpleCov::Formatter::MultiFormatter.new(formatters)
diff --git a/test/server/lib/simplecov_formatter_json.rb b/test/server/lib/simplecov_formatter_json.rb
index 215dc51e..4507c71c 100644
--- a/test/server/lib/simplecov_formatter_json.rb
+++ b/test/server/lib/simplecov_formatter_json.rb
@@ -1,55 +1,56 @@
 require 'simplecov'
 require 'json'
 
-module SimpleCov
-  module Formatter
-    class JSONFormatter
-      # based on https://github.com/vicentllongo/simplecov-json
-
-      def format(result)
-        data = {
-          timestamp: result.created_at.to_i,
-          command_name: result.command_name
+# A SimpleCov formatter writing coverage_metrics.json, which check_metrics.rb
+# reads. SimpleCov ships its own JSON formatter, shaped per file rather than
+# per group, so this one carries its own name rather than reopening that class
+# and redefining its format method. Redefining it makes ruby -w report the
+# redefinition.
+#
+# based on https://github.com/vicentllongo/simplecov-json
+class CoverageMetricsFormatter
+  def format(result)
+    data = {
+      timestamp: result.created_at.to_i,
+      command_name: result.command_name
+    }
+    result.groups.each do |name, file_list|
+      # name == 'code' or name == 'test'
+      data[name] = {
+        lines: {
+          total: file_list.lines_of_code,
+          covered: file_list.covered_lines,
+          missed: file_list.missed_lines
+        },
+        branches: {
+          total: file_list.total_branches,
+          covered: file_list.covered_branches,
+          missed: file_list.missed_branches
         }
-        result.groups.each do |name, file_list|
-          # name == 'code' or name == 'test'
-          data[name] = {
-            lines: {
-              total: file_list.lines_of_code,
-              covered: file_list.covered_lines,
-              missed: file_list.missed_lines
-            },
-            branches: {
-              total: file_list.total_branches,
-              covered: file_list.covered_branches,
-              missed: file_list.missed_branches
-            }
-          }
-        end
-        File.open(output_filepath, 'w+') do |file|
-          file.print(JSON.pretty_generate(data))
-        end
-        puts output_message(result)
-        data.to_json
-      end
+      }
+    end
+    File.open(output_filepath, 'w+') do |file|
+      file.print(JSON.pretty_generate(data))
+    end
+    puts output_message(result)
+    data.to_json
+  end
 
-      def output_filepath
-        File.join(output_path, output_filename)
-      end
+  def output_filepath
+    File.join(output_path, output_filename)
+  end
 
-      def output_filename
-        'coverage_metrics.json'
-      end
+  def output_filename
+    'coverage_metrics.json'
+  end
 
-      def output_message(result)
-        "Coverage report generated for #{result.command_name} to #{output_filepath}. #{result.covered_lines} / #{result.total_lines} LOC (#{result.covered_percent.round(2)}%) covered."
-      end
+  def output_message(result)
+    "Coverage report generated for #{result.command_name} to #{output_filepath}. #{result.covered_lines} / #{result.total_lines} LOC (#{result.covered_percent.round(2)}%) covered."
+  end
 
-      private
+  private
 
-      def output_path
-        SimpleCov.coverage_path
-      end
-    end
+  def output_path
+    SimpleCov.coverage_path
   end
 end