diff --git a/.circleci/config.yml b/.circleci/config.yml
index cac461c9b..c580dc3c2 100755
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -177,8 +177,8 @@ jobs:
     # use. This matches the dependency-pinning policy stated at the top of this
     # file and mirrors the Heroku CLI install in the deploy job: we never run
     # an unpinned, unverified binary fetched over the network. The coverage
-    # JSON itself is produced by SimpleCov::Formatter::Codecov; see
-    # test/test_helper.rb. --disable-search + --file restrict the upload to
+    # JSON itself is produced by SimpleCov::Formatter::JSONFormatter in the
+    # test:coverage_gaps rake task. --disable-search + --file restrict the upload to
     # exactly that file, and --plugin noop skips Codecov's coverage-generation
     # plugins (we already have the report). We do NOT pass --fail-on-error, so
     # a Codecov outage cannot break an otherwise-green build. The upload token,
@@ -208,7 +208,7 @@ jobs:
           echo "** Hash verified. Uploading coverage."
           chmod +x codecov
           ./codecov upload-process --disable-search --plugin noop \
-            --file coverage/codecov-result.json
+            --file coverage/coverage.json
           rm -f codecov
 
   # I haven't found a reliable way to calculate HEROKU_APP just once,
diff --git a/Gemfile b/Gemfile
index 76d655c59..effa89dee 100644
--- a/Gemfile
+++ b/Gemfile
@@ -183,7 +183,6 @@ end
 
 group :test do
   gem 'capybara-slow_finder_errors', require: false # ID slow Capybara finders
-  gem 'codecov', require: false # Report test code coverage
   # Pin minitest < 6.0 until minitest-reporters supports it.
   # Minitest 6.0 introduced breaking changes that cause minitest-reporters 1.7.1
   # to fail silently (tests don't run). Remove this constraint when
@@ -196,7 +195,12 @@ group :test do
   # Note: Updating 'rails-controller-testing' to '1.0.5' causes failures
   gem 'rails-controller-testing', '~> 1.0' # for `assigns` and `assert_template`
   gem 'selenium-webdriver' # Automates browser i/f for Rails system testing
-  # We don't list "simplecov"; code depends on it & brings it in
+  # Test coverage. We require both directly: simplecov in test_helper.rb, and
+  # simplecov_json_formatter in test:coverage_gaps to write coverage/coverage.json
+  # for the Codecov upload. List both so a future simplecov release that stops
+  # bundling the JSON formatter can't break us in a surprising way.
+  gem 'simplecov', require: false
+  gem 'simplecov_json_formatter', require: false
   gem 'webmock', '~> 3.0', require: false # Mock HTTP requests for testing
 end
 
diff --git a/Gemfile.lock b/Gemfile.lock
index 02c505cb5..89aaa7f3d 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -138,8 +138,6 @@ GEM
       concurrent-ruby
     code_analyzer (0.5.5)
       sexp_processor
-    codecov (0.6.0)
-      simplecov (>= 0.15, < 0.22)
     coderay (1.1.3)
     commonmarker (2.6.3)
       rb_sys (~> 0.9)
@@ -532,7 +530,7 @@ GEM
       concurrent-ruby (~> 1.0, >= 1.0.2)
       logger
     sexp_processor (4.17.5)
-    simplecov (0.21.2)
+    simplecov (0.22.0)
       docile (~> 1.1)
       simplecov-html (~> 0.11)
       simplecov_json_formatter (~> 0.1)
@@ -637,7 +635,6 @@ DEPENDENCIES
   bundler-audit
   capybara-slow_finder_errors
   chartkick (~> 5.2)
-  codecov
   commonmarker (~> 2.6.2)
   dotenv (~> 3.0)
   eslintrb
@@ -690,6 +687,8 @@ DEPENDENCIES
   selenium-webdriver
   sentry-rails
   sentry-ruby
+  simplecov
+  simplecov_json_formatter
   solid_queue (~> 1.1)
   spring (~> 4.1)
   sprockets-rails (= 3.5.2)
diff --git a/lib/tasks/default.rake b/lib/tasks/default.rake
index 368bfc8f1..9ca1bae14 100644
--- a/lib/tasks/default.rake
+++ b/lib/tasks/default.rake
@@ -963,20 +963,22 @@ task 'test:coverage_gaps' do
 
   puts "Combined Coverage: #{result.covered_percent.round(2)}%"
 
-  # In CI, write coverage/codecov-result.json from the merged result so the
+  # In CI, write coverage/coverage.json from the merged result so the
   # Codecov CLI (see .circleci/config.yml) has a file to upload. This MUST
   # happen here, not in test_helper.rb: the test workers run with
-  # DEFER_COVERAGE set, which swaps in the minimal SimpleFormatter, so the
-  # Codecov formatter never runs during the test processes. The merged
+  # DEFER_COVERAGE set, which swaps in the minimal SimpleFormatter, so a
+  # report formatter never runs during the test processes. The merged
   # result is only complete once all parallel + system test runs finish.
   #
-  # We call the gem's inner Codecov::SimpleCov::Formatter directly rather
-  # than SimpleCov::Formatter::Codecov, because the latter also invokes the
-  # gem's deprecated built-in network uploader. We upload separately with a
-  # pinned, hash-verified CLI, so we want ONLY the file-writing half here.
+  # We use SimpleCov::Formatter::JSONFormatter (from simplecov_json_formatter,
+  # a dependency of simplecov) because Codecov's backend natively parses its
+  # output. We previously used the abandoned 'codecov' gem's formatter, but
+  # it emits the deprecated "network" report format that the current Codecov
+  # backend rejects as "unusable report ... incorrect data format", silently
+  # yielding 0% coverage on the site even though the upload itself succeeds.
   if ENV['CI']
-    require 'codecov'
-    Codecov::SimpleCov::Formatter.new.format(result)
+    require 'simplecov_json_formatter'
+    SimpleCov::Formatter::JSONFormatter.new.format(result)
   end
 
   # Iterate through files using SimpleCov's own 'missed_lines' logic
diff --git a/test/test_helper.rb b/test/test_helper.rb
index d093e1457..45fccceda 100644
--- a/test/test_helper.rb
+++ b/test/test_helper.rb
@@ -15,16 +15,12 @@
 # *MUST* state VERY EARLY that we're in the test environment.
 ENV['RAILS_ENV'] ||= 'test'
 
-# Configure SimpleCov formatting before we start it
-if ENV['CI']
-  require 'codecov'
-  SimpleCov.formatters = [
-    SimpleCov::Formatter::HTMLFormatter,
-    SimpleCov::Formatter::Codecov
-  ]
-else
-  SimpleCov.formatters = SimpleCov::Formatter::HTMLFormatter
-end
+# Configure SimpleCov formatting before we start it.
+# The HTML report is for humans; the machine-readable JSON report uploaded to
+# Codecov is written separately by the test:coverage_gaps rake task, since CI
+# runs with DEFER_COVERAGE set (which swaps in SimpleFormatter below) and the
+# merged result is only complete once all parallel + system runs finish.
+SimpleCov.formatters = SimpleCov::Formatter::HTMLFormatter
 
 # Start SimpleCov to track coverage
 # NOTE: If you change SimpleCov configuration (used locally), you may also