diff --git a/bin/scripts/rbs_rewriter_tester.rb b/bin/scripts/rbs_rewriter_tester.rb
new file mode 100644
index 00000000..38ba730e
--- /dev/null
+++ b/bin/scripts/rbs_rewriter_tester.rb
@@ -0,0 +1,262 @@
+# typed: ignore
+# devx opencode -s ses_14213e0f5ffeZhvMIpRnsDfCly
+
+$LOAD_PATH.unshift(File.expand_path("../lib", __FILE__))
+require "spoom"
+require "ruby-progressbar"
+require "optparse"
+require "set"
+
+# Only keep a handful of failing-file examples per category so memory stays
+# bounded when testing over very large file sets.
+SAMPLE_LIMIT = 10
+
+# Renders a Markdown table. `headers` is the header row (first cell is the
+# row-label column). `rows` is an array of cell arrays, each the same length as
+# `headers`. Every column is right-aligned.
+def render_table(headers, rows)
+  widths = headers.each_index.map do |i|
+    ([headers[i]] + rows.map { |row| row[i] }).map(&:length).max
+  end
+
+  render_row = ->(cells) do
+    "| " + cells.each_with_index.map { |cell, i| cell.rjust(widths[i]) }.join(" | ") + " |"
+  end
+
+  separator = "|" + widths.each_with_index.map do |width, i|
+    dashes = "-" * (width + 2)
+    i.zero? ? dashes : "#{dashes[0..-2]}:" # right-align marker for value columns
+  end.join("|") + "|"
+
+  [render_row.call(headers), separator, *rows.map { |row| render_row.call(row) }].join("\n")
+end
+
+# Renders syntax errors the way `ruby -c` does: a line-numbered window around
+# each error with a caret and inline message (only the lines near the error, not
+# the whole file). This reuses CRuby's own Prism formatter, which surfaces via
+# the SyntaxError raised when compiling. Falls back to a terse line:col listing
+# if compiling doesn't raise (e.g. Prism.parse and the compiler disagree).
+def format_parse_errors(source, file_path, indent: "    ")
+  message =
+    begin
+      RubyVM::InstructionSequence.compile(source, file_path)
+      nil # Compiled cleanly, so fall back below.
+    rescue SyntaxError => e
+      e.message
+    end
+
+  lines = if message
+    body = message.lines
+    body = body.drop(1) if body.first&.include?("syntax error") # drop the redundant "<file>:<line>:" header
+    body
+  else
+    Prism.parse(source).errors.map do |error|
+      location = error.location
+      "#{location.start_line}:#{location.start_column}: #{error.message}\n"
+    end
+  end
+
+  lines.map { |line| "#{indent}#{line}" }.join
+end
+
+extensions = Set["rb"] # Always search `.rb` files by default.
+
+# [label, translator class] pairs. Defaults to running both (--rewriter both).
+human = ["Human readable", Spoom::Sorbet::Translate::RBSCommentsToSorbetSigs::HumanReadableTranslator]
+line_matching = ["Line-matching", Spoom::Sorbet::Translate::RBSCommentsToSorbetSigs::LineMatchingTranslator]
+selected_rewriters = [human, line_matching]
+
+just_print_rewritten_output = false
+OptionParser.new do |opts|
+  opts.banner = "Usage: rbs_rewriter_tester.rb [options] PATH"
+
+  opts.on("--print-rewrite") do
+    # This is for debugging: print the rewritten version of each file to STDOUT.
+    just_print_rewritten_output = true
+  end
+
+  opts.on("--include-ext EXT", "When PATH is a directory, also process files with this extension (default: rb only). Repeatable.") do |ext|
+    extensions << ext.delete_prefix(".")
+  end
+
+  opts.on("--rewriter NAME", ["human", "line-matching", "both"],
+    "Which rewriter(s) to run: human, line-matching, or both (default: both)") do |name|
+    selected_rewriters = case name
+    when "human" then [human]
+    when "line-matching" then [line_matching]
+    when "both" then [human, line_matching]
+    end
+  end
+end.parse!
+
+if just_print_rewritten_output
+  selected_rewriters = [line_matching] # line-matching is closer to the final output format, so better for debugging.
+  rewritten = Spoom::Sorbet::Translate::RBSCommentsToSorbetSigs::LineMatchingTranslator.new(ARGF.read, file: ARGF.path).rewrite
+  puts rewritten
+  exit
+end
+
+path = File.expand_path(ARGV[0] || ".")
+files = if File.directory?(path)
+  Dir["#{path}/**/*.{#{extensions.join(",")}}"]
+else
+  [path]
+end
+
+# files = [
+#   "/Users/alex/world/trees/root/src/areas/core/shopify/bin/ci/lib/execution_strategy.rb",
+#   "/Users/alex/world/trees/root/src/areas/core/shopify/components/access_and_auth/customer_authentication_provider/test/integration/access_and_auth/customer_authentication_provider/revocation_controller_test.rb",
+#   "/Users/alex/world/trees/root/src/areas/core/shopify/components/access_and_auth/customer_authentication_provider/test/models/access_and_auth/customer_authentication_provider/provider_adapters/vendor_adapter_test.rb",
+#   "/Users/alex/world/trees/root/src/areas/core/shopify/components/developer_dashboard/apps/test/views/developer_dashboard/apps/show_test.rb",
+#   "/Users/alex/world/trees/root/src/areas/core/shopify/components/developer_dashboard/essentials/test/views/layouts/developer_dashboard/app_frame/app_frame_test.rb",
+#   "/Users/alex/world/trees/root/src/areas/core/shopify/components/merchandising/test/models/graph_api/admin/variants_page_limit_test.rb",
+#   "/Users/alex/world/trees/root/src/areas/core/shopify/components/merchandising/test/support/helpers/merchandising/dynamic_complexity_cost_test_helper.rb",
+#   "/Users/alex/world/trees/root/src/areas/core/shopify/components/platform/essentials/lib/development_support/globaldb_primary_key_collector.rb",
+#   "/Users/alex/world/trees/root/src/areas/core/shopify/components/platform/essentials/lib/development_support/migration_sql_compiler.rb",
+#   "/Users/alex/world/trees/root/src/areas/core/shopify/gems/sqlite_test_compat/lib/active_record/sqlite3_semian_stub.rb",
+# ]
+
+progress = ProgressBar.create(
+  title: "Rewriting",
+  total: files.size,
+  format: "%t |%B| %c/%C (%p%%) %e",
+  progress_mark: "█",
+  remainder_mark: "·",
+)
+
+# Shared (rewriter-independent) tallies.
+invalid_ruby_count = 0
+invalid_ruby_samples = []
+not_rbs_count = 0
+
+# Per-rewriter result data, keyed by label.
+stats = selected_rewriters.to_h do |label, _|
+  [label, { rewritten: 0, rewrite_failures: 0, parse_failures: 0, validation_failures: 0, elapsed: 0.0 }]
+end
+
+# Capped examples of failing files for the detail sections: samples[label][category] => [[path, detail], ...]
+samples = Hash.new { |h, label| h[label] = Hash.new { |hh, category| hh[category] = [] } }
+record_sample = ->(label, category, entry) do
+  bucket = samples[label][category]
+  bucket << entry if bucket.size < SAMPLE_LIMIT
+end
+
+files.each do |file_path|
+  original = File.read(file_path)
+
+  # Confirm the original file parses before attempting to rewrite it.
+  if Prism.parse(original).failure?
+    invalid_ruby_count += 1
+    invalid_ruby_samples << [file_path, original] if invalid_ruby_samples.size < SAMPLE_LIMIT
+    next
+  end
+
+  # Whether the file contains RBS syntax is independent of the rewriter, so check once.
+  unless Spoom::Sorbet::Translate::RBSCommentsToSorbetSigs.contains_rbs_syntax?(original)
+    not_rbs_count += 1
+    next
+  end
+
+  # Run every selected rewriter over the same file so the read/parse cost is paid once.
+  selected_rewriters.each do |label, translator_class|
+    counts = stats[label]
+
+    started = Process.clock_gettime(Process::CLOCK_MONOTONIC)
+    begin
+      rewritten = translator_class.new(original, file: file_path).rewrite
+    rescue => e
+      counts[:rewrite_failures] += 1
+      record_sample.call(label, :rewrite_failures, [file_path, e])
+      next
+    ensure
+      counts[:elapsed] += Process.clock_gettime(Process::CLOCK_MONOTONIC) - started
+    end
+    counts[:rewritten] += 1
+
+    if Prism.parse(rewritten).failure?
+      counts[:parse_failures] += 1
+      record_sample.call(label, :parse_failures, [file_path, rewritten])
+    end
+
+    unless (validation = Spoom::Sorbet::Translate::Validator.validate(original, rewritten)).valid?
+      counts[:validation_failures] += 1
+      record_sample.call(label, :validation_failures, [file_path, validation])
+    end
+  end
+ensure
+  progress.increment
+end
+
+progress.finish
+
+unless invalid_ruby_samples.empty?
+  puts
+  puts "Invalid Ruby:"
+  invalid_ruby_samples.each do |file_path, source|
+    puts "  #{file_path}"
+    puts format_parse_errors(source, file_path)
+  end
+end
+
+selected_rewriters.each do |label, translator_class|
+  label_samples = samples[label]
+
+  unless label_samples[:rewrite_failures].empty?
+    puts
+    puts "#{label} - rewrite errors:"
+    label_samples[:rewrite_failures].each do |file_path, error|
+      puts "  #{file_path}: #{error.class}: #{error.message}"
+    end
+  end
+
+  unless label_samples[:parse_failures].empty?
+    puts
+    puts "#{label} - parse failures:"
+    label_samples[:parse_failures].each do |file_path, source|
+      puts "  #{file_path}"
+      puts format_parse_errors(source, file_path)
+    end
+  end
+
+  if Spoom::Sorbet::Translate::RBSCommentsToSorbetSigs::LineMatchingTranslator == translator_class
+    unless label_samples[:validation_failures].empty?
+      puts
+      puts "#{label} - validation failures:"
+      label_samples[:validation_failures].each do |file_path, result|
+        puts "  #{file_path}"
+        result.errors.each { |error| puts "    #{error}" }
+      end
+    end
+  end
+end
+
+
+puts
+puts "Summary"
+puts "-------"
+printf("%-16s%8d\n", "Files processed:", files.size)
+printf("%-16s%8d\n", "Invalid Ruby:", invalid_ruby_count)
+printf("%-16s%8d\n", "Not RBS:", not_rbs_count)
+
+puts
+puts "Results:"
+puts
+
+headers = [""] + selected_rewriters.map { |label, _| label }
+headers << "New errors" if selected_rewriters.size > 1
+
+rows = {
+  "Rewritten:" => :rewritten,
+  "Rewrite failures:" => :rewrite_failures,
+  "Parse failures:" => :parse_failures,
+  "Validation failures:" => :validation_failures,
+  "Elapsed:" => :elapsed,
+}.map do |row_label, key|
+  format_value = key == :elapsed ? ->(v) { format("%.2fs", v) } : ->(v) { v.to_s }
+  values = selected_rewriters.map { |label, _| stats[label][key] }
+  cells = [row_label, *values.map(&format_value)]
+  cells << format_value.call(values.last - values.first) if selected_rewriters.size > 1 # line-matching - human
+  cells
+end
+
+puts render_table(headers, rows)