diff --git a/lib/interscript/equivalence.rb b/lib/interscript/equivalence.rb
new file mode 100644
index 00000000..efd18da9
--- /dev/null
+++ b/lib/interscript/equivalence.rb
@@ -0,0 +1,46 @@
+# frozen_string_literal: true
+
+module Interscript
+  # A3 (WO59): mechanical equivalence checking between two systems.
+  #
+  # Probes a sample set through both systems and reports the agreement
+  # ratio plus the minimal distinguishing samples. The sample set is
+  # caller-supplied or derived from the systems' own embedded tests.
+  class Equivalence
+    def self.check(first, second, samples:)
+      new(first, second).check(samples)
+    end
+
+    # Embedded test inputs of the given systems (the natural probe set
+    # for near-equivalent authorities).
+    def self.samples_from_tests(*codes)
+      codes.flat_map do |code|
+        Array(Interscript.parse(code).tests&.data).map(&:first).compact
+      end.uniq
+    end
+
+    def initialize(first, second)
+      @first = first
+      @second = second
+    end
+
+    def check(samples)
+      distinguishing = []
+      tested = 0
+      matched = 0
+      samples.each do |sample|
+        tested += 1
+        a = Interscript.transliterate(@first, sample)
+        b = Interscript.transliterate(@second, sample)
+        if a == b
+          matched += 1
+        else
+          distinguishing << {sample: sample, first: a, second: b}
+        end
+      end
+      ratio = tested.zero? ? 1.0 : matched.fdiv(tested)
+      {tested: tested, matched: matched, ratio: ratio,
+       distinguishing: distinguishing}
+    end
+  end
+end
diff --git a/lib/interscript/ml/byt5_onnx.rb b/lib/interscript/ml/byt5_onnx.rb
index 375926c3..45d14fc6 100644
--- a/lib/interscript/ml/byt5_onnx.rb
+++ b/lib/interscript/ml/byt5_onnx.rb
@@ -18,7 +18,7 @@ def initialize(zip_path)
       @pasts = @kv ? zero_pasts : {}
     end
 
-    def translate(text, max_seq_length: 256)
+    def translate(text, max_seq_length: 512)
       ids = IMF.encode(text)
       return "" if ids.length == 1 # only the trailing EOS: empty input
 
diff --git a/lib/interscript/ml/imf.rb b/lib/interscript/ml/imf.rb
index 3d56369d..bea06bf2 100644
--- a/lib/interscript/ml/imf.rb
+++ b/lib/interscript/ml/imf.rb
@@ -42,7 +42,10 @@ def decode(token_ids)
           next if token == PAD_ID || token == UNK_ID
           out << ((token - BYTE_OFFSET) % 256).chr
         end
-        out.force_encoding(Encoding::UTF_8)
+        # the python reference decodes with errors="replace": invalid
+        # bytes (mid-character sequence caps) become U+FFFD -- scrub to
+        # match byte-for-byte (the C1 matrix catch)
+        out.force_encoding(Encoding::UTF_8).scrub
       end
 
       def manifest(zip_path)
diff --git a/lib/interscript/ml/translator.rb b/lib/interscript/ml/translator.rb
index 5adb352c..83e5d6b1 100644
--- a/lib/interscript/ml/translator.rb
+++ b/lib/interscript/ml/translator.rb
@@ -10,7 +10,7 @@ def initialize(model_file:)
       @model = Model.from_file(model_file)
     end
 
-    def translate(phrase, max_seq_length: 100)
+    def translate(phrase, max_seq_length: 512)
       return @model.translate(phrase, max_seq_length: max_seq_length) if @model.is_a?(Byt5Onnx)
 
       Interscript::ML.require_optional!("numo-narray", feature: "numo/narray")
diff --git a/spec/interscript/equivalence_spec.rb b/spec/interscript/equivalence_spec.rb
new file mode 100644
index 00000000..6b59e0da
--- /dev/null
+++ b/spec/interscript/equivalence_spec.rb
@@ -0,0 +1,72 @@
+# frozen_string_literal: true
+
+require "json"
+require "tmpdir"
+require "interscript/equivalence"
+
+RSpec.describe Interscript::Equivalence do
+  def write_map(dir, name, body)
+    path = File.join(dir, "#{name}.isc")
+    File.write(path, body)
+    path
+  end
+
+  def identical_pair(dir)
+    body = <<~ISC
+      system "TST:xx-Latn:Latn:1" {
+        metadata {
+          authority_id tst
+          id 1
+          language iso-639-2:xx
+          source_script Latn
+          destination_script Latn
+        }
+        tests { "ab" ->  "ab" }
+        stage main { parallel { sub "a" "a" } }
+      }
+    ISC
+    [write_map(dir, "tst-a-1", body), write_map(dir, "tst-b-1", body.gsub("tst-a-1", "tst-b-1"))]
+  end
+
+  it "reports full agreement for identical systems" do
+    Dir.mktmpdir do |dir|
+      identical_pair(dir)
+      Interscript.load_path.unshift(dir)
+      begin
+        result = described_class.check("tst-a-1", "tst-b-1", samples: ["ab", "abc", "b"])
+        expect(result[:ratio]).to eq(1.0)
+        expect(result[:distinguishing]).to be_empty
+      ensure
+        Interscript.load_path.delete(dir)
+      end
+    end
+  end
+
+  it "finds the minimal distinguishing sample for divergent systems" do
+    Dir.mktmpdir do |dir|
+      body_a = <<~ISC
+        system "TST:xx-Latn:Latn:1" {
+          metadata {
+            authority_id tst
+            id 1
+            language iso-639-2:xx
+            source_script Latn
+            destination_script Latn
+          }
+          stage main { parallel { sub "a" "x" } }
+        }
+      ISC
+      body_b = body_a.gsub('sub "a" "x"', 'sub "a" "y"').gsub("id 1", "id 2")
+      write_map(dir, "tst-c-1", body_a)
+      write_map(dir, "tst-c-2", body_b)
+      Interscript.load_path.unshift(dir)
+      begin
+        result = described_class.check("tst-c-1", "tst-c-2", samples: ["a", "b"])
+        expect(result[:ratio]).to eq(0.5)
+        expect(result[:distinguishing]).to eq([{sample: "a", first: "x", second: "y"}])
+      ensure
+        Interscript.load_path.delete(dir)
+      end
+    end
+  end
+end
diff --git a/spec/interscript/ml/conformance_spec.rb b/spec/interscript/ml/conformance_spec.rb
index ff144fbe..1d9ef728 100644
--- a/spec/interscript/ml/conformance_spec.rb
+++ b/spec/interscript/ml/conformance_spec.rb
@@ -22,7 +22,8 @@
       failures << "#{v["src"].inspect} -> #{got.inspect} (expected #{v["out"].inspect})" if got != v["out"]
     end
     expect(failures).to be_empty,
-      "#{vectors['model']}: #{failures.length}/#{vectors['vectors'].length} mismatched\n" +
-      failures.first(3).join("\n")
+      "#{vectors["model"]}: #{failures.length}/#{vectors["vectors"].length} mismatched
+" +
+        failures.first(3).join("\n")
   end
 end
diff --git a/spec/interscript/ml/imf_spec.rb b/spec/interscript/ml/imf_spec.rb
index 58f4bc4c..bbd1e709 100644
--- a/spec/interscript/ml/imf_spec.rb
+++ b/spec/interscript/ml/imf_spec.rb
@@ -251,3 +251,12 @@
     end
   end
 end
+
+RSpec.describe "IMF.decode byte hygiene" do
+  it "scrubs invalid UTF-8 produced by mid-character sequence caps" do
+    # 0xF0 0x9E begins a 4-byte UTF-8 sequence that never completes: the
+    # python reference decodes with errors="replace" (U+FFFD); the ruby
+    # runtime must match byte-for-byte (the C1 matrix catch)
+    expect(Interscript::ML::IMF.decode([100, 243, 161])).to eq("a�")
+  end
+end