diff --git a/lib/ast_transform/instruction_sequence.rb b/lib/ast_transform/instruction_sequence.rb
index ab875a8..795d2b3 100644
--- a/lib/ast_transform/instruction_sequence.rb
+++ b/lib/ast_transform/instruction_sequence.rb
@@ -16,7 +16,7 @@ def source_to_transformed_iseq(source, source_path)
         rewritten_source = transformer.transform_file_source(source, source_path, rewritten_file_pathname.to_s)
         write(rewritten_source, rewritten_file_pathname)
 
-        RubyVM::InstructionSequence.compile(rewritten_source, rewritten_file_pathname.to_s, rewritten_file_pathname.to_s)
+        RubyVM::InstructionSequence.compile(rewritten_source, source_path, source_path)
       end
 
       def write_pathname(file_path)
diff --git a/lib/ast_transform/instruction_sequence/mixin.rb b/lib/ast_transform/instruction_sequence/mixin.rb
index fa647f5..a25d839 100644
--- a/lib/ast_transform/instruction_sequence/mixin.rb
+++ b/lib/ast_transform/instruction_sequence/mixin.rb
@@ -10,9 +10,12 @@ module Mixin
       def load_iseq(source_path)
         return ASTTransform::MixinUtils.try_super(self, :load_iseq, source_path) if source_path == __FILE__
 
-        source = File.read(source_path)
+        # Binary read avoids encoding errors during the fast-path check below.
+        # Downstream (Prism, RubyVM::InstructionSequence) handle encoding natively
+        # via magic comments, so we never need to set it ourselves.
+        source = File.binread(source_path)
 
-        return ASTTransform::MixinUtils.try_super(self, :load_iseq, source_path) unless source =~ /transform!/
+        return ASTTransform::MixinUtils.try_super(self, :load_iseq, source_path) unless source.include?('transform!'.b)
 
         ASTTransform::InstructionSequence.source_to_transformed_iseq(source, source_path)
       end
diff --git a/lib/ast_transform/source_map.rb b/lib/ast_transform/source_map.rb
index 4e4b627..4833ad7 100644
--- a/lib/ast_transform/source_map.rb
+++ b/lib/ast_transform/source_map.rb
@@ -11,6 +11,7 @@ class << self
       # @return [void]
       def register_source_map(source_map)
         source_maps[source_map.transformed_file_path] = source_map
+        source_maps[source_map.source_file_path] = source_map
 
         nil
       end
diff --git a/test/ast_transform/instruction_sequence_test.rb b/test/ast_transform/instruction_sequence_test.rb
new file mode 100644
index 0000000..b7a927e
--- /dev/null
+++ b/test/ast_transform/instruction_sequence_test.rb
@@ -0,0 +1,65 @@
+# frozen_string_literal: true
+
+require 'test_helper'
+require 'tempfile'
+require 'ast_transform/instruction_sequence'
+require 'ast_transform/instruction_sequence/mixin'
+require 'ast_transform/transformation'
+
+module ASTTransform
+  class InstructionSequenceTest < Minitest::Test
+    extend ASTTransform::Declarative
+
+    class ClassWithMixin
+      include ASTTransform::InstructionSequence::Mixin
+    end
+
+    def setup
+      @loader = ClassWithMixin.new
+    end
+
+    test "compiled iseq reports original source path" do
+      source = "1 + 2\n"
+      source_path = File.expand_path("test/fixtures/dummy.rb")
+
+      iseq = ASTTransform::InstructionSequence.source_to_transformed_iseq(source, source_path)
+
+      assert_equal source_path, iseq.path,
+        "ISeq should report the original source path, not the tmp/ rewritten path"
+    end
+
+    test "source_to_transformed_iseq handles binary-encoded source" do
+      source = "# café résumé\n1 + 2\n".dup.force_encoding('ASCII-8BIT')
+      source_path = File.expand_path("test/fixtures/encoding_test.rb")
+
+      iseq = ASTTransform::InstructionSequence.source_to_transformed_iseq(source, source_path)
+
+      assert_equal source_path, iseq.path
+    end
+
+    test "load_iseq skips non-ASCII files without transform!" do
+      tmpfile = Tempfile.new(['non_ascii', '.rb'])
+      tmpfile.write("# café résumé\nx = 1\n")
+      tmpfile.close
+
+      iseq = @loader.load_iseq(tmpfile.path)
+
+      assert_nil iseq
+    ensure
+      tmpfile&.unlink
+    end
+
+    test "load_iseq processes non-ASCII files with transform! without encoding errors" do
+      tmpfile = Tempfile.new(['non_ascii_transform', '.rb'])
+      tmpfile.write("# café résumé\nx = 1\n# transform!\n")
+      tmpfile.close
+
+      iseq = @loader.load_iseq(tmpfile.path)
+
+      assert_instance_of RubyVM::InstructionSequence, iseq
+      assert_equal tmpfile.path, iseq.path
+    ensure
+      tmpfile&.unlink
+    end
+  end
+end
diff --git a/test/ast_transform/source_map_test.rb b/test/ast_transform/source_map_test.rb
index d70ed03..2375896 100644
--- a/test/ast_transform/source_map_test.rb
+++ b/test/ast_transform/source_map_test.rb
@@ -105,6 +105,20 @@ def run(node)
       assert_equal 1, source_map.line(2)
     end
 
+    test "source map is retrievable by source file path" do
+      transformer = ASTTransform::Transformer.new
+
+      source = <<~HEREDOC
+        method_call
+      HEREDOC
+
+      transformer.transform_file_source(source, '/original/path.rb', '/transformed/path.rb')
+
+      source_map = ASTTransform::SourceMap.for_file_path('/original/path.rb')
+      refute_nil source_map, "source map should be retrievable by source file path"
+      assert_equal '/original/path.rb', source_map.source_file_path
+    end
+
     test "#line returns nil when transformation creates nodes that don't contain previous nodes" do
       transformation = Class.new(ASTTransform::AbstractTransformation) do
         def run(node)