diff --git a/.github/workflows/rake.yml b/.github/workflows/rake.yml
index cc352e78..c15a542d 100644
--- a/.github/workflows/rake.yml
+++ b/.github/workflows/rake.yml
@@ -38,9 +38,13 @@ jobs:
       - name: Install Python helper and gems
         working-directory: ruby
         run: |
-          pip install regex
           bundle config set --local with jsexec
           bundle install
+          # PyCall binds whichever libpython it was built against, which
+          # is not necessarily the python that bare `pip` targets —
+          # install the regex helper into that interpreter.
+          PYBIN=$(bundle exec ruby -e 'require "pycall"; v = PyCall.sys.version[/\d+\.\d+/]; puts "python#{v}"')
+          "$PYBIN" -m pip install regex
 
       - name: RSpec
         working-directory: ruby
diff --git a/lib/interscript/compiler/python.rb b/lib/interscript/compiler/python.rb
index 37705ad2..bc9046b8 100644
--- a/lib/interscript/compiler/python.rb
+++ b/lib/interscript/compiler/python.rb
@@ -290,10 +290,14 @@ def compile_item i, doc = @map, target = nil
   class << self
     attr_accessor :maps_loaded
     attr_accessor :ctx
+    attr_accessor :maps_dir
   end
 
   def load
-    if !self.class.maps_loaded[@map.name]
+    # Keyed by document identity: synthetic spec documents reuse names
+    # (example-1, example-2, ...), and a stale engine under a reused
+    # name would silently serve the previous document.
+    if self.class.maps_loaded[@map.name] != @map.object_id
       @map.dependencies.each do |dep|
         dep = dep.full_name
         if !self.class.maps_loaded[dep]
@@ -302,27 +306,42 @@ def load
       end
 
       ctx = self.class.ctx
-      python_src_path = File.join(__dir__, "..", "..", "..", "..", "python", "src")
+      python_src_path = ENV["INTERSCRIPT_PYTHON_SRC"] ||
+        File.join(__dir__, "..", "..", "..", "..", "python", "src")
       unless ctx
-        PyCall.sys.path.append(python_src_path)
+        # Insert at 0: an editable install of interscript-python may
+        # shadow the monorepo checkout on sys.path.
+        PyCall.sys.path.insert(0, python_src_path)
         self.class.ctx = PyCall.import_module("interscript")
+        ctx = self.class.ctx
       end
-      # puts @code
-      begin
-        Dir.mkdir("#{python_src_path}/interscript/maps")
-      rescue
-        nil
+      # The rewritten python runtime executes ISC sources from its load
+      # paths, so in-memory documents cross the boundary as generated
+      # ISC text in a per-process temp dir — never inside the python
+      # package tree.
+      unless self.class.maps_dir
+        require "tmpdir" unless defined?(Dir.mktmpdir)
+        self.class.maps_dir = Dir.mktmpdir("interscript-python")
+        ctx.add_load_path(self.class.maps_dir)
       end
-      File.write("#{python_src_path}/interscript/maps/#{@map.name}.py", @code)
+      File.write(File.join(self.class.maps_dir, "#{@map.name}.isc"),
+        Interscript::Isc::Generator.generate(@map))
       self.class.ctx.load_map(@map.name)
 
-      self.class.maps_loaded[@map.name] = true
+      if self.class.maps_loaded.key?(@map.name)
+        self.class.ctx.unload_map(@map.name)
+      end
+      self.class.maps_loaded[@map.name] = @map.object_id
     end
   end
 
   def call(str, stage = :main)
     load
-    self.class.ctx.transliterate(@map.name, str, stage.to_s)
+    unless stage.to_s == "main"
+      raise Interscript::MapLogicError,
+        "the python runtime transliterates the whole map; stage #{stage} can't be selected"
+    end
+    self.class.ctx.transliterate(@map.name, str)
   end
 
   def self.read_debug_data
diff --git a/lib/interscript/isc.rb b/lib/interscript/isc.rb
index d5aa2f93..a1aeb766 100644
--- a/lib/interscript/isc.rb
+++ b/lib/interscript/isc.rb
@@ -15,6 +15,7 @@ module Isc
     autoload :YamlBridge, "interscript/isc/yaml_bridge"
     autoload :Serializer, "interscript/isc/serializer"
     autoload :Normalizer, "interscript/isc/normalizer"
+    autoload :Generator, "interscript/isc/generator"
 
     SCHEMA_VERSION = 1
 
diff --git a/lib/interscript/isc/generator.rb b/lib/interscript/isc/generator.rb
new file mode 100644
index 00000000..1505afce
--- /dev/null
+++ b/lib/interscript/isc/generator.rb
@@ -0,0 +1,178 @@
+# frozen_string_literal: true
+
+module Interscript
+  module Isc
+    # Serializes a Node::Document back to ISC source — the inverse of
+    # Parser + DocumentBuilder + NodeAdapter. The Python compiler
+    # bridge generates ISC from in-memory documents because the
+    # rewritten python runtime executes ISC sources, not compiled
+    # modules. Every construct the generator emits must round-trip:
+    # spec/interscript/isc/generator_spec.rb pins behavior equality.
+    class Generator
+      def self.generate(doc)
+        new(doc).generate
+      end
+
+      def initialize(doc)
+        @doc = doc
+      end
+
+      def generate
+        out = "system #{quote(@doc.name.to_s)} {\n"
+        out << generate_aliases
+        out << generate_dependencies
+        @doc.stages.each do |name, group|
+          out << "  stage #{name} {\n" << generate_group(group, 2) << "  }\n"
+        end
+        out << "}\n"
+      end
+
+      private
+
+      def generate_aliases
+        return "" if @doc.aliases.empty?
+
+        out = +"  aliases {\n"
+        @doc.aliases.each_value do |alias_def|
+          out << "    #{alias_def.name} = #{item(alias_def.data)}\n"
+        end
+        out << "  }\n"
+      end
+
+      def generate_dependencies
+        return "" if @doc.dependencies.empty?
+
+        out = +""
+        @doc.dependencies.each do |dep|
+          out << "  dependency #{quote(dep.full_name.to_s)}"
+          out << " as #{dep.name}" if dep.name
+          out << "\n"
+        end
+        out
+      end
+
+      def generate_group(group, indent)
+        Array(group.children).map { |child| emit(child, indent) }.join
+      end
+
+      def emit(child, indent)
+        pad = " " * indent
+        case child
+        when Interscript::Node::Group::Parallel
+          "#{pad}parallel {\n#{generate_group(child, indent + 2)}#{pad}}\n"
+        when Interscript::Node::Group
+          generate_group(child, indent)
+        when Interscript::Node::Rule::Sub
+          rule(child, pad)
+        when Interscript::Node::Rule::Run
+          "#{pad}run #{run_target(child)}\n"
+        when Interscript::Node::Rule::Funcall
+          funcall(child, pad)
+        else
+          raise UnsupportedConstruct, "cannot generate #{child.class} as a stage item"
+        end
+      end
+
+      def rule(sub, pad)
+        out = "#{pad}sub {\n"
+        out << "#{pad}  from #{item(sub.from)}\n"
+        out << "#{pad}  to #{target(sub.to)}\n"
+        {before: sub.before, after: sub.after,
+         not_before: sub.not_before, not_after: sub.not_after}.each do |kw, value|
+          out << "#{pad}  #{kw} #{item(value)}\n" if value
+        end
+        out << "#{pad}}\n"
+      end
+
+      def target(to)
+        return to.to_s if %i[upcase downcase].include?(to)
+
+        item(to)
+      end
+
+      def run_target(run)
+        stage = run.stage
+        stage.map ? "map.#{stage.map}.stage.#{stage.name}" : "stage.#{stage.name}"
+      end
+
+      def funcall(rule, pad)
+        unless rule.kwargs.empty?
+          raise UnsupportedConstruct,
+            "cannot generate #{rule.name}(...) with kwargs as ISC"
+        end
+
+        "#{pad}#{rule.name}\n"
+      end
+
+      # -- items --
+
+      def item(i)
+        case i
+        when Interscript::Node::Item::String
+          quote(i.data)
+        when Interscript::Node::Item::Any
+          any(i)
+        when Interscript::Node::Item::Alias
+          identifier(i.name)
+        when Interscript::Node::Item::CaptureGroup
+          "capture(#{item(i.data)})"
+        when Interscript::Node::Item::CaptureRef
+          "ref(#{i.id})"
+        when Interscript::Node::Item::Group
+          i.children.map { |c| item(c) }.join(" + ")
+        when Interscript::Node::Item::Maybe
+          "maybe(#{item(i.data)})"
+        when Interscript::Node::Item::Some
+          "some(#{item(i.data)})"
+        when ::String
+          quote(i)
+        when ::Symbol
+          identifier(i)
+        else
+          raise UnsupportedConstruct, "cannot generate #{i.class} as an item"
+        end
+      end
+
+      def any(node)
+        case node.value
+        when ::String
+          "any(#{quote(node.value)})"
+        when ::Range
+          "any(#{quote(node.value.first)}..#{quote(node.value.last)})"
+        when ::Array
+          inner = node.value.map { |v| item(v) }
+          return "any(#{inner.first})" if inner.size == 1
+
+          "any([#{inner.join(", ")}])"
+        else
+          raise UnsupportedConstruct, "cannot generate Any of #{node.value.class}"
+        end
+      end
+
+      def identifier(sym)
+        name = sym.to_s
+        raise UnsupportedConstruct, "#{name.inspect} is not a bare identifier" unless /\A[A-Za-z_][A-Za-z0-9_]*\z/.match?(name)
+
+        name
+      end
+
+      ESCAPE_MAP = {
+        "\\" => "\\\\",
+        "\"" => "\\\""
+      }.freeze
+
+      def quote(str)
+        escaped = str.to_s.gsub(/[\\"]/) { |c| ESCAPE_MAP.fetch(c) }
+        # Only control and DEL characters need escaping; both parsers
+        # read raw UTF-8, including astral code points (surrogate-pair
+        # \u escapes are NOT re-joined).
+        escaped = escaped.each_char.map { |c|
+          (c.ord < 0x20 || c.ord == 0x7f) ? format("\\u%04x", c.ord) : c
+        }.join
+        "\"" + escaped + "\""
+      end
+
+      class UnsupportedConstruct < StandardError; end
+    end
+  end
+end
diff --git a/spec/interscript/isc/generator_spec.rb b/spec/interscript/isc/generator_spec.rb
new file mode 100644
index 00000000..be8e0532
--- /dev/null
+++ b/spec/interscript/isc/generator_spec.rb
@@ -0,0 +1,101 @@
+# frozen_string_literal: true
+
+# Generator round-trip: every document the DSL can build must survive a
+# generate → parse → adapt cycle with identical runtime behavior. This
+# is the contract the Python bridge relies on — the rewritten python
+# runtime executes ISC sources, so in-memory documents cross the
+# boundary as generated ISC text.
+require "spec_helper"
+
+RSpec.describe Interscript::Isc::Generator do
+  def round_trip(doc)
+    isc = described_class.generate(doc)
+    tree = Interscript::Isc::Parser.parse(isc)
+    built = Interscript::Isc::DocumentBuilder.build(tree)
+    Interscript::Isc::NodeAdapter.to_interscript_node(built)
+  end
+
+  def round_trip_stage(&block)
+    original = stage(&block)
+    reparsed = round_trip(original)
+    [original, reparsed]
+  end
+
+  it "generates parseable ISC for a stage document" do
+    doc = stage { sub "b", "e" }
+    isc = described_class.generate(doc)
+    expect(isc).to start_with(%(system "example-))
+    expect { Interscript::Isc::Parser.parse(isc) }.not_to raise_error
+  end
+
+  it "round-trips basic substitution" do
+    original, reparsed = round_trip_stage { sub "b", "e" }
+    expect(reparsed.call("abcd")).to eq(original.call("abcd"))
+  end
+
+  it "round-trips substitution with all four constraints" do
+    original, reparsed = round_trip_stage {
+      sub "a", "A", before: space
+      sub "a", "B", not_before: space
+      sub "a", "C", after: space
+      sub "a", "D", not_after: space
+    }
+    expect(reparsed.call("abcda abcda abada")).to eq(original.call("abcda abcda abada"))
+  end
+
+  it "round-trips a casing target" do
+    original, reparsed = round_trip_stage { sub "b", :upcase }
+    expect(reparsed.call("aba")).to eq(original.call("aba"))
+  end
+
+  it "round-trips parallel blocks" do
+    original, reparsed = round_trip_stage {
+      parallel {
+        sub "ab", "X"
+        sub "a", "Y"
+      }
+    }
+    expect(reparsed.call("aba")).to eq(original.call("aba"))
+  end
+
+  it "round-trips any() sets and ranges" do
+    original, reparsed = round_trip_stage {
+      sub any("abc"), "X"
+      sub "z", "W", before: any("aeiou")
+    }
+    expect(reparsed.call("zaza zab")).to eq(original.call("zaza zab"))
+  end
+
+  it "round-trips capture groups and references" do
+    original, reparsed = round_trip_stage {
+      sub capture("b") + capture("c"), ref(2) + ref(1)
+    }
+    expect(reparsed.call("xbc")).to eq(original.call("xbc"))
+  end
+
+  it "round-trips document-level aliases" do
+    original = document {
+      aliases do
+        def_alias :vowel, any("aeiou")
+      end
+      stage {
+        sub "v", "V", before: vowel
+      }
+    }
+    reparsed = round_trip(original)
+    expect(reparsed.call("ava eve")).to eq(original.call("ava eve"))
+  end
+
+  it "round-trips multiple stages with a stage run" do
+    original = document {
+      stage :first do
+        sub "b", "e"
+      end
+      stage {
+        run Interscript::Node::Item::Stage.new(:first)
+      }
+    }
+    reparsed = round_trip(original)
+    expect(reparsed.call("abcd")).to eq(original.call("abcd"))
+  end
+end
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index 7963afdf..d33efa25 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -19,6 +19,15 @@
   # Disable RSpec exposing methods globally on `Module` and `main`
   config.disable_monkey_patching!
 
+  # dsl_stage_spec sets the $compiler global per example; without a
+  # reset it leaks into every file that runs afterwards, silently
+  # rerouting suites that never opted into a specific compiler.
+  # standard:disable Style/GlobalVars (deliberate $DEBUG / -d-flag debug idiom)
+  config.before(:example) do
+    $compiler = nil
+  end
+  # standard:enable Style/GlobalVars
+
   config.expect_with :rspec do |c|
     c.syntax = :expect
   end