diff --git a/lib/ast_transform/layout.rb b/lib/ast_transform/layout.rb
index f6e85d7..8fe9d58 100644
--- a/lib/ast_transform/layout.rb
+++ b/lib/ast_transform/layout.rb
@@ -9,6 +9,7 @@ module ASTTransform
   class Layout
     def initialize
       @lines = []
+      @sealed = false
     end
 
     # The line number currently being written; the next fresh line would be +cursor + 1+.
@@ -21,7 +22,10 @@ def cursor
     # +column+ — cosmetic only (leading whitespace is never significant in emitted code), but it keeps the artifact
     # visually close to the source. Packed text ignores the column, as do continuation lines (they keep their own
     # relative indentation).
-    def place(target_line, text, column: nil)
+    #
+    # +seal+ declares the text's last line unextendable (the caller's property — e.g. it terminates a heredoc,
+    # whose terminator must stand alone): a subsequent pack opens a fresh line instead of joining it.
+    def place(target_line, text, column: nil, seal: false)
       first, *rest = text.split("\n")
 
       if target_line && target_line > @lines.size
@@ -32,22 +36,27 @@ def place(target_line, text, column: nil)
       end
 
       @lines.concat(rest)
+      @sealed = seal
     end
 
     # Appends +text+ on a new line unconditionally — for text that must never be `;`-packed after a statement
     # (e.g. keywords).
     def place_on_fresh_line(text)
       @lines << text
+      @sealed = false
     end
 
-    # Appends +text+ to the current line with a `; ` separator. The last line is never blank here: padding blanks
-    # are only created inside +place+, which immediately overwrites the padded line.
+    # Appends +text+ to the current line with a `; ` separator — unless the current line is sealed, in which case
+    # the text opens a fresh line (alignment degrades by one more line; validity is preserved). The last line is
+    # never blank here: padding blanks are only created inside +place+, which immediately overwrites the padded
+    # line.
     def pack(text)
-      if @lines.empty?
+      if @lines.empty? || @sealed
         @lines << text
       else
         @lines[-1] = "#{@lines.last}; #{text}"
       end
+      @sealed = false
     end
 
     # @return [String] the laid-out text, with a trailing newline.
diff --git a/lib/ast_transform/line_aligned_emitter.rb b/lib/ast_transform/line_aligned_emitter.rb
index f7c46db..6e2e87e 100644
--- a/lib/ast_transform/line_aligned_emitter.rb
+++ b/lib/ast_transform/line_aligned_emitter.rb
@@ -71,7 +71,9 @@ def emit_statement(node, layout, renderer)
       if recursive_container?(node)
         emit_container(node, layout, renderer)
       else
-        layout.place(node.loc&.line, renderer.aligned_render(node), column: node.loc&.column)
+        render = renderer.aligned_render(node)
+        # A render in heredoc form seals its line: nothing may pack after the terminator (issue #16).
+        layout.place(node.loc&.line, render, column: node.loc&.column, seal: renderer.line_terminal?(render))
       end
     end
 
@@ -145,7 +147,8 @@ def block_assignment?(node)
     # nested statements align.
     def emit_container(node, layout, renderer)
       opener, closer = container_delimiters(node, renderer)
-      layout.place(node.loc&.line, opener, column: node.loc&.column)
+      # An opener can carry a heredoc too (e.g. a block call with a heredoc argument) — seal it like a statement.
+      layout.place(node.loc&.line, opener, column: node.loc&.column, seal: renderer.line_terminal?(opener))
       emit_body(container_body(node), layout, renderer)
       layout.place(closer_line(node), closer, column: closer_column(node))
     end
diff --git a/lib/ast_transform/statement_renderer.rb b/lib/ast_transform/statement_renderer.rb
index a5df222..847c768 100644
--- a/lib/ast_transform/statement_renderer.rb
+++ b/lib/ast_transform/statement_renderer.rb
@@ -62,8 +62,30 @@ def aligned_render(node)
       compress_to_single_line(render) || render
     end
 
+    # Whether +render+ must be the last text on its final line: a render ending on a heredoc terminator (Unparser
+    # falls back to `<<-HEREDOC` form when its quoted-string round-trip fails) cannot have anything `;`-packed
+    # after it — the terminator must stand alone, so a join unterminates the heredoc. Probed by re-parse, the same
+    # verification style as +compress_to_single_line+, behind a cheap textual gate: only a heredoc makes a last
+    # line unextendable, and every heredoc opener contains +<<+. A false positive (the probe failing for another
+    # reason, e.g. an isolated container opener carrying a heredoc argument) costs one line of alignment, never
+    # correctness.
+    def line_terminal?(render)
+      return false unless render.include?('<<')
+
+      # chomp: heredoc renders end with a trailing newline; the probe must extend the last *line* (where the
+      # layout would pack), not append below it.
+      !parses?("#{render.chomp}; nil")
+    end
+
     private
 
+    def parses?(source)
+      Unparser.parse(source)
+      true
+    rescue Parser::SyntaxError
+      false
+    end
+
     def compress_to_single_line(render)
       candidate = render.split("\n").map(&:strip).join('; ')
       # Both sides parsed without scope context, so lvar/send ambiguity cancels out; equality means the newline
diff --git a/test/ast_transform/layout_test.rb b/test/ast_transform/layout_test.rb
index 011c1f5..8554d21 100644
--- a/test/ast_transform/layout_test.rb
+++ b/test/ast_transform/layout_test.rb
@@ -71,6 +71,31 @@ class LayoutTest < Minitest::Test
       assert_equal "statement\nensure\n", layout.to_source
     end
 
+    test "a sealed placement forces the next pack onto a fresh line" do
+      layout = Layout.new
+      layout.place(1, "value = <<-EOS\nbody\nEOS", seal: true)
+      layout.place(2, 'displaced')
+
+      assert_equal "value = <<-EOS\nbody\nEOS\ndisplaced\n", layout.to_source
+    end
+
+    test "the seal covers one pack: packing resumes on the fallback line" do
+      layout = Layout.new
+      layout.place(1, 'terminal', seal: true)
+      layout.place(1, 'first_displaced')
+      layout.place(1, 'second_displaced')
+
+      assert_equal "terminal\nfirst_displaced; second_displaced\n", layout.to_source
+    end
+
+    test "padding to a line ahead is unaffected by the seal" do
+      layout = Layout.new
+      layout.place(1, 'terminal', seal: true)
+      layout.place(3, 'ahead')
+
+      assert_equal "terminal\n\nahead\n", layout.to_source
+    end
+
     test "pack onto an empty layout opens the first line" do
       layout = Layout.new
       layout.pack('lonely')
diff --git a/test/ast_transform/line_aligned_emitter_test.rb b/test/ast_transform/line_aligned_emitter_test.rb
index a3a9cb4..505fa88 100644
--- a/test/ast_transform/line_aligned_emitter_test.rb
+++ b/test/ast_transform/line_aligned_emitter_test.rb
@@ -292,6 +292,26 @@ def run; work
       RUBY
     end
 
+    # This dstr fails Unparser's quoted-string round-trip and re-renders as `<<-HEREDOC` form — three lines
+    # tall from a one-line source, displacing every following placement into packing mode (issue #16).
+    HEREDOC_FALLBACK_SOURCE = <<~'RUBY'
+      def write_learning(dir, slug, index_domain: "tooling")
+        line = "- [#{index_domain}/#{slug}] The #{slug} cue. → .cursor/skills/learnings/#{slug}/\n"
+        File.open(File.join(dir, INDEX), "a") { |index| index.write(line) }
+      end
+    RUBY
+
+    test "a render in heredoc form is line-terminal: displaced statements and closers never pack onto its terminator" do
+      emitted = emit(parse(HEREDOC_FALLBACK_SOURCE))
+
+      # Before the fix this was `HEREDOC; File.open(...); end` — an unterminated heredoc swallowing the file.
+      RubyVM::InstructionSequence.compile(emitted)
+      terminator_line = emitted.split("\n").find { |line| line.start_with?('HEREDOC') }
+
+      assert_equal 'HEREDOC', terminator_line
+      assert_includes emitted, 'File.open'
+    end
+
     test "a thunk inside a def stays inside the def (scope boundary)" do
       def_node = parse("def run\n  helper\nend\n")
       name, args, body = def_node.children
diff --git a/test/ast_transform/statement_renderer_test.rb b/test/ast_transform/statement_renderer_test.rb
index 2602b25..154da6d 100644
--- a/test/ast_transform/statement_renderer_test.rb
+++ b/test/ast_transform/statement_renderer_test.rb
@@ -49,6 +49,23 @@ def parse(source)
       assert_nil renderer.send(:compress_to_single_line, "value = <<~TXT\n  hi\nTXT")
     end
 
+    test "line_terminal? flags a render ending on a heredoc terminator" do
+      renderer = StatementRenderer.for_tree(parse("noop\n"))
+
+      assert renderer.line_terminal?("value = <<-HEREDOC\nbody\nHEREDOC")
+    end
+
+    test "line_terminal? passes renders whose last line can be extended" do
+      renderer = StatementRenderer.for_tree(parse("noop\n"))
+
+      refute renderer.line_terminal?('value = 1')
+      # `<<` appears without opening a heredoc: the shovel operator, and heredoc-looking text inside a string.
+      refute renderer.line_terminal?('queue << item')
+      refute renderer.line_terminal?('value = "<<-EOS"')
+      # A quoted multi-line string's last line closes the literal and stays extendable.
+      refute renderer.line_terminal?("value = \"a\nb\"")
+    end
+
     test "for_tree collects assignments and every parameter flavor" do
       tree = parse(<<~HEREDOC)
         assigned = 1