diff --git a/Makefile b/Makefile
index 261bd62a..d9a9593e 100755
--- a/Makefile
+++ b/Makefile
@@ -5,10 +5,16 @@
 # or manually build with meson once.
 #
 # The cell shop is at ~/.cell and core scripts are installed to ~/.cell/core
+#
+# See BUILDING.md for details on the bootstrap process and .mach files.
 
 CELL_SHOP = $(HOME)/.cell
 CELL_CORE_PACKAGE = $(CELL_SHOP)/packages/core
 
+# .cm sources that compile to .mach bytecode
+MACH_SOURCES = tokenize.cm parse.cm fold.cm mcode.cm \
+               internal/bootstrap.cm internal/engine.cm
+
 maker: install
 
 makecell:
@@ -16,7 +22,7 @@ makecell:
 	cp cell /opt/homebrew/bin/
 
 # Install core: symlink this directory to ~/.cell/core
-install: bootstrap $(CELL_SHOP)
+install: bootstrap .mach.stamp $(CELL_SHOP)
 	@echo "Linking cell core to $(CELL_CORE_PACKAGE)"
 	rm -rf $(CELL_CORE_PACKAGE)
 	ln -s $(PWD) $(CELL_CORE_PACKAGE)
@@ -40,6 +46,16 @@ libcell_runtime.dylib: $(CELL_SHOP)/build/dynamic
 cell_main: source/main.c libcell_runtime.dylib
 	cc -o cell_main source/main.c -L. -lcell_runtime -Wl,-rpath,@loader_path -Wl,-rpath,/opt/homebrew/lib
 
+# Regenerate .mach bytecode when any .cm source changes
+.mach.stamp: $(MACH_SOURCES)
+	./cell --core . regen.cm
+	@touch $@
+
+# Force-regenerate all .mach bytecode files
+regen:
+	./cell --core . regen.cm
+	@touch .mach.stamp
+
 # Create the cell shop directories
 $(CELL_SHOP):
 	mkdir -p $(CELL_SHOP)
@@ -57,7 +73,7 @@ static:
 
 # Bootstrap: build cell from scratch using meson (only needed once)
 # Also installs core scripts to ~/.cell/core
-bootstrap: 
+bootstrap:
 	meson setup build_bootstrap -Dbuildtype=debugoptimized
 	meson compile -C build_bootstrap
 	cp build_bootstrap/cell .
@@ -68,7 +84,7 @@ bootstrap:
 # Clean build artifacts
 clean:
 	rm -rf $(CELL_SHOP)/build build_bootstrap
-	rm -f cell cell_main libcell_runtime.dylib
+	rm -f cell cell_main libcell_runtime.dylib .mach.stamp
 
 # Ensure dynamic build directory exists
 $(CELL_SHOP)/build/dynamic: $(CELL_SHOP)
@@ -79,4 +95,4 @@ meson:
 	meson setup build_dbg -Dbuildtype=debugoptimized
 	meson install -C build_dbg
 
-.PHONY: cell static bootstrap clean meson install
+.PHONY: cell static bootstrap clean meson install regen
diff --git a/internal/bootstrap.cm b/internal/bootstrap.cm
index 07fec6e1..2f0984e8 100644
--- a/internal/bootstrap.cm
+++ b/internal/bootstrap.cm
@@ -48,6 +48,41 @@ if (use_mcode) {
   mcode_mod = boot_load("mcode", boot_env)
 }
 
+// Warn if any .cm source is newer than its .mach bytecode
+function check_mach_stale() {
+  var pairs = [
+    ["tokenize.cm", "tokenize.mach"],
+    ["parse.cm", "parse.mach"],
+    ["fold.cm", "fold.mach"],
+    ["mcode.cm", "mcode.mach"],
+    ["internal/bootstrap.cm", "internal/bootstrap.mach"],
+    ["internal/engine.cm", "internal/engine.mach"]
+  ]
+  var stale = []
+  var _i = 0
+  var cm_path = null
+  var mach_path = null
+  var cm_stat = null
+  var mach_stat = null
+  while (_i < length(pairs)) {
+    cm_path = core_path + '/' + pairs[_i][0]
+    mach_path = core_path + '/' + pairs[_i][1]
+    if (fd.is_file(cm_path) && fd.is_file(mach_path)) {
+      cm_stat = fd.stat(cm_path)
+      mach_stat = fd.stat(mach_path)
+      if (cm_stat.mtime > mach_stat.mtime) {
+        push(stale, pairs[_i][0])
+      }
+    }
+    _i = _i + 1
+  }
+  if (length(stale) > 0) {
+    print("warning: bytecode is stale for: " + text(stale, ", ") + "\n")
+    print("run 'make regen' or './cell --core . regen.cm' to update\n")
+  }
+}
+check_mach_stale()
+
 // analyze: tokenize + parse, check for errors
 function analyze(src, filename) {
   var tok_result = tokenize_mod(src, filename)
diff --git a/internal/bootstrap.mach b/internal/bootstrap.mach
index a188f1bf..452430bb 100644
Binary files a/internal/bootstrap.mach and b/internal/bootstrap.mach differ