diff --git a/build_config/cosmopolitan.rb b/build_config/cosmopolitan.rb
new file mode 100644
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+++ b/build_config/cosmopolitan.rb
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+# Cosmopolitan Libc build configuration for mruby
+# https://github.com/jart/cosmopolitan
+#
+# Produces Actually Portable Executables (APE) - single binaries that run on:
+#   - Linux (x86_64, ARM64)
+#   - macOS (x86_64, ARM64)
+#   - Windows (x86_64)
+#   - FreeBSD (x86_64)
+#   - OpenBSD (x86_64)
+#   - NetBSD (x86_64)
+#
+# Requirements:
+#   Download cosmocc toolchain from https://cosmo.zip/pub/cosmocc/
+#
+# Usage:
+#   COSMO_ROOT=/path/to/cosmocc rake MRUBY_CONFIG=cosmopolitan
+#
+# The resulting binaries in bin/ will have .com extension and run on all
+# supported platforms without recompilation.
+
+COSMO_ROOT = ENV['COSMO_ROOT']
+
+unless COSMO_ROOT && File.directory?(COSMO_ROOT)
+  msg = <<~MSG
+    Cosmopolitan toolchain not found.
+
+    Please set COSMO_ROOT environment variable to the cosmocc directory:
+
+      mkdir -p ~/cosmo && cd ~/cosmo
+      wget https://cosmo.zip/pub/cosmocc/cosmocc.zip
+      unzip cosmocc.zip
+      export COSMO_ROOT=~/cosmo
+
+    Then run:
+      COSMO_ROOT=~/cosmo rake MRUBY_CONFIG=cosmopolitan
+  MSG
+  raise msg
+end
+
+MRuby::Build.new do |conf|
+  # C compiler
+  conf.cc do |cc|
+    cc.command = "#{COSMO_ROOT}/bin/cosmocc"
+    cc.flags = %w[-Os -fno-omit-frame-pointer]
+  end
+
+  # C++ compiler
+  conf.cxx do |cxx|
+    cxx.command = "#{COSMO_ROOT}/bin/cosmoc++"
+    cxx.flags = conf.cc.flags.dup
+  end
+
+  # Linker
+  conf.linker do |linker|
+    linker.command = "#{COSMO_ROOT}/bin/cosmocc"
+    linker.flags = %w[-static]
+  end
+
+  # Archiver
+  conf.archiver do |archiver|
+    archiver.command = "#{COSMO_ROOT}/bin/cosmoar"
+  end
+
+  # APE binaries use .com extension
+  conf.exts.executable = '.com'
+
+  # Cosmopolitan provides POSIX compatibility, explicitly select POSIX HALs
+  conf.gem core: 'hal-posix-io'
+  conf.gem core: 'hal-posix-socket'
+  conf.gem core: 'hal-posix-dir'
+
+  # Standard library
+  conf.gembox 'stdlib'
+  conf.gembox 'stdlib-ext'
+  conf.gembox 'stdlib-io'  # Includes mruby-io, mruby-socket, mruby-dir
+  conf.gembox 'math'
+  conf.gembox 'metaprog'
+
+  # Binary tools
+  # Note: mruby-bin-config is a shell script and incompatible with .com extension
+  conf.gem core: 'mruby-bin-mrbc'
+  conf.gem core: 'mruby-bin-mruby'
+  conf.gem core: 'mruby-bin-strip'
+  conf.gem core: 'mruby-bin-mirb'
+  conf.gem core: 'mruby-bin-debugger'
+end