diff --git a/mitogen/parent.py b/mitogen/parent.py
index f301a42c5..637389d5d 100644
--- a/mitogen/parent.py
+++ b/mitogen/parent.py
@@ -1385,10 +1385,6 @@ def __repr__(self):
     # with a custom argv.
     #   * Optimized for minimum byte count after minification & compression.
     #     The script preamble_size.py measures this.
-    #   * 'CONTEXT_NAME' and 'PREAMBLE_COMPRESSED_LEN' are substituted with
-    #     their respective values.
-    #   * CONTEXT_NAME must be prefixed with the name of the Python binary in
-    #     order to allow virtualenvs to detect their install prefix.
     #
     # macOS tweaks for Python 2.7 must be kept in sync with the the Ansible
     # module test_echo_module, used by the integration tests.
@@ -1424,20 +1420,20 @@ def _first_stage():
             os.close(r)
             os.close(W)
             os.close(w)
-            if os.uname()[0]=='Darwin'and os.uname()[2][:2]<'19'and sys.executable=='/usr/bin/python':sys.executable='/usr/bin/python2.7'
-            if os.uname()[0]=='Darwin'and os.uname()[2][:2]in'2021'and sys.version[:3]=='2.7':os.environ['PYTHON_LAUNCHED_FROM_WRAPPER']='1'
+            if os.uname()[0]+sys.executable=='Darwin/usr/bin/python'and os.uname()[2][:2]<'19':sys.executable+='2.7'
+            if os.uname()[0]+sys.version[:3]=='Darwin2.7'and os.uname()[2][:2]in'2021':os.environ['PYTHON_LAUNCHED_FROM_WRAPPER']='1'
             os.environ['ARGV0']=sys.executable
-            os.execl(sys.executable,sys.executable+'(mitogen:CONTEXT_NAME)')
+            os.execl(sys.executable,sys.executable+'(mitogen:%s)'%sys.argv[2])
         os.write(1,'MITO000\n'.encode())
-        fp=os.fdopen(0,'rb')
-        C=zlib.decompress(fp.read(PREAMBLE_COMPRESSED_LEN))
-        fp.close()
-        fp=os.fdopen(W,'wb',0)
-        fp.write(C)
-        fp.close()
-        fp=os.fdopen(w,'wb',0)
-        fp.write(C)
-        fp.close()
+        f=os.fdopen(0,'rb')
+        C=zlib.decompress(f.read(int(sys.argv[3])))
+        f.close()
+        f=os.fdopen(W,'wb',0)
+        f.write(C)
+        f.close()
+        f=os.fdopen(w,'wb',0)
+        f.write(C)
+        f.close()
         os.write(1,'MITO001\n'.encode())
         os.close(2)
 
@@ -1458,11 +1454,8 @@ def get_boot_command(self):
         source = inspect.getsource(self._first_stage)
         source = textwrap.dedent('\n'.join(source.strip().split('\n')[2:]))
         source = source.replace('    ', ' ')
-        source = source.replace('CONTEXT_NAME', self.options.remote_name)
-        preamble_compressed = self.get_preamble()
-        source = source.replace('PREAMBLE_COMPRESSED_LEN',
-                                str(len(preamble_compressed)))
-        compressed = zlib.compress(source.encode(), 9)
+        compressor = zlib.compressobj(9, zlib.DEFLATED, -15, 9)
+        compressed = compressor.compress(source.encode()) + compressor.flush()
         encoded = binascii.b2a_base64(compressed).replace(b('\n'), b(''))
 
         # Just enough to decode, decompress, and exec the first stage.
@@ -1472,7 +1465,10 @@ def get_boot_command(self):
         return self.get_python_argv() + [
             '-c',
             'import sys;sys.path=[p for p in sys.path if p];import binascii,os,zlib;'
-            'exec(zlib.decompress(binascii.a2b_base64("%s")))' % (encoded.decode(),),
+            'exec(zlib.decompress(binascii.a2b_base64(sys.argv[1]),-15))',
+            encoded.decode(),
+            self.options.remote_name,
+            str(len(self.get_preamble())),
         ]
 
     def get_econtext_config(self):