diff --git a/mercurial/bundlecaches.py b/mercurial/bundlecaches.py
--- a/mercurial/bundlecaches.py
+++ b/mercurial/bundlecaches.py
@@ -3,6 +3,8 @@
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
+import collections
+
 from .i18n import _
 
 from .thirdparty import attr
@@ -26,8 +28,25 @@ class bundlespec:
     wirecompression = attr.ib()
     version = attr.ib()
     wireversion = attr.ib()
-    params = attr.ib()
-    contentopts = attr.ib()
+    # parameters explicitly overwritten by the config or the specification
+    _explicit_params = attr.ib()
+    # default parameter for the version
+    #
+    # Keeping it separated is useful to check what was actually overwritten.
+    _default_opts = attr.ib()
+
+    @property
+    def params(self):
+        return collections.ChainMap(self._explicit_params, self._default_opts)
+
+    @property
+    def contentopts(self):
+        # kept for Backward Compatibility concerns.
+        return self.params
+
+    def set_param(self, key, value):
+        """overwrite a parameter value"""
+        self._explicit_params[key] = value
 
 
 # Maps bundle version human names to changegroup versions.
diff --git a/mercurial/commands.py b/mercurial/commands.py
--- a/mercurial/commands.py
+++ b/mercurial/commands.py
@@ -1570,7 +1570,7 @@ def bundle(ui, repo, fname, *dests, **op
             pycompat.bytestr(e),
             hint=_(b"see 'hg help bundlespec' for supported values for --type"),
         )
-    cgversion = bundlespec.contentopts[b"cg.version"]
+    cgversion = bundlespec.params[b"cg.version"]
 
     # Packed bundles are a pseudo bundle format for now.
     if cgversion == b's1':
@@ -1680,14 +1680,12 @@ def bundle(ui, repo, fname, *dests, **op
     # Bundling of obsmarker and phases is optional as not all clients
     # support the necessary features.
     cfg = ui.configbool
-    contentopts = {
-        b'obsolescence': cfg(b'experimental', b'evolution.bundle-obsmarker'),
-        b'obsolescence-mandatory': cfg(
-            b'experimental', b'evolution.bundle-obsmarker:mandatory'
-        ),
-        b'phases': cfg(b'experimental', b'bundle-phases'),
-    }
-    bundlespec.contentopts.update(contentopts)
+    obsolescence_cfg = cfg(b'experimental', b'evolution.bundle-obsmarker')
+    bundlespec.set_param(b'obsolescence', obsolescence_cfg)
+    obs_mand_cfg = cfg(b'experimental', b'evolution.bundle-obsmarker:mandatory')
+    bundlespec.set_param(b'obsolescence-mandatory', obs_mand_cfg)
+    phases_cfg = cfg(b'experimental', b'bundle-phases')
+    bundlespec.set_param(b'phases', phases_cfg)
 
     bundle2.writenewbundle(
         ui,
@@ -1696,7 +1694,7 @@ def bundle(ui, repo, fname, *dests, **op
         fname,
         bversion,
         outgoing,
-        bundlespec.contentopts,
+        bundlespec.params,
         compression=bcompression,
         compopts=compopts,
     )