diff --git a/mercurial/upgrade_utils/actions.py b/mercurial/upgrade_utils/actions.py
--- a/mercurial/upgrade_utils/actions.py
+++ b/mercurial/upgrade_utils/actions.py
@@ -235,6 +235,27 @@
         b'faster'
     )
 
+@registerformatvariant
+class internalphases(requirementformatvariant):
+    name = b'internalphases'
+
+    _requirement = requirements.INTERNAL_PHASE_REQUIREMENT
+
+    default = False
+    description = _(
+        b'older repositories do not support '
+        b'internal phases, which makes it impossible to '
+        b'use features like "soft strip"'
+    )
+    upgrademessage = _(
+        b'repository will be able to use internal phases, '
+        b'making features like "soft strip" possible'
+    )
+
+    touches_filelogs = False
+    touches_manifests = False
+    touches_changelog = False
+    touches_requirements = True
 
 @registerformatvariant
 class sharesafe(requirementformatvariant):
@@ -979,6 +1000,7 @@
         requirements.REVLOGV2_REQUIREMENT,
         requirements.CHANGELOGV2_REQUIREMENT,
         requirements.DIRSTATE_V2_REQUIREMENT,
+        requirements.INTERNAL_PHASE_REQUIREMENT,
     }
     for name in compression.compengines:
         engine = compression.compengines[name]
@@ -1011,6 +1033,7 @@
         requirements.REVLOGV2_REQUIREMENT,
         requirements.CHANGELOGV2_REQUIREMENT,
         requirements.DIRSTATE_V2_REQUIREMENT,
+        requirements.INTERNAL_PHASE_REQUIREMENT,
     }
     for name in compression.compengines:
         engine = compression.compengines[name]
diff --git a/tests/test-copies-chain-merge.t b/tests/test-copies-chain-merge.t
--- a/tests/test-copies-chain-merge.t
+++ b/tests/test-copies-chain-merge.t
@@ -1655,6 +1655,7 @@
   dirstate-v2:         no     no      no
   dotencode:          yes    yes     yes
   generaldelta:       yes    yes     yes
+  internalphases:      no     no      no
   share-safe:          no     no      no
   sparserevlog:       yes    yes     yes
   persistent-nodemap:  no     no      no (no-rust !)
@@ -1695,6 +1696,7 @@
   dirstate-v2:         no     no      no
   dotencode:          yes    yes     yes
   generaldelta:       yes    yes     yes
+  internalphases:      no     no      no
   share-safe:          no     no      no
   sparserevlog:       yes    yes     yes
   persistent-nodemap:  no     no      no (no-rust !)
diff --git a/tests/test-copies-in-changeset.t b/tests/test-copies-in-changeset.t
--- a/tests/test-copies-in-changeset.t
+++ b/tests/test-copies-in-changeset.t
@@ -38,6 +38,7 @@
   dirstate-v2:         no     no      no
   dotencode:          yes    yes     yes
   generaldelta:       yes    yes     yes
+  internalphases:      no     no      no
   share-safe:          no     no      no
   sparserevlog:       yes    yes     yes
   persistent-nodemap:  no     no      no (no-rust !)
@@ -56,6 +57,7 @@
   dirstate-v2:         no     no      no
   dotencode:          yes    yes     yes
   generaldelta:       yes    yes     yes
+  internalphases:      no     no      no
   share-safe:          no     no      no
   sparserevlog:       yes    yes     yes
   persistent-nodemap:  no     no      no (no-rust !)
@@ -431,6 +433,7 @@
   dirstate-v2:         no     no      no
   dotencode:          yes    yes     yes
   generaldelta:       yes    yes     yes
+  internalphases:      no     no      no
   share-safe:          no     no      no
   sparserevlog:       yes    yes     yes
   persistent-nodemap:  no     no      no (no-rust !)
@@ -462,6 +465,7 @@
   dirstate-v2:         no     no      no
   dotencode:          yes    yes     yes
   generaldelta:       yes    yes     yes
+  internalphases:      no     no      no
   share-safe:          no     no      no
   sparserevlog:       yes    yes     yes
   persistent-nodemap:  no     no      no (no-rust !)
@@ -490,6 +494,7 @@
   dirstate-v2:         no     no      no
   dotencode:          yes    yes     yes
   generaldelta:       yes    yes     yes
+  internalphases:      no     no      no
   share-safe:          no     no      no
   sparserevlog:       yes    yes     yes
   persistent-nodemap:  no     no      no (no-rust !)
diff --git a/tests/test-persistent-nodemap.t b/tests/test-persistent-nodemap.t
--- a/tests/test-persistent-nodemap.t
+++ b/tests/test-persistent-nodemap.t
@@ -60,6 +60,7 @@
   dirstate-v2:         no
   dotencode:          yes
   generaldelta:       yes
+  internalphases:      no
   share-safe:          no
   sparserevlog:       yes
   persistent-nodemap: yes
@@ -581,6 +582,7 @@
   dirstate-v2:         no     no      no
   dotencode:          yes    yes     yes
   generaldelta:       yes    yes     yes
+  internalphases:      no     no      no
   share-safe:          no     no      no
   sparserevlog:       yes    yes     yes
   persistent-nodemap: yes     no      no
@@ -631,6 +633,7 @@
   dirstate-v2:         no     no      no
   dotencode:          yes    yes     yes
   generaldelta:       yes    yes     yes
+  internalphases:      no     no      no
   share-safe:          no     no      no
   sparserevlog:       yes    yes     yes
   persistent-nodemap:  no    yes      no
diff --git a/tests/test-sidedata.t b/tests/test-sidedata.t
--- a/tests/test-sidedata.t
+++ b/tests/test-sidedata.t
@@ -55,6 +55,7 @@
   dirstate-v2:         no     no      no
   dotencode:          yes    yes     yes
   generaldelta:       yes    yes     yes
+  internalphases:      no     no      no
   share-safe:          no     no      no
   sparserevlog:       yes    yes     yes
   persistent-nodemap:  no     no      no (no-rust !)
@@ -72,6 +73,7 @@
   dirstate-v2:         no     no      no
   dotencode:          yes    yes     yes
   generaldelta:       yes    yes     yes
+  internalphases:      no     no      no
   share-safe:          no     no      no
   sparserevlog:       yes    yes     yes
   persistent-nodemap:  no     no      no (no-rust !)
@@ -95,6 +97,7 @@
   dirstate-v2:         no     no      no
   dotencode:          yes    yes     yes
   generaldelta:       yes    yes     yes
+  internalphases:      no     no      no
   share-safe:          no     no      no
   sparserevlog:       yes    yes     yes
   persistent-nodemap:  no     no      no (no-rust !)
@@ -112,6 +115,7 @@
   dirstate-v2:         no     no      no
   dotencode:          yes    yes     yes
   generaldelta:       yes    yes     yes
+  internalphases:      no     no      no
   share-safe:          no     no      no
   sparserevlog:       yes    yes     yes
   persistent-nodemap:  no     no      no (no-rust !)
diff --git a/tests/test-upgrade-repo.t b/tests/test-upgrade-repo.t
--- a/tests/test-upgrade-repo.t
+++ b/tests/test-upgrade-repo.t
@@ -60,6 +60,7 @@
   dirstate-v2:         no
   dotencode:          yes
   generaldelta:       yes
+  internalphases:      no
   share-safe:          no
   sparserevlog:       yes
   persistent-nodemap:  no (no-rust !)
@@ -76,6 +77,7 @@
   dirstate-v2:         no     no      no
   dotencode:          yes    yes     yes
   generaldelta:       yes    yes     yes
+  internalphases:      no     no      no
   share-safe:          no     no      no
   sparserevlog:       yes    yes     yes
   persistent-nodemap:  no     no      no (no-rust !)
@@ -93,6 +95,7 @@
   dirstate-v2:         no     no      no
   dotencode:          yes     no     yes
   generaldelta:       yes    yes     yes
+  internalphases:      no     no      no
   share-safe:          no     no      no
   sparserevlog:       yes    yes     yes
   persistent-nodemap:  no     no      no (no-rust !)
@@ -110,6 +113,7 @@
   [formatvariant.name.uptodate|dirstate-v2:       ][formatvariant.repo.uptodate|  no][formatvariant.config.default|     no][formatvariant.default|      no]
   [formatvariant.name.mismatchconfig|dotencode:         ][formatvariant.repo.mismatchconfig| yes][formatvariant.config.special|     no][formatvariant.default|     yes]
   [formatvariant.name.uptodate|generaldelta:      ][formatvariant.repo.uptodate| yes][formatvariant.config.default|    yes][formatvariant.default|     yes]
+  [formatvariant.name.uptodate|internalphases:    ][formatvariant.repo.uptodate|  no][formatvariant.config.default|     no][formatvariant.default|      no]
   [formatvariant.name.uptodate|share-safe:        ][formatvariant.repo.uptodate|  no][formatvariant.config.default|     no][formatvariant.default|      no]
   [formatvariant.name.uptodate|sparserevlog:      ][formatvariant.repo.uptodate| yes][formatvariant.config.default|    yes][formatvariant.default|     yes]
   [formatvariant.name.uptodate|persistent-nodemap:][formatvariant.repo.uptodate|  no][formatvariant.config.default|     no][formatvariant.default|      no] (no-rust !)
@@ -150,6 +154,12 @@
    {
     "config": false,
     "default": false,
+    "name": "internalphases",
+    "repo": false
+   },
+   {
+    "config": false,
+    "default": false,
     "name": "share-safe",
     "repo": false
    },
@@ -340,6 +350,7 @@
   dirstate-v2:         no
   dotencode:           no
   generaldelta:        no
+  internalphases:      no
   share-safe:          no
   sparserevlog:        no
   persistent-nodemap:  no
@@ -355,6 +366,7 @@
   dirstate-v2:         no     no      no
   dotencode:           no    yes     yes
   generaldelta:        no    yes     yes
+  internalphases:      no     no      no
   share-safe:          no     no      no
   sparserevlog:        no    yes     yes
   persistent-nodemap:  no     no      no (no-rust !)
@@ -372,6 +384,7 @@
   dirstate-v2:         no     no      no
   dotencode:           no    yes     yes
   generaldelta:        no     no     yes
+  internalphases:      no     no      no
   share-safe:          no     no      no
   sparserevlog:        no     no     yes
   persistent-nodemap:  no     no      no (no-rust !)
@@ -389,6 +402,7 @@
   [formatvariant.name.uptodate|dirstate-v2:       ][formatvariant.repo.uptodate|  no][formatvariant.config.default|     no][formatvariant.default|      no]
   [formatvariant.name.mismatchconfig|dotencode:         ][formatvariant.repo.mismatchconfig|  no][formatvariant.config.default|    yes][formatvariant.default|     yes]
   [formatvariant.name.mismatchdefault|generaldelta:      ][formatvariant.repo.mismatchdefault|  no][formatvariant.config.special|     no][formatvariant.default|     yes]
+  [formatvariant.name.uptodate|internalphases:    ][formatvariant.repo.uptodate|  no][formatvariant.config.default|     no][formatvariant.default|      no]
   [formatvariant.name.uptodate|share-safe:        ][formatvariant.repo.uptodate|  no][formatvariant.config.default|     no][formatvariant.default|      no]
   [formatvariant.name.mismatchdefault|sparserevlog:      ][formatvariant.repo.mismatchdefault|  no][formatvariant.config.special|     no][formatvariant.default|     yes]
   [formatvariant.name.uptodate|persistent-nodemap:][formatvariant.repo.uptodate|  no][formatvariant.config.default|     no][formatvariant.default|      no] (no-rust !)
@@ -1372,6 +1386,7 @@
   dirstate-v2:         no     no      no
   dotencode:          yes    yes     yes
   generaldelta:       yes    yes     yes
+  internalphases:      no     no      no
   share-safe:          no     no      no
   sparserevlog:       yes    yes     yes
   persistent-nodemap:  no     no      no (no-rust !)
@@ -1414,6 +1429,7 @@
   dirstate-v2:         no     no      no
   dotencode:          yes    yes     yes
   generaldelta:       yes    yes     yes
+  internalphases:      no     no      no
   share-safe:          no     no      no
   sparserevlog:       yes    yes     yes
   persistent-nodemap:  no     no      no (no-rust !)
@@ -1459,6 +1475,7 @@
   dirstate-v2:         no     no      no
   dotencode:          yes    yes     yes
   generaldelta:       yes    yes     yes
+  internalphases:      no     no      no
   share-safe:          no     no      no
   sparserevlog:       yes    yes     yes
   persistent-nodemap:  no     no      no (no-rust !)
@@ -1510,6 +1527,7 @@
   dirstate-v2:         no     no      no
   dotencode:          yes    yes     yes
   generaldelta:       yes    yes     yes
+  internalphases:      no     no      no
   share-safe:          no     no      no
   sparserevlog:       yes    yes     yes
   persistent-nodemap:  no     no      no (no-rust !)
@@ -1558,6 +1576,7 @@
   dirstate-v2:         no     no      no
   dotencode:          yes    yes     yes
   generaldelta:       yes    yes     yes
+  internalphases:      no     no      no
   share-safe:          no     no      no
   sparserevlog:       yes    yes     yes
   persistent-nodemap:  no     no      no (no-rust !)
@@ -1607,6 +1626,7 @@
   dirstate-v2:         no     no      no
   dotencode:          yes    yes     yes
   generaldelta:       yes    yes     yes
+  internalphases:      no     no      no
   share-safe:          no     no      no
   sparserevlog:       yes    yes     yes
   persistent-nodemap:  no     no      no (no-rust !)
@@ -1644,6 +1664,7 @@
   dirstate-v2:         no    yes      no
   dotencode:          yes    yes     yes
   generaldelta:       yes    yes     yes
+  internalphases:      no     no      no
   share-safe:          no     no      no
   sparserevlog:       yes    yes     yes
   persistent-nodemap: yes    yes      no
@@ -1683,6 +1704,7 @@
   dirstate-v2:        yes     no      no
   dotencode:          yes    yes     yes
   generaldelta:       yes    yes     yes
+  internalphases:      no     no      no
   share-safe:          no     no      no
   sparserevlog:       yes    yes     yes
   persistent-nodemap: yes    yes      no
@@ -1723,6 +1745,7 @@
   dirstate-v2:         no     no      no
   dotencode:          yes    yes     yes
   generaldelta:       yes    yes     yes
+  internalphases:      no     no      no
   share-safe:          no     no      no
   sparserevlog:       yes    yes     yes
   persistent-nodemap: yes    yes      no