diff --git a/forgejo/README.md b/forgejo/README.md
index b64a9a1..81bdee5 100644
--- a/forgejo/README.md
+++ b/forgejo/README.md
@@ -286,17 +286,25 @@ Or apply the blocks from `server-config/gitconfig-ifcmerge` directly into
 
 ### Merge direction
 
-The merge driver is asymmetrical: `ifcmerge` rewrites STEP IDs from `%B`
-(theirs) to match `%A` (ours), so `%A`'s ID space is preserved.
+The invariant is that step-IDs in `main`/`master` must never be renumbered,
+because existing cross-references depend on them.  The correct driver depends
+on which branch is the "local" (`%A`) side:
 
-For Forgejo, use the **"Merge commit"** strategy (not rebase or squash).
-With a merge commit, `%A` is always the base branch (e.g. `main`) and
-`%B` is the pull-request branch — so `main`'s STEP IDs are preserved and
-existing cross-references remain valid.
+| Situation | `%A` (local) | `%B` (remote) | Driver to use |
+|---|---|---|---|
+| Forgejo "Merge commit" button | `main` | PR branch | `ifcmerge_ours` (`--prioritise-local`) |
+| Developer rebasing PR from `main` | PR branch | `main` | `ifcmerge` (default) |
 
-A second driver `ifcmerge_ours` is defined in the config file for cases
-where the base branch arrives as `%B`; see comments in
-`server-config/gitconfig-ifcmerge` for details.
+For Forgejo server-side merges, use the **"Merge commit"** strategy (not
+rebase or squash) and configure `.gitattributes` to use `ifcmerge_ours`:
+
+```
+*.ifc merge=ifcmerge_ours
+```
+
+`ifcmerge` (no flag) is for client-side use by developers updating their
+working branch from `main`, where `main` arrives as the remote (`%B`) side
+and its IDs are preserved by default.
 
 ### Client-side `.gitattributes`
 
diff --git a/forgejo/server-config/gitattributes b/forgejo/server-config/gitattributes
index b0fb910..5f53d2e 100644
--- a/forgejo/server-config/gitattributes
+++ b/forgejo/server-config/gitattributes
@@ -9,4 +9,4 @@
 # Committed per-repo .gitattributes files are NOT read by git in bare-repo
 # context and are therefore insufficient for the server-side conflict check.
 
-*.ifc merge=ifcmerge
+*.ifc merge=ifcmerge_ours
diff --git a/forgejo/server-config/gitconfig-ifcmerge b/forgejo/server-config/gitconfig-ifcmerge
index 5262f9a..d50e478 100644
--- a/forgejo/server-config/gitconfig-ifcmerge
+++ b/forgejo/server-config/gitconfig-ifcmerge
@@ -12,22 +12,32 @@
 #
 # How the merge drivers work
 # --------------------------
-# ifcmerge rewrites STEP IDs from %B to match %A's ID space, so %A's IDs
-# are preserved in the merged result.  Use the "Merge commit" strategy in
-# Forgejo so that %A is always the base branch (e.g. main).
+# The invariant: step-IDs in main/master must never be renumbered, because
+# existing cross-references depend on them.  ifcmerge always renumbers the
+# "remote" side by default; --prioritise-local reverses this and renumbers
+# the "remote" side instead.  The correct driver depends on which branch is
+# checked out when the merge happens:
 #
-# Two variants are registered for symmetry:
-#   ifcmerge        — base branch is %A (standard merge-commit direction)
-#   ifcmerge_ours   — base branch is %B (rebase direction); swaps %A/%B so
-#                     base-branch IDs are still preserved
+#   ifcmerge        — use when local (%A) is a working/PR branch and
+#                     remote (%B) is main/master.  Default behaviour
+#                     preserves remote (main) IDs.
+#                     e.g. rebasing or updating a PR branch from main.
+#
+#   ifcmerge_ours   — use when local (%A) is main/master and remote (%B)
+#                     is a working/PR branch.  --prioritise-local preserves
+#                     local (main) IDs.
+#                     e.g. Forgejo "Merge commit" button (standard PR merge).
+#
+# Use the "Merge commit" strategy in Forgejo (not rebase or squash) so that
+# %A is always the base branch — ifcmerge_ours is then the correct driver.
 
 [core]
     attributesFile = /etc/gitattributes
 
 [merge "ifcmerge"]
-    name = IFC merge driver (base is ours)
-    driver = ifcmerge %O %A %B %L
+    name = IFC merge driver (merging from main into working branch)
+    driver = ifcmerge %O %A %B %A
 
 [merge "ifcmerge_ours"]
-    name = IFC merge driver (base is theirs)
-    driver = ifcmerge %O %B %A %L
+    name = IFC merge driver (merging PR branch into main)
+    driver = ifcmerge --prioritise-local %O %A %B %A