diff --git a/bin/omarchy-dev-add-migration b/bin/omarchy-dev-add-migration
index bed8af3ffd2..87de6e0bf92 100755
--- a/bin/omarchy-dev-add-migration
+++ b/bin/omarchy-dev-add-migration
@@ -31,8 +31,19 @@ done
 
 cd "${OMARCHY_PATH:-$(pwd)}"
 mkdir -p migrations
-migration_file="migrations/$(git log -1 --format=%cd --date=unix).sh"
-touch "$migration_file"
+
+# Use wall-clock time, not HEAD's committer date, so two branches cut from the
+# same commit do not mint the same migration id (#13516).
+id=$(date +%s)
+migration_file="migrations/${id}.sh"
+# noclobber creates the file with O_EXCL, so concurrent runs cannot claim one id.
+set -o noclobber
+until { : >"$migration_file"; } 2>/dev/null; do
+  [[ -e $migration_file ]] || { echo "Cannot create $PWD/$migration_file" >&2; exit 1; }
+  id=$((id + 1))
+  migration_file="migrations/${id}.sh"
+done
+set +o noclobber
 
 if (( ! no_edit )); then
   nvim "$migration_file"
diff --git a/test/shell.d/dev-add-migration-race-test.sh b/test/shell.d/dev-add-migration-race-test.sh
new file mode 100755
index 00000000000..46b1bffb902
--- /dev/null
+++ b/test/shell.d/dev-add-migration-race-test.sh
@@ -0,0 +1,33 @@
+#!/bin/bash
+
+set -euo pipefail
+
+source "$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" && pwd)/base-test.sh"
+
+test_tmp=$(mktemp -d)
+trap 'rm -rf "$test_tmp"' EXIT
+
+# Pin the clock so both runs start from the same second.
+stub_bin="$test_tmp/bin"
+mkdir -p "$stub_bin"
+cat >"$stub_bin/date" <<'SH'
+#!/bin/bash
+[[ ${1:-} == "+%s" ]] || exit 1
+echo 1790700000
+SH
+chmod +x "$stub_bin/date"
+
+for trial in $(seq 1 20); do
+  checkout="$test_tmp/omarchy-$trial"
+  mkdir -p "$checkout/migrations"
+
+  PATH="$stub_bin:$PATH" OMARCHY_PATH="$checkout" "$ROOT/bin/omarchy-dev-add-migration" --no-edit >"$test_tmp/$trial.a" &
+  first=$!
+  PATH="$stub_bin:$PATH" OMARCHY_PATH="$checkout" "$ROOT/bin/omarchy-dev-add-migration" --no-edit >"$test_tmp/$trial.b" &
+  second=$!
+  wait "$first" "$second"
+
+  [[ $(<"$test_tmp/$trial.a") != "$(<"$test_tmp/$trial.b")" ]] ||
+    fail "two concurrent runs in one checkout create distinct migrations" "trial $trial: both printed $(<"$test_tmp/$trial.a")"
+done
+pass "two concurrent runs in one checkout create distinct migrations"
diff --git a/test/shell.d/dev-add-migration-test.sh b/test/shell.d/dev-add-migration-test.sh
new file mode 100755
index 00000000000..3859560bb68
--- /dev/null
+++ b/test/shell.d/dev-add-migration-test.sh
@@ -0,0 +1,41 @@
+#!/bin/bash
+
+set -euo pipefail
+
+source "$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" && pwd)/base-test.sh"
+
+test_tmp=$(mktemp -d)
+trap 'rm -rf "$test_tmp"' EXIT
+
+# A checkout whose HEAD was committed at 1789095456, the id #13516 saw reused.
+checkout="$test_tmp/omarchy"
+mkdir -p "$checkout/migrations"
+git -C "$checkout" init -q
+GIT_AUTHOR_NAME=test GIT_AUTHOR_EMAIL=test@example.invalid \
+GIT_COMMITTER_NAME=test GIT_COMMITTER_EMAIL=test@example.invalid \
+GIT_AUTHOR_DATE=@1789095456 GIT_COMMITTER_DATE=@1789095456 \
+  git -C "$checkout" commit --allow-empty -q -m base
+
+# Pin the clock so the expected ids are exact.
+stub_bin="$test_tmp/bin"
+mkdir -p "$stub_bin"
+cat >"$stub_bin/date" <<'SH'
+#!/bin/bash
+[[ ${1:-} == "+%s" ]] || exit 1
+echo 1790700000
+SH
+chmod +x "$stub_bin/date"
+
+add_migration() {
+  PATH="$stub_bin:$PATH" OMARCHY_PATH="$checkout" "$ROOT/bin/omarchy-dev-add-migration" --no-edit
+}
+
+created=$(add_migration)
+[[ $created == "$checkout/migrations/1790700000.sh" && -f $created ]] ||
+  fail "a new migration is named after the current time, not HEAD's commit time" "created: $created"
+pass "a new migration is named after the current time, not HEAD's commit time"
+
+created=$(add_migration)
+[[ $created == "$checkout/migrations/1790700001.sh" && -f $created ]] ||
+  fail "a second migration in the same second takes the next free id" "created: $created"
+pass "a second migration in the same second takes the next free id"