diff --git a/gstack-upgrade/migrations/v1.0.0.0.sh b/gstack-upgrade/migrations/v1.0.0.0.sh
new file mode 100755
index 00000000..2e62fe06
--- /dev/null
+++ b/gstack-upgrade/migrations/v1.0.0.0.sh
@@ -0,0 +1,38 @@
+#!/usr/bin/env bash
+# Migration: v1.0.0.0 — V1 writing style prompt
+#
+# What changed: tier-≥2 skills default to ELI10 writing style (jargon glossed on
+# first use, outcome-framed questions, short sentences). Power users who prefer
+# the older V0 prose can set `gstack-config set explain_level terse`.
+#
+# What this does: writes a "pending prompt" flag file. On the first tier-≥2 skill
+# invocation after upgrade, the preamble reads the flag and asks the user once
+# whether to keep the new default or opt into terse mode. Flag file is deleted
+# after the user answers. Idempotent — safe to run multiple times.
+#
+# Affected: every user on v0.19.x and below who upgrades to v1.x
+set -euo pipefail
+
+GSTACK_HOME="${GSTACK_HOME:-$HOME/.gstack}"
+PROMPTED_FLAG="$GSTACK_HOME/.writing-style-prompted"
+PENDING_FLAG="$GSTACK_HOME/.writing-style-prompt-pending"
+
+mkdir -p "$GSTACK_HOME"
+
+# If the user has already answered the prompt at any point, skip.
+if [ -f "$PROMPTED_FLAG" ]; then
+  exit 0
+fi
+
+# If the user has already explicitly set explain_level (either way), count that
+# as an answer — they've made their choice, don't ask again.
+EXPLAIN_LEVEL_SET="$("${HOME}/.claude/skills/gstack/bin/gstack-config" get explain_level 2>/dev/null || true)"
+if [ -n "$EXPLAIN_LEVEL_SET" ]; then
+  touch "$PROMPTED_FLAG"
+  exit 0
+fi
+
+# Write the pending flag — preamble will see it on the first tier-≥2 skill invocation.
+touch "$PENDING_FLAG"
+
+echo "  [v1.0.0.0] V1 writing style: you'll see a one-time prompt on your next skill run asking if you want the new default (glossed jargon, outcome framing) or the older terse prose."