diff --git a/lib/tests/curated-config-guard.test.sh b/lib/tests/curated-config-guard.test.sh
new file mode 100644
index 0000000..bff7133
--- /dev/null
+++ b/lib/tests/curated-config-guard.test.sh
@@ -0,0 +1,52 @@
+#!/usr/bin/env bash
+# lib/tests/curated-config-guard.test.sh — SPEC-03 (J4-03).
+#
+# Drives install-plugins.sh's restore_curated_configs() in a sandbox. The SUT
+# is extracted from the REAL script AT TEST RUNTIME (awk range, verified
+# single-occurrence + column-0 closing brace) so drift in install-plugins.sh
+# propagates into this test instead of testing a stale copy. GUARDED_CONFIGS,
+# CFG_SNAPSHOT, REPO and an info() stub are defined here — the array literal
+# at install-plugins.sh:41 is outside the extracted range.
+set -u
+INSTALL_SH="$(cd "$(dirname "$0")/../.." && pwd)/install-plugins.sh"
+pass=0; fail=0
+check() { if [ "$2" = "$3" ]; then pass=$((pass+1)); else fail=$((fail+1));
+  printf 'FAIL %s: got[%s] want[%s]\n' "$1" "$2" "$3"; fi; }
+
+SUT="$(mktemp)"
+awk '/^restore_curated_configs\(\) \{/,/^\}/' "$INSTALL_SH" > "$SUT"
+
+REPO="$(mktemp -d)"
+CFG_SNAPSHOT="$(mktemp -d)"
+EXPECT="$(mktemp -d)"          # our own reference copy — independent of CFG_SNAPSHOT (SUT rm -rf's it)
+GUARDED_CONFIGS=("CLAUDE.md" ".claude/settings.json" "settings.json")
+info() { :; }                 # stub — extracted body calls info(), irrelevant to the assertions
+
+mkdir -p "$REPO/.claude"
+printf 'CLAUDE original\n' > "$REPO/CLAUDE.md"
+printf '{"a":1}\n'         > "$REPO/.claude/settings.json"
+printf '{"b":2}\n'         > "$REPO/settings.json"
+
+for f in "${GUARDED_CONFIGS[@]}"; do
+  mkdir -p "$CFG_SNAPSHOT/$(dirname "$f")" "$EXPECT/$(dirname "$f")"
+  cp "$REPO/$f" "$CFG_SNAPSHOT/$f"
+  cp "$REPO/$f" "$EXPECT/$f"
+done
+
+# simulate installer drift: mutate ONE guarded file, leave the other two alone
+printf 'CLAUDE CLOBBERED BY INSTALLER\n' > "$REPO/CLAUDE.md"
+
+# shellcheck source=/dev/null
+source "$SUT"
+restore_curated_configs
+
+cmp -s "$REPO/CLAUDE.md" "$EXPECT/CLAUDE.md"
+check T1-mutated-file-restored "$?" 0
+cmp -s "$REPO/.claude/settings.json" "$EXPECT/.claude/settings.json"
+check T2-untouched-local-settings-unchanged "$?" 0
+cmp -s "$REPO/settings.json" "$EXPECT/settings.json"
+check T3-untouched-settings-unchanged "$?" 0
+[ ! -d "$CFG_SNAPSHOT" ]
+check T4-snapshot-dir-removed "$?" 0
+
+printf 'PASS=%s FAIL=%s\n' "$pass" "$fail"; [ "$fail" -eq 0 ]