diff --git a/tests/test_install.sh b/tests/test_install.sh
new file mode 100755
index 0000000..ad619e1
--- /dev/null
+++ b/tests/test_install.sh
@@ -0,0 +1,144 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+REPO_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+FAKE_HOME="$(mktemp -d)"
+PASS=0
+FAIL=0
+
+cleanup() {
+    rm -rf "$FAKE_HOME"
+}
+trap cleanup EXIT
+
+pass() {
+    printf '[PASS] %s\n' "$1"
+    PASS=$((PASS + 1))
+}
+
+fail() {
+    printf '[FAIL] %s - %s\n' "$1" "$2"
+    FAIL=$((FAIL + 1))
+}
+
+assert_file_exists() {
+    local label="$1" path="$2"
+    if [ -f "$path" ]; then
+        return 0
+    else
+        fail "$label" "expected file not found: $path"
+        return 1
+    fi
+}
+
+assert_file_contains() {
+    local label="$1" path="$2" needle="$3"
+    if grep -qF "$needle" "$path" 2>/dev/null; then
+        return 0
+    else
+        fail "$label" "file $path does not contain: $needle"
+        return 1
+    fi
+}
+
+assert_file_not_exists() {
+    local label="$1" path="$2"
+    if [ ! -e "$path" ]; then
+        return 0
+    else
+        fail "$label" "expected absent but found: $path"
+        return 1
+    fi
+}
+
+assert_dir_not_exists() {
+    local label="$1" path="$2"
+    if [ ! -d "$path" ]; then
+        return 0
+    else
+        fail "$label" "expected absent dir but found: $path"
+        return 1
+    fi
+}
+
+export HOME="$FAKE_HOME"
+
+mkdir -p "$FAKE_HOME/.hermes/hermes-agent"
+python3 -m venv "$FAKE_HOME/.hermes/hermes-agent/venv"
+
+VENV_PYTHON="$FAKE_HOME/.hermes/hermes-agent/venv/bin/python"
+SITE_PACKAGES="$("$VENV_PYTHON" -c 'import site; print(site.getsitepackages()[0])')"
+SITECUSTOMIZE="$SITE_PACKAGES/sitecustomize.py"
+BACKUP="$SITECUSTOMIZE.pre-hermes-claude-auth"
+PATCH_FILE="$FAKE_HOME/.hermes/patches/anthropic_billing_bypass.py"
+
+# Test 1: Fresh install
+T1="Test 1: Fresh install"
+if "$REPO_DIR/install.sh" >/dev/null 2>&1; then
+    ok=1
+    assert_file_exists "$T1" "$PATCH_FILE" || ok=0
+    assert_file_exists "$T1" "$SITECUSTOMIZE" || ok=0
+    assert_file_contains "$T1" "$SITECUSTOMIZE" "# hermes-claude-auth managed" || ok=0
+    [ "$ok" -eq 1 ] && pass "$T1"
+else
+    fail "$T1" "install.sh exited non-zero"
+fi
+
+# Test 2: Idempotent re-install
+T2="Test 2: Idempotent re-install"
+if "$REPO_DIR/install.sh" >/dev/null 2>&1; then
+    ok=1
+    assert_file_exists "$T2" "$SITECUSTOMIZE" || ok=0
+    assert_file_contains "$T2" "$SITECUSTOMIZE" "# hermes-claude-auth managed" || ok=0
+    count="$(grep -cF '# hermes-claude-auth managed' "$SITECUSTOMIZE" 2>/dev/null || true)"
+    if [ "$count" -gt 1 ]; then
+        fail "$T2" "marker duplicated ($count occurrences)"
+        ok=0
+    fi
+    [ "$ok" -eq 1 ] && pass "$T2"
+else
+    fail "$T2" "install.sh exited non-zero on re-run"
+fi
+
+# Test 3: Install over existing sitecustomize.py (no marker)
+T3="Test 3: Install over existing sitecustomize.py"
+printf 'import sys\n# some unrelated hook\n' > "$SITECUSTOMIZE"
+if "$REPO_DIR/install.sh" >/dev/null 2>&1; then
+    ok=1
+    assert_file_exists "$T3" "$BACKUP" || ok=0
+    assert_file_contains "$T3" "$SITECUSTOMIZE" "# hermes-claude-auth managed" || ok=0
+    assert_file_contains "$T3" "$BACKUP" "# some unrelated hook" || ok=0
+    [ "$ok" -eq 1 ] && pass "$T3"
+else
+    fail "$T3" "install.sh exited non-zero"
+fi
+
+# Test 4: Uninstall (hook only)
+T4="Test 4: Uninstall (hook only)"
+if "$REPO_DIR/uninstall.sh" >/dev/null 2>&1; then
+    ok=1
+    assert_file_exists "$T4" "$SITECUSTOMIZE" || ok=0
+    assert_file_contains "$T4" "$SITECUSTOMIZE" "# some unrelated hook" || ok=0
+    assert_file_not_exists "$T4" "$BACKUP" || ok=0
+    assert_file_exists "$T4" "$PATCH_FILE" || ok=0
+    [ "$ok" -eq 1 ] && pass "$T4"
+else
+    fail "$T4" "uninstall.sh exited non-zero"
+fi
+
+# Test 5: Reinstall then uninstall --purge
+T5="Test 5: Reinstall then uninstall --purge"
+rm -f "$SITECUSTOMIZE"
+if "$REPO_DIR/install.sh" >/dev/null 2>&1 && "$REPO_DIR/uninstall.sh" --purge >/dev/null 2>&1; then
+    ok=1
+    assert_file_not_exists "$T5" "$SITECUSTOMIZE" || ok=0
+    assert_file_not_exists "$T5" "$PATCH_FILE" || ok=0
+    assert_dir_not_exists "$T5" "$FAKE_HOME/.hermes/patches" || ok=0
+    [ "$ok" -eq 1 ] && pass "$T5"
+else
+    fail "$T5" "install.sh or uninstall.sh --purge exited non-zero"
+fi
+
+TOTAL=$((PASS + FAIL))
+printf '\n%d/%d tests passed\n' "$PASS" "$TOTAL"
+[ "$FAIL" -eq 0 ]