diff --git a/install.sh b/install.sh
new file mode 100755
index 0000000..c5b5934
--- /dev/null
+++ b/install.sh
@@ -0,0 +1,74 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+GREEN='\033[0;32m'
+RED='\033[0;31m'
+YELLOW='\033[1;33m'
+RESET='\033[0m'
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+HERMES_AGENT_DIR="$HOME/.hermes/hermes-agent"
+PATCHES_DIR="$HOME/.hermes/patches"
+MARKER="# hermes-claude-auth managed"
+
+if [ ! -d "$HERMES_AGENT_DIR" ]; then
+    printf "${RED}[✗] hermes-agent not found at %s${RESET}\n" "$HERMES_AGENT_DIR"
+    printf "    Install hermes-agent first: https://github.com/nousresearch/hermes-agent\n"
+    exit 1
+fi
+
+if [ -n "${HERMES_VENV:-}" ] && [ -d "$HERMES_VENV" ]; then
+    VENV_DIR="$HERMES_VENV"
+elif [ -d "$HERMES_AGENT_DIR/venv" ]; then
+    VENV_DIR="$HERMES_AGENT_DIR/venv"
+elif [ -d "$HERMES_AGENT_DIR/.venv" ]; then
+    VENV_DIR="$HERMES_AGENT_DIR/.venv"
+else
+    printf "${RED}[✗] No virtualenv found in %s (checked venv/, .venv/, and \$HERMES_VENV)${RESET}\n" "$HERMES_AGENT_DIR"
+    exit 1
+fi
+
+VENV_PYTHON="$VENV_DIR/bin/python"
+if [ ! -x "$VENV_PYTHON" ]; then
+    printf "${RED}[✗] Python not found at %s${RESET}\n" "$VENV_PYTHON"
+    exit 1
+fi
+
+SITE_PACKAGES="$("$VENV_PYTHON" -c "import site; print(site.getsitepackages()[0])")"
+if [ ! -d "$SITE_PACKAGES" ]; then
+    printf "${RED}[✗] site-packages directory does not exist: %s${RESET}\n" "$SITE_PACKAGES"
+    exit 1
+fi
+
+mkdir -p "$PATCHES_DIR"
+cp "$SCRIPT_DIR/anthropic_billing_bypass.py" "$PATCHES_DIR/anthropic_billing_bypass.py"
+chmod 644 "$PATCHES_DIR/anthropic_billing_bypass.py"
+printf "${GREEN}[✓] Copied patch to %s/${RESET}\n" "$PATCHES_DIR"
+
+SITECUSTOMIZE="$SITE_PACKAGES/sitecustomize.py"
+
+if [ ! -f "$SITECUSTOMIZE" ]; then
+    cp "$SCRIPT_DIR/sitecustomize_hook.py" "$SITECUSTOMIZE"
+elif grep -q "$MARKER" "$SITECUSTOMIZE"; then
+    cp "$SCRIPT_DIR/sitecustomize_hook.py" "$SITECUSTOMIZE"
+else
+    BACKUP="$SITECUSTOMIZE.pre-hermes-claude-auth"
+    cp "$SITECUSTOMIZE" "$BACKUP"
+    printf "${YELLOW}[!] Backed up existing sitecustomize.py to %s${RESET}\n" "$BACKUP"
+    cp "$SCRIPT_DIR/sitecustomize_hook.py" "$SITECUSTOMIZE"
+fi
+
+chmod 644 "$SITECUSTOMIZE"
+printf "${GREEN}[✓] Installed hook into %s${RESET}\n" "$SITECUSTOMIZE"
+
+if systemctl --user is-active hermes-gateway.service >/dev/null 2>&1; then
+    systemctl --user restart hermes-gateway.service
+    printf "${GREEN}[✓] Restarted hermes-gateway.service${RESET}\n"
+else
+    printf "${YELLOW}[!] hermes-gateway not running — restart manually when ready${RESET}\n"
+fi
+
+printf "\n${GREEN}Installation complete.${RESET}\n"
+printf "  Patch:  %s/anthropic_billing_bypass.py\n" "$PATCHES_DIR"
+printf "  Hook:   %s\n" "$SITECUSTOMIZE"
+printf "  Venv:   %s\n" "$VENV_DIR"