diff --git a/README.md b/README.md
index 0ab5bb6..a6e0473 100644
--- a/README.md
+++ b/README.md
@@ -41,7 +41,7 @@ Download the archive for your platform from [GitHub Releases](https://github.com
 | Variable | Description | Default |
 |----------|-------------|---------|
 | `FLASHDUTY_VERSION` | Install a specific version (e.g. `v1.5.12`) | latest |
-| `FLASHDUTY_INSTALL_DIR` | Install directory | `/usr/local/bin` (shell), `~\.flashduty\bin` (PowerShell) |
+| `FLASHDUTY_INSTALL_DIR` | Install directory | shell: the directory of the `flashduty` already on `PATH`, else `/usr/local/bin`; PowerShell: `~\.flashduty\bin` |
 | `MIRROR_URL` | Release asset mirror (must be `https://`) | `https://static.flashcat.cloud/flashduty-cli` |
 
 ## Quick start
diff --git a/README_zh.md b/README_zh.md
index 40e7359..7e7afb1 100644
--- a/README_zh.md
+++ b/README_zh.md
@@ -41,7 +41,7 @@ irm https://static.flashcat.cloud/flashduty-cli/install.ps1 | iex
 | 变量 | 说明 | 默认值 |
 |------|------|--------|
 | `FLASHDUTY_VERSION` | 安装指定版本（如 `v1.5.12`） | 最新版 |
-| `FLASHDUTY_INSTALL_DIR` | 安装目录 | `/usr/local/bin`（shell），`~\.flashduty\bin`（PowerShell） |
+| `FLASHDUTY_INSTALL_DIR` | 安装目录 | shell：`PATH` 上已有 `flashduty` 所在目录，没有则 `/usr/local/bin`；PowerShell：`~\.flashduty\bin` |
 | `MIRROR_URL` | 下载镜像地址（必须是 `https://`） | `https://static.flashcat.cloud/flashduty-cli` |
 
 ## 快速上手
diff --git a/install.sh b/install.sh
index ee7527e..5d2e0ed 100755
--- a/install.sh
+++ b/install.sh
@@ -4,7 +4,8 @@
 #
 # Environment:
 #   FLASHDUTY_VERSION      Install a specific version (e.g. v0.1.2). Default: latest.
-#   FLASHDUTY_INSTALL_DIR  Install directory. Default: /usr/local/bin.
+#   FLASHDUTY_INSTALL_DIR  Install directory. Default: the directory of the
+#                          flashduty already on PATH, else /usr/local/bin.
 #   INSTALLED_NAME         Installed command name. Default: flashduty.
 #   MIRROR_URL             Fetch release assets from this https mirror prefix.
 #                          Default: https://static.flashcat.cloud/flashduty-cli.
@@ -18,7 +19,15 @@ set -e
 REPO="flashcatcloud/flashduty-cli"
 BINARY="flashduty-cli"
 INSTALLED_NAME="${INSTALLED_NAME:-flashduty}"
-INSTALL_DIR="${FLASHDUTY_INSTALL_DIR:-/usr/local/bin}"
+# Default to the directory of the copy the shell already runs, so an upgrade
+# replaces it instead of landing behind it on PATH. Fresh installs use
+# /usr/local/bin.
+existing=$(command -v "${INSTALLED_NAME}" 2>/dev/null || true)
+case "${existing}" in
+    /*) default_dir=$(dirname -- "${existing}") ;;
+    *) default_dir="/usr/local/bin" ;;
+esac
+INSTALL_DIR="${FLASHDUTY_INSTALL_DIR:-${default_dir}}"
 
 # By default release downloads are fetched from the Flashcat CDN. Set MIRROR_URL
 # to another prefix to override, or to an empty string to force GitHub fallback.
@@ -230,6 +239,12 @@ EOF
 # --- main ---
 
 main() {
+    # flashduty-runner stages its own bundled copy; replacing it would desync the
+    # runner from the CLI it was built and verified against.
+    if [ "${FLASHDUTY_MANAGED_BY_RUNNER:-}" = "1" ]; then
+        fail "flashduty is managed by flashduty-runner; upgrade the runner instead"
+    fi
+
     need_cmd uname
     need_cmd tar
     need_cmd curl
@@ -316,12 +331,15 @@ main() {
 
     info "Installed to ${INSTALL_DIR}/${INSTALLED_NAME}"
 
-    # Warn if install dir is not in PATH
-    case ":${PATH}:" in
-        *":${INSTALL_DIR}:"*) ;;
-        *) info "WARNING: ${INSTALL_DIR} is not in your PATH. Add it with:"
-           info "  export PATH=\"${INSTALL_DIR}:\$PATH\"" ;;
-    esac
+    # Warn unless the shell now runs the binary just installed.
+    active=$(command -v "${INSTALLED_NAME}" 2>/dev/null || true)
+    if [ -z "${active}" ]; then
+        info "WARNING: ${INSTALL_DIR} is not in your PATH. Add it with:"
+        info "  export PATH=\"${INSTALL_DIR}:\$PATH\""
+    elif [ "$(dirname -- "${active}")" != "${INSTALL_DIR%/}" ]; then
+        info "WARNING: your shell runs ${active}, not this install."
+        info "  Remove that copy, or put ${INSTALL_DIR} ahead of it on PATH."
+    fi
 
     # Best-effort shell completion; never fail the install over it.
     BIN="${INSTALL_DIR}/${INSTALLED_NAME}"
diff --git a/tests/install_sh_sudo_test.sh b/tests/install_sh_sudo_test.sh
index b1a0428..8710ceb 100644
--- a/tests/install_sh_sudo_test.sh
+++ b/tests/install_sh_sudo_test.sh
@@ -93,7 +93,7 @@ run_install() {
     home_dir="$1"
     install_dir="$2"
     out_file="$3"
-    env PATH="${FAKE_BIN}:$PATH" \
+    env PATH="${FAKE_BIN}:${EXTRA_PATH:+${EXTRA_PATH}:}$PATH" \
         HOME="${home_dir}" \
         SHELL=/bin/false \
         FLASHDUTY_VERSION=v9.9.9 \
@@ -186,7 +186,76 @@ test_tty_prompts_for_interactive_sudo() {
     assert_file_missing "${home_dir}/.local/bin/flashduty"
 }
 
+# A stale copy earlier on PATH: with no FLASHDUTY_INSTALL_DIR the upgrade must
+# replace it in place rather than install a shadowed copy elsewhere.
+make_existing_copy() {
+    mkdir -p "$1"
+    printf '#!/bin/sh\necho "old flashduty"\n' > "$1/flashduty"
+    chmod +x "$1/flashduty"
+}
+
+test_replaces_copy_already_on_path() {
+    SUDO_LOG="${TMP_DIR}/sudo-existing.log"
+    export SUDO_LOG
+    : > "${SUDO_LOG}"
+    home_dir="${TMP_DIR}/home-existing"
+    existing_dir="${TMP_DIR}/existing-bin"
+    out_file="${TMP_DIR}/existing.out"
+    mkdir -p "${home_dir}"
+    make_existing_copy "${existing_dir}"
+
+    # Trailing slash: dash's `command -v` then reports "<dir>//flashduty".
+    EXTRA_PATH="${existing_dir}/" run_install "${home_dir}" "" "${out_file}"
+
+    assert_contains "${out_file}" "Installed to ${existing_dir}/flashduty"
+    assert_contains "${existing_dir}/flashduty" "fake flashduty"
+    assert_file_missing "${home_dir}/.local/bin/flashduty"
+    if grep -Fq "WARNING" "${out_file}"; then
+        echo "unexpected warning:" >&2
+        cat "${out_file}" >&2
+        exit 1
+    fi
+}
+
+test_warns_when_copy_on_path_shadows_install() {
+    SUDO_LOG="${TMP_DIR}/sudo-shadow.log"
+    export SUDO_LOG
+    : > "${SUDO_LOG}"
+    home_dir="${TMP_DIR}/home-shadow"
+    existing_dir="${TMP_DIR}/shadow-bin"
+    out_file="${TMP_DIR}/shadow.out"
+    mkdir -p "${home_dir}"
+    make_existing_copy "${existing_dir}"
+    chmod 555 "${existing_dir}"
+
+    EXTRA_PATH="${existing_dir}" run_install "${home_dir}" "" "${out_file}"
+
+    assert_file_exists "${home_dir}/.local/bin/flashduty"
+    assert_contains "${existing_dir}/flashduty" "old flashduty"
+    assert_contains "${out_file}" "WARNING: your shell runs ${existing_dir}/flashduty, not this install."
+}
+
+test_refuses_runner_managed_copy() {
+    SUDO_LOG="${TMP_DIR}/sudo-runner.log"
+    export SUDO_LOG
+    : > "${SUDO_LOG}"
+    home_dir="${TMP_DIR}/home-runner"
+    install_dir="${TMP_DIR}/runner-bin"
+    out_file="${TMP_DIR}/runner.out"
+    mkdir -p "${home_dir}" "${install_dir}"
+
+    if FLASHDUTY_MANAGED_BY_RUNNER=1 run_install "${home_dir}" "${install_dir}" "${out_file}"; then
+        echo "expected install to fail when managed by flashduty-runner" >&2
+        exit 1
+    fi
+    assert_contains "${out_file}" "flashduty is managed by flashduty-runner; upgrade the runner instead"
+    assert_file_missing "${install_dir}/flashduty"
+}
+
 make_fixtures
 make_fake_commands
 test_non_tty_falls_back_to_user_bin
 test_tty_prompts_for_interactive_sudo
+test_replaces_copy_already_on_path
+test_warns_when_copy_on_path_shadows_install
+test_refuses_runner_managed_copy