diff --git a/resources/tester/test_570_1_preview_up.py b/resources/tester/test_570_1_preview_up.py
new file mode 100644
index 000000000..427408bcb
--- /dev/null
+++ b/resources/tester/test_570_1_preview_up.py
@@ -0,0 +1,72 @@
+"""570-1 빌드판 자리 — 기본 = 개발 포트 − 1000 · --replace-dev = 개발 포트 · 빌드 실패면 안 끔."""
+
+import importlib.util
+import subprocess
+from pathlib import Path
+
+import pytest
+
+SRC = Path(__file__).resolve().parents[2] / "tools" / "preview_up.py"
+
+
+@pytest.fixture
+def pu(monkeypatch, tmp_path):
+    spec = importlib.util.spec_from_file_location("preview_up_570", SRC)
+    mod = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(mod)
+    calls: list = []
+    monkeypatch.setattr(mod, "TMP", str(tmp_path))
+    monkeypatch.setattr(mod, "PID_FILE", str(tmp_path / "preview.pid"))
+    monkeypatch.setattr(mod, "LOG", str(tmp_path / "preview.log"))
+    monkeypatch.setattr(mod, "owners_ports", lambda: (8001, 5174))
+    monkeypatch.setattr(mod, "stop", lambda: calls.append("stop"))
+    monkeypatch.setattr(mod, "free_port", lambda port: calls.append(("free", port)))
+    monkeypatch.setattr(mod, "run", lambda cmd, env: None)
+    monkeypatch.setattr(mod.time, "sleep", lambda s: None)
+    monkeypatch.setattr(mod.urllib.request, "urlopen", lambda url, timeout: calls.append(url))
+
+    class Proc:
+        pid = 1
+
+    monkeypatch.setattr(mod, "spawn", lambda cmd, env: calls.append(("spawn", cmd)) or Proc())
+    monkeypatch.delenv("AISLO_VITE_PORT", raising=False)
+    monkeypatch.delenv("AISLO_API_PORT", raising=False)
+    mod.calls = calls
+    return mod
+
+
+def spawned_port(calls) -> str:
+    cmd = next(c[1] for c in calls if isinstance(c, tuple) and c[0] == "spawn")
+    return cmd[cmd.index("--port") + 1]
+
+
+def test_default_dev_minus_1000(pu, monkeypatch):
+    monkeypatch.setattr(pu.sys, "argv", ["preview_up.py"])
+    assert pu.main() == 0
+    assert spawned_port(pu.calls) == "4174"
+    assert not any(isinstance(c, tuple) and c[0] == "free" for c in pu.calls)
+
+
+def test_port_arg(pu, monkeypatch):
+    monkeypatch.setattr(pu.sys, "argv", ["preview_up.py", "--port", "4999"])
+    assert pu.main() == 0
+    assert spawned_port(pu.calls) == "4999"
+
+
+def test_replace_dev_takes_dev_port_after_build(pu, monkeypatch):
+    monkeypatch.setattr(pu.sys, "argv", ["preview_up.py", "--replace-dev"])
+    assert pu.main() == 0
+    assert spawned_port(pu.calls) == "5174"
+    assert pu.calls.index(("free", 5174)) < next(
+        i for i, c in enumerate(pu.calls) if isinstance(c, tuple) and c[0] == "spawn"
+    )
+
+
+def test_replace_dev_build_fail_kills_nothing(pu, monkeypatch):
+    def boom(cmd, env):
+        raise subprocess.CalledProcessError(1, cmd)
+
+    monkeypatch.setattr(pu, "run", boom)
+    monkeypatch.setattr(pu.sys, "argv", ["preview_up.py", "--replace-dev"])
+    assert pu.main() == 1
+    assert pu.calls == []
diff --git a/tools/preview_up.py b/tools/preview_up.py
index d745fef31..cf253c66d 100644
--- a/tools/preview_up.py
+++ b/tools/preview_up.py
@@ -3,6 +3,10 @@
 
 실행:  ./venv/Scripts/python.exe tools/preview_up.py          (빌드 + 띄움 · 수십 초)
        ./venv/Scripts/python.exe tools/preview_up.py --stop   (끄기만)
+       --port N   : 띄울 포트(기본 = 개발 포트 − 1000)
+       --replace-dev : 개발판 자리(개발 포트)에 띄움 — 빌드가 된 뒤에만 그 포트의 vite dev 를 끔
+                       · 빌드 실패면 아무것도 안 끄고 1 로 끝남(개발판 그대로)
+                       · `.claude/dev_up.py --build` 가 부름
 
 - 개발 서버(vite)는 파일 수백 개를 하나씩 보내 느린 연결(Tailscale · 폰)에서 첫 화면이 늦다 —
   빌드판은 묶은 파일 몇 개만 보낸다.
@@ -54,6 +58,36 @@ def stop() -> None:
     os.remove(PID_FILE)
 
 
+def free_port(port: int) -> None:
+    """그 포트를 물고 있는 프로세스(개발판 vite)를 끈다 — 짝 = `_free_dev_port`(config)."""
+    out = subprocess.run(
+        ["netstat", "-ano", "-p", "TCP"], capture_output=True, text=True, errors="replace"
+    ).stdout
+    for line in out.splitlines():
+        cols = line.split()
+        if (
+            len(cols) >= 5
+            and cols[1].endswith(f":{port}")
+            and cols[3] == "LISTENING"
+            and cols[4].isdigit()
+        ):
+            subprocess.run(
+                ["taskkill", "/T", "/F", "/PID", cols[4]], capture_output=True, check=False
+            )
+
+
+def arg(name: str) -> str | None:
+    return sys.argv[sys.argv.index(name) + 1] if name in sys.argv[:-1] else None
+
+
+def spawn(cmd: list[str], env: dict[str, str]) -> subprocess.Popen:
+    log = open(LOG, "ab")  # noqa: SIM115 — 자식 프로세스가 물려받는다
+    flags = subprocess.DETACHED_PROCESS | subprocess.CREATE_NEW_PROCESS_GROUP
+    return subprocess.Popen(
+        cmd, cwd=ROOT, env=env, stdout=log, stderr=subprocess.STDOUT, creationflags=flags
+    )
+
+
 def run(cmd: list[str], env: dict[str, str]) -> None:
     print("RUN", " ".join(cmd), flush=True)
     subprocess.run(cmd, cwd=ROOT, env=env, check=True, shell=os.name == "nt" and cmd[0] == "npm")
@@ -61,7 +95,9 @@ def run(cmd: list[str], env: dict[str, str]) -> None:
 
 def main() -> int:
     os.makedirs(TMP, exist_ok=True)
-    stop()
+    replace = "--replace-dev" in sys.argv
+    if not replace:
+        stop()
     if "--stop" in sys.argv:
         print("PREVIEW=stopped")
         return 0
@@ -69,21 +105,31 @@ def main() -> int:
     api, vite = owners_ports() or (8000, 5173)
     api = int(os.environ.get("AISLO_API_PORT") or api)
     vite = int(os.environ.get("AISLO_VITE_PORT") or vite)
-    port = vite - 1000
+    port = int(arg("--port") or (vite if replace else vite - 1000))
     env = {**os.environ, "AISLO_API_PORT": str(api), "FRONTEND_DEV_PORT": str(vite)}
 
-    run(["node", VITE, "build", "--configLoader", "runner"], env)
-    run(["npm", "run", "build:b07-cad"], env)
+    try:
+        run(["node", VITE, "build", "--configLoader", "runner"], env)
+        run(["npm", "run", "build:b07-cad"], env)
+    except subprocess.CalledProcessError as exc:
+        # 옛 빌드판이 자리에 있으면 파일 목록이 낡음 — 끄고 개발판으로
+        if replace and os.path.exists(PID_FILE):
+            stop()
+            free_port(port)
+            time.sleep(1)
+            spawn(["node", VITE, "--configLoader", "runner", "--host", "--port", str(port)], env)
+            print(f"PREVIEW=BUILD_FAIL ({exc.returncode}) — 옛 빌드판을 끄고 {port} 개발판")
+        else:
+            print(f"PREVIEW=BUILD_FAIL ({exc.returncode}) — 아무것도 안 끔")
+        return 1
+    if replace:  # 빌드가 된 뒤에만 — 옛 빌드판 · 그 포트의 개발판을 끔
+        stop()
+        free_port(port)
+        time.sleep(1)
 
-    log = open(LOG, "ab")  # noqa: SIM115 — 자식 프로세스가 물려받는다
-    flags = subprocess.DETACHED_PROCESS | subprocess.CREATE_NEW_PROCESS_GROUP
-    process = subprocess.Popen(
-        ["node", VITE, "preview", "--configLoader", "runner"],
-        cwd=ROOT,
-        env=env,
-        stdout=log,
-        stderr=subprocess.STDOUT,
-        creationflags=flags,
+    process = spawn(
+        ["node", VITE, "preview", "--configLoader", "runner", "--port", str(port), "--strictPort"],
+        env,
     )
     with open(PID_FILE, "w", encoding="utf-8") as f:
         f.write(str(process.pid))