diff --git a/docs/HARNESS.md b/docs/HARNESS.md
index feff168..b94bfac 100644
--- a/docs/HARNESS.md
+++ b/docs/HARNESS.md
@@ -20,6 +20,20 @@ python scripts/execute.py <phase-name>
 python scripts/execute.py <phase-name> --push
 ```
 
+Codex의 `workspace-write` sandbox 밖에 설치된 실행 도구나 runtime이 필요한 경우에만
+`--codex-add-dir`를 반복 지정한다. 경로는 존재하는 절대 디렉터리여야 하며, Codex
+CLI의 추가 writable directory와 child PATH 끝에 함께 전달된다.
+
+```powershell
+python scripts/execute.py <phase-name> `
+  --codex-add-dir "C:\path\to\tool-bin" `
+  --codex-add-dir "C:\path\to\runtime"
+```
+
+이 옵션은 지정한 디렉터리에 child agent의 쓰기 권한도 부여하므로, 사용자 프로필이나
+드라이브 루트처럼 넓은 경로를 지정하지 말고 실행에 필요한 최소 설치 디렉터리만
+허용한다.
+
 ## Harness Python 검증
 
 이 저장소의 테스트와 최종 acceptance 검증은 pytest를 시스템 Python에 설치하지 않고
diff --git a/scripts/execute.py b/scripts/execute.py
index 3ca984b..31d15a1 100644
--- a/scripts/execute.py
+++ b/scripts/execute.py
@@ -3,13 +3,14 @@
 Harness Step Executor — phase 내 step을 순차 실행하고 자가 교정한다.
 
 Usage:
-    python scripts/execute.py <phase-dir> [--push]
+    python scripts/execute.py <phase-dir> [--push] [--codex-add-dir DIR]
 """
 
 import argparse
 import contextlib
 import json
 import os
+import shutil
 import subprocess
 import sys
 import threading
@@ -26,6 +27,78 @@ class CodexEnvironmentError(RuntimeError):
     """재시도로 해결할 수 없는 Codex CLI 환경 오류."""
 
 
+def normalize_codex_add_dirs(raw_dirs) -> tuple[Path, ...]:
+    """검증된 Codex 추가 writable directory를 반환한다."""
+    normalized = []
+    for raw in raw_dirs:
+        candidate = Path(raw)
+        if not candidate.is_absolute():
+            raise CodexEnvironmentError(
+                f"Codex 추가 경로는 절대 경로여야 합니다: {raw}"
+            )
+        resolved = candidate.resolve()
+        if not resolved.is_dir():
+            raise CodexEnvironmentError(
+                f"Codex 추가 경로가 존재하는 디렉터리가 아닙니다: {resolved}"
+            )
+        normalized.append(resolved)
+    return tuple(normalized)
+
+
+def build_codex_command(
+    codex_executable: Path,
+    root: Path,
+    add_dirs: tuple[Path, ...],
+) -> list[str]:
+    """workspace-write Codex command를 안전한 argv로 구성한다."""
+    command = [
+        str(codex_executable.resolve()),
+        "exec",
+        "--json",
+        "--sandbox",
+        "workspace-write",
+        "--dangerously-bypass-hook-trust",
+    ]
+    for path in add_dirs:
+        command.extend(("--add-dir", str(path)))
+    command.extend(("--cd", str(root.resolve()), "-"))
+    return command
+
+
+def build_codex_environment(
+    base_env,
+    add_dirs: tuple[Path, ...],
+) -> dict[str, str]:
+    """명시적 tool directory를 기존 PATH 뒤에 추가한다."""
+    environment = dict(base_env)
+    path_entries = [environment.get("PATH", "")]
+    path_entries.extend(str(path) for path in add_dirs)
+    environment["PATH"] = os.pathsep.join(entry for entry in path_entries if entry)
+    return environment
+
+
+def run_utf8_process(command, *, cwd, prompt, env, timeout):
+    """UTF-8 prompt/output 계약으로 subprocess를 실행한다."""
+    return subprocess.run(
+        list(command),
+        cwd=cwd,
+        input=prompt,
+        capture_output=True,
+        encoding="utf-8",
+        errors="replace",
+        env=env,
+        timeout=timeout,
+    )
+
+
+def configure_standard_streams(*streams) -> None:
+    """Windows legacy console에서 status 출력이 중단되지 않게 한다."""
+    for stream in streams:
+        reconfigure = getattr(stream, "reconfigure", None)
+        if reconfigure is not None:
+            reconfigure(errors="replace")
+
+
 @contextlib.contextmanager
 def progress_indicator(label: str):
     """터미널 진행 표시기. with 문으로 사용하며 .elapsed 로 경과 시간을 읽는다."""
@@ -62,13 +135,20 @@ class StepExecutor:
     CHORE_MSG = "chore({phase}): step {num} output"
     TZ = timezone(timedelta(hours=9))
 
-    def __init__(self, phase_dir_name: str, *, auto_push: bool = False):
+    def __init__(
+        self,
+        phase_dir_name: str,
+        *,
+        auto_push: bool = False,
+        codex_add_dirs=(),
+    ):
         self._root = str(ROOT)
         self._phases_dir = ROOT / "phases"
         self._phase_dir = self._phases_dir / phase_dir_name
         self._phase_dir_name = phase_dir_name
         self._top_index_file = self._phases_dir / "index.json"
         self._auto_push = auto_push
+        self._codex_add_dirs = normalize_codex_add_dirs(codex_add_dirs)
 
         if not self._phase_dir.is_dir():
             print(f"ERROR: {self._phase_dir} not found")
@@ -241,24 +321,26 @@ class StepExecutor:
             sys.exit(1)
 
         prompt = preamble + step_file.read_text(encoding="utf-8")
-        command = [
-            "codex",
-            "exec",
-            "--json",
-            "--sandbox",
-            "workspace-write",
-            "--dangerously-bypass-hook-trust",
-            "--cd",
-            self._root,
-            "-",
-        ]
+        codex_executable = shutil.which("codex")
+        if codex_executable is None:
+            raise CodexEnvironmentError(
+                "Codex CLI를 찾을 수 없습니다. codex를 설치하고 PATH를 확인하세요."
+            )
+        command = build_codex_command(
+            Path(codex_executable),
+            Path(self._root),
+            self._codex_add_dirs,
+        )
+        environment = build_codex_environment(
+            os.environ,
+            self._codex_add_dirs,
+        )
         try:
-            result = subprocess.run(
+            result = run_utf8_process(
                 command,
                 cwd=self._root,
-                input=prompt,
-                capture_output=True,
-                text=True,
+                prompt=prompt,
+                env=environment,
                 timeout=1800,
             )
         except FileNotFoundError as exc:
@@ -465,13 +547,28 @@ class StepExecutor:
 
 
 def main():
+    configure_standard_streams(sys.stdout, sys.stderr)
     parser = argparse.ArgumentParser(description="Harness Step Executor")
     parser.add_argument("phase_dir", help="Phase directory name (e.g. 0-mvp)")
     parser.add_argument("--push", action="store_true", help="Push branch after completion")
+    parser.add_argument(
+        "--codex-add-dir",
+        action="append",
+        default=[],
+        metavar="DIR",
+        help=(
+            "Codex workspace-write sandbox에 추가할 절대 tool/runtime directory. "
+            "여러 번 지정할 수 있습니다."
+        ),
+    )
     args = parser.parse_args()
 
     try:
-        StepExecutor(args.phase_dir, auto_push=args.push).run()
+        StepExecutor(
+            args.phase_dir,
+            auto_push=args.push,
+            codex_add_dirs=args.codex_add_dir,
+        ).run()
     except CodexEnvironmentError as exc:
         print(f"ERROR: {exc}")
         sys.exit(1)
diff --git a/tests/harness/test_execute.py b/tests/harness/test_execute.py
new file mode 100644
index 0000000..ed4d775
--- /dev/null
+++ b/tests/harness/test_execute.py
@@ -0,0 +1,122 @@
+import os
+import sys
+from pathlib import Path
+
+import pytest
+
+from scripts.execute import (
+    CodexEnvironmentError,
+    build_codex_command,
+    build_codex_environment,
+    configure_standard_streams,
+    normalize_codex_add_dirs,
+    run_utf8_process,
+)
+
+
+def test_normalize_codex_add_dirs_accepts_existing_absolute_directories(tmp_path):
+    first = tmp_path / "tool"
+    second = tmp_path / "runtime"
+    first.mkdir()
+    second.mkdir()
+
+    result = normalize_codex_add_dirs([str(first), str(second)])
+
+    assert result == (first.resolve(), second.resolve())
+
+
+@pytest.mark.parametrize("raw", ["relative/tool", "missing"])
+def test_normalize_codex_add_dirs_rejects_unsafe_paths(tmp_path, raw):
+    candidate = raw if raw.startswith("relative") else str(tmp_path / raw)
+
+    with pytest.raises(CodexEnvironmentError):
+        normalize_codex_add_dirs([candidate])
+
+
+def test_build_codex_command_grants_only_explicit_directories(tmp_path):
+    root = tmp_path / "repo"
+    tool = tmp_path / "tool"
+    runtime = tmp_path / "runtime"
+    root.mkdir()
+    tool.mkdir()
+    runtime.mkdir()
+    codex = tmp_path / "codex.exe"
+
+    command = build_codex_command(
+        codex,
+        root,
+        (tool.resolve(), runtime.resolve()),
+    )
+
+    assert command == [
+        str(codex),
+        "exec",
+        "--json",
+        "--sandbox",
+        "workspace-write",
+        "--dangerously-bypass-hook-trust",
+        "--add-dir",
+        str(tool.resolve()),
+        "--add-dir",
+        str(runtime.resolve()),
+        "--cd",
+        str(root.resolve()),
+        "-",
+    ]
+
+
+def test_build_codex_environment_appends_tool_directories(tmp_path):
+    tool = (tmp_path / "tool").resolve()
+    runtime = (tmp_path / "runtime").resolve()
+    base = {"PATH": os.pathsep.join(("existing-one", "existing-two")), "KEEP": "value"}
+
+    result = build_codex_environment(base, (tool, runtime))
+
+    assert result["PATH"].split(os.pathsep) == [
+        "existing-one",
+        "existing-two",
+        str(tool),
+        str(runtime),
+    ]
+    assert result["KEEP"] == "value"
+    assert result is not base
+
+
+def test_run_utf8_process_sends_prompt_as_utf8(tmp_path):
+    verifier = tmp_path / "verify_utf8.py"
+    verifier.write_text(
+        "import sys\n"
+        "payload = sys.stdin.buffer.read()\n"
+        "text = payload.decode('utf-8')\n"
+        "sys.stdout.buffer.write(text.encode('utf-8'))\n",
+        encoding="utf-8",
+    )
+    prompt = "FESA 한글 prompt"
+
+    result = run_utf8_process(
+        [sys.executable, str(verifier)],
+        cwd=tmp_path,
+        prompt=prompt,
+        env=os.environ.copy(),
+        timeout=10,
+    )
+
+    assert result.returncode == 0
+    assert result.stdout == prompt
+
+
+def test_configure_standard_streams_replaces_unencodable_status_characters():
+    class RecordingStream:
+        def __init__(self):
+            self.calls = []
+
+        def reconfigure(self, **kwargs):
+            self.calls.append(kwargs)
+
+    stdout = RecordingStream()
+    stderr = RecordingStream()
+
+    configure_standard_streams(stdout, stderr)
+
+    assert stdout.calls == [{"errors": "replace"}]
+    assert stderr.calls == [{"errors": "replace"}]