diff --git a/spark_executor/core/job_writer.py b/spark_executor/core/job_writer.py
new file mode 100644
index 0000000..f174209
--- /dev/null
+++ b/spark_executor/core/job_writer.py
@@ -0,0 +1,53 @@
+# coding=utf-8
+"""
+@Time   :2026/6/24
+@Author :tao.chen
+
+Writes LLM-generated PySpark code to disk so the agent can then call
+prepare_submit_job(script_path=...) on the returned path.
+
+Resolution order for the output directory:
+  1. explicit jobs_dir argument (used in tests)
+  2. SPARK_EXECUTOR_JOBS_DIR environment variable (operator override)
+  3. DEFAULT_JOBS_DIR constant (./data/jobs/ — gitignored, persists across
+     container restarts when ./data is mounted)
+"""
+import os
+import secrets
+from datetime import datetime
+
+from common.logging import logger
+
+DEFAULT_JOBS_DIR = "./data/jobs"
+ENV_JOBS_DIR = "SPARK_EXECUTOR_JOBS_DIR"
+
+
+def resolve_jobs_dir(jobs_dir: str | None = None) -> str:
+    """Pick the effective jobs directory in priority order: arg > env > default."""
+    if jobs_dir is not None:
+        return jobs_dir
+    env = os.environ.get(ENV_JOBS_DIR)
+    if env:
+        return env
+    return DEFAULT_JOBS_DIR
+
+
+def write_job_file(code: str, jobs_dir: str | None = None) -> str:
+    """Write `code` to <jobs_dir>/job_<timestamp>_<rand>.py; return abs path.
+
+    Creates the directory if missing. Filename is unique per call (timestamp
+    down to the second + 3 random bytes) so concurrent agents cannot collide.
+    """
+    effective_dir = resolve_jobs_dir(jobs_dir)
+    os.makedirs(effective_dir, exist_ok=True)
+    stamp = datetime.utcnow().strftime("%Y%m%d%H%M%S")
+    name = f"job_{stamp}_{secrets.token_hex(3)}.py"
+    path = os.path.join(effective_dir, name)
+    abs_path = os.path.abspath(path)
+    logger.debug(
+        f"write_job_file enter jobs_dir={effective_dir} code_bytes={len(code)}"
+    )
+    with open(abs_path, "w", encoding="utf-8") as f:
+        f.write(code)
+    logger.info(f"write_job_file ok script_path={abs_path} bytes={len(code)}")
+    return abs_path
diff --git a/tests/unit/test_job_writer.py b/tests/unit/test_job_writer.py
new file mode 100644
index 0000000..526b086
--- /dev/null
+++ b/tests/unit/test_job_writer.py
@@ -0,0 +1,78 @@
+# coding=utf-8
+import os
+from pathlib import Path
+
+from spark_executor.core.job_writer import (
+    DEFAULT_JOBS_DIR,
+    ENV_JOBS_DIR,
+    resolve_jobs_dir,
+    write_job_file,
+)
+
+
+# --- resolve_jobs_dir (priority: arg > env > default) ---
+
+def test_resolve_explicit_arg_wins(tmp_path: Path, monkeypatch):
+    monkeypatch.setenv(ENV_JOBS_DIR, "/from/env")
+    assert resolve_jobs_dir(str(tmp_path)) == str(tmp_path)
+
+
+def test_resolve_env_var_used_when_no_arg(monkeypatch, tmp_path: Path):
+    monkeypatch.setenv(ENV_JOBS_DIR, str(tmp_path))
+    assert resolve_jobs_dir() == str(tmp_path)
+
+
+def test_resolve_default_when_neither_set(monkeypatch):
+    monkeypatch.delenv(ENV_JOBS_DIR, raising=False)
+    assert resolve_jobs_dir() == DEFAULT_JOBS_DIR
+    assert resolve_jobs_dir() == "./data/jobs"
+
+
+# --- write_job_file ---
+
+def test_write_creates_file_with_code_and_returns_abs_path(tmp_path: Path):
+    out = write_job_file("print('hi')\n", jobs_dir=str(tmp_path))
+    assert os.path.isfile(out)
+    assert os.path.isabs(out)
+    assert out.startswith(str(tmp_path))
+    assert out.endswith(".py")
+    with open(out) as f:
+        assert f.read() == "print('hi')\n"
+
+
+def test_write_creates_jobs_dir_if_missing(tmp_path: Path):
+    target = tmp_path / "newdir"
+    assert not target.exists()
+    out = write_job_file("x = 1\n", jobs_dir=str(target))
+    assert target.is_dir()
+    assert os.path.isfile(out)
+
+
+def test_write_uses_env_var_when_no_arg(monkeypatch, tmp_path: Path):
+    monkeypatch.setenv(ENV_JOBS_DIR, str(tmp_path))
+    out = write_job_file("env-driven\n")
+    assert out.startswith(str(tmp_path))
+    with open(out) as f:
+        assert f.read() == "env-driven\n"
+
+
+def test_write_uses_default_when_neither_set(monkeypatch, tmp_path: Path):
+    """Default is ./data/jobs relative to cwd. Run in a tmp dir so the test
+    doesn't pollute the real project and stays self-contained."""
+    monkeypatch.delenv(ENV_JOBS_DIR, raising=False)
+    monkeypatch.chdir(tmp_path)  # pytest built-in: chdir for this test only
+    out = write_job_file("default\n")
+    assert os.path.isabs(out)
+    # Should resolve to <tmp_path>/data/jobs/job_*.py
+    expected_dir = tmp_path / "data" / "jobs"
+    assert expected_dir.is_dir()
+    assert str(out).startswith(str(expected_dir))
+    with open(out) as f:
+        assert f.read() == "default\n"
+
+
+def test_write_returns_unique_paths_for_concurrent_calls(tmp_path: Path):
+    """Two writes in the same second must still get distinct filenames (via random suffix)."""
+    out1 = write_job_file("a", jobs_dir=str(tmp_path))
+    out2 = write_job_file("b", jobs_dir=str(tmp_path))
+    assert out1 != out2