diff --git a/scripts/eval_healthbench.py b/scripts/eval_healthbench.py
index 80aab34..9c84076 100644
--- a/scripts/eval_healthbench.py
+++ b/scripts/eval_healthbench.py
@@ -318,6 +318,22 @@ def main():
     args.output = str(Path(args.output).expanduser())
     args.sample_ids = str(Path(args.sample_ids).expanduser())
 
+    # Idempotency: if --output already exists with content, skip this eval.
+    # Stage 4 of the pipeline runs 4 configs in sequence; on resume after preempt
+    # or restart, we don't want to re-run configs whose JSONs already landed.
+    # The pipeline's run_eval() wrapper (in tpu/run_pipeline.sh) also has this
+    # check, but baking it into the script makes the script idempotent on its
+    # own and protects direct invocations. Empty (size 0) files are treated as
+    # partial writes from a crashed run and are allowed to be overwritten.
+    out_path = Path(args.output)
+    if out_path.is_file() and out_path.stat().st_size > 0:
+        print(
+            f"[eval_healthbench] {out_path} already exists "
+            f"({out_path.stat().st_size} bytes), skipping.",
+            file=sys.stderr,
+        )
+        sys.exit(0)
+
     # Calibration-honesty notice (audit C2): make it impossible to read
     # downstream Brier/ECE numbers off the per-item geomean probability
     # without seeing the limitations. stderr keeps stdout clean for any
diff --git a/tests/test_eval_healthbench_skip.py b/tests/test_eval_healthbench_skip.py
new file mode 100644
index 0000000..4824f48
--- /dev/null
+++ b/tests/test_eval_healthbench_skip.py
@@ -0,0 +1,176 @@
+"""Tests that scripts/eval_healthbench.py is idempotent on existing --output.
+
+The pipeline's stage-4 wrapper run_eval() in tpu/run_pipeline.sh already
+short-circuits when the output JSON exists, but a recent preempt-resume
+incident showed that direct invocations (or future wrapper changes) could
+re-run a 10-15 min eval whose output already landed. The script now does
+its own idempotent check immediately after expanding --output. These tests
+pin that contract:
+
+  * non-empty output file present  -> exit 0, file untouched
+  * empty (size 0) output file     -> NOT skipped (treated as a partial
+    write from a crashed run, allowed to be overwritten by the real run)
+
+We invoke the script via subprocess so we exercise the real argparse +
+sys.exit path that the launcher hits at runtime. The script's import chain
+(eval_healthbench -> filter_traces -> generate_traces -> _bodhi_ablation
+-> bodhi, plus torch/transformers/tqdm) reaches into heavy first-party and
+ML deps that aren't installed in dev/CI. We stub those packages onto
+PYTHONPATH so the import chain succeeds and main() runs to the skip check.
+"""
+
+import os
+import subprocess
+import sys
+import textwrap
+import time
+from pathlib import Path
+
+import pytest
+
+
+REPO_ROOT = Path(__file__).resolve().parent.parent
+SCRIPT = REPO_ROOT / "scripts" / "eval_healthbench.py"
+
+
+def _write_stub_pkg(root: Path, name: str, body: str = "") -> None:
+    """Write a stub package directory with __init__.py at root/name."""
+    pkg = root / name
+    pkg.mkdir(parents=True, exist_ok=True)
+    (pkg / "__init__.py").write_text(body)
+
+
+def _make_stub_path(tmp_path: Path) -> Path:
+    """Build a directory of stub modules sufficient to import eval_healthbench.
+
+    Only `bodhi` and its submodules need to be stubbed at the package level
+    because they're checked-out as an external project that isn't installed
+    in dev/CI. Everything else (torch, transformers, numpy, tqdm) is either
+    already installed in the preflight venv or shadowed by an entry here.
+    """
+    stub = tmp_path / "stubs"
+    stub.mkdir()
+
+    # bodhi: top-level + .prompts + .constants. _bodhi_ablation does
+    # `from bodhi import BODHIConfig` and `from bodhi.prompts import
+    # render_analysis_prompt`. Stub just enough symbols to satisfy the
+    # import; we never actually call them because we exit before
+    # reaching the eval body.
+    _write_stub_pkg(
+        stub,
+        "bodhi",
+        body="class BODHIConfig:\n    pass\n",
+    )
+    _write_stub_pkg(
+        stub,
+        "bodhi/prompts",
+        body="def render_analysis_prompt(*a, **kw):\n    return ''\n",
+    )
+    # Some downstream imports reference bodhi.constants; stub it too.
+    _write_stub_pkg(stub, "bodhi/constants", body="")
+
+    return stub
+
+
+def _run_script(output_path: Path, *, stub_path: Path) -> subprocess.CompletedProcess:
+    """Invoke eval_healthbench.py with --output and minimum-required args.
+
+    Returns the CompletedProcess so callers can assert on returncode and
+    captured streams. We pipe a fake --sample-ids file that doesn't have
+    to exist because the skip check fires before sample_ids is read.
+    """
+    env = os.environ.copy()
+    # Prepend stub_path so our shims win over any partial install.
+    existing = env.get("PYTHONPATH", "")
+    env["PYTHONPATH"] = (
+        f"{stub_path}{os.pathsep}{REPO_ROOT}"
+        + (os.pathsep + existing if existing else "")
+    )
+
+    cmd = [
+        sys.executable,
+        str(SCRIPT),
+        "--model", "google/gemma-3-4b-it",
+        "--output", str(output_path),
+        "--sample-ids", "/tmp/does-not-need-to-exist.json",
+    ]
+    return subprocess.run(
+        cmd,
+        env=env,
+        capture_output=True,
+        text=True,
+        timeout=120,
+    )
+
+
+def _imports_available() -> bool:
+    """The skip test only matters when the script's import chain actually
+    runs. On a bare dev box without numpy/transformers/etc., the subprocess
+    can't even reach the skip block. Skip rather than fail in that case."""
+    try:
+        import numpy  # noqa: F401
+        import transformers  # noqa: F401
+        import tqdm  # noqa: F401
+    except ImportError:
+        return False
+    return True
+
+
+pytestmark = pytest.mark.skipif(
+    not _imports_available(),
+    reason="numpy/transformers/tqdm not installed in this environment",
+)
+
+
+def test_skips_when_output_already_exists_nonempty(tmp_path):
+    """Non-empty output file -> script exits 0 without rewriting it."""
+    stub_path = _make_stub_path(tmp_path)
+    out = tmp_path / "result.json"
+    out.write_text('{"already": "graded"}')
+
+    original_mtime_ns = out.stat().st_mtime_ns
+    original_bytes = out.read_bytes()
+
+    # Sleep a hair so any accidental rewrite would bump mtime visibly.
+    time.sleep(0.05)
+
+    result = _run_script(out, stub_path=stub_path)
+
+    assert result.returncode == 0, (
+        f"expected clean exit on existing file, got rc={result.returncode}\n"
+        f"stdout: {result.stdout}\nstderr: {result.stderr}"
+    )
+    assert "skipping" in result.stderr.lower(), (
+        f"expected skip notice on stderr, got:\nstdout: {result.stdout}\n"
+        f"stderr: {result.stderr}"
+    )
+    assert out.stat().st_mtime_ns == original_mtime_ns, (
+        "output file was rewritten despite being non-empty"
+    )
+    assert out.read_bytes() == original_bytes, (
+        "output file content changed despite skip"
+    )
+
+
+def test_does_not_skip_when_output_is_empty(tmp_path):
+    """Empty (size 0) output file -> script does NOT skip; it proceeds and
+    fails downstream because we passed a fake --sample-ids. The point is
+    that we don't see the skip-message and rc != 0 (the real run would
+    overwrite the empty file). Crashed-run partial writes must not
+    poison resume."""
+    stub_path = _make_stub_path(tmp_path)
+    out = tmp_path / "empty.json"
+    out.touch()  # size-0 file
+
+    assert out.stat().st_size == 0
+
+    result = _run_script(out, stub_path=stub_path)
+
+    # We should NOT see the skip message when the file is empty.
+    assert "skipping" not in result.stderr.lower(), (
+        f"empty file was incorrectly treated as skip-worthy:\n"
+        f"stderr: {result.stderr}"
+    )
+    # We don't pin returncode further: the real eval needs sample_ids /
+    # GPU / etc. and will fail downstream. The only contract this test
+    # enforces is "didn't take the skip path on empty file."