diff --git a/libs/openant-core/core/analyzer.py b/libs/openant-core/core/analyzer.py
index 7fb5966..8fa9a68 100644
--- a/libs/openant-core/core/analyzer.py
+++ b/libs/openant-core/core/analyzer.py
@@ -24,6 +24,7 @@
 from core import tracking
 from core.checkpoint import StepCheckpoint
 from core.progress import ProgressReporter
+from utilities.atomic_io import atomic_write_json
 
 # Import existing analysis machinery
 from utilities.llm_client import AnthropicClient, get_global_tracker
@@ -513,8 +514,7 @@ def _summary_callback(finding, usage=None):
         "code_by_route": code_by_route,
     }
 
-    with open(results_path, "w") as f:
-        json.dump(experiment_result, f, indent=2)
+    atomic_write_json(results_path, experiment_result)
 
     print(f"\n[Analyze] Results written to {results_path}", file=sys.stderr)
 
diff --git a/libs/openant-core/core/enhancer.py b/libs/openant-core/core/enhancer.py
index fef1453..80e8760 100644
--- a/libs/openant-core/core/enhancer.py
+++ b/libs/openant-core/core/enhancer.py
@@ -16,6 +16,7 @@
 from core.schemas import EnhanceResult, UsageInfo
 from core import tracking
 from core.progress import ProgressReporter
+from utilities.atomic_io import atomic_write_json
 from utilities.rate_limiter import configure_rate_limiter
 
 
@@ -138,8 +139,7 @@ def _on_restored(count: int):
 
     # Write enhanced dataset
     os.makedirs(os.path.dirname(os.path.abspath(output_path)), exist_ok=True)
-    with open(output_path, "w") as f:
-        json.dump(enhanced, f, indent=2)
+    atomic_write_json(output_path, enhanced)
 
     print(f"[Enhance] Enhanced dataset: {output_path}", file=sys.stderr)
     print(f"[Enhance] Classifications: {classifications}", file=sys.stderr)
diff --git a/libs/openant-core/core/verifier.py b/libs/openant-core/core/verifier.py
index fa7a43f..edfa7dc 100644
--- a/libs/openant-core/core/verifier.py
+++ b/libs/openant-core/core/verifier.py
@@ -19,6 +19,7 @@
 from core.checkpoint import StepCheckpoint
 from core.progress import ProgressReporter
 
+from utilities.atomic_io import atomic_write_json
 from utilities.llm_client import TokenTracker, get_global_tracker
 from utilities.finding_verifier import FindingVerifier
 from utilities.agentic_enhancer.repository_index import load_index_from_file
@@ -268,8 +269,7 @@ def _write_verified_results(
 
     output["metrics"] = {"total": len(merged_results), **counts}
 
-    with open(path, "w") as f:
-        json.dump(output, f, indent=2, ensure_ascii=False)
+    atomic_write_json(path, output, ensure_ascii=False)
 
 
 def _build_code_by_route(results: list) -> dict:
diff --git a/libs/openant-core/tests/test_atomic_io.py b/libs/openant-core/tests/test_atomic_io.py
new file mode 100644
index 0000000..ed83e28
--- /dev/null
+++ b/libs/openant-core/tests/test_atomic_io.py
@@ -0,0 +1,184 @@
+"""Tests for ``utilities.atomic_io.atomic_write_json``.
+
+These exercise the three properties that justify the helper's existence:
+
+1. Round-trip: a written dict reads back identically.
+2. Atomicity: a mid-write failure leaves the *previous* file untouched —
+   no truncated/empty target on disk.
+3. Same-directory temp file: required for ``os.replace`` to be atomic on
+   Windows (cross-volume rename falls back to copy+delete and loses the
+   atomicity guarantee).
+"""
+
+from __future__ import annotations
+
+import json
+import os
+from pathlib import Path
+
+import pytest
+
+from utilities.atomic_io import atomic_write_json
+
+
+def test_roundtrip_writes_and_reads(tmp_path: Path):
+    """Writing a dict and reading it back yields the same content."""
+    target = tmp_path / "results.json"
+    data = {
+        "dataset": "geospatial_vuln12",
+        "metrics": {"total": 3, "vulnerable": 1},
+        "results": [{"id": "u1", "verdict": "VULNERABLE"}],
+    }
+
+    atomic_write_json(str(target), data)
+
+    assert target.exists()
+    with open(target, encoding="utf-8") as f:
+        assert json.load(f) == data
+
+
+def test_unicode_roundtrip_with_ensure_ascii_false(tmp_path: Path):
+    """ensure_ascii=False (used by verifier) preserves non-ASCII chars."""
+    target = tmp_path / "results_verified.json"
+    data = {"note": "résumé — naïve café ☃"}
+
+    atomic_write_json(str(target), data, ensure_ascii=False)
+
+    text = target.read_text(encoding="utf-8")
+    # Raw UTF-8, not \u-escaped:
+    assert "résumé" in text
+    assert json.loads(text) == data
+
+
+def test_failure_mid_write_preserves_existing_file(tmp_path: Path, monkeypatch):
+    """If json.dump raises mid-write, the previous target file is intact."""
+    target = tmp_path / "results.json"
+    original = {"version": 1, "stable": True}
+    atomic_write_json(str(target), original)
+
+    # Sanity check: original content is on disk before the failed write.
+    assert json.loads(target.read_text(encoding="utf-8")) == original
+
+    # Force json.dump to blow up partway through. Because atomic_write_json
+    # writes to a temp file in the same directory and only os.replaces on
+    # success, the existing target must be untouched.
+    real_dump = json.dump
+
+    def exploding_dump(*args, **kwargs):
+        # Write a few bytes first to prove that *temp* file partial writes
+        # don't reach the target — then raise.
+        f = args[1]
+        f.write('{"corru')
+        raise RuntimeError("simulated mid-write crash")
+
+    monkeypatch.setattr("utilities.atomic_io.json.dump", exploding_dump)
+
+    with pytest.raises(RuntimeError, match="simulated mid-write crash"):
+        atomic_write_json(str(target), {"version": 2, "broken": True})
+
+    # Restore so subsequent assertions/teardown aren't affected.
+    monkeypatch.setattr("utilities.atomic_io.json.dump", real_dump)
+
+    # Target is unchanged: still the original content, still valid JSON.
+    assert target.exists()
+    assert json.loads(target.read_text(encoding="utf-8")) == original
+
+
+def test_failure_cleans_up_temp_file(tmp_path: Path, monkeypatch):
+    """Failed writes must not leave stray ``.tmp-`` files behind."""
+    target = tmp_path / "results.json"
+
+    def exploding_dump(*args, **kwargs):
+        raise RuntimeError("boom")
+
+    monkeypatch.setattr("utilities.atomic_io.json.dump", exploding_dump)
+
+    with pytest.raises(RuntimeError):
+        atomic_write_json(str(target), {"x": 1})
+
+    leftovers = [p.name for p in tmp_path.iterdir()]
+    assert leftovers == [], f"expected empty dir, found: {leftovers}"
+
+
+def test_failure_with_no_existing_target_does_not_create_target(
+    tmp_path: Path, monkeypatch,
+):
+    """If no target exists and the write fails, target must not appear."""
+    target = tmp_path / "results.json"
+    assert not target.exists()
+
+    def exploding_dump(*args, **kwargs):
+        raise RuntimeError("boom")
+
+    monkeypatch.setattr("utilities.atomic_io.json.dump", exploding_dump)
+
+    with pytest.raises(RuntimeError):
+        atomic_write_json(str(target), {"x": 1})
+
+    assert not target.exists()
+
+
+def test_temp_file_is_in_same_directory_as_target(tmp_path: Path, monkeypatch):
+    """Temp file must live in the same directory as the target.
+
+    Cross-volume renames are not atomic on Windows; ``os.replace`` falls back
+    to copy+delete, so this is load-bearing for the atomicity guarantee.
+    """
+    target_dir = tmp_path / "outputs"
+    target_dir.mkdir()
+    target = target_dir / "results.json"
+
+    captured: dict[str, str] = {}
+    real_mkstemp = __import__("tempfile").mkstemp
+
+    def spying_mkstemp(*args, **kwargs):
+        fd, path = real_mkstemp(*args, **kwargs)
+        captured["path"] = path
+        captured["dir_kwarg"] = kwargs.get("dir", "")
+        return fd, path
+
+    monkeypatch.setattr("utilities.atomic_io.tempfile.mkstemp", spying_mkstemp)
+
+    atomic_write_json(str(target), {"ok": True})
+
+    assert "path" in captured, "tempfile.mkstemp was not called"
+    tmp_dir = os.path.dirname(captured["path"])
+    expected_dir = os.path.abspath(str(target_dir))
+    assert os.path.abspath(tmp_dir) == expected_dir, (
+        f"temp file in {tmp_dir!r}, expected {expected_dir!r}"
+    )
+    # And the helper passed the same dir explicitly to mkstemp.
+    assert os.path.abspath(captured["dir_kwarg"]) == expected_dir
+
+
+def test_overwrites_existing_file(tmp_path: Path):
+    """Successful second write replaces the target's content."""
+    target = tmp_path / "results.json"
+    atomic_write_json(str(target), {"version": 1})
+    atomic_write_json(str(target), {"version": 2, "extra": [1, 2, 3]})
+
+    assert json.loads(target.read_text(encoding="utf-8")) == {
+        "version": 2, "extra": [1, 2, 3],
+    }
+
+
+@pytest.mark.skipif(os.name != "posix", reason="POSIX permission semantics")
+def test_posix_permissions_match_umask(tmp_path: Path):
+    """The written file honours the process umask, not mkstemp's 0600 default.
+
+    A regression here is silent: pipeline outputs that other users / tools
+    were previously able to read (under a typical 0022 umask -> 0644 file)
+    would suddenly become owner-only. We don't want atomic writes to tighten
+    permissions as a side effect.
+    """
+    target = tmp_path / "results.json"
+
+    old_umask = os.umask(0o022)
+    try:
+        atomic_write_json(str(target), {"ok": True})
+    finally:
+        os.umask(old_umask)
+
+    mode = os.stat(target).st_mode & 0o777
+    # 0666 & ~0022 == 0644
+    assert mode == 0o644, f"expected 0644, got {oct(mode)}"
diff --git a/libs/openant-core/utilities/atomic_io.py b/libs/openant-core/utilities/atomic_io.py
new file mode 100644
index 0000000..5b4a946
--- /dev/null
+++ b/libs/openant-core/utilities/atomic_io.py
@@ -0,0 +1,84 @@
+"""Atomic JSON file writes.
+
+Pipeline output files (``results.json``, ``enhanced_dataset.json``,
+``results_verified.json``) represent the final artefact of an expensive
+multi-stage LLM pipeline. A crash, power loss, or interrupted process during
+``json.dump`` can leave the target file truncated or corrupt, destroying the
+output of a long-running run that may have cost real money.
+
+``atomic_write_json`` writes the JSON to a temporary file in the **same
+directory** as the target, ``fsync``s it, then ``os.replace``s it onto the
+target path. ``os.replace`` is atomic on POSIX and on Windows (when both paths
+sit on the same volume), so concurrent readers either see the previous file
+intact or the fully-written new file — never a partial write.
+
+The same-directory requirement matters: cross-device renames fall back to
+copy+delete on most platforms (and fail outright on Windows), losing
+atomicity. Callers should never pass a temp dir on a different volume.
+"""
+
+from __future__ import annotations
+
+import json
+import os
+import tempfile
+from typing import Any
+
+
+def atomic_write_json(path: str, data: Any, *, indent: int | None = 2,
+                      ensure_ascii: bool = True) -> None:
+    """Atomically write ``data`` as JSON to ``path``.
+
+    Writes to a temporary file in the same directory as ``path``, fsyncs,
+    then ``os.replace``s it onto the target. If any step fails the temp
+    file is removed and ``path`` is left untouched.
+
+    Args:
+        path: Destination path. Parent directory must already exist.
+        data: JSON-serialisable object.
+        indent: Indentation passed through to ``json.dump`` (default 2).
+        ensure_ascii: Passed through to ``json.dump`` (default True).
+    """
+    directory = os.path.dirname(os.path.abspath(path)) or "."
+
+    # delete=False so we can close the handle and rename it; we clean up
+    # manually on error. Same-dir is required for atomic os.replace.
+    fd, tmp_path = tempfile.mkstemp(
+        prefix=".tmp-" + os.path.basename(path) + "-",
+        suffix=".json",
+        dir=directory,
+    )
+    try:
+        # tempfile.mkstemp creates files with mode 0600 (owner-only). A plain
+        # open(path, "w") honours the process umask (typically yielding 0644).
+        # Restore that behaviour so atomic writes don't silently tighten
+        # permissions on pipeline outputs that downstream tools/users read.
+        # On Windows os.chmod's effect is limited to the readonly bit, so
+        # this is effectively a no-op there.
+        if os.name == "posix":
+            try:
+                umask = os.umask(0)
+                os.umask(umask)
+                os.chmod(tmp_path, 0o666 & ~umask)
+            except OSError:
+                pass
+
+        with os.fdopen(fd, "w", encoding="utf-8") as f:
+            json.dump(data, f, indent=indent, ensure_ascii=ensure_ascii)
+            f.flush()
+            try:
+                os.fsync(f.fileno())
+            except OSError:
+                # fsync can fail on some filesystems (e.g. certain network
+                # mounts). The replace below is still atomic at the VFS
+                # layer; durability across power loss is best-effort.
+                pass
+        os.replace(tmp_path, path)
+    except BaseException:
+        # Clean up the temp file on any failure (including KeyboardInterrupt
+        # mid-write) so we don't leave stray .tmp- files in the output dir.
+        try:
+            os.unlink(tmp_path)
+        except OSError:
+            pass
+        raise