diff --git a/tests/test_nan_robustness.py b/tests/test_nan_robustness.py
index 84b34a47..c9c9e6f7 100644
--- a/tests/test_nan_robustness.py
+++ b/tests/test_nan_robustness.py
@@ -20,8 +20,10 @@
 """
 import json
 import math
+import sys
 from unittest.mock import patch
 
+import numpy as np
 import pandas as pd
 import pytest
 
@@ -221,14 +223,47 @@ def test_json_writer_round_trips_real_payload():
     assert parsed == payload
 
 
-# ---- stdlib json baseline (for documentation; not a real test) ---------
+@pytest.mark.parametrize("value", [np.float64('nan'), np.float64('inf'),
+                                   np.float64('-inf')])
+def test_json_writer_handles_nested_numpy_floats_without_mutating_input(value):
+    from vaxrank.cli.entry_point import serialize_json_nan_tolerant
+
+    payload = {'nested': [{'value': value}]}
+    assert json.loads(serialize_json_nan_tolerant(payload)) == {
+        'nested': [{'value': None}]}
+    assert payload['nested'][0]['value'] is value
+
+
+def test_json_writer_normalizes_serializable_containers_and_objects():
+    from vaxrank.cli.entry_point import serialize_json_nan_tolerant
+
+    class Payload:
+        def __init__(self, value):
+            self.value = value
+
+    payload = {'tuple': (float('nan'),), 'set': {float('inf')},
+               'object': Payload(float('-inf'))}
+    parsed = json.loads(serialize_json_nan_tolerant(payload))
+    assert parsed['tuple']['__value__'] == [None]
+    assert parsed['set']['__value__'] == [None]
+    assert parsed['object']['value'] is None
+    assert parsed['object']['__class__']['__name__'] == 'Payload'
+
+
+def test_json_writer_preserves_report_encoding():
+    from vaxrank.cli.entry_point import serialize_json_nan_tolerant
 
+    payload = {'values': [float('nan'), float('inf'), float('-inf')],
+               'negative_zero': -0.0, 'tiny': 5e-324, 'large': 2 ** 64,
+               'text': 'café'}
+    assert serialize_json_nan_tolerant(payload) == (
+        '{"values": [null, null, null], "negative_zero": -0.0, '
+        '"tiny": 5e-324, "large": 18446744073709551616, "text": "caf\\u00e9"}')
+
+
+def test_json_writer_works_when_simplejson_cannot_be_imported(monkeypatch):
+    from vaxrank.cli.entry_point import serialize_json_nan_tolerant
 
-def test_simplejson_default_rejects_nan_as_documented():
-    """Sanity: confirms the bug premise. The stdlib + simplejson
-    *default* path rejects NaN — which is why we wrapped with
-    ``ignore_nan=True``. This test fails the day simplejson changes
-    its default; if it ever does, we can remove the wrapper."""
-    import simplejson
-    with pytest.raises(ValueError, match='Out of range'):
-        simplejson.dumps({'x': float('nan')})
+    monkeypatch.setitem(sys.modules, 'simplejson', None)
+    assert json.loads(serialize_json_nan_tolerant({'value': float('nan')})) == {
+        'value': None}
diff --git a/vaxrank/cli/entry_point.py b/vaxrank/cli/entry_point.py
index 5e6f6222..f1e3d6a3 100644
--- a/vaxrank/cli/entry_point.py
+++ b/vaxrank/cli/entry_point.py
@@ -15,6 +15,7 @@
 import logging
 import json
 import logging.config
+import math
 import os
 import sys
 from importlib.resources import files
@@ -76,15 +77,25 @@
 logger = logging.getLogger(__name__)
 
 
+def _nan_to_none(value):
+    """Replace non-finite floats in normalized dict/list values with None."""
+    if isinstance(value, float) and not math.isfinite(value):
+        return None
+    if isinstance(value, dict):
+        return {key: _nan_to_none(item) for key, item in value.items()}
+    if isinstance(value, list):
+        return [_nan_to_none(item) for item in value]
+    return value
+
+
 def serialize_json_nan_tolerant(obj):
-    """Serialize ``obj`` to a JSON string, rendering NaN/Inf as
-    ``null`` rather than raising. Defense-in-depth wrapper around
-    ``serializable.to_json``; producers should already be coercing
-    NaN to None, but a writer-side safety net prevents the whole
-    report from being lost on a single rogue float (#289)."""
-    import simplejson
+    """Serialize ``obj`` to strict JSON, rendering NaN/Inf as ``null``.
+
+    Producers should already coerce non-finite floats to None. This writer
+    also normalizes them so one bad float cannot lose the report (#289).
+    """
     from serializable.helpers import to_serializable_repr
-    return simplejson.dumps(to_serializable_repr(obj), ignore_nan=True)
+    return json.dumps(_nan_to_none(to_serializable_repr(obj)), allow_nan=False)
 
 
 def filter_unannotatable_variants(variants):
@@ -1752,14 +1763,13 @@ def ranked_vaccine_peptides_with_metadata_from_parsed_args(args):
     if args.output_json_file:
         ensure_parent_dir(args.output_json_file)
         with open(args.output_json_file, 'w') as f:
-            # ``ignore_nan=True`` is defense in depth — producers should
+            # Writer-side normalization is defense in depth — producers should
             # be coercing NaN/Inf to None before reaching the writer
             # (see vaxrank.finite_prediction_value), but if any
             # slips through (e.g. a future predictor adapter, or a
             # custom score expression), render it as JSON null instead
             # of crashing the whole report. Note: strict JSON doesn't
-            # have a NaN literal — simplejson's default behavior is to
-            # raise; we explicitly opt into the null representation.
+            # have a NaN literal; the writer converts non-finite floats to null.
             f.write(serialize_json_nan_tolerant(data))
             logger.info('Wrote JSON report data to %s', args.output_json_file)
 
diff --git a/vaxrank/version.py b/vaxrank/version.py
index ca04ffa2..b5032d23 100644
--- a/vaxrank/version.py
+++ b/vaxrank/version.py
@@ -1,4 +1,4 @@
-__version__ = "3.38.1"
+__version__ = "3.38.2"
 
 
 def print_version():