diff --git a/posthog/request.py b/posthog/request.py
index 76df1fdc..1c2456a7 100644
--- a/posthog/request.py
+++ b/posthog/request.py
@@ -1,5 +1,6 @@
 import json
 import logging
+import math
 import re
 import socket
 import time
@@ -479,9 +480,29 @@ class QuotaLimitError(APIError):
 RequestsConnectionError = requests.exceptions.ConnectionError
 
 
+def _replace_non_finite_floats(value: Any) -> Any:
+    """Recursively replace inf/nan floats with None.
+
+    json.dumps with the default allow_nan=True emits bare Infinity/NaN tokens,
+    which are not valid JSON and get rejected by strict-parsing ingestion (and
+    intermediaries). posthog-js avoids this because JSON.stringify converts
+    non-finite numbers to null; this mirrors that behavior.
+    """
+    if isinstance(value, float):
+        return value if math.isfinite(value) else None
+    if isinstance(value, dict):
+        return {key: _replace_non_finite_floats(item) for key, item in value.items()}
+    if isinstance(value, (list, tuple)):
+        return [_replace_non_finite_floats(item) for item in value]
+    return value
+
+
 class DatetimeSerializer(json.JSONEncoder):
     def default(self, obj: Any):
         if isinstance(obj, (date, datetime)):
             return obj.isoformat()
 
         return json.JSONEncoder.default(self, obj)
+
+    def encode(self, o: Any) -> str:
+        return super().encode(_replace_non_finite_floats(o))
diff --git a/posthog/test/test_request.py b/posthog/test/test_request.py
index d4ad7c9d..455f24d8 100644
--- a/posthog/test/test_request.py
+++ b/posthog/test/test_request.py
@@ -776,3 +776,43 @@ def test_flags_does_not_retry_other_http_status_errors(self, status_code):
 
         self.assertEqual(cm.exception.status, status_code)
         self.assertEqual(mock_session.post.call_count, 1)
+
+
+def test_post_serializes_non_finite_floats_as_null():
+    mock_response = requests.Response()
+    mock_response.status_code = 200
+    mock_session = mock.MagicMock()
+    mock_session.post.return_value = mock_response
+
+    request_module.post(
+        TEST_API_KEY,
+        host="https://test.posthog.com",
+        path="/batch/",
+        session=mock_session,
+        batch=[
+            {
+                "event": "score_computed",
+                "distinct_id": "user-1",
+                "properties": {
+                    "score": float("inf"),
+                    "nested": [float("nan"), {"depth": float("-inf")}],
+                    "ok": 1.5,
+                },
+            }
+        ],
+    )
+
+    raw = mock_session.post.call_args.kwargs["data"]
+    # Bare Infinity/NaN tokens are not valid JSON; a strict parser must accept the body.
+    data = json.loads(
+        raw, parse_constant=lambda x: (_ for _ in ()).throw(ValueError(x))
+    )
+    props = data["batch"][0]["properties"]
+    assert props["score"] is None
+    assert props["nested"] == [None, {"depth": None}]
+    assert props["ok"] == 1.5
+
+
+def test_datetime_serializer_keeps_finite_values():
+    body = {"n": 1.5, "s": "x", "b": True, "i": 3, "none": None}
+    assert json.loads(json.dumps(body, cls=DatetimeSerializer)) == body
diff --git a/references/public_api_snapshot.txt b/references/public_api_snapshot.txt
index 29a1c477..645f6c42 100644
--- a/references/public_api_snapshot.txt
+++ b/references/public_api_snapshot.txt
@@ -1718,6 +1718,7 @@ method posthog.metrics_capture.PostHogMetrics.reset() -> None
 method posthog.poller.Poller.run()
 method posthog.poller.Poller.stop()
 method posthog.request.DatetimeSerializer.default(obj: Any)
+method posthog.request.DatetimeSerializer.encode(o: Any) -> str
 method posthog.request.HTTPAdapterWithSocketOptions.init_poolmanager(*args, **kwargs)
 method posthog.tracing.span.Span.add_event(name: str, attributes: Optional[Mapping[str, Any]] = None, timestamp: Union[datetime, float, None] = None) -> Span
 method posthog.tracing.span.Span.end(end_time: Union[datetime, float, None] = None) -> None