diff --git a/litellm/proxy/utils.py b/litellm/proxy/utils.py
index b336ce1fa27..082552c58e1 100644
--- a/litellm/proxy/utils.py
+++ b/litellm/proxy/utils.py
@@ -7288,7 +7288,12 @@ class ProxyUpdateSpend:
                         if not base_url.endswith("/"):
                             base_url += "/"
                         verbose_proxy_logger.debug("base_url: %s", base_url)
-                        json_data = json.dumps(logs_to_process)
+                        try:
+                            json_data = json.dumps(logs_to_process)
+                        except (TypeError, ValueError):
+                            # No external request has been sent. The batch is safe to replay.
+                            await requeue_spend_logs(prisma_client, proxy_logging_obj, logs_to_process)
+                            raise
                         response = await db_writer_client.post(
                             url=base_url + "spend/update",
                             data=json_data,
@@ -7301,7 +7306,13 @@ class ProxyUpdateSpend:
                     else:
                         for j in range(0, len(logs_to_process), BATCH_SIZE):
                             batch = logs_to_process[j : j + BATCH_SIZE]
-                            batch_with_dates = [prisma_client.jsonify_object({**entry}) for entry in batch]
+                            try:
+                                batch_with_dates = [prisma_client.jsonify_object({**entry}) for entry in batch]
+                            except (TypeError, ValueError):
+                                # This batch has not reached Prisma. Earlier batches may
+                                # already have committed, so replay only this tail.
+                                await requeue_spend_logs(prisma_client, proxy_logging_obj, logs_to_process[j:])
+                                raise
                             isolation_budget = MAX_SPEND_LOG_ISOLATION_FAILURES_PER_BATCH
                             for statement_rows in spend_log_write_batches(
                                 batch_with_dates,
diff --git a/tests/test_litellm/proxy/utils/prisma_and_spend/test_proxy_update_spend.py b/tests/test_litellm/proxy/utils/prisma_and_spend/test_proxy_update_spend.py
index 7099101db1c..89afe852427 100644
--- a/tests/test_litellm/proxy/utils/prisma_and_spend/test_proxy_update_spend.py
+++ b/tests/test_litellm/proxy/utils/prisma_and_spend/test_proxy_update_spend.py
@@ -917,3 +917,77 @@ async def test_update_spend_logs_parks_failed_batch_in_redis_with_wire_safe_date
     parked = await buffer.get_spend_logs_from_redis_buffer(limit=10)
     assert mock_prisma_client.spend_log_transactions == []
     assert [(row["request_id"], row["startTime"]) for row in parked] == [("a", started.isoformat())]
+
+
+@pytest.mark.asyncio
+async def test_spend_log_serialization_failure_requeues_only_unwritten_tail(
+    mock_prisma_client: Any, make_spend_log_row: Any, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    monkeypatch.delenv("SPEND_LOGS_URL", raising=False)
+    rows = [make_spend_log_row(request_id=f"committed-{i}") for i in range(1000)]
+    rows.append(make_spend_log_row(request_id="unwritten"))
+    original_jsonify = mock_prisma_client.jsonify_object
+
+    def jsonify(row: Any) -> Any:
+        if row["request_id"] == "unwritten":
+            raise TypeError("bad local serialization")
+        return original_jsonify(row)
+
+    mock_prisma_client.jsonify_object = jsonify
+    mock_prisma_client.db.litellm_spendlogs.create_many = AsyncMock()
+    proxy_logging = MagicMock()
+    proxy_logging.failure_handler = AsyncMock()
+    with pytest.raises(TypeError, match="bad local serialization"):
+        await ProxyUpdateSpend.update_spend_logs(
+            n_retry_times=0,
+            prisma_client=mock_prisma_client,
+            db_writer_client=None,
+            proxy_logging_obj=proxy_logging,
+            logs_to_process=rows,
+        )
+    assert mock_prisma_client.db.litellm_spendlogs.create_many.await_count >= 1
+    assert [row["request_id"] for row in mock_prisma_client.spend_log_transactions] == ["unwritten"]
+
+
+@pytest.mark.asyncio
+async def test_external_spend_log_preflight_failure_requeues_without_post(
+    mock_prisma_client: Any, make_spend_log_row: Any, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    monkeypatch.setenv("SPEND_LOGS_URL", "http://writer.invalid")
+    rows = [make_spend_log_row(request_id="unwritten")]
+    rows[0]["unserializable"] = object()
+    writer = MagicMock()
+    writer.post = AsyncMock()
+    proxy_logging = MagicMock()
+    proxy_logging.failure_handler = AsyncMock()
+    with pytest.raises(TypeError):
+        await ProxyUpdateSpend.update_spend_logs(
+            n_retry_times=0,
+            prisma_client=mock_prisma_client,
+            db_writer_client=writer,
+            proxy_logging_obj=proxy_logging,
+            logs_to_process=rows,
+        )
+    writer.post.assert_not_awaited()
+    assert [row["request_id"] for row in mock_prisma_client.spend_log_transactions] == ["unwritten"]
+
+
+@pytest.mark.asyncio
+async def test_external_spend_log_post_error_is_not_replayed(
+    mock_prisma_client: Any, make_spend_log_row: Any, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    monkeypatch.setenv("SPEND_LOGS_URL", "http://writer.invalid")
+    writer = MagicMock()
+    writer.post = AsyncMock(side_effect=ValueError("uncertain delivery"))
+    proxy_logging = MagicMock()
+    proxy_logging.failure_handler = AsyncMock()
+    with pytest.raises(ValueError, match="uncertain delivery"):
+        await ProxyUpdateSpend.update_spend_logs(
+            n_retry_times=0,
+            prisma_client=mock_prisma_client,
+            db_writer_client=writer,
+            proxy_logging_obj=proxy_logging,
+            logs_to_process=[make_spend_log_row(request_id="maybe-committed")],
+        )
+    writer.post.assert_awaited_once()
+    assert mock_prisma_client.spend_log_transactions == []