>>> s="1"+"0"*4300
>>> json.loads(s)
...
ValueError: Exceeds the limit (4300 digits) for integer string conversion:
value has 4301 digits; use sys.set_int_max_str_digits() to increase the limit
This was done to prevent DoS attacks 3 years ago and have been backported to at least CPython 3.9 as it was considered a CVE.
Relevant discussion: https://news.ycombinator.com/item?id=32753235
Your sibling comment suggests using decimal.Decimal which handles parsing >4300 digit numbers (by default).