Initial Checks

• I confirm that I'm using Pydantic V2

Description

When decoding a JSON number into a Python Decimal the precision seems to be limited. After a certain number of digits the value is cut off. This is something I would expect for a fixed precision float, but not for an arbitrary precision Decimal. This only happens with numbers that contain a decimal point. I assume what happens is that the number is internally converted to a float and then a Decimal.

As a user, this lossy internal conversion is an unexpected and unwelcome surprise. Both the initial JSON string and the final Decimal can contain the full precision of the value without loss, so I would have expected the conversion to be lossless as well.

PS: I'm not sure if this is a bug per se because I could not find any documentation that explicitly states what the expected behavior is. However, based on what was written in the documentation it certainly was unexpected to me.

Example Code

from decimal import Decimal
from pydantic import BaseModel

class Test(BaseModel):
    value: Decimal

print(Test.model_validate_json('{"value": 1.234567890123456789012345678901234567890}'))
# Expected output: value=Decimal('1.234567890123456789012345678901234567890')
# Actual output: value=Decimal('1.2345678901234567')

print(Test.model_validate_json('{"value": 12345678901234567890123456789012345678.9}'))
# Expected output: value=Decimal('12345678901234567890123456789012345678.9')
# Actual output: value=Decimal('12345678901234568000000000000000000000')

Python, Pydantic & OS Version

             pydantic version: 2.6.4
        pydantic-core version: 2.16.3
          pydantic-core build: profile=release pgo=true
                 install path: <redacted>
               python version: 3.10.11 (tags/v3.10.11:7d4cc5a, Apr  5 2023, 00:38:17) [MSC v.1929 64 bit (AMD64)]
                     platform: Windows-10-10.0.19045-SP0
             related packages: fastapi-0.109.2 mypy-1.3.0 typing_extensions-4.8.0
                       commit: unknown