unexpected cast from int to float for big numbers

[maffinyep]

First Check

• I added a very descriptive title here.
• I used the GitHub search to find a similar question and didn't find it.
• I searched the FastAPI documentation, with the integrated search.
• I already searched in Google "How to X in FastAPI" and didn't find any information.
• I already read and followed all the tutorial in the docs and didn't find an answer.
• I already checked if it is not related to FastAPI but to Pydantic.
• I already checked if it is not related to FastAPI but to Swagger UI.
• I already checked if it is not related to FastAPI but to ReDoc.

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Example Code

from fastapi import FastAPI

app = FastAPI()

@app.get("/")
async def read(bit: int):
    CHUBBY_NUMBER = 2 ** (bit)
    return {
        "as_int": CHUBBY_NUMBER,
        "as_str": str(CHUBBY_NUMBER),
    }

Description

• Open the browser and call the endpoint /?bit=62.
• I expected it to return a JSON where as_int is equal to as_str, but they won't.

This weird behaviour starts from bit=54, so I guess the issue is due to a implicit casting to a 64 bit float, with 53 bit precision.
Return object is still an int and not a float, and no errors or warnings are raised, that's most of my concern.

Operating System

Linux

Operating System Details

tested on Ubuntu 22 and python:3.12 Docker image on WSL

FastAPI Version

0.110.2

Pydantic Version

2.7.1

Python Version

Python 3.12.3

Additional Context

Defining a response model won't mitigate the problem.

Pic of a chubby number:

[maffinyep]

sorry, duplicate of #5464

[maffinyep]

Form #5464 (comment)

The problem [...] is neither a limitation of JSON nor OpenAPI, but of Javascript not being able to parse such large ints correctly (or to be more precise: with such precision).