Httpx Generating Incorrect Content-Length Header Value Using Json Param

[Kilmoore]

Working with some 3rd party apis. I noticed that I was failing a request to a 3rd party api which is passing in Postman and via Pythons requests package.
I have a httpx POST request to the 3rd party api. This 3rd Party is verifying the POST request via checking the content-length against the body.

The httpx request is failing but other python request libraries are passing and receiving data.
Below is a test script that I used to check requests vs httpx vs aiohttp and generate the header value manually.

httpx is on latest from pypi 0.28.1

import requests
import httpx
import aiohttp
import asyncio
import json

body = {
    "identifiers":
    [
        "number",
        "name"
    ],
    "page": 1,
    "per_page": 100
}

headers = {
    'Content-Type': 'application/json',
    'Accept': 'application/json',
    'x': 'xxxx'
}

url = "https://httpbin.org/post"

httpx_response = httpx.post(url, headers=headers, json=body)
requests_response = requests.post(url, headers=headers, json=body)

aiohttp_content_length = None

async def fetch():
    async with aiohttp.ClientSession() as session:
        async with session.post(url, headers=headers, json=body) as aiohttp_response:
            global aiohttp_content_length
            aiohttp_content_length = aiohttp_response._request_info.headers.get('Content-Length', 0)

asyncio.run(fetch())

print("################ Content-Length #################")
print(f"Script calculated: {len(json.dumps(body).encode('utf-8'))}")
print(f"Httpx calculated: {httpx_response.request.headers.get('Content-Length', 0)}")
print(f"Requests calculated: {requests_response.request.headers.get('Content-Length', 0)}")
print(f"aiohttp calculated: {aiohttp_content_length}")
print("#################################################\n")

Here is the output:

################ Content-Length #################
Script calculated: 63
Httpx calculated: 57
Requests calculated: 63
aiohttp calculated: 63
#################################################

I checked the issues and discussions for content and content-length related topics
I did find this closed issue #1482 but it's using the file param and the results I get is not an exception rather an incorrect response.

There could be something I am doing wrong and please let me know and I will adjust the code and test again.

Environments I've tried and python versions
OS: macOS 15.5, Postman V11.52.2, Ubuntu 20.04.6 LTS
Python version: Python 3.12.7
HTTPX version: 0.28.1 and master
HTTP proxy: no
Custom certificates: no

[Kilmoore]

Verified this is working correctly in version <=0.27.2.
Problem occurs when running >=0.28.0

[lovelydinosaur]

httpx/CHANGELOG.md

Line 38 in 4fb9528

* JSON request bodies use a compact representation. (#3363)

You're not seeing an incorrect Content-Length. You're seeing a more compact representation being used.

[Kilmoore]

Thank you for spotting this. Mark as answered 👍