httpx.ReadError for async POST request with content of 256 kB or higher

[asmaier]

Current Behavior:

For POST content above a certain size (256kB in my example) the async client raises an httpx.ReadError.
The sync POST request works as expected and doesn't raise an exception.

Expected Behavior:

I expect the async client to behave the same as the sync client. It should not raise an exception.

Steps To Reproduce:

Consider the following example code

import asyncio
import os
import httpx


method = "POST"
url = "https://example.com"

content = os.urandom(1024*256) # async request fails
# content = os.urandom(1024*128)

result = httpx.post("https://example.com", timeout=60, content=content)
print("Sync Request: success!")

async def main():
    async with httpx.AsyncClient() as client:
        result = await client.request("POST", url, timeout=60, content=content)
    print("Async Request: success!")


if __name__ ==  '__main__':
    asyncio.run(main())

When this code is executed it will output

Sync Request: success!
Traceback (most recent call last):
  File "/Users/git/temp/httpx_bug/.venv/lib/python3.13/site-packages/httpx/_transports/default.py", line 101, in map_httpcore_exceptions
    yield
  File "/Users/git/temp/httpx_bug/.venv/lib/python3.13/site-packages/httpx/_transports/default.py", line 394, in handle_async_request
    resp = await self._pool.handle_async_request(req)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/git/temp/httpx_bug/.venv/lib/python3.13/site-packages/httpcore/_async/connection_pool.py", line 256, in handle_async_request
    raise exc from None
  File "/Users/git/temp/httpx_bug/.venv/lib/python3.13/site-packages/httpcore/_async/connection_pool.py", line 236, in handle_async_request
    response = await connection.handle_async_request(
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        pool_request.request
        ^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/Users/git/temp/httpx_bug/.venv/lib/python3.13/site-packages/httpcore/_async/connection.py", line 103, in handle_async_request
    return await self._connection.handle_async_request(request)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/git/temp/httpx_bug/.venv/lib/python3.13/site-packages/httpcore/_async/http11.py", line 136, in handle_async_request
    raise exc
  File "/Users/git/temp/httpx_bug/.venv/lib/python3.13/site-packages/httpcore/_async/http11.py", line 106, in handle_async_request
    ) = await self._receive_response_headers(**kwargs)
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/git/temp/httpx_bug/.venv/lib/python3.13/site-packages/httpcore/_async/http11.py", line 177, in _receive_response_headers
    event = await self._receive_event(timeout=timeout)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/git/temp/httpx_bug/.venv/lib/python3.13/site-packages/httpcore/_async/http11.py", line 217, in _receive_event
    data = await self._network_stream.read(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        self.READ_NUM_BYTES, timeout=timeout
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/Users/git/temp/httpx_bug/.venv/lib/python3.13/site-packages/httpcore/_backends/anyio.py", line 32, in read
    with map_exceptions(exc_map):
         ~~~~~~~~~~~~~~^^^^^^^^^
  File "/Users/homebrew/Cellar/[email protected]/3.13.5/Frameworks/Python.framework/Versions/3.13/lib/python3.13/contextlib.py", line 162, in __exit__
    self.gen.throw(value)
    ~~~~~~~~~~~~~~^^^^^^^
  File "/Users/git/temp/httpx_bug/.venv/lib/python3.13/site-packages/httpcore/_exceptions.py", line 14, in map_exceptions
    raise to_exc(exc) from exc
httpcore.ReadError

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/Users/git/temp/httpx_bug/main.py", line 22, in <module>
    asyncio.run(main())
    ~~~~~~~~~~~^^^^^^^^
  File "/Users/homebrew/Cellar/[email protected]/3.13.5/Frameworks/Python.framework/Versions/3.13/lib/python3.13/asyncio/runners.py", line 195, in run
    return runner.run(main)
           ~~~~~~~~~~^^^^^^
  File "/Users/homebrew/Cellar/[email protected]/3.13.5/Frameworks/Python.framework/Versions/3.13/lib/python3.13/asyncio/runners.py", line 118, in run
    return self._loop.run_until_complete(task)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/Users/homebrew/Cellar/[email protected]/3.13.5/Frameworks/Python.framework/Versions/3.13/lib/python3.13/asyncio/base_events.py", line 725, in run_until_complete
    return future.result()
           ~~~~~~~~~~~~~^^
  File "/Users/git/temp/httpx_bug/main.py", line 17, in main
    result = await client.request("POST", url, timeout=60, content=content)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/git/temp/httpx_bug/.venv/lib/python3.13/site-packages/httpx/_client.py", line 1540, in request
    return await self.send(request, auth=auth, follow_redirects=follow_redirects)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/git/temp/httpx_bug/.venv/lib/python3.13/site-packages/httpx/_client.py", line 1629, in send
    response = await self._send_handling_auth(
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    ...<4 lines>...
    )
    ^
  File "/Users/git/temp/httpx_bug/.venv/lib/python3.13/site-packages/httpx/_client.py", line 1657, in _send_handling_auth
    response = await self._send_handling_redirects(
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    ...<3 lines>...
    )
    ^
  File "/Users/git/temp/httpx_bug/.venv/lib/python3.13/site-packages/httpx/_client.py", line 1694, in _send_handling_redirects
    response = await self._send_single_request(request)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/git/temp/httpx_bug/.venv/lib/python3.13/site-packages/httpx/_client.py", line 1730, in _send_single_request
    response = await transport.handle_async_request(request)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/git/temp/httpx_bug/.venv/lib/python3.13/site-packages/httpx/_transports/default.py", line 393, in handle_async_request
    with map_httpcore_exceptions():
         ~~~~~~~~~~~~~~~~~~~~~~~^^
  File "/Users/ahomebrew/Cellar/[email protected]/3.13.5/Frameworks/Python.framework/Versions/3.13/lib/python3.13/contextlib.py", line 162, in __exit__
    self.gen.throw(value)
    ~~~~~~~~~~~~~~^^^^^^^
  File "/Users/git/temp/httpx_bug/.venv/lib/python3.13/site-packages/httpx/_transports/default.py", line 118, in map_httpcore_exceptions
    raise mapped_exc(message) from exc
httpx.ReadError

When we activate the line content = os.urandom(1024*128) the output of the program is

$ uv run python main.py
Sync Request: success!
Async Request: success!

Versions

httpx: 0.28.1
OS Version: ARM Mac OS 26.1
Python version: Python 3.13.5
uv version: uv 0.9.13 (Homebrew 2025-11-26)