Content-Encoding with multiple codings is not decoded

Author: PhaeiloCreated Aug 10, 2026Updated Aug 29, 2026

Labelsbug

Describe the bug

RFC 9110 §8.4 allows Content-Encoding to list several codings. The client must decode them in reverse order. When a server sends a body compressed twice and the header Content-Encoding: gzip,gzip, aiohttp does not decode the body. It passes the raw gzip bytes to resp.json() (and resp.text()), which then tries to UTF-8-decode the gzip magic and fails.

To Reproduce

Two files. No TLS, no proxy.

server.py (stdlib only) always answers with a doubly-gzipped JSON body and declares Content-Encoding: gzip,gzip:

python

#!/usr/bin/env python3
import gzip
import json
import socket

body = gzip.compress(gzip.compress(json.dumps({"hello": "world"}).encode()))
srv = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
srv.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
srv.bind(("127.0.0.1", 8080))
srv.listen()
while True:
    conn, _ = srv.accept()
    conn.recv(65536)
    head = "\r\n".join((
        "HTTP/1.1 200 OK",
        "Content-Type: application/json",
        "Content-Encoding: gzip,gzip",
        f"Content-Length: {len(body)}",
        "Connection: close",
        "",
        "",
    )).encode()
    conn.sendall(head + body)
    conn.close()

client.py:

python

#!/usr/bin/env python3
import asyncio
import aiohttp


async def fetch():
    async with aiohttp.ClientSession() as session:
        async with session.get("http://127.0.0.1:8080/") as resp:
            print(await resp.json())


asyncio.run(fetch())

Run:

./server.py &
uv run --with aiohttp==3.14.3 ./client.py

Expected behavior

aiohttp decodes each coding listed in Content-Encoding, in reverse order (here: gunzip twice), and resp.json() returns {"hello": "world"}.

Logs/tracebacks

python-traceback

$ ./server.py &
$ uv run --with aiohttp==3.14.3 ./client.py
Installed 9 packages in 8ms
Traceback (most recent call last):
  File "/tmp/tg/./client.py", line 12, in <module>
    asyncio.run(fetch())
    ~~~~~~~~~~~^^^^^^^^^
  File "/home/philip/.local/share/uv/python/cpython-3.13.11-linux-x86_64-gnu/lib/python3.13/asyncio/runners.py", line 195, in run
    return runner.run(main)
           ~~~~~~~~~~^^^^^^
  File "/home/philip/.local/share/uv/python/cpython-3.13.11-linux-x86_64-gnu/lib/python3.13/asyncio/runners.py", line 118, in run
    return self._loop.run_until_complete(task)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/home/philip/.local/share/uv/python/cpython-3.13.11-linux-x86_64-gnu/lib/python3.13/asyncio/base_events.py", line 725, in run_until_complete
    return future.result()
           ~~~~~~~~~~~~~^^
  File "/tmp/tg/./client.py", line 9, in fetch
    print(await resp.json())
          ^^^^^^^^^^^^^^^^^
  File "/home/philip/.cache/uv/archive-v0/6kdapgyXRUIN9ElB/lib/python3.13/site-packages/aiohttp/client_reqrep.py", line 795, in json
    return loads(stripped.decode(encoding))
                 ~~~~~~~~~~~~~~~^^^^^^^^^^
UnicodeDecodeError: 'utf-8' codec can't decode byte 0x8b in position 1: invalid start byte

Python Version

bash

$ uv run --with aiohttp==3.14.3 python --version
Python 3.13.11

aiohttp Version

bash

$ uv run --with aiohttp==3.14.3 python -m pip show aiohttp
Name: aiohttp
Version: 3.14.3
Summary: Async http client/server framework (asyncio)
Home-page: https://github.com/aio-libs/aiohttp
Author: 
Author-email: 
License: Apache-2.0 AND MIT
Location: /home/philip/.cache/uv/archive-v0/6kdapgyXRUIN9ElB/lib/python3.13/site-packages
Requires: aiohappyeyeballs, aiosignal, attrs, frozenlist, multidict, propcache, yarl
Required-by:

multidict Version

bash

$ uv run --with aiohttp==3.14.3 python -m pip show multidict
Name: multidict
Version: 6.7.1
Summary: multidict implementation
Home-page: https://github.com/aio-libs/multidict
Author: Andrew Svetlov
Author-email: [email protected]
License: Apache License 2.0
Location: /home/philip/.cache/uv/archive-v0/6kdapgyXRUIN9ElB/lib/python3.13/site-packages
Requires: 
Required-by: aiohttp, yarl

propcache Version

bash

$ uv run --with aiohttp==3.14.3 python -m pip show propcache
Name: propcache
Version: 0.5.2
Summary: Accelerated property cache
Home-page: https://github.com/aio-libs/propcache
Author: Andrew Svetlov
Author-email: [email protected]
License: Apache-2.0
Location: /home/philip/.cache/uv/archive-v0/6kdapgyXRUIN9ElB/lib/python3.13/site-packages
Requires: 
Required-by: aiohttp, yarl

yarl Version

bash

$ uv run --with aiohttp==3.14.3 python -m pip show yarl     
Name: yarl
Version: 1.24.5
Summary: Yet another URL library
Home-page: https://github.com/aio-libs/yarl
Author: Andrew Svetlov
Author-email: [email protected]
License: Apache-2.0
Location: /home/philip/.cache/uv/archive-v0/6kdapgyXRUIN9ElB/lib/python3.13/site-packages
Requires: idna, multidict, propcache
Required-by: aiohttp

OS

Arch Linux

Related component

Client

Additional context

Suspected cause: HttpParser.parse_headers() in aiohttp/http_parser.py:632-635 matches the whole header against the set {"gzip", "deflate", "br", "zstd"}. A value like gzip,gzip does not match, so encoding stays None (line 608), the DeflateBuffer wrapper is skipped (line 882), and raw bytes pass through. Multi-coding Content-Encoding is simply not implemented.

Code of Conduct

I agree to follow the aio-libs Code of Conduct

Source: aio-libs/aiohttp

View original on GitHub View discussion on GitHub