Bug fixes

• Skipped the duplicate singleton header check in lax mode (the default for response
  parsing). In strict mode (request parsing, or -X dev), all RFC 9110 singletons
  are still enforced -- by :user:bdraco.

  Related issues and pull requests on GitHub:
  #12302.

Features

• Added max_headers parameter to limit the number of headers that should be read from a response -- by :user:Dreamsorcerer.

  Related issues and pull requests on GitHub:
  #11955.

• Added a dns_cache_max_size parameter to TCPConnector to limit the size of the cache -- by :user:Dreamsorcerer.

  Related issues and pull requests on GitHub:
  #12106.

Bug fixes

• Fixed server hanging indefinitely when chunked transfer encoding chunk-size
  does not match actual data length. The server now raises
  TransferEncodingError instead of waiting forever for data that will
  never arrive -- by :user:Fridayai700.

  Related issues and pull requests on GitHub:
  #10596.

• Fixed access log timestamps ignoring daylight saving time (DST) changes. The
  previous implementation used :py:data:time.timezone which is a constant and
  does not reflect DST transitions -- by :user:nightcityblade.

  Related issues and pull requests on GitHub:
  #11283.

• Fixed RuntimeError: An event loop is running error when using aiohttp.GunicornWebWorker
  or aiohttp.GunicornUVLoopWebWorker on Python >=3.14.
  -- by :user:Tasssadar.

  Related issues and pull requests on GitHub:
  #11701.

• Fixed :exc:ValueError when creating a TLS connection with ClientTimeout(total=0) by converting 0 to None before passing to ssl_handshake_timeout in :py:meth:asyncio.loop.start_tls -- by :user:veeceey.

  Related issues and pull requests on GitHub:
  #11859.

• Restored :py:meth:~aiohttp.BodyPartReader.decode as a synchronous method
  for backward compatibility. The method was inadvertently changed to async
  in 3.13.3 as part of the decompression bomb security fix. A new
  :py:meth:~aiohttp.BodyPartReader.decode_iter method is now available
  for non-blocking decompression of large payloads using an async generator.
  Internal aiohttp code uses the async variant to maintain security protections.

  Changed multipart processing chunk sizes from 64 KiB to 256KiB, to better
  match aiohttp internals
  -- by :user:bdraco and :user:Dreamsorcerer.

  Related issues and pull requests on GitHub:
  #11898.

• Fixed false-positive :py:class:DeprecationWarning for passing enable_cleanup_closed=True to :py:class:~aiohttp.TCPConnector specifically on Python 3.12.7.
  -- by :user:Robsdedude.

  Related issues and pull requests on GitHub:
  #11972.

• Fixed _sendfile_fallback over-reading beyond requested count -- by :user:bysiber.

  Related issues and pull requests on GitHub:
  #12096.

• Fixed digest auth dropping challenge fields with empty string values -- by :user:bysiber.

  Related issues and pull requests on GitHub:
  #12097.

• ClientConnectorCertificateError.os_error no longer raises :exc:AttributeError
  -- by :user:themylogin.

  Related issues and pull requests on GitHub:
  #12136.

• Adjusted pure-Python request header value validation to align with RFC 9110 control-character handling, while preserving lax response parser behavior, and added regression tests for Host/header control-character cases.
  -- by :user:rodrigobnogueira.

  Related issues and pull requests on GitHub:
  #12231.

• Rejected duplicate singleton headers (Host, Content-Type,
  Content-Length, etc.) in the C extension HTTP parser to match
  the pure Python parser behaviour, preventing potential host-based
  access control bypasses via parser differentials
  -- by :user:rodrigobnogueira.

  Related issues and pull requests on GitHub:
  #12240.

• Aligned the pure-Python HTTP request parser with the C parser by splitting
  comma-separated and repeated Connection header values for keep-alive,
  close, and upgrade handling -- by :user:rodrigobnogueira.

  Related issues and pull requests on GitHub:
  #12249.

Improved documentation

• Documented :exc:asyncio.TimeoutError for WebSocketResponse.receive()
  and related methods -- by :user:veeceey.

  Related issues and pull requests on GitHub:
  #12042.

Packaging updates and notes for downstreams

• Upgraded llhttp to 3.9.1 -- by :user:Dreamsorcerer.

  Related issues and pull requests on GitHub:
  #12069.

Contributor-facing changes

• The benchmark CI job now runs only in the upstream repository -- by :user:Cycloctane.

  It used to always fail in forks, which this change fixed.

  Related issues and pull requests on GitHub:
  #11737.

• Fixed flaky performance tests by using appropriate fixed thresholds that account for CI variability -- by :user:rodrigobnogueira.

  Related issues and pull requests on GitHub:
  #11992.

Miscellaneous internal changes

• Fixed test_invalid_idna to work with idna 3.11 by using an invalid character (\u0080) that is rejected by yarl during URL construction -- by :user:rodrigobnogueira.

  Related issues and pull requests on GitHub:
  #12027.

• Fixed race condition in test_data_file on Python 3.14 free-threaded builds -- by :user:rodrigobnogueira.

  Related issues and pull requests on GitHub:
  #12170.

This release contains fixes for several vulnerabilities. It is advised to
upgrade as soon as possible.

Bug fixes

• Fixed proxy authorization headers not being passed when reusing a connection, which caused 407 (Proxy authentication required) errors
  -- by :user:GLeurquin.

  Related issues and pull requests on GitHub:
  #2596.

• Fixed multipart reading failing when encountering an empty body part -- by :user:Dreamsorcerer.

  Related issues and pull requests on GitHub:
  #11857.

• Fixed a case where the parser wasn't raising an exception for a websocket continuation frame when there was no initial frame in context.

  Related issues and pull requests on GitHub:
  #11862.

Removals and backward incompatible breaking changes

• Brotli and brotlicffi minimum version is now 1.2.
  Decompression now has a default maximum output size of 32MiB per decompress call -- by :user:Dreamsorcerer.

  Related issues and pull requests on GitHub:
  #11898.

Packaging updates and notes for downstreams

• Moved dependency metadata from :file:setup.cfg to :file:pyproject.toml per :pep:621
  -- by :user:cdce8p.

  Related issues and pull requests on GitHub:
  #11643.

Contributor-facing changes

• Removed unused update-pre-commit github action workflow -- by :user:Cycloctane.

  Related issues and pull requests on GitHub:
  #11689.

Miscellaneous internal changes

• Optimized web server performance when access logging is disabled by reducing time syscalls -- by :user:bdraco.

  Related issues and pull requests on GitHub:
  #10713.

• Added regression test for cached logging status -- by :user:meehand.

  Related issues and pull requests on GitHub:
  #11778.

Bug fixes

• Fixed cookie parser to continue parsing subsequent cookies when encountering a malformed cookie that fails regex validation, such as Google's g_state cookie with unescaped quotes -- by :user:bdraco.

  Related issues and pull requests on GitHub:
  #11632.

• Fixed loading netrc credentials from the default :file:~/.netrc (:file:~/_netrc on Windows) location when the :envvar:NETRC environment variable is not set -- by :user:bdraco.

  Related issues and pull requests on GitHub:
  #11713, #11714.

• Fixed WebSocket compressed sends to be cancellation safe. Tasks are now shielded during compression to prevent compressor state corruption. This ensures that the stateful compressor remains consistent even when send operations are cancelled -- by :user:bdraco.

  Related issues and pull requests on GitHub:
  #11725.

Features

• Make configuration options in AppRunner also available in run_app()
  -- by :user:Cycloctane.

  Related issues and pull requests on GitHub:
  #11633.

Bug fixes

• Switched to backports.zstd for Python <3.14 and fixed zstd decompression for chunked zstd streams -- by :user:ZhaoMJ.

  Note: Users who installed zstandard for support on Python <3.14 will now need to install
  backports.zstd instead (installing aiohttp[speedups] will do this automatically).

  Related issues and pull requests on GitHub:
  #11623.

• Updated Content-Type header parsing to return application/octet-stream when header contains invalid syntax.
  See :rfc:9110#section-8.3-5.

  -- by :user:sgaist.

  Related issues and pull requests on GitHub:
  #10889.

• Fixed Python 3.14 support when built without zstd support -- by :user:JacobHenner.

  Related issues and pull requests on GitHub:
  #11603.

• Fixed blocking I/O in the event loop when using netrc authentication by moving netrc file lookup to an executor -- by :user:bdraco.

  Related issues and pull requests on GitHub:
  #11634.

• Fixed routing to a sub-application added via .add_domain() not working
  if the same path exists on the parent app. -- by :user:Dreamsorcerer.

  Related issues and pull requests on GitHub:
  #11673.

Packaging updates and notes for downstreams

• Moved core packaging metadata from :file:setup.cfg to :file:pyproject.toml per :pep:621
  -- by :user:cdce8p.

  Related issues and pull requests on GitHub:
  #9951.

Features

• Added support for Python 3.14.

  Related issues and pull requests on GitHub:
  #10851, #10872.

• Added support for free-threading in Python 3.14+ -- by :user:kumaraditya303.

  Related issues and pull requests on GitHub:
  #11466, #11464.

• Added support for Zstandard (aka Zstd) compression
  -- by :user:KGuillaume-chaps.

  Related issues and pull requests on GitHub:
  #11161.

• Added StreamReader.total_raw_bytes to check the number of bytes downloaded
  -- by :user:robpats.

  Related issues and pull requests on GitHub:
  #11483.

Bug fixes

• Fixed pytest plugin to not use deprecated :py:mod:asyncio policy APIs.

  Related issues and pull requests on GitHub:
  #10851.

• Updated Content-Disposition header parsing to handle trailing semicolons and empty parts
  -- by :user:PLPeeters.

  Related issues and pull requests on GitHub:
  #11243.

• Fixed saved CookieJar failing to be loaded if cookies have partitioned flag when
  http.cookie does not have partitioned cookies supports. -- by :user:Cycloctane.

  Related issues and pull requests on GitHub:
  #11523.

Improved documentation

• Added Wireup to third-party libraries -- by :user:maldoinc.

  Related issues and pull requests on GitHub:
  #11233.

Packaging updates and notes for downstreams

• The blockbuster test dependency is now optional; the corresponding test fixture is disabled when it is unavailable
  -- by :user:musicinybrain.

  Related issues and pull requests on GitHub:
  #11363.

• Added riscv64 build to releases -- by :user:eshattow.

  Related issues and pull requests on GitHub:
  #11425.

Contributor-facing changes

• Fixed test_send_compress_text failing when alternative zlib implementation
  is used. (zlib-ng in python 3.14 windows build) -- by :user:Cycloctane.

  Related issues and pull requests on GitHub:
  #11546.

Bug fixes

• Fixed :class:~aiohttp.DigestAuthMiddleware to preserve the algorithm case from the server's challenge in the authorization response. This improves compatibility with servers that perform case-sensitive algorithm matching (e.g., servers expecting algorithm=MD5-sess instead of algorithm=MD5-SESS)
  -- by :user:bdraco.

  Related issues and pull requests on GitHub:
  #11352.

Improved documentation

• Remove outdated contents of aiohttp-devtools and aiohttp-swagger
  from Web_advanced docs.
  -- by :user:Cycloctane

  Related issues and pull requests on GitHub:
  #11347.

Packaging updates and notes for downstreams

• Started including the llhttp :file:LICENSE file in wheels by adding vendor/llhttp/LICENSE to license-files in :file:setup.cfg -- by :user:threexc.

  Related issues and pull requests on GitHub:
  #11226.

Contributor-facing changes

• Updated a regex in test_aiohttp_request_coroutine for Python 3.14.

  Related issues and pull requests on GitHub:
  #11271.

Bug fixes

• Fixed file uploads failing with HTTP 422 errors when encountering 307/308 redirects, and 301/302 redirects for non-POST methods, by preserving the request body when appropriate per :rfc:9110#section-15.4.3-3.1 -- by :user:bdraco.

  Related issues and pull requests on GitHub:
  #11270.

• Fixed :py:meth:ClientSession.close() <aiohttp.ClientSession.close> hanging indefinitely when using HTTPS requests through HTTP proxies -- by :user:bdraco.

  Related issues and pull requests on GitHub:
  #11273.

• Bumped minimum version of aiosignal to 1.4+ to resolve typing issues -- by :user:Dreamsorcerer.

  Related issues and pull requests on GitHub:
  #11280.

Features

• Added initial trailer parsing logic to Python HTTP parser -- by :user:Dreamsorcerer.

  Related issues and pull requests on GitHub:
  #11269.

Improved documentation

• Clarified exceptions raised by WebSocketResponse.send_frame et al.
  -- by :user:DoctorJohn.

  Related issues and pull requests on GitHub:
  #11234.

Bug fixes

• Fixed auto-created :py:class:~aiohttp.TCPConnector not using the session's event loop when :py:class:~aiohttp.ClientSession is created without an explicit connector -- by :user:bdraco.

  Related issues and pull requests on GitHub:
  #11147.

Bug fixes

• Fixed cookie unquoting to properly handle octal escape sequences in cookie values (e.g., \012 for newline) by vendoring the correct _unquote implementation from Python's http.cookies module -- by :user:bdraco.

  Related issues and pull requests on GitHub:
  #11173.

• Fixed Cookie header parsing to treat attribute names as regular cookies per :rfc:6265#section-5.4 -- by :user:bdraco.

  Related issues and pull requests on GitHub:
  #11178.