A fast, light-weight web server for WAI applications.


Version on this page:
LTS Haskell 21.23:3.3.25@rev:1
Stackage Nightly 2023-12-06:3.3.31
Latest on Hackage:3.3.31

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MIT licensed by Michael Snoyman, Kazu Yamamoto, Matt Brown
Maintained by michael@snoyman.com
This version can be pinned in stack with:warp-,7258

Module documentation for


The premier WAI handler. For more information, see Warp: A Haskell Web Server.



  • Warp now supports blaze-builder v0.4 or later only.
  • HTTP/2 code was improved: dynamic priority change, efficient queuing and sender loop continuation. #423 #424


  • Configurable Slowloris size #418


  • Fixing a bug of HTTP/2 when no FD cache is used #411
  • Fixing a buffer-pool bug #406 #407


  • Supporting HTTP/2 #399
  • Cleaning up APIs #387

  • Remove dependency on the void package #375


  • Turn off file descriptor cache by default #371

  • Fix for: HEAD requests returning non-empty entity body #369


  • Only conditionally produce HTTP 100 Continue


  • Better HEAD support for files #357


  • Fix missing IORef tweak
  • Disable timeouts as soon as request body is fully consumed. This addresses the common case of a non-chunked request body. Previously, we would wait until a zero-length ByteString is returned, but that is suboptimal for some cases. For more information, see issue 351.
  • Add pauseTimeout function

  • Don’t serve a 416 status code for 0-length files keter issue #75
  • Don’t serve content-length for 416 responses #346

Fix support for old versions of bytestring

Add support for blaze-builder 0.4


  • Add runEnv: like run but uses $PORT #334


Support for PROXY protocol, such as used by Amazon ELB TCP. This is useful since, for example, Amazon ELB HTTP does not have support for Websockets. More information on the protocol is available from Amazon.


Added setFork.


Modify flushing of request bodies. Previously, regardless of the size of the request body, the entire body would be flushed. When uploading large files to a web app that does not accept such files (e.g., returns a 413 too large status), browsers would still send the entire request body and the servers will still receive it.

The new behavior is to detect if there is a large amount of data still to be consumed and, if so, immediately terminate the connection. In the case of chunked request bodies, up to a maximum number of bytes is consumed before the connection is terminated.

This is controlled by the new setting setMaximumBodyFlush. A value of @Nothing@ will return the original behavior of flushing the entire body.


WAI no longer uses conduit for its streaming interface.


The onOpen and onClose settings now provide the SockAddr of the client, and onOpen can return a Bool which will close the connection. The responseRaw response has been added, which provides a more elegant way to handle WebSockets than the previous settingsIntercept. The old settings accessors have been deprecated in favor of new setters, which will allow settings changes to be made in the future without breaking backwards compatibility.


ResourceT is not used anymore. Request and Response is now abstract data types. To use their constructors, Internal module should be imported.


Support for byte range requests.


Sockets now have FD_CLOEXEC set on them. This behavior is more secure, and the change should not affect the vast majority of use cases. However, it appeared that this is buggy and is fixed in 2.0.0.