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Compatibility dates

Cloudflare regularly updates the Workers runtime. These updates apply to all Workers globally and should never cause a Worker that is already deployed to stop functioning. Sometimes, though, some changes may be backwards-incompatible. In particular, there might bugs in the runtime API that existing Workers may inadvertently depend. Cloudflare implements bug fixes that new Workers can opt into while existing Workers will continue to see the buggy behavior to prevent breaking deployed Workers.

Compatibility dates (and flags) are how you, as a developer, opt into these changes. By specifying a compatibility_date in your wrangler.toml file, that Worker enables all changes that were made before the given date.

# (in wrangler.toml)
# Opt into backwards-incompatible changes through September 14, 2021.
compatibility_date = "2021-09-14"

When you start your project, you should always set compatibility_date to the current date. You should occassionally update the compatibility_date field. When updating, you should refer to this page to find out what has changed, and you should be careful to test your Worker to see if the changes affect you, updating your code as necessary. The new compatibility date takes effect when you next run the wrangler publish command.

There is no need to update your compatibility_date if you do not want to. The Workers runtime will support old compatibility dates forever. If, for some reason, Cloudflare finds it is necessary to make a change that will break live Workers, Cloudflare will actively contact affected developers. That said, Cloudflare aims to avoid this if at all possible.

However, even though you do not need to update the compatibility_date field, it is a good practice to do so for two reasons:

  1. Sometimes, new features can only be made available to Workers that have a current compatibility_date. To access the latest features, you need to stay up-to-date.
  2. Generally, other than this page, the Workers documentation may only describe the current compatibility_date, omitting information about historical behavior. If your Worker uses an old compatibility_date, you will need to continuously refer to this page in order to check if any of the APIs you are using have changed.

Compatibility flags

In addition to setting a compatibility_date in your wrangler.toml file, you may also provide a list of compatibility_flags, which enable or disable specific changes.

# (in wrangler.toml)
# Opt into backwards-incompatible changes through September 14, 2021.
compatibility_date = "2021-09-14"
# Also opt into an upcoming fix to the FormData API.
compatibility_flags = [ "formdata_parser_supports_files" ]

This example enabled the specific flag formdata_parser_supports_files, which is described below. As of the specified date, 2021-09-14, this particular flag was not yet enabled by default, but specifying it in this way enables it anyway. compatibility_flags can also be used to disable changes that became the default in the past.

Most developers will not need to use compatibility_flags; instead, Cloudflare recommends only specifying compatibility_date. compatibility_flags can be useful if you want to help the Workers team test upcoming changes that are not yet enabled by default, or if you need to hold back a change that your code depends on but still want to apply other compatibility changes.

Change history

Newest changes are listed first.

Durable Object stub.fetch() requires a full URL

Default as of2021-11-10
Flag to enable earlydurable_object_fetch_requires_full_url
Flag to disabledurable_object_fetch_allows_relative_url

Originally, when making a request to a Durable Object by calling stub.fetch(url), a relative URL was accepted as an input. The URL would be interpreted relative to the dummy URL http://fake-host, and the resulting absolute URL was delivered to the destination object's fetch() handler. This was a mistake — full URLs were meant to be required. This flag makes full URLs required.

fetch() improperly interprets unknown protocols as HTTP

Default as of2021-11-10
Flag to enable earlyfetch_refuses_unknown_protocols
Flag to disablefetch_treats_unknown_protocols_as_http

Originally, if the fetch() function was passed a URL specifying any protocol other than http: or https:, it would silently treat it as if it were http:. For example, fetch() would appear to accept ftp: URLs, but it was actually making HTTP requests instead.

Note that Cloudflare Workers supports a non-standard extension to fetch() to make it support WebSockets. However, when making an HTTP request that is intended to initiate a WebSocket handshake, you should still use http: or https: as the protocol, not ws: nor wss:.

The ws: and wss: URL schemes are intended to be used together with the new WebSocket() constructor, which exclusively supports WebSocket. The extension to fetch() is designed to support HTTP and WebSocket in the same request (the response may or may not choose to initiate a WebSocket), and so all requests are considered to be HTTP.

FormData parsing supports File

Default as of2021-11-03
Flag to enable earlyformdata_parser_supports_files
Flag to disableformdata_parser_converts_files_to_strings

The FormData API is used to parse data (especially HTTP request bodies) in multipart/form-data format.

Originally, the Workers runtime's implementation of the FormData API incorrectly converted uploaded files to strings. Therefore, formData.get("filename") would return a string containing the file contents instead of a File object. This change fixes the problem, causing files to be represented using File as specified in the standard.

Experimental changes

These changes can be enabled via compatibility_flags, but are not yet scheduled to become default on any particular date.

HTMLRewriter handling of <esi:include>

Default as ofTBD
Flag to enable earlyhtml_rewriter_treats_esi_include_as_void_tag
Flag to disableTBD

The HTML5 standard defines a fixed set of elements as void elements, meaning they do not use an end tag: <area>, <base>, <br>, <col>, <command>, <embed>, <hr>, <img>, <input>, <keygen>, <link>, <meta>, <param>, <source>, <track>, and <wbr>.

HTML5 does not recognize XML self-closing tag syntax. For example, <script src="foo.js" /> does not specify a script element with no body. A </script> ending tag is still required. The /> syntax simply is not recognized by HTML5 at all; it is treated the same as >. However, many developers still like to use this syntax, as a holdover from XHTML, a standard which failed to gain traction in the early 2000's.

<esi:include> and <esi:comment> are two tags that are not part of the HTML5 standard, but are instead used as part of Edge Side Includes, a technology for server-side HTML modification. These tags are not expected to contain any body and are commonly written with XML self-closing syntax.

HTMLRewriter was designed to parse standard HTML5, not ESI. However, it would be useful to be able to implement some parts of ESI using HTMLRewriter. To that end, this compatibility flag causes HTMLRewriter to treat <esi:include> and <esi:comment> as void tags, so that they can be parsed and handled properly.