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 be bugs in the runtime API that existing Workers may inadvertently depend upon. 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.

Compatibility dates can only be set via your wrangler.toml file and by running npx wrangler deploy.

# (in wrangler.toml)
# Opt into backwards-incompatible changes through April 5, 2022.
compatibility_date = "2022-04-05"

When you start your project, you should always set compatibility_date to the current date. You should occasionally 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 npx wrangler deploy 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.

​​ Node.js compatibility flag

A growing subset of Node.js APIs are available directly as Runtime APIs, with no need to add polyfills to your own code. To enable these APIs in your Worker, add the nodejs_compat compatibility flag to your wrangler.toml:

wrangler.toml
compatibility_flags = [ "nodejs_compat" ]

As additional Node.js APIs are added, they will be made available under the nodejs_compat compatibility flag. Unlike most other compatibility flags, we do not expect the nodejs_compat to become active by default at a future date.

The Node.js AsyncLocalStorage API is a particularly useful feature for Workers. To enable only the AsyncLocalStorage API, use the nodejs_als compatibility flag.

wrangler.toml
compatibility_flags = [ "nodejs_als" ]

​​ Change history

Newest changes are listed first.

​​ Properly extract blob MIME type from content-type headers

​​ Use standard URL parsing in fetch()

​​ Returning empty Uint8Array on final BYOB read

In the original implementation of BYOB (“Bring your own buffer”) ReadableStreams, the read() method would return undefined when the stream was closed and there was no more data to read. This behavior was inconsistent with the standard ReadableStream behavior, which returns an empty Uint8Array when the stream is closed.

When the internal_stream_byob_return_view flag is used, the BYOB read() will implement standard behavior.

const resp = await fetch('https://example.org');
const reader = resp.body.getReader({ mode: 'byob' });
await result = await reader.read(new Uint8Array(10));

if (result.done) {
  // The result gives us an empty Uint8Array...
  console.log(result.value.byteLength); // 0

  // However, it is backed by the same underlying memory that was passed
  // into the read call.
  console.log(result.value.buffer.byteLength); // 10
}

​​ Brotli Content-Encoding support

​​ Durable Object stubs and Service Bindings support RPC

With this flag on, Durable Object stubs and Service Bindings support RPC. This means that these objects now appear as if they define every possible method name. Calling any method name sends an RPC to the remote Durable Object or Worker service.

For most applications, this change will have no impact unless you use it. However, it’s possible some existing code will be impacted if it explicitly checks for the existence of method names that were previously not defined on these types. For example, we’ve seen code in the wild which iterates over bindings and tries to auto-detect their types based on what methods they implement. Such code will now see service bindings as implementing every method, so may misinterpret service bindings as being some other type. In the cases we’ve seen, the impact was benign (nothing actually broke), but out of caution we are guarding this change behind a flag.

​​ Handling custom thenables

With the unwrap_custom_thenables flag set, various Workers APIs that accept promises will also correctly handle custom thenables (objects with a then method) that are not native promises, but are intended to be treated as such). For example, the waitUntil method of the ExecutionContext object will correctly handle custom thenables, allowing them to be used in place of native promises.

async fetch(req, env, ctx) {
  ctx.waitUntil({ then(res) {
    // Resolve the thenable after 1 second
    setTimeout(res, 1000);
  } });
  // ...
}

​​ Fetchers no longer have get/put/delete helper methods

Durable Object stubs and Service Bindings both implement a fetch() method which behaves similarly to the global fetch() method, but requests are instead sent to the destination represented by the object, rather than being routed based on the URL.

Historically, API objects that had such a fetch() method also had methods get(), put(), and delete(). These methods were thin wrappers around fetch() which would perform the corresponding HTTP method and automatically handle writing/reading the request/response bodies as needed.

These methods were a very early idea from many years ago, but were never actually documented, and therefore rarely (if ever) used. Enabling the fetcher_no_get_put_delete, or setting a compatibility date on or after 2024-03-26 disables these methods for your Worker.

This change paves a future path for you to be able to define your own custom methods using these names. Without this change, you would be unable to define your own get, put, and delete methods, since they would conflict with these built-in helper methods.

​​ Queues send messages in JSON format

​​ Suppress global importScripts()

​​ Node.js AsyncLocalStorage

​​ Python Workers

​​ WebCrypto preserve publicExponent field

​​ Vectorize query with metadata optionally returned

​​ WebSocket Compression

The Workers runtime did not support WebSocket compression when the initial WebSocket implementation was released. Historically, the runtime has stripped or ignored the Sec-WebSocket-Extensions header – but is now capable of fully complying with the WebSocket Compression RFC. Since many clients are likely sending Sec-WebSocket-Extensions: permessage-deflate to their Workers today (new WebSocket(url) automatically sets this in browsers), we have decided to maintain prior behavior if this flag is absent.

If the flag is present, the Workers runtime is capable of using WebSocket Compression on both inbound and outbound WebSocket connections.

Like browsers, calling new WebSocket(url) in a Worker will automatically set the Sec-WebSocket-Extensions: permessage-deflate header. If you are using the non-standard fetch() API to obtain a WebSocket, you can include the Sec-WebSocket-Extensions header with value permessage-deflate and include any of the compression parameters defined in RFC-7692 Open external link .

​​ Strict crypto error checking

Perform additional error checking in the Web Crypto API to conform with the specification and reject possibly unsafe key parameters:

• For RSA key generation, key sizes are required to be multiples of 128 bits as boringssl may otherwise truncate the key.
• The size of imported RSA keys must be at least 256 bits and at most 16384 bits, as with newly generated keys.
• The public exponent for imported RSA keys is restricted to the commonly used values [3, 17, 37, 65537].
• In conformance with the specification, an error will be thrown when trying to import a public ECDH key with non-empty usages.

​​ Strict compression error checking

​​ Bot Management data

This flag streamlines Workers requests by reducing unnecessary properties in the request.cf object.

With the flag enabled - either by default after 2023-08-01 or by setting the no_cf_botmanagement_default flag - Cloudflare will only include the Bot Management object in a Worker’s request.cf if the account has access to Bot Management.

With the flag disabled, Cloudflare will include a default Bot Management object, regardless of whether the account is entitled to Bot Management.

​​ URLSearchParams delete() and has() value argument

The WHATWG introduced additional optional arguments to the URLSearchParams object delete() Open external link and has() Open external link methods that allow for more precise control over the removal of query parameters. Because the arguments are optional and change the behavior of the methods when present there is a risk of breaking existing code. If your compatibility date is set to July 1, 2023 or after, this compatibility flag will be enabled by default.

For an example of how this change could break existing code, consider code that uses the Array forEach() method to iterate through a number of parameters to delete:

const usp = new URLSearchParams();
// ...
['abc', 'xyz'].forEach(usp.delete.bind(usp));

The forEach() automatically passes multiple parameters to the function that is passed in. Prior to the addition of the new standard parameters, these extra arguments would have been ignored.

Now, however, the additional arguments have meaning and change the behavior of the function. With this flag, the example above would need to be changed to:

const usp = new URLSearchParams();
// ...
['abc', 'xyz'].forEach((key) => usp.delete(key));

​​ Use a spec compliant URL implementation in redirects

​​ Dynamic Dispatch Exception Propagation

​​ Headers supports getSetCookie()

Adds the getSetCookie() Open external link method to the Headers Open external link API in Workers.

const response = await fetch("https://example.com");
let cookieValues = response.headers.getSetCookie();

​​ Node.js compatibility

​​ Streams Constructors

​​ Compliant TransformStream constructor

​​ CommonJS modules do not export a module namespace

​​ Do not throw from async functions

​​ New URL parser implementation

The original implementation of the URL Open external link API in Workers was not fully compliant with the WHATWG URL Standard Open external link , differing in several ways, including:

• The original implementation collapsed sequences of multiple slashes into a single slash:

  new URL("https://example.com/a//b").toString() === "https://example.com/a/b"

• The original implementation would throw "TypeError: Invalid URL string." if it encountered invalid percent-encoded escape sequences, like https://example.com/a%%b.

• The original implementation would percent-encode or percent-decode certain content differently:

  new URL("https://example.com/a%40b?c d%20e?f").toString() === "https://example.com/a@b?c+d+e%3Ff"

• The original implementation lacked more recently implemented URL features, like URL.canParse() Open external link .

Set the compatibility date of your Worker to a date after 2022-10-31 or enable the url_standard compatibility flag to opt-in the fully spec compliant URL API implementation.

Refer to the response_redirect_url_standard compatibility flag , which affects the URL implementation used in Response.redirect().

​​ R2 bucket list respects the include option

​​ Do not substitute null on TypeError

​​ Minimal subrequests

With the minimal_subrequests flag set, fetch() subrequests sent to endpoints on the Worker’s own zone (also called same-zone subrequests) have a reduced set of features applied to them. In general, these features should not have been initially applied to same-zone subrequests, and very few user-facing behavior changes are anticipated. Specifically, Workers might observe the following behavior changes with the new flag:

• Response bodies will not be opportunistically gzipped before being transmitted to the Workers runtime. If a Worker reads the response body, it will read it in plaintext, as has always been the case, so disabling this prevents unnecessary decompression. Meanwhile, if the Worker passes the response through to the client, Cloudflare’s HTTP proxy will opportunistically gzip the response body on that side of the Workers runtime instead. The behavior change observable by a Worker script should be that some Content-Encoding: gzip headers will no longer appear.
• Automatic Platform Optimization may previously have been applied on both the Worker’s initiating request and its subrequests in some circumstances. It will now only apply to the initiating request.
• Link prefetching will now only apply to the Worker’s response, not responses to the Worker’s subrequests.

​​ Global navigator

​​ Do not use the Custom Origin Trust Store for external subrequests

​​ Setters/getters on API object prototypes

Originally, properties on Workers API objects were defined as instance properties as opposed to prototype properties. This broke subclassing at the JavaScript layer, preventing a subclass from correctly overriding the superclass getters/setters. This flag controls the breaking change made to set those getters/setters on the prototype template instead.

This changes applies to:

• AbortSignal
• AbortController
• Blob
• Body
• DigestStream
• Event
• File
• Request
• ReadableStream
• ReadableStreamDefaultReader
• ReadableStreamBYOBReader
• Response
• TextDecoder
• TextEncoder
• TransformStream
• URL
• WebSocket
• WritableStream
• WritableStreamDefaultWriter

​​ Durable Object stub.fetch() requires a full URL

​​ fetch() improperly interprets 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.

​​ Streams BYOB reader detaches buffer

Originally, the Workers runtime did not detach the ArrayBuffers from user-provided TypedArrays when using the BYOB reader’s read() method, as required by the Streams spec, meaning it was possible to inadvertently reuse the same buffer for multiple read() calls. This change makes Workers conform to the spec.

User code should never try to reuse an ArrayBuffer that has been passed into a BYOB reader’s read() method. Instead, user code can reuse the ArrayBuffer backing the result of the read() promise, as in the example below.

// Consume and discard `readable` using a single 4KiB buffer.
let reader = readable.getReader({ mode: "byob" });
let arrayBufferView = new Uint8Array(4096);
while (true) {
  let result = await reader.read(arrayBufferView);
  if (result.done) break;
  // Optionally something with `result` here.
  // Re-use the same memory for the next `read()` by creating
  // a new Uint8Array backed by the result's ArrayBuffer.
  arrayBufferView = new Uint8Array(result.value.buffer);
}

The more recently added extension method readAtLeast() will always detach the ArrayBuffer and is unaffected by this feature flag setting.

​​ FormData parsing supports File

The FormData API Open external link 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>

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 and 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 Open external link , 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.