https://github.com/open-frames/standard/blob/v0.0.1/README.md

Open Frames Spec [Draft v0.0.1]

Since the launch of Farcaster Frames, we’ve seen a number of protocols work to add Frames support independently. The original Frames spec was designed for Farcaster and wasn’t set up to handle interactions from different types of applications with unique capabilities. Open Frames is a lightweight extension to the Frames spec to help coordinate the many new applications and protocols adopting Frames.

Open Frames is an initiative with the following goals:

1. Allow Frames developers to support interactions from different types of clients
2. Don’t screw up the best thing about Frames: how easy it is to build on

We expect the Open Frames specification to evolve, both through improvements to the Farcaster Specification and through the needs of Open Frames applications with different capabilities.

Actors

Client Application

Client applications are responsible for rendering Frames for end-users, handling interactions with buttons on Frames, and creating the POST payload to be sent to the Frames Server. Client applications must support one or more clientProtocols.

Frames Server

A Frames Server is responsible for generating the initial Frames HTML, handling POST requests from Client Applications, and storing any state that might be required for the functioning of the Frame.

Frames Metadata

To turn your web pages into Frames, you need to add basic metadata to your page.

Required Properties

Property	Description
of:version	The version label of the Open Frames spec. Currently the only supported version is vNext
of:accepts:$protocol_identifier	The minimum client protocol version accepted for the given protocol identifier. For example vNext , or 1.5. At least one $protocol_identifier must be specified.
of:image	An image which should have an aspect ratio of 1.91:1 or 1:1.
og:image	An image which should have an aspect ratio of 1.91:1. Fallback for clients that do not support frames.

Optional properties

Property	Description
of:button:$idx	256 byte string containing the user-visible label for button at index $idx. Buttons are 1-indexed. Maximum 4 buttons per Frame. $idx values must be rendered in an unbroken sequence.
of:button:$idx:action	Valid options are post, post_redirect, mint, and link. Default: post
of:button:$idx:target	The target of the action. For post , post_redirect, and link action types the target is expected to be a URL starting with http:// or https://. For the mint action type the target must be a CAIP-10 URL
of:post_url	The URL where the POST payload will be sent. Must be valid and start with http:// or https:// . Maximum 256 bytes.
of:input:text	If this property is present, a text field should be added to the Frame. The contents of this field will be shown to the user as a label on the text field. Maximum 32 bytes.
of:image:aspect_ratio	The aspect ratio of the image specified in the of:image field. Allowed values are 1.91:1 and 1:1. Default: 1.91:1
of:image:alt	Alt text associated with the image for accessibility

Farcaster Compatibility

The following properties are directly compatible with the following Farcaster properties as of Feb 21, 2024

Open Frames Property	Farcaster Property
of:image	fc:frame:image
og:image	og:image
of:button:$idx	fc:frame:button:index
of:button:$idx:action	fc:frame:button:$idx:action
of:button:$idx:target	fc:frame:button:$idx:target
of:input:text	fc:frame:input:text
of:image:aspect_ratio	fc:frame:image:aspect_ratio
of:accepts:farcaster	fc:frame

Frames Servers that wish to handle both Farcaster and Open Frames interactions are recommended to include a complete set of both of: and fc:frame meta tags. If a complete set of Open Frames tags are not present in the page, Client Applications may choose to fall back to equivalent fc:frame tags if the of:accepts:$protocol_identifier tag is present.

The Client Protocol Identifier

clientProtocol is a string identifier for a client’s capabilities that can be used to negotiate compatibility between the Client Application and the Frame Server. Each Frame Server must advertise which clientProtocols it is capable of receiving button clicks from through the of:accepts:$protocol_identifier meta tag included in each HTML response.

<meta 
	property="of:accepts:xmtp"
	content="2024-02-01"
/>
<meta 
	property="of:accepts:lens"
	content="1.1"
/>

If a Frame Server declares that it is compatible with a client protocol, Client Applications capable of sending POST responses using that protocol should feel confident that the Frame will work as expected.

Client protocol strings must conform to the following format: $PROTOCOL_IDENTIFIER@$PROTOCOL_VERSION. Protocol versions are a string, and it is up to each protocol to decide how it should specify versions. For example, a Farcaster client today might define it’s clientProtocol as farcaster@vNext.

The version specified in the of:accepts:* tag should be the earliest version of the client protocol with which the Frames Server is compatible.

Client applications must check the accepts tag for their protocol and ensure that they are capable of sending POST payloads in a format that the Frame Server can understand. If the
of:accepts:$client_protocol field is absent, client applications may choose to assume that the Frame Server only accepts requests using the Farcaster request format using the version specified in the fc:frame meta tag. If the client application does not support any of the listed client protocols, the client can choose to skip rendering the Frame entirely or show the Frame with the buttons disabled.

When sending a POST to the Frame Server, client applications must include the clientProtocol used to generate the payload, which will allow the Frame server to know what data is available and how to verify the trustedData.messageBytes.

POST Payloads

When a user clicks a button on a Frame, the Frame developer receives a POST request with a payload containing both untrustedData and trustedData. The trustedData.messageBytes
can be verified by Frame developers so that they can provably know that the contents of that payload were signed by a given user. We propose a minimum schema for all POST payloads that can be implemented by any Open Frames clientProtocol.

type FramesPost = {
  clientProtocol: string; // The client protcol used by the client to generate the payload
  untrustedData: {
    url: string; // The URL of the Frame that was clicked. May be different from the URL that the data was posted to.
    unixTimestamp: number; // Unix timestamp in milliseconds
    buttonIndex: number; // The button that was clicked
    inputText?: string; // Input text for the Frame's text input, if present. Undefined if no text input field is present
  };
  trustedData: {
    messageBytes: string;
  };
};

Different client protocols may choose to include additional fields in the untrustedData and trustedData portions of the POST payload. Frame Servers may use the clientProtocol as a hint for what additional fields are available. All client protocols must include at least the common fields defined above.

While the payload is similar to the Farcaster Frames Spec, it differs in two important ways. The first is the addition of the clientProtocol field. The second is that in place of a timestamp, which in Farcaster is the number of seconds since the Farcaster epoch, Open Frames uses the unixTimestamp field which is the number of seconds since the unix epoch.