Sec-WebSocket-Extensions

The HTTP Sec-WebSocket-Extensions request and response header negotiates WebSocket protocol extensions between the client and server during the protocol upgrade handshake.

Baseline: Widely available

Supported across all major browsers. webstatus.dev

Table of Contents

Usage

The Sec-WebSocket-Extensions header operates in both directions during the WebSocket upgrade. The client sends this header listing the extensions the client supports and is willing to use. The server responds with the subset of extensions agreed upon for the connection.

Extensions modify how WebSocket frames are processed. The most widely deployed extension is permessage-deflate, which applies Compression to message payloads. Other extensions control frame fragmentation, multiplexing, or custom framing semantics. Each extension has parameters controlling behavior.

The client proposes extensions with parameters. The server selects from the offered list, responding with the same extension names and agreed parameters. Only extensions confirmed in the server response become active. If the server omits Sec-WebSocket-Extensions entirely, the connection proceeds without any extensions.

Extensions operate independently unless the specification defines interactions. Multiple extensions process frames in the order listed in the server response header.

Directives

permessage-deflate

The permessage-deflate extension applies DEFLATE Compression to each WebSocket message. Parameters control compression windows and context takeover. This reduces bandwidth for text-heavy protocols like chat, notifications, and real-time updates.

Other extensions

Extension names are registered with IANA. The x-webkit-deflate-frame extension is an obsolete WebKit-specific predecessor to permessage-deflate and is no longer used in modern browsers. Custom application extensions define their own parameter syntax and semantics.

Example

A browser requests WebSocket Secure connection support for compression. The client proposes permessage-deflate with parameters for window size and context handling.

Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
Sec-WebSocket-Version: 13
Sec-WebSocket-Extensions: permessage-deflate; client_max_window_bits

The server confirms activation of compression with agreed parameters.

Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=
Sec-WebSocket-Extensions: permessage-deflate

Takeaway

The Sec-WebSocket-Extensions header enables negotiation of WebSocket protocol extensions, with the client proposing capabilities and the server confirming which extensions will be active for the connection.

See also

Last updated: March 6, 2026