CAST File

What are CAST files and how to open them

Are you having problems opening a CAST file or are you simply curious about its contents? We're here to explain the properties of these files and provide you with software that can open or handle your CAST files.

What is a CAST file?

A .CAST file is an Asciicast file.

Asciicast is a format for recording and replaying terminal sessions. It is produced by the asciinema command-line tool when a user records their terminal (for example with asciinema rec). The file stores what was displayed in the terminal, how and when it was displayed (timing information), as well as certain metadata such as terminal size, environment variables (like TERM and SHELL), title, and optionally colors and command name. This makes it ideal for sharing demos, tutorials, or saving sessions for replay or analysis.

A program or script reads an Asciicast file to drive a player (such as the asciinema player) that rebuilds the terminal session, showing the output, handling any terminal resizes, and playing back in accurate timing. The format supports events like terminal output, input (if enabled), markers, and resize events. It is versioned; one of the currently used versions is called "asciicast v2," which is the established version described here. Future versions (e.g. v3) are not backward compatible, especially in how metadata is structured or how event times are represented.

The file begins with a header line: a JSON object containing required fields such as version (must be 2), width (columns), and height (rows) of the terminal when recording started. Optional metadata include a Unix timestamp marking the start, a title, command used, environment‐variables map, and color theme. Each subsequent line is a separate event in a newline‐delimited JSON array of three elements: the time since start (float), the event code (e.g. "o" for output, "i" for input, "r" for resize, "m" for marker), and data (string or other form depending on event). Suggested filename extension is .cast and the suggested media type is application/x-asciicast.

For identification, the file starts with the ASCII character "{" at byte offset 0 (as the JSON header object). It contains fields like VERSION (version), HEIGHT, WIDTH, SHELL, TERM, and TIMESTAMP in that header. The event stream that follows uses event codes "o", "i", "m", "r".

How to open CAST files

Important: Different programs may use files with the CAST file extension for different purposes, so unless you are sure which format your CAST file is, you may need to try a few different programs.

We have not yet verified any programs for Windows that work with this specific file format. If you know one, please use the 'Suggest a program' link below. Thanks!

Last updated: August 29, 2026

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