asciinema-cast-format

Reference documentation for the asciinema v3 .cast file format (asciicast v2 specification).

Platform: All platforms (documentation only)

When to Use This Skill

Use this skill when:

Parsing or inspecting .cast file structure
Understanding NDJSON header and event formats
Building tools that read or write .cast files
Debugging recording issues or format errors
Learning the asciicast v2 specification

Format Overview

Asciinema v3 uses NDJSON (Newline Delimited JSON) format:

Line 1: Header object with recording metadata
Lines 2+: Event arrays with timestamp, type, and data

Header Specification

The first line is a JSON object with these fields:

Field	Type	Required	Description
`version`	int	Yes	Format version (always 2 for v3 recordings)
`width`	int	Yes	Terminal width in columns
`height`	int	Yes	Terminal height in rows
`timestamp`	int	No	Unix timestamp of recording start
`duration`	float	No	Total duration in seconds
`title`	string	No	Recording title
`env`	object	No	Environment variables (SHELL, TERM)
`theme`	object	No	Terminal color theme

Example Header

{
  "version": 2,
  "width": 120,
  "height": 40,
  "timestamp": 1703462400,
  "duration": 3600.5,
  "title": "Claude Code Session",
  "env": { "SHELL": "/bin/zsh", "TERM": "xterm-256color" }
}

Event Codes

Each event after the header is a 3-element array:

[timestamp, event_type, data]

Code	Name	Description	Data Format
`o`	Output	Terminal output (stdout)	String
`i`	Input	Terminal input (stdin)	String
`m`	Marker	Named marker for navigation	String (marker name)
`r`	Resize	Terminal resize event	`"WIDTHxHEIGHT"`
`x`	Exit	Extension for custom data	Varies

Event Examples

[0.5, "o", "$ ls -la\r\n"]
[1.2, "o", "total 48\r\n"]
[1.3, "o", "drwxr-xr-x  12 user  staff  384 Dec 24 10:00 .\r\n"]
[5.0, "m", "file-listing-complete"]
[10.5, "r", "80x24"]

Timestamp Behavior

Timestamps are relative to recording start (first event is 0.0)
Measured in seconds with millisecond precision
Used for playback timing and navigation

Calculating Absolute Time

/usr/bin/env bash << 'CALC_TIME_EOF'
HEADER_TIMESTAMP=$(head -1 recording.cast | jq -r '.timestamp')
EVENT_OFFSET=1234.5  # From event array

ABSOLUTE=$(echo "$HEADER_TIMESTAMP + $EVENT_OFFSET" | bc)
date -r "$ABSOLUTE"  # macOS
# date -d "@$ABSOLUTE"  # Linux
CALC_TIME_EOF

Parsing Examples

Extract Header with jq

/usr/bin/env bash << 'HEADER_EOF'
head -1 recording.cast | jq '.'
HEADER_EOF

Get Recording Duration

/usr/bin/env bash << 'DURATION_EOF'
head -1 recording.cast | jq -r '.duration // "unknown"'
DURATION_EOF

Count Events by Type

/usr/bin/env bash << 'COUNT_EOF'
tail -n +2 recording.cast | jq -r '.[1]' | sort | uniq -c
COUNT_EOF

Extract All Output Events

/usr/bin/env bash << 'OUTPUT_EOF'
tail -n +2 recording.cast | jq -r 'select(.[1] == "o") | .[2]'
OUTPUT_EOF

Find Markers

/usr/bin/env bash << 'MARKERS_EOF'
tail -n +2 recording.cast | jq -r 'select(.[1] == "m") | "\(.[0])s: \(.[2])"'
MARKERS_EOF

Get Event at Specific Time

/usr/bin/env bash << 'TIME_EOF'
TARGET_TIME=60  # seconds
tail -n +2 recording.cast | jq -r "select(.[0] >= $TARGET_TIME and .[0] < $((TARGET_TIME + 1))) | .[2]"
TIME_EOF

Large File Considerations

For recordings >100MB:

File Size	Line Count	Approach
<100MB	<1M	jq streaming works fine
100-500MB	1-5M	Use `--stream` flag, consider ripgrep
500MB+	5M+	Convert to .txt first with asciinema

Memory-Efficient Streaming

/usr/bin/env bash << 'STREAM_EOF'
# Stream process large files
jq --stream -n 'fromstream(1|truncate_stream(inputs))' recording.cast | head -1000
STREAM_EOF

Use asciinema convert

For very large files, convert to plain text first:

asciinema convert -f txt recording.cast recording.txt

This strips ANSI codes and produces clean text (typically 950:1 compression).

TodoWrite Task Template

1. [Reference] Identify .cast file to analyze
2. [Header] Extract and display header metadata
3. [Events] Count events by type (o, i, m, r)
4. [Analysis] Extract relevant event data based on user need
5. [Navigation] Find markers or specific timestamps if needed

Post-Change Checklist

After modifying this skill:

Event code table matches asciinema v2 specification
Parsing examples use heredoc wrapper for bash compatibility
Large file guidance reflects actual performance characteristics
All jq commands tested with sample .cast files

Reference Documentation

Troubleshooting

Issue	Cause	Solution
jq parse error	Invalid NDJSON in .cast file	Check each line is valid JSON with `jq -c .`
Header missing duration	Recording in progress	Duration added when recording ends
Unknown event type	Custom extension event	Check for `x` type events (extension data)
Timestamp out of order	Corrupted file	Events should be monotonically increasing
Large file jq timeout	File too big for in-memory	Use `--stream` flag or convert to .txt first
Markers not found	No markers in recording	Markers are optional; not all recordings have them
Wrong version number	Older cast format	This skill covers v2 format (asciinema v3+)
Empty output from tail	File has only header	Recording may be empty or single-line