asciinema-cast-format

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Reference for asciinema v3 NDJSON format. TRIGGERS - cast format, asciicast spec, event codes, cast header, cast structure, parse cast file. Use when understanding or parsing .cast files.

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[Developer Tools]

When & Why to Use This Skill

This Claude skill serves as a comprehensive technical reference for the asciinema v3 NDJSON (.cast) format. It enables developers and DevOps engineers to understand, parse, and analyze terminal recordings by providing detailed specifications for headers, event codes, and timestamp behaviors, along with optimized bash and jq examples for data extraction.

Use Cases

  • Parsing .cast files to extract specific terminal output or command sequences for automated documentation and tutorials.
  • Analyzing recording metadata such as duration, terminal dimensions, and environment variables to verify session consistency.
  • Using jq and bash scripts to filter and count specific event types (input, output, markers) within terminal recordings.
  • Managing and processing large terminal recording files (>100MB) using memory-efficient streaming techniques.
  • Identifying and navigating to specific session markers or timestamps for targeted debugging of CLI tool behaviors.
nameasciinema-cast-format
descriptionReference for asciinema v3 NDJSON format. TRIGGERS - cast format, asciicast spec, event codes, cast header, cast structure, parse cast file. Use when understanding or parsing .cast files.
allowed-toolsRead, Bash

asciinema-cast-format

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

Platform: All platforms (documentation only)

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:

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

Reference Documentation