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:

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 asciinema asciicast v2 Format asciinema CLI Usage jq Manual