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

• asciinema asciicast v2 Format

• asciinema CLI Usage

• jq Manual

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