Calculate Claude Code context length from transcript JSONL using bash/jq

context-length.sh

#\!/usr/bin/env nix
#\! nix shell nixpkgs#bash nixpkgs#jq nixpkgs#coreutils --command bash
set -euo pipefail

# Get context length from Claude Code transcript JSONL file
transcript_file="${1:-}"

if [[ -z "$transcript_file" || \! -f "$transcript_file" ]]; then
    echo "Usage: $0 <transcript.jsonl>" >&2
    exit 1
fi

# Find the most recent main chain entry (isSidechain \!= true) and extract context length
jq -r '
    select(.message.usage and (.isSidechain \!= true)) | 
    {
        timestamp: .timestamp,
        context_length: (
            (.message.usage.input_tokens // 0) + 
            (.message.usage.cache_read_input_tokens // 0) + 
            (.message.usage.cache_creation_input_tokens // 0)
        )
    }
' "$transcript_file" | 
jq -s 'sort_by(.timestamp) | last | .context_length'