Claude Protocol Debugger

Overview

The Claude CLI communicates via a JSON-RPC-like protocol over stdio. This skill covers three debugging techniques:

1. Live event inspection — debug mismatches between stream-json events and JSONL history
2. Daemon log analysis — inspect control_request/response payloads as logged by the daemon
3. Binary reverse-engineering — read the CLI's minified JS source to verify protocol behavior

When to Use

• Feature works after daemon restart (history path) but not during live sessions
• Fields are missing or named differently in live mode vs JSONL
• Permission responses aren't being persisted or honored
• Need to verify what the CLI actually does with a protocol field
• Investigating undocumented CLI behavior or protocol edge cases

Technique 1: Live Event Inspection

For snake_case vs camelCase field mappings, debug logging patterns, and daemon log analysis techniques, see protocol-findings.md. Read it before starting any event debugging.

Technique 2: Daemon Log Analysis

See protocol-findings.md for grep patterns and Python extraction scripts.

Technique 3: Binary Reverse-Engineering

The Claude CLI binary is a bundled Node.js SEA (Single Executable Application) with readable minified JS embedded in it. Use this when docs are incomplete or you need to verify what the CLI actually does with a protocol field.

Before re-reversing the binary, read cli-binary-internals.md in this directory. It contains previously reverse-engineered findings (v2.1.85) including the stdio message loop, interrupt handling, REPL bridge, abort controller chain, and background agent wait loop. Read it first to avoid duplicate work.

Locate the binary

# Find the CLI version and binary
claude --version  # e.g. 2.1.85
ls ~/.local/share/claude/versions/
# Binary is at: ~/.local/share/claude/versions/<version> (the version IS the binary, not a directory)

Extract and search the JS source

Prefer Python re.finditer over strings | grep — the binary contains enormous minified JS lines, and strings | grep returns megabytes of irrelevant context. Use offset-based extraction:

import re
CLI_BIN = "~/.local/share/claude/versions/2.1.85"  # adjust version

with open(CLI_BIN, 'rb') as f:
    data = f.read()

# Search for exact byte pattern
pattern = b'case"interrupt"'
for m in re.finditer(pattern, data):
    start = max(0, m.start() - 300)
    end = min(len(data), m.end() + 500)
    chunk = data[start:end].decode('ascii', errors='replace')
    print(f'=== offset {m.start()} ===')
    print(chunk)

For broader searches, strings still works but pipe through wc -l first to gauge volume:

CLI_BIN=~/.local/share/claude/versions/$(claude --version | head -1)
strings "$CLI_BIN" | grep -c "your_pattern"  # check count first

Previous findings

All previously reverse-engineered details (string literals, minified function tables, call chains, two code paths for control_request, permission persistence, interrupt handling, background agent wait loop) are in cli-binary-internals.md. Read it before starting any new reverse-engineering session.

Maintaining Reference Files

This skill has two reference files that accumulate findings across debugging sessions. Update them when you discover something surprising — behavior that contradicts expectations, undocumented protocol quirks, or anything that would save time if known upfront next time.

What to record: Things we didn't expect — behavior that surprised us, fields that don't match docs, race conditions, blocking issues. Don't record things that are obvious from reading the code.

What NOT to record: Routine findings that match documentation, expected behavior, or things derivable from reading source files.