I built this for testing payment integrations. ISO 8583 is the binary protocol behind most card transactions (ATMs, POS terminals, payment switches) - notoriously painful to debug.

Features: - Parse/build/validate messages (1987, 1993, 2003 versions) - VISA, Mastercard, AMEX, Discover, JCB, UnionPay - 180k+ msg/sec with optional Cython - CLI + Python SDK + Jupyter notebooks

The LLM integration lets you explain raw messages in plain English or generate messages from natural language ("$50 refund to Mastercard at ACME Store"). Works with OpenAI, Claude, Gemini, or Ollama (fully offline).

Docs: https://iso8583.subhadipmitra.com

PyPI: pip install iso8583sim

Happy to answer questions about ISO 8583 or the implementation.