Open sourcing h3i: a command line tool and library for low-level HTTP/3 testing and debugging

Cloudflare introduces h3i, an open-source command-line tool and Rust library for low-level HTTP/3 over QUIC testing that enables engineers to craft and replay specific frame sequences—including malformed or standards-violating cases—to validate robustness and standards compliance of servers and proxies. The post covers interactive testing, qlog-based record/replay, and a Rust example that sends a Content-Length mismatch to verify defensive behavior, positioning h3i as a framework to consolidate and expand protocol conformance and interoperability testing while encouraging responsible disclosure.