Last week Cloudflare released Cap'n Web [1], a schema-free capability-based RPC protocol. I built capn-rs this week - a Rust implementation with full wire protocol compatibility. Links:

GitHub: https://github.com/currentspace/capn-rs Crates: https://crates.io/crates/capnweb-server API docs: https://docs.rs/capnweb-server

What's working:

Wire compatibility verified via integration tests against TypeScript reference Multi-transport: HTTP batch, WebSocket, WebTransport (HTTP/3) Complete IL (intermediate language) expression evaluator Promise pipelining with dependency resolution Comprehensive test coverage

The interesting design challenge was mapping Cap'n Web's record-replay .map() semantics to Rust's type system while maintaining ergonomic APIs. Cap'n Web records operations on placeholder values to build execution plans - in Rust this became a clean builder pattern with type-level guarantees. Built this as an experiment with Claude Code for porting complex protocols. The AI handled mechanical translation well, but architectural decisions (especially around async/await patterns and lifetime management) required human judgment. This is early days - I'd especially appreciate feedback on API ergonomics and any edge cases I might have missed. Also happy to discuss the protocol design or the AI-assisted development experience.

[1] https://news.ycombinator.com/item?id=45332883

1.	Show HN: HTTP/3 and raw QUIC client/server APIs for Node.js(github.com) 11 points by brian_meek 6 hours ago \| 0 comments
2.	Show HN: Cap'n-rs – Rust implementation of Cloudflare's Cap'n Web protocol(github.com) 52 points by brian_meek 251 days ago \| 35 comments

user:	brian_meek
created:	April 22, 2020
karma:	30