Dev tools · September 30, 2025
Cap'n-rs – Rust implementation of Cloudflare's Cap'n Web protocol
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…
In plain words
Cap'n-rs is a Rust implementation of Cloudflare's Cap'n Web, a schema-free capability-based RPC protocol. It provides full wire protocol compatibility with support for multiple transports including HTTP batch, WebSocket, and WebTransport, along with promise pipelining and an intermediate language expression evaluator. The tool is designed for Rust developers building distributed systems that need schema-free RPC communication with verified compatibility against the TypeScript reference implementation.
written from the facts on this page · September 2026
From the sources
In the maker’s words, at launch
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
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