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Show HN: Cap'n-rs – Rust implementation of Cloudflare's Cap'n Web protocol

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Show HN: Cap'n-rs – Rust implementation of Cloudflare's Cap'n Web protocol

#1

Post by brian_meek »

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

Show HN: Cap'n-rs – Rust implementation of Cloudflare's Cap'n Web protocol
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Re: Show HN: Cap'n-rs – Rust implementation of Cloudflare's Cap'n Web protocol

#3

Post by WD-42 »

IshKebab wrote: post #2

I would recommend not having an AI write your README. Even if you didn't use AI for coding at all and the code works perfectly, it really doesn't inspire confidence.

Yup the second I saw the wall of text paragraph with all the emojis I had a sinking feeling the entire thing could be a vibe coded mess. Too bad because the project seems cool.

Re: Show HN: Cap'n-rs – Rust implementation of Cloudflare's Cap'n Web protocol

#4

Post by sandblast »

IshKebab wrote: post #2

I would recommend not having an AI write your README. Even if you didn't use AI for coding at all and the code works perfectly, it really doesn't inspire confidence.

Well, most of commits have „brianathere and claude” as authors. Additionally, the style of comments in some of the files bears a strong resemblance to those typically generated by LLMs.

Moreover, this whole thing has 7 stars on GitHub as of writing, and yet the bottom of README boldly claims „ready for production use”. For me, that overly confident approach discourages maybe even more.

Re: Show HN: Cap'n-rs – Rust implementation of Cloudflare's Cap'n Web protocol

#5

Post by typpilol »

WD-42 wrote: post #3
IshKebab wrote: post #2

I would recommend not having an AI write your README. Even if you didn't use AI for coding at all and the code works perfectly, it really doesn't inspire confidence.

Yup the second I saw the wall of text paragraph with all the emojis I had a sinking feeling the entire thing could be a vibe coded mess. Too bad because the project seems cool.

Makes me sad because I used to put a lot of effort into my readmes. Not insane amount of emojis but some and actually useful badges but not overkill. But in the end they always had nicely structured layouts etc..

I feel like I have to make mine sloppier now to pass the AI smell test

Re: Show HN: Cap'n-rs – Rust implementation of Cloudflare's Cap'n Web protocol

#6

Post by sandblast »

typpilol wrote: post #5
WD-42 wrote: post #3

Earlier quoted context omitted.

Yup the second I saw the wall of text paragraph with all the emojis I had a sinking feeling the entire thing could be a vibe coded mess. Too bad because the project seems cool.

Makes me sad because I used to put a lot of effort into my readmes. Not insane amount of emojis but some and actually useful badges but not overkill. But in the end they always had nicely structured layouts etc.. I feel like I have to make mine sloppier now to pass the AI smell test

Maybe it was also your style that the LLMs learned from (and then pushed more towards the extreme). Oh the irony.

Re: Show HN: Cap'n-rs – Rust implementation of Cloudflare's Cap'n Web protocol

#7

Post by adrian17 »

IshKebab wrote: post #2

I would recommend not having an AI write your README. Even if you didn't use AI for coding at all and the code works perfectly, it really doesn't inspire confidence.

The commit names look very weird too, feel more like marketing speak:

> BREAKTHROUGH: Fix critical ID mismatch in Cap'n Web protocol

> TIER 3 ULTIMATE: Extreme Stress Tests & Advanced Capability Composition

> Achieve Cap'n Web protocol mastery with perfect capability composition (note: this exact commit name is repeated another time)

The commit descriptions are even worse. If they’re so blatantly not written for anyone to read, why bother generating them at all?

Re: Show HN: Cap'n-rs – Rust implementation of Cloudflare's Cap'n Web protocol

#8

Post by h4ch1 »

"I built" more like claude-code built.

Also I don't know what the rush is to get a shiny new Rust implementation out asap that you need to vibe code the entire thing.

The Github stars couldn't be that serious.

Also the disclaimer at the end "Built with Ready for production use is a strong strong claim when the entire thing is vibe-coded.

Re: Show HN: Cap'n-rs – Rust implementation of Cloudflare's Cap'n Web protocol

#9

Post by mort96 »

Okay the name is very unfortunate. "Cap'n-rs" sounds like it's gonna be a Rust implementation of Cap'n Proto, the much older and more widely used and more widely known serialization format.

But I guess this is just to be expected from AI slop.

Re: Show HN: Cap'n-rs – Rust implementation of Cloudflare's Cap'n Web protocol

#10

Post by monadoid »

mort96 wrote: post #9

Okay the name is very unfortunate. "Cap'n-rs" sounds like it's gonna be a Rust implementation of Cap'n Proto, the much older and more widely used and more widely known serialization format. But I guess this is just to be expected from AI slop.

This is someone's rust port of the new "cap'n web" protocol Kenton just released, which he's calling a spiritual sibling to cap'n proto (which he also built). This might be AI slop but this info is all in the OP, come on man!
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