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Cap'n Proto Alternatives
Similar projects and alternatives to Cap'n Proto
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Apache Arrow
Apache Arrow is the universal columnar format and multi-language toolbox for fast data interchange and in-memory analytics
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simdjson
Parsing gigabytes of JSON per second : used by Facebook/Meta Velox, the Node.js runtime, ClickHouse, WatermelonDB, Apache Doris, Milvus, StarRocks
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Sandstorm
Sandstorm is a self-hostable web productivity suite. It's implemented as a security-hardened web app package manager. | Actively sponsored by our friends at TestMu AI
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ts-runtime-checks
A typescript transformer that automatically generates validation code from your types.
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NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a better Cap'n Proto alternative or higher similarity.
Cap'n Proto discussion
Cap'n Proto reviews and mentions
Posts with mentions or reviews of Cap'n Proto.
We have used some of these posts to build our list of alternatives
and similar projects. The last one was on 2026-03-27.
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Capability-Based Security for Redox: Namespace and CWD as Capabilities
Cloudflare Workers is a big on capabilities.
The recently released Dynamic Workers directly provides an API for capability-based sandboxing: https://developers.cloudflare.com/dynamic-workers/usage/bind...
But the platform has used caps internally all along. Cloudflare makes heavy use of Cap'n Proto (https://capnproto.org/), a capability-based RPC protocol, and recently released Cap'n Web (https://capnweb.dev/), a JavaScript-oriented version of the same idea. The "Cap'n" in both is short for "Capabilities and". (Dynamic Workers sandboxing is based around Cap'n Web capabilities.)
Capabilities really shine when used for sandboxing, but here's a blog post I wrote that tries to explain the benefits beyond sandboxing: https://blog.cloudflare.com/workers-environment-live-object-...
(I am the lead developer of Cloudflare Workers, and the creator of Cap'n Proto and Cap'n Web.)
- Show HN: Skir – A schema language I built after 15 years of Protobuf friction
- Cap'n Proto
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Replacing Protobuf with Rust to go 5 times faster
"5 times faster" reminds me of Cap'n Proto's claim: in benchmarks, Cap’n Proto is INFINITY TIMES faster than Protocol Buffers: https://capnproto.org/
- Thoughts on the Word Spec in Rust
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Show HN: Cap'n-rs – Rust implementation of Cloudflare's Cap'n Web protocol
I think you meant to link: https://github.com/cloudflare/capnweb/blob/main/protocol.md
I am so pleased that people find it boring! It was quite a puzzle to whittle it down to that point from the original monstrosity: https://github.com/capnproto/capnproto/blob/v2/c%2B%2B/src/c...
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Cap'n Web: a new RPC system for browsers and web servers
Here is the Cap'n Proto RPC protocol:
https://github.com/capnproto/capnproto/blob/v2/c%2B%2B/src/c...
That's just the RPC state machine -- the serialization is specified elsewhere, and the state machine is actually schema-agnostic. (Schemas are applied at the edges, when messages are actually received from the app or delivered to it.)
This is the Cap'n Web protocol, including serialization details:
https://github.com/cloudflare/capnweb/blob/main/protocol.md
It's actually a lot simpler.
Among other things, I merged the concepts of call-return and promise-resolve. (Which, admittedly, CapTP was doing it that way before I even designed Cap'n Proto. It was a complete mistake on my part to turn them into two separate concepts in Cap'n Proto, but it seemed to make sense at the time.)
What I'd like to do is go back and revise the Cap'n Proto protocol to use a similar design under the hood. This would make no visible difference to applications (they'd still use schemas), but the state machine would be much simpler, and easier to port to more languages.
- Protobuffers Are Wrong
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Parsing Protobuf Like Never Before
Offtopic, but is anyone using CapnProto, the ProtoBuf former maintainer's (kentonv around here) subsequent project?
https://capnproto.org
If so, how does it compare in practice?
(what does Cloudflare Workers use?)
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Rust in the Linux kernel: part 2
My interest in Rust comes from getting frustrated with C's type system. Rust has such a nice type system and I really enjoy the ownership semantics around concurrency. I think that C++ written "correctly" looks a lot like Rust and libkj [1] encourages this, but it is not enforced by the language.
[1] https://github.com/capnproto/capnproto/blob/v2/kjdoc/tour.md
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A note from our sponsor - SaaSHub
www.saashub.com | 6 Jun 2026
Stats
Basic Cap'n Proto repo stats
84
13,053
9.4
about 20 hours ago
capnproto/capnproto is an open source project licensed under GNU General Public License v3.0 or later which is an OSI approved license.
The primary programming language of Cap'n Proto is C++.
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