# Cap'n Proto

20 Hacker News commenters recommended Cap'n Proto and 5 criticized it, November 2025 to November 2026. It was recommended in 7 of 13 months. It meets the Hacker Approved badge rule.

**Verdict:** Fast binary serialization and RPC system that people pick as a simpler, zero-copy alternative to Protobuf. Caveat: output can be large and the C++ code is less ergonomic.

## Why people recommend it

- Fast zero-copy serialization (4 comments): "Capnp is great, but the fact that protobuf can't do zero copy should be more an academic issue than practical." ([source](https://news.ycombinator.com/item?id=46120418)) "That's something flatbuffers/capnproto do well that isn't just pure speed." ([source](https://news.ycombinator.com/item?id=46332252)) "zero-copy formats like FlatBuffers or Cap'n Proto can be much faster" ([source](https://news.ycombinator.com/item?id=46119813))
- Simpler format than Protobuf (3 comments): "the implementation of Cap'n Proto is quite a bit simpler than Protobuf due to the serialization format just being simpler" ([source](https://news.ycombinator.com/item?id=47322839)) "I like the ideas and binary encoding behind Capn Proto more than all the alternatives" ([source](https://news.ycombinator.com/item?id=46741356)) "People who want protobuf but better often settle for something like cap’n proto" ([source](https://news.ycombinator.com/item?id=48884120))
- Trusted, heavily used at Cloudflare (2 comments): "It is heavily used at Cloudflare, though, and is the basis for a lot of cool stuff we're doing" ([source](https://news.ycombinator.com/item?id=46779492)) "has been my goto since forever. Made by the protobuf inventor" ([source](https://news.ycombinator.com/item?id=47299791))

## What people warn about

- Output often produces large sizes (2 comments): "Cap’n Proto and FlatBuffers are fast but often produce rather large output" ([source](https://news.ycombinator.com/item?id=47214235)) "Capnproto doesn't support transform on serialize - the optional fields still take up disk space" ([source](https://news.ycombinator.com/item?id=47276391))
- C++ code less ergonomic (2 comments): "your codebase will be filled with fairly horrific CapnProto C++ code" ([source](https://news.ycombinator.com/item?id=46732942)) "In my experience capn proto is much less ergonomic." ([source](https://news.ycombinator.com/item?id=46731060))

## Alternatives hackers recommend

- [Protocol Buffers (protobuf)](https://hackerapproved.com/p/protocol-buffers.md): 77 recommend
- [Serde](https://hackerapproved.com/p/serde.md): 22 recommend
- [Protobuf](https://hackerapproved.com/p/protobuf.md): 19 recommend
- [FlatBuffers](https://hackerapproved.com/p/flatbuffers.md): 16 recommend
- [Lite³](https://hackerapproved.com/p/lite.md): 14 recommend

## Cite as

Hacker Approved, "Cap'n Proto", https://hackerapproved.com/p/cap-n-proto/ (counts from 1,795,640 Hacker News comments, November 2025 to November 2026).