Cap'n Proto
Hacker ApprovedRanked 3 of 9 serialization formats by recommenders
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.
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.
25 people gave an opinion
- recommend it
- 20
- criticize it
- 5
- approval share
- 80%
Recommended in 7 of 13 months
- November 2025: not recommended
- December 2025: recommended
- January 2026: recommended
- February 2026: not recommended
- March 2026: recommended
- April 2026: recommended
- May 2026: not recommended
- June 2026: not recommended
- July 2026: recommended
- August 2026: recommended
- September 2026: recommended
- October 2026: not recommended
- November 2026: not recommended (month in progress)
Why it carries the badge
- 20 people recommend itNeeds 10 or more, met.
- 4 recommenders for every criticNeeds 2 to 1 or better, met.
- Recommended in 7 of 13 monthsNeeds 4 or more, met.
The same three rules apply to every product, and the badge can’t be bought. How the badge works
What people say about it
The reasons people give, with the number of comments that give each one. Open a reason to read what they said.
Why people recommend it
Each mark is one comment
Fast zero-copy serialization
“Capnp is great, but the fact that protobuf can't do zero copy should be more an academic issue than practical.”
Comment from 2 Dec 2025 in “Why I stopped using JSON for my APIs” “That's something flatbuffers/capnproto do well that isn't just pure speed.”
Comment from 19 Dec 2025 in “Lite^3, a JSON-compatible zero-copy serialization format” “zero-copy formats like FlatBuffers or Cap'n Proto can be much faster”
Comment from 2 Dec 2025 in “Why I stopped using JSON for my APIs” - Comment from 23 Jan 2026 in “Replacing Protobuf with Rust to go 5 times faster”
Simpler format than Protobuf
“the implementation of Cap'n Proto is quite a bit simpler than Protobuf due to the serialization format just being simpler”
Comment from 10 Mar 2026 in “Show HN: Skir – A schema language I built after 15 years of Protobuf friction” “I like the ideas and binary encoding behind Capn Proto more than all the alternatives”
Comment from 24 Jan 2026 in “Replacing Protobuf with Rust to go 5 times faster” “People who want protobuf but better often settle for something like cap’n proto”
Comment from 12 Jul 2026 in “Protobuf-py: Protobuf for Python, without compromises” Trusted, heavily used at Cloudflare
“It is heavily used at Cloudflare, though, and is the basis for a lot of cool stuff we're doing”
Comment from 27 Jan 2026 in “Cap'n Proto” “has been my goto since forever. Made by the protobuf inventor”
Comment from 8 Mar 2026 in “Show HN: Skir – A schema language I built after 15 years of Protobuf friction”
What people warn about
Each mark is one comment
Output often produces large sizes
“Cap’n Proto and FlatBuffers are fast but often produce rather large output”
Comment from 2 Mar 2026 in “Why does C have the best file API” “Capnproto doesn't support transform on serialize - the optional fields still take up disk space”
Comment from 6 Mar 2026 in “Nobody ever got fired for using a struct” C++ code less ergonomic
“your codebase will be filled with fairly horrific CapnProto C++ code”
Comment from 23 Jan 2026 in “Replacing Protobuf with Rust to go 5 times faster” “In my experience capn proto is much less ergonomic.”
Comment from 23 Jan 2026 in “Replacing Protobuf with Rust to go 5 times faster”
Other serialization formats people recommend
The four most recommended other serialization formats, with the same counts for each, so you can compare like for like. The tick on each bar marks the 2 to 1 line the badge needs.
| Product | Recommend | Criticize | Approval | Months | Owned | Badge |
|---|---|---|---|---|---|---|
| Protocol Buffers (protobuf) | 77 | 66 | 54% | 10 | 10 yrs3 owners | — |
| Serde | 22 | 16 | 58% | 10 | — | — |
| Cap'n ProtoThis page | 20 | 5 | 80% | 7 | — | Approved |
| Protobuf | 19 | 25 | 43% | 8 | — | — |
| FlatBuffers | 16 | 4 | 80% | 5 | — | Approved |
What people said
28 of the 29 comments that mention it, newest first. Each links to the original comment.
“Some nice Object Capability technologies not mentioned in the Wikipedia article”
“designing a more compact format doesn’t really buy much on modern CPUs and modern networks”
“People who want protobuf but better often settle for something like cap’n proto”
“Capnproto has promise pipelining where you can issue new instructions with results you don't have yet.”
“the implementation of Cap'n Proto is quite a bit simpler than Protobuf due to the serialization format just being simpler”
“It's cool, but I personally think that for most use cases (not all)”
“has been my goto since forever. Made by the protobuf inventor”
“With a decent memory-safe parser, it’s perfectly safe”
“Capnproto doesn't support transform on serialize - the optional fields still take up disk space”
“the right tool might be Parquet or Arrow or protobuf or Cap’n Proto”
“Cap’n Proto and FlatBuffers are fast but often produce rather large output”
“It is heavily used at Cloudflare, though, and is the basis for a lot of cool stuff we're doing”
“I had gotten slightly better performance results from PB”
“Some standards had all of this settled from day one: I think Cap'n Proto, Avro and Thrift fit this description.”
“I like the ideas and binary encoding behind Capn Proto more than all the alternatives”
“ASN.1, Cap'n Proto and Flatbuffers are all more efficient and exist already.”
“that doesn’t have the encoding/decoding penalty of Protobuf”
“your codebase will be filled with fairly horrific CapnProto C++ code”
“Flatbuffers/protobuf/capn-proto/etc: easy to work with”
“In my experience capn proto is much less ergonomic.”
“it's the same idea in CapnProto or Flatbuffers”
“That's something flatbuffers/capnproto do well that isn't just pure speed.”
“This is definitely a limitation of Cap'n Proto right now.”
“simple RPC libs like Cap'n Proto don't do”
“I'd recommend just pointing to capnproto.org”
“Capnp is great, but the fact that protobuf can't do zero copy should be more an academic issue than practical.”
“zero-copy formats like FlatBuffers or Cap'n Proto can be much faster”
“offer most or all of the perks described in this post, with the addition of ensuring that serialization and deserialization are not a bottleneck”
Counts come from 1,795,640 Hacker News comments, November 2025 to November 2026. How we count. Report a mistake.