# Cap'n Proto

**Canonical:** https://apis.io/providers/capn-proto/  
**Website:** https://capnproto.org/  
**APIs profiled:** 5

Cap'n Proto is an open-source binary data interchange format and capability-based RPC protocol specification originally created by Kenton Varda. Unlike Protocol Buffers, Cap'n Proto's in-memory representation is identical to its wire format, enabling zero-copy deserialization, incremental reads, random field access, and memory-mapped I/O. The reference implementation is in C++; a broad ecosystem of community-maintained bindings covers C#, Erlang, Go, Haskell, JavaScript/Node, OCaml, Python, Rust, C, D, Java, Lua, Nim, Ruby, and Scala.

## Kin Score — 15.8 / 100 (emerging)

Scored 2026-08-20 under rubric 0.12.0. Trend: flat (+0.0 from 15.8).

| Facet | Score |
|---|---|
| Discoverability | 64.8 |
| Contract Quality | 0.0 |
| Governance | 0.0 |
| Contract Governance | 0.0 |
| Operational Transparency | 10.5 |
| Developer Ergonomics | 23.8 |
| Commercial Clarity | 15.8 |
| Access Clarity | 15.8 |

## Agent readiness — 3.0 (human-only)

| Dimension | Value |
|---|---|
| Spec Presence | no |
| Agentic Access | no |
| Reversibility Documented | no |
| MCP Server | no |
| Auth Clarity | no |
| Idempotency | no |
| Error Semantics | no |
| OpenAPI Examples | no |
| Rate Limit Signal | documented |
| Event Surface Described | no |
| Agent Skills | no |
| Well Known Catalog | no |
| Consent Identity | no |
| Agent Card | no |
| Dry Run Mode | no |

## Access

Freemium — onboarding: unknown, pricing: freemium, trial: no (confidence: medium).

## APIs (5)

- **Cap'n Proto Schema Language** — The Cap'n Proto schema language is used to define message types in .capnp files that are then compiled into native code for each supported language. The schema language defines ...
- **Cap'n Proto Encoding (Wire Format)** — The Cap'n Proto encoding specification defines the binary wire format. The in-memory layout is the wire format, enabling zero-copy reads and writes, random field access, and saf...
- **Cap'n Proto RPC Protocol** — Cap'n Proto's RPC protocol is a capability-based RPC layer that supports promise pipelining, object references passed as arguments or return values, and time-travel optimization...
- **Cap'n Proto C++ Reference Implementation** — The C++ reference implementation is the canonical runtime for Cap'n Proto, providing the capnp compiler, serialization library, and KJ/RPC runtime. Other language implementation...
- **Cap'n Proto Language Bindings** — Community-maintained language bindings implement Cap'n Proto serialization and, in many cases, the full RPC protocol. Serialization plus RPC is supported in C++, C#, Erlang, Go,...

## Security (1)

- **Capn Proto Domain Security** — TLSv1.3 · HSTS

## Plans (1)

- **Capn Proto Plans Pricing**

## Tags

Binary Format, Capability-Based Security, Code Generation, IPC, Open-Source, Protocol, RPC, Schema, SDK, Serialization, Specification, Zero-Copy

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Profiled by [API Evangelist](https://apievangelist.com) and published on [APIs.io](https://apis.io/providers/capn-proto/). Scores are computed from the provider's own public artifacts under a published rubric.