Cap'n Proto

aid: capn-proto
url: https://raw.githubusercontent.com/api-evangelist/capn-proto/refs/heads/main/apis.yml
name: Cap'n Proto
description: 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.
type: Index
kind: standard
position: Producer
access: Open Source
image: https://kinlane-images.s3.amazonaws.com/shared/apis-json/apis-json-logo.jpg
tags:
- Binary Format
- Capability-Based Security
- Code Generation
- IPC
- Open Source
- Protocol
- RPC
- Schema
- SDKs
- Serialization
- Specification
- Zero Copy
created: '2026-03-25'
modified: '2026-04-23'
specificationVersion: '0.19'
apis:
- aid: capn-proto:capn-proto-schema-language
  name: Cap'n Proto Schema Language
  description: 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 structs, unions, enums, interfaces (for RPC), groups,
    generics, and annotations, and carefully specifies evolution rules so schemas can be extended without breaking forward
    or backward compatibility.
  humanURL: https://capnproto.org/language.html
  tags:
  - Code Generation
  - Schema
  - Specification
  properties:
  - type: Specification
    url: https://capnproto.org/language.html
  - type: Documentation
    url: https://capnproto.org/language.html
  - type: GitHubRepository
    url: https://github.com/capnproto/capnproto
  x-features:
  - Struct, union, enum, interface, and group definitions
  - Generic parameters for reusable types
  - Annotations for custom metadata
  - Schema evolution rules for forward and backward compatibility
  - Compilation to native code in multiple languages
  - Cross-language ABI through a canonical wire format
  x-use-cases:
  - Defining message types for zero-copy IPC
  - Describing RPC interfaces across language boundaries
  - Long-lived storage formats with schema evolution
  - Replacing Protocol Buffers for latency-sensitive workloads
- aid: capn-proto:capn-proto-encoding
  name: Cap'n Proto Encoding (Wire Format)
  description: 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 safe memory-mapped access to messages stored on disk.
  humanURL: https://capnproto.org/encoding.html
  tags:
  - Binary Format
  - Specification
  - Zero Copy
  properties:
  - type: Specification
    url: https://capnproto.org/encoding.html
  x-features:
  - In-memory layout equals the wire format
  - Zero-copy deserialization
  - Random field access without scanning
  - Support for mmap-based message storage
  - Packed encoding variant for bandwidth-constrained links
  - Canonicalization rules for deterministic encoding
  x-use-cases:
  - Ultra-low-latency inter-process communication
  - Large on-disk datasets with mmap access
  - Network protocols where parsing cost matters
  - Embedded or memory-constrained environments (packed form)
- aid: capn-proto:capn-proto-rpc
  name: Cap'n Proto RPC Protocol
  description: 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 optimizations that eliminate round trips. It is the foundation of
    Cloudflare Workers' inter- service communication and Sandstorm's capability-oriented sandbox.
  humanURL: https://capnproto.org/rpc.html
  tags:
  - Capability-Based Security
  - Protocol
  - RPC
  - Specification
  properties:
  - type: Specification
    url: https://capnproto.org/rpc.html
  - type: Documentation
    url: https://capnproto.org/rpc.html
  x-features:
  - Capability-based security model
  - Object (capability) references passed in messages
  - Promise pipelining to eliminate round trips
  - Bidirectional calls over a single connection
  - Time-travel / path-compression optimizations
  - Transport-agnostic (TCP, TLS, WebSocket, shared memory)
  x-use-cases:
  - Microservice RPC with low round-trip overhead
  - Secure capability-oriented sandboxing (Sandstorm, Workers)
  - Bidirectional streaming between trusted peers
  - Inter-process communication with fine-grained authorization
- aid: capn-proto:capn-proto-cpp-runtime
  name: Cap'n Proto C++ Reference Implementation
  description: The C++ reference implementation is the canonical runtime for Cap'n Proto, providing the capnp compiler, serialization
    library, and KJ/RPC runtime. Other language implementations are maintained by their respective authors and track the C++
    reference.
  humanURL: https://github.com/capnproto/capnproto
  tags:
  - IPC
  - SDKs
  - Serialization
  properties:
  - type: GitHubRepository
    url: https://github.com/capnproto/capnproto
  - type: GettingStarted
    url: https://capnproto.org/install.html
  - type: Documentation
    url: https://capnproto.org/cxx.html
  x-features:
  - capnp schema compiler
  - KJ async and concurrency library
  - RPC runtime over TCP and in-process transports
  - Arena allocator for fast message construction
  - Packed and canonical encoding support
- aid: capn-proto:capn-proto-language-bindings
  name: Cap'n Proto Language Bindings
  description: 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, Haskell, JavaScript (Node.js), OCaml, Python, and
    Rust. Serialization-only bindings exist for C, D, Java, Lua, Nim, Ruby, and Scala.
  humanURL: https://capnproto.org/otherlang.html
  tags:
  - Code Generation
  - SDKs
  - Serialization
  properties:
  - type: Documentation
    url: https://capnproto.org/otherlang.html
  - type: GitHubOrganization
    url: https://github.com/capnproto
  x-features:
  - Serialization + RPC in C++, C#, Erlang, Go, Haskell, JavaScript, OCaml, Python, Rust
  - Serialization-only bindings in C, D, Java, Lua, Nim, Ruby, Scala
  - Shared canonical wire format across all implementations
  - Editor integrations and syntax highlighters for common IDEs
  x-use-cases:
  - Polyglot systems sharing message types across services
  - Compiling schemas into idiomatic bindings for each language
  - Migration paths from Protocol Buffers in non-C++ codebases
common:
- type: Website
  url: https://capnproto.org/
- type: Documentation
  url: https://capnproto.org/language.html
- type: GettingStarted
  url: https://capnproto.org/install.html
- type: GitHubOrganization
  url: https://github.com/capnproto
- type: GitHubRepository
  url: https://github.com/capnproto/capnproto
- type: Discussion Group
  url: https://groups.google.com/g/capnproto
- type: License
  url: https://github.com/capnproto/capnproto/blob/master/LICENSE.txt
maintainers:
- FN: Kin Lane
  email: kin@apievangelist.com