cloudflare/capnweb

Health v2 · maintenance only

Adoption not part of the score

3954 stars · 142 forks observed · 2026-08-28

What it is AI-extracted, prompt v1, taxonomy v1, 2026-08-29, confidence not recorded

Cap'n Web is a JavaScript/TypeScript-native, schema-free object-capability RPC library from Cloudflare, created by the Cap'n Proto author. It supports bidirectional calls, passing functions and objects by reference, and promise pipelining over HTTP, WebSocket, and postMessage, compressing to under 16 kB with zero dependencies.

Use cases

• add rpc between a browser client and a server in javascript
• call typescript methods on a server from cloudflare workers
• replace grpc or json-rpc with a schemaless javascript rpc library
• pass functions and objects by reference over websocket
• chain dependent calls into one round trip with promise pipelining
• build bidirectional communication between node and the browser
• implement capability-based security where holding a reference grants access

When to choose

• your entire stack is JavaScript/TypeScript and you want RPC with no schemas, code generation, or build step
• you need expressive RPC features like bidirectional calling, reference passing, or promise pipelining
• you target many runtimes (browsers, Cloudflare Workers, Node.js, Bun, Deno) and want a tiny dependency-free bundle
• you want capability-based security patterns for your API

When to avoid

• you need non-JavaScript clients or servers, or strict schema contracts with generated code
• you need binary serialization for maximum throughput, where Cap'n Proto or protobuf fit better
• you require a 1.0-stable, long-frozen protocol; the project is still pre-1.0

Facets

library · maturity active

rpc serialization web-development apis microservices browser bun serverless cross-platform object-capability promise-pipelining schemaless bidirectional-rpc typescript json-serialization capability-based-security websocket no-dependencies cloudflare-workers nodejs deno web-server

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