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Kenton Varda

@kentonvarda.com

1.7K followers 130 following 170 posts

Tech lead for workers.cloudflare.com Also made: capnproto.org, lanparty.house, sandstorm.io @KentonVarda on Twitter.

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Kenton Varda @kentonvarda.com · 19d

I don't have a problem with making new RPC systems! I can stop any time I want! But I don't want to. So I made a new one for you. It's called Cap'n Web. It's like Cap'n Proto RPC, except focused on working great in browsers and JavaScript/TypeScript servers. blog.cloudflare.com/capnweb-java...

An image of a circular fabric patch showing some sort of superhero in a nautical hat? Cap'n Web: Protect & Connect, it says. Looks like AI slop.

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Kenton Varda @kentonvarda.com · 5h

Personally, I don't use the "for you" tab, I only use "following", and I basically don't see any politics at all. (I tend to unfollow anyone who posts politics TBH.)

I was actually doing that even before the acquisition, so didn't see much change after.

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Kenton Varda @kentonvarda.com · 6d

Exactly as I remember it, in excruciating detail. Why does my brain continue to store this while forgetting people's names?

Kenton Varda @kentonvarda.com · 7d

Wait... I think someone made, like, a flash animated version of this?

I can still hear it in my head, including the heart beating at the end.

Kenton Varda @kentonvarda.com · 7d

Wow I remember this one. Despite not thinking about it once for 25 years. Why is this occupying space in my brain?

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Kenton Varda @kentonvarda.com · 7d

Is it not Durable Objects?

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Kenton Varda @kentonvarda.com · 10d

I don't personally know much about atproto or next.js, but this all sounds easily doable in Workers. @dom96.picheta.me built @listifications.app on top of Workers, FWIW.

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Kenton Varda @kentonvarda.com · 15d

The ideal API surface to expose to a dynamically-loaded Worker sandbox is a Cap'n Web API. blog.cloudflare.com/capnweb-java...

Cap'n Web: A new RPC system for browsers and web servers

Cap'n Web is a new open source, JavaScript-native RPC protocol for use in browsers and web servers. It provides the expressive power of Cap'n Proto, but with no schemas and no boilerplate.

blog.cloudflare.com

Kenton Varda @kentonvarda.com · 15d

That's the next step I think. MCP is convenient because it's designed for the whole end-to-end experience of connecting an agent to an arbitrary MCP server including auth, etc. But once we fully solve those for e.g. OpenAPI, we should be able to go direct.

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Kenton Varda @kentonvarda.com · 15d

Sorry by "the readme" I mean: github.com/cloudflare/a...

github.com

Kenton Varda @kentonvarda.com · 15d

No, the example should work correctly if you follow the instructions in the readme. It works for me. But only in local dev. To deploy it to prod you would need to be in the closed beta for worker loaders.

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Kenton Varda @kentonvarda.com · 15d

Yes exactly. It's cheaper sandboxes. Also, the capability-based model is well known to be a great fit for sandboxing -- much cleaner than trying to filter and block arbitrary network requests.

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Kenton Varda @kentonvarda.com · 15d

Anyway, please don't murder me for the quip that kinda sorta compares LLMs to Shakespeare.

/me runs away

Kenton Varda @kentonvarda.com · 15d

Dynamic Worker loading, of course, has applications far beyond just MCP or AI. It's a general-purpose capability-based sandboxing mechanism. If I had more time I'd build Sandstorm.io 2.0 on this...

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Kenton Varda @kentonvarda.com · 15d

This is all based on a new Workers Runtime feature called Dynamic Worker Loading, which lets you dynamically load arbitrary code into a fresh isolate. Isolates are wayyyyyy cheaper and faster to load than containers. Perfect for executing one-offs. developers.cloudflare.com/workers/runt...

Dynamic Worker Loaders

The Dynamic Worker Loader API, which allows dynamically spawning isolates that run arbitrary code.

developers.cloudflare.com

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Kenton Varda @kentonvarda.com · 15d

This works better than direct tool calls becaues LLMs actually have WAY more training data for writing code than for calling tools. After all, tool-calling is a sort of artificial thing just for LLMs. Code is everywhere.

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Kenton Varda @kentonvarda.com · 15d

Hey Bluesky, I know how people here just LOVE everything AI, so I wanted to show you what @threepointone.bsky.social and I have been up to. We made an agent that calls MCP not as "tools", but by converting the tools into a TypeScript API, and writing code against that. blog.cloudflare.com/code-mode/

Code Mode: the better way to use MCP

It turns out we've all been using MCP wrong. Most agents today use MCP by exposing the

blog.cloudflare.com

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Kenton Varda @kentonvarda.com · 15d

Oh! The stub is being disposed because you are returning it which transfers ownership to the caller. Try `return customerSession.dup()` instead. See the docs in the readme about automatic disposal.

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Kenton Varda @kentonvarda.com · 15d

Sorry, I meant, "not one per RPC".

Kenton Varda @kentonvarda.com · 15d

Call `newWebSocketRpcSession()` once for gateway->backend. Get a stub back. Use that same stub to serve many incoming requests from clients. Each call to newWebSocketRpcSession() only creates exactly one connection, not one per call.

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Kenton Varda @kentonvarda.com · 15d

The capability model naturally keeps the objects distinct from each other so you don't have to worry about, say, traffic for different users getting mixed up. Every method call is directed at a specific object.

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Kenton Varda @kentonvarda.com · 15d

Absolutely, Cap'n {Web,Proto} is great at this. You can have any number of capabilities (object references) on a single connection.

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Kenton Varda @kentonvarda.com · 16d

Yeah. For Go in particular, the solution will probably be that you use Cap'n Proto on the backend, Cap'n Web on the frontend (in browser), and there's a translating proxy you can stick in between.

Kenton Varda @kentonvarda.com · 16d

In the future we may add "three-party handoff", where when Alice has a stub pointing at Bob, and Alice gives that stub to Carol, Carol would automatically form a direct connection to Bob. The way to support that is well-understood, albeit complex, so it wasn't in v1.

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Kenton Varda @kentonvarda.com · 16d

Cap'n Web (like Cap'n Proto) is great for many-party distributed systems. You can receive a stub from one session and then forward it on across another connection, and it'll automatically arrange proxying, so it "just works".

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Kenton Varda @kentonvarda.com · 17d

The RPC systems will continue until y'all accept the object-capability model as your lord and savior.

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