Cloudflare Computer is a virtual filesystem whose state is stored in SQLite inside a Durable Object and exposed through selectable execution backends. It lets coding agents work with files, shell commands, JavaScript modules, containers, and networked Linux environments.
Borrowing it
Nothing to install: this file belongs to cloudflare/computer. Take a copy, put it at the same path in your own repository, and replace the rules that are about this project with yours.
curl -O https://raw.githubusercontent.com/cloudflare/computer/main/.agents/skills/capnweb/SKILL.mdgit clone --depth 1 https://github.com/cloudflare/computerWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/cloudflare/computer/capnweb)<a href="https://agentmods.dev/skills/cloudflare/computer/capnweb"><img src="https://agentmods.dev/badge/skills/cloudflare/computer/capnweb/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/cloudflare/computer/capnweb"><img src="https://agentmods.dev/badge/skills/cloudflare/computer/capnweb.svg" alt="Reviewed on agentmods" width="80" height="20"></a>- Snyk pass
- NVIDIA SkillSpector pass
What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00094 | $0.02521 |
| Opus 5.5 | $0.00038 | $0.01008 |
| Sonnet 5.5 | $0.00019 | $0.00504 |
| Haiku 4.5 | $0.00009 | $0.00252 |
Grade A, and why
capnweb scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 2mo ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
How it starts
The opening of the file, as written. The whole thing — 268 lines — stays where its author put it; the contents beside it link to each section on GitHub.
capnweb in this repo
capnweb is the RPC framing
between the Durable Object and computerd. It's an object-capability RPC
system with promise pipelining, structured-clone-style transfer of
stubs, and bidirectional calls. The wire format used here is text
JSON over a long-lived WebSocket. That is the only carrier: capnweb's
HTTP batch transport cannot deliver a stream returned from a call,
which is what every read on this interface is.
Where things live
packages/rpc/src/interface.ts— typed wire surface.WorkspaceRPCis the root stub and composesSyncRPCandShellRPC. Add new methods here first.packages/rpc/src/server.ts—Database-backed implementation. Imported by the Durable Object and the in-container computerd.packages/rpc/src/client.ts— typed stubs over a WebSocket carrier. The Durable Object uses a deferred transport so the stub can be created before the upgrade completes.packages/rpc/src/sync-driver.ts—pullOnce,pushOnce,tick. These wrap streaming methods and handle disposal internally.packages/rpc/src/debug.ts—enableStubTracking,stubSnapshotfor leak hunting.docs/08_capnweb_interface.md— design intent for the wire.docs/11_lifecycle.md— the repo's stub disposal contract.
The mental model
A stub is a capability: holding it is the right to call the remote object. Stubs are not garbage-collected across the network — the local GC has no visibility into the remote object graph, and the remote runtime has no idea whether you're under memory pressure. If you don't explicitly dispose stubs, you leak resources on the other side of the connection.
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 2mo ago First seen · 268 lines · 94 tokens per session scan A 1f2686116b37
capnweb is a skill published in the GitHub repository cloudflare/computer (9,753 stars, last pushed today), licensed MIT. It adds 94 tokens to every session and 2,521 once invoked, about $0.0004 per session on Opus 5.5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.
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