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workerd

workerd is Cloudflare's open-source JavaScript / WebAssembly runtime — the same runtime that powers Cloudflare Workers in production. Written in C++, built on V8 + Cap'n Proto, built with Bazel.

Role on this wiki

Named in Cloudflare's 2026-04-01 EmDash launch post as the open-source runtime under Workers. Cloudflare's claim:

"EmDash is different: it's built to run on serverless platforms, and make the most out of the v8 isolate architecture of Cloudflare's open source runtime workerd."

The key property EmDash depends on: "on an incoming request, the Workers runtime instantly spins up an isolate to execute code and serve a response. It scales back down to zero if there are no requests. And it only bills for CPU time." — i.e. scale-to-zero at the isolate tier rather than the VM or container tier.

Substrate for capability-based sandboxing

workerd is the runtime substrate under Dynamic Workers — Cloudflare's per-request-isolate variant. Dynamic Workers' no-ambient- authority posture (globalOutbound: null) is enforced at the workerd boundary, not at a policy layer on top. This is why EmDash can isolate third-party plugins credibly: every plugin gets a fresh workerd isolate, spawned in milliseconds, with capabilities bound from its manifest. See concepts/capability-based-sandbox.

Portability

workerd is fully open-source (Apache 2.0) and can be embedded outside Cloudflare — enables customers to run Workers code on their own infrastructure, and third-party products (EmDash is an early example) to ship a "deploys to Workers or any Node.js server" portability story without Cloudflare-specific lock-in.

Seen in

• sources/2026-04-01-cloudflare-emdash-wordpress-spiritual-successor — canonical wiki instance. workerd named as the open-source runtime under Workers that enables EmDash's scale-to-zero
• capability-sandbox properties.

Related

• systems/cloudflare-workers — the Cloudflare-hosted production deployment of workerd.
• systems/v8-javascript-engine — the isolate engine workerd embeds.
• systems/dynamic-workers — the per-request-isolate variant layered on workerd.
• systems/emdash — a CMS built to run on workerd.
• concepts/scale-to-zero — the runtime economic model workerd enables.
• concepts/serverless-compute — the broader model.
• concepts/capability-based-sandbox — security posture workerd enforces at the boundary.
• companies/cloudflare — author.