WebContainers.

WebContainers is a technology developed by StackBlitz that runs a complete Node.js environment natively in the browser using WebAssembly and modern web APIs. Unlike cloud-based IDEs that connect to a remote server over the network, WebContainers executes JavaScript server-side code entirely on the user's machine — inside the browser tab — with no latency from network round-trips to a VM.

The practical impact of WebContainers is enabling tools like Bolt.new to run full-stack applications (React frontend, Express backend, file system operations) entirely in the browser with instant startup and no provisioning delay. This makes the experience of building an app feel immediate — changes propagate in milliseconds rather than waiting for a server to restart.

WebContainers have significant technical limitations: they can't run native binaries, some Node.js C++ addons don't work, and memory is constrained by browser limits. Applications requiring native database clients (PostgreSQL drivers that use native bindings), GPU access, or large memory allocations can't run in WebContainers. These constraints define the scope of what Bolt.new can build without connecting to external services.