dsh-preview-plugin

When an agent in DeepSeek Harness spits out a front-end app, you should not have to context-switch to another browser tab, manually configure a dev proxy, and fight with SPA routing just to see it. dsh-preview-plugin folds that loop into DSH's Preview tab: start your local dev server, type the port, hit Go, and the app renders inside DSH as if it were a native page.

The proxy is far from a naïve pipe. A path-based SPA proxy rewrites every route under /preview/:port/ so both client-side routing and backend API calls resolve against the DSH host. For HTML responses it injects a small script that intercepts fetch() and XMLHttpRequest.open(), guaranteeing that SPA-driven async requests take the same proxied path. Non-HTML assets such as JS, CSS, and images are streamed through untouched. Ports can be set globally or per conversation session, and the Preview tab polls the host automatically, so if an agent changes the port via the API you see the update without a manual refresh.

On the security side, the proxy targets 127.0.0.1 exclusively, eliminating external SSRF vectors. The iframe runs in a sandbox with only scripts, same-origin, forms, and popups permitted, and the port field is strictly validated to one-to-five-digit numbers. If you use DSH for front-end projects and want to see conversation and live output side by side in one window, this is a lightweight, drop-in preview solution.