0006. A small plugin interface on top of net/http

• Status: Accepted
• Date: 2026-11-09

#Context

Production apps need the same pieces again and again: health checks, access logs, compression, security headers, analytics tags. kit.WithHook and kit.WithMiddleware covered part of this, but a reusable package often needs several hooks at once (a route of its own, a tag in every page, work on shutdown), and users should add it with one line.

#Decision

• A plugin is a Go value with Name() string and Setup(*kit.Setup) error, added with kit.WithPlugins.
• Setup can register hooks, middleware, matchers, http.Handlers that are served before routes (with net/http.ServeMux patterns), HTML for every page's <head> (with the request's CSP nonce), and shutdown functions.
• Plugins are set up in order after the other options; a name can be added once; a Setup error stops the app from starting.
• Built-in plugins live in plugins/ in the main module and use only the standard library.
• Client-side extension stays with Vite: Tern's Vite plugin composes with any other Vite plugin.

#Consequences

• Extending Tern means writing ordinary Go against net/http; there is no separate plugin runtime or registry.
• Plugins cannot change routing or rendering internals. That keeps the public surface small enough to promise stability for at 1.0, and can be extended later without breaking existing plugins.