When building WebUI plugin/extension systems, treat the backend as the source of truth for (1) resolving extension contents and (2) providing a stable context schema to JS extensions.

Apply these rules:

• Extension-point resolution: When the client finds an element like <x-extension id="...">, it should call a dedicated backend API (e.g., get_webui_extensions(id)) that searches the plugin roots by convention (e.g., .../extensions/webui/{id}/*.html) and returns the HTML/content to paste into that node. Keep the client loader thin; avoid re-implementing manifest/scan logic when the backend can resolve by id.

• Stable context schema: Ensure callJsExtensions(..., context) uses one unified “context” shape across default and custom message handlers. If an extension needs message.no (log number), that field must be present consistently in the default implementation too.

Example (contract-driven usage):

// Client: paste extension HTML resolved by backend
async function applyExtensionPoint(node, id) {
  const htmlParts = await get_webui_extensions(id); // backend contract
  node.innerHTML = htmlParts.join("\n");
}

// JS extension: rely on unified context schema
export default async function injectBranchButtons(context) {
  const msgs = context?.messages;
  if (!Array.isArray(msgs) || msgs.length === 0) return;

  for (const m of msgs) {
    // contract requires log number (no)
    if (typeof m.no !== "number") throw new Error("Missing context.messages[].no");
    // ...use m.no to wire actions
  }
}

Outcome: frontend extension code becomes deterministic (no “sometimes field is present” surprises), and plugin authors can build against a predictable backend-defined contract.