diff --git a/docs/automation/README.md b/docs/automation/README.md
index 1dcf4a90e..19e874121 100644
--- a/docs/automation/README.md
+++ b/docs/automation/README.md
@@ -156,3 +156,5 @@ The repeatable live-editor evaluation draws three isolated entities in one batch
 ```sh
 python3 docs/automation/mcp_eval.py target/debug/OpenCADStudio
 ```
+
+The web build is driven through two JavaScript functions instead of a process; see [web.md](web.md).
diff --git a/docs/automation/web.md b/docs/automation/web.md
new file mode 100644
index 000000000..16a80c622
--- /dev/null
+++ b/docs/automation/web.md
@@ -0,0 +1,68 @@
+# Driving the web build
+
+The web build exposes the same control core as `--serve` and `--mcp`, through two
+functions on the WebAssembly module. This page covers only what is specific to the
+browser; the operations, request envelope and replies are the ones in the
+[README](README.md).
+
+## Reaching the control channel
+
+The Trunk loader publishes the wasm-bindgen module as `window.wasmBindings`. The
+control channel is ready once `window.wasmBindings?.ocs_control_submit` is defined
+(the [save smoke test](web_save_smoke.cjs) waits for exactly that).
+
+| Function | Behaviour |
+|---|---|
+| `ocs_control_submit(request: string) → string` | Queues one request (a JSON string) and returns a ticket immediately. |
+| `ocs_control_take(ticket: string) → string \| undefined` | Returns the reply (a JSON string) once it is ready, `undefined` before that. A reply is removed when taken. |
+
+Poll `ocs_control_take` until it returns a reply, as the smoke test does:
+
+```js
+async function control(request) {
+  const api = window.wasmBindings;
+  const ticket = api.ocs_control_submit(JSON.stringify(request));
+  for (let i = 0; i < 600; i++) {
+    const reply = api.ocs_control_take(ticket);
+    if (reply) return JSON.parse(reply);
+    await new Promise((resolve) => setTimeout(resolve, 50));
+  }
+  throw new Error('control timed out');
+}
+```
+
+Limits: at most 64 requests wait in the queue (a further `submit` is answered with the
+`busy` error), and only the newest 64 replies are kept, so take a reply soon after it is
+ready. A request that is not valid JSON is answered with `op: "invalid"` and a
+`parse_error`.
+
+## Opening a drawing from bytes
+
+The web build has no file system, so `open` takes the file's bytes instead of a path:
+
+```json
+{"op":"open","name":"plan.dwg","data_base64":"…"}
+```
+
+`name` is reduced to its file name and labels the tab and the recent-files entry;
+`data_base64` must be valid base64 and non-empty (`invalid_request` otherwise). While an
+earlier drawing is still opening the request is answered with `busy`; retry it once
+`state` shows the tab. A `path` is resolved against the browser's "recent files" store
+rather than a disk, so use `data_base64` for a file the page has not seen.
+`data_base64` is rejected on native builds; use `path` there.
+
+## Startup dialogs
+
+A dialog can be open when the page is ready. The donation prompt appears once per
+application version in each browser profile, so a fresh or disposable profile always
+sees it. While a modal is open `state.modal` is set, and `{"op":"new"}` returns `ok`
+without creating a tab until the modal is closed. Close it first:
+
+```js
+const state = await control({ op: 'state' });
+if (state.modal) await control({ op: 'action', name: 'close_modal' });
+await control({ op: 'new' });
+```
+
+Native builds can show a second dialog at launch (the file-association prompt), so check
+`state.modal` again after closing one.