diff --git a/AGENTS.md b/AGENTS.md
index 9ddf67c..d9e0909 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -383,7 +383,18 @@ Consequences that are easy to get wrong:
 - **Cookie `Path` is scoped to the prefix** (`Path=/forgejo`), so the session token is not sent to sibling apps on the same host. At the root it is `/`, the default-path.
 - **Static assets stay app-absolute** (`nest_service("/static")` in `routes::app`). This is the one place the base path is *not* applied, and getting it wrong is subtle: the asset 404s, `x-data="app()"` never initialises, and every Alpine directive — including the File menu dropdowns — silently stops working. There is a test asserting `GET /static/app.js` returns 200 while a base path is set. `app.js` is inlined via `include_str!` and has no URL to rewrite; only `alpine.min.js` and `style.css` are externally loaded.
 - **Curl will 404 on prefixed paths.** Hitting `http://host:3000/forgejo/login` directly bypasses Apache, so nothing strips the prefix. Test through the proxy, or curl the app-absolute path.
-- **Never hardcode a path in JS.** `static/app.js` matches the new-note form with `form[action$="/notes"]` rather than an exact attribute, so `Ctrl+N` keeps working under any prefix.
+- **Never hardcode a path in JS.** `static/app.js` matches the new-note form with `form[action$="/notes"]` rather than an exact attribute, so `Ctrl+N` keeps working under any prefix. The one `fetch()` it makes is different, because the rule above does not cover it:
+
+  ```javascript
+  const base = window.__BASE_PATH__ || '';
+  fetch(base + '/preview', { ... });
+  ```
+
+  `app.js` is inlined into the page, so it cannot call the server-side `base.url()` helper that every form action and asset `src` uses. A literal `fetch('/preview')` resolves against the *origin root*, which behind a proxy mounted at `/forgejo/` never reaches the app. The failure is nasty because the proxy returns its own 404 body, which `x-html` then swaps straight into the preview pane — the user sees "404 Not Found" in the middle of their document while typing, with nothing in the server logs to explain it. `render_app` therefore injects `window.__BASE_PATH__` next to `__INITIAL_PREVIEW__`, escaped by the same `escape_js` helper and emitted as a backtick literal like its neighbour.
+
+  Pinned by `the_base_path_is_handed_to_client_side_js` and `the_preview_fetch_uses_the_injected_base_path` in `tests/auth_gating.rs`. The second matters most: `app.js` is inlined verbatim, so a regression there is invisible to every other test in the suite.
+
+- **A redirect target is not a forwarded path.** A `303` to `{base}/` is correct for the browser and 404s if you curl it directly, because nothing strips the prefix. This also bites reverse-proxy test harnesses: a proxy built on `urllib`/`requests` will silently follow the redirect, re-resolve it against the upstream root, and return a 404 that looks like an app bug. Disable redirect following when testing through a proxy.
 
 ---
 
diff --git a/README.md b/README.md
index 6bd2734..6f99b64 100644
--- a/README.md
+++ b/README.md
@@ -70,8 +70,8 @@ cargo run -- --base-path /forgejo
 ```
 
 ```apache
-ProxyPass        /forgejo/  http://127.0.0.1:3000/  timeout=10
-ProxyPassReverse /forgejo/  http://127.0.0.1:3000/
+ProxyPass        /el-notes/  http://127.0.0.1:3000/  timeout=10
+ProxyPassReverse /el-notes/  http://127.0.0.1:3000/
 ```
 
 The proxy strips the prefix, so the app still sees ordinary paths and adds the prefix back when generating links, form actions, and redirects. The session cookie's `Path` is scoped to the prefix, so the token is not sent to sibling apps on the same host.
diff --git a/src/pages.rs b/src/pages.rs
index d813c6c..4a4d7c4 100644
--- a/src/pages.rs
+++ b/src/pages.rs
@@ -21,6 +21,17 @@ impl<'a> Base<'a> {
     pub fn url(&self, path: &str) -> String {
         format!("{}{}", self.0, path)
     }
+
+    /// The raw prefix, for handing to client-side JS.
+    ///
+    /// `url()` covers every URL in the rendered markup. This exists for the
+    /// one `fetch()` in `app.js`, which has to build the URL client-side and
+    /// would otherwise resolve `/preview` against the origin root — behind a
+    /// reverse proxy mounted at a subdirectory that misses the app entirely
+    /// and returns the proxy's own 404.
+    pub fn prefix(&self) -> &str {
+        self.0
+    }
 }
 
 pub fn render_app(
@@ -72,6 +83,7 @@ pub fn render_app(
                 (render_status_bar(notes.len(), theme_name))
                 script {
                     "window.__INITIAL_PREVIEW__ = `" (PreEscaped(escape_js(&rendered_preview))) "`;"
+                    "window.__BASE_PATH__ = `" (PreEscaped(escape_js(base.prefix()))) "`;"
                     (PreEscaped(include_str!("../static/app.js")))
                 }
             }
diff --git a/static/app.js b/static/app.js
index 594fe6c..7f55f91 100644
--- a/static/app.js
+++ b/static/app.js
@@ -53,7 +53,16 @@ function app() {
         const formData = new URLSearchParams();
         formData.append('content', content);
 
-        fetch('/preview', {
+        // The base path has to be prepended here. `app.js` is inlined into
+        // the page, so it cannot call the server-side `base.url()` helper
+        // that every form action uses — and an origin-absolute `/preview`
+        // resolves against the site root, which behind a reverse proxy
+        // mounted at a subdirectory never reaches the app. The response body
+        // would then be the proxy's own 404, swapped straight into the
+        // preview pane.
+        const base = window.__BASE_PATH__ || '';
+
+        fetch(base + '/preview', {
           method: 'POST',
           headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
           body: formData.toString()
diff --git a/tests/auth_gating.rs b/tests/auth_gating.rs
index 0b26df2..fd7d7b7 100644
--- a/tests/auth_gating.rs
+++ b/tests/auth_gating.rs
@@ -720,6 +720,60 @@ async fn every_screen_declares_a_viewport() {
     }
 }
 
+/// `app.js` is inlined into the page, so it cannot call the server-side
+/// `base.url()` helper. Its one `fetch()` has to build the URL itself, which
+/// is what `window.__BASE_PATH__` is for.
+///
+/// Without it, `fetch('/preview')` resolves against the origin root. Behind
+/// a reverse proxy mounted at a subdirectory that never reaches the app, and
+/// the proxy's own 404 body gets swapped into the preview pane. The symptom
+/// is a preview pane full of "404 Not Found" while typing.
+#[tokio::test]
+async fn the_base_path_is_handed_to_client_side_js() {
+    // With a base path set, the browser must be told about it.
+    let f = Fixture::with_base_path("base-js", "/forgejo")
+        .login_over_http()
+        .await;
+    let (status, body) = f.get_authed("/").await;
+    assert_eq!(status, StatusCode::OK);
+    // Backtick template literal, matching the `__INITIAL_PREVIEW__` line
+    // above it and escaped by the same `escape_js` helper.
+    assert!(
+        body.contains("window.__BASE_PATH__ = `/forgejo`;"),
+        "client JS should be handed the base path"
+    );
+
+    // Without one, the value must be empty rather than the string "undefined",
+    // so the fetch URL stays byte-identical to the pre-fix behaviour.
+    let plain = Fixture::fresh("base-js-none").login_over_http().await;
+    let (status, body) = plain.get_authed("/").await;
+    assert_eq!(status, StatusCode::OK);
+    assert!(
+        body.contains("window.__BASE_PATH__ = ``;"),
+        "with no base path the client value should be empty"
+    );
+}
+
+/// The `fetch()` in `app.js` must actually use the injected value. This is
+/// the assertion that would have caught the original bug: `app.js` is
+/// inlined verbatim, so a regression there is invisible to every other test.
+#[tokio::test]
+async fn the_preview_fetch_uses_the_injected_base_path() {
+    let f = Fixture::with_base_path("fetch-base", "/forgejo")
+        .login_over_http()
+        .await;
+    let (status, body) = f.get_authed("/").await;
+    assert_eq!(status, StatusCode::OK);
+    assert!(
+        body.contains("fetch(base + '/preview'"),
+        "app.js should build the preview URL from __BASE_PATH__"
+    );
+    assert!(
+        !body.contains("fetch('/preview'"),
+        "app.js must not hardcode an origin-absolute /preview"
+    );
+}
+
 /// The about page advertises a version, and it must be the one in
 /// `Cargo.toml`.
 ///