diff --git a/.changeset/drop-u-prefix-host-fallback.md b/.changeset/drop-u-prefix-host-fallback.md
new file mode 100644
index 0000000..659c711
--- /dev/null
+++ b/.changeset/drop-u-prefix-host-fallback.md
@@ -0,0 +1,10 @@
+---
+"sideshow": patch
+---
+
+Drop the dead `/u/`-prefix URL fallback from the default viewer host. The
+embeddable engine's `createDefaultHost()` now derives its base path solely from
+`window.__SIDESHOW_BASE_PATH__` (empty at root) instead of also sniffing a
+`/u/:account` prefix out of `location.pathname`. That fallback was specific to an
+old hosted-wrapper URL shape; self-hosted sideshow already runs at the root or
+sets the global explicitly, so behavior is unchanged for every supported host.
diff --git a/viewer/embed.d.ts b/viewer/embed.d.ts
index 687bc35..0134cf1 100644
--- a/viewer/embed.d.ts
+++ b/viewer/embed.d.ts
@@ -14,7 +14,7 @@ export interface HostRouter {
 }
 
 export interface SideshowHost {
-  /** Link/base prefix the engine prepends to every path, e.g. "/u/alice" (""). */
+  /** Link/base prefix the engine prepends to every path, e.g. "/alice" (""). */
   basePath: string;
   router: HostRouter;
   /** The caller's own identity, when the host knows it. */
diff --git a/viewer/src/host.ts b/viewer/src/host.ts
index fe118c6..241f0fa 100644
--- a/viewer/src/host.ts
+++ b/viewer/src/host.ts
@@ -23,7 +23,7 @@ export interface HostRouter {
 }
 
 export interface SideshowHost {
-  // Link/base prefix the engine prepends to every path, e.g. "/u/alice" ("" at
+  // Link/base prefix the engine prepends to every path, e.g. "/alice" ("" at
   // root). API calls are `${basePath}/api/...`.
   basePath: string;
   router: HostRouter;
@@ -157,12 +157,11 @@ export function host(): SideshowHost {
   return (defaultHostCache ??= createDefaultHost());
 }
 
-// Self-hosted default host: base path from the hosted-wrapper global / URL
-// prefix (as the pre-engine viewer read it), routing over the History API with
-// URL shapes identical to before (/session/:id and /session/:id/s/:sid).
+// Self-hosted default host: base path from the hosted-wrapper global (set by any
+// wrapper before the engine loads; empty at root), routing over the History API
+// with URL shapes identical to before (/session/:id and /session/:id/s/:sid).
 export function createDefaultHost(): SideshowHost {
-  const basePath =
-    window.__SIDESHOW_BASE_PATH__ ?? location.pathname.match(/^\/u\/[^/]+/)?.[0] ?? "";
+  const basePath = window.__SIDESHOW_BASE_PATH__ ?? "";
   const subs = new Set<(r: Route) => void>();
 
   const get = (): Route => {