Serve the Jellyfin surface under /emby as well as at the root

Many Jellyfin clients probe GET /emby/System/Info/Public during their
handshake. There was no /emby prefix anywhere in the tree, so that 404ed
and the client abandoned discovery and fell back to its manual "enter your
server URL" screen - the exact experience mDNS exists to remove.

Register installs every pattern twice through a mount helper, with the same
handler value both times, so the alias cannot drift from the root surface.
The JSON 404 fallbacks get /emby forms too, plus one subtree handler,
because static.go's isAPIPath knows only /healthz and /api/ and an unknown
/emby/... path would otherwise be served the SPA shell.

No new endpoint, no handler behaviour changed, and the bare root paths are
untouched: Jellyfin's own clients use them.

internal/jellyfin/jellyfin.go

@@ -27,6 +27,12 @@
 // requests route here first. isAPIPath in static.go does not know these
 // prefixes, so each Jellyfin subtree installs its own JSON 404 handler; an
 // unknown /Items/... path returns JSON rather than the SPA shell.
+//
+// Every pattern is installed a second time under /emby, the legacy base path.
+// Many Jellyfin apps probe GET /emby/System/Info/Public during handshake and
+// give up on discovery when it 404s, showing their manual "enter your server
+// URL" screen. It is the same handler behind both paths: an alias, never a
+// second surface, so it cannot drift from the root.
 package jellyfin
 
 import (
@@ -36,6 +42,7 @@ import (
 	"encoding/hex"
 	"log/slog"
 	"net/http"
+	"strings"
 	"sync"
 
 	"github.com/mithral/vellum/internal/auth"
@@ -62,41 +69,60 @@ func NewHandlers(db *sql.DB, log *slog.Logger, a *auth.Authenticator) *Handlers
 	return &Handlers{db: db, log: log, a: a}
 }
 
+// embyPrefix is the legacy Jellyfin base path. It is an alias for the root
+// mount, not a second surface: every pattern below is installed under both.
+const embyPrefix = "/emby"
+
 // Register mounts the routes. See the package comment for why they live at the
 // root and why each subtree has a JSON 404 fallback.
 func (h *Handlers) Register(mux *http.ServeMux) {
+	// mount installs each pattern twice, bare and under /emby. Jellyfin's own
+	// clients use the bare root paths, but many real apps probe
+	// /emby/System/Info/Public during handshake and abandon discovery when it
+	// 404s. Same handler either way, so the alias cannot drift from the root.
+	mount := func(pattern string, handler http.Handler) {
+		mux.Handle(pattern, handler)
+		// The prefix goes on the path, not the whole pattern: "GET /Items" is
+		// one string and "embyGET /Items" does not parse.
+		method, path, hasMethod := strings.Cut(pattern, " ")
+		if !hasMethod {
+			method, path = "", pattern
+		}
+		mux.Handle(method+" "+embyPrefix+path, handler)
+	}
+
 	// Public discovery.
-	mux.HandleFunc("GET /System/Info/Public", h.handleSystemInfoPublic)
-	mux.HandleFunc("POST /Users/AuthenticateByName", h.handleAuthenticateByName)
+	mount("GET /System/Info/Public", http.HandlerFunc(h.handleSystemInfoPublic))
+	mount("POST /Users/AuthenticateByName", http.HandlerFunc(h.handleAuthenticateByName))
 
 	// Authenticated: users.
-	mux.Handle("GET /Users/Me", h.require(h.handleUserMe))
-	mux.Handle("GET /Users/{userId}", h.require(h.handleUserByID))
+	mount("GET /Users/Me", h.require(h.handleUserMe))
+	mount("GET /Users/{userId}", h.require(h.handleUserByID))
 
 	// Authenticated: browsing.
-	mux.Handle("GET /Items", h.require(h.handleItems))
-	mux.Handle("GET /Users/{userId}/Items", h.require(h.handleUserItems))
-	mux.Handle("GET /Items/{itemId}", h.require(h.handleItem))
-	mux.Handle("GET /Items/{itemId}/Images/Primary", h.require(h.handleImagePrimary))
-	mux.Handle("GET /Items/{itemId}/PlaybackInfo", h.require(h.handlePlaybackInfo))
+	mount("GET /Items", h.require(h.handleItems))
+	mount("GET /Users/{userId}/Items", h.require(h.handleUserItems))
+	mount("GET /Items/{itemId}", h.require(h.handleItem))
+	mount("GET /Items/{itemId}/Images/Primary", h.require(h.handleImagePrimary))
+	mount("GET /Items/{itemId}/PlaybackInfo", h.require(h.handlePlaybackInfo))
 
 	// Media. These MUST accept api_key in the query string: <video>/<audio>
 	// elements cannot set request headers, so a Jellyfin client appends
 	// ?api_key=... to the URL it plays.
-	mux.Handle("GET /Videos/{itemId}/stream", h.require(h.handleVideoStream))
-	mux.Handle("GET /Audio/{itemId}/stream", h.require(h.handleAudioStream))
+	mount("GET /Videos/{itemId}/stream", h.require(h.handleVideoStream))
+	mount("GET /Audio/{itemId}/stream", h.require(h.handleAudioStream))
 	// Real clients ask for stream.<container> (stream.mp4, stream.mkv,
 	// stream.mp3). ServeMux wildcards are whole segments only, so a suffix
 	// pattern is impossible; a subtree handler dispatches those instead. The
 	// exact /stream pattern above is more specific and still wins.
-	mux.Handle("GET /Videos/{itemId}/", h.require(h.handleVideoStream))
-	mux.Handle("GET /Audio/{itemId}/", h.require(h.handleAudioStream))
+	mount("GET /Videos/{itemId}/", h.require(h.handleVideoStream))
+	mount("GET /Audio/{itemId}/", h.require(h.handleAudioStream))
 
 	// Playback reporting. This is how a Jellyfin client tells us its resume
 	// position; it is written through the existing progress store.
-	mux.Handle("POST /Sessions/Playing", h.require(h.handlePlaying))
-	mux.Handle("POST /Sessions/Playing/Progress", h.require(h.handlePlayingProgress))
-	mux.Handle("POST /Sessions/Playing/Stopped", h.require(h.handlePlayingStopped))
+	mount("POST /Sessions/Playing", h.require(h.handlePlaying))
+	mount("POST /Sessions/Playing/Progress", h.require(h.handlePlayingProgress))
+	mount("POST /Sessions/Playing/Stopped", h.require(h.handlePlayingStopped))
 
 	// JSON 404s for the rest of each Jellyfin subtree. Without these an
 	// unknown /Items/... falls through to the SPA shell, because static.go's
@@ -104,8 +130,11 @@ func (h *Handlers) Register(mux *http.ServeMux) {
 	for _, prefix := range []string{
 		"/System/", "/Users/", "/Items/", "/Videos/", "/Audio/", "/Sessions/",
 	} {
-		mux.HandleFunc(prefix, h.handleNotFound)
+		mount(prefix, http.HandlerFunc(h.handleNotFound))
 	}
+	// One subtree covers the whole alias: nothing outside the Jellyfin API
+	// lives under /emby, so anything unmounted there is a 404, not the shell.
+	mux.HandleFunc(embyPrefix+"/", h.handleNotFound)
 }
 
 // serverID returns a stable, per-installation server id, created on first use

internal/jellyfin/jellyfin_test.go

@@ -433,3 +433,46 @@ func TestSystemInfoPublicNeedsNoAuth(t *testing.T) {
 		t.Errorf("incomplete discovery response: %s", body)
 	}
 }
+
+// ── the /emby alias ──────────────────────────────────────────────────────
+
+// Many Jellyfin apps probe GET /emby/System/Info/Public during handshake and
+// abandon discovery when it 404s, dropping the user on a manual "enter your
+// server URL" screen. The alias is the same handler behind a second path, so
+// the body must be identical - not merely similar.
+func TestEmbyPrefixMirrorsRoot(t *testing.T) {
+	f := newFixture(t)
+
+	rootResp, rootBody := f.do(t, "GET", "/System/Info/Public", "", "")
+	aliasResp, aliasBody := f.do(t, "GET", "/emby/System/Info/Public", "", "")
+	if aliasResp.StatusCode != rootResp.StatusCode {
+		t.Fatalf("alias status = %d, root = %d", aliasResp.StatusCode, rootResp.StatusCode)
+	}
+	if aliasBody != rootBody {
+		t.Errorf("alias body:\n%s\nroot body:\n%s", aliasBody, rootBody)
+	}
+
+	// The authenticated routes must be aliased too, not just the public one:
+	// discovery that succeeds and then a 401 on the first real call is the same
+	// broken handshake.
+	if resp, _ := f.do(t, "GET", "/emby/Users/Me", "X-Emby-Token", f.admin); resp.StatusCode != http.StatusOK {
+		t.Errorf("/emby/Users/Me status = %d, want 200", resp.StatusCode)
+	}
+}
+
+// An unknown /emby/... path must be JSON. static.go's isAPIPath knows nothing
+// about /emby, so without a 404 subtree here it would fall through to the SPA
+// shell and the client would try to parse index.html as a discovery response.
+func TestEmbyUnknownPathIsJSONNotShell(t *testing.T) {
+	f := newFixture(t)
+	for _, path := range []string{"/emby/Items/1/Nope", "/emby/Nope"} {
+		resp, body := f.do(t, "GET", path, "X-Emby-Token", f.admin)
+		if resp.StatusCode != http.StatusNotFound {
+			t.Errorf("%s: status = %d, want 404", path, resp.StatusCode)
+		}
+		var out map[string]any
+		if err := json.Unmarshal([]byte(body), &out); err != nil {
+			t.Errorf("%s: body is not JSON (%s): %s", path, resp.Header.Get("Content-Type"), body)
+		}
+	}
+}