diff --git a/CHANGELOG.md b/CHANGELOG.md
index 01349ff..8630924 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -16,6 +16,7 @@
 - **Chat font sizing**: `.message-text` set to `0.95rem` (slightly smaller than the default, larger than the `0.83rem` system/flash text).
 - **R2 retention by run count + size**: Playout now keeps the newest `R2_KEEP_RUNS` run folders (default 2) and enforces an `R2_MAX_RETENTION_BYTES` cap (default 10 GiB), replacing the old 24h time-based `R2_SEGMENT_RETENTION_HOURS` cleanup. Cleanup runs after each new run plus an hourly backstop, and never deletes the live manifest. Adds `linearvoid_playout_r2_runs_retained` and `linearvoid_playout_r2_bytes_retained` gauges.
 - **Cloudflare Workers frontend hosting**: Replaced Cloudflare Pages with a Cloudflare Worker (`linearvoid-frontend`) that serves the static SPA via Workers Static Assets. Added `frontend/wrangler.toml` and renamed the deploy workflow to `.gitea/workflows/workers-deploy.yml` (builds the SPA and runs `npx wrangler deploy`). Architecture, setup/deployment, and ops documentation updated from Pages to Workers.
+- **Viewer session cookie cross-site**: The `linearvoid_user_session` cookie now uses `SameSite=None; Secure` over HTTPS (so it is sent cross-site from the Workers-served SPA) and `SameSite=Lax` over plain HTTP (local dev). The admin session cookie remains `SameSite=Strict`.
 
 ### Fixed
 
diff --git a/backend/chat/src/auth/auth.go b/backend/chat/src/auth/auth.go
index 6a8884a..728edd1 100644
--- a/backend/chat/src/auth/auth.go
+++ b/backend/chat/src/auth/auth.go
@@ -88,29 +88,48 @@ func CookieSecure(r *http.Request) bool {
 	return r.TLS != nil
 }
 
-// SetCookie writes the admin session cookie.
-func SetCookie(w http.ResponseWriter, r *http.Request, cookieName, token string, expiresAt time.Time) {
+// SameSiteForUserCookie returns the SameSite policy for the viewer session cookie.
+// Over HTTPS it returns SameSite=None so the cookie is sent cross-site from the
+// Workers-served SPA; over plain HTTP (local dev) it falls back to Lax.
+func SameSiteForUserCookie(r *http.Request) http.SameSite {
+	if CookieSecure(r) {
+		return http.SameSiteNoneMode
+	}
+	return http.SameSiteLaxMode
+}
+
+// SetCookie writes a session cookie with the given SameSite policy.
+// SameSite=None is only valid together with Secure, which is enforced here.
+func SetCookie(w http.ResponseWriter, r *http.Request, cookieName, token string, expiresAt time.Time, sameSite http.SameSite) {
+	secure := CookieSecure(r)
+	if sameSite == http.SameSiteNoneMode {
+		secure = true
+	}
 	http.SetCookie(w, &http.Cookie{
 		Name:     cookieName,
 		Value:    token,
 		Path:     "/",
 		HttpOnly: true,
-		SameSite: http.SameSiteStrictMode,
-		Secure:   CookieSecure(r),
+		SameSite: sameSite,
+		Secure:   secure,
 		Expires:  expiresAt,
 		MaxAge:   int(time.Until(expiresAt).Seconds()),
 	})
 }
 
-// ClearCookie removes the admin session cookie.
-func ClearCookie(w http.ResponseWriter, r *http.Request, cookieName string) {
+// ClearCookie removes a session cookie with the given SameSite policy.
+func ClearCookie(w http.ResponseWriter, r *http.Request, cookieName string, sameSite http.SameSite) {
+	secure := CookieSecure(r)
+	if sameSite == http.SameSiteNoneMode {
+		secure = true
+	}
 	http.SetCookie(w, &http.Cookie{
 		Name:     cookieName,
 		Value:    "",
 		Path:     "/",
 		HttpOnly: true,
-		SameSite: http.SameSiteStrictMode,
-		Secure:   CookieSecure(r),
+		SameSite: sameSite,
+		Secure:   secure,
 		Expires:  time.Unix(0, 0),
 		MaxAge:   -1,
 	})
diff --git a/backend/chat/src/auth/auth_test.go b/backend/chat/src/auth/auth_test.go
index ec697e5..b8312a7 100644
--- a/backend/chat/src/auth/auth_test.go
+++ b/backend/chat/src/auth/auth_test.go
@@ -134,7 +134,7 @@ func TestSetCookie(t *testing.T) {
 	w := httptest.NewRecorder()
 
 	expiresAt := time.Now().Add(1 * time.Hour)
-	SetCookie(w, req, "test_cookie", "abc123", expiresAt)
+	SetCookie(w, req, "test_cookie", "abc123", expiresAt, http.SameSiteStrictMode)
 
 	cookies := w.Result().Cookies()
 	if len(cookies) != 1 {
@@ -165,7 +165,7 @@ func TestClearCookie(t *testing.T) {
 	req := httptest.NewRequest("GET", "/", nil)
 	w := httptest.NewRecorder()
 
-	ClearCookie(w, req, "test_cookie")
+	ClearCookie(w, req, "test_cookie", http.SameSiteStrictMode)
 
 	cookies := w.Result().Cookies()
 	if len(cookies) != 1 {
@@ -183,6 +183,38 @@ func TestClearCookie(t *testing.T) {
 	}
 }
 
+func TestSameSiteForUserCookie(t *testing.T) {
+	req := httptest.NewRequest("GET", "/", nil)
+	req.Header.Set("X-Forwarded-Proto", "https")
+	if got := SameSiteForUserCookie(req); got != http.SameSiteNoneMode {
+		t.Errorf("SameSiteForUserCookie(https) = %v, want None", got)
+	}
+
+	req2 := httptest.NewRequest("GET", "/", nil)
+	if got := SameSiteForUserCookie(req2); got != http.SameSiteLaxMode {
+		t.Errorf("SameSiteForUserCookie(http) = %v, want Lax", got)
+	}
+}
+
+func TestSetCookie_NoneForcesSecure(t *testing.T) {
+	req := httptest.NewRequest("GET", "/", nil)
+	w := httptest.NewRecorder()
+	expiresAt := time.Now().Add(1 * time.Hour)
+	SetCookie(w, req, "test_cookie", "abc123", expiresAt, http.SameSiteNoneMode)
+
+	cookies := w.Result().Cookies()
+	if len(cookies) != 1 {
+		t.Fatalf("expected 1 cookie, got %d", len(cookies))
+	}
+	c := cookies[0]
+	if c.SameSite != http.SameSiteNoneMode {
+		t.Errorf("cookie SameSite = %v, want None", c.SameSite)
+	}
+	if !c.Secure {
+		t.Error("cookie Secure = false, want true (SameSite=None requires Secure)")
+	}
+}
+
 func TestGetTokenFromCookie(t *testing.T) {
 	req := httptest.NewRequest("GET", "/", nil)
 	req.AddCookie(&http.Cookie{Name: "session", Value: "token123"})
diff --git a/backend/chat/src/handlers/handlers.go b/backend/chat/src/handlers/handlers.go
index 35a3251..f10b95d 100644
--- a/backend/chat/src/handlers/handlers.go
+++ b/backend/chat/src/handlers/handlers.go
@@ -291,7 +291,7 @@ func HandleAdminLogin(deps *Dependencies) http.HandlerFunc {
 
 		expiresAt := time.Now().Add(config.AdminSessionTTL())
 		deps.SessionStore.Add(token, expiresAt)
-		auth.SetCookie(w, r, config.AdminSessionCookieName, token, expiresAt)
+		auth.SetCookie(w, r, config.AdminSessionCookieName, token, expiresAt, http.SameSiteStrictMode)
 
 		w.Header().Set("Content-Type", "application/json")
 		json.NewEncoder(w).Encode(AdminSessionResponse{Authenticated: true, Username: config.AdminUsername()})
@@ -309,7 +309,7 @@ func HandleAdminLogout(deps *Dependencies) http.HandlerFunc {
 		if token, ok := auth.GetTokenFromCookie(r, config.AdminSessionCookieName); ok {
 			deps.SessionStore.Remove(token)
 		}
-		auth.ClearCookie(w, r, config.AdminSessionCookieName)
+		auth.ClearCookie(w, r, config.AdminSessionCookieName, http.SameSiteStrictMode)
 
 		w.Header().Set("Content-Type", "application/json")
 		json.NewEncoder(w).Encode(AdminSessionResponse{Authenticated: false})
@@ -368,7 +368,7 @@ func startUserSession(deps *Dependencies, w http.ResponseWriter, r *http.Request
 	}
 	expiresAt := time.Now().Add(config.UserSessionTTL())
 	deps.UserSessions.Add(token, username, expiresAt)
-	auth.SetCookie(w, r, userSessionCookieName, token, expiresAt)
+	auth.SetCookie(w, r, userSessionCookieName, token, expiresAt, auth.SameSiteForUserCookie(r))
 }
 
 // HandleUserRegister creates a new chat user and signs them in.
@@ -474,7 +474,7 @@ func HandleUserLogout(deps *Dependencies) http.HandlerFunc {
 		if token, ok := auth.GetTokenFromCookie(r, userSessionCookieName); ok {
 			deps.UserSessions.Remove(token)
 		}
-		auth.ClearCookie(w, r, userSessionCookieName)
+		auth.ClearCookie(w, r, userSessionCookieName, auth.SameSiteForUserCookie(r))
 		w.Header().Set("Content-Type", "application/json")
 		json.NewEncoder(w).Encode(map[string]bool{"ok": true})
 	}
diff --git a/docs/architecture/README.md b/docs/architecture/README.md
index c450387..85ae029 100644
--- a/docs/architecture/README.md
+++ b/docs/architecture/README.md
@@ -730,7 +730,7 @@ Security is split between edge controls (Cloudflare), reverse proxy controls (ng
 #### Viewer User Accounts
 
 - Register/login/logout via `POST /api/auth/register`, `POST /api/auth/login`, `POST /api/auth/logout`; current session via `GET /api/auth/session`.
-- Passwords are hashed with bcrypt; sessions use a `linearvoid_user_session` cookie with a 30-day default TTL (`USER_SESSION_TTL_HOURS`).
+- Passwords are hashed with bcrypt; sessions use a `linearvoid_user_session` cookie with a 30-day default TTL (`USER_SESSION_TTL_HOURS`). The cookie is `SameSite=None; Secure` over HTTPS (cross-site from the Workers SPA) and `SameSite=Lax` over plain HTTP (local dev).
 - Viewer sign-in UI (username/password) lives in `frontend/index.html`.
 
 #### Rate Limiting and Abuse Controls
@@ -742,7 +742,8 @@ Security is split between edge controls (Cloudflare), reverse proxy controls (ng
 #### CORS and Cross-Origin Behavior
 
 - HLS endpoints exposed by nginx include explicit `Access-Control-Allow-Origin: *` for player compatibility.
-- Admin/authenticated routes are expected to run same-origin behind nginx auth gates.
+- API routes (`/api/*`) include `Access-Control-Allow-Origin` (reflecting an allowlisted origin) and `Access-Control-Allow-Credentials: true` so the Workers-served SPA can call the app origin cross-origin.
+- The viewer session cookie (`linearvoid_user_session`) uses `SameSite=None; Secure` over HTTPS so it is sent cross-site; the admin session cookie remains `SameSite=Strict`.
 
 #### Moderation and Privacy