diff --git a/api/auth.py b/api/auth.py
index 6dbb79a3..5c89c4d0 100644
--- a/api/auth.py
+++ b/api/auth.py
@@ -685,6 +685,46 @@ def parse_cookie(handler) -> str | None:
     return morsel.value if morsel else None
 
 
+def _safe_login_inner_next(query: str | None) -> str:
+    """#5578: extract a SAFE, non-login inner redirect from a login page's query.
+
+    When an expired-auth bounce lands back on the login page (which already
+    carries its own `next` in the query), we want to preserve a legitimate inner
+    destination X across the redirect to the real login route — but only if X is
+    itself safe (path-absolute, not protocol-relative/backslash, no control
+    chars) AND not login-shaped / not itself carrying a nested next param.
+    Anything else collapses to '' (no inner redirect), which kills the
+    self-referential chain. Mirrors _safe_login_redirect_path().
+    """
+    import urllib.parse as _u
+    raw = _u.parse_qs(query or "").get("next", [""])[0]
+    path = str(raw or "").strip()
+    if not path or path[0] != "/" or path[1:2] in {"/", "\\"}:
+        return ""
+    if re.search(r"[\x00-\x1f\x7f\s]", path) or len(path) > 2048:
+        return ""
+    # Collapse only login-route chains — decode a few levels so a nested
+    # `/session/login%3Fnext%3D...` (encoded `?`) is still recognized by its
+    # leading PATH — but preserve a legitimate non-login inner path that merely
+    # carries its own `next=` query key (e.g. `/admin?next=/real/path`).
+    _probe = path
+    for _ in range(8):
+        _p = _probe.split("?", 1)[0].split("#", 1)[0].split("&", 1)[0].rstrip("/")
+        if _p == "/login" or _p.endswith("/login"):
+            return ""
+        _decoded = _u.unquote(_probe)
+        if _decoded == _probe:
+            break
+        _probe = _decoded
+    else:
+        # Still decoding at the cap (pathologically deep encoding) → fail closed.
+        _p = _probe.split("?", 1)[0].split("#", 1)[0].split("&", 1)[0].rstrip("/")
+        if _p == "/login" or _p.endswith("/login"):
+            return ""
+        return ""
+    return path
+
+
 def check_auth(handler, parsed) -> bool:
     """Check if request is authorized. Returns True if OK.
     If not authorized, sends 401 (API) or 302 redirect (page) and returns False."""
@@ -727,6 +767,35 @@ def check_auth(handler, parsed) -> bool:
         # the full original URL (the browser auto-decodes once).
         # (Opus pre-release advisor finding for v0.50.258.)
         import urllib.parse as _urlparse
+        # #5578: if the page being redirected is ALREADY login-shaped, do NOT
+        # wrap its full `path?query` into a fresh `next=` — that query already
+        # carries a `next=`, so quoting the whole thing nests the login URL into
+        # itself and re-encodes it on every expired-auth bounce, exploding the
+        # URL until the tab breaks. This guard runs in check_auth() (BEFORE
+        # route handling), the actual source of the server-side loop.
+        #
+        # The login page is served ONLY at the public `/login` route (see
+        # PUBLIC_PATHS + the routes.py `/login` handler); the app's client route
+        # `/session/login` is NOT public, so a bare relative `login` from
+        # `/session/login` resolves to `/session/login` again and re-triggers
+        # check_auth() — an infinite redirect. Resolve to the real login route
+        # with `../login`, which lands on `/login` from a `/session/*` scope and
+        # on `<mount>/login` under a subpath mount (verified via urljoin). Carry
+        # through only a validated, non-login inner `next` so a legitimate
+        # post-login destination still survives a bounce that happened to land
+        # on the login page.
+        _login_path = (parsed.path or '/').rstrip('/')
+        if _login_path == '/login' or _login_path.endswith('/login'):
+            # /login itself is public → check_auth never redirects it; this only
+            # fires for the non-public client login route (e.g. /session/login).
+            _target = '../login' if '/' in _login_path.lstrip('/') else 'login'
+            _inner = _safe_login_inner_next(parsed.query)
+            if _inner:
+                _target += '?next=' + _urlparse.quote(_inner, safe='/')
+            handler.send_header('Location', _target)
+            handler.send_header('Content-Length', '0')
+            handler.end_headers()
+            return False
         _path_with_query = parsed.path or '/'
         if parsed.query:
             _path_with_query += '?' + parsed.query
diff --git a/api/routes.py b/api/routes.py
index 13ced3bf..c977e2dc 100644
--- a/api/routes.py
+++ b/api/routes.py
@@ -31,7 +31,7 @@ import socket as _socket
 from collections import defaultdict
 from pathlib import Path
 from contextlib import closing
-from urllib.parse import parse_qs, quote, urljoin, urlsplit
+from urllib.parse import parse_qs, quote, unquote, urljoin, urlsplit
 from urllib.error import HTTPError, URLError
 from urllib.request import HTTPRedirectHandler, HTTPSHandler, ProxyHandler, Request, build_opener
 from api.agent_sessions import (
@@ -9238,6 +9238,37 @@ def _safe_login_redirect_path(raw_path: str | None) -> str:
         return "/"
     if re.search(r"[\x00-\x1f\x7f\s]", path):
         return "/"
+    # #5578: reject a `next` that points back at the login page, so an
+    # expired-auth bounce on the login page can't feed the redirect its own
+    # address and grow the URL exponentially. Length cap is belt-and-suspenders:
+    # a legitimate app path is never this long.
+    if len(path) > 2048:
+        return "/"
+    # Detect a login-route target even through nested percent-encoding: a nested
+    # login-redirect chain looks like `/session/login%3Fnext%3D...`, where the
+    # `?` separating the path from the query is itself encoded, so a plain
+    # split("?") wouldn't isolate the real path. Fully decode (bounded) and check
+    # the leading PATH of EVERY decode level, including the final fully-decoded
+    # form. Only collapse login-route chains — a legitimate non-login path that
+    # merely carries its own `next=` query key (e.g. `/admin?action=foo&next=/x`)
+    # must still round-trip (regression guarded by test_v050258_opus_followups.py).
+    _probe = path
+    for _ in range(8):
+        _path_only = _probe.split("?", 1)[0].split("#", 1)[0].split("&", 1)[0].rstrip("/")
+        if _path_only.endswith("/login") or _path_only == "/login":
+            return "/"
+        _decoded = unquote(_probe)
+        if _decoded == _probe:
+            break
+        _probe = _decoded
+    else:
+        # Loop exhausted the cap while STILL decoding (pathologically deep
+        # encoding): check the final decoded form too, then fail closed — an
+        # 8-level-deep encoded value is never a legitimate redirect.
+        _path_only = _probe.split("?", 1)[0].split("#", 1)[0].split("&", 1)[0].rstrip("/")
+        if _path_only.endswith("/login") or _path_only == "/login":
+            return "/"
+        return "/"
     return path
 
 
diff --git a/static/boot.js b/static/boot.js
index 3bee6262..7b2ccc8d 100644
--- a/static/boot.js
+++ b/static/boot.js
@@ -3071,6 +3071,10 @@ window._mirrorSpeechSettingsFromServer=_mirrorSpeechSettingsFromServer;
       return true;
     };
     const redirectToLogin=(nextUrl)=>{
+      // #5578: never nest the login URL into its own next= — if already on a
+      // login-shaped page, reload 'login' bare (the page keeps its inner next).
+      const _p=(window.location.pathname||'').replace(/\/+$/,'');
+      if(/(?:^|\/)login$/.test(_p)){window.location.href='login';return;}
       window.location.href='login?next='+encodeURIComponent(nextUrl);
     };
     return {
diff --git a/static/login.js b/static/login.js
index 9a7cdbe2..4e0c9829 100644
--- a/static/login.js
+++ b/static/login.js
@@ -32,6 +32,27 @@ document.addEventListener('DOMContentLoaded', function () {
       if (raw.charAt(0) !== '/') return './';             // must be path-absolute
       if (raw.charAt(1) === '/' || raw.charAt(1) === '\\') return './'; // reject // and \\
       if (/[\x00-\x1f\x7f\s]/.test(raw)) return './';  // reject control chars / whitespace
+      // #5578: never redirect back to the login page — that self-referential
+      // chain is what grows the URL exponentially on repeated expired-auth
+      // bounces. Detect the login route even through nested percent-encoding
+      // (a nested chain looks like `/session/login%3Fnext%3D...`, where the `?`
+      // is encoded so a plain split('?') wouldn't isolate the path). Decode a
+      // few levels and check the leading PATH. Only collapse login-route chains
+      // — a legitimate non-login path that merely carries its own `next=` query
+      // key must still round-trip.
+      if (raw.length > 2048) return './';
+      var probe = raw;
+      var stabilized = false;
+      for (var i = 0; i < 8; i++) {
+        var pathOnly = probe.split('?')[0].split('#')[0].split('&')[0].replace(/\/+$/, '');
+        if (pathOnly === '/login' || /\/login$/.test(pathOnly)) return './';
+        var decoded;
+        try { decoded = decodeURIComponent(probe); } catch (_) { stabilized = true; break; }
+        if (decoded === probe) { stabilized = true; break; }
+        probe = decoded;
+      }
+      // If still decoding at the cap (pathologically deep encoding), fail closed.
+      if (!stabilized) return './';
       return raw;
     } catch (_) { return './'; }
   }
diff --git a/static/ui.js b/static/ui.js
index a6bbf9fc..61960aed 100644
--- a/static/ui.js
+++ b/static/ui.js
@@ -201,7 +201,8 @@ else initOfflineMonitor();
 // Redirect to login when the server responds with 401 (auth session expired).
 // Handles iOS PWA standalone mode and keeps subpath mounts like /hermes/ from
 // escaping to the personal site root /login.
-function _redirectIfUnauth(res){if(res&&res.status===401){window.location.href='login?next='+encodeURIComponent(window.location.pathname+window.location.search);return true;}return false;}
+// #5578: on a login-shaped page, reload 'login' WITHOUT a next (avoid self-nesting).
+function _redirectIfUnauth(res){if(res&&res.status===401){var _p=(window.location.pathname||'').replace(/\/+$/,'');if(/(?:^|\/)login$/.test(_p)){window.location.href='login';}else{window.location.href='login?next='+encodeURIComponent(window.location.pathname+window.location.search);}return true;}return false;}
 function _getSessionQueue(sid, create=false){
   if(!sid) return [];
   if(!SESSION_QUEUES[sid]&&create) SESSION_QUEUES[sid]=[];
diff --git a/static/workspace.js b/static/workspace.js
index 296d6be7..50151c02 100644
--- a/static/workspace.js
+++ b/static/workspace.js
@@ -46,7 +46,21 @@ async function api(path,opts={}){
           // re-authenticate. This is especially important for iOS PWA (standalone mode)
           // and for subpath mounts like /hermes/, where /login escapes to the site root.
           if(res.status===401){
-            if(redirect401) window.location.href='login?next='+encodeURIComponent(window.location.pathname+window.location.search);
+            // #5578: if we're ALREADY on the login page, appending
+            // window.location.pathname+search (which contains ?next=…) into a
+            // fresh next= wraps the login URL into itself and re-encodes it —
+            // exponential URL growth on each expired-auth bounce until the tab
+            // breaks. On the login page, just reload login WITHOUT a next (the
+            // page preserves its own inner next); elsewhere, capture the path.
+            if(redirect401){
+              // Already on the login page? Reload login WITHOUT a next.
+              const _p=(window.location.pathname||'').replace(/\/+$/,'');
+              if(/(?:^|\/)login$/.test(_p)){
+                window.location.href='login';
+              }else{
+                window.location.href='login?next='+encodeURIComponent(window.location.pathname+window.location.search);
+              }
+            }
             // Callers can opt out of navigation and handle the unauthenticated state themselves.
             return;
           }
diff --git a/tests/test_issue5578_login_next_nesting.py b/tests/test_issue5578_login_next_nesting.py
new file mode 100644
index 00000000..981fb3a8
--- /dev/null
+++ b/tests/test_issue5578_login_next_nesting.py
@@ -0,0 +1,150 @@
+"""Regression test for #5578 — login `next=` self-nesting URL explosion.
+
+Bug: an expired-auth 401 while already on `/session/login?next=…` fed the login
+redirect its own URL. Each of the three guards (workspace.js api() 401 handler,
+login.js `_safeNextPath`, routes.py `_safe_login_redirect_path`) validated against
+open-redirect but NONE rejected a `next` pointing back at the login page, so the
+nested `/session/login?next=/session/login%3Fnext%3D…` chain re-percent-encoded
+and grew exponentially on every bounce until the tab broke (~12k chars).
+
+Fix: all three guards now reject a `next` that targets the login page or already
+carries a nested `next=`, plus a length cap — while preserving legitimate
+`next=/some/real/path` redirects and the existing open-redirect protections.
+"""
+from pathlib import Path
+
+from api.routes import _safe_login_redirect_path as guard
+
+ROOT = Path(__file__).resolve().parents[1]
+LOGIN_JS = (ROOT / "static" / "login.js").read_text(encoding="utf-8")
+WORKSPACE_JS = (ROOT / "static" / "workspace.js").read_text(encoding="utf-8")
+
+
+# ── server guard: the self-nesting cases the bug exploited ──────────────────
+
+class TestServerGuardRejectsNesting:
+    def test_rejects_login_self_reference(self):
+        assert guard("/login") == "/"
+        assert guard("/session/login") == "/"
+        assert guard("/session/login/") == "/"
+        assert guard("/hermes/session/login") == "/"  # subpath mount
+
+    def test_rejects_nested_next_chain(self):
+        nested = "/session/login?next=/session/login%3Fnext%3D/session/login"
+        assert guard(nested) == "/"
+
+    def test_rejects_encoded_login_chain_at_any_depth(self):
+        # A nested LOGIN chain must collapse even when the `?` separating path
+        # from query is percent-encoded at any depth (that is the exponential
+        # explosion signature). Detection is by the decoded leading PATH.
+        assert guard("/session/login%3Fnext%3D/y") == "/"
+        assert guard("/session/login%253Fnext%253D/y") == "/"
+        assert guard("/hermes/session/login%3Fnext%3D/y") == "/"
+
+    def test_rejects_deeply_encoded_login_chain(self):
+        # #5579 gate: a 6+-level percent-encoded login chain must still collapse —
+        # the decode loop checks the leading path at EVERY level (incl. the final
+        # decoded form), so a login route hidden behind deep encoding is caught.
+        # /session/login with the '?' encoded 6 times: %25252525253F
+        assert guard("/session/login%25252525253Fnext%25252525253D/y") == "/"
+
+    def test_preserves_non_login_path_carrying_its_own_next_key(self):
+        # A legitimate NON-login destination that merely carries its own `next=`
+        # query key must still round-trip — only login-route chains collapse.
+        # (Regression guarded by test_v050258_opus_followups.py; #5579 gate.)
+        assert guard("/x?next=/y") == "/x?next=/y"
+        assert guard("/admin?action=foo&next=/real/path") == "/admin?action=foo&next=/real/path"
+
+    def test_rejects_overlong_next(self):
+        assert guard("/" + "a" * 3000) == "/"
+
+    def test_the_exact_12k_explosion_collapses(self):
+        # Reconstruct the reported exponential chain; the guard must collapse it.
+        enc = "%3Fnext%3D"
+        blown = "/session/login" + (enc + "/session/login") * 40
+        assert len(blown) > 500
+        assert guard(blown) == "/"
+
+
+class TestServerGuardPreservesLegitimateRedirects:
+    def test_preserves_real_session_path(self):
+        assert guard("/session/abc123") == "/session/abc123"
+
+    def test_preserves_root_and_plain_paths(self):
+        assert guard("/") == "/"
+        assert guard("/workspace") == "/workspace"
+        assert guard("/session/xyz?tab=files") == "/session/xyz?tab=files"
+
+    def test_still_rejects_open_redirect_classics(self):
+        # The pre-existing protections must remain intact.
+        assert guard("//evil.example") == "/"
+        assert guard("/\\evil") == "/"
+        assert guard("https://evil.example") == "/"
+        assert guard("/x\x00y") == "/"
+        assert guard("") == "/"
+        assert guard(None) == "/"
+
+
+# ── client guards: static wiring (both JS sites carry the self-ref guard) ───
+
+class TestClientGuardsWired:
+    def test_login_js_rejects_login_self_and_nested_next(self):
+        # _safeNextPath must carry the login-route + length guards and detect the
+        # login route through nested percent-encoding via bounded decodeURIComponent.
+        assert "/login$/.test(pathOnly)" in LOGIN_JS or "/login$/" in LOGIN_JS
+        assert "decodeURIComponent" in LOGIN_JS
+        assert "2048" in LOGIN_JS
+
+    def test_workspace_js_skips_next_on_login_page(self):
+        # The 401 handler must NOT append the whole login URL as next when it's
+        # already on the login page (the recursion source).
+        assert "login$/.test(_p)" in WORKSPACE_JS
+        assert "window.location.href='login';" in WORKSPACE_JS
+        # And the non-login path still captures the real destination.
+        assert "'login?next='+encodeURIComponent" in WORKSPACE_JS
+
+    def test_all_client_401_helpers_guard_login_page(self):
+        # #5578 Codex round-2: workspace.js was fixed but two more client 401
+        # redirect helpers (ui.js _redirectIfUnauth, boot.js redirectToLogin)
+        # also nested the login URL. All three must carry the on-login guard.
+        UI_JS = (ROOT / "static" / "ui.js").read_text(encoding="utf-8")
+        BOOT_JS = (ROOT / "static" / "boot.js").read_text(encoding="utf-8")
+        assert "login$/.test(_p)" in UI_JS, "ui.js _redirectIfUnauth must guard the login page"
+        assert "login$/.test(_p)" in BOOT_JS, "boot.js redirectToLogin must guard the login page"
+
+
+class TestServerCheckAuthGuard:
+    def test_check_auth_does_not_nest_login_redirect(self):
+        # #5578 Codex round-2 (BRICK): check_auth() runs BEFORE route handling,
+        # so the server-side page-redirect loop never reached
+        # _safe_login_redirect_path(). check_auth() must itself refuse to wrap a
+        # login-shaped path into a fresh next=.
+        AUTH_PY = (ROOT / "api" / "auth.py").read_text(encoding="utf-8")
+        assert "endswith('/login')" in AUTH_PY or "_login_path.endswith('/login')" in AUTH_PY, (
+            "check_auth() must detect a login-shaped path and skip the next= wrap"
+        )
+
+    def test_check_auth_resolves_to_real_public_login_route(self):
+        # #5578 Codex round-3 (BRICK): a bare relative 'login' from /session/login
+        # resolves BACK to /session/login (not public, not the /login route) and
+        # loops forever. Must use '../login' so it lands on the real public
+        # /login route (and <mount>/login under a subpath mount).
+        AUTH_PY = (ROOT / "api" / "auth.py").read_text(encoding="utf-8")
+        assert "'../login'" in AUTH_PY, (
+            "check_auth() must redirect a session-scoped login path to ../login "
+            "(the real public /login), not a bare relative login that loops"
+        )
+
+    def test_inner_next_helper_drops_login_keeps_safe_nonlogin(self):
+        # The preserved inner next must be safe + non-login. A nested LOGIN chain
+        # (even with an encoded `?`) collapses; a legit non-login path that
+        # carries its own `next=` key round-trips (#5579 gate — no over-collapse).
+        import api.auth as auth
+        assert auth._safe_login_inner_next("next=/session/login") == ""
+        assert auth._safe_login_inner_next("next=/session/login%3Fnext%3D/y") == ""
+        assert auth._safe_login_inner_next("next=//evil") == ""
+        assert auth._safe_login_inner_next("next=/session/abc123") == "/session/abc123"
+        # parse_qs decodes the value: /x%3Fnext%3D/y -> /x?next=/y, a legit
+        # non-login path with its own next key -> preserved (round-trips).
+        assert auth._safe_login_inner_next("next=/x%3Fnext%3D/y") == "/x?next=/y"
+        assert auth._safe_login_inner_next("") == ""