diff --git a/app/controllers/levelcode/web_controller.rb b/app/controllers/levelcode/web_controller.rb
index a1bcc571..4490977a 100644
--- a/app/controllers/levelcode/web_controller.rb
+++ b/app/controllers/levelcode/web_controller.rb
@@ -40,6 +40,8 @@ class WebController < ApplicationController
     # be written in: RFC 3986's path and query characters and well-formed percent escapes (#safe_next).
     NEXT_MAX = 512
     NEXT_CHARS = %r{\A(?:[A-Za-z0-9\-._~!$&'()*+,;=:@/?]|%\h\h)+\z}
+    # What this app serves under /ai/ itself (config/routes.rb, the "ai" scope): never a page to land on.
+    NEXT_SERVER_NAMES = %w[auth authorize_editor checkout billing signout csrf].freeze
 
     # POST /ai/auth/email  (JSON) — passwordless step 1: email a 6-digit code.
     def email_request
@@ -266,6 +268,14 @@ def deliver_signed_in(user)
       end
     end
 
+    # Is this first segment after /ai/ one of this app's own endpoints, as written or decoded? A name is read as the
+    # router would: in lower case, up to the first character that is not a letter, digit or underscore (`csrf.json`
+    # and `csrf;x` are `csrf`). Read both ways, `%63srf` and `checkout%A9` are endpoints too. The SPA's isServerName.
+    def next_server_name?(segment)
+      name = ->(s) { s.scrub.downcase[/\A[a-z0-9_]*/] }
+      NEXT_SERVER_NAMES.include?(name.(segment)) || NEXT_SERVER_NAMES.include?(name.(URI.decode_uri_component(segment)))
+    end
+
     # The login page again after a failed sign-in, still bringing the visitor to the page they asked for.
     def login_again(error)
       query = { error: error }
@@ -300,8 +310,9 @@ def web_editor_home
     # SPA's safeNext (src/levelcode/next.ts in onetime): a page under /ai/, written only in the characters a URL
     # path and query allow (so no blank, control character, backslash, fragment or non-ASCII), with a query kept
     # (`/ai/pricing?plan=pro`). Refused: `//` (a host to a browser), a percent-encoded slash or backslash, a dot
-    # segment, plain or encoded (`/ai/%2e%2e/x` is /x to a browser), and the server's own endpoints under /ai/auth/
-    # and /ai/csrf, which are not pages. It is only ever a same-site path, so it takes the visitor nowhere this
+    # segment, plain or encoded (`/ai/%2e%2e/x` is /x to a browser), and the server's own endpoints
+    # (NEXT_SERVER_NAMES), which are not pages, in every spelling the router answers: /ai/csrf.json, /ai/csrf/,
+    # /ai/%63srf, /ai/CSRF, /ai/auth/…. It is only ever a same-site path, so it takes the visitor nowhere this
     # site's own links could not.
     def safe_next(raw)
       return nil unless raw.is_a?(String) && raw.length.between?(5, NEXT_MAX) && raw.start_with?("/ai/")
@@ -310,7 +321,7 @@ def safe_next(raw)
 
       path = raw.split("?", 2).first
       return nil if path.split("/").any? { |segment| %w[. ..].include?(segment.gsub(/%2e/i, ".")) }
-      return nil if path.start_with?("/ai/auth/") || path == "/ai/csrf"
+      return nil if next_server_name?(path.split("/")[2].to_s)
 
       uri = URI.parse(raw)
       return nil if uri.scheme || uri.host || uri.userinfo
diff --git a/spec/requests/levelcode/web_spec.rb b/spec/requests/levelcode/web_spec.rb
index 3223195d..94dad68b 100644
--- a/spec/requests/levelcode/web_spec.rb
+++ b/spec/requests/levelcode/web_spec.rb
@@ -543,7 +543,10 @@ def verify(extra = {})
        '/ai/../users/edit', '/ai/%2e%2e/users/edit', '/ai/%2E./x', '/ai/.%2e/users/edit', '/ai/%2e./users/edit', '/ai/%2e/x',
        '/ai/docs/%2e%2e/%2e%2e/users/edit', '/other/path', 'javascript:alert(1)', '/ai/x#y',
        "/ai/x\nSet-Cookie:a=b", '/ai/' + ('a' * 600), '/ai/caf' + [ 0xe9 ].pack('U') + '/x', '/ai/<script>', '/ai/x%zz',
-       '/ai/auth/handoff?code=attacker', '/ai/auth/oauth/github', '/ai/csrf' ].each_with_index do |bad, i|
+       '/ai/auth/handoff?code=attacker', '/ai/auth/oauth/github', '/ai/csrf',
+       # an endpoint in every spelling the router answers: a format suffix, a trailing slash, an encoded letter, capitals
+       '/ai/csrf.json', '/ai/csrf/', '/ai/%63srf', '/ai/CSRF', '/ai/auth.json/handoff', '/ai/%61uth/handoff?code=attacker',
+       '/ai/authorize_editor', '/ai/checkout', '/ai/billing', '/ai/signout', '/ai/checkout%A9' ].each_with_index do |bad, i|
         post '/ai/auth/verify', params: { email: "bad-next-#{i}@example.com", code: '123456', next: bad }
         expect(json).to eq('redirect' => editor_home), bad.inspect
       end
@@ -554,6 +557,14 @@ def verify(extra = {})
       expect(json).to eq('redirect' => '/ai/docs/a.%2e')
     end
 
+    it 'takes a page whose name only starts like an endpoint\'s, or has an endpoint\'s name further down' do
+      verify(next: '/ai/csrfx')
+      expect(json).to eq('redirect' => '/ai/csrfx')
+
+      post '/ai/auth/verify', params: { email: 'lands-2@example.com', code: '123456', next: '/ai/docs/auth' }
+      expect(json).to eq('redirect' => '/ai/docs/auth')
+    end
+
     it 'leaves an editor that is signing in alone: it gets its code, and `next` is not looked at' do
       with_web_editor('https://app.levelcode.test', url: 'https://app.levelcode.test/')
       allow(Levelcode::OneTimeCode).to receive(:issue).and_return('one-time-code')