diff --git a/.gitea/workflows/deploy-web.yml b/.gitea/workflows/deploy-web.yml
index cb30848..f11397e 100644
--- a/.gitea/workflows/deploy-web.yml
+++ b/.gitea/workflows/deploy-web.yml
@@ -80,7 +80,7 @@ jobs:
         run: |
           # the agent window's default endpoint: the site's relay to B4L's
           # inference server (deploy/Caddyfile, handle_path /llm/*)
-          python3 tools/build_web.py --agent-url /llm/v1
+          python3 tools/build_web.py --agent-url /llm/v1 --agent-upstream https://inference.p0.contact/v1
           echo "$GITHUB_SHA" > dist/web/version.txt
 
       - name: Set up SSH
diff --git a/studio/agent.js b/studio/agent.js
index 851c5d8..971f8cb 100644
--- a/studio/agent.js
+++ b/studio/agent.js
@@ -6,8 +6,11 @@
  * Under `isokit studio` the model is reached through the Studio server
  * (/api/agent/*, crates/isokit-cli/src/agent.rs): the key stays on the machine
  * and the endpoint needs no CORS. The web Studio calls the endpoint from the
- * page with a key kept in this browser; a build can name a same-site relay as
- * the default endpoint (<meta name="isokit-agent" content="/llm/v1">).
+ * page with a key kept in this browser. Most inference servers refuse calls
+ * from web pages on other sites (CORS), so a build can name a same-site relay
+ * (<meta name="isokit-agent" content="/llm/v1" data-upstream="https://…/v1">):
+ * it is the default endpoint, and the upstream URL, when entered, goes
+ * through it too.
  */
 (function () {
   "use strict";
@@ -29,7 +32,15 @@
   function saveUi() { lsSet(LS_UI, A.ui); }
 
   // ------------------------------------------------------------ settings
-  function siteDefault() { var m = document.querySelector('meta[name="isokit-agent"]'); return (m && m.getAttribute("content")) || DEFAULT_URL; }
+  function norm(u) { return String(u || "").trim().replace(/\/+$/, ""); }
+  /** This site's relay to an endpoint elsewhere (build_web.py --agent-url), or null. */
+  function relay() {
+    var m = document.querySelector('meta[name="isokit-agent"]');
+    return m && m.getAttribute("content") ? { url: norm(m.getAttribute("content")), upstream: norm(m.getAttribute("data-upstream") || DEFAULT_URL) } : null;
+  }
+  function siteDefault() { var r = relay(); return r ? r.url : DEFAULT_URL; }
+  /** Same as the site's default (the relay, or the endpoint it relays)? */
+  function isDefault(u) { var r = relay(); u = norm(u); return u === norm(siteDefault()) || (!!r && u === r.upstream); }
   function loadCfg() {
     if (!web()) return fetch("/api/agent/config", { cache: "no-store" }).then(json).then(function (c) { A.cfg = c; return c; });
     var c = lsGet(LS_CFG) || {};
@@ -44,12 +55,24 @@
     }
     var c = lsGet(LS_CFG) || {};
     ["url", "model", "key", "think"].forEach(function (k) { if (v[k] !== undefined) c[k] = typeof v[k] === "string" ? v[k].trim() : v[k]; });
-    if (c.url === siteDefault()) delete c.url; // follow the site's default when it moves
+    if (c.url !== undefined && isDefault(c.url)) delete c.url; // follow the site's default when it moves
     lsSet(LS_CFG, c);
     return loadCfg();
   }
-  function baseUrl() { return new URL(A.cfg.url.replace(/\/+$/, ""), location.href).href; }
+  /** The URL the page calls: the relay stands in for the endpoint it relays. */
+  function baseUrl() {
+    var r = relay(), u = norm(A.cfg.url);
+    if (r && u === r.upstream) u = r.url;
+    return new URL(u, location.href).href;
+  }
   function host() { try { var u = new URL(baseUrl()); return u.host === location.host ? "this site's relay" : u.host; } catch (e) { return A.cfg.url; } }
+  function unreachable(e) {
+    var u = baseUrl(), r = relay(), why = " (" + (e && e.message || e) + ").";
+    if (new URL(u).origin === location.origin) return "The browser could not reach this site's relay " + u + why + " The relay or the inference server behind it is down.";
+    return "The browser could not reach " + u + why + " Web pages may only call a server on another site when it allows them (CORS, including the preflight request, which carries no key); most inference servers do not. " +
+      (r ? "Set the endpoint to " + r.url + ": this site relays that to " + r.upstream + "."
+         : "Use the agent in `isokit studio` (it passes the calls on itself) or on https://isokit.buildfor.life (it relays to B4L's inference server).");
+  }
   function model() { return A.cfg.model || A.models[0] || ""; }
 
   // ------------------------------------------------------------ the model's endpoint
@@ -60,7 +83,7 @@
     if (A.cfg.key) o.headers.Authorization = "Bearer " + A.cfg.key;
     return fetch(baseUrl() + path, o).catch(function (e) {
       if (e && e.name === "AbortError") throw e;
-      throw new Error("The browser could not reach " + baseUrl() + " (" + (e && e.message || e) + "). An endpoint on another site has to allow this page (CORS, including the preflight request, which carries no key), or use a relay on this site.");
+      throw new Error(unreachable(e));
     });
   }
   function httpError(r) {
@@ -465,7 +488,7 @@
       names.map(function (m) { return "<option" + (m === c.model ? " selected" : "") + ">" + esc(m) + "</option>"; }).join("");
     $("agthink").checked = c.think !== false;
     $("agsetnote").innerHTML = web()
-      ? "Kept in this browser only." + (siteDefault() !== DEFAULT_URL ? " Default endpoint: " + esc(siteDefault()) + " (this site's relay to B4L's inference server)." : "")
+      ? "Kept in this browser only." + (relay() ? " " + esc(relay().url) + " is this site's relay to " + esc(relay().upstream) + " (entering that URL uses the relay too)." : "")
       : "Kept by the Studio server" + (c.file ? " in <code>" + esc(c.file) + "</code>" : "") + "; the key never reaches this page.";
     setMsg(A.modelsErr ? A.modelsErr : A.models.length ? "Connected: " + A.models.length + " model" + (A.models.length === 1 ? "" : "s") : "", !!A.modelsErr);
     f.hidden = false;
diff --git a/tools/build_web.py b/tools/build_web.py
index 0257848..a17ac7b 100644
--- a/tools/build_web.py
+++ b/tools/build_web.py
@@ -113,7 +113,9 @@ def main():
     ap = argparse.ArgumentParser(description=__doc__, formatter_class=argparse.RawDescriptionHelpFormatter)
     ap.add_argument("--out", default=str(ROOT / "dist" / "web"))
     ap.add_argument("--skip-cargo", action="store_true", help="reuse the last WASM build (front-end changes only)")
-    ap.add_argument("--agent-url", help="default endpoint of the agent window, e.g. a relay on the same site (/llm/v1)")
+    ap.add_argument("--agent-url", help="default endpoint of the agent window: a relay on the same site (/llm/v1)")
+    ap.add_argument("--agent-upstream", default="https://inference.p0.contact/v1",
+                    help="the endpoint that --agent-url relays to; entered in the window, it goes through the relay")
     a = ap.parse_args()
     out = Path(a.out).resolve()
     web = out / "assets" / "web"
@@ -157,7 +159,8 @@ def main():
     html = replace_once(html, '<title>isokit Studio</title>',
                         '<title>isokit Studio</title>\n<meta name="description" content="isokit Studio: edit isometric site plans in the browser.">'
                         + (f'\n<meta name="isokit-build" content="{build}">' if build else "")
-                        + (f'\n<meta name="isokit-agent" content="{html_attr(a.agent_url)}">' if a.agent_url else ""))
+                        + (f'\n<meta name="isokit-agent" content="{html_attr(a.agent_url)}" data-upstream="{html_attr(a.agent_upstream)}">'
+                           if a.agent_url else ""))
     html = replace_once(html, 'href="/assets/studio/favicon.svg"', f'href="assets/studio/favicon.svg?{v}"')
     html = replace_once(html, 'href="/assets/viewer/isokit-view.css"', f'href="assets/viewer/isokit-view.css?{v}"')
     html = replace_once(html, '<link rel="stylesheet" href="/assets/theme.css">', '<style id="iso-theme"></style>')
diff --git a/tools/check_web.py b/tools/check_web.py
index 5fb66c2..9beff62 100644
--- a/tools/check_web.py
+++ b/tools/check_web.py
@@ -196,6 +196,34 @@ try:
         stored = pg.evaluate("() => JSON.parse(localStorage.getItem('isokit-agent'))")
         check(stored.get("key") == "test-key" and not stored.get("model"), f"settings stay in this browser {stored}")
 
+        # a site relay (here: the mock) stands in for its upstream, which pages may not call directly
+        upstream = "https://inference.p0.contact/v1"
+        pg.evaluate("""([u, up]) => { const m = document.createElement('meta'); m.name = 'isokit-agent'; m.content = u;
+            m.setAttribute('data-upstream', up); document.head.appendChild(m); }""", [agent_checks.MOCK + "/v1", upstream])
+        pg.locator("#agent").click()
+        pg.locator("#agset").click()
+        pg.locator("#agurl").fill(upstream + "/")
+        n_chat = len(agent_checks.chats())
+        pg.locator('#agsetf button[type="submit"]').click()
+        pg.wait_for_function("() => document.querySelector('#agwho .ag-on')", timeout=5000)
+        check(pg.evaluate("() => JSON.parse(localStorage.getItem('isokit-agent')).url") is None,
+              "entering the relayed endpoint keeps following the site's default")
+        pg.locator("#agtext").fill("hello via the relay")
+        pg.locator("#agtext").press("Enter")
+        pg.wait_for_function("() => !document.querySelector('.ag-busy')", timeout=10000)
+        check(len(agent_checks.chats()) == n_chat + 1, "the relayed endpoint is called through the relay")
+        # no relay for another site: the message says what to do
+        pg.evaluate("() => document.querySelector('meta[name=isokit-agent]').remove()")
+        pg.locator("#agset").click() if pg.locator("#agsetf").is_hidden() else None
+        pg.locator("#agurl").fill("http://127.0.0.1:7939/v1")
+        pg.locator('#agsetf button[type="submit"]').click()
+        pg.wait_for_function("() => /could not reach/.test(document.getElementById('agsetmsg').textContent)", timeout=15000)
+        msg = pg.locator("#agsetmsg").inner_text()
+        check("CORS" in msg and "isokit studio" in msg, "an unreachable endpoint on another site explains CORS and the way out: " + msg[:90])
+        shot(pg, "web-agent-unreachable.jpg")
+        pg.evaluate("() => localStorage.setItem('isokit-agent', JSON.stringify({url: arguments[0], key: 'test-key'}))".replace("arguments[0]", "'" + agent_checks.MOCK + "/v1'"))
+        pg.locator("#agclose").click()
+
         # ---------------------------------------------------------------- two tabs on one project
         pg2 = ctx.new_page()
         watch(pg2)
@@ -320,7 +348,7 @@ try:
         shot(pg, "web-narrow.jpg")
 
         # the browser logs the 401 of the agent window's wrong-key check
-        errs = [e for e in errs if "status of 401" not in e]
+        errs = [e for e in errs if "status of 401" not in e and "ERR_CONNECTION_REFUSED" not in e]
         check(not errs, f"no page errors {errs[:5]}")
         b.close()
 finally: