diff --git a/changelog.d/fixes/7857-web-cookie-validation-probe.md b/changelog.d/fixes/7857-web-cookie-validation-probe.md
new file mode 100644
index 0000000000..cb88be58f1
--- /dev/null
+++ b/changelog.d/fixes/7857-web-cookie-validation-probe.md
@@ -0,0 +1 @@
+- fix(providers): treat a non-401/403 response from the web-cookie `/models` validation probe (redirect, login-HTML 200, 404, 405, 429) as `unsupported` instead of `valid: true` for providers whose registry `baseUrl` is a conversation/completion endpoint rather than a real API root (#7857)
diff --git a/src/lib/providers/validation.ts b/src/lib/providers/validation.ts
index 591fb77f35..f1d645b4ba 100644
--- a/src/lib/providers/validation.ts
+++ b/src/lib/providers/validation.ts
@@ -37,6 +37,7 @@ import {
   validationWrite,
   toValidationErrorResult,
   toWebCookieValidationErrorResult,
+  WEB_COOKIE_PROVIDERS_WITHOUT_MODELS_API,
 } from "./validation/transport";
 import {
   validateDeepSeekWebProvider,
@@ -162,6 +163,21 @@ export async function validateWebCookieProvider({
     // Attempt a minimal request to check if the session is valid
     // Use /models endpoint or a minimal completion request depending on the provider
     const baseUrl = normalizeBaseUrl(entry.baseUrl || "");
+
+    // Defense-in-depth: only an http(s) baseUrl without a query string is safe to
+    // probe by blindly appending `/models`. A ws(s):// baseUrl (e.g. copilot-web) is
+    // already rejected by the outbound URL guard downstream, but reject it explicitly
+    // here for the honest "unsupported" result instead of a confusing security-block
+    // message — this also covers a future http(s) baseUrl carrying a query string,
+    // which the guard does not currently block (#7857 acceptance criteria).
+    if (!/^https?:\/\//i.test(baseUrl) || baseUrl.includes("?")) {
+      return {
+        valid: false,
+        error: "Provider validation not supported",
+        unsupported: true,
+      };
+    }
+
     const testUrl = `${baseUrl}/models`;
 
     const res = await validationRead(
@@ -185,6 +201,20 @@ export async function validateWebCookieProvider({
       };
     }
 
+    // #7857: for providers whose baseUrl is a conversation/completion endpoint rather
+    // than a real API root, the /models path never existed upstream — a redirect,
+    // login-HTML 200, 404, 405, or 429 from it is not a meaningful auth signal and is
+    // indistinguishable from a genuinely valid session. Report the same honest
+    // "unsupported" result the !entry branch above already gives its no-registry
+    // siblings, instead of a false `valid: true`.
+    if (WEB_COOKIE_PROVIDERS_WITHOUT_MODELS_API.has(provider)) {
+      return {
+        valid: false,
+        error: "Provider validation not supported",
+        unsupported: true,
+      };
+    }
+
     // Any other response (200, 404, 405, 429, ...) means the cookie was accepted —
     // a 401/403 from the /models probe is the only definitive "session expired" signal
     // for web-cookie auth, so a non-auth status is treated as a valid session.
diff --git a/src/lib/providers/validation/transport.ts b/src/lib/providers/validation/transport.ts
index 8fb49af5ce..c486ff41fc 100644
--- a/src/lib/providers/validation/transport.ts
+++ b/src/lib/providers/validation/transport.ts
@@ -107,6 +107,28 @@ export function isSecurityBlockError(error: unknown): boolean {
 // #7542 plan-file, "Risks").
 const WEB_COOKIE_PROVIDERS_WITH_UNRELIABLE_MODELS_PROBE = new Set(["lmarena"]);
 
+// #7857 — web-cookie providers whose registry `baseUrl` is a conversation/completion
+// endpoint, not a real API root (e.g. huggingchat's baseUrl is
+// "https://huggingface.co/chat/conversation", not "https://huggingface.co"). Appending
+// `/models` to these produces a path the upstream never served, so its status
+// (200/404/405/429/redirect/login-HTML) carries no meaningful auth signal — it is NOT
+// distinguishable from a genuinely valid session. A 401/403 from the same probe IS still
+// treated as a real SESSION_EXPIRED signal (some of these hosts auth-gate every path,
+// including nonexistent ones), so providers here still get probed; only the non-401/403
+// branch is short-circuited to the honest "unsupported" result instead of `valid: true`.
+// lmarena is deliberately NOT in this set — it already degrades via the
+// WEB_COOKIE_PROVIDERS_WITH_UNRELIABLE_MODELS_PROBE/REDIRECT_BLOCKED path above (#7542).
+export const WEB_COOKIE_PROVIDERS_WITHOUT_MODELS_API = new Set([
+  "huggingchat",
+  "chatgpt-web",
+  "grok-web",
+  "notion-web",
+  "t3-web",
+  "yuanbao-web",
+  "copilot-web",
+  "copilot-m365-web",
+]);
+
 export function toWebCookieValidationErrorResult(provider: string, error: unknown) {
   if (
     error instanceof SafeOutboundFetchError &&
diff --git a/tests/unit/provider-validation-web-cookie-auth007.test.ts b/tests/unit/provider-validation-web-cookie-auth007.test.ts
index 344b1e865b..9b665a9d59 100644
--- a/tests/unit/provider-validation-web-cookie-auth007.test.ts
+++ b/tests/unit/provider-validation-web-cookie-auth007.test.ts
@@ -56,9 +56,13 @@ test("should_return_AUTH_007_when_models_endpoint_returns_403", async () => {
   assert.equal(result.error, "SESSION_EXPIRED");
 });
 
-test("should_return_valid_when_models_endpoint_returns_200", async () => {
-  // A non-401/403 status from the /models probe means the cookie was accepted,
-  // so the session is treated as valid.
+test("should_return_unsupported_when_models_endpoint_returns_200_for_a_conversation_baseUrl", async () => {
+  // #7857: chatgpt-web's registry baseUrl is a conversation endpoint
+  // ("https://chatgpt.com/backend-api/conversation"), not a real API root, so
+  // "${baseUrl}/models" is a path that never existed upstream. A 200 from that
+  // nonsense path (e.g. a login-page SPA shell) is not a meaningful auth signal and
+  // is indistinguishable from a genuinely valid session — it must be reported as
+  // unsupported, not silently accepted as valid.
   mockFetch(200, JSON.stringify({ ok: true, data: [] }));
 
   const result = await validateWebCookieProvider({
@@ -68,7 +72,8 @@ test("should_return_valid_when_models_endpoint_returns_200", async () => {
   });
 
   assert.equal(fetchCalls, 1, "the /models probe must be the mocked fetch, not the live network");
-  assert.equal(result.valid, true);
+  assert.equal(result.valid, false);
+  assert.equal(result.unsupported, true);
 });
 
 test("should_return_unsupported_when_provider_not_in_registry", async () => {
diff --git a/tests/unit/web-cookie-validation-false-positive-7857.test.ts b/tests/unit/web-cookie-validation-false-positive-7857.test.ts
new file mode 100644
index 0000000000..7351407874
--- /dev/null
+++ b/tests/unit/web-cookie-validation-false-positive-7857.test.ts
@@ -0,0 +1,51 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+let nextResponse: { status: number; body: string } = { status: 404, body: "Not Found" };
+let fetchCalls = 0;
+let lastUrl = "";
+
+const { validateWebCookieProvider } = await import("../../src/lib/providers/validation.ts");
+
+globalThis.fetch = (async (input: RequestInfo | URL) => {
+  fetchCalls++;
+  lastUrl = typeof input === "string" ? input : input instanceof URL ? input.href : input.url;
+  return new Response(nextResponse.body, { status: nextResponse.status });
+}) as typeof fetch;
+
+function mockFetch(status: number, body: string) {
+  nextResponse = { status, body };
+  fetchCalls = 0;
+  lastUrl = "";
+}
+
+test("BUG #7857: a 404 from huggingchat's nonsense /models probe path is reported valid:true", async () => {
+  mockFetch(404, "Not Found");
+  const result = await validateWebCookieProvider({
+    provider: "huggingchat",
+    apiKey: "garbage_cookie_value_that_was_never_valid=xyz",
+    providerSpecificData: {},
+  });
+  assert.equal(fetchCalls, 1);
+  assert.equal(lastUrl, "https://huggingface.co/chat/conversation/models");
+  assert.equal(
+    result.valid,
+    false,
+    "a 404 from a nonsense probe path must never be reported as a valid cookie session"
+  );
+});
+
+test("BUG #7857: same false positive reproduces for grok-web (conversations/new endpoint)", async () => {
+  mockFetch(405, "Method Not Allowed");
+  const result = await validateWebCookieProvider({
+    provider: "grok-web",
+    apiKey: "sso=garbage; sso-rw=garbage",
+    providerSpecificData: {},
+  });
+  assert.equal(fetchCalls, 1);
+  assert.equal(
+    result.valid,
+    false,
+    "a 405 from a nonsense probe path must never be reported as a valid cookie session"
+  );
+});