diff --git a/open-sse/services/qoderCli.ts b/open-sse/services/qoderCli.ts
index 5b596ed795..09c052097b 100644
--- a/open-sse/services/qoderCli.ts
+++ b/open-sse/services/qoderCli.ts
@@ -505,6 +505,22 @@ export async function validateQoderCliPat({
 
     // Treat 5xx as valid bypass to prevent false negatives from legacy Qoder APIs (issue #1391)
     if (res.status >= 500) {
+      const isCosyAppError =
+        /"success"\s*:\s*false/.test(errorDetail) &&
+        (/"msgCode"\s*:\s*500/.test(errorDetail) || /internal\s*server\s*error/i.test(errorDetail));
+
+      if (isCosyAppError) {
+        return {
+          valid: false,
+          error:
+            `Authentication failed (HTTP ${res.status}). The Qoder Cosy server returned an Internal Server Error. ` +
+            "This typically indicates that your Personal Access Token is invalid, expired, or not authorized. " +
+            "Please check your token at https://qoder.com/account/integrations." +
+            (errorDetail ? ` Server response: ${errorDetail}` : ""),
+          unsupported: false,
+        };
+      }
+
       return {
         valid: true,
         error: `Validation endpoint returned HTTP ${res.status}${errorDetail ? `: ${errorDetail}` : ""}, treating PAT as valid`,
diff --git a/tests/unit/qoder-cli.test.ts b/tests/unit/qoder-cli.test.ts
index 310a2bda83..c4a45cb01c 100644
--- a/tests/unit/qoder-cli.test.ts
+++ b/tests/unit/qoder-cli.test.ts
@@ -364,3 +364,41 @@ test("validateQoderCliPat treats 5xx HTTP failures as valid bypass", async () =>
     globalThis.fetch = originalFetch;
   }
 });
+
+test("validateQoderCliPat rejects 500 HTTP failures if response is Cosy app-level auth error", async () => {
+  const originalFetch = globalThis.fetch;
+  globalThis.fetch = async (url) => {
+    if (String(url).includes("/ping")) return new Response("pong", { status: 200 });
+    return new Response(
+      ' { "success"  :  false, "traceId": "a4e5de61929400b9243b4f6e49756906", "msgCode"  :  500 } ',
+      { status: 500 }
+    );
+  };
+
+  try {
+    const result = await qoderCli.validateQoderCliPat({ apiKey: "invalid-pat" });
+    assert.equal(result.valid, false);
+    assert.match(result.error!, /Authentication failed \(HTTP 500\)/);
+  } finally {
+    globalThis.fetch = originalFetch;
+  }
+});
+
+test("validateQoderCliPat rejects 500 HTTP failures using regex for Internal Server Error with whitespace", async () => {
+  const originalFetch = globalThis.fetch;
+  globalThis.fetch = async (url) => {
+    if (String(url).includes("/ping")) return new Response("pong", { status: 200 });
+    return new Response(
+      ' { "success"  :  false, "error": "Internal   Server    Error" } ',
+      { status: 500 }
+    );
+  };
+
+  try {
+    const result = await qoderCli.validateQoderCliPat({ apiKey: "invalid-pat" });
+    assert.equal(result.valid, false);
+    assert.match(result.error!, /Authentication failed \(HTTP 500\)/);
+  } finally {
+    globalThis.fetch = originalFetch;
+  }
+});