fix(core): keep provider error messages when the payload is a plain string

packages/core/src/common/llm-error.ts

@@ -102,10 +102,19 @@ function getErrorDetails(error: unknown, depth: number, seen: Set<object>): LlmE
 }
 
 function getProviderMessage(error: unknown): string | undefined {
-  if (!isRecord(error) || !isRecord(error.error)) {
+  if (!isRecord(error)) {
     return undefined;
   }
-  return safeText(error.error.message);
+  // Providers and gateways do not agree on the error payload shape: some send
+  // an object with `message`, others send the message as a plain string.
+  const payload = error.error;
+  if (typeof payload === "string") {
+    return safeText(payload);
+  }
+  if (!isRecord(payload)) {
+    return undefined;
+  }
+  return safeText(payload.message);
 }
 
 function getHeader(headers: unknown, name: string): string | undefined {

packages/core/src/tests/llm-error.test.ts

@@ -22,6 +22,18 @@ test("describeLlmError shows provider business errors with trace metadata", () =
   );
 });
 
+test("describeLlmError keeps provider errors whose payload is a plain string", () => {
+  const error = Object.assign(new Error("502 Bad Gateway"), {
+    status: 502,
+    error: "Gateway failed",
+    headers: new Headers({
+      "x-request-id": "request-789",
+    }),
+  });
+
+  assert.equal(describeLlmError(error), "HTTP 502: Gateway failed [request ID: request-789]");
+});
+
 test("describeLlmError unwraps underlying network causes", () => {
   const cause = new Error("getaddrinfo ENOTFOUND api.deepseek.com");
   const error = Object.assign(new Error("Connection error."), { cause });