diff --git a/lib/core/settle.js b/lib/core/settle.js
index 8629ad05..782f5712 100644
--- a/lib/core/settle.js
+++ b/lib/core/settle.js
@@ -16,16 +16,12 @@ export default function settle(resolve, reject, response) {
   if (!response.status || !validateStatus || validateStatus(response.status)) {
     resolve(response);
   } else {
-    reject(
-      new AxiosError(
-        'Request failed with status code ' + response.status,
-        [AxiosError.ERR_BAD_REQUEST, AxiosError.ERR_BAD_RESPONSE][
-          Math.floor(response.status / 100) - 4
-        ],
-        response.config,
-        response.request,
-        response
-      )
-    );
+    reject(new AxiosError(
+      'Request failed with status code ' + response.status,
+      response.status >= 400 && response.status < 500 ? AxiosError.ERR_BAD_REQUEST : AxiosError.ERR_BAD_RESPONSE,
+      response.config,
+      response.request,
+      response
+    ));
   }
 }
diff --git a/tests/unit/core/settle.test.js b/tests/unit/core/settle.test.js
new file mode 100644
index 00000000..1403758d
--- /dev/null
+++ b/tests/unit/core/settle.test.js
@@ -0,0 +1,130 @@
+import { describe, it, expect, vi } from 'vitest';
+import settle from '../../../lib/core/settle.js';
+import AxiosError from '../../../lib/core/AxiosError.js';
+
+/**
+ * Builds a minimal response object that settle() expects.
+ * @param {number} status - HTTP status code
+ * @param {Function} [validateStatus] - optional custom validator
+ */
+function makeResponse(status, validateStatus = (s) => s >= 200 && s < 300) {
+  return {
+    status,
+    config: { validateStatus },
+    request: {},
+    data: null,
+    headers: {},
+  };
+}
+
+describe('core::settle', () => {
+  describe('resolves the promise', () => {
+    it('resolves for a 200 status by default', () => {
+      const resolve = vi.fn();
+      const reject = vi.fn();
+      settle(resolve, reject, makeResponse(200));
+      expect(resolve).toHaveBeenCalledTimes(1);
+      expect(reject).not.toHaveBeenCalled();
+    });
+
+    it('resolves when validateStatus is not provided', () => {
+      const resolve = vi.fn();
+      const reject = vi.fn();
+      const response = makeResponse(500);
+      response.config.validateStatus = null;
+      settle(resolve, reject, response);
+      expect(resolve).toHaveBeenCalledTimes(1);
+    });
+
+    it('resolves when status is 0 (no response, e.g. network error)', () => {
+      const resolve = vi.fn();
+      const reject = vi.fn();
+      const response = makeResponse(0);
+      settle(resolve, reject, response);
+      expect(resolve).toHaveBeenCalledTimes(1);
+    });
+  });
+
+  describe('error code assignment', () => {
+    it('assigns ERR_BAD_REQUEST for a 400 status', () => {
+      const resolve = vi.fn();
+      const reject = vi.fn();
+      settle(resolve, reject, makeResponse(400, () => false));
+      expect(reject).toHaveBeenCalledTimes(1);
+      const error = reject.mock.calls[0][0];
+      expect(error).toBeInstanceOf(AxiosError);
+      expect(error.code).toBe(AxiosError.ERR_BAD_REQUEST);
+    });
+
+    it('assigns ERR_BAD_REQUEST for a 404 status', () => {
+      const resolve = vi.fn();
+      const reject = vi.fn();
+      settle(resolve, reject, makeResponse(404, () => false));
+      const error = reject.mock.calls[0][0];
+      expect(error.code).toBe(AxiosError.ERR_BAD_REQUEST);
+    });
+
+    it('assigns ERR_BAD_REQUEST for a 499 status (top of 4xx range)', () => {
+      const resolve = vi.fn();
+      const reject = vi.fn();
+      settle(resolve, reject, makeResponse(499, () => false));
+      const error = reject.mock.calls[0][0];
+      expect(error.code).toBe(AxiosError.ERR_BAD_REQUEST);
+    });
+
+    it('assigns ERR_BAD_RESPONSE for a 500 status', () => {
+      const resolve = vi.fn();
+      const reject = vi.fn();
+      settle(resolve, reject, makeResponse(500, () => false));
+      const error = reject.mock.calls[0][0];
+      expect(error.code).toBe(AxiosError.ERR_BAD_RESPONSE);
+    });
+
+    it('assigns ERR_BAD_RESPONSE for a 503 status', () => {
+      const resolve = vi.fn();
+      const reject = vi.fn();
+      settle(resolve, reject, makeResponse(503, () => false));
+      const error = reject.mock.calls[0][0];
+      expect(error.code).toBe(AxiosError.ERR_BAD_RESPONSE);
+    });
+
+    // A custom validateStatus can reject any status — including 1xx, 2xx, 3xx,
+    // or ≥ 600 — and the resulting AxiosError must always have a defined code.
+    it('assigns ERR_BAD_RESPONSE for a 3xx status rejected via custom validateStatus', () => {
+      const resolve = vi.fn();
+      const reject = vi.fn();
+      settle(resolve, reject, makeResponse(301, () => false));
+      const error = reject.mock.calls[0][0];
+      expect(error.code).toBeDefined();
+      expect(error.code).toBe(AxiosError.ERR_BAD_RESPONSE);
+    });
+
+    it('assigns ERR_BAD_RESPONSE for a 2xx status rejected via custom validateStatus', () => {
+      const resolve = vi.fn();
+      const reject = vi.fn();
+      settle(resolve, reject, makeResponse(200, () => false));
+      const error = reject.mock.calls[0][0];
+      expect(error.code).toBeDefined();
+      expect(error.code).toBe(AxiosError.ERR_BAD_RESPONSE);
+    });
+
+    it('assigns ERR_BAD_RESPONSE for a 6xx status rejected via custom validateStatus', () => {
+      const resolve = vi.fn();
+      const reject = vi.fn();
+      settle(resolve, reject, makeResponse(600, () => false));
+      const error = reject.mock.calls[0][0];
+      expect(error.code).toBeDefined();
+      expect(error.code).toBe(AxiosError.ERR_BAD_RESPONSE);
+    });
+  });
+
+  describe('error message', () => {
+    it('includes the status code in the error message', () => {
+      const resolve = vi.fn();
+      const reject = vi.fn();
+      settle(resolve, reject, makeResponse(404, () => false));
+      const error = reject.mock.calls[0][0];
+      expect(error.message).toBe('Request failed with status code 404');
+    });
+  });
+});