diff --git a/src/services/api/__tests__/errorSanitization.test.ts b/src/services/api/__tests__/errorSanitization.test.ts
new file mode 100644
index 0000000..f893b59
--- /dev/null
+++ b/src/services/api/__tests__/errorSanitization.test.ts
@@ -0,0 +1,58 @@
+import {
+  buildSanitizedApiError,
+  containsUrlOrPath,
+  getSafeErrorMessage,
+  sanitizeErrorMessage,
+} from '../errorSanitization';
+
+const FOUR_XX = [400, 401, 403, 404, 408, 409, 422, 429];
+const FIVE_XX = [500, 502, 503, 504];
+
+const LEAKY_MESSAGES = [
+  'Request failed for https://api.teachlink.com/api/users/42/profile',
+  'GET /api/v1/courses/123/lessons failed with 404',
+  'Error at /api/auth/login',
+];
+
+const expectNoUrl = (message: string) => {
+  expect(message).not.toMatch(/https?:\/\//);
+  expect(message).not.toMatch(/\/api\//);
+  expect(containsUrlOrPath(message)).toBe(false);
+};
+
+describe('getSafeErrorMessage', () => {
+  it.each([...FOUR_XX, ...FIVE_XX])('returns a non-empty, URL-free message for %i', status => {
+    const message = getSafeErrorMessage(status);
+    expect(typeof message).toBe('string');
+    expect(message.length).toBeGreaterThan(0);
+    expectNoUrl(message);
+  });
+
+  it('falls back to a generic message for unknown/zero status', () => {
+    expectNoUrl(getSafeErrorMessage(undefined));
+    expectNoUrl(getSafeErrorMessage(0));
+  });
+});
+
+describe('sanitizeErrorMessage', () => {
+  it.each(LEAKY_MESSAGES)('strips URLs/paths from: %s', leaked => {
+    expectNoUrl(sanitizeErrorMessage(leaked, 400));
+  });
+
+  it('passes a clean message through unchanged', () => {
+    expect(sanitizeErrorMessage('Please try again later.', 500)).toBe('Please try again later.');
+  });
+
+  it('falls back to a safe message when message is empty', () => {
+    expect(sanitizeErrorMessage(undefined, 404)).toBe(getSafeErrorMessage(404));
+  });
+});
+
+describe('buildSanitizedApiError', () => {
+  it.each([...FOUR_XX, ...FIVE_XX])('produces a URL-free payload for %i', status => {
+    const error = buildSanitizedApiError(status, 'TEST_CODE');
+    expect(error.status).toBe(status);
+    expect(error.code).toBe('TEST_CODE');
+    expectNoUrl(error.message);
+  });
+});
diff --git a/src/services/api/axios.config.ts b/src/services/api/axios.config.ts
index d29ead5..d162744 100644
--- a/src/services/api/axios.config.ts
+++ b/src/services/api/axios.config.ts
@@ -27,6 +27,7 @@ import {
   invalidateCacheForBatchRequests,
   invalidateCacheForMutation,
 } from './cache';
+import { buildSanitizedApiError } from './errorSanitization';
 import { requestQueue } from './requestQueue';
 
 // ─── Helpers ────────────────────────────────────────────────────────────────
@@ -218,7 +219,10 @@ apiClient.interceptors.response.use(
     const receivedRequestId = response.headers['x-request-id'];
 
     if (sentRequestId && receivedRequestId && sentRequestId !== receivedRequestId) {
-      appLogger.warnSync('Request ID mismatch', { sent: sentRequestId, received: receivedRequestId });
+      appLogger.warnSync('Request ID mismatch', {
+        sent: sentRequestId,
+        received: receivedRequestId,
+      });
     }
 
     popLogContext();
@@ -572,8 +576,20 @@ apiClient.interceptors.response.use(
 
     // ─── Default fallback ──────────────────────────────────────────────────
 
-    return Promise.reject(error);
+    sentryContextService.captureException(error, {
+      tags: { 'api.error': 'unhandled_response' },
+      contexts: {
+        request: {
+          url: originalRequest?.url,
+          method: originalRequest?.method?.toUpperCase(),
+          status: status ?? null,
+        },
+      },
+      fingerprint: ['api-unhandled-error', String(status ?? 'unknown')],
+    });
+
+    return Promise.reject(buildSanitizedApiError(status, error.code));
   }
 );
 
-export default apiClient;
\ No newline at end of file
+export default apiClient;
diff --git a/src/services/api/errorSanitization.ts b/src/services/api/errorSanitization.ts
new file mode 100644
index 0000000..5075a11
--- /dev/null
+++ b/src/services/api/errorSanitization.ts
@@ -0,0 +1,69 @@
+/**
+ * API error message sanitization (Issue #579).
+ *
+ * User-facing error messages must never contain request URLs or endpoint
+ * paths. The full url + method are sent to Sentry request context and
+ * structured logs instead — never embedded in the message shown to the UI.
+ */
+
+/** Generic, URL-free user-facing messages keyed by HTTP status. */
+const STATUS_MESSAGES: Record<number, string> = {
+  400: 'The request could not be processed. Please check your input.',
+  401: 'Your session has expired. Please log in again.',
+  403: 'You are not allowed to perform this action.',
+  404: 'The requested resource could not be found.',
+  408: 'The request timed out. Please try again.',
+  422: 'Some information provided was invalid. Please review and retry.',
+};
+
+/** Return a safe, URL-free message for a given HTTP status. */
+export function getSafeErrorMessage(status?: number): string {
+  if (status === undefined || status === 0) {
+    return 'Something went wrong. Please try again.';
+  }
+  if (STATUS_MESSAGES[status]) {
+    return STATUS_MESSAGES[status];
+  }
+  if (status >= 500) {
+    return 'Server error. Please try again later.';
+  }
+  if (status >= 400) {
+    return 'The request could not be completed. Please try again.';
+  }
+  return 'Something went wrong. Please try again.';
+}
+
+// Matches absolute URLs and multi-segment paths like /api/users/42.
+// Non-global so .test() is stateless.
+const URL_OR_PATH = /(https?:\/\/[^\s]+)|(\/[\w.~%-]+(?:\/[\w.~%-]+)+)/;
+
+/** True if a string contains a URL or a multi-segment endpoint path. */
+export function containsUrlOrPath(message: string): boolean {
+  return URL_OR_PATH.test(message);
+}
+
+/**
+ * Defense-in-depth: if a message contains a URL/path, replace it with a safe
+ * generic message; otherwise pass it through unchanged.
+ */
+export function sanitizeErrorMessage(message: string | undefined, status?: number): string {
+  if (!message || containsUrlOrPath(message)) {
+    return getSafeErrorMessage(status);
+  }
+  return message;
+}
+
+export interface SanitizedApiError {
+  message: string;
+  status: number;
+  code?: string;
+}
+
+/** Build a normalized, URL-free error payload for the UI. */
+export function buildSanitizedApiError(status?: number, code?: string): SanitizedApiError {
+  return {
+    message: getSafeErrorMessage(status),
+    status: status ?? 0,
+    ...(code ? { code } : {}),
+  };
+}