diff --git a/.changeset/breezy-mangos-refuse.md b/.changeset/breezy-mangos-refuse.md
new file mode 100644
index 00000000000..43f98b3cd46
--- /dev/null
+++ b/.changeset/breezy-mangos-refuse.md
@@ -0,0 +1,5 @@
+---
+"@firebase/auth": patch
+---
+
+Fix bug where custom errors from blocking functions were being dropped.
diff --git a/packages/auth/src/api/errors.ts b/packages/auth/src/api/errors.ts
index e70f8523011..a2e8643282e 100644
--- a/packages/auth/src/api/errors.ts
+++ b/packages/auth/src/api/errors.ts
@@ -22,6 +22,7 @@ import { AuthErrorCode } from '../core/errors';
  */
 export const enum ServerError {
   ADMIN_ONLY_OPERATION = 'ADMIN_ONLY_OPERATION',
+  BLOCKING_FUNCTION_ERROR_RESPONSE = 'BLOCKING_FUNCTION_ERROR_RESPONSE',
   CAPTCHA_CHECK_FAILED = 'CAPTCHA_CHECK_FAILED',
   CORS_UNSUPPORTED = 'CORS_UNSUPPORTED',
   CREDENTIAL_MISMATCH = 'CREDENTIAL_MISMATCH',
@@ -199,5 +200,8 @@ export const SERVER_ERROR_MAP: Partial<ServerErrorMap<ServerError>> = {
   [ServerError.SECOND_FACTOR_EXISTS]:
     AuthErrorCode.SECOND_FACTOR_ALREADY_ENROLLED,
   [ServerError.SECOND_FACTOR_LIMIT_EXCEEDED]:
-    AuthErrorCode.SECOND_FACTOR_LIMIT_EXCEEDED
+    AuthErrorCode.SECOND_FACTOR_LIMIT_EXCEEDED,
+
+  // Blocking functions related errors.
+  [ServerError.BLOCKING_FUNCTION_ERROR_RESPONSE]: AuthErrorCode.INTERNAL_ERROR,
 };
diff --git a/packages/auth/src/api/index.test.ts b/packages/auth/src/api/index.test.ts
index 990cde7cbb0..ab590ac4406 100644
--- a/packages/auth/src/api/index.test.ts
+++ b/packages/auth/src/api/index.test.ts
@@ -198,6 +198,37 @@ describe('api/_performApiRequest', () => {
       expect(mock.calls[0].request).to.eql(request);
     });
 
+    it('should pass through server messages if applicable', async () => {
+      mockEndpoint(
+        Endpoint.SIGN_UP,
+        {
+          error: {
+            code: 400,
+            message: `${ServerError.BLOCKING_FUNCTION_ERROR_RESPONSE} : Text text text`,
+            errors: [
+              {
+                message: 'Text text text'
+              }
+            ]
+          }
+        },
+        400
+      );
+      const promise = _performApiRequest<typeof request, typeof serverResponse>(
+        auth,
+        HttpMethod.POST,
+        Endpoint.SIGN_UP,
+        request
+      );
+      let error: FirebaseError;
+      try {
+        await promise;
+      } catch (e) {
+        error = e;
+      }
+      expect(error!.customData!.message).to.eql('Text text text');
+    });
+
     it('should handle unknown server errors', async () => {
       const mock = mockEndpoint(
         Endpoint.SIGN_UP,
diff --git a/packages/auth/src/api/index.ts b/packages/auth/src/api/index.ts
index 7cedb429359..4357db0b535 100644
--- a/packages/auth/src/api/index.ts
+++ b/packages/auth/src/api/index.ts
@@ -152,7 +152,7 @@ export async function _performFetchWithErrorHandling<V>(
       return json;
     } else {
       const errorMessage = response.ok ? json.errorMessage : json.error.message;
-      const serverErrorCode = errorMessage.split(' : ')[0] as ServerError;
+      const [serverErrorCode, serverErrorMessage] = errorMessage.split(' : ');
       if (serverErrorCode === ServerError.FEDERATED_USER_ID_ALREADY_LINKED) {
         throw _makeTaggedError(
           auth,
@@ -163,11 +163,15 @@ export async function _performFetchWithErrorHandling<V>(
         throw _makeTaggedError(auth, AuthErrorCode.EMAIL_EXISTS, json);
       }
       const authError =
-        errorMap[serverErrorCode] ||
+        errorMap[serverErrorCode as ServerError] ||
         ((serverErrorCode
           .toLowerCase()
           .replace(/[_\s]+/g, '-') as unknown) as AuthErrorCode);
-      _fail(auth, authError);
+      if (serverErrorMessage) {
+        _fail(auth, authError, {message: serverErrorMessage});
+      } else {
+        _fail(auth, authError);
+      }
     }
   } catch (e) {
     if (e instanceof FirebaseError) {