diff --git a/requirements-test.txt b/requirements-test.txt
--- a/requirements-test.txt
+++ b/requirements-test.txt
@@ -5,3 +5,4 @@
 swh.core[testing-core]
 swh.model[testing]
 swh.storage[testing]
+swh.web[testing]
diff --git a/swh/scanner/exceptions.py b/swh/scanner/exceptions.py
--- a/swh/scanner/exceptions.py
+++ b/swh/scanner/exceptions.py
@@ -6,4 +6,9 @@
 
 class APIError(Exception):
     def __str__(self):
-        return 'API Error: "%s"' % self.args
+        return '"%s"' % self.args
+
+
+def error_response(reason: str, status_code: int, api_url: str):
+    error_msg = f'{status_code} {reason}: \'{api_url}\''
+    raise APIError(error_msg)
diff --git a/swh/scanner/scanner.py b/swh/scanner/scanner.py
--- a/swh/scanner/scanner.py
+++ b/swh/scanner/scanner.py
@@ -10,7 +10,7 @@
 from typing import List, Dict, Tuple, Iterator
 from pathlib import PosixPath
 
-from .exceptions import APIError
+from .exceptions import error_response
 from .model import Tree
 
 from swh.model.cli import pid_of_file, pid_of_dir
@@ -49,9 +49,7 @@
     async def make_request(pids):
         async with session.post(endpoint, json=pids) as resp:
             if resp.status != 200:
-                error_message = '%s with given values %s' % (
-                    resp.text, str(pids))
-                raise APIError(error_message)
+                error_response(resp.reason, resp.status, endpoint)
 
             return await resp.json()
 
diff --git a/swh/scanner/tests/conftest.py b/swh/scanner/tests/conftest.py
--- a/swh/scanner/tests/conftest.py
+++ b/swh/scanner/tests/conftest.py
@@ -60,6 +60,12 @@
     return app
 
 
+@pytest.fixture(scope='session')
+def live_server(live_server):
+    live_server.start()
+    return live_server
+
+
 @pytest.fixture
 def test_folder():
     tests_path = PosixPath(os.path.abspath(__file__)).parent
diff --git a/swh/scanner/tests/flask_api.py b/swh/scanner/tests/flask_api.py
--- a/swh/scanner/tests/flask_api.py
+++ b/swh/scanner/tests/flask_api.py
@@ -7,6 +7,8 @@
 
 from .data import present_pids
 
+from swh.web.common.exc import LargePayloadExc
+
 
 def create_app():
     app = Flask(__name__)
@@ -14,6 +16,11 @@
     @app.route('/known/', methods=['POST'])
     def known():
         pids = request.get_json()
+
+        if len(pids) > 900:
+            raise LargePayloadExc('The maximum number of PIDs this endpoint '
+                                  'can receive is 900')
+
         res = {pid: {'known': False} for pid in pids}
         for pid in pids:
             if pid in present_pids:
diff --git a/swh/scanner/tests/test_scanner.py b/swh/scanner/tests/test_scanner.py
--- a/swh/scanner/tests/test_scanner.py
+++ b/swh/scanner/tests/test_scanner.py
@@ -35,6 +35,18 @@
            pids_discovery([], aiosession, 'http://example.org/api/'))
 
 
+def test_scanner_raise_apierror_input_size_limit(
+        event_loop, aiosession, live_server):
+
+    api_url = live_server.url() + '/'
+    request = ["swh:1:cnt:7c4c57ba9ff496ad179b8f65b1d286edbda34c9a"
+               for i in range(901)]  # /known/ is limited at 900
+
+    with pytest.raises(APIError):
+        event_loop.run_until_complete(
+           pids_discovery(request, aiosession, api_url))
+
+
 def test_scanner_get_subpaths(tmp_path, temp_paths):
     for subpath, pid in get_subpaths(tmp_path):
         assert subpath in temp_paths['paths']
@@ -47,7 +59,6 @@
 
 
 def test_scanner_result(live_server, event_loop, test_folder):
-    live_server.start()
     api_url = live_server.url() + '/'
 
     result_path = test_folder.joinpath(PosixPath('sample-folder-result.json'))