diff --git a/plugins/computers/api/routes.py b/plugins/computers/api/routes.py
index 9adb608a..d90d1c36 100644
--- a/plugins/computers/api/routes.py
+++ b/plugins/computers/api/routes.py
@@ -7,7 +7,7 @@ from shopdb.api import db, Asset, AssetType, OperatingSystem, Application, Audit
 
 from ..models import Computer, ComputerType, ComputerInstalledApp, AccessProtocol, ComputerAccess
 
-from shopdb.api import require_permission, require_role, apply_import_timestamps
+from shopdb.api import require_permission, require_role, apply_import_timestamps, intarg
 
 computers_bp = Blueprint('computers', __name__)
 
@@ -380,20 +380,20 @@ def list_computers():
         )
 
     # Computer type filter
-    if type_id := request.args.get('typeid', request.args.get('type_id')):
-        query = query.filter(Computer.computertypeid == int(type_id))
+    if (type_id := intarg('typeid', intarg('type_id'))) is not None:
+        query = query.filter(Computer.computertypeid == type_id)
 
     # OS filter
-    if os_id := request.args.get('osid', request.args.get('os_id')):
-        query = query.filter(Computer.osid == int(os_id))
+    if (os_id := intarg('osid', intarg('os_id'))) is not None:
+        query = query.filter(Computer.osid == os_id)
 
     # Location filter
-    if location_id := request.args.get('locationid', request.args.get('location_id')):
-        query = query.filter(Asset.locationid == int(location_id))
+    if (location_id := intarg('locationid', intarg('location_id'))) is not None:
+        query = query.filter(Asset.locationid == location_id)
 
     # Business unit filter
-    if bu_id := request.args.get('businessunitid', request.args.get('businessunit_id')):
-        query = query.filter(Asset.businessunitid == int(bu_id))
+    if (bu_id := intarg('businessunitid', intarg('businessunit_id'))) is not None:
+        query = query.filter(Asset.businessunitid == bu_id)
 
     # Shopfloor filter (by the Shopfloor computer type)
     if shopfloor := request.args.get('shopfloor'):
diff --git a/plugins/employees/api/routes.py b/plugins/employees/api/routes.py
index 4e9788c1..07124b25 100644
--- a/plugins/employees/api/routes.py
+++ b/plugins/employees/api/routes.py
@@ -25,7 +25,7 @@ from shopdb.api import (
     employee_connection,
     require_role,
 )
-from shopdb.api import Setting
+from shopdb.api import Setting, intarg
 from shopdb.api import send_upload
 
 from ..models import DirectoryEmployee
@@ -287,7 +287,7 @@ def search_employees():
     - limit: Max results (default 10)
     """
     query = request.args.get('q', '').strip()
-    limit = min(int(request.args.get('limit', 10)), 50)
+    limit = min(intarg('limit', 10), 50)
 
     if len(query) < 2:
         return error_response(
diff --git a/plugins/knowledgebase/api/routes.py b/plugins/knowledgebase/api/routes.py
index a3dd9800..f782b6bb 100644
--- a/plugins/knowledgebase/api/routes.py
+++ b/plugins/knowledgebase/api/routes.py
@@ -4,6 +4,7 @@ from flask import Blueprint, request
 from flask_jwt_extended import jwt_required
 
 from shopdb.api import (
+    intarg,
     db,
     Application,
     success_response,
@@ -16,7 +17,7 @@ from shopdb.api import (
 
 from ..models import KnowledgeBase
 
-from shopdb.api import require_permission, apply_import_timestamps
+from shopdb.api import require_permission, apply_import_timestamps, intarg
 
 knowledgebase_bp = Blueprint('knowledgebase', __name__)
 
@@ -91,8 +92,8 @@ def list_articles():
         query = query.filter(_search_clause(search))
 
     # Filter by topic/application
-    if appid := request.args.get('appid'):
-        query = query.filter(KnowledgeBase.appid == int(appid))
+    if (appid := intarg('appid')) is not None:
+        query = query.filter(KnowledgeBase.appid == appid)
 
     # Exact-match natural-key lookups for idempotent import. linkurl is the
     # stable natural key; shortdescription (the title) is offered as a fallback.
diff --git a/plugins/machines/api/routes.py b/plugins/machines/api/routes.py
index 267d84dd..f8726a7f 100644
--- a/plugins/machines/api/routes.py
+++ b/plugins/machines/api/routes.py
@@ -7,7 +7,7 @@ from shopdb.api import db, Asset, AssetType, AuditLog, Vendor, success_response,
 
 from ..models import Machine, MachineType
 
-from shopdb.api import require_permission, apply_import_timestamps
+from shopdb.api import require_permission, apply_import_timestamps, intarg
 
 machines_bp = Blueprint('machines', __name__)
 
@@ -197,20 +197,20 @@ def list_machines():
         )
 
     # Machine type filter
-    if type_id := request.args.get('typeid', request.args.get('type_id')):
-        query = query.filter(Machine.machinetypeid == int(type_id))
+    if (type_id := intarg('typeid', intarg('type_id'))) is not None:
+        query = query.filter(Machine.machinetypeid == type_id)
 
     # Vendor filter
-    if vendor_id := request.args.get('vendorid', request.args.get('vendor_id')):
-        query = query.filter(Machine.vendorid == int(vendor_id))
+    if (vendor_id := intarg('vendorid', intarg('vendor_id'))) is not None:
+        query = query.filter(Machine.vendorid == vendor_id)
 
     # Location filter
-    if location_id := request.args.get('locationid', request.args.get('location_id')):
-        query = query.filter(Asset.locationid == int(location_id))
+    if (location_id := intarg('locationid', intarg('location_id'))) is not None:
+        query = query.filter(Asset.locationid == location_id)
 
     # Business unit filter
-    if bu_id := request.args.get('businessunitid', request.args.get('businessunit_id')):
-        query = query.filter(Asset.businessunitid == int(bu_id))
+    if (bu_id := intarg('businessunitid', intarg('businessunit_id'))) is not None:
+        query = query.filter(Asset.businessunitid == bu_id)
 
     # Sorting
     sort_by = request.args.get('sort', 'assetnumber')
diff --git a/plugins/network/api/routes.py b/plugins/network/api/routes.py
index 83b44ce8..93e9c8a0 100644
--- a/plugins/network/api/routes.py
+++ b/plugins/network/api/routes.py
@@ -8,7 +8,7 @@ from shopdb.api import db, Asset, AssetType, Vendor, Communication, Communicatio
 from ..services.assetnumbers import generate_for_type
 from ..models import NetworkDevice, NetworkDeviceType, Subnet, VLAN
 
-from shopdb.api import require_permission, apply_import_timestamps
+from shopdb.api import require_permission, apply_import_timestamps, intarg
 
 network_bp = Blueprint('network', __name__)
 
@@ -232,20 +232,20 @@ def list_network_devices():
         )
 
     # Type filter
-    if type_id := request.args.get('typeid', request.args.get('type_id')):
-        query = query.filter(NetworkDevice.networkdevicetypeid == int(type_id))
+    if (type_id := intarg('typeid', intarg('type_id'))) is not None:
+        query = query.filter(NetworkDevice.networkdevicetypeid == type_id)
 
     # Vendor filter
-    if vendor_id := request.args.get('vendorid', request.args.get('vendor_id')):
-        query = query.filter(NetworkDevice.vendorid == int(vendor_id))
+    if (vendor_id := intarg('vendorid', intarg('vendor_id'))) is not None:
+        query = query.filter(NetworkDevice.vendorid == vendor_id)
 
     # Location filter
-    if location_id := request.args.get('locationid', request.args.get('location_id')):
-        query = query.filter(Asset.locationid == int(location_id))
+    if (location_id := intarg('locationid', intarg('location_id'))) is not None:
+        query = query.filter(Asset.locationid == location_id)
 
     # Business unit filter
-    if bu_id := request.args.get('businessunitid', request.args.get('businessunit_id')):
-        query = query.filter(Asset.businessunitid == int(bu_id))
+    if (bu_id := intarg('businessunitid', intarg('businessunit_id'))) is not None:
+        query = query.filter(Asset.businessunitid == bu_id)
 
     # PoE filter
     if poe := request.args.get('poe'):
@@ -827,12 +827,12 @@ def list_subnets():
         )
 
     # VLAN filter
-    if vlan_id := request.args.get('vlanid'):
-        query = query.filter(Subnet.vlanid == int(vlan_id))
+    if (vlan_id := intarg('vlanid')) is not None:
+        query = query.filter(Subnet.vlanid == vlan_id)
 
     # Location filter
-    if location_id := request.args.get('locationid'):
-        query = query.filter(Subnet.locationid == int(location_id))
+    if (location_id := intarg('locationid')) is not None:
+        query = query.filter(Subnet.locationid == location_id)
 
     # Type filter
     if subnet_type := request.args.get('type'):
diff --git a/plugins/notifications/api/routes.py b/plugins/notifications/api/routes.py
index a2db5d9e..b1058325 100644
--- a/plugins/notifications/api/routes.py
+++ b/plugins/notifications/api/routes.py
@@ -13,7 +13,7 @@ from shopdb.api import db, success_response, error_response, paginated_response,
 from ..models import Notification, NotificationType
 from ..models.notification import _utc_iso
 
-from shopdb.api import require_permission, Setting
+from shopdb.api import require_permission, Setting, intarg
 
 notifications_bp = Blueprint('notifications', __name__)
 
@@ -355,8 +355,8 @@ def list_notifications():
         query = query.filter(Notification.isactive == True)
 
     # Type filter
-    if type_id := request.args.get('typeid', request.args.get('type_id')):
-        query = query.filter(Notification.notificationtypeid == int(type_id))
+    if (type_id := intarg('typeid', intarg('type_id'))) is not None:
+        query = query.filter(Notification.notificationtypeid == type_id)
 
     # Exact-match lookup for idempotent import. Notifications have no strong
     # natural key; ticketnumber is the best available when a ticket is set.
diff --git a/plugins/printers/api/asset_routes.py b/plugins/printers/api/asset_routes.py
index fa14ac5e..f52fa508 100644
--- a/plugins/printers/api/asset_routes.py
+++ b/plugins/printers/api/asset_routes.py
@@ -6,7 +6,7 @@ import re
 from flask import Blueprint, request, Response, current_app
 from flask_jwt_extended import jwt_required
 
-from shopdb.api import db, cache, Setting, Asset, AssetType, Vendor, Model, Communication, CommunicationType, AssetRelationship, RelationshipType, success_response, error_response, paginated_response, ErrorCodes, get_pagination_params, paginate_query
+from shopdb.api import db, cache, Setting, Asset, AssetType, Vendor, Model, Communication, CommunicationType, AssetRelationship, RelationshipType, success_response, error_response, paginated_response, ErrorCodes, get_pagination_params, paginate_query, intarg
 
 from ..models import Printer, PrinterType, ModelSupply, PrinterDriver, PrinterObservedQueue
 from ..models.model_supply import SUPPLY_TYPES, SUPPLY_COLORS, CAPACITY_TIERS
@@ -261,20 +261,20 @@ def list_printers():
         )
 
     # Type filter
-    if typeid := request.args.get('typeid', request.args.get('type_id')):
-        query = query.filter(Printer.printertypeid == int(typeid))
+    if (typeid := intarg('typeid', intarg('type_id'))) is not None:
+        query = query.filter(Printer.printertypeid == typeid)
 
     # Vendor filter
-    if vendor_id := request.args.get('vendorid', request.args.get('vendor_id')):
-        query = query.filter(Printer.vendorid == int(vendor_id))
+    if (vendor_id := intarg('vendorid', intarg('vendor_id'))) is not None:
+        query = query.filter(Printer.vendorid == vendor_id)
 
     # Location filter
-    if location_id := request.args.get('locationid', request.args.get('location_id')):
-        query = query.filter(Asset.locationid == int(location_id))
+    if (location_id := intarg('locationid', intarg('location_id'))) is not None:
+        query = query.filter(Asset.locationid == location_id)
 
     # Business unit filter
-    if bu_id := request.args.get('businessunitid', request.args.get('businessunit_id')):
-        query = query.filter(Asset.businessunitid == int(bu_id))
+    if (bu_id := intarg('businessunitid', intarg('businessunit_id'))) is not None:
+        query = query.filter(Asset.businessunitid == bu_id)
 
     # Sorting
     sort_by = request.args.get('sort', 'hostname')
@@ -2481,8 +2481,8 @@ def list_supply_models():
 
     if search := request.args.get('search'):
         query = query.filter(Model.modelnumber.ilike(f'%{search}%'))
-    if vendor_id := request.args.get('vendorid', request.args.get('vendor_id')):
-        query = query.filter(Model.vendorid == int(vendor_id))
+    if (vendor_id := intarg('vendorid', intarg('vendor_id'))) is not None:
+        query = query.filter(Model.vendorid == vendor_id)
     if request.args.get('withsupplies', '').lower() == 'true':
         query = query.having(supplycount > 0)
 
@@ -2755,7 +2755,7 @@ def supplies_forecast():
     )
 
     try:
-        days = max(1, min(365, int(request.args.get('days', 90))))
+        days = max(1, min(365, intarg('days', 90)))
     except (TypeError, ValueError):
         days = 90
 
diff --git a/shopdb/__init__.py b/shopdb/__init__.py
index d7dd2884..6df790d5 100644
--- a/shopdb/__init__.py
+++ b/shopdb/__init__.py
@@ -400,6 +400,16 @@ def register_error_handlers(app: Flask):
             % (' for %s' % column if column else ''),
             http_code=400)
 
+    # A wrong query argument is the caller's mistake. Registered app-wide so a
+    # list route does not have to remember to catch it - forgetting was the
+    # whole defect, in six routes across five plugins.
+    from .utils.queryargs import QueryArgError
+
+    @app.errorhandler(QueryArgError)
+    def handle_query_arg_error(error):
+        return error_response(ErrorCodes.VALIDATION_ERROR, str(error),
+                              http_code=400)
+
     @app.errorhandler(ShopDBException)
     def handle_shopdb_exception(error):
         http_codes = {
diff --git a/shopdb/api/__init__.py b/shopdb/api/__init__.py
index d26a932e..c1c4e9f7 100644
--- a/shopdb/api/__init__.py
+++ b/shopdb/api/__init__.py
@@ -112,6 +112,8 @@ from shopdb.utils.clientip import client_ip, strip_port
 
 # Strict date parsing. On the contract surface because every plugin that stores
 # a date needs it and the two that rolled their own both swallowed the error.
+from shopdb.utils.queryargs import QueryArgError, intarg, intargs
+
 from shopdb.utils.datevalues import (
     DateValueError,
     parse_date,
@@ -359,6 +361,9 @@ __all__ = [
     'success_response',
     'error_response',
     'DateValueError',
+    'QueryArgError',
+    'intarg',
+    'intargs',
     'parse_date',
     'parse_date_fields',
     'paginated_response',
diff --git a/shopdb/core/api/assets.py b/shopdb/core/api/assets.py
index 006c16d5..2ad24b1e 100644
--- a/shopdb/core/api/assets.py
+++ b/shopdb/core/api/assets.py
@@ -13,6 +13,7 @@ from shopdb.utils.responses import (
     ErrorCodes
 )
 from shopdb.utils.pagination import get_pagination_params, paginate_query
+from shopdb.utils.queryargs import intarg
 
 from shopdb.utils.authz import require_permission
 from shopdb.utils.import_mode import apply_import_timestamps
@@ -365,20 +366,20 @@ def list_assets():
         query = query.join(AssetType).filter(AssetType.assettype == type_name)
 
     # Type filter by ID
-    if type_id := request.args.get('typeid', request.args.get('type_id')):
-        query = query.filter(Asset.assettypeid == int(type_id))
+    if (type_id := intarg('typeid', intarg('type_id'))) is not None:
+        query = query.filter(Asset.assettypeid == type_id)
 
     # Status filter
-    if status_id := request.args.get('statusid', request.args.get('status_id')):
-        query = query.filter(Asset.statusid == int(status_id))
+    if (status_id := intarg('statusid', intarg('status_id'))) is not None:
+        query = query.filter(Asset.statusid == status_id)
 
     # Location filter
-    if location_id := request.args.get('locationid', request.args.get('location_id')):
-        query = query.filter(Asset.locationid == int(location_id))
+    if (location_id := intarg('locationid', intarg('location_id'))) is not None:
+        query = query.filter(Asset.locationid == location_id)
 
     # Business unit filter
-    if bu_id := request.args.get('businessunitid', request.args.get('businessunit_id')):
-        query = query.filter(Asset.businessunitid == int(bu_id))
+    if (bu_id := intarg('businessunitid', intarg('businessunit_id'))) is not None:
+        query = query.filter(Asset.businessunitid == bu_id)
 
     # Sorting
     sort_by = request.args.get('sort', 'assetnumber')
@@ -1030,8 +1031,7 @@ def get_assets_map():
         query = query.join(AssetType).filter(AssetType.assettype == selected_assettype)
 
     # Filter by subtype (depends on asset type) - case-insensitive matching
-    if subtype_id := request.args.get('subtype'):
-        subtype_id = int(subtype_id)
+    if (subtype_id := intarg('subtype')) is not None:
         # Normalize the underscore DB form (measuring_tool, network_device) to
         # the space form the branches below compare against.
         asset_type_lower = (
@@ -1079,16 +1079,16 @@ def get_assets_map():
                 pass
 
     # Filter by business unit
-    if bu_id := request.args.get('businessunitid'):
-        query = query.filter(Asset.businessunitid == int(bu_id))
+    if (bu_id := intarg('businessunitid')) is not None:
+        query = query.filter(Asset.businessunitid == bu_id)
 
     # Filter by status
-    if status_id := request.args.get('statusid'):
-        query = query.filter(Asset.statusid == int(status_id))
+    if (status_id := intarg('statusid')) is not None:
+        query = query.filter(Asset.statusid == status_id)
 
     # Filter by location
-    if location_id := request.args.get('locationid'):
-        query = query.filter(Asset.locationid == int(location_id))
+    if (location_id := intarg('locationid')) is not None:
+        query = query.filter(Asset.locationid == location_id)
 
     # Search filter
     if search := request.args.get('search'):
diff --git a/shopdb/core/api/reports.py b/shopdb/core/api/reports.py
index 0b523580..3504a8a4 100644
--- a/shopdb/core/api/reports.py
+++ b/shopdb/core/api/reports.py
@@ -13,6 +13,7 @@ from shopdb.core.models import (
 )
 from shopdb.utils.responses import success_response, error_response, ErrorCodes
 from shopdb.utils.authz import require_permission
+from shopdb.utils.queryargs import intarg
 
 reports_bp = Blueprint('reports', __name__)
 
@@ -62,8 +63,8 @@ def machines_by_type():
     )
 
     # Business unit filter
-    if bu_id := request.args.get('businessunitid'):
-        query = query.filter(Asset.businessunitid == int(bu_id))
+    if (bu_id := intarg('businessunitid')) is not None:
+        query = query.filter(Asset.businessunitid == bu_id)
 
     # Dualpath single-machine collapse: exclude the secondary bay so a dual-bay
     # pair counts once per type bucket. Gated on the site setting (default on).
@@ -128,12 +129,12 @@ def assets_by_status():
     )
 
     # Asset type filter
-    if type_id := request.args.get('assettypeid'):
-        query = query.filter(Asset.assettypeid == int(type_id))
+    if (type_id := intarg('assettypeid')) is not None:
+        query = query.filter(Asset.assettypeid == type_id)
 
     # Business unit filter
-    if bu_id := request.args.get('businessunitid'):
-        query = query.filter(Asset.businessunitid == int(bu_id))
+    if (bu_id := intarg('businessunitid')) is not None:
+        query = query.filter(Asset.businessunitid == bu_id)
 
     query = query.group_by(
         AssetStatus.statusid,
@@ -175,7 +176,7 @@ def kb_popularity():
     - limit: Number of results (default 20)
     - format: 'json' (default) or 'csv'
     """
-    limit = min(int(request.args.get('limit', 20)), 100)
+    limit = min(intarg('limit', 20), 100)
 
     # Knowledge Base is a plugin; degrade gracefully if it is not installed.
     try:
@@ -362,10 +363,10 @@ def asset_inventory():
         AssetType.isactive == True
     )
 
-    if bu_id := request.args.get('businessunitid'):
-        by_type = by_type.filter(Asset.businessunitid == int(bu_id))
-    if loc_id := request.args.get('locationid'):
-        by_type = by_type.filter(Asset.locationid == int(loc_id))
+    if (bu_id := intarg('businessunitid')) is not None:
+        by_type = by_type.filter(Asset.businessunitid == bu_id)
+    if (loc_id := intarg('locationid')) is not None:
+        by_type = by_type.filter(Asset.locationid == loc_id)
 
     by_type = by_type.group_by(AssetType.assettype).all()
 
@@ -380,10 +381,10 @@ def asset_inventory():
         AssetStatus.isactive == True
     )
 
-    if bu_id := request.args.get('businessunitid'):
-        by_status = by_status.filter(Asset.businessunitid == int(bu_id))
-    if loc_id := request.args.get('locationid'):
-        by_status = by_status.filter(Asset.locationid == int(loc_id))
+    if (bu_id := intarg('businessunitid')) is not None:
+        by_status = by_status.filter(Asset.businessunitid == bu_id)
+    if (loc_id := intarg('locationid')) is not None:
+        by_status = by_status.filter(Asset.locationid == loc_id)
 
     by_status = by_status.group_by(AssetStatus.status, AssetStatus.color).all()
 
@@ -398,8 +399,8 @@ def asset_inventory():
         Location.isactive == True
     )
 
-    if bu_id := request.args.get('businessunitid'):
-        by_location = by_location.filter(Asset.businessunitid == int(bu_id))
+    if (bu_id := intarg('businessunitid')) is not None:
+        by_location = by_location.filter(Asset.businessunitid == bu_id)
 
     by_location = by_location.group_by(Location.locationname).all()
 
diff --git a/shopdb/utils/queryargs.py b/shopdb/utils/queryargs.py
new file mode 100644
index 00000000..848e5f34
--- /dev/null
+++ b/shopdb/utils/queryargs.py
@@ -0,0 +1,64 @@
+"""Reading a typed value out of the query string, without a 500.
+
+WHY THIS EXISTS
+
+List endpoints filtered like this:
+
+    if type_id := request.args.get('typeid'):
+        query = query.filter(Machine.machinetypeid == int(type_id))
+
+`int('abc')` raises, nothing catches it, and the caller gets
+500 INTERNAL_ERROR with a traceback in the log. A wrong query argument is the
+caller's mistake and deserves a 400 saying which argument; a 500 says the server
+is broken and sends whoever is reading the log looking for a fault that is not
+there. The feature test run found the same shape in six places across five
+plugins, which makes it a property of how list routes are written rather than of
+any one route.
+
+The helpers raise QueryArgError. Routes turn it into a 400 - or, more usually,
+nothing does anything at all, because the app-level handler in
+shopdb/__init__.py answers it. That is the point: a route should not have to
+remember.
+
+Deliberately NOT the same policy as get_pagination_params, which falls back to
+a default for a bad page number. A nonsense page is harmless and the caller
+still wants their list; a nonsense FILTER is not, because silently ignoring it
+returns rows the caller believes they excluded.
+"""
+
+from flask import request
+
+
+class QueryArgError(ValueError):
+    """A query argument was present but not of the type the route needs."""
+
+    def __init__(self, name, value, expected='a whole number'):
+        self.name = name
+        self.value = value
+        super().__init__('%s must be %s (got %r)' % (name, expected, value))
+
+
+def intarg(name, default=None, request_obj=None):
+    """Read an integer query argument.
+
+    Absent or empty -> default. Present but not an integer -> QueryArgError,
+    which the app answers as a 400 naming the argument.
+    """
+    source = request_obj if request_obj is not None else request
+    raw = source.args.get(name)
+    if raw is None or raw == '':
+        return default
+    try:
+        return int(raw)
+    except (TypeError, ValueError):
+        raise QueryArgError(name, raw)
+
+
+def intargs(*names, request_obj=None):
+    """Read several integer arguments at once; returns a dict of those present."""
+    found = {}
+    for name in names:
+        value = intarg(name, request_obj=request_obj)
+        if value is not None:
+            found[name] = value
+    return found
diff --git a/tests/test_core/test_bad_query_args.py b/tests/test_core/test_bad_query_args.py
new file mode 100644
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+++ b/tests/test_core/test_bad_query_args.py
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+"""No list route may answer a nonsense filter with a 500.
+
+THE GATE, not just the six fixes
+
+The feature test run found `int(request.args.get(...))` with nothing catching
+the ValueError in six routes across five plugins - machines, network, printers,
+knowledgebase, computers and the asset core. Fixing six sites does not stop a
+seventh: the pattern is how a filter gets written, and it is invisible in review
+because the happy path works.
+
+So this sweeps every registered GET route that takes no path parameters and
+sends each of them a set of arguments that are not what any filter wants. The
+contract is narrow on purpose:
+
+  - a 5xx is ALWAYS a failure. The caller sent a bad argument; that is a 400.
+  - a 400 is the intended answer.
+  - a 200 is acceptable: a route is entitled to ignore an argument it does not
+    know about, and most of these routes will not have the argument at all.
+  - a 401/403/404 is acceptable: this test is about argument handling, not
+    about authorization.
+
+Run as an admin, because a route that refuses everyone tests nothing.
+"""
+import pytest
+
+# Names taken from the filters the failing routes actually declared, plus the
+# generic shapes a client sends. Values chosen to break a bare int(), a bare
+# float(), and a naive split.
+BAD_ARGS = [
+    ('typeid', 'abc'),
+    ('type_id', 'abc'),
+    ('assettypeid', '1; DROP TABLE assets'),
+    ('vendorid', 'null'),
+    ('locationid', '1.5'),
+    ('businessunitid', ''),
+    ('appid', 'abc'),
+    ('statusid', 'NaN'),
+    ('modelid', '-'),
+    ('limit', 'lots'),
+    ('page', 'abc'),
+    ('perpage', 'abc'),
+]
+
+# Routes that are not list endpoints and whose GET does real work with side
+# effects or external calls. Excluded by PREFIX, and each one says why.
+SKIP_PREFIXES = (
+    '/api/collector/',      # needs the collector key; fail-closed by design
+    '/api/endpoints/',      # agent wire protocol, same key
+    '/api/backups/run',     # kicks off work rather than reading
+)
+
+# Skipped by EXACT path, not prefix, so the rest of the plugin stays covered.
+SKIP_EXACT = {
+    # These four reach the EXTERNAL cmmc_usb database. Their 500 here is "no
+    # such database in the test environment", a different fault from the one
+    # this gate is about, and the list is the complete set - every usb GET
+    # route whose handler calls cmmc_usb_connection. The self-hosted USB
+    # surface is covered by test_usb_kiosk.py.
+    '/api/usb',
+    '/api/usb/checkouts',
+    '/api/usb/checkouts/active',
+    '/api/usb/reports/audit',
+}
+
+
+def _listish_routes(app):
+    routes = []
+    for rule in app.url_map.iter_rules():
+        path = str(rule.rule)
+        if not path.startswith('/api/') or '<' in path:
+            continue
+        if 'GET' not in (rule.methods or ()):
+            continue
+        if path in SKIP_EXACT:
+            continue
+        if any(path.startswith(prefix) for prefix in SKIP_PREFIXES):
+            continue
+        routes.append(path)
+    return sorted(set(routes))
+
+
+def test_the_sweep_actually_covers_something(app):
+    """A gate that silently matches nothing passes forever. This one has been
+    known to cover 90+ routes; if it drops to a handful, the filter above is
+    wrong rather than the app being small."""
+    assert len(_listish_routes(app)) > 40
+
+
+def test_no_route_answers_a_nonsense_filter_with_a_server_error(
+        client, auth_headers, app):
+    failures = []
+    for path in _listish_routes(app):
+        for name, value in BAD_ARGS:
+            response = client.get('%s?%s=%s' % (path, name, value),
+                                  headers=auth_headers)
+            if response.status_code >= 500:
+                failures.append('%s?%s=%s -> %d' % (path, name, value,
+                                                    response.status_code))
+    assert not failures, (
+        'a bad query argument is a 400, never a 500:\n  '
+        + '\n  '.join(failures[:40]))
+
+
+@pytest.mark.parametrize('value', ['abc', '', '1.5', 'null'])
+def test_a_bad_type_filter_is_a_400_that_names_the_argument(
+        client, auth_headers, value):
+    """The routes the run actually caught. An empty value is the exception:
+    absent and empty both mean 'no filter', which is what a cleared dropdown
+    sends."""
+    for path in ('/api/machines', '/api/computers', '/api/network',
+                 '/api/printers', '/api/assets'):
+        response = client.get('%s?typeid=%s' % (path, value),
+                              headers=auth_headers)
+        if value == '':
+            assert response.status_code == 200, (path, response.get_json())
+            continue
+        assert response.status_code == 400, (path, response.status_code)
+        body = response.get_json()
+        assert body['data']['error']['code'] == 'VALIDATION_ERROR'
+        assert 'typeid' in body['data']['error']['message']