@@ -71,6 +71,10 @@
 
 # Note: ``'nextToken'`` below is the Billing API's pagination field name, not a
 # credential; the same PageSpec shape is used by enterprise_support_operations.
+# Both operations reject a bad caller token with HTTP 400 ValidationException
+# and a message naming nextToken ("Invalid nextToken", or a constraint message
+# for a malformed one), so that code counts as a rejected token on the caller's
+# own next_token.
 
 # How ListBusinessSupportAccountCharges pages: the per-account charge rows come
 # back under ``accountCharges``, continued by ``nextToken``, at up to 100 per
@@ -81,6 +85,7 @@
     token_key_in_response='nextToken',
     page_max=100,
     page_size_param='maxResults',
+    caller_token_error_codes=frozenset({'ValidationException'}),
 )
 
 # How ListBusinessSupportSubscriptionHistory pages: the contract rows come back
@@ -92,6 +97,7 @@
     token_key_in_response='nextToken',
     page_max=100,
     page_size_param='maxResults',
+    caller_token_error_codes=frozenset({'ValidationException'}),
 )
 
 
 
@@ -24,6 +24,7 @@
     list_subscription_history,
 )
 from awslabs.billing_cost_management_mcp_server.utilities import sql_utils
+from awslabs.billing_cost_management_mcp_server.utilities.error_contract import add_retry_guidance
 from botocore.exceptions import ClientError, EndpointConnectionError
 from botocore.stub import Stubber
 from datetime import datetime, timezone
@@ -976,6 +977,72 @@ async def test_subscription_history_with_window(self, mock_context):
         assert result['data']['pagination']['items_returned'] == 1
 
 
+# The service rejects a bad caller token with ValidationException (HTTP 400) for
+# both operations: "Invalid nextToken" for a well-formed token, a constraint
+# message naming nextToken for a malformed one. The PageSpecs must name the code.
+INVALID_TOKEN_CASES = (
+    (list_account_charges, 'list_business_support_account_charges', {'billing_month': '2026-06'}),
+    (list_subscription_history, 'list_business_support_subscription_history', {}),
+)
+
+
+async def _run_with_stubbed_error(mock_context, handler, operation, message, **handler_kwargs):
+    """Run a handler through the real paginator against a client that raises ValidationException."""
+    client = boto3.client('billing', region_name='us-east-1')
+    stubber = Stubber(client)
+    stubber.add_client_error(
+        operation,
+        service_error_code='ValidationException',
+        service_message=message,
+        http_status_code=400,
+    )
+    stubber.activate()
+    with patch(CREATE_CLIENT_PATH, return_value=client):
+        result = await handler(mock_context, **handler_kwargs)
+    stubber.assert_no_pending_responses()
+    return result
+
+
+class TestRejectedCallerToken:
+    """A rejected caller next_token is reported as invalid_next_token, not a generic error."""
+
+    @pytest.mark.asyncio
+    @pytest.mark.parametrize('handler,operation,kwargs', INVALID_TOKEN_CASES)
+    @pytest.mark.parametrize(
+        'message',
+        [
+            'Invalid nextToken',
+            "1 validation error detected: Value at 'nextToken' failed to satisfy constraint: "
+            'Member must satisfy regular expression pattern: [-a-zA-Z0-9+=/_]+',
+        ],
+    )
+    async def test_rejected_caller_token_is_invalid_next_token(
+        self, mock_context, handler, operation, kwargs, message
+    ):
+        """A ValidationException naming the caller's token maps to invalid_next_token."""
+        result = await _run_with_stubbed_error(
+            mock_context, handler, operation, message, next_token='stale', **kwargs
+        )
+
+        assert result['status'] == 'error'
+        assert result['error_type'] == 'invalid_next_token'
+        # The server's error middleware adds the hint the caller sees.
+        assert 'without next_token' in add_retry_guidance(result)['hint']
+
+    @pytest.mark.asyncio
+    @pytest.mark.parametrize('handler,operation,kwargs', INVALID_TOKEN_CASES)
+    async def test_validation_error_without_caller_token_is_unchanged(
+        self, mock_context, handler, operation, kwargs
+    ):
+        """Without a caller next_token the same code stays the service's validation error."""
+        result = await _run_with_stubbed_error(
+            mock_context, handler, operation, 'Invalid nextToken', **kwargs
+        )
+
+        assert result['status'] == 'error'
+        assert result['error_type'] != 'invalid_next_token'
+
+
 # ---------------------------------------------------------------------------
 # Real SQL offload (convert_response_if_needed is NOT mocked)
 # ---------------------------------------------------------------------------
 
@@ -27,6 +27,7 @@
     bcm_pricing_calculator_tools,
     billing_conductor_operations,
     billing_preferences_operations,
+    business_support_operations,
     bvs_operations,
     compute_optimizer_automation_operations,
     cost_optimization_hub_helpers,
@@ -188,6 +189,16 @@
         VALIDATION,
         id='COH_LIST_EFFICIENCY_METRICS',
     ),
+    pytest.param(
+        business_support_operations.LIST_ACCOUNT_CHARGES_PAGE_SPEC,
+        VALIDATION,
+        id='LIST_ACCOUNT_CHARGES_PAGE_SPEC',
+    ),
+    pytest.param(
+        business_support_operations.LIST_SUBSCRIPTION_HISTORY_PAGE_SPEC,
+        VALIDATION,
+        id='LIST_SUBSCRIPTION_HISTORY_PAGE_SPEC',
+    ),
     pytest.param(
         credits_operations.GET_CREDIT_ALLOCATION_HISTORY_PAGE_SPEC,
         VALIDATION,