@@ -405,7 +405,7 @@ async def cost_comparison(
             metric_for_comparison,
             today,
         )
-        validate_comparison_filter(parse_json(filter, 'filter'))
+        validate_comparison_filter(parse_filter(filter))
 
         # Initialize Cost Explorer client using shared utility
         ce_client = create_aws_client('ce', region_name='us-east-1')
 
@@ -660,6 +660,7 @@ async def get_tags(
     end_date: Optional[str] = None,
     search_string: Optional[str] = None,
     tag_key: Optional[str] = None,
+    filter_expr: Optional[Union[str, Dict[str, Any]]] = None,
     next_token: Optional[str] = None,
     max_items: Optional[int] = None,
     billing_view_arn: Optional[str] = None,
@@ -673,6 +674,7 @@ async def get_tags(
         end_date: End date in YYYY-MM-DD format (exclusive)
         search_string: Optional string to filter results
         tag_key: Optional specific tag key to get values for
+        filter_expr: Optional filters, as an object or JSON string
         next_token: Continuation token from a previous response
         max_items: Most items to return; None fetches every page
         billing_view_arn: Optional ARN of a billing view to scope the query.
@@ -699,6 +701,10 @@ async def get_tags(
         if tag_key:
             request_params['TagKey'] = str(tag_key)
 
+        filters = parse_filter(filter_expr)
+        if filters is not None:
+            request_params['Filter'] = filters
+
         if billing_view_arn:
             request_params['BillingViewArn'] = billing_view_arn
 
@@ -731,6 +737,7 @@ async def get_cost_categories(
     end_date: Optional[str] = None,
     search_string: Optional[str] = None,
     cost_category_name: Optional[str] = None,
+    filter_expr: Optional[Union[str, Dict[str, Any]]] = None,
     next_token: Optional[str] = None,
     max_items: Optional[int] = None,
     billing_view_arn: Optional[str] = None,
@@ -744,6 +751,7 @@ async def get_cost_categories(
         end_date: End date in YYYY-MM-DD format (exclusive)
         search_string: Optional string to filter results
         cost_category_name: Optional specific cost category to get values for
+        filter_expr: Optional filters, as an object or JSON string
         next_token: Continuation token from a previous response
         max_items: Most items to return; None fetches every page
         billing_view_arn: Optional ARN of a billing view to scope the query.
@@ -773,6 +781,10 @@ async def get_cost_categories(
         if cost_category_name:
             request_params['CostCategoryName'] = str(cost_category_name)
 
+        filters = parse_filter(filter_expr)
+        if filters is not None:
+            request_params['Filter'] = filters
+
         if billing_view_arn:
             request_params['BillingViewArn'] = billing_view_arn
 
 
@@ -18,7 +18,12 @@
 """
 
 import json
-from ..utilities.aws_service_base import create_aws_client, format_response, handle_aws_error
+from ..utilities.aws_service_base import (
+    create_aws_client,
+    format_response,
+    handle_aws_error,
+    parse_json,
+)
 from ..utilities.constants import PAGINATION_LIMIT_TEXT
 from .cost_explorer_operations import (
     get_cost_and_usage,
@@ -183,7 +188,8 @@ def _with_example_dates(description: str) -> str:
   where the tag or cost category key is not present, so there is no value to compare against:
   omit `Values` entirely (an empty `Values` list is rejected too).
 
-Tags leaves in getCostAndUsage, getCostAndUsageWithResources and getDimensionValues filters:
+Tags leaves in getCostAndUsage, getCostAndUsageWithResources, getDimensionValues,
+getTagsOrValues and getCostCategories filters:
 - `Key` is the tag key as getTagsOrValues lists it, with no `user:` prefix even where billing
   exports show one: `Environment`, not `user:Environment` (which matches nothing, with no
   error). The forecasts accept either form. Tags that AWS applies itself keep
@@ -196,10 +202,11 @@ def _with_example_dates(description: str) -> str:
 - getCostAndUsage only: `{"Tags": {"Key": "", "Values": []}}` (empty key, empty list) matches
   costs that carry no activated cost allocation tag (user-defined or AWS-generated), as does
   `{"Tags": {"MatchOptions": ["ABSENT"]}}` with no Key. An empty key with any value, or with
-  Values omitted, is rejected, and getDimensionValues and getCostAndUsageWithResources reject
-  an empty key.
-- A wrong key is not an error. With `Values` it matches nothing ($0); with `ABSENT` it matches
-  every cost, since no cost carries that key. Copy the key from getTagsOrValues.
+  Values omitted, is rejected, and getCostAndUsageWithResources, getDimensionValues,
+  getTagsOrValues and getCostCategories reject an empty key.
+- A wrong key is not an error. With `Values` it matches nothing ($0, or an empty list); with
+  `ABSENT` it matches every cost, since no cost carries that key. Copy the key from
+  getTagsOrValues.
 
 Correct filter examples:
 - `{"Dimensions": {"Key": "SERVICE", "Values": ["Amazon Relational Database Service"]}}`
@@ -211,7 +218,8 @@ def _with_example_dates(description: str) -> str:
 - Every operation: at most 6,000 values in total, counted across all leaves of the filter.
 - getCostAndUsage, getCostAndUsageWithResources: `And`, `Or` and `Not` nest up to 4 levels
   deep; a 5th is rejected.
-- getDimensionValues (default COST_AND_USAGE context): `And` only; `Or` and `Not` are rejected.
+- getDimensionValues (default COST_AND_USAGE context), getTagsOrValues and getCostCategories:
+  `And` of leaves only (an `And` inside it is rejected); `Or` and `Not` are rejected.
 - getSavingsPlansUtilization: `And` and `Not` only; `Or` is rejected.
 
 ## OPERATIONS
@@ -260,19 +268,19 @@ def _with_example_dates(description: str) -> str:
 
 6. getTagsOrValues - Available cost allocation tags or values
    Required: operation="getTagsOrValues"
-   Optional: start_date, end_date, tag_key, search_string, max_items, next_token, billing_view_arn
+   Optional: start_date, end_date, tag_key, search_string, filter, max_items, next_token, billing_view_arn
    Defaults: If start_date is omitted, the tool uses 30 days before today; if end_date is omitted, it uses today (exclusive, so today is not included).
    Example 1: {"operation": "getTagsOrValues"}
    Example 2: {"operation": "getTagsOrValues", "tag_key": "Environment"}
-   Returns: List of available cost allocation tags (see PAGINATION). If tag values for a particular key are needed, pass the tag key as a parameter. The tag key list holds at most 5,000 keys in total.
+   Returns: List of available cost allocation tags (see PAGINATION). If tag values for a particular key are needed, pass the tag key as a parameter. The tag key list holds at most 5,000 keys in total. A filter keeps only the keys (or values) found on costs that match it, e.g. the tags used in one Region.
 
 7. getCostCategories - Available cost categories or values inside a category
    Required: operation="getCostCategories"
-   Optional: start_date, end_date, cost_category_name, search_string, max_items, next_token, billing_view_arn
+   Optional: start_date, end_date, cost_category_name, search_string, filter, max_items, next_token, billing_view_arn
    Defaults: If start_date is omitted, the tool uses 30 days before today; if end_date is omitted, it uses today (exclusive, so today is not included).
    Example 1 (list category names): {"operation": "getCostCategories", "start_date": "__SIX_MONTHS_AGO_START__", "end_date": "__MONTH_START__"}
    Example 2 (list values inside a category): {"operation": "getCostCategories", "cost_category_name": "CostCenters", "start_date": "__SIX_MONTHS_AGO_START__", "end_date": "__MONTH_START__"}
-   Notes: Without cost_category_name the response contains CostCategoryNames. With cost_category_name set the response contains CostCategoryValues for that category. The category name list holds at most 5,000 names in total. See PAGINATION.
+   Notes: Without cost_category_name the response contains CostCategoryNames. With cost_category_name set the response contains CostCategoryValues for that category. The category name list holds at most 5,000 names in total. A filter keeps only the names (or values) found on costs that match it. See PAGINATION.
 
 8. getSavingsPlansUtilization - Savings Plans utilization data
    Required: operation="getSavingsPlansUtilization"
@@ -423,10 +431,10 @@ def _tag_leaf_hints(key: str, values: List[Any], absent: bool) -> List[str]:
 def _tag_filter_hint(filter: Optional[Union[str, Dict[str, Any]]]) -> Optional[str]:
     """Return a hint for a Tags filter that Cost Explorer accepts but that cannot match.
 
-    getCostAndUsage, getCostAndUsageWithResources and getDimensionValues add the `user:`
-    prefix to user-defined tag keys themselves, so a `user:` key matches no tagged cost, and a TAG group_by label
-    (`key$value`) is not a key or value. With no error, Values then match nothing and
-    ABSENT matches every cost.
+    getCostAndUsage, getCostAndUsageWithResources, getDimensionValues, getTagsOrValues and
+    getCostCategories add the `user:` prefix to user-defined tag keys themselves, so a
+    `user:` key matches no tagged cost, and a TAG group_by label (`key$value`) is not a key
+    or value. With no error, Values then match nothing and ABSENT matches every cost.
     """
     try:
         if isinstance(filter, str):
@@ -447,6 +455,19 @@ def _tag_filter_hint(filter: Optional[Union[str, Dict[str, Any]]]) -> Optional[s
     return ' '.join(hints) or None
 
 
+def _asks_for_grouping(group_by: Any) -> bool:
+    """Return whether group_by asks for grouping.
+
+    An empty or falsy JSON value ('[]', 'null', '{}') does not, as getCostAndUsage reads
+    it. A group_by that is not JSON still does, so it gets the not-supported error rather
+    than a JSON one.
+    """
+    try:
+        return bool(parse_json(group_by, 'group_by'))
+    except ValueError:
+        return True
+
+
 def _with_hint(result: Dict[str, Any], hint: Optional[str]) -> Dict[str, Any]:
     """Add the hint to a successful response.
 
@@ -543,7 +564,7 @@ async def cost_explorer(
         tag_hint = _tag_filter_hint(filter)
         await ctx.info(f'Routing to operation: {operation}')
 
-        if group_by and operation in OPERATIONS_WITHOUT_GROUP_BY:
+        if operation in OPERATIONS_WITHOUT_GROUP_BY and _asks_for_grouping(group_by):
             return format_response(
                 'error',
                 {
@@ -722,32 +743,40 @@ async def cost_explorer(
             )
 
         elif operation == 'getTagsOrValues':
-            return await get_tags(
-                ctx,
-                ce_client,
-                start_date,
-                end_date,
-                search_string,
-                tag_key,
-                next_token,
-                max_items,
-                billing_view_arn,
+            return _with_hint(
+                await get_tags(
+                    ctx,
+                    ce_client,
+                    start_date,
+                    end_date,
+                    search_string,
+                    tag_key,
+                    filter,
+                    next_token,
+                    max_items,
+                    billing_view_arn,
+                ),
+                tag_hint,
             )
 
         elif operation == 'getCostCategories':
             # When `cost_category_name` is provided, the same AWS API
             # (GetCostCategories) returns the values inside that category
             # instead of the list of category names.
-            return await get_cost_categories(
-                ctx,
-                ce_client,
-                start_date,
-                end_date,
-                search_string,
-                cost_category_name,
-                next_token,
-                max_items,
-                billing_view_arn,
+            return _with_hint(
+                await get_cost_categories(
+                    ctx,
+                    ce_client,
+                    start_date,
+                    end_date,
+                    search_string,
+                    cost_category_name,
+                    filter,
+                    next_token,
+                    max_items,
+                    billing_view_arn,
+                ),
+                tag_hint,
             )
 
         elif operation == 'getSavingsPlansUtilization':
 
@@ -233,8 +233,9 @@ def parse_filter(
 ) -> Any:
     """Parse a filter expression, rejecting an empty one.
 
-    An empty filter (``''``, ``{}``, ``[]``, ``null``) is an error rather than "no filter":
-    dropping it would run the query unfiltered, and the result would read as a filtered one.
+    An empty filter (``''``, ``""`` or another blank JSON string, ``{}``, ``[]``, ``null``)
+    is an error rather than "no filter": dropping it would run the query unfiltered, and the
+    result would read as a filtered one.
 
     Args:
         filter_expr: The filter, as an object or a JSON string. None means no filter.
@@ -252,7 +253,7 @@ def parse_filter(
     if isinstance(filter_expr, str):
         filter_expr = filter_expr.strip()
     parsed = parse_json(filter_expr, parameter_name)
-    if parsed is None or parsed == {} or parsed == []:
+    if parsed in (None, {}, []) or (isinstance(parsed, str) and not parsed.strip()):
         raise ValueError(
             f'{parameter_name} is empty. Omit {parameter_name} to query without one, or pass '
             'a filter expression.'
 
@@ -1528,6 +1528,19 @@ async def test_invalid_filter_json_is_a_validation_error(
         )
         _assert_local_validation_error(result, dispatch_ce_client, 'Invalid JSON format')
 
+    @pytest.mark.parametrize('flt', ['   ', '{}', '""'], ids=repr)
+    async def test_empty_filter_is_called_empty(self, mock_context, dispatch_ce_client, flt):
+        """A blank or empty filter gets the "filter is empty" error, not a JSON error."""
+        result = await _call(
+            mock_context,
+            dispatch_ce_client,
+            'getCostAndUsageComparisons',
+            month(2),
+            month(1),
+            filter=flt,
+        )
+        _assert_local_validation_error(result, dispatch_ce_client, 'filter is empty')
+
 
 class TestValidateComparisonRequest:
     """Boundaries of the rules, at a fixed date."""
 
@@ -31,6 +31,9 @@
 from unittest.mock import AsyncMock, MagicMock, patch
 
 
+REGION_FILTER = {'Dimensions': {'Key': 'REGION', 'Values': ['us-east-1']}}
+
+
 @pytest.fixture
 def mock_context():
     """Create a mock MCP context."""
@@ -1559,6 +1562,34 @@ async def test_with_search_string(self, mock_context, mock_ce_client):
         assert 'SearchString' in call_kwargs
         assert call_kwargs['SearchString'] == 'Env'
 
+    @pytest.mark.parametrize('filter_expr', [REGION_FILTER, json.dumps(REGION_FILTER)])
+    @pytest.mark.parametrize('tag_key', [None, 'Environment'], ids=['keys', 'values'])
+    async def test_filter_is_sent(self, mock_context, mock_ce_client, filter_expr, tag_key):
+        """The filter reaches GetTags as Filter, for tag keys and for one key's values.
+
+        Dropping it would return the unfiltered list as if it were filtered.
+        """
+        result = await get_tags(
+            mock_context, mock_ce_client, tag_key=tag_key, filter_expr=filter_expr
+        )
+
+        assert result['status'] == 'success'
+        assert mock_ce_client.get_tags.call_args[1]['Filter'] == REGION_FILTER
+
+    async def test_no_filter_sends_none(self, mock_context, mock_ce_client):
+        """Without a filter, no Filter key is sent."""
+        await get_tags(mock_context, mock_ce_client)
+
+        assert 'Filter' not in mock_ce_client.get_tags.call_args[1]
+
+    async def test_empty_filter_is_rejected_before_aws(self, mock_context, mock_ce_client):
+        """An empty filter is an error, not an unfiltered call."""
+        result = await get_tags(mock_context, mock_ce_client, filter_expr='{}')
+
+        assert result['status'] == 'error'
+        assert 'filter is empty' in result['message']
+        mock_ce_client.get_tags.assert_not_called()
+
     async def test_error_handling(self, mock_context, mock_ce_client):
         """Test error handling in get_tags."""
         # Setup error
@@ -1672,6 +1703,34 @@ async def test_with_search_string(self, mock_context, mock_ce_client):
         assert 'SearchString' in call_kwargs
         assert call_kwargs['SearchString'] == 'Dep'
 
+    @pytest.mark.parametrize('filter_expr', [REGION_FILTER, json.dumps(REGION_FILTER)])
+    @pytest.mark.parametrize('name', [None, 'Department'], ids=['names', 'values'])
+    async def test_filter_is_sent(self, mock_context, mock_ce_client, filter_expr, name):
+        """The filter reaches GetCostCategories as Filter, for names and for one category's values.
+
+        Dropping it would return the unfiltered list as if it were filtered.
+        """
+        result = await get_cost_categories(
+            mock_context, mock_ce_client, cost_category_name=name, filter_expr=filter_expr
+        )
+
+        assert result['status'] == 'success'
+        assert mock_ce_client.get_cost_categories.call_args[1]['Filter'] == REGION_FILTER
+
+    async def test_no_filter_sends_none(self, mock_context, mock_ce_client):
+        """Without a filter, no Filter key is sent."""
+        await get_cost_categories(mock_context, mock_ce_client)
+
+        assert 'Filter' not in mock_ce_client.get_cost_categories.call_args[1]
+
+    async def test_empty_filter_is_rejected_before_aws(self, mock_context, mock_ce_client):
+        """An empty filter is an error, not an unfiltered call."""
+        result = await get_cost_categories(mock_context, mock_ce_client, filter_expr='{}')
+
+        assert result['status'] == 'error'
+        assert 'filter is empty' in result['message']
+        mock_ce_client.get_cost_categories.assert_not_called()
+
     async def test_error_handling(self, mock_context, mock_ce_client):
         """Test error handling in get_cost_categories."""
         # Setup error
Original file line number	Diff line number	Diff line change
`@@ -405,7 +405,7 @@ async def cost_comparison(`
`405`	`405`	`metric_for_comparison,`
`406`	`406`	`today,`
`407`	`407`	`)`
`408`		`- validate_comparison_filter(parse_json(filter, 'filter'))`
	`408`	`+ validate_comparison_filter(parse_filter(filter))`
`409`	`409`
`410`	`410`	`# Initialize Cost Explorer client using shared utility`
`411`	`411`	`ce_client = create_aws_client('ce', region_name='us-east-1')`