fix(scan): drop the FilterExpression suffix from the values message

crates/core/src/expression/resolver.rs

@@ -200,21 +200,21 @@ pub fn resolve_element_name<'a>(
 }
 
 /// Reject `ExpressionAttributeNames`/`Values` supplied with no expression that
-/// can reference them. Matches Amazon `DynamoDB`:
+/// can reference them:
 ///
 /// - names: `ExpressionAttributeNames can only be specified when using expressions`
 ///   (no suffix), for every API.
 /// - values: `ExpressionAttributeValues can only be specified when using
-///   expressions: <params> is/are null`, listing the value-capable expression
-///   parameter(s) of the API.
+///   expressions`, followed by `: <param> is null` or `: <a> and <b> are null`
+///   when `value_expr_params` is not empty.
 ///
 /// `name_expr_present` is true when any expression that can carry a `#name` is
 /// present (projection, condition, filter, key condition, update).
 /// `value_expr_present` is true when any expression that can carry a `:value` is
 /// present (condition, filter, key condition, update; not projection).
-/// `value_expr_params` is the API's static list of value-capable expression
-/// parameter kinds, used to build the values suffix. An empty list yields no
-/// suffix (matching Query).
+/// `value_expr_params` lists the expression parameters the values message names
+/// as null. Query and Scan pass an empty list, because the service's message for
+/// them has no suffix.
 ///
 /// # Errors
 ///

crates/engine/src/scan.rs

@@ -106,12 +106,13 @@ pub async fn handle_scan(
         .projection_expression
         .as_ref()
         .is_some_and(|s| !s.is_empty());
+    // Scan emits no values suffix (measured in us-east-1, #722).
     extenddb_core::expression::validate_expression_param_usage(
         input.expression_attribute_names.as_ref(),
         has_proj_expr || has_filter_expr,
         input.expression_attribute_values.as_ref(),
         has_filter_expr,
-        &[ExpressionKind::Filter],
+        &[],
     )?;
 
     // ConditionalOperator requires a ScanFilter with two or more conditions.

tests/test_expression_param_usage.py

@@ -5,8 +5,8 @@
 
 Amazon DynamoDB rejects a request that provides ExpressionAttributeNames or
 ExpressionAttributeValues when there is no expression parameter that could
-reference them. The names message is always suffix-free; the values message
-names the null value-capable expression parameter(s) for that API.
+reference them. The names message is always suffix-free. On Query and Scan
+the values message is suffix-free as well: it ends after "expressions".
 
 Dual-target against Amazon DynamoDB and extenddb.
 """
@@ -82,7 +82,7 @@ def test_scan_values_no_expression(self, dynamodb_client, hash_table):
                 TableName=hash_table,
                 ExpressionAttributeValues={":v": {"S": "x"}},
             ),
-            f"{VALUES_PREFIX}: FilterExpression is null",
+            VALUES_PREFIX,
         )
 
     def test_delete_item_names_no_expression(self, dynamodb_client, hash_table):
@@ -162,7 +162,7 @@ def test_query_names_no_expression(self, dynamodb_client, hash_table):
         )
 
     def test_query_values_no_expression(self, dynamodb_client, hash_table):
-        # Query emits no values suffix, unlike Scan/Delete/Update.
+        # Query emits no values suffix, as Scan does.
         _expect_validation(
             lambda: dynamodb_client.query(
                 TableName=hash_table,