diff --git a/schemas/dab.draft.schema.json b/schemas/dab.draft.schema.json
index 4561b06952..25dd39bc95 100644
--- a/schemas/dab.draft.schema.json
+++ b/schemas/dab.draft.schema.json
@@ -244,6 +244,11 @@
               "description": "Maximum allowed depth of a GraphQL query.",
               "default": null
             },
+            "enable-aggregation": {
+              "$ref": "#/$defs/boolean-or-string",
+              "description": "Allow enabling/disabling aggregation (groupBy, sum, avg, min, max, count) for supported database types (MSSQL, DWSQL).",
+              "default": true
+            },
             "multiple-mutations": {
               "type": "object",
               "description": "Configuration properties for multiple mutation operations",
diff --git a/src/Config/ObjectModel/RuntimeConfig.cs b/src/Config/ObjectModel/RuntimeConfig.cs
index d6bdefc1b8..5d18ec75da 100644
--- a/src/Config/ObjectModel/RuntimeConfig.cs
+++ b/src/Config/ObjectModel/RuntimeConfig.cs
@@ -194,12 +194,14 @@ Runtime.GraphQL is null ||
     public string DefaultDataSourceName { get; set; }
 
     /// <summary>
-    /// Retrieves the value of runtime.graphql.aggregation.enabled property if present, default is true.
+    /// Retrieves the value of runtime.graphql.enable-aggregation property if present, default is true.
+    /// Returns true when runtime section is absent, when graphql section is absent,
+    /// or when enable-aggregation is explicitly set to true.
     /// </summary>
     [JsonIgnore]
     public bool EnableAggregation =>
-        Runtime is not null &&
-        Runtime.GraphQL is not null &&
+        Runtime is null ||
+        Runtime.GraphQL is null ||
         Runtime.GraphQL.EnableAggregation;
 
     [JsonIgnore]
diff --git a/src/Core/Services/GraphQLSchemaCreator.cs b/src/Core/Services/GraphQLSchemaCreator.cs
index 90e918c833..229b8b0855 100644
--- a/src/Core/Services/GraphQLSchemaCreator.cs
+++ b/src/Core/Services/GraphQLSchemaCreator.cs
@@ -80,12 +80,14 @@ public GraphQLSchemaCreator(
         /// <summary>
         /// Executed when a hot-reload event occurs. Pulls the latest
         /// runtimeconfig object from the provider and updates the flag indicating
-        /// whether multiple create operations are enabled, and the entities based on the new config.
+        /// whether multiple create operations are enabled, whether aggregation is enabled,
+        /// and the entities based on the new config.
         /// </summary>
         protected void OnConfigChanged(object? sender, HotReloadEventArgs args)
         {
             RuntimeConfig runtimeConfig = _runtimeConfigProvider.GetConfig();
             _isMultipleCreateOperationEnabled = runtimeConfig.IsMultipleCreateOperationEnabled();
+            _isAggregationEnabled = runtimeConfig.EnableAggregation;
             _entities = runtimeConfig.Entities;
         }
 
diff --git a/src/Service.Tests/Configuration/RuntimeConfigLoaderTests.cs b/src/Service.Tests/Configuration/RuntimeConfigLoaderTests.cs
index d724753f97..024c84c409 100644
--- a/src/Service.Tests/Configuration/RuntimeConfigLoaderTests.cs
+++ b/src/Service.Tests/Configuration/RuntimeConfigLoaderTests.cs
@@ -283,4 +283,159 @@ public async Task ChildConfigLoadFailureHaltsParentConfigLoading()
             }
         }
     }
+
+    /// <summary>
+    /// Tests that EnableAggregation returns true by default when runtime.graphql section is absent.
+    /// This is a regression test for the bug where EnableAggregation returned false (disabled)
+    /// when Runtime.GraphQL was null, even though the default value for EnableAggregation is true.
+    /// </summary>
+    [TestMethod]
+    public void EnableAggregation_WhenGraphQLSectionAbsent_DefaultsToTrue()
+    {
+        // Arrange: a minimal config with no runtime.graphql section
+        string configJson = @"{
+            ""$schema"": ""https://github.com/Azure/data-api-builder/releases/download/vmajor.minor.patch/dab.draft.schema.json"",
+            ""data-source"": {
+                ""database-type"": ""mssql"",
+                ""connection-string"": ""Server=tcp:127.0.0.1,1433;""
+            },
+            ""runtime"": {
+                ""host"": {
+                    ""authentication"": { ""provider"": ""StaticWebApps"" }
+                }
+            },
+            ""entities"": {}
+        }";
+
+        IFileSystem fs = new MockFileSystem(new Dictionary<string, MockFileData>()
+        {
+            { "dab-config.json", new MockFileData(configJson) }
+        });
+
+        FileSystemRuntimeConfigLoader loader = new(fs);
+
+        // Act
+        bool loaded = loader.TryLoadConfig("dab-config.json", out RuntimeConfig runtimeConfig);
+
+        // Assert
+        Assert.IsTrue(loaded, "Config should load successfully.");
+        Assert.IsNull(runtimeConfig.Runtime?.GraphQL, "GraphQL section should be null for this config.");
+        Assert.IsTrue(runtimeConfig.EnableAggregation,
+            "EnableAggregation should default to true when runtime.graphql section is absent.");
+    }
+
+    /// <summary>
+    /// Tests that EnableAggregation returns true by default when runtime section is absent.
+    /// </summary>
+    [TestMethod]
+    public void EnableAggregation_WhenRuntimeSectionAbsent_DefaultsToTrue()
+    {
+        // Arrange: a minimal config with no runtime section at all
+        string configJson = @"{
+            ""$schema"": ""https://github.com/Azure/data-api-builder/releases/download/vmajor.minor.patch/dab.draft.schema.json"",
+            ""data-source"": {
+                ""database-type"": ""mssql"",
+                ""connection-string"": ""Server=tcp:127.0.0.1,1433;""
+            },
+            ""entities"": {}
+        }";
+
+        IFileSystem fs = new MockFileSystem(new Dictionary<string, MockFileData>()
+        {
+            { "dab-config.json", new MockFileData(configJson) }
+        });
+
+        FileSystemRuntimeConfigLoader loader = new(fs);
+
+        // Act
+        bool loaded = loader.TryLoadConfig("dab-config.json", out RuntimeConfig runtimeConfig);
+
+        // Assert
+        Assert.IsTrue(loaded, "Config should load successfully.");
+        Assert.IsNull(runtimeConfig.Runtime, "Runtime section should be null for this config.");
+        Assert.IsTrue(runtimeConfig.EnableAggregation,
+            "EnableAggregation should default to true when runtime section is absent.");
+    }
+
+    /// <summary>
+    /// Tests that EnableAggregation returns false when explicitly disabled in config.
+    /// </summary>
+    [TestMethod]
+    public void EnableAggregation_WhenExplicitlyDisabled_ReturnsFalse()
+    {
+        // Arrange: a config with enable-aggregation explicitly set to false
+        string configJson = @"{
+            ""$schema"": ""https://github.com/Azure/data-api-builder/releases/download/vmajor.minor.patch/dab.draft.schema.json"",
+            ""data-source"": {
+                ""database-type"": ""mssql"",
+                ""connection-string"": ""Server=tcp:127.0.0.1,1433;""
+            },
+            ""runtime"": {
+                ""graphql"": {
+                    ""enabled"": true,
+                    ""enable-aggregation"": false
+                },
+                ""host"": {
+                    ""authentication"": { ""provider"": ""StaticWebApps"" }
+                }
+            },
+            ""entities"": {}
+        }";
+
+        IFileSystem fs = new MockFileSystem(new Dictionary<string, MockFileData>()
+        {
+            { "dab-config.json", new MockFileData(configJson) }
+        });
+
+        FileSystemRuntimeConfigLoader loader = new(fs);
+
+        // Act
+        bool loaded = loader.TryLoadConfig("dab-config.json", out RuntimeConfig runtimeConfig);
+
+        // Assert
+        Assert.IsTrue(loaded, "Config should load successfully.");
+        Assert.IsFalse(runtimeConfig.EnableAggregation,
+            "EnableAggregation should be false when explicitly set to false in config.");
+    }
+
+    /// <summary>
+    /// Tests that EnableAggregation returns true when explicitly enabled in config.
+    /// </summary>
+    [TestMethod]
+    public void EnableAggregation_WhenExplicitlyEnabled_ReturnsTrue()
+    {
+        // Arrange: a config with enable-aggregation explicitly set to true
+        string configJson = @"{
+            ""$schema"": ""https://github.com/Azure/data-api-builder/releases/download/vmajor.minor.patch/dab.draft.schema.json"",
+            ""data-source"": {
+                ""database-type"": ""mssql"",
+                ""connection-string"": ""Server=tcp:127.0.0.1,1433;""
+            },
+            ""runtime"": {
+                ""graphql"": {
+                    ""enabled"": true,
+                    ""enable-aggregation"": true
+                },
+                ""host"": {
+                    ""authentication"": { ""provider"": ""StaticWebApps"" }
+                }
+            },
+            ""entities"": {}
+        }";
+
+        IFileSystem fs = new MockFileSystem(new Dictionary<string, MockFileData>()
+        {
+            { "dab-config.json", new MockFileData(configJson) }
+        });
+
+        FileSystemRuntimeConfigLoader loader = new(fs);
+
+        // Act
+        bool loaded = loader.TryLoadConfig("dab-config.json", out RuntimeConfig runtimeConfig);
+
+        // Assert
+        Assert.IsTrue(loaded, "Config should load successfully.");
+        Assert.IsTrue(runtimeConfig.EnableAggregation,
+            "EnableAggregation should be true when explicitly set to true in config.");
+    }
 }
diff --git a/src/Service.Tests/GraphQLBuilder/QueryBuilderTests.cs b/src/Service.Tests/GraphQLBuilder/QueryBuilderTests.cs
index c257a86054..56cac93c8a 100644
--- a/src/Service.Tests/GraphQLBuilder/QueryBuilderTests.cs
+++ b/src/Service.Tests/GraphQLBuilder/QueryBuilderTests.cs
@@ -18,6 +18,16 @@ public class QueryBuilderTests
     {
         private const int NUMBER_OF_ARGUMENTS = 4;
 
+        /// <summary>
+        /// GQL schema for a Book entity with numeric fields, used for aggregation tests.
+        /// </summary>
+        private const string BOOK_WITH_NUMERIC_FIELDS_GQL = @"
+type Book @model(name:""Book"") {
+    id: ID!
+    price: Float!
+    title: String
+}";
+
         private Dictionary<string, EntityMetadata> _entityPermissions;
 
         /// <summary>
@@ -37,6 +47,14 @@ public void SetupEntityPermissionsMap()
                     );
         }
 
+        private static Dictionary<string, EntityMetadata> CreateBookEntityPermissions()
+        {
+            return GraphQLTestHelpers.CreateStubEntityPermissionsMap(
+                new string[] { "Book" },
+                new EntityActionOperation[] { EntityActionOperation.Read },
+                new string[] { "anonymous" });
+        }
+
         [DataTestMethod]
         [TestCategory("Query Generation")]
         [TestCategory("Single item access")]
@@ -538,6 +556,95 @@ public void GenerateReturnType_IncludesGroupByField()
             Assert.AreEqual("BookGroupBy", groupByType.Name.Value, "should return GroupBy type");
         }
 
+        /// <summary>
+        /// Tests that the return type does NOT include the groupBy field when aggregation is disabled.
+        /// </summary>
+        [TestMethod]
+        [TestCategory("Query Builder - Return Type")]
+        public void GenerateReturnType_ExcludesGroupByField_WhenAggregationDisabled()
+        {
+            // Arrange
+            NameNode entityName = new("Book");
+
+            // Act
+            ObjectTypeDefinitionNode returnType = QueryBuilder.GenerateReturnType(entityName, isAggregationEnabled: false);
+
+            // Assert
+            FieldDefinitionNode groupByField = returnType.Fields.FirstOrDefault(f => f.Name.Value == "groupBy");
+            Assert.IsNull(groupByField, "groupBy field should NOT exist when aggregation is disabled");
+        }
+
+        /// <summary>
+        /// Tests that QueryBuilder.Build adds a groupBy field to the connection type
+        /// for MSSQL entities when aggregation is enabled. This verifies the fix for the
+        /// regression where aggregation features were not accessible in the schema.
+        /// </summary>
+        [TestMethod]
+        [TestCategory("Query Builder - Aggregation")]
+        public void Build_WithMssqlAndAggregationEnabled_AddsGroupByToConnectionType()
+        {
+            // Arrange
+            DocumentNode root = Utf8GraphQLParser.Parse(BOOK_WITH_NUMERIC_FIELDS_GQL);
+            Dictionary<string, DatabaseType> entityNameToDatabaseType = new()
+            {
+                { "Book", DatabaseType.MSSQL }
+            };
+
+            // Act
+            DocumentNode queryRoot = QueryBuilder.Build(
+                root,
+                entityNameToDatabaseType,
+                new(new Dictionary<string, Entity> { { "Book", GraphQLTestHelpers.GenerateEmptyEntity() } }),
+                inputTypes: new(),
+                entityPermissionsMap: CreateBookEntityPermissions(),
+                _isAggregationEnabled: true
+            );
+
+            // Assert: find BookConnection type
+            ObjectTypeDefinitionNode bookConnection = queryRoot.Definitions
+                .OfType<ObjectTypeDefinitionNode>()
+                .FirstOrDefault(d => d.Name.Value == "BookConnection");
+            Assert.IsNotNull(bookConnection, "BookConnection type should exist");
+
+            FieldDefinitionNode groupByField = bookConnection.Fields.FirstOrDefault(f => f.Name.Value == "groupBy");
+            Assert.IsNotNull(groupByField, "groupBy field should exist on BookConnection when aggregation is enabled for MSSQL");
+        }
+
+        /// <summary>
+        /// Tests that QueryBuilder.Build does NOT add a groupBy field to the connection type
+        /// for PostgreSQL entities, since aggregation is only enabled for MSSQL and DWSQL.
+        /// </summary>
+        [TestMethod]
+        [TestCategory("Query Builder - Aggregation")]
+        public void Build_WithPostgreSqlAndAggregationEnabled_DoesNotAddGroupByToConnectionType()
+        {
+            // Arrange
+            DocumentNode root = Utf8GraphQLParser.Parse(BOOK_WITH_NUMERIC_FIELDS_GQL);
+            Dictionary<string, DatabaseType> entityNameToDatabaseType = new()
+            {
+                { "Book", DatabaseType.PostgreSQL }
+            };
+
+            // Act
+            DocumentNode queryRoot = QueryBuilder.Build(
+                root,
+                entityNameToDatabaseType,
+                new(new Dictionary<string, Entity> { { "Book", GraphQLTestHelpers.GenerateEmptyEntity() } }),
+                inputTypes: new(),
+                entityPermissionsMap: CreateBookEntityPermissions(),
+                _isAggregationEnabled: true
+            );
+
+            // Assert: find BookConnection type
+            ObjectTypeDefinitionNode bookConnection = queryRoot.Definitions
+                .OfType<ObjectTypeDefinitionNode>()
+                .FirstOrDefault(d => d.Name.Value == "BookConnection");
+            Assert.IsNotNull(bookConnection, "BookConnection type should exist");
+
+            FieldDefinitionNode groupByField = bookConnection.Fields.FirstOrDefault(f => f.Name.Value == "groupBy");
+            Assert.IsNull(groupByField, "groupBy field should NOT exist on BookConnection for PostgreSQL (not in AggregationEnabledDatabaseTypes)");
+        }
+
         public static ObjectTypeDefinitionNode GetQueryNode(DocumentNode queryRoot)
         {
             return (ObjectTypeDefinitionNode)queryRoot.Definitions.First(d => d is ObjectTypeDefinitionNode node && node.Name.Value == "Query");