diff --git a/src/Service.Tests/UnitTests/MultiSourceAutoentitiesMergeTests.cs b/src/Service.Tests/UnitTests/MultiSourceAutoentitiesMergeTests.cs
new file mode 100644
index 0000000000..a314682efe
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+++ b/src/Service.Tests/UnitTests/MultiSourceAutoentitiesMergeTests.cs
@@ -0,0 +1,188 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.IO.Abstractions.TestingHelpers;
+using System.Linq;
+using System.Reflection;
+using System.Text.Json.Nodes;
+using System.Threading;
+using System.Threading.Tasks;
+using Azure.DataApiBuilder.Config;
+using Azure.DataApiBuilder.Config.ObjectModel;
+using Azure.DataApiBuilder.Core.Configurations;
+using Azure.DataApiBuilder.Core.Models;
+using Azure.DataApiBuilder.Core.Resolvers;
+using Azure.DataApiBuilder.Core.Resolvers.Factories;
+using Azure.DataApiBuilder.Core.Services;
+using Azure.DataApiBuilder.Core.Services.MetadataProviders;
+using Microsoft.Extensions.Logging.Abstractions;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Moq;
+
+namespace Azure.DataApiBuilder.Service.Tests.UnitTests
+{
+    /// <summary>
+    /// Regression tests for propagating generated autoentities from multiple child
+    /// configurations into the shared root runtime configuration (child of #3711).
+    /// </summary>
+    [TestClass]
+    public class MultiSourceAutoentitiesMergeTests
+    {
+        private readonly List<string> _tempFiles = new();
+
+        [TestCleanup]
+        public void Cleanup()
+        {
+            foreach (string file in _tempFiles)
+            {
+                if (File.Exists(file))
+                {
+                    File.Delete(file);
+                }
+            }
+        }
+
+        [DataTestMethod]
+        [DataRow(1, DisplayName = "Parent with one child")]
+        [DataRow(2, DisplayName = "Parent with two children")]
+        [DataRow(3, DisplayName = "Parent with three children")]
+        public async Task GeneratedAutoentitiesFromAllChildrenAreMergedIntoRootConfig(int childCount)
+        {
+            List<string> childFiles = new();
+            for (int i = 0; i < childCount; i++)
+            {
+                childFiles.Add(WriteChildConfig(i));
+            }
+
+            RuntimeConfigProvider provider = CreateProvider(childFiles);
+            RuntimeConfig initialConfig = provider.GetConfig();
+            Assert.AreEqual(childCount, initialConfig.ChildConfigs.Count);
+            Assert.AreEqual(0, initialConfig.Entities.Entities.Count);
+
+            // Each child's data source returns objects unique to that child.
+            Dictionary<string, int> dataSourceToChildIndex = initialConfig.ChildConfigs
+                .Select((child, index) => (child.Config.DefaultDataSourceName, index))
+                .ToDictionary(x => x.DefaultDataSourceName, x => x.index);
+
+            MetadataProviderFactory factory = CreateFactory(provider, dataSourceToChildIndex);
+
+            foreach (ISqlMetadataProvider metadataProvider in factory.ListMetadataProviders())
+            {
+                await GenerateAutoentitiesAsync((MsSqlMetadataProvider)metadataProvider);
+            }
+
+            RuntimeConfig finalConfig = provider.GetConfig();
+            Assert.AreEqual(childCount * 2, finalConfig.Entities.Entities.Count,
+                $"Entities in merged config: {string.Join(", ", finalConfig.Entities.Entities.Keys)}");
+
+            foreach ((string dataSourceName, int childIndex) in dataSourceToChildIndex)
+            {
+                foreach (string table in new[] { "orders", "customers" })
+                {
+                    string entityName = $"c{childIndex}_{table}";
+                    Assert.IsTrue(finalConfig.Entities.ContainsKey(entityName), $"Missing generated entity '{entityName}'.");
+                    Assert.IsTrue(finalConfig.Entities[entityName].IsAutoentity);
+                    Assert.AreEqual(dataSourceName, finalConfig.GetDataSourceNameFromEntityName(entityName),
+                        $"Entity '{entityName}' is not mapped to its originating data source.");
+                }
+            }
+        }
+
+        private string WriteChildConfig(int index)
+        {
+            string path = Path.Combine(Path.GetTempPath(), $"dab-child-{index}-config.json");
+            string json = $@"{{
+                ""data-source"": {{
+                    ""database-type"": ""mssql"",
+                    ""connection-string"": ""Server=localhost;Database=child{index};Trusted_Connection=True;""
+                }},
+                ""entities"": {{}},
+                ""autoentities"": {{
+                    ""child{index}-auto"": {{
+                    ""patterns"": {{ ""include"": [ ""dbo.%"" ], ""name"": ""c{index}_{{object}}"" }},
+                    ""permissions"": [ {{ ""role"": ""anonymous"", ""actions"": [ {{ ""action"": ""*"" }} ] }} ]
+                    }}
+                }}
+            }}";
+            File.WriteAllText(path, json);
+            _tempFiles.Add(path);
+            return path;
+        }
+
+        private static RuntimeConfigProvider CreateProvider(IEnumerable<string> childFiles)
+        {
+            string files = string.Join(", ", childFiles.Select(f => $"\"{f.Replace("\\", "\\\\")}\""));
+            string parentJson = $@"{{
+                ""$schema"": ""https://github.com/Azure/data-api-builder/releases/download/vmajor.minor.patch/dab.draft.schema.json"",
+                ""data-source-files"": [ {files} ],
+                ""runtime"": {{
+                    ""rest"": {{ ""enabled"": true }},
+                    ""graphql"": {{ ""enabled"": true }},
+                    ""mcp"": {{ ""enabled"": true }},
+                    ""host"": {{ ""mode"": ""development"" }}
+                }},
+                ""entities"": {{}}
+            }}";
+            MockFileSystem fileSystem = new();
+            fileSystem.AddFile(FileSystemRuntimeConfigLoader.DEFAULT_CONFIG_FILE_NAME, parentJson);
+            FileSystemRuntimeConfigLoader loader = new(fileSystem);
+            return new RuntimeConfigProvider(loader);
+        }
+
+        private static MetadataProviderFactory CreateFactory(
+            RuntimeConfigProvider provider,
+            Dictionary<string, int> dataSourceToChildIndex)
+        {
+            Mock<IQueryBuilder> queryBuilder = new();
+            queryBuilder.Setup(x => x.BuildGetAutoentitiesQuery()).Returns("SELECT autoentities");
+
+            Mock<IQueryExecutor> queryExecutor = new();
+            queryExecutor.Setup(x => x.ExecuteQueryAsync(
+                    It.IsAny<string>(),
+                    It.IsAny<IDictionary<string, DbConnectionParam>>(),
+                    It.IsAny<Func<System.Data.Common.DbDataReader, List<string>?, Task<JsonArray>>>(),
+                    It.IsAny<string>(),
+                    It.IsAny<CancellationToken>(),
+                    It.IsAny<Microsoft.AspNetCore.Http.HttpContext>(),
+                    It.IsAny<List<string>>()))
+                .ReturnsAsync((string _, IDictionary<string, DbConnectionParam> _, Func<System.Data.Common.DbDataReader, List<string>?, Task<JsonArray>> _, string dataSourceName, CancellationToken _, Microsoft.AspNetCore.Http.HttpContext _, List<string> _) =>
+                {
+                    int index = dataSourceToChildIndex[dataSourceName];
+                    return new JsonArray(
+                        new JsonObject { ["entity_name"] = $"c{index}_orders", ["object"] = "orders", ["schema"] = "dbo" },
+                        new JsonObject { ["entity_name"] = $"c{index}_customers", ["object"] = "customers", ["schema"] = "dbo" });
+                });
+
+            Mock<IAbstractQueryManagerFactory> queryManagerFactory = new();
+            queryManagerFactory.Setup(x => x.GetQueryBuilder(It.IsAny<DatabaseType>())).Returns(queryBuilder.Object);
+            queryManagerFactory.Setup(x => x.GetQueryExecutor(It.IsAny<DatabaseType>())).Returns(queryExecutor.Object);
+
+            MockFileSystem fileSystem = new();
+            RuntimeConfigValidator validator = new(provider, fileSystem, NullLogger<RuntimeConfigValidator>.Instance);
+            return new MetadataProviderFactory(
+                provider,
+                validator,
+                queryManagerFactory.Object,
+                NullLogger<ISqlMetadataProvider>.Instance,
+                fileSystem,
+                handler: null);
+        }
+
+        // Mirrors the autoentity step of SqlMetadataProvider.InitializeAsync without requiring database metadata.
+        private static Task GenerateAutoentitiesAsync(MsSqlMetadataProvider metadataProvider)
+        {
+            Type baseType = typeof(MsSqlMetadataProvider).BaseType!;
+            object autoentities = baseType
+                .GetProperty("Autoentities", BindingFlags.Instance | BindingFlags.NonPublic)!
+                .GetValue(metadataProvider)!;
+            MethodInfo generate = typeof(MsSqlMetadataProvider).GetMethod(
+                "GenerateAutoentitiesIntoEntities",
+                BindingFlags.Instance | BindingFlags.NonPublic,
+                new[] { typeof(IReadOnlyDictionary<string, Autoentity>), typeof(CancellationToken) })!;
+            return (Task)generate.Invoke(metadataProvider, new[] { autoentities, CancellationToken.None })!;
+        }
+    }
+}