diff --git a/src/Elastic.Clients.Elasticsearch/Common/ElasticsearchClientProductRegistration.cs b/src/Elastic.Clients.Elasticsearch/Common/ElasticsearchClientProductRegistration.cs
index 8008c36bf01..892863d09d2 100644
--- a/src/Elastic.Clients.Elasticsearch/Common/ElasticsearchClientProductRegistration.cs
+++ b/src/Elastic.Clients.Elasticsearch/Common/ElasticsearchClientProductRegistration.cs
@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System;
+using System.Diagnostics.CodeAnalysis;
 using Elastic.Transport;
 using Elastic.Transport.Products.Elasticsearch;
 
@@ -28,7 +29,7 @@ public override bool HttpStatusCodeClassifier(HttpMethod method, int statusCode)
 	///     Makes the low level transport aware of Elastic.Clients.Elasticsearch's <see cref="ElasticsearchResponseBase" />
 	///     so that it can peek in to its exposed error when reporting failures.
 	/// </summary>
-	public override bool TryGetServerErrorReason<TResponse>(TResponse response, out string? reason)
+	public override bool TryGetServerErrorReason<TResponse>(TResponse response, [NotNullWhen(returnValue: true)] out string? reason)
 	{
 		if (response is not ElasticsearchResponseBase r)
 			return base.TryGetServerErrorReason(response, out reason);
diff --git a/src/Elastic.Clients.Elasticsearch/CrossPlatform/NullableAttributes.cs b/src/Elastic.Clients.Elasticsearch/CrossPlatform/NullableAttributes.cs
new file mode 100644
index 00000000000..99e09498c04
--- /dev/null
+++ b/src/Elastic.Clients.Elasticsearch/CrossPlatform/NullableAttributes.cs
@@ -0,0 +1,143 @@
+// Licensed to Elasticsearch B.V under one or more agreements.
+// Elasticsearch B.V licenses this file to you under the Apache 2.0 License.
+// See the LICENSE file in the project root for more information.
+
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+#define INTERNAL_NULLABLE_ATTRIBUTES
+
+#pragma warning disable SA1402 // File may only contain a single type
+#pragma warning disable SA1649 // File name should match first type name
+
+#if !NETCOREAPP3_1_OR_GREATER && !NETSTANDARD2_1_OR_GREATER || NET461
+namespace System.Diagnostics.CodeAnalysis
+{
+	/// <summary>Specifies that null is allowed as an input even if the corresponding type disallows it.</summary>
+	[AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property, Inherited = false)]
+#if INTERNAL_NULLABLE_ATTRIBUTES
+	internal
+#else
+    public
+#endif
+		sealed class AllowNullAttribute : Attribute
+	{ }
+
+	/// <summary>Specifies that null is disallowed as an input even if the corresponding type allows it.</summary>
+	[AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property, Inherited = false)]
+#if INTERNAL_NULLABLE_ATTRIBUTES
+	internal
+#else
+    public
+#endif
+		sealed class DisallowNullAttribute : Attribute
+	{ }
+
+	/// <summary>Specifies that an output may be null even if the corresponding type disallows it.</summary>
+	[AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue, Inherited = false)]
+#if INTERNAL_NULLABLE_ATTRIBUTES
+	internal
+#else
+    public
+#endif
+		sealed class MaybeNullAttribute : Attribute
+	{ }
+
+	/// <summary>Specifies that an output will not be null even if the corresponding type allows it.</summary>
+	[AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue, Inherited = false)]
+#if INTERNAL_NULLABLE_ATTRIBUTES
+	internal
+#else
+    public
+#endif
+		sealed class NotNullAttribute : Attribute
+	{ }
+
+	/// <summary>Specifies that when a method returns <see cref="ReturnValue"/>, the parameter may be null even if the corresponding type disallows it.</summary>
+	[AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
+#if INTERNAL_NULLABLE_ATTRIBUTES
+	internal
+#else
+    public
+#endif
+		sealed class MaybeNullWhenAttribute : Attribute
+	{
+		/// <summary>Initializes the attribute with the specified return value condition.</summary>
+		/// <param name="returnValue">
+		/// The return value condition. If the method returns this value, the associated parameter may be null.
+		/// </param>
+		public MaybeNullWhenAttribute(bool returnValue) => ReturnValue = returnValue;
+
+		/// <summary>Gets the return value condition.</summary>
+		public bool ReturnValue { get; }
+	}
+
+	/// <summary>Specifies that when a method returns <see cref="ReturnValue"/>, the parameter will not be null even if the corresponding type allows it.</summary>
+	[AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
+#if INTERNAL_NULLABLE_ATTRIBUTES
+	internal
+#else
+    public
+#endif
+		sealed class NotNullWhenAttribute : Attribute
+	{
+		/// <summary>Initializes the attribute with the specified return value condition.</summary>
+		/// <param name="returnValue">
+		/// The return value condition. If the method returns this value, the associated parameter will not be null.
+		/// </param>
+		public NotNullWhenAttribute(bool returnValue) => ReturnValue = returnValue;
+
+		/// <summary>Gets the return value condition.</summary>
+		public bool ReturnValue { get; }
+	}
+
+	/// <summary>Specifies that the output will be non-null if the named parameter is non-null.</summary>
+	[AttributeUsage(AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue, AllowMultiple = true, Inherited = false)]
+#if INTERNAL_NULLABLE_ATTRIBUTES
+	internal
+#else
+    public
+#endif
+		sealed class NotNullIfNotNullAttribute : Attribute
+	{
+		/// <summary>Initializes the attribute with the associated parameter name.</summary>
+		/// <param name="parameterName">
+		/// The associated parameter name.  The output will be non-null if the argument to the parameter specified is non-null.
+		/// </param>
+		public NotNullIfNotNullAttribute(string parameterName) => ParameterName = parameterName;
+
+		/// <summary>Gets the associated parameter name.</summary>
+		public string ParameterName { get; }
+	}
+
+	/// <summary>Applied to a method that will never return under any circumstance.</summary>
+	[AttributeUsage(AttributeTargets.Method, Inherited = false)]
+#if INTERNAL_NULLABLE_ATTRIBUTES
+	internal
+#else
+    public
+#endif
+		sealed class DoesNotReturnAttribute : Attribute
+	{ }
+
+	/// <summary>Specifies that the method will not return if the associated Boolean parameter is passed the specified value.</summary>
+	[AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
+#if INTERNAL_NULLABLE_ATTRIBUTES
+	internal
+#else
+    public
+#endif
+		sealed class DoesNotReturnIfAttribute : Attribute
+	{
+		/// <summary>Initializes the attribute with the specified parameter value.</summary>
+		/// <param name="parameterValue">
+		/// The condition parameter value. Code after the method will be considered unreachable by diagnostics if the argument to
+		/// the associated parameter matches this value.
+		/// </param>
+		public DoesNotReturnIfAttribute(bool parameterValue) => ParameterValue = parameterValue;
+
+		/// <summary>Gets the condition parameter value.</summary>
+		public bool ParameterValue { get; }
+	}
+}
+#endif
diff --git a/src/Elastic.Clients.Elasticsearch/Types/SourceConfig.cs b/src/Elastic.Clients.Elasticsearch/Types/SourceConfig.cs
index ec5d007ffb0..637d9f46a7d 100644
--- a/src/Elastic.Clients.Elasticsearch/Types/SourceConfig.cs
+++ b/src/Elastic.Clients.Elasticsearch/Types/SourceConfig.cs
@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System;
+using System.Diagnostics.CodeAnalysis;
 using System.Text.Json;
 using System.Text.Json.Serialization;
 
@@ -15,8 +16,7 @@ public partial class SourceConfig
 
 		public bool HasSourceFilterValue => Tag == 1;
 
-		// TODO - Not null when
-		public bool TryGetBool(out bool? value)
+		public bool TryGetBool([NotNullWhen(returnValue: true)] out bool? value)
 		{
 			if (Tag == 0)
 			{
@@ -28,7 +28,7 @@ public bool TryGetBool(out bool? value)
 			return false;
 		}
 
-		public bool TryGetSourceFilter(out SourceFilter? value)
+		public bool TryGetSourceFilter([NotNullWhen(returnValue: true)] out SourceFilter? value)
 		{
 			if (Tag == 1)
 			{