diff --git a/pkgs/sdk/server-ai/src/Interfaces/ILdAiClient.cs b/pkgs/sdk/server-ai/src/Interfaces/ILdAiClient.cs
index 2e5c32bd..28fd3dfb 100644
--- a/pkgs/sdk/server-ai/src/Interfaces/ILdAiClient.cs
+++ b/pkgs/sdk/server-ai/src/Interfaces/ILdAiClient.cs
@@ -22,10 +22,11 @@ public interface ILdAiClient
     /// </summary>
     /// <param name="key">the AI Completion Config key</param>
     /// <param name="context">the context</param>
-    /// <param name="defaultValue">the default config, if unable to retrieve from LaunchDarkly</param>
+    /// <param name="defaultValue">the default config, if unable to retrieve from LaunchDarkly. When not provided,
+    /// a disabled config is used as the fallback.</param>
     /// <param name="variables">the list of variables used when interpolating the prompt</param>
     /// <returns>an AI Completion Config tracker</returns>
-    public ILdAiConfigTracker CompletionConfig(string key, Context context, LdAiConfig defaultValue,
+    public ILdAiConfigTracker CompletionConfig(string key, Context context, LdAiConfig defaultValue = null,
         IReadOnlyDictionary<string, object> variables = null);
 
     /// <summary>
@@ -37,6 +38,6 @@ public ILdAiConfigTracker CompletionConfig(string key, Context context, LdAiConf
     /// <param name="variables">the list of variables used when interpolating the prompt</param>
     /// <returns>an AI Completion Config tracker</returns>
     [Obsolete("Use CompletionConfig instead.")]
-    public ILdAiConfigTracker Config(string key, Context context, LdAiConfig defaultValue,
+    public ILdAiConfigTracker Config(string key, Context context, LdAiConfig defaultValue = null,
         IReadOnlyDictionary<string, object> variables = null);
 }
diff --git a/pkgs/sdk/server-ai/src/LdAiClient.cs b/pkgs/sdk/server-ai/src/LdAiClient.cs
index 9690ad49..803b3d5a 100644
--- a/pkgs/sdk/server-ai/src/LdAiClient.cs
+++ b/pkgs/sdk/server-ai/src/LdAiClient.cs
@@ -58,10 +58,11 @@ public LdAiClient(ILaunchDarklyClient client)
     private const string LdContextVariable = "ldctx";
 
     /// <inheritdoc/>
-    public ILdAiConfigTracker CompletionConfig(string key, Context context, LdAiConfig defaultValue,
+    public ILdAiConfigTracker CompletionConfig(string key, Context context, LdAiConfig defaultValue = null,
         IReadOnlyDictionary<string, object> variables = null)
     {
         _client.Track(TrackUsageCompletionConfig, context, LdValue.Of(key), 1);
+        defaultValue ??= LdAiConfig.Disabled;
 
         return Evaluate(key, context, defaultValue, variables);
     }
@@ -129,7 +130,7 @@ private ILdAiConfigTracker Evaluate(string key, Context context, LdAiConfig defa
     /// <param name="variables">the list of variables used when interpolating the prompt</param>
     /// <returns>an AI Completion Config tracker</returns>
     [Obsolete("Use CompletionConfig instead.")]
-    public ILdAiConfigTracker Config(string key, Context context, LdAiConfig defaultValue,
+    public ILdAiConfigTracker Config(string key, Context context, LdAiConfig defaultValue = null,
         IReadOnlyDictionary<string, object> variables = null)
     {
         return CompletionConfig(key, context, defaultValue, variables);
diff --git a/pkgs/sdk/server-ai/test/LdAiClientTest.cs b/pkgs/sdk/server-ai/test/LdAiClientTest.cs
index 4d8b2639..f28e9c9d 100644
--- a/pkgs/sdk/server-ai/test/LdAiClientTest.cs
+++ b/pkgs/sdk/server-ai/test/LdAiClientTest.cs
@@ -324,4 +324,35 @@ public void ProviderConfigIsParsed()
 
         Assert.Equal("amazing-provider", tracker.Config.Provider.Name);
     }
+
+    [Fact]
+    public void ConfigWithoutDefaultValueUsesDisabledConfig()
+    {
+        var mockClient = new Mock<ILaunchDarklyClient>();
+        var mockLogger = new Mock<ILogger>();
+
+        mockClient.Setup(x =>
+            x.JsonVariation("foo", It.IsAny<Context>(), It.IsAny<LdValue>())).Returns(LdValue.Null);
+        mockClient.Setup(x => x.GetLogger()).Returns(mockLogger.Object);
+
+        var client = new LdAiClient(mockClient.Object);
+        var tracker = client.Config("foo", Context.New(ContextKind.Default, "key"));
+
+        Assert.False(tracker.Config.Enabled);
+    }
+
+    [Fact]
+    public void DisabledMethodReturnsDisabledConfig()
+    {
+        var config = LdAiConfig.Disabled;
+        Assert.False(config.Enabled);
+    }
+
+    [Fact]
+    public void DisabledMethodReturnsNewInstanceEachCall()
+    {
+        var first = LdAiConfig.Disabled;
+        var second = LdAiConfig.Disabled;
+        Assert.NotSame(first, second);
+    }
 }