diff --git a/app/MindWork AI Studio/Tools/ToolCallingSystem/ToolParameterSchemaBuilder.cs b/app/MindWork AI Studio/Tools/ToolCallingSystem/ToolParameterSchemaBuilder.cs
index 592c8228..0ca6e5a3 100644
--- a/app/MindWork AI Studio/Tools/ToolCallingSystem/ToolParameterSchemaBuilder.cs	
+++ b/app/MindWork AI Studio/Tools/ToolCallingSystem/ToolParameterSchemaBuilder.cs	
@@ -33,6 +33,34 @@ public sealed class ToolParameterSchemaBuilder
 
     public ToolParameterSchemaBuilder OptionalEnum(string name, string description, params string[] allowedValues) => this.Add(name, "string", description, isRequired: false, allowedValues);
 
+    /// <summary>
+    /// An argument the model may leave out or pass as a list of strings.
+    /// </summary>
+    /// <remarks>
+    /// With allowed values, every entry of the list has to be one of them, such as the data sources
+    /// Semantic Search may be asked to search. How many entries the list holds is for the tool to
+    /// check, like everything else a model passes.
+    /// </remarks>
+    public ToolParameterSchemaBuilder OptionalStringArray(string name, string description, params string[] allowedValues)
+    {
+        var items = new JsonObject
+        {
+            ["type"] = "string",
+        };
+
+        if (allowedValues is { Length: > 0 })
+            items["enum"] = new JsonArray([..allowedValues.Select(value => JsonValue.Create(value))]);
+
+        this.properties[name] = new JsonObject
+        {
+            ["type"] = "array",
+            ["description"] = description,
+            ["items"] = items,
+        };
+
+        return this;
+    }
+
     /// <summary>
     /// Produces the finished schema.
     /// </summary>
diff --git a/app/Tests/Provider/ToolCalling/OpenAIStrictToolSchemaTests.cs b/app/Tests/Provider/ToolCalling/OpenAIStrictToolSchemaTests.cs
index 9bf71d9b..297f65d6 100644
--- a/app/Tests/Provider/ToolCalling/OpenAIStrictToolSchemaTests.cs
+++ b/app/Tests/Provider/ToolCalling/OpenAIStrictToolSchemaTests.cs
@@ -45,6 +45,21 @@ public sealed class OpenAIStrictToolSchemaTests
         });
     }
 
+    [Test]
+    public void AnOptionalListMayBeNullWhileItsEntriesKeepTheirChoice()
+    {
+        var dataSourceIds = Converted(ToolParameterSchemaBuilder.Create()
+            .RequiredString("query", "The search query.")
+            .OptionalStringArray("data_source_ids", "The data sources.", "first", "second"))["properties"]!["data_source_ids"]!;
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(Types(dataSourceIds), Is.EqualTo(["array", "null"]), "Leaving the list out is said by allowing null for the list itself.");
+            Assert.That(dataSourceIds["items"]!["enum"]!.AsArray().Select(value => value?.GetValue<string>()), Is.EqualTo(["first", "second"]), "Null is a way to leave the list out, not an entry it may hold.");
+            Assert.That(dataSourceIds["enum"], Is.Null, "The list itself names no values of its own.");
+        });
+    }
+
     [Test]
     public void ARequiredArgumentStaysAsItIs()
     {
diff --git a/app/Tests/Tools/ToolCalling/ToolParameterSchemaBuilderTests.cs b/app/Tests/Tools/ToolCalling/ToolParameterSchemaBuilderTests.cs
new file mode 100644
index 00000000..b13f96f3
--- /dev/null
+++ b/app/Tests/Tools/ToolCalling/ToolParameterSchemaBuilderTests.cs
@@ -0,0 +1,52 @@
+using System.Text.Json.Nodes;
+
+using AIStudio.Tools.ToolCallingSystem;
+
+namespace AIStudio.Tests.Tools.ToolCalling;
+
+/// <summary>
+/// Checks the plain JSON Schema a tool describes its arguments with.
+/// </summary>
+/// <remarks>
+/// This is the form Anthropic and every host without strict mode receive as written, so it has to
+/// mean exactly what the tool expects. The translation for strict mode is checked on its own, see
+/// OpenAIStrictToolSchemaTests.
+/// </remarks>
+[TestFixture]
+public sealed class ToolParameterSchemaBuilderTests
+{
+    [Test]
+    public void AListOfChoicesRestrictsEveryEntry()
+    {
+        var schema = Built(ToolParameterSchemaBuilder.Create().OptionalStringArray("data_source_ids", "The data sources.", "first", "second"));
+        var property = schema["properties"]!["data_source_ids"]!;
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(property["type"]!.GetValue<string>(), Is.EqualTo("array"));
+            Assert.That(property["description"]!.GetValue<string>(), Is.EqualTo("The data sources."));
+            Assert.That(property["items"]!["type"]!.GetValue<string>(), Is.EqualTo("string"));
+            Assert.That(property["items"]!["enum"]!.AsArray().Select(value => value!.GetValue<string>()), Is.EqualTo(new[] { "first", "second" }));
+        });
+    }
+
+    [Test]
+    public void AListWithoutChoicesTakesAnyString()
+    {
+        var items = Built(ToolParameterSchemaBuilder.Create().OptionalStringArray("tags", "Some tags."))["properties"]!["tags"]!["items"]!;
+
+        Assert.That(items["enum"], Is.Null, "An empty enum would allow no entry at all rather than any.");
+    }
+
+    [Test]
+    public void AnOptionalListIsNotRequired()
+    {
+        var schema = Built(ToolParameterSchemaBuilder.Create()
+            .RequiredString("query", "The search query.")
+            .OptionalStringArray("data_source_ids", "The data sources.", "first"));
+
+        Assert.That(schema["required"]!.AsArray().Select(name => name!.GetValue<string>()), Is.EqualTo(new[] { "query" }));
+    }
+
+    private static JsonNode Built(ToolParameterSchemaBuilder builder) => JsonNode.Parse(builder.Build().GetRawText())!;
+}
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