Name the tokenizer a model uses, where its vendor does

app/MindWork AI Studio/Models/OpenAI/Gpt4Family.cs

@@ -18,13 +18,20 @@ public sealed class Gpt4Family : ModelFamily
     /// <inheritdoc />
     public override ModelSource Source => new("https://developers.openai.com/api/docs/models/gpt-4-turbo", new DateOnly(2026, 9, 12), "Capabilities ported unchanged from ProviderExtensions.OpenAI.cs: GPT-4 is text only, Turbo adds images and tool calling. The windows are the documented 8,192 tokens of GPT-4 and the 128,000 Turbo raised it to.");
 
+    /// <inheritdoc />
+    public override IReadOnlyList<ModelSource> FurtherSources =>
+    [
+        new("https://github.com/openai/tiktoken/blob/main/tiktoken/model.py", new DateOnly(2026, 9, 12), "OpenAI's own mapping from model names to encodings. It maps \"gpt-4\" and the prefix \"gpt-4-\" to cl100k_base, so Turbo uses it too -- the newer o200k_base begins with the 4o line, which is a family of its own here.")
+    ];
+
     /// <inheritdoc />
     protected override void Declare(ModelFamilyBuilder builder)
     {
         builder.Rule("gpt-4").AsPrefix()
             .Capabilities(TEXT_INPUT | TEXT_OUTPUT)
             .Apis(RESPONSES_API)
-            .ContextWindow(8_192);
+            .ContextWindow(8_192)
+            .Tokenizer(TokenizerKind.TIKTOKEN, "cl100k_base");
 
         builder.Rule("gpt-4-turbo").AsPrefix().Inherits()
             .Capabilities(MULTIPLE_IMAGE_INPUT | FUNCTION_CALLING)