Count the tool schemas the request carries

app/MindWork AI Studio/Chat/ConversationParts.cs

@@ -1,3 +1,7 @@
+using System.Text.Json;
+
+using AIStudio.Tools.ToolCallingSystem;
+
 namespace AIStudio.Chat;
 
 /// <summary>
@@ -6,9 +10,10 @@ namespace AIStudio.Chat;
 /// <remarks>
 /// Collected here rather than while counting, so that what counts towards a token budget is one
 /// question with one answer which a test can ask. It follows what the message builder actually
-/// sends: the system prompt, the text of every block, and the attachments hanging off those
-/// blocks -- plus whatever is standing in the composer but has not been sent yet, because that is
-/// the part a person is deciding about while they look at the number.
+/// sends: the system prompt, the schema of every tool the model may call, the text of every block,
+/// and the attachments hanging off those blocks -- plus whatever is standing in the composer but
+/// has not been sent yet, because that is the part a person is deciding about while they look at
+/// the number.
 ///
 /// And, while a request is running, what its tools have returned so far. That is the one part
 /// which is not about the next request but about the one in flight: it is what the model is
@@ -70,8 +75,12 @@ public sealed record ConversationParts
     /// <param name="draft">What stands in the composer.</param>
     /// <param name="draftAttachments">What is attached to the composer.</param>
     /// <param name="imagesAreSent">Whether the model takes images at all. When it does not, none are sent.</param>
+    /// <param name="toolDefinitions">
+    /// The tools the model may call, filtered for the provider the same way they are before
+    /// sending, or null when there are none.
+    /// </param>
     /// <returns>The parts of the conversation.</returns>
-    public static ConversationParts Of(ChatThread? thread, string systemPrompt, string draft, IEnumerable<FileAttachment>? draftAttachments, bool imagesAreSent)
+    public static ConversationParts Of(ChatThread? thread, string systemPrompt, string draft, IEnumerable<FileAttachment>? draftAttachments, bool imagesAreSent, IEnumerable<ToolDefinition>? toolDefinitions)
     {
         var texts = new List<string>();
         var growing = new List<string>();
@@ -81,6 +90,15 @@ public sealed record ConversationParts
         if (!string.IsNullOrWhiteSpace(systemPrompt))
             texts.Add(systemPrompt);
 
+        //
+        // The tools ride along beside the messages, one schema each, in every single request of a
+        // conversation. Counted with the lasting texts rather than with the growing ones: a schema
+        // is the same string all session long, so measuring it once and remembering it is exactly
+        // what the cache is for.
+        //
+        foreach (var definition in toolDefinitions ?? [])
+            texts.Add(Describe(definition));
+
         if (thread is not null)
         {
             //
@@ -128,6 +146,27 @@ public sealed record ConversationParts
         };
     }
 
+    /// <summary>
+    /// What one tool costs the request it is offered in.
+    /// </summary>
+    /// <remarks>
+    /// Its name, what it tells the model it does, and the arguments it takes -- that is what the
+    /// provider adapters put into the tool list of the request body. The wire shape differs
+    /// between the APIs: they name the fields differently, and a strict schema is rewritten for
+    /// the OpenAI ones. None of that changes the length by an amount which matters next to a
+    /// conversation, and the number is reported as an estimate anyway.
+    /// </remarks>
+    /// <param name="definition">The tool as it was declared.</param>
+    /// <returns>The text to count for it.</returns>
+    private static string Describe(ToolDefinition definition)
+    {
+        var parameters = definition.Function.Parameters.ValueKind is JsonValueKind.Undefined
+            ? string.Empty
+            : definition.Function.Parameters.GetRawText();
+
+        return $"{definition.Function.Name}{definition.Function.DescriptionForLLM}{parameters}";
+    }
+
     /// <summary>
     /// Puts attachments into the two groups they are counted in.
     /// </summary>