Added the exact token count for Anthropic

app/MindWork AI Studio/Provider/Anthropic/ResponseStreamLine.cs

@@ -9,12 +9,38 @@ namespace AIStudio.Provider.Anthropic;
 /// <param name="Delta">The delta of the response line.</param>
 public readonly record struct ResponseStreamLine(string Type, int Index, Delta Delta) : IResponseStreamLine
 {
+    /// <summary>
+    /// The message the stream opens with, on the message start event only.
+    /// </summary>
+    /// <remarks>
+    /// Not a positional parameter, because nobody but the serializer ever builds this line with
+    /// one. Only the opening event carries a message, which makes it the only line with a usage.
+    /// </remarks>
+    public AnthropicStreamMessage? Message { get; init; }
+
     /// <inheritdoc />
     public bool ContainsContent() => this != default && !string.IsNullOrWhiteSpace(this.Delta.Text);
 
     /// <inheritdoc />
     public ContentStreamChunk GetContent() => new(this.Delta.Text, []);
 
+    /// <inheritdoc />
+    /// <remarks>
+    /// Read off the message start event, the first line of the stream, and never off the message
+    /// delta at its end. That one carries a usage as well, but a cumulative one: once the model
+    /// used a server tool, it holds what the tool fed back into the same request. The example in
+    /// the streaming documentation, read on 2026-09-24 at
+    /// https://platform.claude.com/docs/en/build-with-claude/streaming, shows 2,679 input tokens at
+    /// the start of a message with a web search and 10,682 at its end. No later request carries
+    /// those search results; the start states exactly what this one carried.
+    ///
+    /// The number arriving before the answer is no problem, because it describes the request, not
+    /// the answer. A stream which breaks off afterward leaves the answer with whatever arrived up
+    /// to then, nothing at all included, and the next request carries exactly that -- which is
+    /// what the answer's text is counted as.
+    /// </remarks>
+    public TokenUsage GetUsage() => this.Message?.Usage?.ToTokenUsage() ?? TokenUsage.UNKNOWN;
+
     #region Implementation of IAnnotationStreamLine
 
     //