Added the exact token count for the OpenAI Responses API

app/MindWork AI Studio/Provider/OpenAI/ResponsesResponse.cs

@@ -7,6 +7,16 @@ namespace AIStudio.Provider.OpenAI;
 /// </summary>
 public sealed record ResponsesResponse
 {
+    /// <summary>
+    /// The output items after which the usage still describes the request as it was sent.
+    /// </summary>
+    /// <remarks>
+    /// A list of the items which are known to leave the input alone, rather than one of those which
+    /// do not: a hosted tool OpenAI adds later would otherwise inflate the number without anybody
+    /// noticing.
+    /// </remarks>
+    private static readonly HashSet<string> OUTPUT_ITEMS_LEAVING_THE_INPUT_ALONE = ["message", "reasoning", "function_call"];
+
     public string Id { get; init; } = string.Empty;
 
     public string Model { get; init; } = string.Empty;
@@ -15,6 +25,30 @@ public sealed record ResponsesResponse
 
     public IList<JsonElement> Output { get; init; } = [];
 
+    /// <summary>
+    /// What OpenAI says the response cost. Only the completed response carries it.
+    /// </summary>
+    public ResponsesUsage? Usage { get; init; }
+
+    /// <summary>
+    /// States what the request of this response carried, as far as it can be believed.
+    /// </summary>
+    /// <remarks>
+    /// Only for a response which ran no hosted tool. When OpenAI runs one, such as its web search,
+    /// what the tool found is charged as input tokens of the same response, cf. the pricing read on
+    /// 2026-09-24 at https://developers.openai.com/api/docs/pricing. No later request carries what
+    /// the tool found, so the number would describe a conversation larger than the one there is --
+    /// the same reason why the tool calling loop keeps only the usage of its first round.
+    ///
+    /// A response put back together from its items, for a gateway which never sent the completed
+    /// event, has no usage. Nor is one read off a response which ended as incomplete, because it
+    /// ran out of output tokens: that is rare enough for the estimate to cover it.
+    /// </remarks>
+    /// <returns>The usage, or TokenUsage.UNKNOWN when the response states nothing usable.</returns>
+    public TokenUsage GetUsage() => this.Output.All(x => OUTPUT_ITEMS_LEAVING_THE_INPUT_ALONE.Contains(ReadString(x, "type")))
+        ? this.Usage?.ToTokenUsage() ?? TokenUsage.UNKNOWN
+        : TokenUsage.UNKNOWN;
+
     public IReadOnlyList<ResponsesFunctionCallItem> GetFunctionCalls() => this.Output
         .Where(x => ReadString(x, "type").Equals("function_call", StringComparison.Ordinal))
         .Select(x => new ResponsesFunctionCallItem