Added the exact token count for the OpenAI Responses API

app/MindWork AI Studio/Provider/BaseProvider.cs

@@ -1157,17 +1157,44 @@ public abstract class BaseProvider : IProvider, ISecretId
         // Check if annotations are supported:
         var annotationSupported = typeof(TAnnotation) != typeof(NoResponsesAnnotationStreamLine) && typeof(TAnnotation) != typeof(NoChatCompletionAnnotationStreamLine);
         
+        var isCompleted = false;
         await foreach (var serverSentEvent in this.ReadServerSentEventsAsync(providerName, "responses call", requestBuilder, token))
         {
-            // Check if the line is the end of the stream. This one is read off the raw line
-            // rather than off a payload, because it has none:
+            // Check if the line announces the end of the stream. This one is read off the raw
+            // line rather than off a payload, because it has none:
             if (serverSentEvent.Line.StartsWith("event: response.completed", StringComparison.InvariantCulture))
-                yield break;
+            {
+                isCompleted = true;
+                continue;
+            }
             
             // Skip lines without a payload:
             if (serverSentEvent.Data.Length is 0)
                 continue;
 
+            //
+            // The payload after the announcement is the whole response, and the only line which
+            // states what the request cost. The stream ends here whether that can be read or not,
+            // which keeps the end independent of how a gateway orders the fields of the payload.
+            //
+            if (isCompleted)
+            {
+                var usage = TokenUsage.UNKNOWN;
+                try
+                {
+                    usage = JsonSerializer.Deserialize<ResponsesCompletedStreamLine>(serverSentEvent.Data, JSON_SERIALIZER_OPTIONS)?.GetUsage() ?? TokenUsage.UNKNOWN;
+                }
+                catch
+                {
+                    // Invalid JSON data states nothing, and the answer is complete either way.
+                }
+
+                if (usage.IsKnown)
+                    yield return new(string.Empty, [], Usage: usage);
+
+                yield break;
+            }
+
             //
             // Find delta lines:
             //

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

@@ -10,4 +10,15 @@ namespace AIStudio.Provider.OpenAI;
 /// </remarks>
 /// <param name="Type">The type of the stream event.</param>
 /// <param name="Response">The response as a non-streamed call would have returned it.</param>
-public sealed record ResponsesCompletedStreamLine(string Type, ResponsesResponse? Response);
+public sealed record ResponsesCompletedStreamLine(string Type, ResponsesResponse? Response)
+{
+    /// <summary>
+    /// States what the request of this call carried, as far as it can be believed.
+    /// </summary>
+    /// <remarks>
+    /// The one way from the wire to a usage, shared by the plain text path and the tool calling
+    /// path, so that what counts as believable is decided in a single place.
+    /// </remarks>
+    /// <returns>The usage, or TokenUsage.UNKNOWN when the line states nothing usable.</returns>
+    public TokenUsage GetUsage() => this.Response?.GetUsage() ?? TokenUsage.UNKNOWN;
+}

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

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

@@ -27,7 +27,7 @@ public sealed class ResponsesStreamAccumulator
     /// Takes the next event of the stream and returns what it has to show.
     /// </summary>
     /// <param name="serverSentEvent">The event to read.</param>
-    /// <returns>The text and sources of this event, both empty when it carried neither.</returns>
+    /// <returns>The text and sources of this event, both empty when it carried neither, and the usage of the completed event.</returns>
     public ResponsesStreamPart Process(ServerSentEvent serverSentEvent)
     {
         if (serverSentEvent.Data.Length is 0)
@@ -61,8 +61,15 @@ public sealed class ResponsesStreamAccumulator
         switch (eventType)
         {
             case EVENT_COMPLETED:
-                this.completedResponse = TryDeserialize<ResponsesCompletedStreamLine>(serverSentEvent.Data)?.Response ?? this.completedResponse;
-                return ResponsesStreamPart.Nothing;
+                var completedLine = TryDeserialize<ResponsesCompletedStreamLine>(serverSentEvent.Data);
+                this.completedResponse = completedLine?.Response ?? this.completedResponse;
+
+                //
+                // What the request carried, read the same way as on the plain text path and with
+                // the same exception, a response which ran a hosted tool, cf. ResponsesResponse.
+                //
+                var usage = completedLine?.GetUsage() ?? TokenUsage.UNKNOWN;
+                return usage.IsKnown ? new ResponsesStreamPart(string.Empty, [], usage) : ResponsesStreamPart.Nothing;
             
             case EVENT_TEXT_DELTA:
                 var deltaLine = TryDeserialize<ResponsesDeltaStreamLine>(serverSentEvent.Data);

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

@@ -5,7 +5,8 @@ namespace AIStudio.Provider.OpenAI;
 /// </summary>
 /// <param name="TextDelta">The text this line carried, empty when it carried none.</param>
 /// <param name="Sources">The sources this line announced, empty when it announced none.</param>
-public readonly record struct ResponsesStreamPart(string TextDelta, IList<ISource> Sources)
+/// <param name="Usage">What the provider said the request cost, unknown on every line but the completed event.</param>
+public readonly record struct ResponsesStreamPart(string TextDelta, IList<ISource> Sources, TokenUsage Usage = default)
 {
     /// <summary>
     /// The part of a line which says nothing to the user, such as a bookkeeping event.
@@ -15,5 +16,9 @@ public readonly record struct ResponsesStreamPart(string TextDelta, IList<ISourc
     /// <summary>
     /// Whether this part has anything to show at all.
     /// </summary>
+    /// <remarks>
+    /// The usage is not part of that: it is nothing to show, and whether it reaches the answer at
+    /// all is the tool calling loop's decision, which knows which round this is.
+    /// </remarks>
     public bool HasContent => this.TextDelta.Length > 0 || this.Sources.Count > 0;
 }

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

@@ -59,14 +59,16 @@ public sealed class ResponsesToolCallingAdapter(Model chatModel, IList<object> b
 
         //
         // The text goes out while it is being written, the round only once the stream closed it.
-        // Sources travel with the text because the API announces them as it cites them.
+        // Sources travel with the text because the API announces them as it cites them. The usage
+        // goes out with every round: which of them describes the conversation is the loop's
+        // decision, which knows which round this is.
         //
         var accumulator = new ResponsesStreamAccumulator();
         await foreach (var serverSentEvent in streamRequestAsync(request, token))
         {
             var part = accumulator.Process(serverSentEvent);
-            if (part.HasContent)
-                yield return ToolCallingStreamEvent.TextDelta(new ContentStreamChunk(part.TextDelta, part.Sources));
+            if (part.HasContent || part.Usage.IsKnown)
+                yield return ToolCallingStreamEvent.TextDelta(new ContentStreamChunk(part.TextDelta, part.Sources, Usage: part.Usage));
         }
 
         var response = accumulator.Build();

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

@@ -0,0 +1,31 @@
+// ReSharper disable ClassNeverInstantiated.Global
+namespace AIStudio.Provider.OpenAI;
+
+/// <summary>
+/// What OpenAI reports a Responses API call cost, as it closes the stream.
+/// </summary>
+/// <remarks>
+/// The input tokens are the whole request. What the API took from its cache is a share of them,
+/// not an addition to them, which is why that detail stays unread. Read on 2026-09-24 at
+/// https://developers.openai.com/api/docs/guides/prompt-caching.
+///
+/// The block states more than this, the output and its reasoning share among it. Those are left
+/// unread on purpose, for the reason given at TokenUsage: no later request carries them.
+/// </remarks>
+public sealed record ResponsesUsage
+{
+    /// <summary>
+    /// What everything sent to the model cost, the cached part included.
+    /// </summary>
+    public int? InputTokens { get; init; }
+
+    /// <summary>
+    /// States what this block reports, as far as it can be believed.
+    /// </summary>
+    /// <remarks>
+    /// Whether the block describes the request at all is not decided here but by the response
+    /// around it, cf. ResponsesResponse.
+    /// </remarks>
+    /// <returns>The usage, or TokenUsage.UNKNOWN when the block states nothing usable.</returns>
+    public TokenUsage ToTokenUsage() => TokenUsage.OfReported(this.InputTokens);
+}

app/Tests/Provider/ResponsesUsageTests.cs

@@ -0,0 +1,95 @@
+using System.Text.Json;
+
+using AIStudio.Provider;
+using AIStudio.Provider.OpenAI;
+
+namespace AIStudio.Tests.Provider;
+
+/// <summary>
+/// Checks that what OpenAI says a Responses API call cost is read off the completed event, and only
+/// believed when it describes the request.
+/// </summary>
+/// <remarks>
+/// The completed event states the input of the whole response. That is the request as it was sent,
+/// unless OpenAI ran a hosted tool along the way: what such a tool found is charged as input of the
+/// same response, and no later request carries it. Believe the number then, and a single web search
+/// makes the chat look several times as large as it is.
+/// </remarks>
+[TestFixture]
+public sealed class ResponsesUsageTests
+{
+    private const string COMPLETED_PREFIX = """{"type":"response.completed","sequence_number":42,"response":{"id":"resp_1","object":"response","status":"completed","model":"gpt-5","output":[""";
+    private const string MESSAGE_ITEM = """{"id":"msg_1","type":"message","status":"completed","role":"assistant","content":[{"type":"output_text","text":"Hi there!","annotations":[]}]}""";
+    private const string USAGE_SUFFIX = ""","usage":{"input_tokens":2006,"input_tokens_details":{"cached_tokens":1920},"output_tokens":300,"output_tokens_details":{"reasoning_tokens":120},"total_tokens":2306}}}""";
+
+    [Test]
+    public void TheCompletedEventStatesWhatTheRequestCarried()
+    {
+        //
+        // The cached tokens are a share of the input tokens, not an addition to them: the request
+        // carried 2,006 tokens, 1,920 of which came out of the cache.
+        //
+        var line = Read(COMPLETED_PREFIX + MESSAGE_ITEM + "]" + USAGE_SUFFIX);
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(line.GetUsage().IsKnown, Is.True);
+            Assert.That(line.GetUsage().PromptTokens, Is.EqualTo(2006));
+        });
+    }
+
+    [Test]
+    public void ReasoningAndFunctionCallsLeaveTheInputAlone()
+    {
+        //
+        // Both are output the model wrote itself. Neither makes OpenAI add anything to the input,
+        // and the function call's result comes back in the next request, which states its own.
+        //
+        var line = Read(COMPLETED_PREFIX +
+                        """{"type":"reasoning","id":"rs_1","summary":[],"encrypted_content":"gAAAAAB0aXRs"},""" +
+                        """{"type":"function_call","call_id":"call_1","name":"web_search","arguments":"{\"query\":\"weather\"}"}""" +
+                        "]" + USAGE_SUFFIX);
+
+        Assert.That(line.GetUsage().PromptTokens, Is.EqualTo(2006));
+    }
+
+    [Test]
+    public void AHostedWebSearchStatesNothing()
+    {
+        //
+        // OpenAI searched on its own, and what it found is part of the input tokens. The next
+        // request carries none of it.
+        //
+        var line = Read(COMPLETED_PREFIX +
+                        """{"type":"web_search_call","id":"ws_1","status":"completed","action":{"type":"search","query":"weather"}},""" +
+                        MESSAGE_ITEM +
+                        "]" + USAGE_SUFFIX);
+
+        Assert.That(line.GetUsage().IsKnown, Is.False);
+    }
+
+    [Test]
+    public void AnOutputItemNobodyKnowsStatesNothing()
+    {
+        //
+        // A hosted tool OpenAI adds later is treated like the web search until somebody checked
+        // what it does to the input.
+        //
+        var line = Read(COMPLETED_PREFIX +
+                        """{"type":"future_tool_call","id":"ft_1","status":"completed"},""" +
+                        MESSAGE_ITEM +
+                        "]" + USAGE_SUFFIX);
+
+        Assert.That(line.GetUsage().IsKnown, Is.False);
+    }
+
+    [Test]
+    public void ACompletedEventWithoutAUsageStatesNothing()
+    {
+        var line = Read(COMPLETED_PREFIX + MESSAGE_ITEM + "]}}");
+
+        Assert.That(line.GetUsage().IsKnown, Is.False);
+    }
+
+    private static ResponsesCompletedStreamLine Read(string data) => JsonSerializer.Deserialize<ResponsesCompletedStreamLine>(data, ProviderJsonOptions.OPTIONS)!;
+}

app/Tests/Provider/ToolCalling/ResponsesStreamAccumulatorTests.cs

@@ -105,6 +105,33 @@ public sealed class ResponsesStreamAccumulatorTests
         Assert.That(response!.GetTextOutput(), Is.EqualTo("Complete"), "The closing event is the round, and the collected items were only there in case it never came.");
     }
     
+    [Test]
+    public void TheCompletedEventStatesWhatTheRequestCost()
+    {
+        var accumulator = new ResponsesStreamAccumulator();
+        var part = accumulator.Process(Event(COMPLETED_PREFIX + REASONING_ITEM + """],"usage":{"input_tokens":2679,"input_tokens_details":{"cached_tokens":0},"output_tokens":510}}}"""));
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(part.Usage.IsKnown, Is.True, "The closing event is the one line which states what the request cost.");
+            Assert.That(part.Usage.PromptTokens, Is.EqualTo(2679));
+            Assert.That(part.HasContent, Is.False, "And there is nothing on it to show.");
+            Assert.That(accumulator.Build()!.Output, Has.Count.EqualTo(1), "Reading the usage leaves the round as it was.");
+        });
+    }
+
+    [Test]
+    public void ARoundPutBackTogetherFromItsItemsStatesNoCost()
+    {
+        //
+        // The gateway which never sends the closing event never sends the usage either, which
+        // leaves the round to the estimate.
+        //
+        var response = Read("""{"type":"response.output_item.done","output_index":0,"item":{"type":"message","role":"assistant","content":[{"type":"output_text","text":"Partial"}]}}""");
+
+        Assert.That(response!.GetUsage().IsKnown, Is.False);
+    }
+
     [Test]
     public void AStreamWhichSaidNothingIsAFailedRound()
     {

app/Tests/Provider/ToolCalling/ResponsesToolCallingAdapterTests.cs

@@ -0,0 +1,112 @@
+using System.Runtime.CompilerServices;
+
+using AIStudio.Provider;
+using AIStudio.Provider.OpenAI;
+
+namespace AIStudio.Tests.Provider.ToolCalling;
+
+/// <summary>
+/// Checks what a round of a tool calling conversation with the Responses API passes on about its cost.
+/// </summary>
+/// <remarks>
+/// Every round is a request of its own, and every one of them states at its end what it cost -- as
+/// long as OpenAI ran no hosted tool in it. Which round describes the conversation is the tool
+/// calling loop's decision, checked in ToolCallingLoopTests.
+/// </remarks>
+[TestFixture]
+public sealed class ResponsesToolCallingAdapterTests
+{
+    [Test]
+    public async Task EveryRoundPassesOnWhatItsRequestCost()
+    {
+        var adapter = Adapter(
+        [
+            """{"type":"response.output_item.done","output_index":0,"item":{"type":"function_call","call_id":"call_1","name":"web_search","arguments":"{\"query\":\"weather\"}"}}""",
+            """{"type":"response.completed","response":{"id":"resp_1","model":"gpt-5","output":[{"type":"function_call","call_id":"call_1","name":"web_search","arguments":"{\"query\":\"weather\"}"}],"usage":{"input_tokens":1200,"output_tokens":20}}}""",
+        ],
+        [
+            """{"type":"response.output_text.delta","delta":"It is sunny."}""",
+            """{"type":"response.completed","response":{"id":"resp_2","model":"gpt-5","output":[{"type":"message","role":"assistant","content":[{"type":"output_text","text":"It is sunny."}]}],"usage":{"input_tokens":9800,"output_tokens":30}}}""",
+        ]);
+
+        var firstRound = await Usages(adapter);
+
+        //
+        // What the loop does between two rounds: the model's turn and the tool's result become part
+        // of the next request.
+        //
+        adapter.RecordAssistantTurn();
+        adapter.RecordToolResult("call_1", "Sunny, 24 degrees.");
+
+        var secondRound = await Usages(adapter);
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(firstRound, Is.EqualTo(new[] { 1200 }), "The first round states the conversation up to the question.");
+            Assert.That(secondRound, Is.EqualTo(new[] { 9800 }), "The second round states its own request.");
+        });
+    }
+
+    [Test]
+    public async Task ARoundWithAHostedWebSearchPassesOnNothingButItsText()
+    {
+        var adapter = Adapter(
+        [
+            """{"type":"response.output_text.delta","delta":"It is sunny."}""",
+            """{"type":"response.completed","response":{"id":"resp_1","model":"gpt-5","output":[{"type":"web_search_call","id":"ws_1","status":"completed"},{"type":"message","role":"assistant","content":[{"type":"output_text","text":"It is sunny."}]}],"usage":{"input_tokens":12000,"output_tokens":30}}}""",
+        ]);
+
+        var usages = new List<int>();
+        var written = new List<string>();
+        await foreach (var streamEvent in adapter.ExecuteRoundAsync(null, true))
+        {
+            if (streamEvent.Delta is not { } delta)
+                continue;
+
+            if (delta.Usage.IsKnown)
+                usages.Add(delta.Usage.PromptTokens);
+
+            if (delta.Content.Length > 0)
+                written.Add(delta.Content);
+        }
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(usages, Is.Empty, "What the search found is part of the number, and no later request carries it.");
+            Assert.That(written, Is.EqualTo(new[] { "It is sunny." }), "The answer itself goes out as always.");
+        });
+    }
+
+    /// <summary>
+    /// Runs the next round and returns the prompt of every usage it passed on.
+    /// </summary>
+    private static async Task<List<int>> Usages(ResponsesToolCallingAdapter adapter)
+    {
+        var usages = new List<int>();
+        await foreach (var streamEvent in adapter.ExecuteRoundAsync(null, true))
+            if (streamEvent.Delta is { Usage.IsKnown: true } delta)
+                usages.Add(delta.Usage.PromptTokens);
+
+        return usages;
+    }
+
+    /// <summary>
+    /// Builds an adapter whose requests are answered by the given rounds, one after another.
+    /// </summary>
+    private static ResponsesToolCallingAdapter Adapter(params string[][] rounds)
+    {
+        var nextRound = 0;
+        return new(new Model("gpt-5", null), [], new Dictionary<string, object>(), [], [], (_, token) => Lines(rounds[nextRound++], token));
+    }
+
+    private static async IAsyncEnumerable<ServerSentEvent> Lines(string[] data, [EnumeratorCancellation] CancellationToken token = default)
+    {
+        foreach (var line in data)
+        {
+            token.ThrowIfCancellationRequested();
+            yield return new ServerSentEvent($"data: {line}", line);
+        }
+
+        await Task.CompletedTask;
+    }
+}