Added the exact token count for Anthropic

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

@@ -14,6 +14,7 @@ namespace AIStudio.Provider.Anthropic;
 /// </remarks>
 public sealed class AnthropicMessageStreamAccumulator
 {
+    private const string EVENT_MESSAGE_START = "message_start";
     private const string EVENT_BLOCK_START = "content_block_start";
     private const string EVENT_BLOCK_DELTA = "content_block_delta";
     private const string EVENT_BLOCK_STOP = "content_block_stop";
@@ -32,7 +33,7 @@ public sealed class AnthropicMessageStreamAccumulator
     /// 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 of this event, empty when it carried none.</returns>
+    /// <returns>The text of this event, empty when it carried none, and the usage of the message start.</returns>
     public AnthropicStreamPart Process(ServerSentEvent serverSentEvent)
     {
         if (serverSentEvent.Data.Length is 0)
@@ -51,6 +52,14 @@ public sealed class AnthropicMessageStreamAccumulator
 
         switch (line.Type)
         {
+            case EVENT_MESSAGE_START:
+                //
+                // What the request carried, read the same way as on the plain text path and for
+                // the same reason: the start states this request alone, cf. ResponseStreamLine.
+                //
+                var usage = line.Message?.Usage?.ToTokenUsage() ?? TokenUsage.UNKNOWN;
+                return usage.IsKnown ? new AnthropicStreamPart(string.Empty, usage) : AnthropicStreamPart.Nothing;
+
             case EVENT_BLOCK_START:
                 this.openBlocks[line.Index] = new AnthropicContentBlockBuilder(line.ContentBlock);
                 return AnthropicStreamPart.Nothing;

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

@@ -9,4 +9,14 @@ namespace AIStudio.Provider.Anthropic;
 /// <param name="Index">Which content block the event belongs to; blocks are correlated by it.</param>
 /// <param name="ContentBlock">The block as it opens, for a content block start.</param>
 /// <param name="Delta">The piece this event adds, for a content block delta or a message delta.</param>
-public readonly record struct AnthropicStreamLine(string? Type, int Index, JsonElement ContentBlock, AnthropicStreamDelta Delta);
+public readonly record struct AnthropicStreamLine(string? Type, int Index, JsonElement ContentBlock, AnthropicStreamDelta Delta)
+{
+    /// <summary>
+    /// The message the stream opens with, for a message start.
+    /// </summary>
+    /// <remarks>
+    /// Not a positional parameter, because nobody but the serializer ever builds this line with
+    /// one. It is what states the usage, for the reason given at ResponseStreamLine.GetUsage.
+    /// </remarks>
+    public AnthropicStreamMessage? Message { get; init; }
+}

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

@@ -0,0 +1,18 @@
+// ReSharper disable ClassNeverInstantiated.Global
+namespace AIStudio.Provider.Anthropic;
+
+/// <summary>
+/// The message a streamed Anthropic messages call opens with, as far as it is read.
+/// </summary>
+/// <remarks>
+/// It arrives on the message start event, before any content, and states what the request
+/// carried. Its content is always empty there -- the blocks follow as events of their own -- so
+/// the usage is all there is to read.
+/// </remarks>
+public sealed record AnthropicStreamMessage
+{
+    /// <summary>
+    /// What the request carried, where the stream states it.
+    /// </summary>
+    public AnthropicUsage? Usage { get; init; }
+}

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

@@ -8,15 +8,20 @@ namespace AIStudio.Provider.Anthropic;
 /// and doing so would be a feature of its own rather than a side effect of streaming.
 /// </remarks>
 /// <param name="TextDelta">The text this line carried, empty when it carried none.</param>
-public readonly record struct AnthropicStreamPart(string TextDelta)
+/// <param name="Usage">What the provider said the request carried, unknown on every line but the message start.</param>
+public readonly record struct AnthropicStreamPart(string TextDelta, TokenUsage Usage = default)
 {
     /// <summary>
     /// The part of a line that says nothing to the user, such as an opening or closing block.
     /// </summary>
     public static AnthropicStreamPart Nothing => new(string.Empty);
-    
+
     /// <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;
 }

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

@@ -57,14 +57,16 @@ public sealed class AnthropicToolCallingAdapter(Model chatModel, IList<IMessageB
 
         //
         // The text goes out while it is being written; the blocks are put back together behind
-        // it, because they have to return to the provider exactly as they arrived.
+        // it, because they have to return to the provider exactly as they arrived. 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 AnthropicMessageStreamAccumulator();
         await foreach (var serverSentEvent in streamRequestAsync(request, token))
         {
             var part = accumulator.Process(serverSentEvent);
-            if (part.HasContent)
-                yield return ToolCallingStreamEvent.TextDelta(part.TextDelta);
+            if (part.HasContent || part.Usage.IsKnown)
+                yield return ToolCallingStreamEvent.TextDelta(new ContentStreamChunk(part.TextDelta, [], Usage: part.Usage));
         }
 
         var response = accumulator.Build();

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

@@ -0,0 +1,49 @@
+// ReSharper disable ClassNeverInstantiated.Global
+namespace AIStudio.Provider.Anthropic;
+
+/// <summary>
+/// What Anthropic reports a messages call carried, as it opens the stream.
+/// </summary>
+/// <remarks>
+/// The input arrives in up to three parts, and only their sum is what the request carried: the
+/// input tokens are just those after the last cache breakpoint, the other two are what was written
+/// to and read from the cache before it. Read on 2026-09-24 at
+/// https://platform.claude.com/docs/en/build-with-claude/prompt-caching.
+///
+/// A missing cache part means that nothing was cached, not that its size is unknown: the API
+/// caches only for a request which asks for it with cache_control, which AI Studio never does on
+/// its own -- somebody could, though, through the additional API parameters. A missing input part
+/// is different, and without it the block states nothing.
+///
+/// The output tokens are left unread on purpose, for the reason given at TokenUsage: they include
+/// the model's thinking, which no later request carries.
+/// </remarks>
+public sealed record AnthropicUsage
+{
+    /// <summary>
+    /// What the request carried after its last cache breakpoint, which is all of it without caching.
+    /// </summary>
+    public int? InputTokens { get; init; }
+
+    /// <summary>
+    /// What the request wrote to the cache.
+    /// </summary>
+    public int? CacheCreationInputTokens { get; init; }
+
+    /// <summary>
+    /// What the request read from the cache.
+    /// </summary>
+    public int? CacheReadInputTokens { get; init; }
+
+    /// <summary>
+    /// States what this block reports, 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 block states nothing usable.</returns>
+    public TokenUsage ToTokenUsage() => this.InputTokens is { } inputTokens
+        ? TokenUsage.OfReported(inputTokens + (this.CacheCreationInputTokens ?? 0) + (this.CacheReadInputTokens ?? 0))
+        : TokenUsage.UNKNOWN;
+}

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
 
     //

app/Tests/Provider/AnthropicUsageTests.cs

@@ -0,0 +1,116 @@
+using System.Text.Json;
+
+using AIStudio.Provider;
+using AIStudio.Provider.Anthropic;
+
+namespace AIStudio.Tests.Provider;
+
+/// <summary>
+/// Checks that what Anthropic says a request carried is read off the right line of the stream.
+/// </summary>
+/// <remarks>
+/// Anthropic states a usage twice per message: at its start, and cumulatively at its end. Only the
+/// start describes the request as it was sent. The end adds whatever a server tool fed back into
+/// the same request, which no later request carries -- read that one, and a single web search
+/// makes the chat look four times as large as it is.
+/// </remarks>
+[TestFixture]
+public sealed class AnthropicUsageTests
+{
+    /// <summary>
+    /// The opening line of a message, as the streaming documentation shows it.
+    /// </summary>
+    private const string MESSAGE_START =
+        """
+        {"type": "message_start", "message": {"id": "msg_1nZdL29xx5MUA1yADyHTEsnR8uuvGzszyY", "type": "message", "role": "assistant", "content": [], "model": "claude-opus-5-5", "stop_reason": null, "stop_sequence": null, "usage": {"input_tokens": 25, "output_tokens": 1}}}
+        """;
+
+    [Test]
+    public void TheOpeningLineStatesWhatTheRequestCarried()
+    {
+        var line = Read(MESSAGE_START);
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(line.GetUsage().IsKnown, Is.True);
+            Assert.That(line.GetUsage().PromptTokens, Is.EqualTo(25));
+
+            // It carries no answer, which is why it has to be read before the content check drops it:
+            Assert.That(line.ContainsContent(), Is.False);
+        });
+    }
+
+    [Test]
+    public void WhatWasCachedCountsAsWell()
+    {
+        //
+        // With caching, the input tokens are only what comes after the last cache breakpoint. The
+        // request carried all three parts.
+        //
+        var line = Read(
+            """
+            {"type":"message_start","message":{"id":"msg_1","type":"message","role":"assistant","content":[],"model":"claude-opus-5-5","usage":{"input_tokens":50,"cache_creation_input_tokens":1000,"cache_read_input_tokens":2000,"output_tokens":1}}}
+            """);
+
+        Assert.That(line.GetUsage().PromptTokens, Is.EqualTo(3050));
+    }
+
+    [Test]
+    public void WithoutItsInputTokensABlockStatesNothing()
+    {
+        //
+        // The cache parts alone are not the request: the part after the breakpoint is missing, and
+        // it is the one part every request has.
+        //
+        var line = Read(
+            """
+            {"type":"message_start","message":{"id":"msg_1","type":"message","role":"assistant","content":[],"model":"claude-opus-5-5","usage":{"cache_read_input_tokens":2000}}}
+            """);
+
+        Assert.That(line.GetUsage().IsKnown, Is.False);
+    }
+
+    [Test]
+    public void TheClosingLineOfAMessageWithAWebSearchStatesNothing()
+    {
+        //
+        // The example of the streaming documentation: 2,679 input tokens at the start, 10,682 at
+        // the end, the difference being the search results. The end is cumulative, and the next
+        // request carries none of what the search added.
+        //
+        var line = Read(
+            """
+            {"type":"message_delta","delta":{"stop_reason":"end_turn","stop_sequence":null},"usage":{"input_tokens":10682,"cache_creation_input_tokens":0,"cache_read_input_tokens":0,"output_tokens":510,"server_tool_use":{"web_search_requests":1}}}
+            """);
+
+        Assert.That(line.GetUsage().IsKnown, Is.False);
+    }
+
+    [Test]
+    public void AnOpeningLineWithoutAUsageStatesNothing()
+    {
+        var line = Read(
+            """
+            {"type": "message_start", "message": {"id": "msg_01...", "type": "message", "role": "assistant", "content": [], "model": "claude-opus-5-5", "stop_reason": null, "stop_sequence": null}}
+            """);
+
+        Assert.That(line.GetUsage().IsKnown, Is.False);
+    }
+
+    [Test]
+    public void ALineOfTheAnswerStatesNothing()
+    {
+        var line = Read(
+            """
+            {"type":"content_block_delta","index":0,"delta":{"type":"text_delta","text":"Hi"}}
+            """);
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(line.GetUsage().IsKnown, Is.False);
+            Assert.That(line.ContainsContent(), Is.True);
+        });
+    }
+
+    private static ResponseStreamLine Read(string data) => JsonSerializer.Deserialize<ResponseStreamLine>(data, ProviderJsonOptions.OPTIONS);
+}

app/Tests/Provider/ToolCalling/AnthropicMessageStreamAccumulatorTests.cs

@@ -218,7 +218,40 @@ public sealed class AnthropicMessageStreamAccumulatorTests
 
         Assert.That(response, Is.Null, "An unfinished message is not handed on as if it were finished.");
     }
-    
+
+    [Test]
+    public void TheMessageStartStatesWhatTheRequestCarried()
+    {
+        const string MESSAGE_START = """{"type":"message_start","message":{"id":"msg_1","type":"message","role":"assistant","model":"claude-opus-5-5","content":[],"stop_reason":null,"stop_sequence":null,"usage":{"input_tokens":2679,"cache_creation_input_tokens":0,"cache_read_input_tokens":0,"output_tokens":3}}}""";
+
+        var part = new AnthropicMessageStreamAccumulator().Process(Event(MESSAGE_START));
+        var response = Read(
+            MESSAGE_START,
+            """{"type":"content_block_start","index":0,"content_block":{"type":"text","text":""}}""",
+            """{"type":"content_block_delta","index":0,"delta":{"type":"text_delta","text":"Hello."}}""",
+            """{"type":"content_block_stop","index":0}""",
+            """{"type":"message_stop"}""");
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(part.Usage.PromptTokens, Is.EqualTo(2679), "The start of the message states what the request carried.");
+            Assert.That(part.HasContent, Is.False, "And it has nothing to show.");
+            Assert.That(response!.Content, Has.Count.EqualTo(1), "Nor does it become a block of the message.");
+        });
+    }
+
+    [Test]
+    public void TheMessageDeltaStatesNothingAboutTheRequest()
+    {
+        //
+        // Its usage is cumulative and holds whatever a server tool fed back into the same request.
+        //
+        var part = new AnthropicMessageStreamAccumulator().Process(Event(
+            """{"type":"message_delta","delta":{"stop_reason":"end_turn","stop_sequence":null},"usage":{"input_tokens":10682,"output_tokens":510}}"""));
+
+        Assert.That(part.Usage.IsKnown, Is.False);
+    }
+
     private static AnthropicResponse? Read(params string[] data)
     {
         var accumulator = new AnthropicMessageStreamAccumulator();

app/Tests/Provider/ToolCalling/AnthropicToolCallingAdapterTests.cs

@@ -0,0 +1,111 @@
+using System.Runtime.CompilerServices;
+
+using AIStudio.Provider;
+using AIStudio.Provider.Anthropic;
+
+namespace AIStudio.Tests.Provider.ToolCalling;
+
+/// <summary>
+/// Checks what a round of a tool calling conversation with Anthropic passes on about its cost.
+/// </summary>
+/// <remarks>
+/// Every round is a request of its own, and every one of them states at its start what it carried.
+/// The adapter passes each of those on and none of the cumulative ones at the end of a message.
+/// Which round describes the conversation is the tool calling loop's decision, checked in
+/// ToolCallingLoopTests.
+/// </remarks>
+[TestFixture]
+public sealed class AnthropicToolCallingAdapterTests
+{
+    [Test]
+    public async Task EveryRoundPassesOnWhatItsRequestCarried()
+    {
+        var adapter = Adapter(
+        [
+            """{"type":"message_start","message":{"id":"msg_1","type":"message","role":"assistant","content":[],"usage":{"input_tokens":1200,"output_tokens":1}}}""",
+            """{"type":"content_block_start","index":0,"content_block":{"type":"tool_use","id":"toolu_1","name":"web_search","input":{}}}""",
+            """{"type":"content_block_delta","index":0,"delta":{"type":"input_json_delta","partial_json":"{\"query\":\"weather\"}"}}""",
+            """{"type":"content_block_stop","index":0}""",
+            """{"type":"message_delta","delta":{"stop_reason":"tool_use","stop_sequence":null},"usage":{"output_tokens":20}}""",
+            """{"type":"message_stop"}""",
+        ],
+        [
+            """{"type":"message_start","message":{"id":"msg_2","type":"message","role":"assistant","content":[],"usage":{"input_tokens":9800,"output_tokens":1}}}""",
+            """{"type":"content_block_start","index":0,"content_block":{"type":"text","text":""}}""",
+            """{"type":"content_block_delta","index":0,"delta":{"type":"text_delta","text":"It is sunny."}}""",
+            """{"type":"content_block_stop","index":0}""",
+            """{"type":"message_delta","delta":{"stop_reason":"end_turn","stop_sequence":null},"usage":{"input_tokens":12000,"output_tokens":30}}""",
+            """{"type":"message_stop"}""",
+        ]);
+
+        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("toolu_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, and the cumulative number at its end stays behind.");
+        });
+    }
+
+    [Test]
+    public async Task TheTextStillGoesOutNextToTheUsage()
+    {
+        var adapter = Adapter(
+        [
+            """{"type":"message_start","message":{"id":"msg_1","type":"message","role":"assistant","content":[],"usage":{"input_tokens":1200,"output_tokens":1}}}""",
+            """{"type":"content_block_start","index":0,"content_block":{"type":"text","text":""}}""",
+            """{"type":"content_block_delta","index":0,"delta":{"type":"text_delta","text":"Hello."}}""",
+            """{"type":"content_block_stop","index":0}""",
+            """{"type":"message_stop"}""",
+        ]);
+
+        var written = new List<string>();
+        await foreach (var streamEvent in adapter.ExecuteRoundAsync(null, true))
+            if (streamEvent.Delta is { Content.Length: > 0 } delta)
+                written.Add(delta.Content);
+
+        Assert.That(written, Is.EqualTo(new[] { "Hello." }));
+    }
+
+    /// <summary>
+    /// Runs the next round and returns the prompt of every usage it passed on.
+    /// </summary>
+    private static async Task<List<int>> Usages(AnthropicToolCallingAdapter 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 AnthropicToolCallingAdapter Adapter(params string[][] rounds)
+    {
+        var nextRound = 0;
+        return new(new Model("claude-test", null), [], "You are a helpful assistant.", 1024, 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;
+    }
+}