Added the exact token count to chats which offer tools

app/Tests/Provider/ToolCalling/ChatCompletionToolCallAccumulatorTests.cs

@@ -183,7 +183,38 @@ public sealed class ChatCompletionToolCallAccumulatorTests
         
         Assert.That(part.Sources.Select(x => x.URL), Is.EqualTo(new[] { "https://example.org/" }), "Whatever the provider announced on that line reaches the user with it.");
     }
-    
+
+    [Test]
+    public void TheLineWithoutChoicesStatesWhatTheRequestCost()
+    {
+        var accumulator = new ChatCompletionToolCallAccumulator();
+        var part = accumulator.Process(Event("""{"choices":[],"usage":{"prompt_tokens":1200,"completion_tokens":345,"total_tokens":1545}}"""));
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(part.Usage.IsKnown, Is.True, "The last line of the stream has no choices, and it must not be dropped for that.");
+            Assert.That(part.Usage.PromptTokens, Is.EqualTo(1200));
+            Assert.That(part.HasContent, Is.False, "It has nothing to show, though.");
+        });
+    }
+
+    [Test]
+    public void AUsageNextToTheLastTextIsReadAsWell()
+    {
+        //
+        // Some providers put the usage on the line which carries the last piece of the answer
+        // rather than on a line of its own.
+        //
+        var accumulator = new ChatCompletionToolCallAccumulator();
+        var part = accumulator.Process(Event("""{"choices":[{"index":0,"delta":{"content":"Bye"}}],"usage":{"prompt_tokens":1200,"completion_tokens":2}}"""));
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(part.TextDelta, Is.EqualTo("Bye"));
+            Assert.That(part.Usage.PromptTokens, Is.EqualTo(1200));
+        });
+    }
+
     private static ChatCompletionResponseMessage? Read(params string[] data)
     {
         var accumulator = new ChatCompletionToolCallAccumulator();

app/Tests/Provider/ToolCalling/ChatCompletionToolCallingAdapterTests.cs

@@ -0,0 +1,118 @@
+using System.Runtime.CompilerServices;
+
+using AIStudio.Provider;
+using AIStudio.Provider.OpenAI;
+
+using Microsoft.Extensions.Logging.Abstractions;
+
+namespace AIStudio.Tests.Provider.ToolCalling;
+
+/// <summary>
+/// Checks which round of a tool calling conversation passes on what its request cost.
+/// </summary>
+/// <remarks>
+/// Every round of a tool conversation is a request of its own, and every one of them reports what
+/// it cost. Only the first one describes what the next question will be sent after: every later
+/// round carries the tool calls and their results on top, none of which is sent again once the
+/// answer stands. Passing on the last report instead would put the chat at the size of everything
+/// the tools returned, which is the one number a person watching their context window must not
+/// see as exact.
+/// </remarks>
+[TestFixture]
+public sealed class ChatCompletionToolCallingAdapterTests
+{
+    private const string FIRST_ROUND_USAGE = """{"choices":[],"usage":{"prompt_tokens":1200,"completion_tokens":20}}""";
+
+    private const string SECOND_ROUND_USAGE = """{"choices":[],"usage":{"prompt_tokens":9800,"completion_tokens":150}}""";
+
+    [Test]
+    public async Task OnlyTheFirstRoundPassesOnWhatItsRequestCost()
+    {
+        var adapter = Adapter(
+        [
+            """{"choices":[{"index":0,"delta":{"tool_calls":[{"index":0,"id":"call_1","type":"function","function":{"name":"web_search","arguments":"{\"query\":\"weather\"}"}}]}}]}""",
+            FIRST_ROUND_USAGE,
+            "[DONE]",
+        ],
+        [
+            """{"choices":[{"index":0,"delta":{"content":"It is sunny."}}]}""",
+            SECOND_ROUND_USAGE,
+            "[DONE]",
+        ]);
+
+        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's prompt is the conversation up to the question.");
+            Assert.That(secondRound, Is.Empty, "The second round's prompt holds the tool result as well, which the next question is not sent with.");
+        });
+    }
+
+    [Test]
+    public async Task ARoundWithoutToolCallsPassesItOnAsWell()
+    {
+        //
+        // Offering tools does not mean the model uses them. Then the first round is the only one,
+        // and its report is as good as the one of a request which offered none.
+        //
+        var adapter = Adapter(
+        [
+            """{"choices":[{"index":0,"delta":{"content":"Hello."}}]}""",
+            FIRST_ROUND_USAGE,
+            "[DONE]",
+        ]);
+
+        Assert.That(await Usages(adapter), Is.EqualTo(new[] { 1200 }));
+    }
+
+    /// <summary>
+    /// Runs the next round and returns the prompt of every usage it passed on.
+    /// </summary>
+    private static async Task<List<int>> Usages(ChatCompletionToolCallingAdapter<ChatCompletionAPIRequest> 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 ChatCompletionToolCallingAdapter<ChatCompletionAPIRequest> Adapter(params string[][] rounds)
+    {
+        var nextRound = 0;
+        return new(
+            (_, _, _) => Task.FromResult(new ChatCompletionAPIRequest("model-a", [], true)),
+            new TextMessage("You are a helpful assistant.", "system"),
+            new Dictionary<string, object>(),
+            [],
+            [],
+            (_, token) => Lines(rounds[nextRound++], token),
+            _ => [],
+            NullLogger.Instance);
+    }
+
+    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;
+    }
+}