Added the exact token count where the provider reports it (#989)

Co-authored-by: Thorsten Sommer <SommerEngineering@users.noreply.github.com>

app/Tests/Provider/ChatCompletionUsageTests.cs

@@ -0,0 +1,129 @@
+using System.Text.Json;
+
+using AIStudio.Provider;
+using AIStudio.Provider.OpenAI;
+
+namespace AIStudio.Tests.Provider;
+
+/// <summary>
+/// Checks that what a provider says a request cost is read off the stream, and asked for.
+/// </summary>
+/// <remarks>
+/// Both halves matter and neither is visible from the other: an OpenAI-compatible provider says
+/// nothing about the cost of a streamed request unless the request asks for it, and the line it
+/// then sends carries no content, so the reading side has to look for it apart from the text.
+/// Get either half wrong and the app silently keeps estimating, which looks exactly like a
+/// provider which reports nothing.
+/// </remarks>
+[TestFixture]
+public sealed class ChatCompletionUsageTests
+{
+    /// <summary>
+    /// The last line of a streamed answer at a provider which was asked for the usage.
+    /// </summary>
+    private const string USAGE_LINE =
+        """
+        {"id":"chatcmpl-1","object":"chat.completion.chunk","created":1,"model":"gpt-5","choices":[],"usage":{"prompt_tokens":1200,"completion_tokens":345,"total_tokens":1545}}
+        """;
+
+    /// <summary>
+    /// An ordinary line carrying a piece of the answer.
+    /// </summary>
+    private const string CONTENT_LINE =
+        """
+        {"id":"chatcmpl-1","object":"chat.completion.chunk","created":1,"model":"gpt-5","choices":[{"index":0,"delta":{"content":"Hi"}}]}
+        """;
+
+    [Test]
+    public void TheFinalLineStatesWhatTheRequestCost()
+    {
+        var line = JsonSerializer.Deserialize<ChatCompletionDeltaStreamLine>(USAGE_LINE, ProviderJsonOptions.OPTIONS);
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(line!.GetUsage().IsKnown, Is.True);
+            Assert.That(line.GetUsage().PromptTokens, Is.EqualTo(1200));
+
+            //
+            // The line which carries the usage 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 ALineOfTheAnswerStatesNoCost()
+    {
+        var line = JsonSerializer.Deserialize<ChatCompletionDeltaStreamLine>(CONTENT_LINE, ProviderJsonOptions.OPTIONS);
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(line!.GetUsage().IsKnown, Is.False);
+            Assert.That(line.ContainsContent(), Is.True);
+        });
+    }
+
+    [Test]
+    public void AStreamedRequestAsksForTheUsage()
+    {
+        var request = new ChatCompletionAPIRequest("gpt-5", [], true);
+        var json = JsonSerializer.Serialize(request, ProviderJsonOptions.OPTIONS);
+
+        Assert.That(json, Does.Contain("""
+                                       "stream_options":{"include_usage":true}
+                                       """));
+    }
+
+    [Test]
+    public void ARequestWhichIsNotStreamedDoesNot()
+    {
+        var request = new ChatCompletionAPIRequest("gpt-5", [], false);
+        var json = JsonSerializer.Serialize(request, ProviderJsonOptions.OPTIONS);
+
+        Assert.That(json, Does.Not.Contain("stream_options"));
+    }
+
+    /// <summary>
+    /// A provider which sends the block but fills in nothing usable states nothing.
+    /// </summary>
+    /// <remarks>
+    /// Several OpenAI-compatible servers send an empty or zeroed usage block on every line while
+    /// streaming and the real numbers only at the end. Reading a zero as a fact would replace an
+    /// estimate with a statement that the conversation costs nothing.
+    /// </remarks>
+    [Test]
+    public void AnEmptyUsageBlockStatesNothing()
+    {
+        var line = JsonSerializer.Deserialize<ChatCompletionDeltaStreamLine>(
+            """
+            {"id":"chatcmpl-1","object":"chat.completion.chunk","created":1,"model":"gpt-5","choices":[],"usage":{"prompt_tokens":0,"completion_tokens":0}}
+            """, ProviderJsonOptions.OPTIONS);
+
+        Assert.That(line!.GetUsage().IsKnown, Is.False);
+    }
+
+    /// <summary>
+    /// The line a real LM Studio server sends, taken off the wire.
+    /// </summary>
+    /// <remarks>
+    /// It carries far more than we read, and it shows why the prompt is all we take: 102 of the 116
+    /// completion tokens are the model's reasoning, which the next request never carries. Counting
+    /// the completion would have put the history at 133 tokens, when what travels on is the prompt
+    /// and an answer of a handful of tokens.
+    /// </remarks>
+    [Test]
+    public void ARealServerLineIsRead()
+    {
+        var line = JsonSerializer.Deserialize<ChatCompletionDeltaStreamLine>(
+            """
+            {"id":"chatcmpl-xb8mn282eff3tiu46xz8t3","object":"chat.completion.chunk","created":1789917744,"model":"google/gemma-4-12b-qat","system_fingerprint":"google/gemma-4-12b-qat","choices":[],"usage":{"prompt_tokens":17,"completion_tokens":116,"total_tokens":133,"completion_tokens_details":{"reasoning_tokens":102}}}
+            """, ProviderJsonOptions.OPTIONS);
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(line!.GetUsage().IsKnown, Is.True);
+            Assert.That(line.GetUsage().PromptTokens, Is.EqualTo(17));
+        });
+    }
+}