feat: add classifier support

Braintrust.Sdk.sln

@@ -19,6 +19,8 @@ Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "OpenAIInstrumentation", "ex
 EndProject
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "EvalExample", "examples\EvalExample\EvalExample.csproj", "{DFAA25AA-72B1-4246-BAB9-A10CCF115406}"
 EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "ClassifiersExample", "examples\ClassifiersExample\ClassifiersExample.csproj", "{0A934BA7-BEBB-4EF0-88A6-9A5355E6D0BB}"
+EndProject
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "TraceScoring", "examples\TraceScoring\TraceScoring.csproj", "{66D24AFB-3541-429D-9402-72A344D99115}"
 EndProject
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Braintrust.Sdk.OpenAI", "src\Braintrust.Sdk.OpenAI\Braintrust.Sdk.OpenAI.csproj", "{B3C7D1A2-4E5F-6789-ABCD-EF0123456789}"
@@ -72,6 +74,10 @@ Global
 		{DFAA25AA-72B1-4246-BAB9-A10CCF115406}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{DFAA25AA-72B1-4246-BAB9-A10CCF115406}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{DFAA25AA-72B1-4246-BAB9-A10CCF115406}.Release|Any CPU.Build.0 = Release|Any CPU
+		{0A934BA7-BEBB-4EF0-88A6-9A5355E6D0BB}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{0A934BA7-BEBB-4EF0-88A6-9A5355E6D0BB}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{0A934BA7-BEBB-4EF0-88A6-9A5355E6D0BB}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{0A934BA7-BEBB-4EF0-88A6-9A5355E6D0BB}.Release|Any CPU.Build.0 = Release|Any CPU
 		{66D24AFB-3541-429D-9402-72A344D99115}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
 		{66D24AFB-3541-429D-9402-72A344D99115}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{66D24AFB-3541-429D-9402-72A344D99115}.Release|Any CPU.ActiveCfg = Release|Any CPU
@@ -127,6 +133,7 @@ Global
 		{5A09E90C-6BCB-440C-AC03-5212B2AAE6C2} = {A1BDA853-65BE-4CC8-8070-CCBA22069A7A}
 		{929EDD10-7B06-4C4F-B70F-E4E51072A724} = {A1BDA853-65BE-4CC8-8070-CCBA22069A7A}
 		{DFAA25AA-72B1-4246-BAB9-A10CCF115406} = {A1BDA853-65BE-4CC8-8070-CCBA22069A7A}
+		{0A934BA7-BEBB-4EF0-88A6-9A5355E6D0BB} = {A1BDA853-65BE-4CC8-8070-CCBA22069A7A}
 		{66D24AFB-3541-429D-9402-72A344D99115} = {A1BDA853-65BE-4CC8-8070-CCBA22069A7A}
 		{A8A1C23E-7D6F-47FE-9959-B90E9CEF7B2C} = {6530DEC3-1D19-4854-80AC-2D6D02BEAECC}
 		{446D2C4A-41D6-4E4F-AC4C-6809E2416A98} = {A1BDA853-65BE-4CC8-8070-CCBA22069A7A}

examples/ClassifiersExample/ClassifiersExample.csproj

@@ -0,0 +1,14 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\Braintrust.Sdk\Braintrust.Sdk.csproj" />
+  </ItemGroup>
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net8.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+</Project>

examples/ClassifiersExample/Program.cs

@@ -0,0 +1,152 @@
+using Braintrust.Sdk.Eval;
+
+namespace Braintrust.Sdk.Examples.ClassifiersExample;
+
+// Example: Classifiers
+//
+// Classifiers categorize and label eval outputs. Unlike scorers (which return
+// numeric 0-1 values), classifiers return structured Classification items —
+// each with an Id, an optional Label, and optional Metadata.
+//
+// Results are stored as a dictionary keyed by classifier name:
+//
+//   { "sentiment": [{ id: "positive", label: "Positive" }] }
+//
+// Three patterns are shown:
+//
+//   1. Inline single-label FunctionClassifier
+//   2. Inline multi-label FunctionClassifier (returns IReadOnlyList<Classification>)
+//   3. Class-based classifier implementing IClassifier<TInput, TOutput>
+//
+// Classifiers and scorers run independently. You can use both together, or
+// use only classifiers when you don't need numeric scores.
+
+sealed class ResponseQualityClassifier : IClassifier<string, string>
+{
+    public string Name => "response_quality";
+
+    public Task<IReadOnlyList<Classification>> Classify(TaskResult<string, string> taskResult)
+    {
+        var output = taskResult.Result;
+        var wordCount = output.Split(' ', StringSplitOptions.RemoveEmptyEntries).Length;
+
+        string id;
+        if (string.IsNullOrWhiteSpace(output))
+        {
+            id = "no_response";
+        }
+        else if (wordCount < 5)
+        {
+            id = "too_short";
+        }
+        else if (output.Contains("immediately", StringComparison.OrdinalIgnoreCase)
+            || output.Contains("right away", StringComparison.OrdinalIgnoreCase)
+            || output.Contains("look into", StringComparison.OrdinalIgnoreCase))
+        {
+            id = "action_oriented";
+        }
+        else
+        {
+            id = "informational";
+        }
+
+        var label = char.ToUpperInvariant(id[0]) + id[1..].Replace('_', ' ');
+
+        IReadOnlyList<Classification> results = new[]
+        {
+            new Classification(
+                id,
+                Label: label,
+                Metadata: new Dictionary<string, object> { ["word_count"] = wordCount })
+        };
+        return Task.FromResult(results);
+    }
+}
+
+class Program
+{
+    private static readonly (string Input, string Expected)[] Messages =
+    {
+        ("Hi! I just wanted to say thank you, the product is amazing!", "praise"),
+        ("I've been waiting 2 weeks for my order. This is unacceptable!", "follow_up"),
+        ("How do I reset my password? I can't find the option anywhere.", "how_to"),
+        ("The item arrived damaged. I need a refund immediately.", "complaint"),
+        ("Just checking in — any update on my ticket #4821?", "follow_up")
+    };
+
+    static string GenerateResponse(string message)
+    {
+        if (Regex("thank").IsMatch(message))
+            return "You're welcome! So glad you're enjoying it.";
+        if (Regex("waiting|order").IsMatch(message))
+            return "I sincerely apologise for the delay. Let me look into this right away.";
+        if (Regex("password|reset").IsMatch(message))
+            return "To reset your password, go to Settings > Account > Reset Password.";
+        if (Regex("damaged|refund").IsMatch(message))
+            return "I'm sorry to hear that. I'll process your refund immediately.";
+        return "Thanks for reaching out! Let me check on that for you.";
+    }
+
+    static System.Text.RegularExpressions.Regex Regex(string pattern)
+        => new(pattern, System.Text.RegularExpressions.RegexOptions.IgnoreCase);
+
+    static async Task Main()
+    {
+        var braintrust = Braintrust.Get();
+
+        // Pattern 1: inline single-label classifier
+        var intentClassifier = new FunctionClassifier<string, string>(
+            "intent",
+            taskResult =>
+            {
+                var input = taskResult.DatasetCase.Input;
+                string id =
+                    Regex("thank").IsMatch(input) ? "praise" :
+                    Regex("waiting|order|update").IsMatch(input) ? "follow_up" :
+                    Regex("password|reset|find").IsMatch(input) ? "how_to" :
+                    Regex("damaged|refund").IsMatch(input) ? "complaint" :
+                    "other";
+
+                return new Classification(
+                    id,
+                    Label: char.ToUpperInvariant(id[0]) + id[1..].Replace('_', ' '));
+            });
+
+        // Pattern 2: inline multi-label classifier — returns a list
+        var toneClassifier = new FunctionClassifier<string, string>(
+            "tone",
+            taskResult =>
+            {
+                var input = taskResult.DatasetCase.Input;
+                var labels = new List<Classification>();
+                if (Regex("immediately|unacceptable|waiting").IsMatch(input))
+                    labels.Add(new Classification("urgent", Label: "Urgent"));
+                if (Regex("please|thank|just checking").IsMatch(input))
+                    labels.Add(new Classification("polite", Label: "Polite"));
+                if (Regex("unacceptable|damaged|waiting").IsMatch(input))
+                    labels.Add(new Classification("frustrated", Label: "Frustrated"));
+                if (labels.Count == 0)
+                    labels.Add(new Classification("neutral", Label: "Neutral"));
+                return (IReadOnlyList<Classification>)labels;
+            });
+
+        // Pattern 3: class-based classifier (see ResponseQualityClassifier above)
+        var qualityClassifier = new ResponseQualityClassifier();
+
+        var cases = Messages
+            .Select(m => DatasetCase.Of(m.Input, m.Expected))
+            .ToArray();
+
+        var eval = await braintrust
+            .EvalBuilder<string, string>()
+            .Name($"dotnet-classifiers-example-{DateTimeOffset.UtcNow.ToUnixTimeMilliseconds()}")
+            .Tags("classifiers-example", "dotnet-sdk")
+            .Cases(cases)
+            .TaskFunction(GenerateResponse)
+            .Classifiers(intentClassifier, toneClassifier, qualityClassifier)
+            .BuildAsync();
+
+        var result = await eval.RunAsync();
+        Console.WriteLine($"\n\n{result.CreateReportString()}");
+    }
+}

src/Braintrust.Sdk/Eval/Classification.cs

@@ -0,0 +1,14 @@
+namespace Braintrust.Sdk.Eval;
+
+/// <summary>
+/// A structured label produced by a classifier.
+/// </summary>
+/// <param name="Id">Stable identifier for filtering and grouping. Required.</param>
+/// <param name="Name">Grouping key in the per-case classifications dictionary. If null or empty, the runner defaults this to the classifier's resolved name.</param>
+/// <param name="Label">Optional display label. Consumers may fall back to <paramref name="Id"/> when omitted.</param>
+/// <param name="Metadata">Optional arbitrary metadata associated with this classification.</param>
+public readonly record struct Classification(
+    string Id,
+    string? Name = null,
+    string? Label = null,
+    IReadOnlyDictionary<string, object>? Metadata = null);