[bot] Anthropic `Beta.Messages` (IBetaMessageService) execution calls are not instrumented

[braintrust-bot]

Summary

InstrumentedAnthropicClient instruments the regular Messages service but delegates the Beta service directly to the underlying client without any wrapping. As a result, all calls through client.Beta.Messages.Create() and client.Beta.Messages.CreateStreaming() — the endpoint used for beta-gated features such as computer use — bypass telemetry entirely and produce no spans.

What is missing

In InstrumentedAnthropicClient (src/Braintrust.Sdk.Anthropic/InstrumentedAnthropicClient.cs):

// line 90 — instrumented ✅
public IMessageService Messages => _instrumentedMessages;

// line 94 — NOT instrumented ❌
public IBetaService Beta => _client.Beta;

The IBetaService interface exposes Beta.Messages as an IBetaMessageService with the same execution methods as the regular messages service:

• Beta.Messages.Create(BetaMessageCreateParams) → /v1/messages with anthropic-beta: ... headers
• Beta.Messages.CreateStreaming(BetaMessageCreateParams) → streaming variant

These methods are used for beta-gated Anthropic features, the most prominent being computer use (anthropic-beta: computer-use-2024-10-22), which lets Claude interact with desktop GUIs. Any user calling client.Beta.Messages.Create() (or its streaming variant) via a Braintrust-instrumented client gets:

• No activity/span created
• No input content captured
• No output content captured
• No token usage metrics recorded
• No time_to_first_token measured

For comparison, the regular Messages endpoint is fully wrapped via InstrumentedMessageService.

Braintrust docs status

The Braintrust Anthropic integration docs at https://www.braintrust.dev/docs/integrations/ai-providers/anthropic explicitly list the Beta Messages API as a supported instrumentation surface:

• For TypeScript: "anthropic.beta.messages.toolRunner" spans documented
• For Python: Beta messages calls are described as auto-patched
• For .NET: Only the regular Messages API is mentioned — Beta Messages are absent

Status: supported in TypeScript and Python SDKs; not_found for the C# SDK specifically.

Upstream sources

• Anthropic C# SDK IBetaService: https://github.com/anthropics/anthropic-sdk-csharp/blob/main/src/Anthropic/Services/IBetaService.cs — defines Beta::IMessageService Messages { get; }
• Anthropic computer use docs: https://docs.anthropic.com/en/docs/build-with-claude/computer-use — primary use case for the beta messages endpoint
• NuGet package: https://www.nuget.org/packages/Anthropic (v12.5.0 used by this repo; IBetaService.Messages present in this version)

Local files inspected

• src/Braintrust.Sdk.Anthropic/InstrumentedAnthropicClient.cs — line 90: Messages is wrapped by InstrumentedMessageService; line 94: Beta is an uninstrumented pass-through to _client.Beta
• src/Braintrust.Sdk.Anthropic/InstrumentedMessageService.cs — reference implementation showing the instrumentation pattern (Create and CreateStreaming overrides) that would need to be replicated for IBetaMessageService
• src/Braintrust.Sdk.Anthropic/Braintrust.Sdk.Anthropic.csproj — confirms Anthropic v12.5.0 dependency, which includes IBetaService