[bot] Anthropic Messages API: `usage.output_tokens_details.thinking_tokens` not captured in span metrics

[braintrust-bot]

Summary

Anthropic SDK v1.44.0 added usage.output_tokens_details to Messages API responses. This nested object contains thinking_tokens — the number of output tokens consumed by extended thinking/reasoning. The Braintrust Ruby SDK does not capture this field. Users who enable extended thinking via anthropic.messages.create(thinking: {...}, ...) or via the beta messages API have no visibility into their thinking token consumption.

This is distinct from issue #164 (RubyLLM extended thinking), which covers the ruby_llm gem. This issue covers the direct anthropic gem instrumentation.

What is missing

The Anthropic Messages API now returns:

"usage": {
  "input_tokens": 2095,
  "output_tokens": 503,
  "cache_creation_input_tokens": 2051,
  "cache_read_input_tokens": 2051,
  "output_tokens_details": {
    "thinking_tokens": 312
  }
}

output_tokens_details.thinking_tokens is the count of tokens the model generated as internal reasoning (always ≤ output_tokens). Capturing it allows users to:

• Attribute cost to extended thinking vs. standard output
• Diagnose cases where reasoning dominates total output tokens
• Compare thinking token spend across requests

Why it is dropped today

Common.parse_usage_tokens in lib/braintrust/contrib/anthropic/instrumentation/common.rb iterates over the top-level usage hash and skips any value that is not Numeric:

usage_hash.each do |key, value|
  next unless value.is_a?(Numeric)   # ← skips output_tokens_details (a Hash)
  ...
end

output_tokens_details maps to {thinking_tokens: 312}, which fails the Numeric check and is silently dropped. No field in the existing field_map covers it.

The same gap applies to both the stable Messages API instrumentation (messages.rb) and the beta Messages API instrumentation (beta_messages.rb), since both delegate to Common.parse_usage_tokens.

Braintrust docs status

not_found — The Braintrust Anthropic integration docs at https://www.braintrust.dev/docs/providers/anthropic list prompt_tokens, completion_tokens, and cache metrics as captured but do not mention thinking tokens or output_tokens_details.

Upstream sources

• Anthropic Messages API reference — usage.output_tokens_details.thinking_tokens field: https://platform.claude.com/docs/en/api/messages
• Anthropic Ruby SDK changelog — v1.44.0 added output_tokens_details and mid-conversation usage details: https://github.com/anthropics/anthropic-sdk-ruby/blob/main/CHANGELOG.md

Local files inspected

• lib/braintrust/contrib/anthropic/instrumentation/common.rb — parse_usage_tokens method (lines 14–48): 4-field field_map; Numeric guard silently drops nested objects like output_tokens_details
• lib/braintrust/contrib/anthropic/instrumentation/messages.rb — set_metrics (line 132) calls Common.parse_usage_tokens; also captures streaming output via finalize_stream_span
• lib/braintrust/contrib/anthropic/instrumentation/beta_messages.rb — same parse_usage_tokens call pattern