fix(ai-anthropic): populate cachedInputTokens in streaming responses

[davidgoli]

Summary

The Anthropic streaming response builder (makeStreamResponse) populates inputTokens, outputTokens, and totalTokens, but never copies cache_read_input_tokens into the normalized usage. As a result usage.cachedInputTokens is always undefined for streamText / streamObject, even when prompt caching is active and the API reports a non-zero cache read at message_start. The non-streaming path already maps this field, so streaming and non-streaming disagree.

Fix

Set cachedInputTokens from cache_read_input_tokens at message_start — where input_tokens is already read — mirroring the non-streaming mapping. The output payload shape is unchanged; only the previously-missing field is now populated.

case "message_start": {
  usage.inputTokens = event.message.usage.input_tokens
  usage.cachedInputTokens = event.message.usage.cache_read_input_tokens ?? undefined
  metaUsage = event.message.usage
  // ...
}

event.message.usage is Generated.BetaUsage, which already declares cache_read_input_tokens, so this matches the existing non-streaming mapping (?? undefined).

Notes

I didn't add a streaming test — packages/ai/anthropic has no test harness for the language model adapter (no test/ dir, and the sibling ai-openai package has none either), so there's no pattern yet for mocking the Anthropic SSE stream. Happy to follow up with a focused stream test once the maintainers point at the preferred mocking shape.

Cache writes (cache_creation_input_tokens) are out of scope — @effect/ai's core Usage type has no corresponding field.

A changeset is included (@effect/ai-anthropic, patch).

[changeset-bot]

🦋 Changeset detected

Latest commit: 512af66

The changes in this PR will be included in the next version bump.

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Name	Type
@effect/ai-anthropic	Patch

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