fix(anthropic): read real output_tokens in message_delta block

[hank9999]

Background

The Anthropic SDK only read input_tokens from message_start block now, but a lot of compatible providers only return real tokens in message_delta block.

Summary

Compare existing input_tokens with those in the message_delta block(and, existing in message_delta). If they do not match, use the data from message_delta.

Manual Verification

It will change nothing in official provider.

Checklist

• Tests have been added / updated (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)
• A patch changeset for relevant packages has been added (for bug fixes / features - run pnpm changeset in the project root)
• I have reviewed this pull request (self-review)

[vercel]

Additional Suggestion:

The message_delta.usage schema is missing input_tokens and cache_read_input_tokens fields, causing TypeScript to reject accessing these properties from the API response.

[hank9999]

Additional Suggestion:

The message_delta.usage schema is missing input_tokens and cache_read_input_tokens fields, causing TypeScript to reject accessing these properties from the API response.

It exists

    {
      "type": "message_delta",
      "delta": {
        "stop_reason": "end_turn",
        "stop_sequence": null
      },
      "usage": {
        "input_tokens": 10,
        "cache_creation_input_tokens": 0,
        "cache_read_input_tokens": 22063,
        "output_tokens": 74
      },
      "context_management": {
        "applied_edits": []
      }
    }

[aayush-kapoor]

can you add test cases with actual model response fixtures to verify this?

look at https://github.com/vercel/ai/blob/main/contributing/testing.md#manual-testing for adding streaming fixtures

[hank9999]

can you add test cases with actual model response fixtures to verify this?

look at https://github.com/vercel/ai/blob/main/contributing/testing.md#manual-testing for adding streaming fixtures

of course, i will update commit

[hank9999]

can you add test cases with actual model response fixtures to verify this?

look at https://github.com/vercel/ai/blob/main/contributing/testing.md#manual-testing for adding streaming fixtures

@aayush-kapoor Hi, I added a stream format test by using a ping pong prompt. Could you please check it? Thank you!