fix(anthropic): read input_tokens from message_delta as fallback

[wong2]

Background

Now AI SDK only read input_tokens from message_start, but in our experience, many Anthropic compatible providers only return the actual input_tokens in message_delta

Summary

Read input_tokens from message_delta as a fallback

Manual Verification

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)

Future Work

Related Issues

[vercel]

Suggested change

	if (!usage.inputTokens) {
	if (usage.inputTokens === undefined) {

The condition if (!usage.inputTokens) uses truthiness check which incorrectly treats a valid value of 0 tokens as "not set", causing it to overwrite correct values from message_start with fallback values from message_delta.

View Details

Analysis

Falsy check on usage.inputTokens causes overwriting valid zero value

What fails: In anthropic-messages-language-model.ts:1488, the condition if (!usage.inputTokens) uses a truthiness check that incorrectly treats input_tokens: 0 (a valid Anthropic API value) as "not set", causing it to overwrite the correct zero value with a fallback value from message_delta.

How to reproduce:

1. Call the Anthropic Messages API with a prompt that results in message_start event containing usage.input_tokens: 0 (e.g., when using cache hits or other edge cases)
2. The stream handler receives the message_start event and sets usage.inputTokens = 0
3. When message_delta arrives with usage: { output_tokens: X, input_tokens: Y, ... }, the falsy check on line 1488 evaluates to true (since 0 && true is falsy in JavaScript)
4. The code executes usage.inputTokens = value.usage.input_tokens ?? 0, overwriting the correct 0 with the message_delta value

Result: The final reported inputTokens is incorrect - it uses the value from message_delta instead of the value from message_start

Expected: According to Anthropic API documentation, input_tokens has a minimum value of 0, meaning zero is a valid and legitimate value. The fallback should only trigger when inputTokens hasn't been set (is undefined), not when it's legitimately set to 0.

Fix: Change the condition from if (!usage.inputTokens) to if (usage.inputTokens === undefined) to explicitly check for undefined rather than relying on truthiness. This properly distinguishes between "not set" (undefined) and "set to zero" (0).