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fix(anthropic): read input_tokens from message_delta as fallback #10671

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fix(anthropic): read input_tokens from message_delta as fallback #10671

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3 changes: 3 additions & 0 deletions packages/anthropic/src/anthropic-messages-language-model.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -1485,6 +1485,9 @@
}

case 'message_delta': {
if (!usage.inputTokens) {
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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.

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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).

usage.inputTokens = value.usage.input_tokens ?? 0;

Check failure on line 1489 in packages/anthropic/src/anthropic-messages-language-model.ts

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GitHub Actions / TypeScript 

Type '{}' is not assignable to type 'number'.
}
usage.outputTokens = value.usage.output_tokens;
usage.totalTokens =
(usage.inputTokens ?? 0) + (value.usage.output_tokens ?? 0);
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