fix: double counting anthropic langchain #2838

carlos-marchal-ph · 2026-01-02T17:15:38Z

We were counting cache write tokens incorrectly when using Anthropic with Langchain.

Furthermore, we were sending read tokens under the wrong property name.

vercel · 2026-01-02T17:15:42Z

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greptile-apps · 2026-01-02T17:17:22Z

Problem Analysis

The issue stems from how Anthropic's API reports token usage compared to other LLM providers. When using prompt caching with Anthropic via LangChain:

Cache read tokens were being sent under an incorrect property name ($ai_cache_read_tokens vs the expected $ai_cache_read_input_tokens)
Cache creation/write tokens were causing double-counting because LangChain includes them in the total input token count for Anthropic, requiring subtraction to get accurate uncached token counts

Changes Made

In packages/ai/src/langchain/callbacks.ts:

Fixed property names for cache token fields to use the correct schema ($ai_cache_read_input_tokens and $ai_cache_creation_input_tokens)
Added parsing logic to extract cache creation tokens from Anthropic responses in multiple formats (usage.cache_creation_input_tokens and usage.input_token_details.cache_creation)
Enhanced the Anthropic-specific token adjustment logic to subtract BOTH cache read and cache write tokens from input counts to prevent double-counting

In packages/ai/tests/callbacks.test.ts:

Updated existing test assertions to use the corrected property names
Added comprehensive test coverage for cache creation token scenarios, including:
- Anthropic provider with cache creation token subtraction
- Combined cache read and creation token handling
- OpenAI provider behavior (which should NOT subtract cache creation tokens)

Technical Implementation

The fix correctly handles the difference in how Anthropic vs other providers report cached tokens:

Anthropic: LangChain includes cache tokens in total input count, so they must be subtracted
OpenAI: Cache tokens are reported separately and should not be subtracted

The token adjustment logic now calculates: actualInputTokens = reportedInputTokens - (cacheReadTokens + cacheWriteTokens) for Anthropic providers only.

Impact and Risks

Low Risk Changes:

Property name corrections align with existing schema expectations
New test coverage validates the fix thoroughly
Changes are isolated to the AI package's LangChain integration

Potential Considerations:

Ensure downstream consumers of these events expect the corrected property names
The Math.max() protection prevents negative token counts if cache tokens exceed reported input tokens

This is a targeted bug fix with comprehensive test coverage that resolves token accounting accuracy for Anthropic's caching features.

github-actions · 2026-01-02T17:20:20Z

Download bundle size visualization

github-actions · 2026-01-02T17:21:17Z

Size Change: +3.72 kB (+0.07%)

Total Size: 5.4 MB

Filename	Size	Change
`packages/ai/dist/index.cjs`	143 kB	+930 B (+0.65%)
`packages/ai/dist/index.mjs`	142 kB	+930 B (+0.66%)
`packages/ai/dist/langchain/index.cjs`	42.1 kB	+931 B (+2.26%)
`packages/ai/dist/langchain/index.mjs`	41.6 kB	+931 B (+2.29%)

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_{compressed-size-action}

Radu-Raicea · 2026-01-05T20:20:27Z

@carlos-marchal-ph Is this bug only for the Node SDK?

carlos-marchal-ph · 2026-01-07T13:15:00Z

@carlos-marchal-ph Is this bug only for the Node SDK?

It was also present in the Python one, there was another PR for that.

carlos-marchal-ph requested a review from a team January 2, 2026 17:15

carlos-marchal-ph self-assigned this Jan 2, 2026

carlos-marchal-ph added bug Something isn't working release team/llm-analytics LLM Analytics labels Jan 2, 2026

vercel bot deployed to Preview – posthog-nextjs-config January 2, 2026 17:18 View deployment

vercel bot deployed to Preview – posthog-js January 2, 2026 17:18 View deployment

carlos-marchal-ph removed the bug Something isn't working label Jan 5, 2026

fix: double counting anthropic langchain

1db37f7

carlos-marchal-ph force-pushed the fix(llma)/double-counting-anthropic-langchain branch from 80d0fc3 to 1db37f7 Compare January 5, 2026 17:39

vercel bot deployed to Preview – posthog-nextjs-config January 5, 2026 17:44 View deployment

vercel bot deployed to Preview – posthog-js January 5, 2026 17:45 View deployment

Radu-Raicea approved these changes Jan 5, 2026

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andrewm4894 merged commit c64a15d into main Jan 7, 2026
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andrewm4894 deleted the fix(llma)/double-counting-anthropic-langchain branch January 7, 2026 11:16

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fix: double counting anthropic langchain #2838

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fix: double counting anthropic langchain #2838

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