# How to track LLM cost in LangChain

> Culpa's LangChain adapter is a callback handler that reports the framework's own token counts. An honest floor, and how to measure what it misses.

URL: https://getculpa.com/track-llm-cost-langchain
Last reviewed: 2026-08-02

## Answer
Culpa's LangChain adapter is a callback handler you attach to your calls, and it reports the token counts LangChain itself surfaces. That's an honest floor rather than a ceiling, because the adapter sees what the framework reports and not the raw provider response. Culpa, a local-first LLM cost, margin, and forecast ledger, prices what arrives and reconciles it against the invoice so the floor is measurable.

## Why this happens
A framework sits between your code and the provider, and it decides what to pass on. Anything it summarises or drops is invisible to anything downstream of it, including a cost adapter. The fields most at risk are the ones that change the price rather than the count, and cached input is the clearest example: if a framework reports prompt tokens as one number, a ledger has no way to know which of them were cached reads at a tenth of the rate. The result is a ledger that's internally consistent and reads above the invoice.

## What this usually looks like
- Your captured spend runs above the provider invoice rather than below it.
- Your prompts use a large cached prefix and nothing in your ledger shows cached tokens.
- The gap is proportional to your cached share rather than a fixed amount.
- Per-call token counts look right while per-call cost looks high.

## Common mistakes
- Assuming a framework passes through every usage field the provider returned. Why it hurts: The fields that decide price, cached reads especially, are the ones most often collapsed. Do instead: Reconcile against the invoice in the first week and treat any gap as a fidelity measurement.
- Reading a ledger that sits above the invoice as an accounting error. Why it hurts: It's usually a capture-fidelity gap, and looking in the accounting will find nothing. Do instead: Compare the shape of the gap against your cached share. A proportional gap names its own cause.
- Running only one capture path and trusting it because it's consistent. Why it hurts: Consistency is what a floor looks like from the inside. Do instead: Run the request path or the in-process SDK alongside it for a week and compare the two.

## Self-check
- Total your captured spend for a full month and put it beside the provider invoice.
- Work out what share of your input is a repeated cached prefix.
- Check whether any cached-token figure reaches your ledger at all.
- Divide the gap by your cached volume. If it lands near the cached discount, you've found the cause.
- Run a second capture path for a week and compare the two ledgers on the same traffic.

## What a collapsed cached-token field does to a ledger (illustrative)
A feature on Claude Opus 4.8 at real rates of $5.00 per million input, $25.00 output and $0.50 cached read. It sends 40 million input tokens a month of which 30 million is a cached prefix, plus 6 million output. The cached share is modelled.
  True cost: (30 x $0.50) + (10 x $5.00) + (6 x $25.00) = $15 + $50 + $150 = $215
  If all 40M input is reported as one uncached number: (40 x $5.00) + (6 x $25.00) = $200 + $150 = $350
  The ledger reads $350 against a $215 invoice, a 63% over-statement
  The gap is 30M x ($5.00 - $0.50) = $135, exactly the cached discount not applied
  Divide the gap by the cached volume and you recover the missing rate: $135 / 30 = $4.50
The count was right and the price was wrong, and the arithmetic points straight at the missing field. A gap that divides cleanly into a rate difference is a fidelity problem, not an accounting one.

## Cost figures
Every figure carries its confidence and its source. No figure on this site is provider-reported.
- $215 to $350 — modelled monthly cost of one workload, priced with the cached split and priced without it [estimated] Source: Both endpoints from the teardown arithmetic at real Claude Opus 4.8 rates from the price book, effective 2026-07-02. A range because the cached share is modelled.
- 63% — modelled over-statement when a cached-token field never reaches the ledger [calculated] Source: $350 divided by $215, from the teardown arithmetic.

## FAQ
Q: How accurate is LangChain cost tracking?
A: As accurate as what LangChain reports, which is a floor rather than a ceiling. The adapter builds an openai-shaped body from the framework's own token counts, so any usage field the framework collapses is a field no downstream ledger can recover.

Q: Will the adapter break my chain if Culpa is unavailable?
A: No. Every hook swallows its own failures by design, so a Culpa problem can't propagate into your chain. That's the same fail-open contract the rest of the capture surface follows.

Q: How do I get full-fidelity capture instead?
A: Capture on the request path by pointing your base URL at Culpa, or use the in-process SDK, which reports the raw provider response. Both see the usage fields the provider actually returned rather than the framework's summary of them.

## Sources
- Anthropic pricing: https://platform.claude.com/docs/en/docs/about-claude/pricing

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