LangSmith

What LangSmith actually meters

One trace is one execution of your application, and LangChain defines it generously. Every LLM call, tool call and retrieval step inside a single agent invocation rolls up into one billable trace, capped at 25,000 runs before LangSmith rejects the rest. A 200-step agent loop costs the same $0.005 as a one-shot completion, which favours anyone running deep agents.

The second meter is time. Base traces live 14 days, extended traces 180, and the upgrade costs 0.0025 LSU on top of the base charge. Your invoice carries two lines, all traces and extended retention upgrades, because a trace recorded in June can be upgraded in July.

You don't choose every upgrade. An online evaluator that scores a trace upgrades it. An automation rule matching a single run upgrades the entire trace, and a thread-level rule upgrades every trace in the thread. Retention extension is on by default for both. UI feedback, notes and annotation queue adds leave the tier alone, and experiments start at extended retention.

Everything else rides other units. Deployments, Sandboxes, Engine and Fleet meter in LangChain Compute Units at $1.50 each, storage in LangChain Storage Units at $1.00. Engine burns roughly 5 to 30 LCUs per run, so $7.50 to $45, every six hours unless an admin disables it.

How credits work

Gateway Credits are a separate prepaid balance buying model calls, not traces.

Route model traffic through the LLM Gateway without your own provider key and LangChain bills the tokens at face value, charging the spend as LCUs against a balance you buy upfront. The purchase carries a processing fee LangChain never publishes a rate for, and the minimum top-up is $4.50, which is three LCUs. A gateway request still costs a trace, with no extra LangChain fee on top.

Expiry, rollover and pooling rules are not documented on the pricing page or in the billing docs. Enterprise commitments differ: LCU usage draws down prepaid LangChain credits on a Credit usage tab. VC-backed startups can apply for up to $10,000 in credits.

What happens when you hit the limit

It depends which limit, and LangSmith has three kinds.

Exceed your included traces and nothing stops. LangSmith auto-charges and bills in arrears. There's no spend cap, only usage limits you set yourself, which LangChain warns are approximate.

Exceed a self-set usage limit and you get a 429. The extended-retention limit cuts deeper: once reached, LangSmith blocks anything that would create another long-lived trace, so automation rules and evaluators with retention extension enabled stop running.

Exceed the plan rate limits and you get a 429 too. Developer accounts without a card cap at 50,000 trace events an hour and 5,000 traces a month. A card lifts that to 250,000. Plus gets 500,000.