Is LangSmith API Free? Keys, Limits and Costs

Is LangSmith API free? Yes, in the sense that matters most: creating a LangSmith account and an API key costs nothing, and calling the API is not billed per request. What you pay for is usage the API records, mainly traces. The free Developer plan includes 5,000 base traces a month for one user, and without a credit card on file LangSmith simply stops accepting traces at that cap instead of billing you.

So for learning, side projects and small prototypes, the LangSmith API is free in practice. It stops being free when you add a card and go past the allowance, when you need a second seat, or when you turn on products such as Deployment, Fleet or Engine that are metered separately. This guide covers how to get a key, which key type to use, what the free plan limits look like at the API level, and the settings that quietly raise a bill. Prices and limits are taken from LangChain's pricing page and documentation as of October 25, 2026.

Is the LangSmith API key free?

Yes. The account and API key docs say you can sign up for a free account with Google, GitHub or email, and then create keys from the Settings page. There is no charge for a key, no charge for the number of keys, and no separate API subscription. The same key works with the Python and TypeScript SDKs and with the REST API.

The LangSmith SDK itself is open source under the MIT license, so installing langsmith from PyPI or npm is free too. The LangSmith platform it talks to is a proprietary hosted service, which is where plan limits apply. If you want an open source backend you can run yourself, our guide on whether Langfuse is free for commercial use covers that route.

How to get a LangSmith API key

1. Sign up or log in at smith.langchain.com. A new personal account starts on the Developer plan.
2. Open Settings, then the API Keys section.
3. Pick the key type (see the next section). For a service key, choose organization scope or specific workspaces.
4. Set an expiration in days, or choose never. Keys never expire by default.
5. Click Create API Key and copy it right away. LangSmith shows the key only once.

Store the key in a secrets manager or an environment file that is not committed to Git. If you lose it, create a new one; you cannot view an existing key again.

The three LangSmith API key types

LangSmith now issues three kinds of keys, and the prefix tells you which one you are holding (administration overview).

The tracing key is the newest option and the safest one to ship inside an app. It can submit traces but cannot read stored traces, run experiments or manage keys, so a leaked tracing key exposes much less. For evaluations, dataset work and other read operations, use a PAT or a service key.

Two details catch people out. First, old keys that start with ls__ stopped working on October 22, 2024, so tutorials that show that prefix are out of date. Second, a PAT dies with its owner: if that user leaves the organization, the token stops working. That is why production services should use a service key or a tracing key.

How to use a LangSmith API key

For the SDKs, set two environment variables and your traced code starts sending data:

pip install langsmith
export LANGSMITH_API_KEY=<your-api-key>
export LANGSMITH_TRACING=true

The TypeScript SDK installs with npm install langsmith and reads the same variables. Set LANGSMITH_ENDPOINT only if you are not on the default US region: the docs list separate API hosts for the EU, APAC and an AWS US region. If one key covers more than one workspace, also set LANGSMITH_WORKSPACE_ID.

For the REST API, send the key in an X-Api-Key header (API reference):

curl --request GET \
  --url https://api.smith.langchain.com/api/v1/workspaces \
  --header 'X-Api-Key: <your-api-key>'

Organization scoped service keys also need an X-Tenant-Id header to reach workspace resources; without it, those requests fail with a 403 error. The REST API covers tracing, datasets, experiments, annotations and nearly every admin action in the UI.

One more point that confuses beginners: a LangSmith key is not a model key. It authenticates you to LangSmith for tracing and evaluation. Your app still calls OpenAI, Anthropic or another provider with that provider's own key and pays that provider for tokens. The exception is LangSmith Fleet, where the pricing page says model costs are included in its metered usage.

What the free Developer plan includes

The LangSmith pricing page lists the Developer plan at 0 dollars for one seat, with up to 5,000 base traces a month and community support. A trace is one run of your app (one agent call, evaluator run or playground session), and it can contain many steps such as model calls and tool calls. So 5,000 traces is 5,000 requests through your app, not 5,000 model calls.

Base traces are kept for 14 days. Extended traces are kept for 180 days. After retention ends, traces disappear from the UI and the API, though the monitoring charts keep working from trace metadata. Datasets are kept indefinitely, so copy any examples you want to keep into a dataset.

The free plan also includes 7 LangChain Standard Units (LSUs) a month for Fleet and 8 LSUs of sandbox usage. One LSU costs 1 dollar. On the Developer plan, Fleet pauses new runs once its allowance is used until the next month. LangSmith Deployment, with one free small serverless deployment, is a Plus plan feature.

LangSmith API rate limits on the free plan

Here is where "free" has edges. The usage and billing docs describe several limits that return HTTP 429 errors.

An event is the creation or update of a run, so a run created and then updated in the same hour counts twice. On top of the plan limits, every key faces per minute limits at the load balancer on all plans: 5,000 POST or PATCH calls a minute to run endpoints, 5,000 to feedback endpoints, only 30 GET calls a minute for a single run by ID, and 2,000 a minute for everything else. Each trace can hold at most 25,000 runs.

The SDK batches up to 100 runs from one session into a single call, so normal tracing rarely hits the per minute cap. Scripts that fetch runs one at a time hit the 30 a minute limit quickly. The docs recommend the v2 run query endpoint (list_runs in Python) and setting a start_time window of 7 days or less when you query.

When the LangSmith API stops being free

You add a card and pass the allowance

Adding a card to a Developer account removes the 5,000 trace cap, and every extra trace is billed. The pricing page tooltip lists 0.005 LSU per base trace and 0.0025 LSU per extended retention upgrade. At 1 dollar per LSU, that is 5 dollars per 1,000 extra base traces. For example, 20,000 base traces in a month on the Developer plan would mean 15,000 billable traces, or about 75 dollars, before any upgrades. Check the pricing page before you budget, since rates have changed before.

Traces get upgraded to extended retention

Some features move a trace from base to extended retention, which costs more. Online evaluators and automation rules do this by default when they match a trace, and a match on one run upgrades the whole trace. Feedback sent through the API upgrades a trace only if you pass extend_trace_retention=true. Runs created by experiments use extended retention by default. Each new online evaluator and automation rule has a setting to opt out of the upgrade.

You need a second person or a team workspace

The Developer plan has one seat. Plus costs 39 dollars per seat per month with 10,000 base traces included, and new organizations you create by hand must be on Plus. LangChain also runs a startup program with discounted rates and up to 10,000 dollars in credits for venture backed startups.

You use metered products

Deployment compute, sandboxes beyond the free allowance, Fleet beyond 7 LSUs, the Tuned Evaluators beta and Engine all bill in LSUs. The pricing page estimates one Engine run at roughly 7 to 45 LSUs, and Engine runs every six hours once enabled, so it is the fastest way to turn a free account into a real bill.

How to keep the LangSmith API free

• Do not add a card while you are experimenting. The 5,000 trace cap is then a hard ceiling, and you will see 429 errors instead of an invoice.
• Set usage limits under Settings, Billing and Usage, Usage limits, if you do add a card. You can cap total traces and extended traces per month. The docs warn the limit is approximate and can take a minute or two to apply.
• Sample traces in high volume apps and keep extended retention for errored traces or a small share of traffic through an automation rule.
• Turn off the retention upgrade on online evaluators you run on every trace.
• Use a tracing key in deployed apps, so a leak cannot read your data.

If those limits do not suit you, open source tracers such as Langfuse, Opik and Arize Phoenix can be self hosted with no trace caps. Our Langfuse vs LangSmith comparison covers plans, retention and framework fit side by side, and the best LLM observability tools roundup covers the wider field, including Helicone. For the LangSmith listing itself, see its tool page. Once tracing works, our guide on how to evaluate LLM applications shows what to do with the data.

FAQ

Is the LangSmith API key free?

Yes. Any LangSmith account, including the free Developer plan, can create personal access tokens and service keys at no cost. You pay only for traces and metered products above your plan's allowance.

Do I need a credit card to use the LangSmith API?

No. A Developer account works without a card. Without one, you are limited to 5,000 traces a month, 50,000 trace events an hour and 500 MB of trace data an hour, and LangSmith returns 429 errors rather than charging you.

Why does my LangSmith API key not work?

Common causes are an old key starting with ls__, which stopped working in October 2024; a PAT whose owner left the organization; an expired or deleted key; a missing LANGSMITH_ENDPOINT for a non US region; or a missing X-Tenant-Id header when using an organization scoped service key. Deleted or deactivated keys stop working within about a minute because authentication is cached.

Can I use the LangSmith API without LangChain?

Yes. The SDK traces any Python or TypeScript code, and the REST API works from any language that can send HTTP requests with an X-Api-Key header. LangChain and LangGraph just add automatic tracing.

Is LangSmith free for commercial use?

The pricing page describes the Developer plan as a fit for personal projects and solo users, so read LangChain's terms of service before you run a business on it. In practice a team product soon needs more seats, more traces or Plus features, so budget for Plus at 39 dollars per seat per month.

Does the LangSmith API charge per request?

No. Requests are not billed. Billing counts traces (base and extended upgrades), seats and LSU metered products. Requests are only limited by the per minute and per hour rate limits.