RelayPlane vs LangSmith
LangSmith is a SaaS debugging and observability platform for LLM apps by LangChain. RelayPlane is an MIT-licensed npm proxy that intercepts LLM requests for real-time cost control and model routing. Here is how they compare for teams who need to govern their LLM spend.
TL;DR
Choose RelayPlane when you want:
- npm install and running in 30 seconds with no account or API key setup
- Real-time cost control and model routing in the request path
- Local SQLite cost tracking with no data leaving your machine
- OpenAI-compatible drop-in with one baseURL swap, no code changes
- Claude Code and Cursor cost tracking without SDK instrumentation
LangSmith may work for you if you need:
- Full run tracing with nested spans across LangChain chains, agents, and tools
- LLM-as-judge evals, human feedback collection, and annotation queues
- Prompt hub with versioning and runtime prompt pulling
- Dataset management and regression testing for LLM pipelines
Feature Comparison
| Feature | RelayPlane | LangSmith |
|---|---|---|
| Product type RelayPlane sits in the critical path of every LLM request: it intercepts, routes, and logs each call at the network layer. LangSmith is an asynchronous observability layer that collects traces after requests complete. LangSmith does not proxy, route, or intercept requests. | npm-native LLM proxy and gateway (local-first) | SaaS observability and debugging platform for LLM applications |
| Install method RelayPlane ships as a standalone npm binary: one command and you are proxying requests. LangSmith requires SDK installation in your application plus account signup and API key configuration before you can record a single trace. | npm install -g @relayplane/proxy | pip install langsmith or npm install langsmith (SDK only, account required) |
| No account required RelayPlane starts with zero signup, zero credit card, and zero cloud dependency. LangSmith requires creating an account and configuring an API key in your environment before any traces are recorded. | ||
| Free tier RelayPlane has no request cap or free tier limit. LangSmith Developer tier gives 5,000 traces per month with a single-user limit. Beyond 5,000 traces the plan stops recording until the next billing period. | MIT open source, no usage limits, fully free to self-host | Developer: 5,000 traces/month, 1 user (account required) |
| Pricing (entry paid tier) RelayPlane has no paid cloud tier. LangSmith Plus is $39 per seat per month with 50,000 traces included. Enterprise pricing is custom. Overage traces are billed above the included amount. | MIT open source, free self-hosted | Plus: $39/month per seat, 50,000 traces/mo included |
| Model routing and fallback RelayPlane routes requests to different models based on complexity and cost, with automatic fallback on provider failures. LangSmith does not route requests. It is purely an observability layer that records what your application sends to models. | ||
| Request interception (proxy mode) RelayPlane intercepts every LLM request transparently via a baseURL swap. No code changes needed beyond pointing your client at localhost:4100. LangSmith requires SDK instrumentation or LangChain callbacks woven into your application code. | ||
| Local SQLite cost tracking RelayPlane logs every request's exact dollar cost in local SQLite with no data leaving your machine. LangSmith tracks token usage and estimated costs on their cloud platform, requiring data to be sent to LangChain's servers. | ||
| No data leaves your machine RelayPlane runs entirely on localhost by default with zero external telemetry. LangSmith is a cloud SaaS product: all traces, prompts, and evaluation data are sent to LangChain's servers. There is no officially supported self-hosted version for general use. | ||
| Spend governance and budget limits RelayPlane can enforce spend limits and route away from expensive models when budgets are exceeded. LangSmith tracks cost as an observability metric but has no mechanism to block, reroute, or cap spending on live requests. | ||
| OpenAI-compatible drop-in RelayPlane exposes an OpenAI-compatible endpoint: set OPENAI_BASE_URL=http://localhost:4100 and your existing code works unchanged. LangSmith requires SDK calls or callback instrumentation woven into your application logic. | ||
| Works with Claude Code and Cursor RelayPlane is designed for Claude Code, Cursor, Windsurf, and Aider with direct integration docs. LangSmith is an application-level SDK that does not integrate with AI coding assistants at the IDE or CLI level. | Not applicable (observability layer, not a proxy) | |
| LLM tracing and debugging LangSmith provides full run tracing: nested spans for LLM calls, chain steps, tool calls, retrieval, and custom components. Its debugging UI shows input/output at every step of a chain or agent. RelayPlane logs per-request metadata including model, tokens, cost, and latency, but does not produce nested trace trees. | Basic request logging | |
| Evals and LLM-as-judge scoring LangSmith has a built-in evaluation framework with LLM-as-judge scoring, human feedback collection, and custom eval functions. You can run evals against datasets and compare runs. RelayPlane focuses on cost control and routing rather than output quality evaluation. | ||
| Prompt management and versioning LangSmith provides a prompt hub with version control, commit history, and deploy-without-redeploy. You can pull specific prompt versions at runtime and link production traces back to specific prompt versions. RelayPlane does not manage prompts. | ||
| Datasets and regression testing LangSmith lets you build datasets from production traces and run systematic regression tests to compare prompt versions or model changes before deploying. RelayPlane does not have a dataset or experiment feature. | ||
| Open source license RelayPlane is MIT licensed with full source available. LangSmith is a proprietary SaaS product. The open-source LangChain framework is MIT, but LangSmith itself is a closed commercial platform. | MIT | Proprietary SaaS (no self-host option for general use) |
| SDK languages LangSmith is primarily built around the Python ecosystem and LangChain. Its JS SDK exists but Python has deeper integrations. RelayPlane is an HTTP proxy so any language that can make HTTP requests works without a dedicated SDK. | Node.js (proxy intercepts any language via HTTP) | Python-primary, JavaScript/TypeScript available |
Why Teams Choose RelayPlane When They Need Cost Control, Not Just Observability
A proxy intercepts requests. An observability SDK instruments code. These are different problems.
LangSmith and RelayPlane are not direct substitutes. LangSmith is an asynchronous observability layer: you add SDK calls or LangChain callbacks to your code, traces are collected after requests complete, and you get dashboards for debugging and evaluation. RelayPlane is a synchronous proxy that sits in the request path: you change one baseURL and every LLM call flows through it, enabling real-time routing, cost tracking, and spend governance. LangSmith has no ability to intercept, modify, or reroute requests.
30-second setup vs account signup, SDK integration, and API key configuration
npm install -g @relayplane/proxy and you are proxying requests in under 30 seconds. LangSmith requires creating an account, generating an API key, setting LANGCHAIN_API_KEY in your environment, and instrumenting your application code before you can record a single trace. If you are using tools like Claude Code or Cursor that you do not own the source of, LangSmith instrumentation is simply not an option.
Built-in cost control. Not just cost observation.
LangSmith is useful for showing you what you spent. It tracks token counts and estimated costs by run and project. But it cannot stop you from spending. RelayPlane tracks cost and can enforce it: budget limits, routing away from expensive models when thresholds are hit, automatic fallback on cost overruns. If you want to see your LLM spend, LangSmith provides that view. If you want to control it, you need a proxy in the request path.
Local-first with no data leaving your machine
RelayPlane runs entirely on localhost. All cost and request data is stored in a local SQLite file. Nothing is sent to any cloud service. LangSmith is a cloud-only SaaS: every trace, every prompt, and every evaluation result is sent to LangChain's servers. For teams with data residency requirements, IP-sensitive prompts, or simply a preference for keeping AI usage private, LangSmith's architecture is a hard blocker.
LangSmith Solves Debugging Problems. RelayPlane Solves Cost Control Problems.
LangSmith is a genuinely useful product for teams building production LLM applications with LangChain who need deep trace debugging, eval pipelines, and prompt management. Its run trace view, LLM-as-judge scoring, and prompt hub are strong capabilities for teams invested in the LangChain ecosystem. If your primary challenge is understanding why your chain or agent is producing wrong outputs, LangSmith is worth evaluating.
But LangSmith cannot route requests, enforce budget limits, or intercept traffic from tools you do not control. It requires SDK instrumentation in your application code and sends all data to LangChain's cloud. If you want to start tracking LLM costs in Claude Code or Cursor today without touching your codebase, without an account, and without sending logs to a third-party platform, RelayPlane installs in one command and runs on localhost.
LangSmith Pricing at a Glance
| Plan | Price | Traces/mo | Seats | Notes |
|---|---|---|---|---|
| Developer | Free | 5,000 | 1 | Account required |
| Plus | $39/seat/mo | 50,000 | Unlimited | Overage billed per trace |
| Enterprise | Custom | Custom | Unlimited | SSO, RBAC, SLA |
Get Running in 30 Seconds
No account. No API key. No code changes: