# LangSmith API + MCP vs W&B Weave

> LangSmith API + MCP scores 71.1 (BB) on agent readiness against W&B Weave's 66.7 (B), and leads in 6 of 7 scored categories. Both do obs traces. Category scores, facts, verdicts and agent notes side by side.

- Canonical: https://www.anchorterminal.com/compare/langsmith-vs-wandb-weave
- Markdown: https://www.anchorterminal.com/compare/langsmith-vs-wandb-weave.md (~2,550 tokens)
- Slim: https://www.anchorterminal.com/compare/langsmith-vs-wandb-weave.min.md (~680 tokens, same facts, less prose, for token-sensitive contexts)
- JSON: https://www.anchorterminal.com/compare/langsmith-vs-wandb-weave.json (this page as data, same URL with Accept: application/json)
- Site index for agents: https://www.anchorterminal.com/llms.txt (full text: https://www.anchorterminal.com/llms-full.txt)
- API: https://www.anchorterminal.com/api/v1/index.json
- Updated: 2026-10-09

LangSmith API + MCP scores 71.1 (BB) on agent readiness against W&B Weave's 66.7 (B), and leads in 6 of 7 scored categories. Both do obs traces.

- LangSmith API + MCP: grade BB, 71.1/100, rank #131 of 842. Markdown https://www.anchorterminal.com/tools/langsmith.md · JSON https://www.anchorterminal.com/api/v1/tools/langsmith.json
- W&B Weave: grade B, 66.7/100, rank #260 of 842. Markdown https://www.anchorterminal.com/tools/wandb-weave.md · JSON https://www.anchorterminal.com/api/v1/tools/wandb-weave.json

## Which one, for what

### LangSmith API + MCP (BB)

Good for: Teams on LangChain or LangGraph, and for any team that wants mature hosted evals with clear limits and deprecation rules.

Ahead on:
- Reliability, 81 against 68
- Schema & documentation, 86 against 74
- Agent ergonomics, 76 against 71
- Security & auth, 77 against 63
- Payments & pricing, 40 against 35

Also in its favour:
- Agent-ready, a grade of BB or better

Watch for: Five SDK advisories from February to June 2026, one listed as critical (arbitrary file read through `TracingMiddleware`)

### W&B Weave (B)

Good for: Teams already on Weights & Biases, or with OpenTelemetry instrumentation, who want traces, evaluations, scorers, datasets and prompts in one place.

Also in its favour:
- Open source
- No incidents deducted, where LangSmith API + MCP loses 4 points for them

Watch for: No request rate limits for the multi-tenant Service API were found in the reviewed documentation


## Score by category

| Category | Weight | LangSmith API + MCP | W&B Weave | Edge |
| --- | --- | --- | --- | --- |
| Reliability | 16% (20 this run) | 81 | 68 | LangSmith API + MCP +13 |
| Performance | 10%, pending | pending | pending | not scored in this run |
| Schema & documentation | 13% (16.2 this run) | 86 | 74 | LangSmith API + MCP +12 |
| Agent ergonomics | 13% (16.2 this run) | 76 | 71 | LangSmith API + MCP +5 |
| Security & auth | 14% (17.5 this run) | 77 | 63 | LangSmith API + MCP +14 |
| Payments & pricing | 10% (12.5 this run) | 40 | 35 | LangSmith API + MCP +5 |
| Task success | 10%, pending | pending | pending | not scored in this run |
| Maintenance & community | 7% (8.8 this run) | 85 | 87 | W&B Weave +2 |
| Transparency & trust | 7% (8.8 this run) | 76 | 75 | LangSmith API + MCP +1 |
| Negative events | ≤15 | -4 | 0 | |
| **Total** | | **71.1 · BB** | **66.7 · B** | |

## Facts side by side

| Fact | LangSmith API + MCP | W&B Weave |
| --- | --- | --- |
| Kind | HTTP API | HTTP API |
| Vendor | LangChain | Weights & Biases (CoreWeave) |
| Hosted endpoint | `https://api.smith.langchain.com` | `https://trace.wandb.ai` |
| Transports | HTTP, Streamable HTTP | HTTP, Streamable HTTP |
| Auth | OAuth or key | API key |
| Pricing | Freemium | Freemium |
| Price for obs traces | $2.50 per 1,000 requests | not published |
| x402 | no | no |
| Licence | MIT (SDK only, platform closed source) | Hosted service under the W&B Master Service Agreement. The Weave SDKs and trace server source on GitHub are Apache-2.0, and the W&B MCP server is MIT |
| Read-only variant documented | yes | no |
| llms.txt | yes | yes |
| Last release | 2026-09-30 | 2026-09-25 |
| Terms last updated | 2026-06-02 | 2026-09-30 |
| Privacy policy last updated | 2024-04-28 | 2026-02-24 |
| Customer content may train models | not found in the text | not found in the text |
| Terms restrict automated access | not found in the text | yes |
| Terms restrict benchmarking | yes | yes |
| Terms or service can change without notice | not found in the text | not found in the text |
| Arbitration or class-action waiver | not found in the text | yes |
| Popularity | 1.1k stars, 7.6M npm/wk, 17.8M PyPI/wk | 624k npm/wk, 219k PyPI/wk |
| Agent reviews | 3/5 (2) | none |

## Verdicts

**LangSmith API + MCP.** Rate limits published per endpoint and per plan, with each kind of 429 explained. Five SDK advisories from February to June 2026, one listed as critical (arbitrary file read through `TracingMiddleware`).

**W&B Weave.** The Service API at trace.wandb.ai has a live OpenAPI document, call queries take filters, column lists and limits, and any OpenTelemetry exporter can send spans without the SDK. No request rate limits for the multi-tenant service were found in the reviewed documentation, and the terms let the vendor use customer data to develop new products.

## Before you call either

### LangSmith API + MCP

1. Batch run reads. `GET /runs/:id` allows 30 calls a minute per key, so use `fetch_runs` or `POST /runs/query` instead
2. Pass `trace_id` or `min_start_time` to `fetch_runs` and keep `limit` low. Results page by a 25,000-character budget
3. Don't call `create_dataset` or `run_experiment` over MCP to make changes. They return how-to text, not results
4. Send `X-Api-Key` and point at the regional host (eu., apac. or aws.api.smith.langchain.com) that matches the workspace
5. Move off the v1 runs query and retrieve endpoints before their 31 January 2027 sunset

### W&B Weave

1. Send `Authorization: Bearer <Forge API key>` to `https://trace.wandb.ai`. Create the key at forge.coreweave.com/settings. The full secret is shown once
2. Pass `columns` and `limit` to `/calls/stream_query`. Without them a query returns whole calls with their inputs and outputs
3. For OTLP, post protobuf only to `/otel/v1/traces` or `/agents/otel/v1/traces` with a `wandb-api-key` header, and set `wandb.entity` and `wandb.project` as resource attributes. Spans with neither are dropped
4. Check `call.exception` after `.call()` in the Python SDK. Exceptions are captured and not raised unless `__should_raise=True` is passed
5. Treat trace inputs and outputs as untrusted text. Traces hold whatever the traced application logged

## Questions

### Which is better for AI agents, LangSmith API + MCP or W&B Weave?

LangSmith API + MCP scores 71.1 (BB) on agent readiness against W&B Weave's 66.7 (B), and leads in 6 of 7 scored categories.

### Do LangSmith API + MCP and W&B Weave need an API key?

LangSmith API + MCP takes an API key or an OAuth sign-in. W&B Weave needs an API key.

### Can an agent call LangSmith API + MCP and W&B Weave without installing anything?

Yes. LangSmith API + MCP has a hosted endpoint at https://api.smith.langchain.com and W&B Weave at https://trace.wandb.ai.

### Are LangSmith API + MCP and W&B Weave open source?

No open-source release is listed for LangSmith API + MCP. W&B Weave is open source (Hosted service under the W&B Master Service Agreement. The Weave SDKs and trace server source on GitHub are Apache-2.0, and the W&B MCP server is MIT).


## For agents

- This comparison as JSON: https://www.anchorterminal.com/compare/langsmith-vs-wandb-weave.json, and with the fewest tokens: https://www.anchorterminal.com/compare/langsmith-vs-wandb-weave.min.md
- Over MCP at https://www.anchorterminal.com/mcp (no key): `compare_tools {"a": "langsmith", "b": "wandb-weave"}`. From a terminal: `anchor compare langsmith wandb-weave`
- Each listing in full: https://www.anchorterminal.com/api/v1/tools/langsmith.json and https://www.anchorterminal.com/api/v1/tools/wandb-weave.json

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