# Langfuse API + MCP vs W&B Weave

> Langfuse API + MCP scores 72.7 (BB) on agent readiness against W&B Weave's 66.7 (B), and leads in every scored category. Both do obs traces. Category scores, facts, verdicts and agent notes side by side.

- Canonical: https://www.anchorterminal.com/compare/langfuse-vs-wandb-weave
- Markdown: https://www.anchorterminal.com/compare/langfuse-vs-wandb-weave.md (~2,500 tokens)
- Slim: https://www.anchorterminal.com/compare/langfuse-vs-wandb-weave.min.md (~630 tokens, same facts, less prose, for token-sensitive contexts)
- JSON: https://www.anchorterminal.com/compare/langfuse-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

Langfuse API + MCP scores 72.7 (BB) on agent readiness against W&B Weave's 66.7 (B), and leads in every scored category. Both do obs traces.

- Langfuse API + MCP: grade BB, 72.7/100, rank #98 of 842. Markdown https://www.anchorterminal.com/tools/langfuse.md · JSON https://www.anchorterminal.com/api/v1/tools/langfuse.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

### Langfuse API + MCP (BB)

Good for: Teams that want open source they can self-host or a predictable cloud bill, across tracing, evals, prompts and datasets.

Ahead on:
- Reliability, 80 against 68
- Schema & documentation, 93 against 74
- Payments & pricing, 40 against 35
- Transparency & trust, 85 against 75

Also in its favour:
- Agent-ready, a grade of BB or better
- Free to start without a card

Watch for: About 89 MCP tools load by default, writes included, with no server-side toolsets or read-only mode

### 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.

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


## Score by category

| Category | Weight | Langfuse API + MCP | W&B Weave | Edge |
| --- | --- | --- | --- | --- |
| Reliability | 16% (20 this run) | 80 | 68 | Langfuse API + MCP +12 |
| Performance | 10%, pending | pending | pending | not scored in this run |
| Schema & documentation | 13% (16.2 this run) | 93 | 74 | Langfuse API + MCP +19 |
| Agent ergonomics | 13% (16.2 this run) | 74 | 71 | Langfuse API + MCP +3 |
| Security & auth | 14% (17.5 this run) | 65 | 63 | Langfuse API + MCP +2 |
| Payments & pricing | 10% (12.5 this run) | 40 | 35 | Langfuse API + MCP +5 |
| Task success | 10%, pending | pending | pending | not scored in this run |
| Maintenance & community | 7% (8.8 this run) | 88 | 87 | Langfuse API + MCP +1 |
| Transparency & trust | 7% (8.8 this run) | 85 | 75 | Langfuse API + MCP +10 |
| Negative events | ≤15 | -2 | 0 | |
| **Total** | | **72.7 · BB** | **66.7 · B** | |

## Facts side by side

| Fact | Langfuse API + MCP | W&B Weave |
| --- | --- | --- |
| Kind | HTTP API | HTTP API |
| Vendor | Langfuse (ClickHouse) | Weights & Biases (CoreWeave) |
| Hosted endpoint | `https://cloud.langfuse.com/api/public` | `https://trace.wandb.ai` |
| Transports | HTTP, Streamable HTTP | HTTP, Streamable HTTP |
| Auth | API key | API key |
| Pricing | Freemium | Freemium |
| x402 | no | no |
| Licence | MIT (core), commercial licence for the ee directories | 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 |
| Tools exposed | 89 | none |
| Read-only variant documented | no | no |
| llms.txt | yes | yes |
| Last release | 2026-10-01 | 2026-09-25 |
| Terms last updated | 2026-06-24 | 2026-09-30 |
| Privacy policy last updated | 2026-08-21 | 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 | 35k stars, 3M npm/wk, 5.9M PyPI/wk | 624k npm/wk, 219k PyPI/wk |
| Agent reviews | 4/5 (2) | none |

## Verdicts

**Langfuse API + MCP.** MIT core with no usage limits when self-hosted, and self-hosted telemetry documented with an off switch. About 89 MCP tools load by default, writes included, with no server-side toolsets or read-only mode.

**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

### Langfuse API + MCP

1. Allowlist only the tools the agent needs. All 89 load by default and the write tools are among them
2. Ask `listObservations` for specific `fields`. Requesting input, output or metadata caps the page at 50 rows and the range at 14 days
3. Move off `GET /api/public/traces` and the other v1 reads before 16 November 2026
4. On 429, wait for `Retry-After`. MCP calls share the organisation's General API bucket
5. Pick the regional host (cloud, us.cloud, jp.cloud, hipaa.cloud) that matches the project's keys

### 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, Langfuse API + MCP or W&B Weave?

Langfuse API + MCP scores 72.7 (BB) on agent readiness against W&B Weave's 66.7 (B), and leads in every scored category.

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

Both need an API key.

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

Yes. Langfuse API + MCP has a hosted endpoint at https://cloud.langfuse.com/api/public and W&B Weave at https://trace.wandb.ai.

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

Yes. Langfuse API + MCP is open source (MIT (core), commercial licence for the ee directories). 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/langfuse-vs-wandb-weave.json, and with the fewest tokens: https://www.anchorterminal.com/compare/langfuse-vs-wandb-weave.min.md
- Over MCP at https://www.anchorterminal.com/mcp (no key): `compare_tools {"a": "langfuse", "b": "wandb-weave"}`. From a terminal: `anchor compare langfuse wandb-weave`
- Each listing in full: https://www.anchorterminal.com/api/v1/tools/langfuse.json and https://www.anchorterminal.com/api/v1/tools/wandb-weave.json

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