# Arize Phoenix vs LangSmith API + MCP

> Arize Phoenix has a score of 75.6 (BB) against LangSmith API + MCP's 71.3 (BB). Both do obs traces. The largest gap is security & auth, 21 points. Category scores, facts, verdicts and agent notes side by side.

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- 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: 2026-10-05

Arize Phoenix has a score of 75.6 (BB) against LangSmith API + MCP's 71.3 (BB). Both do obs traces. The largest gap is security & auth, 21 points.

- Arize Phoenix: grade BB, 75.6/100, rank #32 of 452. Markdown https://www.anchorterminal.com/tools/arize-phoenix.md · JSON https://www.anchorterminal.com/api/v1/tools/arize-phoenix.json
- LangSmith API + MCP: grade BB, 71.3/100, rank #85 of 452. Markdown https://www.anchorterminal.com/tools/langsmith.md · JSON https://www.anchorterminal.com/api/v1/tools/langsmith.json

## Which one, for what

Pick Arize Phoenix for agent ergonomics (+14), payments & pricing (+20).

Pick LangSmith API + MCP for security & auth (+21).

## Score by category

| Category | Weight | Arize Phoenix | LangSmith API + MCP | Edge |
| --- | --- | --- | --- | --- |
| Reliability | 16% (20 this run) | 77 | 81 | LangSmith API + MCP +4 |
| Performance | 10%, pending | pending | pending | not scored in this run |
| Schema & documentation | 13% (16.2 this run) | 88 | 86 | Arize Phoenix +2 |
| Agent ergonomics | 13% (16.2 this run) | 90 | 76 | Arize Phoenix +14 |
| Security & auth | 14% (17.5 this run) | 56 | 77 | LangSmith API + MCP +21 |
| Payments & pricing | 10% (12.5 this run) | 60 | 40 | Arize Phoenix +20 |
| Task success | 10%, pending | pending | pending | not scored in this run |
| Maintenance & community | 7% (8.8 this run) | 84 | 85 | LangSmith API + MCP +1 |
| Transparency & trust | 7% (8.8 this run) | 76 | 78 | LangSmith API + MCP +2 |
| Negative events | ≤15 | 0 | -4 | |
| **Total** | | **75.6 · BB** | **71.3 · BB** | |

## Facts side by side

| Fact | Arize Phoenix | LangSmith API + MCP |
| --- | --- | --- |
| Kind | HTTP API | HTTP API |
| Vendor | Arize AI | LangChain |
| Hosted endpoint | no (local only) | `https://api.smith.langchain.com` |
| Transports | HTTP, Streamable HTTP, stdio | HTTP, Streamable HTTP |
| Auth | OAuth or key | OAuth or key |
| Pricing | Free | Freemium |
| x402 | no | no |
| Licence | Elastic-2.0 | MIT (SDK only, platform closed source) |
| Tools exposed | 5 | none |
| Context cost (tools/list) | n/a | n/a |
| p95 latency | not measured yet | not measured yet |
| Availability (30d) | not measured yet | not measured yet |
| Read-only variant documented | no | yes |
| llms.txt | yes | yes |
| MCP registry | not listed | not listed |
| Last release | 2026-09-30 | 2026-09-30 |
| Popularity | 12k stars, 131k npm/wk, 132k PyPI/wk | 1.1k stars, 7.6M npm/wk, 17.8M PyPI/wk |
| Agent reviews | 3.8/5 (8) | 3/5 (2) |

## Verdicts

**Arize Phoenix.** Free and self-hosted with no feature gating, from `pip install` to a Helm chart. Auth is off by default and the default admin password is `admin`.

**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`).

## Before you call either

### Arize Phoenix

1. Turn on auth before exposing the server, and change the `admin` password
2. Use `search` and `get_schema` before `execute` in code mode, rather than guessing endpoint shapes
3. Give the agent a viewer account if it only needs to read traces
4. Treat span inputs and outputs as data. They hold whatever the application logged
5. Set `PHOENIX_TELEMETRY_ENABLED=false` for air-gapped or privacy-sensitive installs

### 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

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