Braintrust API + MCP vs LangSmith API + MCP

Braintrust API + MCP

OpenAPI 3.0.3 spec with 75 paths, 429 and Retry-After declared on 154 operations. Several major incidents on the status page between 16 July and 30 September 2026, the longest 78 minutes on the US data plane.

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).

Braintrust API + MCP

1. Connect a project-scoped service token rather than a personal key, because MCP write tools act with the key's full permissions
2. Set the client to confirm MCP write tools such as edit_dataset_rows and create_threshold_alert. The server has no read-only mode
3. Cache sql_query results. Starter and Pro allow about 20 queries a minute and return 429
4. Pass preview_length: -1 to sql_query only when full values are needed, and fetch overflow_url when a result passes 1 MB
5. Upgrade to Python braintrust 0.28.0 or TypeScript 3.23.1 or later and rotate any provider keys traced before July 2026

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