LangSmith API + MCP vs W&B Weave

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.

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