Implement SEP-414: Document OpenTelemetry Trace Context Propagation Conventions

[felixweinberger]

This is a tracking issue for implementation of SEP-414.

[mattzcarey]

Status — SEP-414 (OpenTelemetry Trace Context Propagation Conventions for _meta)

Track: C — independent, starts now. Convention-only; no RFC-2119 MUSTs.
Spec: seps/414-request-meta.md · merged

What's done

• Nothing in main. No traceparent / tracestate / baggage handling in packages/.
• @opentelemetry/api is already a transitive dependency (via vitest tooling), so no new third-party install needed.

Outstanding

SEP-414 has zero RFC-2119 keywords — it's pure convention. SDK work is a small propagation helper, not a normative requirement. Recommended scope:

1. Helper API (sketch):
   • Client side: an opt-in middleware/interceptor that reads the current @opentelemetry/api span context and writes _meta['traceparent'] / _meta['tracestate'] / optional _meta['baggage'] onto outgoing requests.
   • Server side: a corresponding helper that reads those keys off ctx.mcpReq._meta and attaches them to the active span (or starts one).
2. No hard dependency on @opentelemetry/api in the published packages — keep it as an optional peer dependency or a separate @modelcontextprotocol/otel middleware package.
3. Docs in the middleware package's README, plus a runnable example under examples/.

Note

• The convention keys live in _meta at the request-params level, not under any extension prefix. They're not gated by capability negotiation.
• Companion to SEP-2575's _meta['io.modelcontextprotocol/*'] envelope, but not a part of it — implementers should not move these keys into the io.modelcontextprotocol/ namespace.

Coordination

• Independent. Lives best in a separate @modelcontextprotocol/otel middleware package (matches the existing pattern of @modelcontextprotocol/{express, hono, node, fastify}).
• Worth pairing with Implement SEP-2577: Deprecate Roots, Sampling, and Logging #2195 (SEP-2577) since OpenTelemetry is the recommended replacement for the deprecated logging/setLevel feature.