OpenTelemetry tracing

MCP 2026-07-28 (SEP-414) folds tracing into the protocol: a client that wants its child operations attached to a parent span passes the W3C Trace Context (traceparent / tracestate) in the request _meta block. Servers that re-emit traces use these headers to build the parent / child relationship.

mcptest ships the library for that propagation today. Wiring it to a real OpenTelemetry SDK so a run lands spans in Jaeger / otel-cli / any OTLP collector is a follow-up.

Library surface

mcptest_core::otel:

Item	Purpose
`TraceParent { trace_id, parent_id, flags }`	Typed W3C `traceparent` per RFC 9462.
`TraceState(String)`	Opaque vendor list (round-trip lossless).
`TraceFlags(u8)`	Sampling + reserved bits with `.sampled()` accessor.
`TraceContextError`	Typed parse errors (BadShape / UnsupportedVersion / BadTraceId / ...).
`TraceParent::parse(s)`	Strict parser (lowercase hex, length-checked, refuses all-zero ids).
`TraceParent::to_wire()`	Format back to the wire form, byte-stable.
`inject_into_meta(&mut meta, parent, state)`	Set `_meta.traceparent` + optional `tracestate`.
`extract_from_meta(meta) -> Option<...>`	Read trace context off `_meta`.
`RUN_SPAN_NAME` / `TEST_SPAN_NAME` / `tool_call_span_name(method)`	Stable span-name conventions.

Wire shape

A request that propagates a parent span:

{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "tools/call",
  "params": {
    "name": "weather",
    "arguments": { "loc": "SFO" },
    "_meta": {
      "traceparent": "00-0af7651916cd43dd8448eb211c80319c-b7ad6b7169203331-01",
      "tracestate":  "vendor1=opaqueValue,vendor2=opaqueValue"
    }
  }
}

traceparent is required when propagating. tracestate is optional (an empty vendor list is omitted by spec); the library mirrors that by skipping the key when the value is empty.

Strict parsing

TraceParent::parse matches RFC 9462 exactly:

Four dash-separated fields: version-trace_id-parent_id-flags.
Version must be 00 (the only supported value).
trace_id is 32 lowercase hex characters; uppercase is rejected.
parent_id is 16 lowercase hex characters.
flags is 2 lowercase hex characters.
All-zero trace_id or parent_id is rejected (the W3C spec reserves them).

Anything else returns a typed [TraceContextError].

Span naming

The conventions in span_names.rs align with the OTel semantic convention for RPC clients:

Span	Name	Notes
Top-level run	`mcptest.run`	One per `mcptest run` invocation.
Per-test	`mcptest.test`	Test name on the span attributes (`mcptest.test.name`), not the span name (cardinality).
Per tool call	`mcp/<method>`	Tool name is an attribute.

What ships today

Typed TraceParent / TraceState / TraceFlags.
Strict parser and inject / extract helpers.
Stable span-name constants and tool_call_span_name builder.
Unit tests covering the happy path, every reject path, the empty-tracestate skip, the round-trip stability, and the all-zero ID guard.

Planned follow-up

Real OTel SDK integration (opentelemetry-sdk, opentelemetry-otlp for a default exporter).
Span emission for mcptest.run / mcptest.test / mcp/<method> across the runner lifecycle, with the attributes the spec calls for (mcptest.test.name, mcp.tool.name, mcp.method, etc.).
CLI flags: --otel-exporter <otlp|none> and --otel-endpoint <url>; env vars: OTEL_EXPORTER_OTLP_ENDPOINT per OTel convention.
A suite-level assertion (assert_traceparent_present or similar) so a YAML can require the server echoed the context back.

Cross-references

mcptest_core::otel for the library.
SEP-414 + RFC 9462 for the spec.