[FEATURE]: propagate W3C trace context on outbound MCP HTTP/SSE requests

[HrdlickaJakub]

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Problem

When opencode connects to an MCP server over HTTP, the outbound requests do not carry the W3C traceparent / tracestate headers.

Expected Behavior

With OTel tracing enabled (OTEL_EXPORTER_OTLP_ENDPOINT set and experimental.openTelemetry: true), every outbound MCP HTTP request should carry the W3C traceparent (and tracestate when present) header derived from the currently-active trace context, so the MCP server's spans attach as children of the originating opencode span in the same trace.

Use Case

Operating a tracing-enabled MCP server (whether self-hosted or shared) and needing a single, complete trace per tool call across all hops.

Concretely, this unblocks:

• Debugging tool-call latency end-to-end (where time is spent on the opencode side vs. inside the MCP server vs. in whatever the MCP server calls downstream).
• Correlating errors raised inside the MCP server with the originating opencode session in a single view.
• Consistent sampling

Related Issues

• Add OTEL export capabilities for observability and monitoring #12142 — the umbrella OTel export work this builds on.
• [FEATURE]: propagate incoming trace context for HTTP requests #18801 — the inbound mirror of this gap on opencode's own HTTP server.
• [FEATURE]: propagate trace context to subprocesses #27031 — the same correlation gap for child processes.

[nexxeln]

will keep this open to see if there's more interest