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Merge pull request #397 from koic/otel_trace_context_meta

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Support W3C Trace Context Propagation via `_meta` per SEP-414
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Koichi ITO 2026-06-13 20:27:08 +09:00 • committed by GitHub
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@ -300,6 +300,36 @@ end
}
```
**Distributed Tracing (W3C Trace Context):**
Per SEP-414, the keys `traceparent`, `tracestate`, and `baggage` are reserved un-prefixed `_meta` keys for propagating
[W3C Trace Context](https://www.w3.org/TR/trace-context/) across MCP requests. The SDK guarantees these keys pass through
incoming request `_meta` untouched, and exposes their names as constants on `MCP::TraceContext` (`TRACEPARENT_META_KEY`,
`TRACESTATE_META_KEY`, `BAGGAGE_META_KEY`, and `META_KEYS`). The SDK does not depend on OpenTelemetry; bridge the values
to your tracing system yourself:
```ruby
class TracedTool < MCP::Tool
def self.call(message:, server_context:)
traceparent = server_context.dig(:_meta, :traceparent)
# Hand traceparent/tracestate/baggage to your tracing library
# (e.g. the opentelemetry-ruby gems) to continue the caller's trace.
MCP::Tool::Response.new([{ type: "text", text: "ok" }])
end
end
```
On the client side, every request method (`call_tool`, `read_resource`, `get_prompt`, `complete`, `ping`, and the `list_*` methods)
accepts a `meta:` keyword to inject these keys into the outgoing request, so trace context can flow on every request:
```ruby
meta = { MCP::TraceContext::TRACEPARENT_META_KEY => "00-0af7651916cd43dd8448eb211c80319c-b7ad6b7169203331-01" }
client.call_tool(tool: tool, arguments: { message: "Hello" }, meta: meta)
client.read_resource(uri: "file:///report.txt", meta: meta)
```
#### Configuration Block Data
##### Exception Reporter