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When you call instrumentServer(server, config), the package patches server.registerTool so every subsequent tool you register runs inside an OpenTelemetry span.

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Span shape

Each tool invocation produces a single span: 
name:       tools/call <toolName>
status:     OK | ERROR
duration:   ms
attributes: { ... see below ... }

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Attributes

AttributeDescription
mcp.method.nameAlways "tools/call" for tool invocations.
mcp.tool.nameThe tool name passed to registerTool.
mcp.tool.titleTool title from the tool config.
mcp.tool.descriptionTool description from the tool config.
mcp.request.idUUID generated per request.
mcp.session.idUUID generated when the SDK starts; constant for the process.
mcp.operation.successtrue on OK, false on thrown error.
mcp.operation.durationDuration in ms (also written as a histogram metric).
client.addressFirst non-internal IPv4 address of the host (localhost if none).
client.portprocess.env.PORT if set.
error.typeError class name (only on failure).
error.messageError message (only on failure).

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Optional attributes

These show up only under specific configurations:
AttributeWhen
mcp.request.argument.<key>When enableArgumentCollection: true. Each argument flattened into a span attribute.
mcp.safety_check.contentWhen the handler is wrapped with safetyCheck.
mcp.safety_check.flaggedtrue if Sedata’s safety API flagged the input.
mcp.safety_check.latency_msRound-trip latency of the safety check.
mcp.safety_check.successtrue if the safety API responded successfully (whether flagged or not).
enableArgumentCollection is off by default. Tool arguments often contain user input, secrets, or PII — turn it on knowingly.

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Resource attributes

The OTel resource attached to every span carries:
KeySource
service.nameconfig.serverName
service.versionconfig.serverVersion
mcp.session.idUUID per process
Host / OS / env detector attrsFrom @opentelemetry/resources detectors
These are added once per process and merged into every export.

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Errors

If your handler throws, the package:
  1. Sets the span status to ERROR with the error message.
  2. Sets error.type, error.message, mcp.operation.success: false.
  3. Records the duration histogram with mcp.operation.success: false.
  4. Re-throws the original error so the MCP runtime sees it.
span.name             tools/call calculate-bmi
status                ERROR
status.message        height cannot be zero
attributes.error.type RangeError

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Sampling

By default, all spans are recorded (samplingRate: 1.0). To trim volume in production, set a ratio: 
const config: TelemetryConfig = {
  // ...
  samplingRate: 0.1, // keep 10% of traces
}
The sampler is TraceIdRatioBasedSampler — head-based and deterministic per trace id. See Sampling for tradeoffs.

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What’s next

Automatic attributes reference

Full table of every attribute the package writes.

Custom spans

Record domain-specific operations alongside auto-spans.