Summary

OpenTelemetry officially merged MCP semantic conventions on January 12, 2026 (PR #2083). ToolHive should align its telemetry implementation with these standards for better observability tool compatibility and ecosystem alignment.

Standard References

• Main Documentation: docs/gen-ai/mcp.md
• Attribute Registry: model/mcp/registry.yaml
• Metrics Definitions: model/mcp/metrics.yaml
• W3C Trace Context: https://www.w3.org/TR/trace-context/

Current State

ToolHive has solid telemetry foundation but predates the official conventions:

• Middleware: pkg/telemetry/middleware.go - spans, attributes, metrics for MCP proxy
• vMCP: pkg/vmcp/server/telemetry.go - backend and workflow telemetry
• Parser: pkg/mcp/parser.go - already extracts _meta field (lines 228-233)

Core Implementation Tasks

1. Update Attributes and Span Naming

File: pkg/telemetry/middleware.go

Attribute Renames (for standard compliance):

• mcp.method → mcp.method.name (line 222)
• mcp.request.id → jsonrpc.request.id (line 229)
• mcp.tool.name → gen_ai.tool.name (line 263)
• mcp.tool.arguments → gen_ai.tool.call.arguments (line 267, opt-in)
• mcp.prompt.name → gen_ai.prompt.name (line 279)
• mcp.transport → network.transport with value mapping:
  • stdio → pipe
  • sse, streamable-http → tcp

Add Missing Required Attributes:

• mcp.protocol.version - MCP spec version (e.g., "2025-11-25")
• mcp.session.id - Session identifier
• jsonrpc.protocol.version - When not "2.0"
• error.type - On failures (JSON-RPC error code or "tool_error")
• rpc.response.status_code - When response contains error
• gen_ai.operation.name - "execute_tool" for tool calls
• network.protocol.name - "http" for SSE/streamable-http

Span Naming (lines 161-170):

• Current: mcp.tools/call
• Standard: tools/call get_weather (include target when available)
• Format: {mcp.method.name} {target} where target is tool/prompt name

2. Add Standard Metrics

File: pkg/telemetry/middleware.go

New Standard Metrics (alongside existing):

• mcp.client.operation.duration (histogram, seconds)
• mcp.server.operation.duration (histogram, seconds)
• Use recommended buckets: [0.01, 0.02, 0.05, 0.1, 0.2, 0.5, 1, 2, 5, 10, 30, 60, 120, 300]

Keep existing toolhive_mcp_* metrics for backward compatibility.

3. Implement W3C Trace Context Propagation

Critical Feature: Enable distributed tracing across MCP boundaries.

3a. Context Injection (vMCP → Backends)

Files:

• New: pkg/telemetry/propagation.go - W3C Trace Context helpers
• pkg/vmcp/client/client.go - Inject before backend calls

Implementation: Inject traceparent and tracestate into params._meta:

{
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "name": "get-weather",
    "_meta": {
      "traceparent": "00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-01",
      "tracestate": "rojo=00f067aa0ba902b7"
    }
  }
}

Code Structure:

// propagation.go
func InjectTraceContext(ctx context.Context, params map[string]interface{}) {
    meta := getOrCreateMeta(params)
    carrier := &MetaCarrier{meta: meta}
    otel.GetTextMapPropagator().Inject(ctx, carrier)
}

type MetaCarrier struct {
    meta map[string]interface{}
}
// Implement TextMapCarrier interface

3b. Context Extraction (Clients → ToolHive)

File: pkg/telemetry/middleware.go (around line 114)

Implementation: Extract trace context from incoming params._meta and use as parent for server span:

if parsedMCP := mcpparser.GetParsedMCPRequest(ctx); parsedMCP != nil && parsedMCP.Meta != nil {
    carrier := &MetaCarrier{meta: parsedMCP.Meta}
    ctx = otel.GetTextMapPropagator().Extract(ctx, carrier)
}

4. Add Client-Side Spans for vMCP

File: pkg/vmcp/client/client.go

Current: Only SERVER spans when serving requests
Needed: CLIENT spans when vMCP calls backend MCP servers

Operations to Instrument:

• initialize - Protocol handshake
• tools/list, tools/call
• resources/list, resources/read
• prompts/list, prompts/get

Span Kind: Use trace.SpanKindClient for these operations.

5. Add Session Duration Metrics

Files:

• pkg/vmcp/server/session_adapter.go - Track session lifecycle
• Proxy components - Track session termination

Metrics:

• mcp.client.session.duration (histogram, seconds)
• mcp.server.session.duration (histogram, seconds)

Attributes:

• mcp.protocol.version
• network.protocol.name
• network.transport
• error.type (if session terminated with error)

Backward Compatibility

Approach: Emit both legacy and standard names during transition period.

Configuration: Add optional flag:

telemetry:
  useLegacyAttributes: false  # default: standard only

CLI Flag: --otel-use-legacy-attributes (enables dual emission)

Timeline:

• Ship standard-compliant attributes/metrics immediately
• Announce deprecation after 6 months
• Remove legacy support in v2.0

Components Affected

• pkg/telemetry/middleware.go - MCP proxy telemetry (spans, metrics, attributes)
• pkg/telemetry/propagation.go - New file for trace context helpers
• pkg/vmcp/client/client.go - CLIENT spans and context injection
• pkg/vmcp/server/session_adapter.go - Session duration tracking
• pkg/telemetry/config.go - Backward compatibility configuration
• cmd/thv-operator/api/v1alpha1/*_types.go - CRD telemetry specs
• docs/observability.md - Update documentation
• Test files: Update assertions for new attribute names

Testing Requirements

• Update test expectations in pkg/telemetry/middleware_test.go
• Update E2E tests in test/e2e/telemetry_middleware_e2e_test.go
• Add trace propagation E2E test (vMCP → backend → vMCP chain)
• Validate span hierarchy (CLIENT/SERVER relationship)
• Test session duration tracking
• Verify histogram buckets

Success Criteria

• All required attributes emitted per standard
• Span names follow {method} {target} format
• Standard metrics recorded with correct units/attributes
• W3C Trace Context propagates through params._meta
• CLIENT spans created for vMCP backend calls
• Session duration metrics tracked
• Network transport values mapped correctly (stdio→pipe, http→tcp)
• Documentation updated
• Backward compatibility maintained (with flag)
• No performance regression
• Traces produced by vMCP also include spans from the MCPServer's proxyrunner.

References

• OTel MCP PR: MCP semantic conventions open-telemetry/semantic-conventions#2083
• Implementation Examples:
  • .NET: OpenTelemetry: context propagation and semconv update modelcontextprotocol/csharp-sdk#262
  • Python: https://github.com/mayankagarwals/openllmetry/tree/main/packages/opentelemetry-instrumentation-mcp
• Related Issue: Mentioned in team discussion about enhancing ToolHive telemetry