Standard OTel Logged 7 Failures, MCP Actually Had 18
Thirumalaiboobathi · reddit · 2026-10-02
MCP tools often report errors as isError: true inside a normal, successful JSON-RPC response. The transport says 200 OK, so a standard OpenTelemetry setup records a clean green span while the agent actually receives permissiondenied, notfound, or validationerror.
The author built opentel-mcp, an OpenTelemetry instrumentation library for Node.js MCP servers, to catch these. The new release adds a local dashboard runnable with one command: npx opentel-mcp-ui --demo — no Prometheus, Grafana, or Collector needed.
- In the demo data, standard OTel would have missed 11 of 18 failures
- Side-by-side view of each silent failure: what normal tracing records (200 OK · no error) vs what actually happened (isError: true · notfound)
- Spans for tools/call and tools/list, aligned with OTel MCP semantic conventions
- 12 metrics: silent failures, errors, duration, tokens, cost, agent thrash (retry loops), schema drift
- Error fingerprinting and classification (validation, timeout, auth, network, etc.)
- Works with @modelcontextprotocol/sdk v1 and v2, stdio and HTTP
- Never throws into your server; failures degrade to a warning; dev mode prints metrics to stderr
Current limits: server-side only, covers tools but not resources or prompts, and cost tracking requires tools to report token usage.
Related event: Standard OpenTelemetry Misses Silent MCP Tool Failures(2 posts)→
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