v2.4.0

Minor Changes

• 939fa1d: Write one OpenTelemetry SERVER span per MCP request or notification, following the OpenTelemetry MCP semantic conventions and the vercel.rpc.* attributes Vercel's MCP observability reads. Spans are written only when the app registers a tracer provider; mcp-handler has no runtime dependency on @opentelemetry/api. Set the endpoint label with telemetry.route, or turn spans off with telemetry: false.

  Add registerMcpOTel() in mcp-handler/otel, which registers @vercel/otel 2.x with the Vercel project as the service name and removes the raw User-Agent and Referer attributes @vercel/otel adds to spans.

  @vercel/otel is now an optional peer dependency at ^2.0.0. With npm, upgrade @vercel/otel to 2.x before you upgrade mcp-handler, or the install fails with ERESOLVE.

  With MCP Events, write mcp.event.subscription {action} and mcp.event.verify_callback spans around the calls to the application's subscription store and verifyEndpoint.

  Spans are on by default whenever the app registers a tracer provider. If your app already writes its own MCP SERVER spans with mcp.method.name, or mcp.event.* spans around its MCP Events store or verifyEndpoint, set telemetry: false or remove your spans when upgrading, or each message and subscription step is recorded twice.

• 755e2db: Add experimental_setToolOutcome(ctx, { outcome, errorType }), which a tool handler calls to say why it failed. Its MCP span then records api_error, tool_error, execution_failure or request_error with a short error.type such as forbidden or timeout, instead of the default execution_failure / internal_error. It applies only to calls answered with an isError result.