Errors

MCP Server error instrumentation ensures that all errors occurring within the Model Context Protocol (MCP) server are captured and reported to Sentry, without ever interfering with the operation of the MCP service itself.

Goals and Philosophy

• Comprehensive context: Errors are always associated with the active Sentry span, so you get full context (method, tool, arguments, etc.) in Sentry.
• Categorized errors: Errors are tagged by type (e.g., validation, timeout, tool_execution, resource_operation, prompt_execution, transport, etc.) for easy filtering and analysis in Sentry.
• Handler wrapping: All MCP server handlers (tool, resource, prompt) are wrapped to ensure errors are captured and correlated with the correct request span.

Safe Error Capture

The core utility is an error capture function:

import { getClient } from '../../currentScopes';
import { captureException } from '../../exports';

export function captureError(error: Error, errorType?: string): void {
  try {
    const client = getClient();
    if (!client) return;
    captureException(error, {
      tags: {
        mcp_error_type: errorType || 'handler_execution',
      },
    });
  } catch {
    // Silently ignore capture errors - never affect MCP operation
  }
}

• Never throws: All error capture is wrapped in a try/catch and will never throw.
• Tags errors: Errors are tagged with mcp_error_type for later filtering.

Handler Wrapping for Error Capture

All MCP server method handlers (tool, resource, prompt) are wrapped to:

• Correlate handler execution with the correct Sentry span (using request/session data)
• Capture both synchronous and asynchronous errors
• Categorize errors by handler type and error nature

Error Categorization

Errors are categorized and tagged based on the handler and error type:

• Tool handler errors:
  • validation (e.g., protocol/validation errors)
  • timeout (e.g., server timeouts)
  • tool_execution (all other tool errors)
• Resource handler errors:
  • resource_operation
• Prompt handler errors:
  • prompt_execution
• Transport errors:
  • transport
• Protocol errors:
  • protocol

Span Correlation

All errors are captured within the context of the active Sentry span, so you can:

• See which MCP method, tool, or resource caused the error
• View all arguments and context for the failed request
• Correlate errors with traces and performance data

Transport Layer Error Instrumentation

The MCP transport layer is also instrumented to:

• Create spans for incoming/outgoing messages
• Capture errors in transport event handlers (e.g., onerror)
• Correlate protocol errors with the correct request/response