Tool handlers catch errors and return isError: true

MCP tools that let exceptions propagate unhandled either crash the server process or produce an opaque JSON-RPC internal error (-32603) with no actionable detail. CWE-703 (improper check of exceptional conditions) is the root; CWE-209 (generation of error messages with sensitive information) is the follow-on risk when raw stack traces leak internal file paths and dependency versions into tool output. The correct MCP pattern is to catch exceptions inside the handler and return isError: true with a descriptive, scrubbed message — keeping the server alive and giving the AI something useful to act on.

Wrap every tool handler body in a try/catch. Return isError: true with a clean error message — never rethrow.

// src/tools/query.ts
server.tool('query_db', ..., async ({ sql }) => {
  try {
    const result = await db.query(sql)
    return { content: [{ type: 'text', text: JSON.stringify(result) }] }
  } catch (error) {
    // Return error as content — do not rethrow
    return {
      content: [{ type: 'text', text: `Query failed: ${(error as Error).message}` }],
      isError: true,
    }
  }
})

Strip stack traces before including error messages in the response. Expose the error type and message, not the stack or internal paths.