If you have a schema which allows extensions (i.e. with <any> or <anyAttribute> in it somewhere) then it seems xmllint (and possibly libxml2) cannot handle multiple schemas.  Is this just an error in the way I invoke it, bad defintion of my XSD/XML or a new feature requirement?  I don't want to use skip validation as I want the extra attribs to be tightly controlled too.  I don't own the main/core XSD hence the extension using my custom XSD.

header of XML = "<?xml version="1.0" encoding="UTF-8"?>
<SERVICE_PLUGIN xmlns="xxx"
                xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                xmlns:sfbu="yyy"
                xsi:schemaLocation="xxx file:/home/sg/dev/ACPtools/XSD/AE_2.3/ServicePlugin.xsd yyy file:/home/sg/dev/ACPtools/XSD/AE_2.3/SFBUCustomAttribs.xsd" >"

Extract of element where I tried to add custom attribs = "            <PARAM_DEFINITION PARAM_NAME="SP1.GAT1.GP.PD2" PARAM_TYPE="number" sfbu:MinValue="x" MaxValue="7" />"  (tried with/without namespace prefix)

Copy of custom XSD used by XML above = "<?xml version="1.0" encoding="ISO-8859-1"?>
<xs:schema  xmlns:xs="http://www.w3.org/2001/XMLSchema"
            targetNamespace="yyy"
            xmlns="yyy"
            elementFormDefault="qualified">
    <xs:attribute name="MinValue">
        <xs:simpleType>
            <xs:restriction base="xs:integer">
                <xs:minInclusive value="0"/>
            </xs:restriction>
        </xs:simpleType>
    </xs:attribute>
    <xs:attribute name="MaxValue">
        <xs:simpleType>
            <xs:restriction base="xs:integer" />
                <xs:minInclusive value="1"/>
            </xs:restriction>
        </xs:simpleType>
    </xs:attribute>
</xs:schema>"

test output = "xmllint --noout --schema file:/home/sg/dev/ACPtools/XSD/AE_2.3/ServicePlugin.xsd SP_test1.xml
SP_test1.xml:82: element PARAM_DEFINITION: Schemas validity error : Element '{http://www.cramer.com/ServicePluginSchema}PARAM_DEFINITION', attribute '{http://www.amdocs.com/SFBU}MinValue': No matching global attribute declaration available, but demanded by the strict wildcard.
SP_test1.xml:82: element PARAM_DEFINITION: Schemas validity error : Element '{http://www.cramer.com/ServicePluginSchema}PARAM_DEFINITION', attribute 'MaxValue': No matching global attribute declaration available, but demanded by the strict wildcard.
SP_test1.xml fails to validate"

Env Info

xmllint: using libxml version 20632
   compiled with: Threads Tree Output Push Reader Patterns Writer SAXv1 FTP HTTP DTDValid HTML Legacy C14N Catalog XPath XPointer XInclude Iconv ISO8859X Unicode Regexps Automata Expr Schemas Schematron Modules Debug Zlib 

Kernel:           Linux      2.6.28-15-generic      x86_64