XSD validation error
The attribute is not allowed
The document carries an attribute the schema never declared. XSD is closed by default: anything not named is forbidden rather than ignored.
Element 'order', attribute 'ref': The attribute 'ref' is not allowed. (line 2)What it means
A complex type permits exactly the attributes it declares. Unlike a permissive JSON consumer, a schema validator does not skip what it does not recognise — an undeclared attribute is an error, even one that is obviously harmless.
This is the failure that appears when a sender adds a field. Their document is richer, yours is a validator, and the contract said no.
What usually causes it
- A partner adding a field without a schema revision agreed on both sides.
- Debug or trace attributes left on a document by a middleware layer.
- A typo in an attribute name — the correct one goes missing and the misspelling is rejected as unknown.
- A namespace-qualified attribute from an extension the schema does not import.
How to fix it
- Remove the attribute, or declare it in the schema — those are the only two honest options.
- For genuinely extensible documents, declare <xs:anyAttribute namespace="##other"/> so extensions are allowed by design.
- Check the spelling first: an unexpected attribute and a missing required one usually arrive together, and are the same typo.
The same error elsewhere
libxml2 words these plainly. Xerces numbers them, and those cvc- codes are what most people end up searching for.
Xerces (Java)
cvc-complex-type.3.2.2: Attribute 'ref' is not allowed to appear in element 'order'..NET System.Xml
The 'ref' attribute is not declared.
Before and after
Both documents below are well-formed XML — that is the point. The test suite re-checks them against the real validator on every build: the first is confirmed to produce the exact message above, the second to validate cleanly.
<?xml version="1.0" encoding="UTF-8"?>
<order id="A-1" ref="X">
<customer>Ada</customer>
<issued>2026-01-31</issued>
<state>open</state>
<line>
<code>AB-1234</code>
<qty>2</qty>
</line>
</order>
<?xml version="1.0" encoding="UTF-8"?>
<order id="A-1">
<customer>Ada</customer>
<issued>2026-01-31</issued>
<state>open</state>
<line>
<code>AB-1234</code>
<qty>2</qty>
</line>
</order>
The schema both are checked against
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:simpleType name="status">
<xs:restriction base="xs:string">
<xs:enumeration value="open"/>
<xs:enumeration value="closed"/>
</xs:restriction>
</xs:simpleType>
<xs:simpleType name="sku">
<xs:restriction base="xs:string">
<xs:pattern value="[A-Z]{2}-\d{4}"/>
</xs:restriction>
</xs:simpleType>
<xs:element name="order">
<xs:complexType>
<xs:sequence>
<xs:element name="customer" type="xs:string"/>
<xs:element name="issued" type="xs:date"/>
<xs:element name="state" type="status"/>
<xs:element name="line" minOccurs="1" maxOccurs="2">
<xs:complexType>
<xs:sequence>
<xs:element name="code" type="sku"/>
<xs:element name="qty" type="xs:positiveInteger"/>
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:sequence>
<xs:attribute name="id" type="xs:string" use="required"/>
</xs:complexType>
</xs:element>
</xs:schema>
Where this bites most
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