XSD validation error
The attribute is required but missing
The schema marks an attribute use="required" and the document does not carry it. Well-formed, and rejected by the first consumer that validates.
Element 'order': The attribute 'id' is required but missing. (line 2)What it means
An attribute declared with use="required" is part of the contract, not a suggestion. XML itself has no opinion about which attributes an element carries, so nothing catches this until a schema is applied — which is usually at the receiving end, after the document has already been sent.
Required attributes are where identifiers live: order ids, currency codes, schema versions. Their absence is rarely cosmetic, so this error normally means a field was dropped upstream rather than mistyped.
What usually causes it
- A generator that emits the attribute only when its value is non-empty, so an empty value becomes a missing attribute.
- A field renamed on one side of an integration and not the other.
- Mapping code that writes the value as a child element when the schema wants an attribute.
- A partial object serialized before it was fully populated.
How to fix it
- Add the attribute. If the value is genuinely unknown, the schema is telling you the document is not ready to send.
- Emit required attributes unconditionally — an empty string is a value; a missing attribute is a contract breach.
- If the field really is optional in your domain, change the schema to use="optional" rather than shipping documents that fail it.
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.4: Attribute 'id' must appear on element 'order'..NET System.Xml
The required attribute 'id' is missing.
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>
<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:simpleType name="shortLabel">
<xs:restriction base="xs:string">
<xs:minLength value="3"/>
</xs:restriction>
</xs:simpleType>
<xs:element name="order">
<xs:complexType>
<xs:sequence>
<xs:element name="customer" type="xs:string"/>
<xs:element name="middleName" type="xs:string" minOccurs="0"/>
<xs:element name="nickname" type="shortLabel" minOccurs="0"/>
<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
The terms behind it
Each of these is defined in one sentence and then demonstrated against the parser.
Other schema errors
- The attribute is not allowed
- This element is not expected
- Missing child element(s)
- Not a valid value of the atomic type
- The value is not an element of the set
- The value is not accepted by the pattern
- No matching global declaration for the validation root
- The element is not nillable
- The value is shorter than minLength
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