diff --git a/CHANGELOG.md b/CHANGELOG.md
index 613e5d3..ae500b6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -21,6 +21,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- XSTS invalid-schemas-accepted (#145): an identity constraint's content model is enforced. `unique`, `key`, and `keyref` must contain exactly one `selector` followed by one or more `field`s (after an optional `annotation`); a `field` before the `selector`, a second `selector`, or no `field` is now rejected (previously only child-element membership was checked, not their cardinality or order). invalid-schemas-accepted 110 → 104, no false positive (valid-schemas-rejected held at 0), no other bucket moved.
+
 - XSTS invalid-schemas-accepted (#145): the occurrence-range constraints on model groups are enforced. A particle's `minOccurs` may not exceed its `maxOccurs`, both defaulting to 1 when absent (`p-props-correct.1`), so `<xs:sequence minOccurs="2">` (2 > the default 1) and `<xs:all maxOccurs="0">` (the default 1 > 0) are rejected; and an `all` group's `maxOccurs` must be 1 and `minOccurs` 0 or 1, with every particle it contains limited to `maxOccurs` 0 or 1 (`cos-all-limited.2`). The previous check fired only when both attributes were present, missing the common default-`maxOccurs` cases. Triple-confirmed against the W3C-labelled suite and libxml2: invalid-schemas-accepted 130 → 110, with no false positive (valid-schemas-rejected held at 0) and no other bucket moved.
  - 2026-06-16
 
diff --git a/Sources/Schema/SchemaStructure.swift b/Sources/Schema/SchemaStructure.swift
index 174c5f1..5983ee6 100644
--- a/Sources/Schema/SchemaStructure.swift
+++ b/Sources/Schema/SchemaStructure.swift
@@ -136,6 +136,8 @@ extension PureXML.Schema.XSDParser {
             errors += simpleContentOrderErrors(node)
         case "selector", "field":
             errors += identityXPathErrors(node, local: local)
+        case "unique", "key", "keyref":
+            errors += identityConstraintContentErrors(names)
         default:
             break
         }
diff --git a/Sources/Schema/SchemaStructureHelpers.swift b/Sources/Schema/SchemaStructureHelpers.swift
index c4b2fe1..eb0d228 100644
--- a/Sources/Schema/SchemaStructureHelpers.swift
+++ b/Sources/Schema/SchemaStructureHelpers.swift
@@ -227,6 +227,19 @@ extension PureXML.Schema.XSDParser {
         return errors
     }
 
+    /// The content model of an identity constraint (`unique`/`key`/`keyref`) is
+    /// `(annotation?, selector, field+)`: exactly one `selector`, then one or more
+    /// `field`s, in that order. A `field` before the `selector`, a second
+    /// `selector`, or no `field` is invalid. (`names` is already the XSD-namespace
+    /// children; misplaced annotation is checked by the general annotation rule.)
+    static func identityConstraintContentErrors(_ names: [String]) -> [String] {
+        let meaningful = names.filter { $0 != "annotation" }
+        let wellFormed = meaningful.first == "selector"
+            && meaningful.count >= 2
+            && meaningful.dropFirst().allSatisfy { $0 == "field" }
+        return wellFormed ? [] : ["an identity constraint must contain one selector followed by one or more fields"]
+    }
+
     static func groupChildrenErrors(_: XSDTree, names: [String]) -> [String] {
         var errors: [String] = []
         let compositors = names.count { $0 == "all" || $0 == "choice" || $0 == "sequence" }
diff --git a/Tests/SchemaIdentityXPathTests.swift b/Tests/SchemaIdentityXPathTests.swift
index f701f6e..fc0b16c 100644
--- a/Tests/SchemaIdentityXPathTests.swift
+++ b/Tests/SchemaIdentityXPathTests.swift
@@ -68,4 +68,38 @@ struct SchemaIdentityXPathTests {
             #expect(rejectsField(field), "expected rejection: '\(field)'")
         }
     }
+
+    /// The content model of `unique`/`key`/`keyref` is `(annotation?, selector,
+    /// field+)`: a field before the selector, a second selector, or no field is
+    /// invalid, while one selector followed by one or more fields (optionally after
+    /// an annotation) is valid.
+    private func compilesConstraint(_ body: String) -> Bool {
+        let document = #"""
+        <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
+          <xs:element name="r">
+            <xs:complexType><xs:sequence><xs:element name="c" type="xs:string"/></xs:sequence></xs:complexType>
+            <xs:unique name="u">\#(body)</xs:unique>
+          </xs:element>
+        </xs:schema>
+        """#
+        return (try? PureXML.Schema.Document(document)) != nil
+    }
+
+    @Test("an identity constraint requires one selector then one or more fields")
+    func test_identityConstraintContentModel() {
+        let selector = #"<xs:selector xpath="c"/>"#
+        let field = #"<xs:field xpath="@v"/>"#
+        let annotation = "<xs:annotation><xs:documentation>x</xs:documentation></xs:annotation>"
+        // Valid shapes.
+        #expect(compilesConstraint(selector + field))
+        #expect(compilesConstraint(selector + field + field))
+        #expect(compilesConstraint(annotation + selector + field))
+        // A field before the selector (idA044 shape).
+        #expect(!compilesConstraint(field + selector))
+        // Two selectors (idA046 shape).
+        #expect(!compilesConstraint(selector + selector + field))
+        // No field, or no selector.
+        #expect(!compilesConstraint(selector))
+        #expect(!compilesConstraint(field))
+    }
 }
diff --git a/Tests/XSTSSuiteTests.swift b/Tests/XSTSSuiteTests.swift
index 28bafa4..cbf7015 100644
--- a/Tests/XSTSSuiteTests.swift
+++ b/Tests/XSTSSuiteTests.swift
@@ -145,8 +145,11 @@ struct XSTSSuiteTests {
     /// minOccurs may not exceed maxOccurs, both defaulting to 1; cos-all-limited.2: an
     /// all group's maxOccurs is 1 and minOccurs 0 or 1, its particles' maxOccurs 0 or
     /// 1) then caught twenty more invalid schemas with no false positive: 130 → 110.
+    /// Enforcing the identity-constraint content model (`unique`/`key`/`keyref` is
+    /// `(annotation?, selector, field+)`: exactly one selector, then one or more
+    /// fields, in order) caught six more with no false positive: 110 → 104.
     private let knownSchemaValidRejected = 0
-    private let knownSchemaInvalidAccepted = 110
+    private let knownSchemaInvalidAccepted = 104
     private let knownInstanceValidRejected = 0
     private let knownInstanceInvalidAccepted = 133