diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7600e4f..cb8517e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -22,6 +22,8 @@ Second pre-release. Continues the W3C XSTS schema-validity campaign: **invalid-s
 
 ### Fixed
 
+- XSTS invalid-schemas-accepted (#145): a `simpleContent` restriction's base must be a complex type. Restricting a built-in (`xs:integer`, `xs:anySimpleType`) or a user `simpleType` through `<simpleContent><restriction>` is rejected (`src-ct.2`); a complex type with simple content is built by extending a simple type, not by restricting one. Caught five invalid schemas: invalid-schemas-accepted 142 → 137, no other bucket moved.
+
 - XSTS invalid-schemas-accepted (#145): Particle Valid (Restriction) now enforces the NameAndTypeOK nillable clause. A restricting element may not be `nillable` unless the base element it restricts is also nillable. Caught two invalid schemas: invalid-schemas-accepted 144 → 142, no other bucket moved.
 
 - XSTS invalid-schemas-accepted (#145): Particle Valid (Restriction) now enforces the NameAndTypeOK fixed-value clause. When the base element has a `fixed` value constraint, the restricting element must also be `fixed` to the same value (compared in the base type's value space, so a value-equal pair in different lexical forms is not wrongly flagged). Caught four invalid schemas: invalid-schemas-accepted 148 → 144, no other bucket moved.
diff --git a/Sources/Schema/SchemaCompileValidation.swift b/Sources/Schema/SchemaCompileValidation.swift
index 209ca7e..9055a0c 100644
--- a/Sources/Schema/SchemaCompileValidation.swift
+++ b/Sources/Schema/SchemaCompileValidation.swift
@@ -360,11 +360,12 @@ extension PureXML.Validation.SchemaCompile {
     /// cos-ct-extends.1.4.2.2: a complexContent extension adding element content
     /// must have a base with complex (or empty) content, not simpleContent.
     static var complexExtensionBaseValid: PureXML.Validation.Validation<PureXML.Schema.SchemaCompileRoot, PureXML.Schema.SchemaCompileContext> {
-        compileRule("A complexContent extension is consistent with its base type") { document in
+        compileRule("A content-model derivation is consistent with its base type") { document in
             guard let namedTypes = document.namedTypes else { return [] }
             return PureXML.Schema.SchemaLocatedFinding.unlocated(
                 PureXML.Schema.XSDParser.simpleContentExtensionBaseErrors(document.schema, document.context, namedTypes)
-                    + PureXML.Schema.XSDParser.extensionMixedAgreementErrors(document.schema, document.context, namedTypes),
+                    + PureXML.Schema.XSDParser.extensionMixedAgreementErrors(document.schema, document.context, namedTypes)
+                    + PureXML.Schema.XSDParser.simpleContentRestrictionBaseErrors(document.schema, document.context, namedTypes),
             )
         }
     }
diff --git a/Sources/Schema/SchemaExtensionAllGroup.swift b/Sources/Schema/SchemaExtensionAllGroup.swift
index 754558e..2f9a312 100644
--- a/Sources/Schema/SchemaExtensionAllGroup.swift
+++ b/Sources/Schema/SchemaExtensionAllGroup.swift
@@ -113,6 +113,31 @@ extension PureXML.Schema.XSDParser {
         return errors
     }
 
+    /// XSD 1.0 `src-ct.2` / Derivation Valid (Restriction): the base of a
+    /// `simpleContent` RESTRICTION must be a complex type (whose content is simple).
+    /// A built-in (`xs:integer`, `xs:anySimpleType`) or a user `simpleType` base is
+    /// invalid: a complex type with simple content is built by EXTENDING a simple
+    /// type, never by restricting one through `simpleContent`. Self-contained schema
+    /// only; a foreign or unresolved base is left alone.
+    static func simpleContentRestrictionBaseErrors(_ schema: XSDTree, _ context: PureXML.Schema.XSDContext, _ types: [String: PureXML.Schema.ElementType]) -> [String] {
+        guard !skipsCrossDocumentRules(schema, compositionLoaded: context.compositionLoaded) else { return [] }
+        let bindings = context.namespaceBindings
+        var errors: [String] = []
+        for content in descendants(schema, named: "simpleContent") {
+            guard let restriction = ExtAllNode.firstChild(content, named: "restriction"),
+                  let base = ExtAllNode.attribute(restriction, "base")
+            else { continue }
+            let namespace = ExtAllNode.referenceNamespace(base, bindings)
+            let baseName = ExtAllNode.stripPrefix(base)
+            if namespace == xsdNamespace {
+                errors.append("a simpleContent restriction's base must be a complex type, not the built-in simple type '\(baseName)'")
+            } else if namespace == context.targetNamespace, case .simple? = types[baseName] {
+                errors.append("a simpleContent restriction's base must be a complex type, not the simple type '\(baseName)'")
+            }
+        }
+        return errors
+    }
+
     /// XSD 1.0 `cos-ct-extends.1.4.3.2.2.1`: a `complexContent` extension's mixedness
     /// must match its base type's. The effective content joins the base content and
     /// the extension's, so a mixed extension may extend only a mixed base and an
diff --git a/Tests/SchemaContentOrderTests.swift b/Tests/SchemaContentOrderTests.swift
index 6b3d708..c688466 100644
--- a/Tests/SchemaContentOrderTests.swift
+++ b/Tests/SchemaContentOrderTests.swift
@@ -44,6 +44,11 @@ struct SchemaContentOrderTests {
     @Test("well-formed simpleContent derivations compile")
     func test_valid() {
         #expect(!rejects(simpleContent(#"<xs:extension base="xs:string"><xs:attribute name="a" type="xs:string"/></xs:extension>"#)))
-        #expect(!rejects(simpleContent(#"<xs:restriction base="xs:string"><xs:maxLength value="3"/><xs:attribute name="a"/></xs:restriction>"#)))
+        // A simpleContent restriction's base must be a complex type (src-ct.2), so it
+        // restricts a complexType with simple content, not xs:string directly.
+        #expect(!rejects(#"""
+        <xs:complexType name="base"><xs:simpleContent><xs:extension base="xs:string"><xs:attribute name="a"/></xs:extension></xs:simpleContent></xs:complexType>
+        <xs:complexType name="t"><xs:simpleContent><xs:restriction base="base"><xs:maxLength value="3"/></xs:restriction></xs:simpleContent></xs:complexType>
+        """#))
     }
 }
diff --git a/Tests/ValidationConfigurationTests.swift b/Tests/ValidationConfigurationTests.swift
index af09471..676c02a 100644
--- a/Tests/ValidationConfigurationTests.swift
+++ b/Tests/ValidationConfigurationTests.swift
@@ -101,7 +101,7 @@ struct ValidationConfigurationTests {
             "Element value constraints are valid against their declared types",
             "Complex types extending xs:all groups satisfy XSD placement rules",
             "Anonymous complex-type restrictions accept a subset of their base's",
-            "A complexContent extension is consistent with its base type",
+            "A content-model derivation is consistent with its base type",
             "Attribute restrictions are faithful to their bases",
             "Simple types do not derive from complex types",
             "Simple-type varieties are declared consistently",
diff --git a/Tests/XSTSSuiteTests.swift b/Tests/XSTSSuiteTests.swift
index e132669..0a7c471 100644
--- a/Tests/XSTSSuiteTests.swift
+++ b/Tests/XSTSSuiteTests.swift
@@ -102,8 +102,11 @@ struct XSTSSuiteTests {
     /// Enforcing the NameAndTypeOK nillable clause (a restriction may not be nillable
     /// unless the base element is) then caught two more invalid schemas:
     /// invalid-schemas accepted 144 → 142, no other bucket moved.
+    /// Requiring a `simpleContent` restriction's base to be a complex type, not a
+    /// built-in or user simple type (src-ct.2), then caught five more invalid
+    /// schemas: invalid-schemas accepted 142 → 137, no other bucket moved.
     private let knownSchemaValidRejected = 1
-    private let knownSchemaInvalidAccepted = 142
+    private let knownSchemaInvalidAccepted = 137
     private let knownInstanceValidRejected = 3
     private let knownInstanceInvalidAccepted = 132