diff --git a/CHANGELOG.md b/CHANGELOG.md
index ce88f5a..1bf43e9 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -33,7 +33,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   unexpected elements, invalid or missing attributes, invalid text/datatype
   content, and missing required content. A datatype mismatch quotes the offending
   value and names the expected type (`'abc' is not a valid integer`), and a value
-  mismatch quotes both the actual text and the required literal. `RelaxNG.validation()` exposes the schema
+  mismatch quotes both the actual text and the required literal. An attribute
+  failure distinguishes an undeclared attribute from a declared one with a bad
+  value, quoting the value and naming its type. `RelaxNG.validation()` exposes the schema
   as a `Validation<Node, Void>`, so RELAX NG composes with the same validation
   framework as the XSD, DTD, and Schematron rules. The boolean engine remains the
   authority on validity; the walk only places and explains the failures.
diff --git a/Sources/Schema/RelaxNGDiagnostics.swift b/Sources/Schema/RelaxNGDiagnostics.swift
index c93ed63..fbabd61 100644
--- a/Sources/Schema/RelaxNGDiagnostics.swift
+++ b/Sources/Schema/RelaxNGDiagnostics.swift
@@ -37,10 +37,7 @@ extension PureXML.Schema.RelaxNGEngine {
         for attribute in element.attributes where !Self.isNamespaceDeclaration(attribute) {
             let next = attributeDeriv(residual, attribute)
             if isNotAllowed(next) {
-                errors.append(.init(
-                    reason: "attribute @\(attribute.name.description) is not allowed on <\(name)> or has an invalid value",
-                    at: path + [.attribute(attribute.name.description)],
-                ))
+                errors.append(.init(reason: attributeDetail(residual, attribute, on: name), at: path + [.attribute(attribute.name.description)]))
             } else {
                 residual = next
             }
@@ -115,6 +112,32 @@ extension PureXML.Schema.RelaxNGEngine {
         return (forceClose(opened), false)
     }
 
+    /// Explains an attribute failure: an undeclared attribute, or, when the name
+    /// is declared, why its value is invalid (reusing the datatype detail).
+    private func attributeDetail(_ residual: Pattern, _ attribute: PureXML.Model.Attribute, on name: String) -> String {
+        guard let content = attributeContent(residual, attribute.name, visiting: []) else {
+            return "attribute @\(attribute.name.description) is not allowed on <\(name)>"
+        }
+        return "attribute @\(attribute.name.description) on <\(name)>: \(textDetail(content, attribute.value))"
+    }
+
+    /// The value pattern of a declared attribute whose name matches, or nil when
+    /// no attribute of that name is permitted at this point.
+    private func attributeContent(_ pattern: Pattern, _ wanted: PureXML.Model.QualifiedName, visiting: Set<String>) -> Pattern? {
+        switch pattern {
+        case let .attribute(nameClass, content):
+            nameClass.contains(wanted) ? content : nil
+        case let .choice(lhs, rhs), let .group(lhs, rhs), let .interleave(lhs, rhs):
+            attributeContent(lhs, wanted, visiting: visiting) ?? attributeContent(rhs, wanted, visiting: visiting)
+        case let .after(lhs, _), let .oneOrMore(lhs):
+            attributeContent(lhs, wanted, visiting: visiting)
+        case let .ref(reference):
+            visiting.contains(reference) ? nil : attributeContent(resolve(reference), wanted, visiting: visiting.union([reference]))
+        default:
+            nil
+        }
+    }
+
     /// Explains why `value` does not satisfy the text/datatype `pattern`, quoting
     /// the offending value and naming the datatype it failed (`'abc' is not a
     /// valid integer`) or the literal it should have matched, falling back to a
diff --git a/Tests/RelaxNGLocatedErrorTests.swift b/Tests/RelaxNGLocatedErrorTests.swift
index 4c652e8..a758b33 100644
--- a/Tests/RelaxNGLocatedErrorTests.swift
+++ b/Tests/RelaxNGLocatedErrorTests.swift
@@ -78,9 +78,25 @@ struct RelaxNGLocatedErrorTests {
         let found = try errors(rng, "<n count='abc'/>")
         #expect(found.count == 1)
         #expect(found.first?.reason.contains("@count") == true)
+        // The declared attribute's bad value is quoted and the type named.
+        #expect(found.first?.reason.contains("'abc'") == true)
+        #expect(found.first?.reason.contains("integer") == true)
         #expect(found.first?.codingPath.map(\.stringValue) == ["n", "@count"])
     }
 
+    @Test("An undeclared attribute is distinguished from a bad value")
+    func test_undeclaredAttribute() throws {
+        let rng = """
+        <element name="n" xmlns="http://relaxng.org/ns/structure/1.0">
+          <attribute name="a"><text/></attribute>
+        </element>
+        """
+        let found = try errors(rng, "<n a='x' b='y'/>")
+        #expect(found.count == 1)
+        #expect(found.first?.reason.contains("@b") == true)
+        #expect(found.first?.reason.contains("is not allowed") == true)
+    }
+
     @Test("Invalid datatype content is located on the element")
     func test_dataValue() throws {
         let rng = """