diff --git a/CHANGELOG.md b/CHANGELOG.md
index 052ddf7..3b2d556 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -28,6 +28,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- A non-nillable element may now carry no `xsi:nil` attribute at all, whatever its
+  value (cvc-elt.3.1). Previously only `xsi:nil="true"` on a non-nillable element
+  was rejected; `xsi:nil="false"` slipped through. `ComplexValidator.nilErrors` now
+  flags the presence of any `xsi:nil` on a non-nillable element, while a nillable
+  element still accepts `xsi:nil="false"` (explicitly not nilled). XSTS
+  invalid-instances-accepted 28 -> 26, no other bucket moved (valid-rejected held
+  at 0 across all 14383 groups).
+
 - An element with neither a `type` attribute nor an inline type, but a
   `substitutionGroup`, now takes the type definition of its head (XSD 1.0 3.3.2)
   instead of defaulting to `anyType`. `XSDParser.elementType` resolves the head
diff --git a/Sources/Schema/ComplexValidator.swift b/Sources/Schema/ComplexValidator.swift
index 0923936..7288a36 100644
--- a/Sources/Schema/ComplexValidator.swift
+++ b/Sources/Schema/ComplexValidator.swift
@@ -86,11 +86,14 @@ public extension PureXML.Schema {
         /// element is not nillable, or when it carries content. Returns nil when
         /// the element is not nilled.
         func nilErrors(_ element: PureXML.Model.Element, at path: XSDPath) -> [XSDFailure]? {
-            guard Self.isNil(element) else { return nil }
             let name = element.name.localName
-            if !nillableElements.contains(name) {
+            // cvc-elt.3.1: a non-nillable element must carry no `xsi:nil` attribute at
+            // all, whatever its value (so `xsi:nil="false"` is equally forbidden, not
+            // only `xsi:nil="true"`).
+            if Self.nilAttributeValue(element) != nil, !nillableElements.contains(name) {
                 return [XSDFailure(reason: "element '\(name)' is not nillable but has xsi:nil", at: path)]
             }
+            guard Self.isNil(element) else { return nil }
             if Self.hasContent(element) {
                 return [XSDFailure(reason: "nil element '\(name)' must be empty", at: path)]
             }
diff --git a/Sources/Schema/ComplexValidatorHelpers.swift b/Sources/Schema/ComplexValidatorHelpers.swift
index 3c98ea3..5aad85a 100644
--- a/Sources/Schema/ComplexValidatorHelpers.swift
+++ b/Sources/Schema/ComplexValidatorHelpers.swift
@@ -185,11 +185,17 @@ extension PureXML.Schema.ComplexValidator {
 extension PureXML.Schema.ComplexValidator {
     /// Whether the element carries `xsi:nil="true"`.
     static func isNil(_ element: PureXML.Model.Element) -> Bool {
-        element.attributes.contains { attribute in
+        nilAttributeValue(element) == "true"
+    }
+
+    /// The value of the `xsi:nil` attribute if present (any value), or nil if the
+    /// element carries no `xsi:nil`. Used for the cvc-elt.3.1 rule that a
+    /// non-nillable element must carry no `xsi:nil` attribute at all.
+    static func nilAttributeValue(_ element: PureXML.Model.Element) -> String? {
+        element.attributes.first { attribute in
             attribute.name.localName == "nil"
                 && (attribute.name.namespaceURI == "http://www.w3.org/2001/XMLSchema-instance" || attribute.name.prefix == "xsi")
-                && attribute.value == "true"
-        }
+        }?.value
     }
 
     /// Whether the element has any child element or non-whitespace text.
diff --git a/Tests/XSDCategoryTests.swift b/Tests/XSDCategoryTests.swift
index 0387b12..cba2278 100644
--- a/Tests/XSDCategoryTests.swift
+++ b/Tests/XSDCategoryTests.swift
@@ -90,6 +90,18 @@ struct XSDCategoryTests {
         #expect(validate(element("a", [xsi]), type, validator: nillable).isEmpty)
     }
 
+    @Test("Nillable: xsi:nil='false' is also forbidden on a non-nillable element (cvc-elt.3.1)")
+    func test_notNillableNilFalse() {
+        let nilFalse = ("xsi:nil", "false")
+        let type = Schema.ComplexType(content: .empty)
+        // cvc-elt.3.1: a non-nillable element must carry no xsi:nil attribute at all,
+        // so even xsi:nil="false" is rejected (not only "true").
+        #expect(validate(element("a", [nilFalse]), type).contains { $0.reason.contains("not nillable") })
+        // A nillable element may carry xsi:nil="false" (explicitly not nilled): accepted.
+        let nillable = Schema.ComplexValidator(nillableElements: ["a"])
+        #expect(validate(element("a", [nilFalse]), type, validator: nillable).isEmpty)
+    }
+
     @Test("Fixed: an element fixed value must match")
     func test_fixedElement() {
         let validator = Schema.ComplexValidator(elementConstraints: ["a": .fixed("yes")])
diff --git a/Tests/XSTSSuiteTests.swift b/Tests/XSTSSuiteTests.swift
index a03fd5c..f7f0873 100644
--- a/Tests/XSTSSuiteTests.swift
+++ b/Tests/XSTSSuiteTests.swift
@@ -150,13 +150,13 @@ struct XSTSSuiteTests {
     private let knownSchemaValidRejected = 0
     private let knownSchemaInvalidAccepted = 43
     private let knownInstanceValidRejected = 0
-    /// Instance bucket (133 -> 28), per-step deltas in CHANGELOG.md: xsi:type must derive from the
+    /// Instance bucket (133 -> 26), per-step deltas in CHANGELOG.md: xsi:type must derive from the
     /// declared type (complex/atomic/list/union via the element-ref chain); anyType cannot stand in
-    /// for an anySimpleType element; an untyped substitutionGroup member inherits its head's type;
-    /// block/blockDefault bar substitutions on the namespace-keyed backbone; ur-type anyType wildcards
-    /// are `lax`; identity fields compare in decimal value space, taking a declared default/fixed when
+    /// for anySimpleType; an untyped substitutionGroup member inherits its head's type; a non-nillable
+    /// element carries no xsi:nil (cvc-elt.3.1); block/blockDefault bar substitutions; ur-type anyType
+    /// wildcards are `lax`; identity fields compare in decimal value space, taking a default/fixed when
     /// absent/empty; an attribute wildcard is the INTERSECTION of its sources; year 0000 is invalid.
-    private let knownInstanceInvalidAccepted = 28
+    private let knownInstanceInvalidAccepted = 26
 
     @Test("Every XSTS case behaves: compile, reject, validate, invalidate")
     func test_suite() throws {
diff --git a/docs/plans/2026-06-18-false-negative-tail.md b/docs/plans/2026-06-18-false-negative-tail.md
index 8e57474..70b6d39 100644
--- a/docs/plans/2026-06-18-false-negative-tail.md
+++ b/docs/plans/2026-06-18-false-negative-tail.md
@@ -91,7 +91,7 @@ classification is wrong.
 |---|---|---|
 | Target namespace / form matching | `targetns00101m/targetNS00101m1_n` | `ComplexValidator*`, reference resolution |
 | Disallowed substitution | `disallowedsubst00105m/disallowedSubst00105m1_n`, `disallowedsubst00106m2/Negative` | `XSDSubstitutionMembers`, `ComplexValidatorXSIType` |
-| Nillability | `nillable00201m/nillable00201m2_n` | `XSDParserHelpers`, instance validator |
+| Nillability | ~~`nillable00201m/nillable00201m2_n`~~ closed: a non-nillable element carries no `xsi:nil` at all, even `"false"` (cvc-elt.3.1) | `ComplexValidator.nilErrors` |
 | Type derivation | ~~`typedef00204m/typeDef00204m1_n`~~ closed: an untyped `substitutionGroup` member inherits its head's type (XSD 1.0 3.3.2) | `XSDParser.elementType` |
 | Additional-suite instance triage | `addB065/addB065.i` | to classify from fixture |
 | Datatype value-space edges | `anyURI_a004_1339/anyURI_a004_1339.i` (open); ~~`dateTime011_2008/dateTime011_2008.i`~~ closed: XSD 1.0 has no year zero, `0000` now rejected | `SimpleType`, `DateTimeParser` |