diff --git a/Dummies.UnitTests/AnyCollectionTests.cs b/Dummies.UnitTests/AnyCollectionTests.cs
index 6b1e2de8..6fac877c 100644
--- a/Dummies.UnitTests/AnyCollectionTests.cs
+++ b/Dummies.UnitTests/AnyCollectionTests.cs
@@ -158,9 +158,8 @@ public void ContainingOutsideDomainExtendsCardinality() {
         }
 
         // The same reasoning holds for a distinct list and for several out-of-domain values at once. Dictionary keys
-        // run through the very same CollectionState path, so the correction reaches them too — but AnyDictionary has
-        // no Containing surface, so its keys are gated purely by count (see DictionaryOfBehaves) and cannot exercise
-        // the out-of-domain case directly.
+        // run through the very same CollectionState path, so the correction reaches them too, now exercised directly
+        // through AnyDictionary.ContainingKey (see DictionaryContainingKeyOutsideDomainExtendsCardinality).
         for (int i = 0; i < SampleCount; i++) {
             HashSet<int> list = new(Any.ListOf(Any.Int32().OneOf(1, 2)).Containing(3).WithCount(3).Distinct().Generate());
             Check.That(list).Contains(1, 2, 3);
@@ -321,6 +320,37 @@ public void DictionaryOfBehaves() {
         Check.ThatCode(() => Any.DictionaryOf<int, int>(null!, Any.Int32())).Throws<ArgumentNullException>();
     }
 
+    [Fact(DisplayName = "ContainingKey: a key outside the key domain extends the effective cardinality (issue #225).")]
+    public void DictionaryContainingKeyOutsideDomainExtendsCardinality() {
+        // Mirrors ContainingOutsideDomainExtendsCardinality on the dictionary surface: {1, 2} is all the key
+        // generator can produce, and 3 is supplied directly from outside that domain, so a three-entry dictionary is
+        // satisfiable — the out-of-domain cardinality-credit path AnyDictionary could not exercise before.
+        for (int i = 0; i < SampleCount; i++) {
+            Dictionary<int, string> dictionary =
+                Any.DictionaryOf(Any.Int32().OneOf(1, 2), Any.String().NonEmpty()).ContainingKey(3).WithCount(3).Generate();
+            Check.That(dictionary.Keys).Contains(1, 2, 3);
+            Check.That(dictionary.Count).IsEqualTo(3);
+            Check.That(dictionary.Values).ContainsOnlyElementsThatMatch(value => value.Length > 0);
+        }
+    }
+
+    [Fact(DisplayName = "ContainingKey: a within-domain key is present, an out-of-capacity one still conflicts eagerly.")]
+    public void DictionaryContainingKeyInsideDomainDoesNotInflate() {
+        // A within-domain fixed key is present and adds no capacity of its own.
+        for (int i = 0; i < SampleCount; i++) {
+            Dictionary<int, string> dictionary =
+                Any.DictionaryOf(Any.Int32().Between(1, 9), Any.String().NonEmpty()).WithCount(5).ContainingKey(7).Generate();
+            Check.That(dictionary.ContainsKey(7)).IsTrue();
+            Check.That(dictionary.Count).IsEqualTo(5);
+        }
+
+        // 1 is already producible, so three distinct keys over {1, 2} remain impossible and still fail eagerly,
+        // naming the shortfall — exactly as ContainingInsideDomainDoesNotInflate asserts for a set.
+        ConflictingAnyConstraintException conflict = Assert.Throws<ConflictingAnyConstraintException>(
+            () => Any.DictionaryOf(Any.Int32().OneOf(1, 2), Any.String().NonEmpty()).ContainingKey(1).WithCount(3));
+        Check.That(conflict.Message).Contains("2 distinct value");
+    }
+
     [Fact(DisplayName = "PairOf and TripleOf assemble value tuples from constrained parts.")]
     public void PairAndTriple() {
         for (int i = 0; i < SampleCount; i++) {
diff --git a/Dummies/AnyDictionary.cs b/Dummies/AnyDictionary.cs
index c2f16240..38933ae1 100644
--- a/Dummies/AnyDictionary.cs
+++ b/Dummies/AnyDictionary.cs
@@ -80,6 +80,21 @@ public AnyDictionary<TKey, TValue> WithCountBetween(int minimum, int maximum) {
         return With(CountConstraints.WithCountBetween(_keys, minimum, maximum));
     }
 
+    /// <summary>
+    ///     Requires the dictionary to contain an entry for <paramref name="key" />. May be declared several times;
+    ///     each required key takes one entry's room and the required keys must be distinct. A key outside the key
+    ///     generator's domain extends the effective cardinality exactly as <see cref="AnySet{T}" />'s containment
+    ///     does, so an otherwise impossible entry count becomes reachable; the entry's value is generated like any
+    ///     other. Named <c>ContainingKey</c> rather than a bare <c>Containing</c> so the surface reads unambiguously
+    ///     on a dictionary, whose elements are key/value pairs.
+    /// </summary>
+    /// <param name="key">The key the generated dictionary must contain.</param>
+    /// <returns>A new generator carrying the added constraint.</returns>
+    /// <exception cref="ConflictingAnyConstraintException">Thrown when the constraint contradicts a constraint already declared.</exception>
+    public AnyDictionary<TKey, TValue> ContainingKey(TKey key) {
+        return With(_keys.WithContaining(key, $"ContainingKey({AnyDerivation.Display(key)})"));
+    }
+
     /// <inheritdoc />
     public Dictionary<TKey, TValue> Generate() {
         List<TKey>                 keys       = _keys.Materialize(_source ?? AmbientRandomSource.Instance);
diff --git a/Dummies/PublicAPI/net8.0/PublicAPI.Unshipped.txt b/Dummies/PublicAPI/net8.0/PublicAPI.Unshipped.txt
index 2d64bbdb..913325c5 100644
--- a/Dummies/PublicAPI/net8.0/PublicAPI.Unshipped.txt
+++ b/Dummies/PublicAPI/net8.0/PublicAPI.Unshipped.txt
@@ -116,6 +116,7 @@ Dummies.AnyDecimal.OneOf(params decimal[]! values) -> Dummies.AnyDecimal!
 Dummies.AnyDecimal.Positive() -> Dummies.AnyDecimal!
 Dummies.AnyDecimal.Zero() -> Dummies.AnyDecimal!
 Dummies.AnyDictionary<TKey, TValue>
+Dummies.AnyDictionary<TKey, TValue>.ContainingKey(TKey key) -> Dummies.AnyDictionary<TKey, TValue>!
 Dummies.AnyDictionary<TKey, TValue>.Empty() -> Dummies.AnyDictionary<TKey, TValue>!
 Dummies.AnyDictionary<TKey, TValue>.Generate() -> System.Collections.Generic.Dictionary<TKey, TValue>!
 Dummies.AnyDictionary<TKey, TValue>.NonEmpty() -> Dummies.AnyDictionary<TKey, TValue>!
diff --git a/Dummies/PublicAPI/netstandard2.0/PublicAPI.Unshipped.txt b/Dummies/PublicAPI/netstandard2.0/PublicAPI.Unshipped.txt
index 9f341225..5bc982ea 100644
--- a/Dummies/PublicAPI/netstandard2.0/PublicAPI.Unshipped.txt
+++ b/Dummies/PublicAPI/netstandard2.0/PublicAPI.Unshipped.txt
@@ -101,6 +101,7 @@ Dummies.AnyDecimal.OneOf(params decimal[]! values) -> Dummies.AnyDecimal!
 Dummies.AnyDecimal.Positive() -> Dummies.AnyDecimal!
 Dummies.AnyDecimal.Zero() -> Dummies.AnyDecimal!
 Dummies.AnyDictionary<TKey, TValue>
+Dummies.AnyDictionary<TKey, TValue>.ContainingKey(TKey key) -> Dummies.AnyDictionary<TKey, TValue>!
 Dummies.AnyDictionary<TKey, TValue>.Empty() -> Dummies.AnyDictionary<TKey, TValue>!
 Dummies.AnyDictionary<TKey, TValue>.Generate() -> System.Collections.Generic.Dictionary<TKey, TValue>!
 Dummies.AnyDictionary<TKey, TValue>.NonEmpty() -> Dummies.AnyDictionary<TKey, TValue>!