diff --git a/UNDF-REGISTRY.json b/UNDF-REGISTRY.json
index 276beafe6..f188b1aa4 100644
--- a/UNDF-REGISTRY.json
+++ b/UNDF-REGISTRY.json
@@ -586,5 +586,6 @@
   "substrate-0003": "UNDF-2026-000000504",
   "erlang-0004": "UNDF-2026-000000528",
   "weld-0001": "UNDF-2026-000000530",
-  "weld-0002": "UNDF-2026-000000550"
+  "weld-0002": "UNDF-2026-000000550",
+  "weld-0003": "UNDF-2026-000000554"
 }
diff --git a/defects/weld/patch/weld-0003-merged-stereotypes-merge-diamond.md b/defects/weld/patch/weld-0003-merged-stereotypes-merge-diamond.md
new file mode 100644
index 000000000..8fe6e7377
--- /dev/null
+++ b/defects/weld/patch/weld-0003-merged-stereotypes-merge-diamond.md
@@ -0,0 +1,148 @@
+# UNDF: UNDF-2026-000000554
+# weld-0003: MergedStereotypes.merge — no visited set O(2^D) on diamond meta-stereotype hierarchy
+
+## CWE-407 — Algorithmic Complexity: Exponential Re-traversal on Diamond CDI Meta-Stereotype Hierarchy
+
+| Field        | Value |
+|--------------|-------|
+| ID           | weld-0003 |
+| Severity     | HIGH |
+| Ecosystem    | weld |
+| Package      | weld-impl |
+| File         | `impl/src/main/java/org/jboss/weld/metadata/cache/MergedStereotypes.java` |
+| Lines        | 75–98 |
+| Complexity   | O(2^D) where D = diamond depth in CDI meta-stereotype annotation hierarchy |
+| Hot path     | Called at CDI container startup via BeanAttributesFactory for every bean type |
+
+## Defect
+
+`MergedStereotypes.merge()` traverses the CDI meta-stereotype hierarchy recursively
+with **no visited-set guard**. It iterates over stereotype annotations, and for each
+one calls `merge(stereotype.getInheritedStereotypes())` unconditionally — without
+first checking whether that stereotype type has already been processed in the current
+traversal.
+
+`this.stereotypes.add(stereotypeAnnotation.annotationType())` at line 95 accumulates
+the output set, but the **return value is ignored** — the code never checks whether
+the type was already present before recursing at line 97.
+
+On a diamond-shaped meta-stereotype hierarchy (bean annotated with @StA; @StA
+inherits from @StB and @StC; both @StB and @StC inherit from @StBase), the traversal
+visits @StBase twice:
+
+```
+merge({@StA})
+  → process @StA: merge(getInheritedStereotypes(@StA)) = merge({@StB, @StC})
+    → process @StB: merge(getInheritedStereotypes(@StB)) = merge({@StBase})
+      → process @StBase: (no children)
+    → process @StC: merge(getInheritedStereotypes(@StC)) = merge({@StBase})
+      → process @StBase: VISITED AGAIN — no guard
+```
+
+At depth D: the base stereotype is visited 2^D times.
+
+```java
+// impl/src/main/java/org/jboss/weld/metadata/cache/MergedStereotypes.java:75-98 (DEFECT)
+protected void merge(Set<Annotation> stereotypeAnnotations) {
+    final MetaAnnotationStore store = manager.getServices().get(MetaAnnotationStore.class);
+    for (Annotation stereotypeAnnotation : stereotypeAnnotations) {
+        StereotypeModel<?> stereotype = store.getStereotype(stereotypeAnnotation.annotationType());
+        if (stereotype == null) {
+            throw MetadataLogger.LOG.stereotypeNotRegistered(stereotypeAnnotation);
+        }
+        if (stereotype.isAlternative()) {
+            alternative = true;
+        }
+        if (stereotype.isReserve()) {
+            reserve = true;
+        }
+        if (stereotype.getDefaultScopeType() != null) {
+            possibleScopeTypes.add(stereotype.getDefaultScopeType());
+        }
+        if (stereotype.isBeanNameDefaulted()) {
+            beanNameDefaulted = true;
+        }
+        this.stereotypes.add(stereotypeAnnotation.annotationType());  // return value IGNORED
+        // Merge in inherited stereotypes
+        merge(stereotype.getInheritedStereotypes());  // NO visited guard
+    }
+}
+```
+
+Called from `BeanAttributesFactory.BeanAttributesBuilder.initStereotypes()` for every
+CDI-managed bean at container startup. A new `MergedStereotypes` is constructed per
+bean — results are NOT shared — so the exponential work is repeated for every bean
+that carries a diamond meta-stereotype chain.
+
+## Fix
+
+Add a `Set<Class<? extends Annotation>> visited` field to guard re-traversal:
+
+```java
+// AFTER — O(N+E): visited set prevents exponential re-traversal
+public class MergedStereotypes<T, E> {
+    // ... existing fields ...
+    private final Set<Class<? extends Annotation>> visited = new HashSet<>();
+
+    protected void merge(Set<Annotation> stereotypeAnnotations) {
+        final MetaAnnotationStore store = manager.getServices().get(MetaAnnotationStore.class);
+        for (Annotation stereotypeAnnotation : stereotypeAnnotations) {
+            // O(1): skip if already processed — prevents O(2^D) diamond re-traversal
+            if (!visited.add(stereotypeAnnotation.annotationType())) {
+                continue;
+            }
+            StereotypeModel<?> stereotype = store.getStereotype(stereotypeAnnotation.annotationType());
+            if (stereotype == null) {
+                throw MetadataLogger.LOG.stereotypeNotRegistered(stereotypeAnnotation);
+            }
+            if (stereotype.isAlternative()) {
+                alternative = true;
+            }
+            if (stereotype.isReserve()) {
+                reserve = true;
+            }
+            if (stereotype.getDefaultScopeType() != null) {
+                possibleScopeTypes.add(stereotype.getDefaultScopeType());
+            }
+            if (stereotype.isBeanNameDefaulted()) {
+                beanNameDefaulted = true;
+            }
+            this.stereotypes.add(stereotypeAnnotation.annotationType());
+            merge(stereotype.getInheritedStereotypes());
+        }
+    }
+}
+```
+
+## Speedup
+
+| Diamond depth (D) | Before (merge() calls) | After (merge() calls) | Speedup |
+|------------------|------------------------|----------------------|---------|
+| 5                | 31                     | 5                    | 6×      |
+| 10               | 1,023                  | 10                   | 102×    |
+| 15               | 32,767                 | 15                   | 2,184×  |
+| 20               | 1,048,575              | 20                   | 52,428× |
+
+Growth before: O(2^D). Growth after: O(D).
+
+## Relationship to Existing Weld Defects
+
+This is a third independent defect site in weld-impl:
+
+| ID        | File                              | Method                              |
+|-----------|-----------------------------------|-------------------------------------|
+| weld-0001 | `util/Beans.java`                 | `recursiveStereotypeSearch()`       |
+| weld-0002 | `util/Interceptors.java`          | `addInheritedInterceptorBindings()` |
+| weld-0003 | `metadata/cache/MergedStereotypes.java` | `merge()`                     |
+
+All three are triggered at CDI container startup. Each operates on different
+annotation hierarchy types (stereotypes for 0001/0003, interceptor bindings for 0002)
+through different code paths.
+
+## Impact
+
+Weld is the CDI reference implementation, used by WildFly, GlassFish, Payara, and
+as the CDI layer in Quarkus. Diamond meta-stereotype hierarchies arise in enterprise
+patterns: shared `@ApplicationScoped @Transactional @Named` base stereotypes extended
+by multiple domain-specific stereotypes. A D=15 diamond incurs 32,767 redundant
+`merge()` calls per bean at startup.