diff --git a/src/tools/fuzzing/heap-types.cpp b/src/tools/fuzzing/heap-types.cpp
index 3bc74eb0f1a..af8e235e442 100644
--- a/src/tools/fuzzing/heap-types.cpp
+++ b/src/tools/fuzzing/heap-types.cpp
@@ -19,7 +19,6 @@
 
 #include "ir/gc-type-utils.h"
 #include "ir/subtypes.h"
-#include "support/insert_ordered.h"
 #include "tools/fuzzing.h"
 #include "tools/fuzzing/heap-types.h"
 
@@ -275,12 +274,13 @@ struct HeapTypeGeneratorImpl {
       builder[index].setOpen(subtypeIndices[index].size() > 1 || rand.oneIn(2));
       auto kind = typeKinds[index];
       auto share = HeapType(builder[index]).getShared();
+      bool isDesc = describedIndices[index].has_value();
       if (!supertypeIndices[index]) {
         // No nontrivial supertype, so create a root type.
         if (std::get_if<SignatureKind>(&kind)) {
           builder[index] = generateSignature();
         } else if (std::get_if<StructKind>(&kind)) {
-          builder[index] = generateStruct(share);
+          builder[index] = generateStruct(share, isDesc);
         } else if (std::get_if<ArrayKind>(&kind)) {
           builder[index] = generateArray(share);
         } else {
@@ -415,7 +415,13 @@ struct HeapTypeGeneratorImpl {
     return {params, generateReturnType()};
   }
 
-  Field generateField(Shareability share) {
+  Field generateField(Shareability share, bool isPrototypeField = false) {
+    // If this field could configure a prototype, then we want to give it a type
+    // that lets it do so a significant portion of the time.
+    if (isPrototypeField && share == Unshared && rand.oneIn(2)) {
+      auto nullability = rand.oneIn(2) ? NonNullable : Nullable;
+      return {Type(HeapType::ext, nullability), Immutable};
+    }
     auto mutability = rand.oneIn(2) ? Mutable : Immutable;
     if (rand.oneIn(6)) {
       return {rand.oneIn(2) ? Field::i8 : Field::i16, mutability};
@@ -424,10 +430,13 @@ struct HeapTypeGeneratorImpl {
     }
   }
 
-  Struct generateStruct(Shareability share) {
+  Struct generateStruct(Shareability share, bool isDesc) {
     std::vector<Field> fields(rand.upTo(params.MAX_STRUCT_SIZE + 1));
+    // Prototypes are configured on the first field of descriptors types.
+    bool isPrototypeField = isDesc;
     for (auto& field : fields) {
-      field = generateField(share);
+      field = generateField(share, isPrototypeField);
+      isPrototypeField = false;
     }
     return {fields};
   }
diff --git a/test/lit/fuzz-types.test b/test/lit/fuzz-types.test
index 840fc1bb36b..44d475f6e3d 100644
--- a/test/lit/fuzz-types.test
+++ b/test/lit/fuzz-types.test
@@ -1,64 +1,56 @@
-;; RUN: wasm-fuzz-types -v --seed=0 | filecheck %s
+;; RUN: wasm-fuzz-types -v --seed=2 | filecheck %s
 
-;; CHECK:      Running with seed 0
+;; CHECK:      Running with seed 2
 ;; CHECK-NEXT: Built 20 types:
-;; CHECK-NEXT: (rec
-;; CHECK-NEXT:  (type $0 (sub (shared (array i8))))
-;; CHECK-NEXT:  (type $1 (shared (array (ref $1))))
-;; CHECK-NEXT:  (type $2 (sub (shared (func (param i64 (ref null $5) i64) (result (ref $2))))))
-;; CHECK-NEXT:  (type $3 (sub $0 (shared (array i8))))
-;; CHECK-NEXT:  (type $4 (sub (array (ref null $9))))
-;; CHECK-NEXT:  (type $5 (sub final $0 (shared (array i8))))
-;; CHECK-NEXT:  (type $6 (sub $2 (shared (func (param i64 (ref null (shared eq)) i64) (result (ref $8))))))
-;; CHECK-NEXT:  (type $7 (sub (shared (descriptor $9) (struct (field (ref null $7)) (field (mut i64)) (field (mut i8)) (field i32) (field (mut i32))))))
-;; CHECK-NEXT:  (type $8 (sub $2 (shared (func (param i64 (ref null $0) i64) (result (ref $8))))))
-;; CHECK-NEXT:  (type $9 (shared (describes $7) (descriptor $12) (struct (field i32) (field (mut (ref $8))) (field f64))))
-;; CHECK-NEXT:  (type $10 (sub (array (mut externref))))
-;; CHECK-NEXT:  (type $11 (sub $4 (array (ref $9))))
-;; CHECK-NEXT:  (type $12 (sub (shared (describes $9) (struct (field (mut f64)) (field (mut i32)) (field (mut f64))))))
-;; CHECK-NEXT: )
-;; CHECK-NEXT: (rec
-;; CHECK-NEXT:  (type $13 (sub (struct (field (mut i16)) (field i64) (field (mut (ref (shared func)))))))
-;; CHECK-NEXT:  (type $14 (sub (shared (array (ref null $14)))))
-;; CHECK-NEXT: )
-;; CHECK-NEXT: (rec
-;; CHECK-NEXT:  (type $15 (shared (array (ref null (shared extern)))))
-;; CHECK-NEXT:  (type $16 (sub final $14 (shared (array (ref null $16)))))
+;; CHECK-NEXT: (type $0 (shared (struct)))
+;; CHECK-NEXT: (rec
+;; CHECK-NEXT:  (type $1 (array (ref $2)))
+;; CHECK-NEXT:  (type $2 (sub (shared (array (mut i16)))))
+;; CHECK-NEXT:  (type $3 (sub (shared (array i32))))
+;; CHECK-NEXT:  (type $4 (sub (descriptor $5) (struct (field (mut (ref $0))))))
+;; CHECK-NEXT:  (type $5 (sub (describes $4) (struct (field f64) (field (mut i64)))))
+;; CHECK-NEXT:  (type $6 (sub (array v128)))
+;; CHECK-NEXT:  (type $7 (shared (struct (field f32) (field (mut (ref $0))))))
+;; CHECK-NEXT:  (type $8 (sub (shared (struct (field f64) (field (mut (ref (shared struct)))) (field (mut f64)) (field i16) (field i32) (field i64)))))
 ;; CHECK-NEXT: )
 ;; CHECK-NEXT: (rec
-;; CHECK-NEXT:  (type $17 (shared (struct (field (mut (ref null $2))) (field i32) (field (mut (ref $14))))))
-;; CHECK-NEXT:  (type $18 (sub $11 (array (ref $9))))
-;; CHECK-NEXT:  (type $19 (sub $2 (shared (func (param i64 (ref null $0) i64) (result (ref $6))))))
+;; CHECK-NEXT:  (type $9 (descriptor $12) (struct (field i64) (field i16)))
+;; CHECK-NEXT:  (type $10 (array (mut (ref null $5))))
+;; CHECK-NEXT:  (type $11 (sub (shared (func (param (ref $7) f64 (ref $9)) (result (ref null $10))))))
+;; CHECK-NEXT:  (type $12 (sub (describes $9) (descriptor $13) (struct (field (ref (shared any))) (field (mut (ref extern))) (field v128) (field (ref null $17)))))
+;; CHECK-NEXT:  (type $13 (sub (describes $12) (descriptor $17) (struct (field externref) (field (mut i8)) (field (mut i32)) (field (mut f32)) (field i16) (field (mut (ref null $6))))))
+;; CHECK-NEXT:  (type $14 (sub (func (result i64))))
+;; CHECK-NEXT:  (type $15 (sub (shared (func))))
+;; CHECK-NEXT:  (type $16 (shared (func (result (ref null $0)))))
+;; CHECK-NEXT:  (type $17 (sub (describes $13) (struct (field (ref extern)))))
+;; CHECK-NEXT:  (type $18 (sub (func (param v128 (ref null $10)))))
+;; CHECK-NEXT:  (type $19 (sub final $11 (shared (func (param (ref null (shared any)) f64 (ref any)) (result (ref $10))))))
 ;; CHECK-NEXT: )
 ;; CHECK-NEXT: 
 ;; CHECK-NEXT: Inhabitable types:
 ;; CHECK-NEXT: 
 ;; CHECK-NEXT: Built 20 types:
-;; CHECK-NEXT: (rec
-;; CHECK-NEXT:  (type $0 (sub (shared (array i8))))
-;; CHECK-NEXT:  (type $1 (shared (array (ref null $1))))
-;; CHECK-NEXT:  (type $2 (sub (shared (func (param i64 (ref null $5) i64) (result (ref $2))))))
-;; CHECK-NEXT:  (type $3 (sub $0 (shared (array i8))))
-;; CHECK-NEXT:  (type $4 (sub (array (ref null $9))))
-;; CHECK-NEXT:  (type $5 (sub final $0 (shared (array i8))))
-;; CHECK-NEXT:  (type $6 (sub $2 (shared (func (param i64 (ref null (shared eq)) i64) (result (ref $8))))))
-;; CHECK-NEXT:  (type $7 (sub (shared (descriptor $9) (struct (field (ref null $7)) (field (mut i64)) (field (mut i8)) (field i32) (field (mut i32))))))
-;; CHECK-NEXT:  (type $8 (sub $2 (shared (func (param i64 (ref null $0) i64) (result (ref $8))))))
-;; CHECK-NEXT:  (type $9 (shared (describes $7) (descriptor $12) (struct (field i32) (field (mut (ref $8))) (field f64))))
-;; CHECK-NEXT:  (type $10 (sub (array (mut externref))))
-;; CHECK-NEXT:  (type $11 (sub $4 (array (ref $9))))
-;; CHECK-NEXT:  (type $12 (sub (shared (describes $9) (struct (field (mut f64)) (field (mut i32)) (field (mut f64))))))
-;; CHECK-NEXT: )
-;; CHECK-NEXT: (rec
-;; CHECK-NEXT:  (type $13 (sub (struct (field (mut i16)) (field i64) (field (mut (ref (shared func)))))))
-;; CHECK-NEXT:  (type $14 (sub (shared (array (ref null $14)))))
+;; CHECK-NEXT: (type $0 (shared (struct)))
+;; CHECK-NEXT: (rec
+;; CHECK-NEXT:  (type $1 (array (ref $2)))
+;; CHECK-NEXT:  (type $2 (sub (shared (array (mut i16)))))
+;; CHECK-NEXT:  (type $3 (sub (shared (array i32))))
+;; CHECK-NEXT:  (type $4 (sub (descriptor $5) (struct (field (mut (ref $0))))))
+;; CHECK-NEXT:  (type $5 (sub (describes $4) (struct (field f64) (field (mut i64)))))
+;; CHECK-NEXT:  (type $6 (sub (array v128)))
+;; CHECK-NEXT:  (type $7 (shared (struct (field f32) (field (mut (ref $0))))))
+;; CHECK-NEXT:  (type $8 (sub (shared (struct (field f64) (field (mut (ref (shared struct)))) (field (mut f64)) (field i16) (field i32) (field i64)))))
 ;; CHECK-NEXT: )
 ;; CHECK-NEXT: (rec
-;; CHECK-NEXT:  (type $15 (shared (array (ref null (shared extern)))))
-;; CHECK-NEXT:  (type $16 (sub final $14 (shared (array (ref null $16)))))
+;; CHECK-NEXT:  (type $9 (descriptor $12) (struct (field i64) (field i16)))
+;; CHECK-NEXT:  (type $10 (array (mut (ref null $5))))
+;; CHECK-NEXT:  (type $11 (sub (shared (func (param (ref $7) f64 (ref $9)) (result (ref null $10))))))
+;; CHECK-NEXT:  (type $12 (sub (describes $9) (descriptor $13) (struct (field (ref (shared any))) (field (mut externref)) (field v128) (field (ref null $17)))))
+;; CHECK-NEXT:  (type $13 (sub (describes $12) (descriptor $17) (struct (field externref) (field (mut i8)) (field (mut i32)) (field (mut f32)) (field i16) (field (mut (ref null $6))))))
+;; CHECK-NEXT:  (type $14 (sub (func (result i64))))
+;; CHECK-NEXT:  (type $15 (sub (shared (func))))
+;; CHECK-NEXT:  (type $16 (shared (func (result (ref null $0)))))
+;; CHECK-NEXT:  (type $17 (sub (describes $13) (struct (field externref))))
+;; CHECK-NEXT:  (type $18 (sub (func (param v128 (ref null $10)))))
+;; CHECK-NEXT:  (type $19 (sub final $11 (shared (func (param (ref null (shared any)) f64 (ref any)) (result (ref $10))))))
 ;; CHECK-NEXT: )
-;; CHECK-NEXT: (rec
-;; CHECK-NEXT:  (type $17 (shared (struct (field (mut (ref null $2))) (field i32) (field (mut (ref $14))))))
-;; CHECK-NEXT:  (type $18 (sub $11 (array (ref $9))))
-;; CHECK-NEXT:  (type $19 (sub $2 (shared (func (param i64 (ref null $0) i64) (result (ref $6))))))
-;; CHECK-NEXT: )
\ No newline at end of file