diff --git a/hll/include/CouponList-internal.hpp b/hll/include/CouponList-internal.hpp
index 5b067a59..de48239f 100644
--- a/hll/include/CouponList-internal.hpp
+++ b/hll/include/CouponList-internal.hpp
@@ -33,7 +33,7 @@ namespace datasketches {
 
 template<typename A>
 CouponList<A>::CouponList(uint8_t lgConfigK, target_hll_type tgtHllType, hll_mode mode, const A& allocator):
-HllSketchImpl<A>(lgConfigK, tgtHllType, mode, false),
+HllSketchImpl<A>(lgConfigK, tgtHllType, mode),
 couponCount_(0),
 oooFlag_(false),
 coupons_(1ULL << (mode == hll_mode::LIST ? hll_constants::LG_INIT_LIST_SIZE : hll_constants::LG_INIT_SET_SIZE), 0, allocator)
@@ -41,7 +41,7 @@ coupons_(1ULL << (mode == hll_mode::LIST ? hll_constants::LG_INIT_LIST_SIZE : hl
 
 template<typename A>
 CouponList<A>::CouponList(const CouponList& that, const target_hll_type tgtHllType):
-HllSketchImpl<A>(that.lgConfigK_, tgtHllType, that.mode_, false),
+HllSketchImpl<A>(that.lgConfigK_, tgtHllType, that.mode_),
 couponCount_(that.couponCount_),
 oooFlag_(that.oooFlag_),
 coupons_(that.coupons_)
diff --git a/hll/include/Hll4Array-internal.hpp b/hll/include/Hll4Array-internal.hpp
index 082f168f..adfed74a 100644
--- a/hll/include/Hll4Array-internal.hpp
+++ b/hll/include/Hll4Array-internal.hpp
@@ -30,8 +30,8 @@
 namespace datasketches {
 
 template<typename A>
-Hll4Array<A>::Hll4Array(uint8_t lgConfigK, bool startFullSize, const A& allocator):
-HllArray<A>(lgConfigK, target_hll_type::HLL_4, startFullSize, allocator),
+Hll4Array<A>::Hll4Array(uint8_t lgConfigK, const A& allocator):
+HllArray<A>(lgConfigK, target_hll_type::HLL_4, allocator),
 auxHashMap_(nullptr)
 {
   const uint32_t numBytes = this->hll4ArrBytes(lgConfigK);
@@ -53,7 +53,7 @@ Hll4Array<A>::Hll4Array(const Hll4Array<A>& that) :
 
 template<typename A>
 Hll4Array<A>::Hll4Array(const HllArray<A>& other) :
-  HllArray<A>(other.getLgConfigK(), target_hll_type::HLL_4, other.isStartFullSize(), other.getAllocator()),
+  HllArray<A>(other.getLgConfigK(), target_hll_type::HLL_4, other.getAllocator()),
   auxHashMap_(nullptr)
 {
   const int numBytes = this->hll4ArrBytes(this->lgConfigK_);
diff --git a/hll/include/Hll4Array.hpp b/hll/include/Hll4Array.hpp
index 0e3e2cd8..7edfb051 100644
--- a/hll/include/Hll4Array.hpp
+++ b/hll/include/Hll4Array.hpp
@@ -28,7 +28,7 @@ namespace datasketches {
 template<typename A>
 class Hll4Array final : public HllArray<A> {
   public:
-    explicit Hll4Array(uint8_t lgConfigK, bool startFullSize, const A& allocator);
+    explicit Hll4Array(uint8_t lgConfigK, const A& allocator);
     explicit Hll4Array(const Hll4Array<A>& that);
     explicit Hll4Array(const HllArray<A>& that);
 
diff --git a/hll/include/Hll6Array-internal.hpp b/hll/include/Hll6Array-internal.hpp
index b8b4d6bb..34c72438 100644
--- a/hll/include/Hll6Array-internal.hpp
+++ b/hll/include/Hll6Array-internal.hpp
@@ -27,8 +27,8 @@
 namespace datasketches {
 
 template<typename A>
-Hll6Array<A>::Hll6Array(uint8_t lgConfigK, bool startFullSize, const A& allocator):
-HllArray<A>(lgConfigK, target_hll_type::HLL_6, startFullSize, allocator)
+Hll6Array<A>::Hll6Array(uint8_t lgConfigK, const A& allocator):
+HllArray<A>(lgConfigK, target_hll_type::HLL_6, allocator)
 {
   const int numBytes = this->hll6ArrBytes(lgConfigK);
   this->hllByteArr_.resize(numBytes, 0);
@@ -36,7 +36,7 @@ HllArray<A>(lgConfigK, target_hll_type::HLL_6, startFullSize, allocator)
 
 template<typename A>
 Hll6Array<A>::Hll6Array(const HllArray<A>& other) :
-  HllArray<A>(other.getLgConfigK(), target_hll_type::HLL_6, other.isStartFullSize(), other.getAllocator())
+  HllArray<A>(other.getLgConfigK(), target_hll_type::HLL_6, other.getAllocator())
 {
   const int numBytes = this->hll6ArrBytes(this->lgConfigK_);
   this->hllByteArr_.resize(numBytes, 0);
diff --git a/hll/include/Hll6Array.hpp b/hll/include/Hll6Array.hpp
index 4921976d..82286d87 100644
--- a/hll/include/Hll6Array.hpp
+++ b/hll/include/Hll6Array.hpp
@@ -30,7 +30,7 @@ class Hll6Iterator;
 template<typename A>
 class Hll6Array final : public HllArray<A> {
   public:
-    Hll6Array(uint8_t lgConfigK, bool startFullSize, const A& allocator);
+    Hll6Array(uint8_t lgConfigK, const A& allocator);
     explicit Hll6Array(const HllArray<A>& that);
 
     virtual ~Hll6Array() = default;
diff --git a/hll/include/Hll8Array-internal.hpp b/hll/include/Hll8Array-internal.hpp
index 14a2bbd7..5e4239ec 100644
--- a/hll/include/Hll8Array-internal.hpp
+++ b/hll/include/Hll8Array-internal.hpp
@@ -25,8 +25,8 @@
 namespace datasketches {
 
 template<typename A>
-Hll8Array<A>::Hll8Array(uint8_t lgConfigK, bool startFullSize, const A& allocator):
-HllArray<A>(lgConfigK, target_hll_type::HLL_8, startFullSize, allocator)
+Hll8Array<A>::Hll8Array(uint8_t lgConfigK, const A& allocator):
+HllArray<A>(lgConfigK, target_hll_type::HLL_8, allocator)
 {
   const int numBytes = this->hll8ArrBytes(lgConfigK);
   this->hllByteArr_.resize(numBytes, 0);
@@ -34,7 +34,7 @@ HllArray<A>(lgConfigK, target_hll_type::HLL_8, startFullSize, allocator)
 
 template<typename A>
 Hll8Array<A>::Hll8Array(const HllArray<A>& other):
-  HllArray<A>(other.getLgConfigK(), target_hll_type::HLL_8, other.isStartFullSize(), other.getAllocator())
+  HllArray<A>(other.getLgConfigK(), target_hll_type::HLL_8, other.getAllocator())
 {
   const int numBytes = this->hll8ArrBytes(this->lgConfigK_);
   this->hllByteArr_.resize(numBytes, 0);
diff --git a/hll/include/Hll8Array.hpp b/hll/include/Hll8Array.hpp
index 655baf9f..1630be4c 100644
--- a/hll/include/Hll8Array.hpp
+++ b/hll/include/Hll8Array.hpp
@@ -30,7 +30,7 @@ class Hll8Iterator;
 template<typename A>
 class Hll8Array final : public HllArray<A> {
   public:
-    Hll8Array(uint8_t lgConfigK, bool startFullSize, const A& allocator);
+    Hll8Array(uint8_t lgConfigK, const A& allocator);
     explicit Hll8Array(const HllArray<A>& that);
 
     virtual ~Hll8Array() = default;
diff --git a/hll/include/HllArray-internal.hpp b/hll/include/HllArray-internal.hpp
index 92dbe8b2..8a081a2e 100644
--- a/hll/include/HllArray-internal.hpp
+++ b/hll/include/HllArray-internal.hpp
@@ -35,8 +35,8 @@
 namespace datasketches {
 
 template<typename A>
-HllArray<A>::HllArray(uint8_t lgConfigK, target_hll_type tgtHllType, bool startFullSize, const A& allocator):
-HllSketchImpl<A>(lgConfigK, tgtHllType, hll_mode::HLL, startFullSize),
+HllArray<A>::HllArray(uint8_t lgConfigK, target_hll_type tgtHllType, const A& allocator):
+HllSketchImpl<A>(lgConfigK, tgtHllType, hll_mode::HLL),
 hipAccum_(0.0),
 kxq0_(1 << lgConfigK),
 kxq1_(0.0),
@@ -49,7 +49,7 @@ rebuild_kxq_curmin_(false)
 
 template<typename A>
 HllArray<A>::HllArray(const HllArray& other, target_hll_type tgtHllType) :
-  HllSketchImpl<A>(other.getLgConfigK(), tgtHllType, hll_mode::HLL, other.isStartFullSize()),
+  HllSketchImpl<A>(other.getLgConfigK(), tgtHllType, hll_mode::HLL),
   // remaining fields are initialized to empty sketch defaults
   // and left to subclass constructor to populate
   hipAccum_(0.0),
@@ -109,7 +109,6 @@ HllArray<A>* HllArray<A>::newHll(const void* bytes, size_t len, const A& allocat
   const target_hll_type tgtHllType = HllSketchImpl<A>::extractTgtHllType(data[hll_constants::MODE_BYTE]);
   const bool oooFlag = ((data[hll_constants::FLAGS_BYTE] & hll_constants::OUT_OF_ORDER_FLAG_MASK) ? true : false);
   const bool comapctFlag = ((data[hll_constants::FLAGS_BYTE] & hll_constants::COMPACT_FLAG_MASK) ? true : false);
-  const bool startFullSizeFlag = ((data[hll_constants::FLAGS_BYTE] & hll_constants::FULL_SIZE_FLAG_MASK) ? true : false);
 
   const uint8_t lgK = data[hll_constants::LG_K_BYTE];
   const uint8_t curMin = data[hll_constants::HLL_CUR_MIN_BYTE];
@@ -139,7 +138,7 @@ HllArray<A>* HllArray<A>::newHll(const void* bytes, size_t len, const A& allocat
     aux_ptr = aux_hash_map_ptr(auxHashMap, auxHashMap->make_deleter());
   }
 
-  HllArray<A>* sketch = HllSketchImplFactory<A>::newHll(lgK, tgtHllType, startFullSizeFlag, allocator);
+  HllArray<A>* sketch = HllSketchImplFactory<A>::newHll(lgK, tgtHllType, allocator);
   sketch->putCurMin(curMin);
   sketch->putOutOfOrderFlag(oooFlag);
   if (!oooFlag) { sketch->putHipAccum(hip); }
@@ -180,12 +179,11 @@ HllArray<A>* HllArray<A>::newHll(std::istream& is, const A& allocator) {
   const target_hll_type tgtHllType = HllSketchImpl<A>::extractTgtHllType(listHeader[hll_constants::MODE_BYTE]);
   const bool oooFlag = ((listHeader[hll_constants::FLAGS_BYTE] & hll_constants::OUT_OF_ORDER_FLAG_MASK) ? true : false);
   const bool comapctFlag = ((listHeader[hll_constants::FLAGS_BYTE] & hll_constants::COMPACT_FLAG_MASK) ? true : false);
-  const bool startFullSizeFlag = ((listHeader[hll_constants::FLAGS_BYTE] & hll_constants::FULL_SIZE_FLAG_MASK) ? true : false);
 
   const uint8_t lgK = listHeader[hll_constants::LG_K_BYTE];
   const uint8_t curMin = listHeader[hll_constants::HLL_CUR_MIN_BYTE];
 
-  HllArray* sketch = HllSketchImplFactory<A>::newHll(lgK, tgtHllType, startFullSizeFlag, allocator);
+  HllArray* sketch = HllSketchImplFactory<A>::newHll(lgK, tgtHllType, allocator);
   typedef std::unique_ptr<HllArray<A>, std::function<void(HllSketchImpl<A>*)>> hll_array_ptr;
   hll_array_ptr sketch_ptr(sketch, sketch->get_deleter());
   sketch->putCurMin(curMin);
diff --git a/hll/include/HllArray.hpp b/hll/include/HllArray.hpp
index 471a1402..87994cb6 100644
--- a/hll/include/HllArray.hpp
+++ b/hll/include/HllArray.hpp
@@ -33,7 +33,7 @@ class HllArray : public HllSketchImpl<A> {
   public:
     using vector_bytes = std::vector<uint8_t, typename std::allocator_traits<A>::template rebind_alloc<uint8_t>>;
 
-    HllArray(uint8_t lgConfigK, target_hll_type tgtHllType, bool startFullSize, const A& allocator);
+    HllArray(uint8_t lgConfigK, target_hll_type tgtHllType, const A& allocator);
     explicit HllArray(const HllArray& other, target_hll_type tgtHllType);
 
     static HllArray* newHll(const void* bytes, size_t len, const A& allocator);
diff --git a/hll/include/HllSketch-internal.hpp b/hll/include/HllSketch-internal.hpp
index e60f08d0..bbd5d29d 100644
--- a/hll/include/HllSketch-internal.hpp
+++ b/hll/include/HllSketch-internal.hpp
@@ -45,7 +45,7 @@ template<typename A>
 hll_sketch_alloc<A>::hll_sketch_alloc(uint8_t lg_config_k, target_hll_type tgt_type, bool start_full_size, const A& allocator) {
   HllUtil<A>::checkLgK(lg_config_k);
   if (start_full_size) {
-    sketch_impl = HllSketchImplFactory<A>::newHll(lg_config_k, tgt_type, start_full_size, allocator);
+    sketch_impl = HllSketchImplFactory<A>::newHll(lg_config_k, tgt_type, allocator);
   } else {
     typedef typename std::allocator_traits<A>::template rebind_alloc<CouponList<A>> clAlloc;
     sketch_impl = new (clAlloc(allocator).allocate(1)) CouponList<A>(lg_config_k, tgt_type, hll_mode::LIST, allocator);
@@ -107,10 +107,8 @@ hll_sketch_alloc<A>& hll_sketch_alloc<A>::operator=(hll_sketch_alloc<A>&& other)
 }
 
 template<typename A>
-void hll_sketch_alloc<A>::reset() {
-  // TODO: need to allow starting from a full-sized sketch
-  //       (either here or in other implementation)
-  sketch_impl = sketch_impl->reset();
+void hll_sketch_alloc<A>::reset(bool full_size) {
+  sketch_impl = sketch_impl->reset(full_size);
 }
 
 template<typename A>
diff --git a/hll/include/HllSketchImpl-internal.hpp b/hll/include/HllSketchImpl-internal.hpp
index 6b11e7ce..c4dd83cc 100644
--- a/hll/include/HllSketchImpl-internal.hpp
+++ b/hll/include/HllSketchImpl-internal.hpp
@@ -29,11 +29,10 @@ namespace datasketches {
 
 template<typename A>
 HllSketchImpl<A>::HllSketchImpl(uint8_t lgConfigK, target_hll_type tgtHllType,
-                                hll_mode mode, bool startFullSize)
+                                hll_mode mode)
   : lgConfigK_(lgConfigK),
     tgtHllType_(tgtHllType),
-    mode_(mode),
-    startFullSize_(startFullSize)
+    mode_(mode)
 {
 }
 
@@ -75,7 +74,7 @@ uint8_t HllSketchImpl<A>::makeFlagsByte(bool compact) const {
   flags |= (isEmpty() ? hll_constants::EMPTY_FLAG_MASK : 0);
   flags |= (compact ? hll_constants::COMPACT_FLAG_MASK : 0);
   flags |= (isOutOfOrderFlag() ? hll_constants::OUT_OF_ORDER_FLAG_MASK : 0);
-  flags |= (startFullSize_ ? hll_constants::FULL_SIZE_FLAG_MASK : 0);
+  // bit 32 is reserved: see RESERVED_FLAG_MASK_32 in HllUtil.hpp
   return flags;
 }
 
@@ -122,8 +121,8 @@ uint8_t HllSketchImpl<A>::makeModeByte() const {
 }
 
 template<typename A>
-HllSketchImpl<A>* HllSketchImpl<A>::reset() {
-  return HllSketchImplFactory<A>::reset(this, startFullSize_);
+HllSketchImpl<A>* HllSketchImpl<A>::reset(bool full_size) {
+  return HllSketchImplFactory<A>::reset(this, full_size);
 }
 
 template<typename A>
@@ -141,11 +140,6 @@ hll_mode HllSketchImpl<A>::getCurMode() const {
   return mode_;
 }
 
-template<typename A>
-bool HllSketchImpl<A>::isStartFullSize() const {
-  return startFullSize_;
-}
-
 }
 
 #endif // _HLLSKETCHIMPL_INTERNAL_HPP_
diff --git a/hll/include/HllSketchImpl.hpp b/hll/include/HllSketchImpl.hpp
index 80667199..bdd4407b 100644
--- a/hll/include/HllSketchImpl.hpp
+++ b/hll/include/HllSketchImpl.hpp
@@ -32,7 +32,7 @@ class HllSketchImpl {
   public:
     using vector_bytes = std::vector<uint8_t, typename std::allocator_traits<A>::template rebind_alloc<uint8_t>>;
 
-    HllSketchImpl(uint8_t lgConfigK, target_hll_type tgtHllType, hll_mode mode, bool startFullSize);
+    HllSketchImpl(uint8_t lgConfigK, target_hll_type tgtHllType, hll_mode mode);
     virtual ~HllSketchImpl();
 
     virtual void serialize(std::ostream& os, bool compact) const = 0;
@@ -40,7 +40,7 @@ class HllSketchImpl {
 
     virtual HllSketchImpl* copy() const = 0;
     virtual HllSketchImpl* copyAs(target_hll_type tgtHllType) const = 0;
-    HllSketchImpl<A>* reset();
+    HllSketchImpl<A>* reset(bool full_size);
 
     virtual std::function<void(HllSketchImpl<A>*)> get_deleter() const = 0;
 
@@ -69,7 +69,6 @@ class HllSketchImpl {
     virtual bool isOutOfOrderFlag() const = 0;
     virtual void putOutOfOrderFlag(bool oooFlag) = 0;
     virtual A getAllocator() const = 0;
-    bool isStartFullSize() const;
 
   protected:
     static target_hll_type extractTgtHllType(uint8_t modeByte);
@@ -80,7 +79,6 @@ class HllSketchImpl {
     const uint8_t lgConfigK_;
     const target_hll_type tgtHllType_;
     const hll_mode mode_;
-    const bool startFullSize_;
 };
 
 }
diff --git a/hll/include/HllSketchImplFactory.hpp b/hll/include/HllSketchImplFactory.hpp
index aa2eafa8..2142b535 100644
--- a/hll/include/HllSketchImplFactory.hpp
+++ b/hll/include/HllSketchImplFactory.hpp
@@ -41,10 +41,11 @@ class HllSketchImplFactory final {
 
   static CouponHashSet<A>* promoteListToSet(const CouponList<A>& list);
   static HllArray<A>* promoteListOrSetToHll(const CouponList<A>& list);
-  static HllArray<A>* newHll(uint8_t lgConfigK, target_hll_type tgtHllType, bool startFullSize, const A& allocator);
+  static HllArray<A>* newHll(uint8_t lgConfigK, target_hll_type tgtHllType, const A& allocator);
   
   // resets the input impl, deleting the input pointer and returning a new pointer
-  static HllSketchImpl<A>* reset(HllSketchImpl<A>* impl, bool startFullSize);
+  // full_size selects the state to reset to: an empty HLL array, or LIST (coupon) mode
+  static HllSketchImpl<A>* reset(HllSketchImpl<A>* impl, bool full_size);
 
   static Hll4Array<A>* convertToHll4(const HllArray<A>& srcHllArr);
   static Hll6Array<A>* convertToHll6(const HllArray<A>& srcHllArr);
@@ -63,7 +64,7 @@ CouponHashSet<A>* HllSketchImplFactory<A>::promoteListToSet(const CouponList<A>&
 
 template<typename A>
 HllArray<A>* HllSketchImplFactory<A>::promoteListOrSetToHll(const CouponList<A>& src) {
-  HllArray<A>* tgtHllArr = HllSketchImplFactory<A>::newHll(src.getLgConfigK(), src.getTgtHllType(), false, src.getAllocator());
+  HllArray<A>* tgtHllArr = HllSketchImplFactory<A>::newHll(src.getLgConfigK(), src.getTgtHllType(), src.getAllocator());
   tgtHllArr->putKxQ0(1 << src.getLgConfigK());
   for (const auto coupon: src) {
     tgtHllArr->couponUpdate(coupon);
@@ -105,25 +106,25 @@ HllSketchImpl<A>* HllSketchImplFactory<A>::deserialize(const void* bytes, size_t
 }
 
 template<typename A>
-HllArray<A>* HllSketchImplFactory<A>::newHll(uint8_t lgConfigK, target_hll_type tgtHllType, bool startFullSize, const A& allocator) {
+HllArray<A>* HllSketchImplFactory<A>::newHll(uint8_t lgConfigK, target_hll_type tgtHllType, const A& allocator) {
   switch (tgtHllType) {
     case HLL_8:
       using Hll8Alloc = typename std::allocator_traits<A>::template rebind_alloc<Hll8Array<A>>;
-      return new (Hll8Alloc(allocator).allocate(1)) Hll8Array<A>(lgConfigK, startFullSize, allocator);
+      return new (Hll8Alloc(allocator).allocate(1)) Hll8Array<A>(lgConfigK, allocator);
     case HLL_6:
       using Hll6Alloc = typename std::allocator_traits<A>::template rebind_alloc<Hll6Array<A>>;
-      return new (Hll6Alloc(allocator).allocate(1)) Hll6Array<A>(lgConfigK, startFullSize, allocator);
+      return new (Hll6Alloc(allocator).allocate(1)) Hll6Array<A>(lgConfigK, allocator);
     case HLL_4:
       using Hll4Alloc = typename std::allocator_traits<A>::template rebind_alloc<Hll4Array<A>>;
-      return new (Hll4Alloc(allocator).allocate(1)) Hll4Array<A>(lgConfigK, startFullSize, allocator);
+      return new (Hll4Alloc(allocator).allocate(1)) Hll4Array<A>(lgConfigK, allocator);
   }
   throw std::logic_error("Invalid target_hll_type");
 }
 
 template<typename A>
-HllSketchImpl<A>* HllSketchImplFactory<A>::reset(HllSketchImpl<A>* impl, bool startFullSize) {
-  if (startFullSize) {
-    HllArray<A>* hll = newHll(impl->getLgConfigK(), impl->getTgtHllType(), startFullSize, impl->getAllocator());
+HllSketchImpl<A>* HllSketchImplFactory<A>::reset(HllSketchImpl<A>* impl, bool full_size) {
+  if (full_size) {
+    HllArray<A>* hll = newHll(impl->getLgConfigK(), impl->getTgtHllType(), impl->getAllocator());
     impl->get_deleter()(impl);
     return hll;
   } else {
diff --git a/hll/include/HllUnion-internal.hpp b/hll/include/HllUnion-internal.hpp
index a469a2d0..d83340de 100644
--- a/hll/include/HllUnion-internal.hpp
+++ b/hll/include/HllUnion-internal.hpp
@@ -171,7 +171,9 @@ uint8_t hll_union_alloc<A>::get_lg_config_k() const {
 
 template<typename A>
 void hll_union_alloc<A>::reset() {
-  gadget_.reset();
+  // always coupon collection mode: the gadget is an internal detail and must not
+  // inherit a full-size state from whatever sketches happen to have been unioned in
+  gadget_.reset(false);
 }
 
 template<typename A>
@@ -216,7 +218,7 @@ HllSketchImpl<A>* hll_union_alloc<A>::copy_or_downsample(const HllSketchImpl<A>*
     return src->copyAs(HLL_8);
   }
   typedef typename std::allocator_traits<A>::template rebind_alloc<Hll8Array<A>> hll8Alloc;
-  Hll8Array<A>* tgtHllArr = new (hll8Alloc(src->getAllocator()).allocate(1)) Hll8Array<A>(tgt_lg_k, false, src->getAllocator());
+  Hll8Array<A>* tgtHllArr = new (hll8Alloc(src->getAllocator()).allocate(1)) Hll8Array<A>(tgt_lg_k, src->getAllocator());
   tgtHllArr->mergeHll(*src);
   //both of these are required for isomorphism
   tgtHllArr->putHipAccum(src->getHipAccum());
diff --git a/hll/include/HllUtil.hpp b/hll/include/HllUtil.hpp
index b81c014a..74fa6f3f 100644
--- a/hll/include/HllUtil.hpp
+++ b/hll/include/HllUtil.hpp
@@ -43,7 +43,15 @@ static const uint8_t FAMILY_ID = 7;
 static const uint8_t EMPTY_FLAG_MASK          = 4;
 static const uint8_t COMPACT_FLAG_MASK        = 8;
 static const uint8_t OUT_OF_ORDER_FLAG_MASK   = 16;
-static const uint8_t FULL_SIZE_FLAG_MASK      = 32;
+// Bit 32 is RESERVED and must not be written or interpreted.
+// It was formerly FULL_SIZE_FLAG_MASK here, while datasketches-java uses the same bit as
+// REBUILD_CURMIN_NUM_KXQ_MASK for its union gadget. The two meanings collided across
+// implementations, so this side no longer writes or reads it. Do not reuse: bits 64 and 128
+// are free.
+static const uint8_t RESERVED_FLAG_MASK_32    = 32;
+// Deprecated alias for the bit above, retained for source compatibility. Do not use.
+// (No [[deprecated]] attribute: this library targets C++11, where it is unavailable.)
+static const uint8_t FULL_SIZE_FLAG_MASK      = RESERVED_FLAG_MASK_32;
 
 static const uint32_t PREAMBLE_INTS_BYTE = 0;
 static const uint32_t SER_VER_BYTE       = 1;
diff --git a/hll/include/hll.hpp b/hll/include/hll.hpp
index 5fc49629..9950eec0 100644
--- a/hll/include/hll.hpp
+++ b/hll/include/hll.hpp
@@ -120,7 +120,10 @@ class hll_sketch_alloc final {
      * @param tgt_type The HLL mode to use, if/when the sketch reaches that state
      * @param start_full_size Indicates whether to start in HLL mode,
      *        keeping memory use constant (if HLL_6 or HLL_8) at the cost of
-     *        starting out using much more memory
+     *        starting out using much more memory. This is a property of this
+     *        constructor call only: it is not retained by the sketch and is not
+     *        serialized, so reset() returns to coupon collection mode unless
+     *        reset(true) is used.
      * @param allocator instance of an Allocator
      */
     explicit hll_sketch_alloc(uint8_t lg_config_k, target_hll_type tgt_type = HLL_4, bool start_full_size = false, const A& allocator = A());
@@ -177,10 +180,14 @@ class hll_sketch_alloc final {
     hll_sketch_alloc& operator=(hll_sketch_alloc<A>&& other);
 
     /**
-     * Resets the sketch to an empty state in coupon collection mode.
+     * Resets the sketch to an empty state.
      * Does not re-use existing internal objects.
+     * @param full_size if true, reset to an empty full-size HLL array, as
+     *        the start_full_size constructor argument does; otherwise reset to
+     *        coupon collection mode. Full size is not remembered across a reset
+     *        or a serialization round trip, so it must be requested each time.
      */
-    void reset();
+    void reset(bool full_size = false);
 
     // This is a convenience alias for users
     // The type returned by the following serialize method
diff --git a/hll/test/CMakeLists.txt b/hll/test/CMakeLists.txt
index efdc5215..2bfaef05 100644
--- a/hll/test/CMakeLists.txt
+++ b/hll/test/CMakeLists.txt
@@ -42,6 +42,7 @@ target_sources(hll_test
     CouponListTest.cpp
     CrossCountingTest.cpp
     HllArrayTest.cpp
+    HllFullSizeTest.cpp
     HllSketchTest.cpp
     HllUnionTest.cpp
     TablesTest.cpp
diff --git a/hll/test/HllFullSizeTest.cpp b/hll/test/HllFullSizeTest.cpp
new file mode 100644
index 00000000..360f07f2
--- /dev/null
+++ b/hll/test/HllFullSizeTest.cpp
@@ -0,0 +1,137 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+#include <catch2/catch.hpp>
+
+#include "hll.hpp"
+
+namespace datasketches {
+
+// preamble byte 0 is preInts, which identifies the current mode
+static const uint8_t LIST_PREINTS = 2;
+static const uint8_t HLL_PREINTS = 10;
+// preamble byte 5 is the flags byte; bit 32 is reserved and must never be written
+static const size_t FLAGS_BYTE = 5;
+static const uint8_t RESERVED_BIT_32 = 32;
+
+static uint8_t preints_of(const hll_sketch& sk) {
+  return sk.serialize_compact()[0];
+}
+
+static hll_sketch make(uint8_t lg_k, target_hll_type type, bool full_size, uint64_t n) {
+  hll_sketch sk(lg_k, type, full_size);
+  for (uint64_t i = 0; i < n; ++i) sk.update(i);
+  return sk;
+}
+
+TEST_CASE("hll full size: reserved bit 32 is never written", "[hll_full_size]") {
+  for (auto type: {HLL_4, HLL_6, HLL_8}) {
+    for (bool full_size: {false, true}) {
+      for (uint64_t n: {uint64_t(0), uint64_t(5), uint64_t(5000)}) {
+        const hll_sketch sk = make(8, type, full_size, n);
+        REQUIRE((sk.serialize_compact()[FLAGS_BYTE] & RESERVED_BIT_32) == 0);
+        REQUIRE((sk.serialize_updatable()[FLAGS_BYTE] & RESERVED_BIT_32) == 0);
+      }
+    }
+  }
+}
+
+TEST_CASE("hll full size: start_full_size starts in HLL mode", "[hll_full_size]") {
+  REQUIRE(preints_of(make(8, HLL_8, true, 0)) == HLL_PREINTS);
+  REQUIRE(preints_of(make(8, HLL_8, false, 0)) == LIST_PREINTS);
+}
+
+TEST_CASE("hll full size: reset takes the mode as an argument", "[hll_full_size]") {
+  for (auto type: {HLL_4, HLL_6, HLL_8}) {
+    // full size is not remembered: a plain reset() returns to coupon collection mode
+    hll_sketch sk = make(8, type, true, 5000);
+    sk.reset();
+    REQUIRE(sk.is_empty());
+    REQUIRE(sk.get_estimate() == 0.0);
+    REQUIRE(preints_of(sk) == LIST_PREINTS);
+
+    // ...and must be asked for explicitly
+    hll_sketch sk2 = make(8, type, false, 5000);
+    sk2.reset(true);
+    REQUIRE(sk2.is_empty());
+    REQUIRE(sk2.get_estimate() == 0.0);
+    REQUIRE(preints_of(sk2) == HLL_PREINTS);
+    REQUIRE(sk2.get_lg_config_k() == 8);
+    REQUIRE(sk2.get_target_type() == type);
+
+    // reset(false) is the same as reset()
+    hll_sketch sk3 = make(8, type, true, 5000);
+    sk3.reset(false);
+    REQUIRE(preints_of(sk3) == LIST_PREINTS);
+  }
+}
+
+TEST_CASE("hll full size: a reset sketch is usable again", "[hll_full_size]") {
+  hll_sketch sk = make(8, HLL_8, false, 100);
+  sk.reset(true);
+  for (uint64_t i = 0; i < 1000; ++i) sk.update(i);
+  REQUIRE_FALSE(sk.is_empty());
+  REQUIRE(sk.get_estimate() == Approx(1000).epsilon(0.2));
+}
+
+TEST_CASE("hll full size: an image with the reserved bit set is read as if it were clear",
+          "[hll_full_size]") {
+  // an image produced by another implementation may have bit 32 set: datasketches-java uses it
+  // as its union rebuild flag. It must not change how this implementation reads the sketch.
+  for (auto type: {HLL_4, HLL_6, HLL_8}) {
+    const hll_sketch sk = make(8, type, false, 5000);
+    auto bytes = sk.serialize_updatable();
+    auto tampered = bytes;
+    tampered[FLAGS_BYTE] |= RESERVED_BIT_32;
+
+    const hll_sketch clean = hll_sketch::deserialize(bytes.data(), bytes.size());
+    hll_sketch tainted = hll_sketch::deserialize(tampered.data(), tampered.size());
+
+    REQUIRE(tainted.get_estimate() == clean.get_estimate());
+    REQUIRE(tainted.serialize_updatable() == bytes); // the bit is not propagated back out
+    tainted.reset();
+    REQUIRE(preints_of(tainted) == LIST_PREINTS);    // and does not alter reset() behaviour
+  }
+}
+
+TEST_CASE("hll full size: a union never inherits full size from an input sketch",
+          "[hll_full_size]") {
+  // the gadget is an internal detail; unioning a full-size sketch must not change how the
+  // union resets, nor put the reserved bit into the union's result
+  const hll_sketch full = make(8, HLL_8, true, 5000);
+
+  hll_union u(8);
+  u.update(full);
+  const hll_sketch result = u.get_result(HLL_8);
+  REQUIRE((result.serialize_compact()[FLAGS_BYTE] & RESERVED_BIT_32) == 0);
+  REQUIRE((result.serialize_updatable()[FLAGS_BYTE] & RESERVED_BIT_32) == 0);
+
+  u.reset();
+  REQUIRE(u.is_empty());
+  REQUIRE(preints_of(u.get_result(HLL_8)) == LIST_PREINTS);
+
+  // same through the rvalue overload, which moves the sketch into the gadget
+  hll_union u2(8);
+  u2.update(make(8, HLL_8, true, 5000));
+  u2.reset();
+  REQUIRE(u2.is_empty());
+  REQUIRE(preints_of(u2.get_result(HLL_8)) == LIST_PREINTS);
+}
+
+} /* namespace datasketches */