diff --git a/Makefile-ponyc b/Makefile-ponyc
index 9e8cf10822..0839591807 100644
--- a/Makefile-ponyc
+++ b/Makefile-ponyc
@@ -65,6 +65,15 @@ endif
 version_str = "$(tag) [$(config)]\ncompiled with: llvm $(llvm_version) \
   -- "$(compiler_version)
 
+# build ponyc version md5
+ifneq (,$(filter $(OSTYPE), osx bsd))
+  TAG_MD5_CMD = md5 | sed 's/./&,0x/30;s/./&,0x/28;s/./&,0x/26;s/./&,0x/24;s/./&,0x/22;s/./&,0x/20;s/./&,0x/18;s/./&,0x/16;s/./&,0x/14;s/./&,0x/12;s/./&,0x/10;s/./&,0x/8;s/./&,0x/6;s/./&,0x/4;s/./&,0x/2;s/^/0x/'
+else
+  TAG_MD5_CMD = md5sum | sed 's/  -$$//' | sed 's/./&,0x/30;s/./&,0x/28;s/./&,0x/26;s/./&,0x/24;s/./&,0x/22;s/./&,0x/20;s/./&,0x/18;s/./&,0x/16;s/./&,0x/14;s/./&,0x/12;s/./&,0x/10;s/./&,0x/8;s/./&,0x/6;s/./&,0x/4;s/./&,0x/2;s/^/0x/'
+endif
+
+tag_formatted_md5 := $(shell echo $(tag) | $(TAG_MD5_CMD))
+
 # package_name, _version, and _iteration can be overridden by Travis or AppVeyor
 package_base_version ?= $(tag)
 package_iteration ?= "1"
@@ -116,6 +125,7 @@ ALL_CFLAGS = -std=gnu11 -fexceptions \
   -DBUILD_COMPILER=\"$(compiler_version)\" \
   -DPONY_BUILD_CONFIG=\"$(config)\" \
   -DPONY_VERSION_STR=\"$(version_str)\" \
+  -DPONY_VERSION_FORMATTED_MD5=$(tag_formatted_md5) \
   -D_FILE_OFFSET_BITS=64
 ALL_CXXFLAGS = -std=gnu++11 -fno-rtti
 LL_FLAGS = -mcpu=$(cpu)
diff --git a/packages/json/_test.pony b/packages/json/_test.pony
index 885d326fa9..dc87df2048 100644
--- a/packages/json/_test.pony
+++ b/packages/json/_test.pony
@@ -552,8 +552,9 @@ class iso _TestPrintObject is UnitTest
     obj.data("a") = true
     obj.data("b") = I64(3)
     let s = doc.string("  ", true)
-    h.assert_true((s == "{\n  \"a\": true,\n  \"b\": 3\n}") or
-      (s == "{\n  \"b\": false,\n  \"a\": true\n}"))
+    h.assert_true((s == "{\n  \"a\": true,\n  \"b\": 3\n}")
+      or (s == "{\n  \"b\": 3,\n  \"a\": true\n}")
+      or (s == "{\n  \"b\": false,\n  \"a\": true\n}"))
 
     // We don't test with more fields in the object because we don't know what
     // order they will be printed in
diff --git a/src/libponyc/ast/ast.c b/src/libponyc/ast/ast.c
index 2dd43f8a8f..f2f3c520c9 100644
--- a/src/libponyc/ast/ast.c
+++ b/src/libponyc/ast/ast.c
@@ -1954,12 +1954,12 @@ static void ast_signature_serialise_trace(pony_ctx_t* ctx, void* object)
 }
 
 static void ast_signature_serialise(pony_ctx_t* ctx, void* object, void* buf,
-  size_t offset, int mutability)
+  uint64_t offset, int mutability)
 {
   (void)mutability;
 
   ast_t* ast = (ast_t*)object;
-  ast_signature_t* dst = (ast_signature_t*)((uintptr_t)buf + offset);
+  ast_signature_t* dst = (ast_signature_t*)((uintptr_t)buf + (uintptr_t)offset);
 
   dst->t = (token_signature_t*)pony_serialise_offset(ctx, ast->t);
   dst->child = (ast_signature_t*)pony_serialise_offset(ctx, ast->child);
@@ -1981,6 +1981,7 @@ static pony_type_t ast_signature_pony =
   sizeof(ast_signature_t),
   0,
   0,
+  0,
   NULL,
   NULL,
   ast_signature_serialise_trace,
@@ -2030,12 +2031,12 @@ static void ast_nominal_pkg_id_signature_serialise_trace(pony_ctx_t* ctx,
 }
 
 static void ast_nominal_pkg_id_signature_serialise(pony_ctx_t* ctx,
-  void* object, void* buf, size_t offset, int mutability)
+  void* object, void* buf, uint64_t offset, int mutability)
 {
   (void)mutability;
 
   ast_t* ast = (ast_t*)object;
-  ast_signature_t* dst = (ast_signature_t*)((uintptr_t)buf + offset);
+  ast_signature_t* dst = (ast_signature_t*)((uintptr_t)buf + (uintptr_t)offset);
 
   ast_t* def = (ast_t*)ast_data(ast_parent(ast));
   pony_assert(def != NULL);
@@ -2059,6 +2060,7 @@ static pony_type_t ast_nominal_pkg_id_signature_pony =
   sizeof(ast_signature_t),
   0,
   0,
+  0,
   NULL,
   NULL,
   ast_nominal_pkg_id_signature_serialise_trace,
@@ -2276,12 +2278,12 @@ static void ast_serialise_trace(pony_ctx_t* ctx, void* object)
 }
 
 static void ast_serialise(pony_ctx_t* ctx, void* object, void* buf,
-  size_t offset, int mutability)
+  uint64_t offset, int mutability)
 {
   (void)mutability;
 
   ast_t* ast = (ast_t*)object;
-  ast_t* dst = (ast_t*)((uintptr_t)buf + offset);
+  ast_t* dst = (ast_t*)((uintptr_t)buf + (uintptr_t)offset);
 
   dst->t = (token_t*)pony_serialise_offset(ctx, ast->t);
   ast_serialise_data(ctx, ast, dst);
@@ -2321,6 +2323,7 @@ static pony_type_t ast_pony =
   sizeof(ast_t),
   0,
   0,
+  0,
   NULL,
   NULL,
   ast_serialise_trace,
diff --git a/src/libponyc/ast/source.c b/src/libponyc/ast/source.c
index f302198ed6..b344f8e32e 100644
--- a/src/libponyc/ast/source.c
+++ b/src/libponyc/ast/source.c
@@ -87,12 +87,12 @@ static void source_serialise_trace(pony_ctx_t* ctx, void* object)
 }
 
 static void source_serialise(pony_ctx_t* ctx, void* object, void* buf,
-  size_t offset, int mutability)
+  uint64_t offset, int mutability)
 {
   (void)mutability;
 
   source_t* source = (source_t*)object;
-  source_t* dst = (source_t*)((uintptr_t)buf + offset);
+  source_t* dst = (source_t*)((uintptr_t)buf + (uintptr_t)offset);
 
   dst->file = (const char*)pony_serialise_offset(ctx, (char*)source->file);
   dst->m = (char*)pony_serialise_offset(ctx, source->m);
@@ -114,6 +114,7 @@ static pony_type_t source_pony =
   sizeof(source_t),
   0,
   0,
+  0,
   NULL,
   NULL,
   source_serialise_trace,
diff --git a/src/libponyc/ast/stringtab.c b/src/libponyc/ast/stringtab.c
index daa14c2df5..2ee4904698 100644
--- a/src/libponyc/ast/stringtab.c
+++ b/src/libponyc/ast/stringtab.c
@@ -118,14 +118,15 @@ void stringtab_done()
 }
 
 static void string_serialise(pony_ctx_t* ctx, void* object, void* buf,
-  size_t offset, int mutability)
+  uint64_t offset, int mutability)
 {
   (void)ctx;
   (void)mutability;
 
   const char* string = (const char*)object;
 
-  memcpy((void*)((uintptr_t)buf + offset), object, strlen(string) + 1);
+  memcpy((void*)((uintptr_t)buf + (uintptr_t)offset), object,
+    strlen(string) + 1);
 }
 
 static __pony_thread_local struct _pony_type_t string_pony =
@@ -134,6 +135,7 @@ static __pony_thread_local struct _pony_type_t string_pony =
   0,
   0,
   0,
+  0,
   NULL,
   NULL,
   NULL,
@@ -191,12 +193,12 @@ static void strlist_serialise_trace(pony_ctx_t* ctx, void* object)
 }
 
 static void strlist_serialise(pony_ctx_t* ctx, void* object, void* buf,
-  size_t offset, int mutability)
+  uint64_t offset, int mutability)
 {
   (void)mutability;
 
   strlist_t* list = (strlist_t*)object;
-  strlist_t* dst = (strlist_t*)((uintptr_t)buf + offset);
+  strlist_t* dst = (strlist_t*)((uintptr_t)buf + (uintptr_t)offset);
 
   dst->contents.data = (void*)pony_serialise_offset(ctx, list->contents.data);
   dst->contents.next = (list_t*)pony_serialise_offset(ctx, list->contents.next);
@@ -219,6 +221,7 @@ static pony_type_t strlist_pony =
   sizeof(strlist_t),
   0,
   0,
+  0,
   NULL,
   NULL,
   strlist_serialise_trace,
diff --git a/src/libponyc/ast/symtab.c b/src/libponyc/ast/symtab.c
index a75427070c..ce5a229b8c 100644
--- a/src/libponyc/ast/symtab.c
+++ b/src/libponyc/ast/symtab.c
@@ -352,12 +352,12 @@ static void symbol_serialise_trace(pony_ctx_t* ctx, void* object)
 }
 
 static void symbol_serialise(pony_ctx_t* ctx, void* object, void* buf,
-  size_t offset, int mutability)
+  uint64_t offset, int mutability)
 {
   (void)mutability;
 
   symbol_t* sym = (symbol_t*)object;
-  symbol_t* dst = (symbol_t*)((uintptr_t)buf + offset);
+  symbol_t* dst = (symbol_t*)((uintptr_t)buf + (uintptr_t)offset);
 
   dst->name = (const char*)pony_serialise_offset(ctx, (char*)sym->name);
   dst->def = (ast_t*)pony_serialise_offset(ctx, sym->def);
@@ -381,6 +381,7 @@ static pony_type_t symbol_pony =
   sizeof(symbol_t),
   0,
   0,
+  0,
   NULL,
   NULL,
   symbol_serialise_trace,
diff --git a/src/libponyc/ast/token.c b/src/libponyc/ast/token.c
index a838532bdf..8ea72bd6c1 100644
--- a/src/libponyc/ast/token.c
+++ b/src/libponyc/ast/token.c
@@ -364,12 +364,13 @@ static void token_signature_serialise_trace(pony_ctx_t* ctx, void* object)
 }
 
 static void token_signature_serialise(pony_ctx_t* ctx, void* object, void* buf,
-  size_t offset, int mutability)
+  uint64_t offset, int mutability)
 {
   (void)mutability;
 
   token_t* token = (token_t*)object;
-  token_signature_t* dst = (token_signature_t*)((uintptr_t)buf + offset);
+  token_signature_t* dst = (token_signature_t*)((uintptr_t)buf +
+    (uintptr_t)offset);
 
   memset(dst, 0, sizeof(token_signature_t));
 
@@ -402,6 +403,7 @@ static pony_type_t token_signature_pony =
   sizeof(token_signature_t),
   0,
   0,
+  0,
   NULL,
   NULL,
   token_signature_serialise_trace,
@@ -435,13 +437,14 @@ static void token_docstring_signature_serialise_trace(pony_ctx_t* ctx,
 }
 
 static void token_docstring_signature_serialise(pony_ctx_t* ctx, void* object,
-  void* buf, size_t offset, int mutability)
+  void* buf, uint64_t offset, int mutability)
 {
   (void)ctx;
   (void)object;
   (void)mutability;
 
-  token_signature_t* dst = (token_signature_t*)((uintptr_t)buf + offset);
+  token_signature_t* dst = (token_signature_t*)((uintptr_t)buf +
+    (uintptr_t)offset);
 
   memset(dst, 0, sizeof(token_signature_t));
 
@@ -454,6 +457,7 @@ static pony_type_t token_docstring_signature_pony =
   sizeof(token_signature_t),
   0,
   0,
+  0,
   NULL,
   NULL,
   token_docstring_signature_serialise_trace,
@@ -486,12 +490,12 @@ static void token_serialise_trace(pony_ctx_t* ctx, void* object)
 }
 
 static void token_serialise(pony_ctx_t* ctx, void* object, void* buf,
-  size_t offset, int mutability)
+  uint64_t offset, int mutability)
 {
   (void)mutability;
 
   token_t* token = (token_t*)object;
-  token_t* dst = (token_t*)((uintptr_t)buf + offset);
+  token_t* dst = (token_t*)((uintptr_t)buf + (uintptr_t)offset);
 
   dst->id = token->id;
 
@@ -541,6 +545,7 @@ static pony_type_t token_pony =
   sizeof(token_t),
   0,
   0,
+  0,
   NULL,
   NULL,
   token_serialise_trace,
diff --git a/src/libponyc/codegen/codegen.c b/src/libponyc/codegen/codegen.c
index 1a69cad813..86b3ee3165 100644
--- a/src/libponyc/codegen/codegen.c
+++ b/src/libponyc/codegen/codegen.c
@@ -203,6 +203,13 @@ static void init_runtime(compile_t* c)
   c->msg_ptr = LLVMPointerType(c->msg_type, 0);
   LLVMStructSetBody(c->msg_type, params, 2, false);
 
+  // descriptor_offset_lookup
+  // uint32_t (*)(i64)
+  params[0] = c->i64;
+  c->descriptor_offset_lookup_type = LLVMFunctionType(c->i32, params, 1, false);
+  c->descriptor_offset_lookup_fn =
+    LLVMPointerType(c->descriptor_offset_lookup_type, 0);
+
   // trace
   // void (*)(i8*, __object*)
   params[0] = c->void_ptr;
@@ -211,11 +218,11 @@ static void init_runtime(compile_t* c)
   c->trace_fn = LLVMPointerType(c->trace_type, 0);
 
   // serialise
-  // void (*)(i8*, __object*, i8*, intptr, i32)
+  // void (*)(i8*, __object*, i8*, i64, i32)
   params[0] = c->void_ptr;
   params[1] = c->object_ptr;
   params[2] = c->void_ptr;
-  params[3] = c->intptr;
+  params[3] = c->i64;
   params[4] = c->i32;
   c->serialise_type = LLVMFunctionType(c->void_type, params, 5, false);
   c->serialise_fn = LLVMPointerType(c->serialise_type, 0);
diff --git a/src/libponyc/codegen/codegen.h b/src/libponyc/codegen/codegen.h
index d0b4b1122a..3995940863 100644
--- a/src/libponyc/codegen/codegen.h
+++ b/src/libponyc/codegen/codegen.h
@@ -183,6 +183,7 @@ typedef struct compile_t
   LLVMValueRef primitives_init;
   LLVMValueRef primitives_final;
   LLVMValueRef desc_table;
+  LLVMValueRef desc_table_offset_lookup_fn;
   LLVMValueRef numeric_sizes;
 
   LLVMTypeRef void_type;
@@ -199,6 +200,8 @@ typedef struct compile_t
   LLVMTypeRef void_ptr;
   LLVMTypeRef descriptor_type;
   LLVMTypeRef descriptor_ptr;
+  LLVMTypeRef descriptor_offset_lookup_type;
+  LLVMTypeRef descriptor_offset_lookup_fn;
   LLVMTypeRef field_descriptor;
   LLVMTypeRef object_type;
   LLVMTypeRef object_ptr;
diff --git a/src/libponyc/codegen/gendesc.c b/src/libponyc/codegen/gendesc.c
index 18dab986e0..54a06cf2df 100644
--- a/src/libponyc/codegen/gendesc.c
+++ b/src/libponyc/codegen/gendesc.c
@@ -12,23 +12,24 @@
 
 #define DESC_ID 0
 #define DESC_SIZE 1
-#define DESC_FIELD_COUNT 2
-#define DESC_FIELD_OFFSET 3
-#define DESC_INSTANCE 4
-#define DESC_TRACE 5
-#define DESC_SERIALISE_TRACE 6
-#define DESC_SERIALISE 7
-#define DESC_DESERIALISE 8
-#define DESC_CUSTOM_SERIALISE_SPACE 9
-#define DESC_CUSTOM_DESERIALISE 10
-#define DESC_DISPATCH 11
-#define DESC_FINALISE 12
-#define DESC_EVENT_NOTIFY 13
-#define DESC_TRAITS 14
-#define DESC_FIELDS 15
-#define DESC_VTABLE 16
-
-#define DESC_LENGTH 17
+#define DESC_SERIALISEID 2
+#define DESC_FIELD_COUNT 3
+#define DESC_FIELD_OFFSET 4
+#define DESC_INSTANCE 5
+#define DESC_TRACE 6
+#define DESC_SERIALISE_TRACE 7
+#define DESC_SERIALISE 8
+#define DESC_DESERIALISE 9
+#define DESC_CUSTOM_SERIALISE_SPACE 10
+#define DESC_CUSTOM_DESERIALISE 11
+#define DESC_DISPATCH 12
+#define DESC_FINALISE 13
+#define DESC_EVENT_NOTIFY 14
+#define DESC_TRAITS 15
+#define DESC_FIELDS 16
+#define DESC_VTABLE 17
+
+#define DESC_LENGTH 18
 
 static LLVMValueRef make_unbox_function(compile_t* c, reach_type_t* t,
   reach_method_t* m)
@@ -331,6 +332,7 @@ void gendesc_basetype(compile_t* c, LLVMTypeRef desc_type)
 
   params[DESC_ID] = c->i32;
   params[DESC_SIZE] = c->i32;
+  params[DESC_SERIALISEID] = c->i64;
   params[DESC_FIELD_COUNT] = c->i32;
   params[DESC_FIELD_OFFSET] = c->i32;
   params[DESC_INSTANCE] = c->object_ptr;
@@ -382,6 +384,7 @@ void gendesc_type(compile_t* c, reach_type_t* t)
 
   params[DESC_ID] = c->i32;
   params[DESC_SIZE] = c->i32;
+  params[DESC_SERIALISEID] = c->i64;
   params[DESC_FIELD_COUNT] = c->i32;
   params[DESC_FIELD_OFFSET] = c->i32;
   params[DESC_INSTANCE] = c->object_ptr;
@@ -420,6 +423,7 @@ void gendesc_init(compile_t* c, reach_type_t* t)
   LLVMValueRef args[DESC_LENGTH];
   args[DESC_ID] = LLVMConstInt(c->i32, t->type_id, false);
   args[DESC_SIZE] = LLVMConstInt(c->i32, c_t->abi_size, false);
+  args[DESC_SERIALISEID] = LLVMConstInt(c->i64, t->serialise_id, false);
   args[DESC_FIELD_COUNT] = make_field_count(c, t);
   args[DESC_FIELD_OFFSET] = make_field_offset(c, t);
   args[DESC_INSTANCE] = make_desc_ptr(c_t->instance, c->object_ptr);
@@ -485,6 +489,59 @@ void gendesc_table(compile_t* c)
   ponyint_pool_free_size(size, args);
 }
 
+void gendesc_table_lookup(compile_t* c)
+{
+  reach_type_t* t;
+  size_t i = HASHMAP_BEGIN;
+
+  LLVMValueRef desc_lkp_fn = codegen_addfun(c, "__DescOffsetLookupFn",
+    c->descriptor_offset_lookup_type, false);
+  LLVMSetFunctionCallConv(desc_lkp_fn, LLVMCCallConv);
+  LLVMSetLinkage(desc_lkp_fn, LLVMExternalLinkage);
+  codegen_startfun(c, desc_lkp_fn, NULL, NULL, NULL, false);
+
+  LLVMBasicBlockRef unreachable = codegen_block(c, "unreachable");
+
+  // Read the serialise ID.
+  LLVMValueRef serialise_id = LLVMGetParam(desc_lkp_fn, 0);
+
+  // switch based on serialise_id
+  LLVMValueRef serialise_switch = LLVMBuildSwitch(c->builder, serialise_id, unreachable, 0);
+
+  // the default case is unreachable unless something major has gone wrong
+  LLVMPositionBuilderAtEnd(c->builder, unreachable);
+
+  LLVMValueRef ret = LLVMConstInt(c->i32, (uint32_t)-1, false);
+  LLVMBuildRet(c->builder, ret);
+
+  while((t = reach_types_next(&c->reach->types, &i)) != NULL)
+  {
+    if(t->is_trait || (t->underlying == TK_STRUCT))
+      continue;
+
+    pony_assert(t->serialise_id != (uint64_t)-1);
+
+    LLVMBasicBlockRef type_block = codegen_block(c,
+      genname_type_with_id("type", t->serialise_id));
+
+    LLVMAddCase(serialise_switch, LLVMConstInt(c->i64, t->serialise_id, false),
+      type_block);
+
+    LLVMPositionBuilderAtEnd(c->builder, type_block);
+
+    ret = LLVMConstInt(c->i32, t->type_id, false);
+    LLVMBuildRet(c->builder, ret);
+  }
+
+  // Mark the default case as unreachable.
+  LLVMPositionBuilderAtEnd(c->builder, unreachable);
+
+  codegen_finishfun(c);
+
+  c->desc_table_offset_lookup_fn = make_desc_ptr(desc_lkp_fn,
+    c->descriptor_offset_lookup_type);
+}
+
 static LLVMValueRef desc_field(compile_t* c, LLVMValueRef desc, int index)
 {
   LLVMValueRef ptr = LLVMBuildStructGEP(c->builder, desc, index, "");
diff --git a/src/libponyc/codegen/gendesc.h b/src/libponyc/codegen/gendesc.h
index 97c76aea27..cd8f7a90a7 100644
--- a/src/libponyc/codegen/gendesc.h
+++ b/src/libponyc/codegen/gendesc.h
@@ -14,6 +14,8 @@ void gendesc_init(compile_t* c, reach_type_t* t);
 
 void gendesc_table(compile_t* c);
 
+void gendesc_table_lookup(compile_t* c);
+
 LLVMValueRef gendesc_fetch(compile_t* c, LLVMValueRef object);
 
 LLVMValueRef gendesc_typeid(compile_t* c, LLVMValueRef desc);
diff --git a/src/libponyc/codegen/genexe.c b/src/libponyc/codegen/genexe.c
index aa8b54d782..55720ed9ba 100644
--- a/src/libponyc/codegen/genexe.c
+++ b/src/libponyc/codegen/genexe.c
@@ -49,16 +49,18 @@ static LLVMValueRef make_lang_features_init(compile_t* c)
     boolean = c->i8;
 
   LLVMTypeRef desc_ptr_ptr = LLVMPointerType(c->descriptor_ptr, 0);
+  LLVMValueRef desc_table_lookup_fn = c->desc_table_offset_lookup_fn;
 
   uint32_t desc_table_size = reach_max_type_id(c->reach);
 
-  LLVMTypeRef f_params[4];
+  LLVMTypeRef f_params[5];
   f_params[0] = boolean;
   f_params[1] = boolean;
   f_params[2] = desc_ptr_ptr;
   f_params[3] = c->intptr;
+  f_params[4] = c->descriptor_offset_lookup_fn;
 
-  LLVMTypeRef lfi_type = LLVMStructTypeInContext(c->context, f_params, 4,
+  LLVMTypeRef lfi_type = LLVMStructTypeInContext(c->context, f_params, 5,
     false);
 
   LLVMBasicBlockRef this_block = LLVMGetInsertBlock(c->builder);
@@ -88,6 +90,10 @@ static LLVMValueRef make_lang_features_init(compile_t* c)
   LLVMBuildStore(c->builder, LLVMConstInt(c->intptr, desc_table_size, false),
     field);
 
+  field = LLVMBuildStructGEP(c->builder, lfi_object, 4, "");
+  LLVMBuildStore(c->builder, LLVMBuildBitCast(c->builder, desc_table_lookup_fn,
+    c->descriptor_offset_lookup_fn, ""), field);
+
   return LLVMBuildBitCast(c->builder, lfi_object, c->void_ptr, "");
 }
 
diff --git a/src/libponyc/codegen/genname.c b/src/libponyc/codegen/genname.c
index a481f31dca..cab07cf70a 100644
--- a/src/libponyc/codegen/genname.c
+++ b/src/libponyc/codegen/genname.c
@@ -199,3 +199,10 @@ const char* genname_program_fn(const char* program, const char* name)
 {
   return stringtab_two(program, name);
 }
+
+const char* genname_type_with_id(const char* type, uint64_t type_id)
+{
+  printbuf_t* buf = printbuf_new();
+  printbuf(buf, "%s_%" PRIu64, type, type_id);
+  return stringtab_buf(buf);
+}
diff --git a/src/libponyc/codegen/genname.h b/src/libponyc/codegen/genname.h
index 13d39ee334..ce160524b0 100644
--- a/src/libponyc/codegen/genname.h
+++ b/src/libponyc/codegen/genname.h
@@ -42,6 +42,8 @@ const char* genname_unsafe(const char* name);
 
 const char* genname_program_fn(const char* program, const char* name);
 
+const char* genname_type_with_id(const char* type, uint64_t type_id);
+
 PONY_EXTERN_C_END
 
 #endif
diff --git a/src/libponyc/codegen/genprim.c b/src/libponyc/codegen/genprim.c
index ca8b8e298e..ae5fcc60bb 100644
--- a/src/libponyc/codegen/genprim.c
+++ b/src/libponyc/codegen/genprim.c
@@ -821,7 +821,7 @@ void genprim_array_serialise(compile_t* c, reach_type_t* t)
   LLVMValueRef offset_addr = LLVMBuildInBoundsGEP(c->builder, addr, &offset, 1,
     "");
 
-  genserialise_typeid(c, t, offset_addr);
+  genserialise_serialiseid(c, t, offset_addr);
 
   // Don't serialise our contents if we are opaque.
   LLVMBasicBlockRef body_block = codegen_block(c, "body");
@@ -942,7 +942,7 @@ void genprim_array_deserialise(compile_t* c, reach_type_t* t)
 
   LLVMValueRef object = LLVMBuildBitCast(c->builder, arg, c_t->structure_ptr,
     "");
-  gendeserialise_typeid(c, c_t, object);
+  gendeserialise_serialiseid(c, c_t, object);
 
   // Deserialise the array contents.
   LLVMValueRef alloc = field_value(c, object, 2);
@@ -1065,7 +1065,7 @@ void genprim_string_serialise(compile_t* c, reach_type_t* t)
   LLVMValueRef offset_addr = LLVMBuildInBoundsGEP(c->builder, addr, &offset, 1,
     "");
 
-  genserialise_typeid(c, t, offset_addr);
+  genserialise_serialiseid(c, t, offset_addr);
 
   // Don't serialise our contents if we are opaque.
   LLVMBasicBlockRef body_block = codegen_block(c, "body");
@@ -1129,7 +1129,7 @@ void genprim_string_deserialise(compile_t* c, reach_type_t* t)
 
   LLVMValueRef object = LLVMBuildBitCast(c->builder, arg, c_t->structure_ptr,
     "");
-  gendeserialise_typeid(c, c_t, object);
+  gendeserialise_serialiseid(c, c_t, object);
 
   // Deserialise the string contents.
   LLVMValueRef alloc = field_value(c, object, 2);
diff --git a/src/libponyc/codegen/genserialise.c b/src/libponyc/codegen/genserialise.c
index e150ada7c2..80831e4ecc 100644
--- a/src/libponyc/codegen/genserialise.c
+++ b/src/libponyc/codegen/genserialise.c
@@ -3,6 +3,7 @@
 #include "gendesc.h"
 #include "genfun.h"
 #include "genname.h"
+#include "genopt.h"
 #include "genprim.h"
 #include "ponyassert.h"
 #include "../../libponyrt/mem/pool.h"
@@ -19,7 +20,7 @@ static void serialise(compile_t* c, reach_type_t* t, LLVMValueRef ctx,
   {
     case TK_PRIMITIVE:
     {
-      genserialise_typeid(c, t, offset);
+      genserialise_serialiseid(c, t, offset);
 
       if(c_t->primitive != NULL)
       {
@@ -36,7 +37,7 @@ static void serialise(compile_t* c, reach_type_t* t, LLVMValueRef ctx,
 
     case TK_CLASS:
     {
-      genserialise_typeid(c, t, offset);
+      genserialise_serialiseid(c, t, offset);
       extra++;
       break;
     }
@@ -44,7 +45,7 @@ static void serialise(compile_t* c, reach_type_t* t, LLVMValueRef ctx,
   case TK_ACTOR:
     {
       // Skip the actor pad.
-      genserialise_typeid(c, t, offset);
+      genserialise_serialiseid(c, t, offset);
       extra += 2;
       break;
     }
@@ -54,7 +55,7 @@ static void serialise(compile_t* c, reach_type_t* t, LLVMValueRef ctx,
       // Get the tuple primitive type.
       if(LLVMTypeOf(object) == c_t->structure_ptr)
       {
-        genserialise_typeid(c, t, offset);
+        genserialise_serialiseid(c, t, offset);
         object = LLVMBuildStructGEP(c->builder, object, 1, "");
         LLVMValueRef size = LLVMConstInt(c->intptr,
           LLVMOffsetOfElement(c->target_data, structure, 1), false);
@@ -101,13 +102,25 @@ static void serialise(compile_t* c, reach_type_t* t, LLVMValueRef ctx,
   }
 }
 
-void genserialise_typeid(compile_t* c, reach_type_t* t, LLVMValueRef offset)
+void genserialise_serialiseid(compile_t* c, reach_type_t* t, LLVMValueRef offset)
 {
-  // Write the type id instead of the descriptor.
-  LLVMValueRef value = LLVMConstInt(c->intptr, t->type_id, false);
-  LLVMValueRef loc = LLVMBuildBitCast(c->builder, offset,
-    LLVMPointerType(c->intptr, 0), "");
-  LLVMBuildStore(c->builder, value, loc);
+  // Write the serialise id instead of the descriptor.
+  if(target_is_ilp32(c->opt->triple))
+  {
+    // TODO: fix this and make 32 bit platforms properly serialise 64 bit
+    //       serialise_ids
+    LLVMValueRef value = LLVMConstInt(c->intptr, t->serialise_id, false);
+    LLVMValueRef loc = LLVMBuildBitCast(c->builder, offset,
+      LLVMPointerType(c->intptr, 0), "");
+    LLVMBuildStore(c->builder, value, loc);
+  }
+  else
+  {
+    LLVMValueRef value = LLVMConstInt(c->i64, t->serialise_id, false);
+    LLVMValueRef loc = LLVMBuildBitCast(c->builder, offset,
+      LLVMPointerType(c->i64, 0), "");
+    LLVMBuildStore(c->builder, value, loc);
+  }
 }
 
 static void serialise_bare_interface(compile_t* c, reach_type_t* t,
@@ -137,7 +150,7 @@ static void serialise_bare_interface(compile_t* c, reach_type_t* t,
     LLVMValueRef test = LLVMBuildICmp(c->builder, LLVMIntEQ, obj,
       ((compile_type_t*)sub->c_type)->instance, "");
     LLVMBuildCondBr(c->builder, test, post_block, next_block);
-    LLVMValueRef value = LLVMConstInt(c->intptr, sub->type_id, false);
+    LLVMValueRef value = LLVMConstInt(c->i64, sub->serialise_id, false);
     LLVMAddIncoming(phi, &value, &current_block, 1);
     LLVMPositionBuilderAtEnd(c->builder, next_block);
     sub = next;
@@ -146,7 +159,7 @@ static void serialise_bare_interface(compile_t* c, reach_type_t* t,
   }
 
   LLVMBuildBr(c->builder, post_block);
-  LLVMValueRef value = LLVMConstInt(c->intptr, sub->type_id, false);
+  LLVMValueRef value = LLVMConstInt(c->i64, sub->serialise_id, false);
   LLVMAddIncoming(phi, &value, &current_block, 1);
 
   LLVMMoveBasicBlockAfter(post_block, current_block);
@@ -173,12 +186,12 @@ void genserialise_element(compile_t* c, reach_type_t* t, bool embed,
       LLVMPointerType(c_t->mem_type, 0), "");
     LLVMBuildStore(c->builder, value, loc);
   } else if(t->bare_method != NULL) {
-    // Bare object, either write the type id directly if it is a concrete object
-    // or compute the type id based on the object value and write it if it isn't.
+    // Bare object, either write the serialise id directly if it is a concrete object
+    // or compute the serialise id based on the object value and write it if it isn't.
     switch(t->underlying)
     {
       case TK_PRIMITIVE:
-        genserialise_typeid(c, t, offset);
+        genserialise_serialiseid(c, t, offset);
         break;
 
       case TK_INTERFACE:
@@ -247,7 +260,7 @@ static void deserialise(compile_t* c, reach_type_t* t, LLVMValueRef ctx,
     case TK_PRIMITIVE:
     case TK_CLASS:
     {
-      gendeserialise_typeid(c, c_t, object);
+      gendeserialise_serialiseid(c, c_t, object);
       extra++;
       break;
     }
@@ -255,7 +268,7 @@ static void deserialise(compile_t* c, reach_type_t* t, LLVMValueRef ctx,
     case TK_ACTOR:
     {
       // Skip the actor pad.
-      gendeserialise_typeid(c, c_t, object);
+      gendeserialise_serialiseid(c, c_t, object);
       extra += 2;
       break;
     }
@@ -265,7 +278,7 @@ static void deserialise(compile_t* c, reach_type_t* t, LLVMValueRef ctx,
       // Get the tuple primitive type.
       if(LLVMTypeOf(object) == c_t->structure_ptr)
       {
-        gendeserialise_typeid(c, c_t, object);
+        gendeserialise_serialiseid(c, c_t, object);
         object = LLVMBuildStructGEP(c->builder, object, 1, "");
       }
       break;
@@ -282,20 +295,20 @@ static void deserialise(compile_t* c, reach_type_t* t, LLVMValueRef ctx,
   }
 }
 
-void gendeserialise_typeid(compile_t* c, compile_type_t* t, LLVMValueRef object)
+void gendeserialise_serialiseid(compile_t* c, compile_type_t* t, LLVMValueRef object)
 {
-  // Write the descriptor instead of the type id.
+  // Write the descriptor instead of the serialise id.
   LLVMValueRef desc_ptr = LLVMBuildStructGEP(c->builder, object, 0, "");
   LLVMBuildStore(c->builder, t->desc, desc_ptr);
 }
 
 static void deserialise_bare_interface(compile_t* c, LLVMValueRef ptr)
 {
-  LLVMValueRef type_id = LLVMBuildLoad(c->builder, ptr, "");
+  LLVMValueRef serialise_id = LLVMBuildLoad(c->builder, ptr, "");
 
   LLVMValueRef args[2];
   args[0] = LLVMConstInt(c->i32, 0, false);
-  args[1] = LLVMBuildPtrToInt(c->builder, type_id, c->intptr, "");
+  args[1] = LLVMBuildPtrToInt(c->builder, serialise_id, c->i64, "");
 
   LLVMValueRef desc = LLVMBuildInBoundsGEP(c->builder, c->desc_table, args, 2,
     "");
diff --git a/src/libponyc/codegen/genserialise.h b/src/libponyc/codegen/genserialise.h
index 45743be46e..92d4d9803f 100644
--- a/src/libponyc/codegen/genserialise.h
+++ b/src/libponyc/codegen/genserialise.h
@@ -11,9 +11,9 @@ typedef struct compile_type_t compile_type_t;
 void genserialise_element(compile_t* c, reach_type_t* t, bool embed,
   LLVMValueRef ctx, LLVMValueRef ptr, LLVMValueRef offset);
 
-void genserialise_typeid(compile_t* c, reach_type_t* t, LLVMValueRef offset);
+void genserialise_serialiseid(compile_t* c, reach_type_t* t, LLVMValueRef offset);
 
-void gendeserialise_typeid(compile_t* c, compile_type_t* t, LLVMValueRef offset);
+void gendeserialise_serialiseid(compile_t* c, compile_type_t* t, LLVMValueRef offset);
 
 void gendeserialise_element(compile_t* c, reach_type_t* t, bool embed,
   LLVMValueRef ctx, LLVMValueRef ptr);
diff --git a/src/libponyc/codegen/gentype.c b/src/libponyc/codegen/gentype.c
index b098121dae..3e432e3fe3 100644
--- a/src/libponyc/codegen/gentype.c
+++ b/src/libponyc/codegen/gentype.c
@@ -907,6 +907,7 @@ bool gentypes(compile_t* c)
   }
 
   gendesc_table(c);
+  gendesc_table_lookup(c);
 
   c->numeric_sizes = gen_numeric_size_table(c);
 
diff --git a/src/libponyc/pkg/package.c b/src/libponyc/pkg/package.c
index 19132606f9..0d17e3b33b 100644
--- a/src/libponyc/pkg/package.c
+++ b/src/libponyc/pkg/package.c
@@ -1370,12 +1370,13 @@ static void package_signature_serialise_trace(pony_ctx_t* ctx,
 
 
 static void package_signature_serialise(pony_ctx_t* ctx, void* object,
-  void* buf, size_t offset, int mutability)
+  void* buf, uint64_t offset, int mutability)
 {
   (void)mutability;
 
   package_t* package = (package_t*)object;
-  package_signature_t* dst = (package_signature_t*)((uintptr_t)buf + offset);
+  package_signature_t* dst = (package_signature_t*)((uintptr_t)buf +
+    (uintptr_t)offset);
 
   dst->filename = (const char*)pony_serialise_offset(ctx,
     (char*)package->filename);
@@ -1390,6 +1391,7 @@ static pony_type_t package_dep_signature_pony =
   sizeof(package_signature_t),
   0,
   0,
+  0,
   NULL,
   NULL,
   package_dep_signature_serialise_trace,
@@ -1418,6 +1420,7 @@ static pony_type_t package_signature_pony =
   sizeof(package_signature_t),
   0,
   0,
+  0,
   NULL,
   NULL,
   package_signature_serialise_trace,
@@ -1469,13 +1472,13 @@ static void package_group_signature_serialise_trace(pony_ctx_t* ctx,
 
 
 static void package_group_signature_serialise(pony_ctx_t* ctx, void* object,
-  void* buf, size_t offset, int mutability)
+  void* buf, uint64_t offset, int mutability)
 {
   (void)ctx;
   (void)mutability;
 
   package_group_t* group = (package_group_t*)object;
-  package_group_t* dst = (package_group_t*)((uintptr_t)buf + offset);
+  package_group_t* dst = (package_group_t*)((uintptr_t)buf + (uintptr_t)offset);
 
   if(group->signature != NULL)
   {
@@ -1495,6 +1498,7 @@ static pony_type_t package_group_dep_signature_pony =
   sizeof(const char*),
   0,
   0,
+  0,
   NULL,
   NULL,
   package_group_dep_signature_serialise_trace,
@@ -1523,6 +1527,7 @@ static pony_type_t package_group_signature_pony =
   sizeof(const char*),
   0,
   0,
+  0,
   NULL,
   NULL,
   package_group_signature_serialise_trace,
@@ -1617,12 +1622,12 @@ static void package_serialise_trace(pony_ctx_t* ctx, void* object)
 
 
 static void package_serialise(pony_ctx_t* ctx, void* object, void* buf,
-  size_t offset, int mutability)
+  uint64_t offset, int mutability)
 {
   (void)mutability;
 
   package_t* package = (package_t*)object;
-  package_t* dst = (package_t*)((uintptr_t)buf + offset);
+  package_t* dst = (package_t*)((uintptr_t)buf + (uintptr_t)offset);
 
   dst->path = (const char*)pony_serialise_offset(ctx, (char*)package->path);
   dst->qualified_name = (const char*)pony_serialise_offset(ctx,
@@ -1671,6 +1676,7 @@ static pony_type_t package_pony =
   sizeof(package_t),
   0,
   0,
+  0,
   NULL,
   NULL,
   package_serialise_trace,
@@ -1705,13 +1711,13 @@ static void package_group_serialise_trace(pony_ctx_t* ctx, void* object)
 
 
 static void package_group_serialise(pony_ctx_t* ctx, void* object, void* buf,
-  size_t offset, int mutability)
+  uint64_t offset, int mutability)
 {
   (void)ctx;
   (void)mutability;
 
   package_group_t* group = (package_group_t*)object;
-  package_group_t* dst = (package_group_t*)((uintptr_t)buf + offset);
+  package_group_t* dst = (package_group_t*)((uintptr_t)buf + (uintptr_t)offset);
 
   uintptr_t ptr_offset = pony_serialise_offset(ctx, group->signature);
   dst->signature = (char*)ptr_offset;
@@ -1743,6 +1749,7 @@ static pony_type_t package_group_pony =
   sizeof(package_group_t),
   0,
   0,
+  0,
   NULL,
   NULL,
   package_group_serialise_trace,
diff --git a/src/libponyc/pkg/program.c b/src/libponyc/pkg/program.c
index d768147876..4ecdbd82c9 100644
--- a/src/libponyc/pkg/program.c
+++ b/src/libponyc/pkg/program.c
@@ -358,12 +358,12 @@ static void program_serialise_trace(pony_ctx_t* ctx, void* object)
 }
 
 static void program_serialise(pony_ctx_t* ctx, void* object, void* buf,
-  size_t offset, int mutability)
+  uint64_t offset, int mutability)
 {
   (void)mutability;
 
   program_t* program = (program_t*)object;
-  program_t* dst = (program_t*)((uintptr_t)buf + offset);
+  program_t* dst = (program_t*)((uintptr_t)buf + (uintptr_t)offset);
 
   dst->package_groups = (package_group_list_t*)pony_serialise_offset(ctx,
     program->package_groups);
@@ -416,6 +416,7 @@ static pony_type_t program_pony =
   sizeof(program_t),
   0,
   0,
+  0,
   NULL,
   NULL,
   program_serialise_trace,
diff --git a/src/libponyc/reach/reach.c b/src/libponyc/reach/reach.c
index 34ceba4f59..933333230f 100644
--- a/src/libponyc/reach/reach.c
+++ b/src/libponyc/reach/reach.c
@@ -2,6 +2,7 @@
 #include "subtype.h"
 #include "../ast/astbuild.h"
 #include "../codegen/genname.h"
+#include "../codegen/genopt.h"
 #include "../pass/expr.h"
 #include "../type/assemble.h"
 #include "../type/cap.h"
@@ -10,6 +11,7 @@
 #include "../../libponyrt/gc/serialise.h"
 #include "../../libponyrt/mem/pool.h"
 #include "ponyassert.h"
+#include <inttypes.h>
 #include <stdio.h>
 #include <string.h>
 
@@ -738,6 +740,7 @@ static reach_type_t* add_reach_type(reach_t* r, ast_t* type)
   t->ast = set_cap_and_ephemeral(type, TK_REF, TK_NONE);
   t->ast_cap = ast_dup(type);
   t->type_id = (uint32_t)-1;
+  t->serialise_id = (uint64_t)-1;
 
   ast_set_scope(t->ast, NULL);
   ast_set_scope(t->ast_cap, NULL);
@@ -792,6 +795,14 @@ static reach_type_t* add_tuple(reach_t* r, ast_t* type, pass_opt_t* opt)
   t->type_id = get_new_tuple_id(r);
   t->can_be_boxed = true;
 
+  if(target_is_ilp32(opt->triple))
+    t->serialise_id = t->type_id;
+  else
+    // TODO: ideally should be hashing AST tree for type and not just name
+    t->serialise_id = ponyint_hash_str64(t->name);
+
+  pony_assert(t->serialise_id != ((uint64_t)-1)); // -1 is for `Pointer`s
+
   t->field_count = (uint32_t)ast_childcount(t->ast);
   t->fields = (reach_field_t*)ponyint_pool_alloc_size(
       t->field_count * sizeof(reach_field_t));
@@ -922,6 +933,14 @@ static reach_type_t* add_nominal(reach_t* r, ast_t* type, pass_opt_t* opt)
       t->type_id = get_new_numeric_id(r);
     else if(t->underlying != TK_STRUCT)
       t->type_id = get_new_object_id(r);
+
+    if(target_is_ilp32(opt->triple))
+      t->serialise_id = t->type_id;
+    else
+      // TODO: ideally should be hashing AST tree for type and not just name
+      t->serialise_id = ponyint_hash_str64(t->name);
+
+    pony_assert(t->serialise_id != ((uint64_t)-1)); // -1 is for `Pointer`s
   }
 
   if(ast_id(def) != TK_PRIMITIVE)
@@ -1510,6 +1529,10 @@ void reach_dump(reach_t* r)
     size_t j = HASHMAP_BEGIN;
     reach_method_name_t* n;
 
+    printf("    serialise_id: %" PRIu64 "\n", t->serialise_id);
+    printf("    is_trait: %s\n", (t->is_trait)?"true":"false");
+    printf("    can_be_boxed: %s\n", (t->can_be_boxed)?"true":"false");
+
     printf("    vtable: %d\n", t->vtable_size);
 
     while((n = reach_method_names_next(&t->methods, &j)) != NULL)
@@ -1529,6 +1552,8 @@ void reach_dump(reach_t* r)
       printf("    %s\n", t2->name);
     }
   }
+
+  printf("  Total Type Count: %d\n", r->total_type_count);
 }
 
 static void reach_param_serialise_trace(pony_ctx_t* ctx, void* object)
@@ -1541,12 +1566,12 @@ static void reach_param_serialise_trace(pony_ctx_t* ctx, void* object)
 }
 
 static void reach_param_serialise(pony_ctx_t* ctx, void* object, void* buf,
-  size_t offset, int mutability)
+  uint64_t offset, int mutability)
 {
   (void)mutability;
 
   reach_param_t* p = (reach_param_t*)object;
-  reach_param_t* dst = (reach_param_t*)((uintptr_t)buf + offset);
+  reach_param_t* dst = (reach_param_t*)((uintptr_t)buf + (uintptr_t)offset);
 
   dst->name = (const char*)pony_serialise_offset(ctx, (char*)p->name);
   dst->ast = (ast_t*)pony_serialise_offset(ctx, p->ast);
@@ -1571,6 +1596,7 @@ static pony_type_t reach_param_pony =
   sizeof(reach_param_t),
   0,
   0,
+  0,
   NULL,
   NULL,
   reach_param_serialise_trace,
@@ -1624,12 +1650,12 @@ static void reach_method_serialise_trace(pony_ctx_t* ctx, void* object)
 }
 
 static void reach_method_serialise(pony_ctx_t* ctx, void* object, void* buf,
-   size_t offset, int mutability)
+   uint64_t offset, int mutability)
 {
   (void)mutability;
 
   reach_method_t* m = (reach_method_t*)object;
-  reach_method_t* dst = (reach_method_t*)((uintptr_t)buf + offset);
+  reach_method_t* dst = (reach_method_t*)((uintptr_t)buf + (uintptr_t)offset);
 
   dst->name = (const char*)pony_serialise_offset(ctx, (char*)m->name);
   dst->mangled_name = (const char*)pony_serialise_offset(ctx,
@@ -1705,6 +1731,7 @@ static pony_type_t reach_method_pony =
   sizeof(reach_method_t),
   0,
   0,
+  0,
   NULL,
   NULL,
   reach_method_serialise_trace,
@@ -1735,12 +1762,13 @@ static void reach_method_name_serialise_trace(pony_ctx_t* ctx, void* object)
 }
 
 static void reach_method_name_serialise(pony_ctx_t* ctx, void* object,
-  void* buf, size_t offset, int mutability)
+  void* buf, uint64_t offset, int mutability)
 {
   (void)mutability;
 
   reach_method_name_t* n = (reach_method_name_t*)object;
-  reach_method_name_t* dst = (reach_method_name_t*)((uintptr_t)buf + offset);
+  reach_method_name_t* dst = (reach_method_name_t*)((uintptr_t)buf +
+    (uintptr_t)offset);
 
   dst->id = n->id;
   dst->cap = n->cap;
@@ -1767,6 +1795,7 @@ static pony_type_t reach_method_name_pony =
   sizeof(reach_method_name_t),
   0,
   0,
+  0,
   NULL,
   NULL,
   reach_method_name_serialise_trace,
@@ -1796,12 +1825,12 @@ static void reach_field_serialise_trace(pony_ctx_t* ctx, void* object)
 }
 
 static void reach_field_serialise(pony_ctx_t* ctx, void* object, void* buf,
-  size_t offset, int mutability)
+  uint64_t offset, int mutability)
 {
   (void)mutability;
 
   reach_field_t* f = (reach_field_t*)object;
-  reach_field_t* dst = (reach_field_t*)((uintptr_t)buf + offset);
+  reach_field_t* dst = (reach_field_t*)((uintptr_t)buf + (uintptr_t)offset);
 
   dst->ast = (ast_t*)pony_serialise_offset(ctx, f->ast);
   dst->type = (reach_type_t*)pony_serialise_offset(ctx, f->type);
@@ -1824,6 +1853,7 @@ static pony_type_t reach_field_pony =
   sizeof(reach_field_t),
   0,
   0,
+  0,
   NULL,
   NULL,
   reach_field_serialise_trace,
@@ -1872,12 +1902,12 @@ static void reach_type_serialise_trace(pony_ctx_t* ctx, void* object)
 }
 
 static void reach_type_serialise(pony_ctx_t* ctx, void* object, void* buf,
-  size_t offset, int mutability)
+  uint64_t offset, int mutability)
 {
   (void)mutability;
 
   reach_type_t* t = (reach_type_t*)object;
-  reach_type_t* dst = (reach_type_t*)((uintptr_t)buf + offset);
+  reach_type_t* dst = (reach_type_t*)((uintptr_t)buf + (uintptr_t)offset);
 
   dst->name = (const char*)pony_serialise_offset(ctx, (char*)t->name);
   dst->mangle = (const char*)pony_serialise_offset(ctx, (char*)t->mangle);
@@ -1894,6 +1924,7 @@ static void reach_type_serialise(pony_ctx_t* ctx, void* object, void* buf,
   dst->vtable_size = t->vtable_size;
   dst->can_be_boxed = t->can_be_boxed;
   dst->is_trait = t->is_trait;
+  dst->serialise_id = t->serialise_id;
 
   dst->field_count = t->field_count;
   dst->fields = (reach_field_t*)pony_serialise_offset(ctx, t->fields);
@@ -1947,6 +1978,7 @@ static pony_type_t reach_type_pony =
   sizeof(reach_type_t),
   0,
   0,
+  0,
   NULL,
   NULL,
   reach_type_serialise_trace,
@@ -1975,12 +2007,12 @@ static void reach_serialise_trace(pony_ctx_t* ctx, void* object)
 }
 
 static void reach_serialise(pony_ctx_t* ctx, void* object, void* buf,
-  size_t offset, int mutability)
+  uint64_t offset, int mutability)
 {
   (void)mutability;
 
   reach_t* r = (reach_t*)object;
-  reach_t* dst = (reach_t*)((uintptr_t)buf + offset);
+  reach_t* dst = (reach_t*)((uintptr_t)buf + (uintptr_t)offset);
 
   reach_types_serialise(ctx, &r->types, buf, offset + offsetof(reach_t, types),
     PONY_TRACE_MUTABLE);
@@ -2005,6 +2037,7 @@ static pony_type_t reach_pony =
   sizeof(reach_t),
   0,
   0,
+  0,
   NULL,
   NULL,
   reach_serialise_trace,
diff --git a/src/libponyc/reach/reach.h b/src/libponyc/reach/reach.h
index afd0f79025..eee7f81e25 100644
--- a/src/libponyc/reach/reach.h
+++ b/src/libponyc/reach/reach.h
@@ -99,6 +99,7 @@ struct reach_type_t
   reach_type_cache_t subtypes;
   uint32_t type_id;
   uint32_t vtable_size;
+  uint64_t serialise_id;
   bool can_be_boxed;
   bool is_trait;
 
diff --git a/src/libponyc/type/reify.c b/src/libponyc/type/reify.c
index 3b4b532b3e..5a5ce49ac7 100644
--- a/src/libponyc/type/reify.c
+++ b/src/libponyc/type/reify.c
@@ -505,13 +505,13 @@ static void deferred_reification_serialise_trace(pony_ctx_t* ctx, void* object)
 }
 
 static void deferred_reification_serialise(pony_ctx_t* ctx, void* object,
-  void* buf, size_t offset, int mutability)
+  void* buf, uint64_t offset, int mutability)
 {
   (void)mutability;
 
   deferred_reification_t* d = (deferred_reification_t*)object;
   deferred_reification_t* dst =
-    (deferred_reification_t*)((uintptr_t)buf + offset);
+    (deferred_reification_t*)((uintptr_t)buf + (uintptr_t)offset);
 
   dst->ast = (ast_t*)pony_serialise_offset(ctx, d->ast);
   dst->type_typeparams = (ast_t*)pony_serialise_offset(ctx, d->type_typeparams);
@@ -546,6 +546,7 @@ static pony_type_t deferred_reification_pony =
   sizeof(deferred_reification_t),
   0,
   0,
+  0,
   NULL,
   NULL,
   deferred_reification_serialise_trace,
diff --git a/src/libponyrt/ds/fun.c b/src/libponyrt/ds/fun.c
index c41e386428..6fa3499820 100644
--- a/src/libponyrt/ds/fun.c
+++ b/src/libponyrt/ds/fun.c
@@ -3,11 +3,8 @@
 
 #include <platform.h>
 
-static const unsigned char the_key[16] = {
-  0xFE, 0x09, 0xD3, 0x22, 0x6B, 0x9C, 0x10, 0x8A,
-  0xE1, 0x35, 0x72, 0xB5, 0xCC, 0x3F, 0x92, 0x9F
-};
-
+// Use the MD5 of the current pony version for the key
+static const unsigned char the_key[16] = { PONY_VERSION_FORMATTED_MD5 };
 
 #ifdef PLATFORM_IS_ILP32
 
@@ -64,6 +61,7 @@ static uint32_t halfsiphash24(const unsigned char* key, const char* in,
 
   return v0 ^ v1 ^ v2 ^ v3;
 }
+
 #endif
 
 #define ROTL64(x, b) (uint64_t)(((x) << (b)) | ((x) >> (64 - (b))))
@@ -137,6 +135,11 @@ PONY_API uint64_t ponyint_hash_block64(const void* p, size_t len)
   return siphash24(the_key, (const char*)p, len);
 }
 
+uint64_t ponyint_hash_str64(const char* str)
+{
+  return siphash24(the_key, str, strlen(str));
+}
+
 size_t ponyint_hash_str(const char* str)
 {
 #ifdef PLATFORM_IS_ILP32
diff --git a/src/libponyrt/ds/fun.h b/src/libponyrt/ds/fun.h
index b19976bc0d..a192548043 100644
--- a/src/libponyrt/ds/fun.h
+++ b/src/libponyrt/ds/fun.h
@@ -25,6 +25,8 @@ PONY_API size_t ponyint_hash_block(const void* p, size_t len);
 
 PONY_API uint64_t ponyint_hash_block64(const void* p, size_t len);
 
+uint64_t ponyint_hash_str64(const char* str);
+
 size_t ponyint_hash_str(const char* str);
 
 size_t ponyint_hash_ptr(const void* p);
diff --git a/src/libponyrt/ds/hash.c b/src/libponyrt/ds/hash.c
index d447e86f18..9b7514c46c 100644
--- a/src/libponyrt/ds/hash.c
+++ b/src/libponyrt/ds/hash.c
@@ -563,10 +563,10 @@ void ponyint_hashmap_serialise_trace(pony_ctx_t* ctx, void* object,
 }
 
 void ponyint_hashmap_serialise(pony_ctx_t* ctx, void* object, void* buf,
-  size_t offset)
+  uint64_t offset)
 {
   hashmap_t* map = (hashmap_t*)object;
-  hashmap_t* dst = (hashmap_t*)((uintptr_t)buf + offset);
+  hashmap_t* dst = (hashmap_t*)((uintptr_t)((uintptr_t)buf + offset));
 
   uintptr_t bitmap_offset = pony_serialise_offset(ctx, map->item_bitmap);
 
diff --git a/src/libponyrt/ds/hash.h b/src/libponyrt/ds/hash.h
index 8361187f83..b75a0f79e0 100644
--- a/src/libponyrt/ds/hash.h
+++ b/src/libponyrt/ds/hash.h
@@ -114,7 +114,7 @@ void ponyint_hashmap_serialise_trace(pony_ctx_t* ctx, void* object,
   pony_type_t* elem_type);
 
 void ponyint_hashmap_serialise(pony_ctx_t* ctx, void* object, void* buf,
-  size_t offset);
+  uint64_t offset);
 
 void ponyint_hashmap_deserialise(pony_ctx_t* ctx, void* object,
   pony_type_t* elem_type);
@@ -137,7 +137,7 @@ void ponyint_hashmap_deserialise(pony_ctx_t* ctx, void* object,
   DECLARE_HASHMAP(name, name_t, type) \
   void name##_serialise_trace(pony_ctx_t* ctx, void* object); \
   void name##_serialise(pony_ctx_t* ctx, void* object, void* buf, \
-    size_t offset, int mutability); \
+    uint64_t offset, int mutability); \
   void name##_deserialise(pony_ctx_t* ctx, void* object); \
   const pony_type_t* name##_pony_type(); \
 
@@ -210,7 +210,7 @@ void ponyint_hashmap_deserialise(pony_ctx_t* ctx, void* object,
     ponyint_hashmap_serialise_trace(ctx, object, elem_type); \
   } \
   void name##_serialise(pony_ctx_t* ctx, void* object, void* buf, \
-    size_t offset, int mutability) \
+    uint64_t offset, int mutability) \
   { \
     (void)mutability; \
     ponyint_hashmap_serialise(ctx, object, buf, offset); \
@@ -225,6 +225,7 @@ void ponyint_hashmap_deserialise(pony_ctx_t* ctx, void* object,
     sizeof(name_t), \
     0, \
     0, \
+    0, \
     NULL, \
     NULL, \
     name##_serialise_trace, \
diff --git a/src/libponyrt/ds/list.c b/src/libponyrt/ds/list.c
index 4fd6adbd62..509908c7c5 100644
--- a/src/libponyrt/ds/list.c
+++ b/src/libponyrt/ds/list.c
@@ -192,10 +192,10 @@ void ponyint_list_serialise_trace(pony_ctx_t* ctx, void* object,
 }
 
 void ponyint_list_serialise(pony_ctx_t* ctx, void* object, void* buf,
-  size_t offset)
+  uint64_t offset)
 {
   list_t* list = (list_t*)object;
-  list_t* dst = (list_t*)((uintptr_t)buf + offset);
+  list_t* dst = (list_t*)((uintptr_t)((uintptr_t)buf + offset));
 
   dst->data = (void*)pony_serialise_offset(ctx, list->data);
   dst->next = (list_t*)pony_serialise_offset(ctx, list->next);
diff --git a/src/libponyrt/ds/list.h b/src/libponyrt/ds/list.h
index 6d175f41dc..cbbeed060a 100644
--- a/src/libponyrt/ds/list.h
+++ b/src/libponyrt/ds/list.h
@@ -47,7 +47,7 @@ void ponyint_list_serialise_trace(pony_ctx_t* ctx, void* object,
   pony_type_t* list_type, pony_type_t* elem_type);
 
 void ponyint_list_serialise(pony_ctx_t* ctx, void* object, void* buf,
-  size_t offset);
+  uint64_t offset);
 
 void ponyint_list_deserialise(pony_ctx_t* ctx, void* object,
   pony_type_t* list_type, pony_type_t* elem_type);
@@ -76,7 +76,7 @@ void ponyint_list_deserialise(pony_ctx_t* ctx, void* object,
   DECLARE_LIST(name, name_t, type) \
   void name##_serialise_trace(pony_ctx_t* ctx, void* object); \
   void name##_serialise(pony_ctx_t* ctx, void* object, void* buf, \
-    size_t offset, int mutability); \
+    uint64_t offset, int mutability); \
   void name##_deserialise(pony_ctx_t* ctx, void* object); \
   const pony_type_t* name##_pony_type(); \
 
@@ -152,7 +152,7 @@ void ponyint_list_deserialise(pony_ctx_t* ctx, void* object,
     ponyint_list_serialise_trace(ctx, object, name##_pony_type(), elem_type); \
   } \
   void name##_serialise(pony_ctx_t* ctx, void* object, void* buf, \
-    size_t offset, int mutability) \
+    uint64_t offset, int mutability) \
   { \
     (void)mutability; \
     ponyint_list_serialise(ctx, object, buf, offset); \
@@ -167,6 +167,7 @@ void ponyint_list_deserialise(pony_ctx_t* ctx, void* object,
     sizeof(name_t), \
     0, \
     0, \
+    0, \
     NULL, \
     NULL, \
     name##_serialise_trace, \
diff --git a/src/libponyrt/gc/cycle.c b/src/libponyrt/gc/cycle.c
index b618665c64..c7df2a4b9d 100644
--- a/src/libponyrt/gc/cycle.c
+++ b/src/libponyrt/gc/cycle.c
@@ -891,6 +891,7 @@ static pony_type_t cycle_type =
   sizeof(detector_t),
   0,
   0,
+  0,
   NULL,
   NULL,
   NULL,
diff --git a/src/libponyrt/gc/serialise.c b/src/libponyrt/gc/serialise.c
index 1b5144b7ae..8e74be334d 100644
--- a/src/libponyrt/gc/serialise.c
+++ b/src/libponyrt/gc/serialise.c
@@ -20,13 +20,14 @@ PONY_EXTERN_C_BEGIN
 
 static size_t desc_table_size = 0;
 static pony_type_t** desc_table = NULL;
+static desc_offset_lookup_fn desc_table_offset_lookup_fn = NULL;
 
 PONY_EXTERN_C_END
 
 struct serialise_t
 {
   uintptr_t key;
-  uintptr_t value;
+  uint64_t value;
   pony_type_t* t;
   int mutability;
   bool block;
@@ -75,13 +76,14 @@ static void custom_deserialise(pony_ctx_t* ctx)
   {
     if (s->t != NULL && s->t->custom_deserialise != NULL)
     {
-      s->t->custom_deserialise((void *)(s->value),
+      s->t->custom_deserialise((void *)((uintptr_t)s->value),
         (void*)((uintptr_t)ctx->serialise_buffer + s->key + s->t->size));
     }
   }
 }
 
-bool ponyint_serialise_setup(pony_type_t** table, size_t table_size)
+bool ponyint_serialise_setup(pony_type_t** table, size_t table_size,
+  desc_offset_lookup_fn desc_table_offset_lookup)
 {
 #ifndef PONY_NDEBUG
   for(uint32_t i = 0; i < table_size; i++)
@@ -93,10 +95,18 @@ bool ponyint_serialise_setup(pony_type_t** table, size_t table_size)
 
   desc_table = table;
   desc_table_size = table_size;
+  desc_table_offset_lookup_fn = desc_table_offset_lookup;
 
   return true;
 }
 
+static pony_type_t* get_descriptor(uint64_t serialise_id)
+{
+  uint32_t offset = desc_table_offset_lookup_fn(serialise_id);
+  pony_assert(offset < desc_table_size);
+  return desc_table[offset];
+}
+
 void ponyint_serialise_object(pony_ctx_t* ctx, void* p, pony_type_t* t,
   int mutability)
 {
@@ -186,15 +196,22 @@ PONY_API size_t pony_serialise_offset(pony_ctx_t* ctx, void* p)
   if(s != NULL)
   {
     if(s->block || (s->t != NULL && s->t->serialise != NULL))
-      return s->value;
+      return (uintptr_t)s->value;
     else
       return ALL_BITS;
   }
 
-  // If we are not in the map, we are an untraced primitive. Return the type id
-  // with the high bit set.
+  // TODO: FIND OUT WHEN/HOW THIS CAN HAPPEN!!!!!!
+  // If not possible normally, make this into an error/assert!
+  // If possible, figure out how to handle 64 bit serialise_id's and return types
+  // since the pointers are no longer able to hold the serialise_id + high bit
+  // twiddles.
+  // See TODO in pony_deserialise_offset for related issue
+
+  // If we are not in the map, we are an untraced primitive. Return the
+  // serialise id with the high bit set.
   pony_type_t* t = *(pony_type_t**)p;
-  return (size_t)t->id | HIGH_BIT;
+  return (size_t)t->serialise_id | HIGH_BIT;
 }
 
 PONY_API void pony_serialise(pony_ctx_t* ctx, void* p, pony_type_t* t,
@@ -244,6 +261,9 @@ PONY_API void* pony_deserialise_offset(pony_ctx_t* ctx, pony_type_t* t,
   // primitive, or an unserialised field in an opaque object.
   if((offset & HIGH_BIT) != 0)
   {
+    // TODO: NOTE: this section needs changing because uintptr_t can't safely
+    // hold u64 + high bit twiddles
+    // See TODO in pony_serialise_offset for related issue
     offset &= ~HIGH_BIT;
 
     if(offset > desc_table_size)
@@ -263,22 +283,32 @@ PONY_API void* pony_deserialise_offset(pony_ctx_t* ctx, pony_type_t* t,
   serialise_t* s = ponyint_serialise_get(&ctx->serialise, &k, &index);
 
   if(s != NULL)
-    return (void*)s->value;
+    return (void*)((uintptr_t)s->value);
 
   // If we haven't been passed a type descriptor, read one.
   if(t == NULL)
   {
-    // Make sure we have space to read a type id.
+    // Make sure we have space to read a serialise id.
+#ifdef PLATFORM_IS_ILP32
     if((offset + sizeof(uintptr_t)) > ctx->serialise_size)
+#else
+    if((offset + sizeof(uint64_t)) > ctx->serialise_size)
+#endif
     {
       serialise_cleanup(ctx);
       ctx->serialise_throw();
       abort();
     }
 
-    // Turn the type id into a descriptor pointer.
-    uintptr_t id = *(uintptr_t*)((uintptr_t)ctx->serialise_buffer + offset);
-    t = desc_table[id];
+    // Turn the serialise id into a descriptor pointer.
+#ifdef PLATFORM_IS_ILP32
+    // TODO: remove this ifdef once serialisation of 64 bit serialise_id's works for
+    //       32 bit platforms
+    uint64_t id = *(uintptr_t*)((uintptr_t)ctx->serialise_buffer + offset);
+#else
+    uint64_t id = *(uint64_t*)((uintptr_t)ctx->serialise_buffer + offset);
+#endif
+    t = get_descriptor(id);
   }
 
   // If it's a primitive, return the global instance.
@@ -348,7 +378,7 @@ PONY_API void* pony_deserialise_raw(pony_ctx_t* ctx, uintptr_t offset,
   serialise_t* s = ponyint_serialise_get(&ctx->serialise, &k, &index);
 
   if(s != NULL)
-    return (void*)s->value;
+    return (void*)((uintptr_t)s->value);
 
   void* object = ds_fn((void*)((uintptr_t)ctx->serialise_buffer + offset),
     ctx->serialise_size - offset);
diff --git a/src/libponyrt/gc/serialise.h b/src/libponyrt/gc/serialise.h
index 4a41da9af9..d31b30ad88 100644
--- a/src/libponyrt/gc/serialise.h
+++ b/src/libponyrt/gc/serialise.h
@@ -23,7 +23,8 @@ typedef struct serialise_t serialise_t;
 
 DECLARE_HASHMAP(ponyint_serialise, ponyint_serialise_t, serialise_t);
 
-bool ponyint_serialise_setup(pony_type_t** table, size_t table_size);
+bool ponyint_serialise_setup(pony_type_t** table, size_t table_size,
+  desc_offset_lookup_fn desc_table_offset_lookup);
 
 void ponyint_serialise_object(pony_ctx_t* ctx, void* p, pony_type_t* t,
   int mutability);
diff --git a/src/libponyrt/pony.h b/src/libponyrt/pony.h
index 1f1ad853f6..f0c61ea1b5 100644
--- a/src/libponyrt/pony.h
+++ b/src/libponyrt/pony.h
@@ -82,7 +82,7 @@ typedef void (*pony_trace_fn)(pony_ctx_t* ctx, void* p);
  * A serialise function must not raise errors.
  */
 typedef void (*pony_serialise_fn)(pony_ctx_t* ctx, void* p, void* addr,
-  size_t offset, int m);
+  uint64_t offset, int m);
 
 /** Serialise Space function.
  *
@@ -134,6 +134,7 @@ typedef const struct _pony_type_t
 {
   uint32_t id;
   uint32_t size;
+  uint64_t serialise_id;
   uint32_t field_count;
   uint32_t field_offset;
   void* instance;
@@ -151,6 +152,12 @@ typedef const struct _pony_type_t
   void* vtable;
 } pony_type_t;
 
+/** Desc table lookup function.
+ *
+ * A function to convert `serialise_id`s to offsets in the desc table
+ */
+typedef uint32_t (*desc_offset_lookup_fn)(uint64_t serialise_id);
+
 /** Language feature initialiser.
  *
  * Contains initialisers for the various language features initialised by
@@ -177,6 +184,9 @@ typedef struct pony_language_features_init_t
 
   /// The total size of the descriptor_table array.
   size_t descriptor_table_size;
+
+  /// The function to translate `type_id`s to offsets in the desc_table
+  desc_offset_lookup_fn desc_table_offset_lookup;
 } pony_language_features_init_t;
 
 /** Padding for actor types.
diff --git a/src/libponyrt/sched/start.c b/src/libponyrt/sched/start.c
index b961e6a497..d83b16b19d 100644
--- a/src/libponyrt/sched/start.c
+++ b/src/libponyrt/sched/start.c
@@ -183,7 +183,8 @@ PONY_API bool pony_start(bool library, int* exit_code,
 
     if(language_init.init_serialisation &&
       !ponyint_serialise_setup(language_init.descriptor_table,
-        language_init.descriptor_table_size))
+        language_init.descriptor_table_size,
+        language_init.desc_table_offset_lookup))
     {
       atomic_store_explicit(&running, NOT_RUNNING, memory_order_relaxed);
       return false;
diff --git a/test/libponyrt/ds/hash.cc b/test/libponyrt/ds/hash.cc
index e635c43277..3eb906d9ad 100644
--- a/test/libponyrt/ds/hash.cc
+++ b/test/libponyrt/ds/hash.cc
@@ -48,7 +48,7 @@ static void hash_elem_serialise_trace(pony_ctx_t* ctx, void* object)
 }
 
 static void hash_elem_serialise(pony_ctx_t* ctx, void* object, void* buf,
-  size_t offset, int mutability)
+  uint64_t offset, int mutability)
 {
   (void)ctx;
   (void)mutability;
@@ -72,6 +72,7 @@ static pony_type_t hash_elem_pony =
   sizeof(hash_elem_t),
   0,
   0,
+  0,
   NULL,
   NULL,
   hash_elem_serialise_trace,
diff --git a/wscript b/wscript
index 80adfc4e0b..373c8c5a9f 100644
--- a/wscript
+++ b/wscript
@@ -35,6 +35,11 @@ with open('VERSION') as v:
     except:
         pass
 
+# build ponyc version md5
+import hashlib
+temp_md5 = hashlib.md5(VERSION).hexdigest()
+VERSION_FORMATTED_MD5 = "0x" + ",0x".join([temp_md5[i:i+2] for i in range(0, len(temp_md5), 2)])
+
 # source and build directories
 top = '.'
 out = 'build'
@@ -78,6 +83,7 @@ def init(ctx):
 def configure(ctx):
     ctx.env.append_value('DEFINES', [
         'PONY_VERSION="' + VERSION + '"',
+        'PONY_VERSION_FORMATTED_MD5=' + VERSION_FORMATTED_MD5,
         'PONY_USE_BIGINT',
     ])