diff --git a/core/extension/extension_api_dump.cpp b/core/extension/extension_api_dump.cpp
index 79b0ebc6414..0bcc8a44e8a 100644
--- a/core/extension/extension_api_dump.cpp
+++ b/core/extension/extension_api_dump.cpp
@@ -880,6 +880,15 @@ Dictionary GDExtensionAPIDump::generate_extension_api() {
 						d2["is_virtual"] = false;
 						d2["hash"] = method->get_hash();
 
+						Vector<uint32_t> compat_hashes = ClassDB::get_method_compatibility_hashes(class_name, method_name);
+						if (compat_hashes.size()) {
+							Array compatibility;
+							for (int i = 0; i < compat_hashes.size(); i++) {
+								compatibility.push_back(compat_hashes[i]);
+							}
+							d2["hash_compatibility"] = compatibility;
+						}
+
 						Vector<Variant> default_args = method->get_default_arguments();
 
 						Array arguments;
@@ -1056,4 +1065,275 @@ void GDExtensionAPIDump::generate_extension_json_file(const String &p_path) {
 	fa->store_string(text);
 }
 
+static bool compare_dict_array(const Dictionary &p_old_api, const Dictionary &p_new_api, const String &p_base_array, const String &p_name_field, const Vector<String> &p_fields_to_compare, bool p_compare_hashes, const String &p_outer_class = String(), bool p_compare_operators = false) {
+	String base_array = p_outer_class + p_base_array;
+	if (!p_old_api.has(p_base_array)) {
+		return true; // May just not have this array and its still good. Probably added recently.
+	}
+	bool failed = false;
+	ERR_FAIL_COND_V_MSG(!p_new_api.has(p_base_array), false, "New API lacks base array: " + p_base_array);
+	Array new_api = p_new_api[p_base_array];
+	HashMap<String, Dictionary> new_api_assoc;
+
+	for (int i = 0; i < new_api.size(); i++) {
+		Dictionary elem = new_api[i];
+		ERR_FAIL_COND_V_MSG(!elem.has(p_name_field), false, "Validate extension JSON: Element of base_array '" + base_array + "' is missing field '" + p_name_field + "'. This is a bug.");
+		String name = elem[p_name_field];
+		if (p_compare_operators && elem.has("right_type")) {
+			name += " " + String(elem["right_type"]);
+		}
+		new_api_assoc.insert(name, elem);
+	}
+
+	Array old_api = p_old_api[p_base_array];
+	for (int i = 0; i < old_api.size(); i++) {
+		Dictionary old_elem = old_api[i];
+		if (!old_elem.has(p_name_field)) {
+			failed = true;
+			print_error("Validate extension JSON: JSON file: element of base array '" + base_array + "' is missing the field: '" + p_name_field + "'.");
+			continue;
+		}
+		String name = old_elem[p_name_field];
+		if (p_compare_operators && old_elem.has("right_type")) {
+			name += " " + String(old_elem["right_type"]);
+		}
+		if (!new_api_assoc.has(name)) {
+			failed = true;
+			print_error("Validate extension JSON: API was removed: " + base_array + "/" + name);
+			continue;
+		}
+
+		Dictionary new_elem = new_api_assoc[name];
+
+		for (int j = 0; j < p_fields_to_compare.size(); j++) {
+			String field = p_fields_to_compare[j];
+			bool optional = field.begins_with("*");
+			if (optional) {
+				// This is an optional field, but if exists it has to exist in both.
+				field = field.substr(1, field.length());
+			}
+
+			bool added = field.begins_with("+");
+			if (added) {
+				// Meaning this field must either exist or contents may not exist.
+				field = field.substr(1, field.length());
+			}
+
+			Variant old_value;
+
+			if (!old_elem.has(field)) {
+				if (optional) {
+					if (new_elem.has(field)) {
+						failed = true;
+						print_error("Validate extension JSON: JSON file: Field was added in a way that breaks compatibility '" + base_array + "/" + name + "': " + field);
+					}
+				} else if (added && new_elem.has(field)) {
+					// Should be ok, field now exists, should not be verified in prior versions where it does not.
+				} else {
+					failed = true;
+					print_error("Validate extension JSON: JSON file: Missing filed in '" + base_array + "/" + name + "': " + field);
+				}
+				continue;
+			} else {
+				old_value = old_elem[field];
+			}
+
+			if (!new_elem.has(field)) {
+				failed = true;
+				ERR_PRINT("Validate extension JSON: Missing filed in current API '" + base_array + "/" + name + "': " + field + ". This is a bug.");
+				continue;
+			}
+
+			Variant new_value = new_elem[field];
+
+			bool equal = Variant::evaluate(Variant::OP_EQUAL, old_value, new_value);
+			if (!equal) {
+				failed = true;
+				print_error("Validate extension JSON: Error: Field '" + base_array + "/" + name + "': " + field + " changed value in new API, from " + old_value.get_construct_string() + " to " + new_value.get_construct_string() + ".");
+				continue;
+			}
+		}
+
+		if (p_compare_hashes) {
+			if (!old_elem.has("hash")) {
+				if (old_elem.has("is_virtual") && bool(old_elem["is_virtual"]) && !new_elem.has("hash")) {
+					continue; // No hash for virtual methods, go on.
+				}
+
+				failed = true;
+				print_error("Validate extension JSON: JSON file: element of base array '" + base_array + "' is missing the field: 'hash'.");
+				continue;
+			}
+
+			uint64_t old_hash = old_elem["hash"];
+
+			if (!new_elem.has("hash")) {
+				failed = true;
+				print_error("Validate extension JSON: Error: Field '" + base_array + "' is missing the field: 'hash'.");
+				continue;
+			}
+
+			uint64_t new_hash = new_elem["hash"];
+			bool hash_found = false;
+			if (old_hash == new_hash) {
+				hash_found = true;
+			} else if (new_elem.has("hash_compatibility")) {
+				Array compatibility = new_elem["hash_compatibility"];
+				for (int j = 0; j < compatibility.size(); j++) {
+					new_hash = compatibility[j];
+					if (new_hash == old_hash) {
+						hash_found = true;
+						break;
+					}
+				}
+			}
+
+			if (!hash_found) {
+				failed = true;
+				print_error("Validate extension JSON: Error: Hash mismatch for '" + base_array + "/" + name + "'. This means that the function has changed and no compatibility function was provided.");
+				continue;
+			}
+		}
+	}
+
+	return !failed;
+}
+
+static bool compare_sub_dict_array(HashSet<String> &r_removed_classes_registered, const String &p_outer, const String &p_outer_name, const Dictionary &p_old_api, const Dictionary &p_new_api, const String &p_base_array, const String &p_name_field, const Vector<String> &p_fields_to_compare, bool p_compare_hashes, bool p_compare_operators = false) {
+	if (!p_old_api.has(p_outer)) {
+		return true; // May just not have this array and its still good. Probably added recently or optional.
+	}
+	bool failed = false;
+	ERR_FAIL_COND_V_MSG(!p_new_api.has(p_outer), false, "New API lacks base array: " + p_outer);
+	Array new_api = p_new_api[p_outer];
+	HashMap<String, Dictionary> new_api_assoc;
+
+	for (int i = 0; i < new_api.size(); i++) {
+		Dictionary elem = new_api[i];
+		ERR_FAIL_COND_V_MSG(!elem.has(p_outer_name), false, "Validate extension JSON: Element of base_array '" + p_outer + "' is missing field '" + p_outer_name + "'. This is a bug.");
+		new_api_assoc.insert(elem[p_outer_name], elem);
+	}
+
+	Array old_api = p_old_api[p_outer];
+
+	for (int i = 0; i < old_api.size(); i++) {
+		Dictionary old_elem = old_api[i];
+		if (!old_elem.has(p_outer_name)) {
+			failed = true;
+			print_error("Validate extension JSON: JSON file: element of base array '" + p_outer + "' is missing the field: '" + p_outer_name + "'.");
+			continue;
+		}
+		String name = old_elem[p_outer_name];
+		if (!new_api_assoc.has(name)) {
+			failed = true;
+			if (!r_removed_classes_registered.has(name)) {
+				print_error("Validate extension JSON: API was removed: " + p_outer + "/" + name);
+				r_removed_classes_registered.insert(name);
+			}
+			continue;
+		}
+
+		Dictionary new_elem = new_api_assoc[name];
+
+		if (!compare_dict_array(old_elem, new_elem, p_base_array, p_name_field, p_fields_to_compare, p_compare_hashes, p_outer + "/" + name + "/", p_compare_operators)) {
+			failed = true;
+		}
+	}
+
+	return !failed;
+}
+
+Error GDExtensionAPIDump::validate_extension_json_file(const String &p_path) {
+	Error error;
+	String text = FileAccess::get_file_as_string(p_path, &error);
+	if (error != OK) {
+		ERR_PRINT("Validate extension JSON: Could not open file '" + p_path + "'.");
+		return error;
+	}
+
+	Ref<JSON> json;
+	json.instantiate();
+	error = json->parse(text);
+	if (error != OK) {
+		ERR_PRINT("Validate extension JSON: Error parsing '" + p_path + "' at line " + itos(json->get_error_line()) + ": " + json->get_error_message());
+		return error;
+	}
+
+	Dictionary old_api = json->get_data();
+	Dictionary new_api = generate_extension_api();
+
+	{ // Validate header:
+		Dictionary header = old_api["header"];
+		ERR_FAIL_COND_V(!header.has("version_major"), ERR_INVALID_DATA);
+		ERR_FAIL_COND_V(!header.has("version_minor"), ERR_INVALID_DATA);
+		int major = header["version_major"];
+		int minor = header["version_minor"];
+
+		ERR_FAIL_COND_V_MSG(major != VERSION_MAJOR, ERR_INVALID_DATA, "JSON API dump is for a different engine version (" + itos(major) + ") than this one (" + itos(major) + ")");
+		ERR_FAIL_COND_V_MSG(minor > VERSION_MINOR, ERR_INVALID_DATA, "JSON API dump is for a newer version of the engine: " + itos(major) + "." + itos(minor));
+	}
+
+	bool failed = false;
+
+	HashSet<String> removed_classes_registered;
+
+	if (!compare_dict_array(old_api, new_api, "constants", "name", Vector<String>({ "value", "is_bitfield" }), false)) {
+		failed = true;
+	}
+
+	if (!compare_dict_array(old_api, new_api, "utility_functions", "name", Vector<String>({ "category" }), true)) {
+		failed = true;
+	}
+
+	if (!compare_sub_dict_array(removed_classes_registered, "builtin_classes", "name", old_api, new_api, "members", "name", { "type" }, false)) {
+		failed = true;
+	}
+
+	if (!compare_sub_dict_array(removed_classes_registered, "builtin_classes", "name", old_api, new_api, "constants", "name", { "type", "value" }, false)) {
+		failed = true;
+	}
+
+	if (!compare_sub_dict_array(removed_classes_registered, "builtin_classes", "name", old_api, new_api, "operators", "name", { "return_type" }, false, true)) {
+		failed = true;
+	}
+
+	if (!compare_sub_dict_array(removed_classes_registered, "builtin_classes", "name", old_api, new_api, "methods", "name", {}, true)) {
+		failed = true;
+	}
+
+	if (!compare_sub_dict_array(removed_classes_registered, "builtin_classes", "name", old_api, new_api, "constructors", "index", { "*arguments" }, false)) {
+		failed = true;
+	}
+
+	if (!compare_sub_dict_array(removed_classes_registered, "classes", "name", old_api, new_api, "constants", "name", { "value" }, false)) {
+		failed = true;
+	}
+
+	if (!compare_sub_dict_array(removed_classes_registered, "classes", "name", old_api, new_api, "methods", "name", { "is_virtual", "is_vararg", "is_static" }, true)) { // is_const sometimes can change because they are added if someone forgot, but should not be a problem for the extensions.
+		failed = true;
+	}
+
+	if (!compare_sub_dict_array(removed_classes_registered, "classes", "name", old_api, new_api, "signals", "name", { "*arguments" }, false)) {
+		failed = true;
+	}
+
+	if (!compare_sub_dict_array(removed_classes_registered, "classes", "name", old_api, new_api, "properties", "name", { "type", "*setter", "*getter", "*index" }, false)) {
+		failed = true;
+	}
+
+	if (!compare_dict_array(old_api, new_api, "singletons", "name", Vector<String>({ "type" }), false)) {
+		failed = true;
+	}
+
+	if (!compare_dict_array(old_api, new_api, "native_structures", "name", Vector<String>({ "format" }), false)) {
+		failed = true;
+	}
+
+	if (failed) {
+		return ERR_INVALID_DATA;
+	} else {
+		return OK;
+	}
+}
+
 #endif // TOOLS_ENABLED
diff --git a/core/extension/extension_api_dump.h b/core/extension/extension_api_dump.h
index 7e588c94463..11ea2cf9230 100644
--- a/core/extension/extension_api_dump.h
+++ b/core/extension/extension_api_dump.h
@@ -39,6 +39,7 @@ class GDExtensionAPIDump {
 public:
 	static Dictionary generate_extension_api();
 	static void generate_extension_json_file(const String &p_path);
+	static Error validate_extension_json_file(const String &p_path);
 };
 #endif
 
diff --git a/core/extension/gdextension_interface.cpp b/core/extension/gdextension_interface.cpp
index 2bedb623e4f..c892fa3a686 100644
--- a/core/extension/gdextension_interface.cpp
+++ b/core/extension/gdextension_interface.cpp
@@ -984,7 +984,12 @@ static GDExtensionScriptInstancePtr gdextension_script_instance_create(const GDE
 static GDExtensionMethodBindPtr gdextension_classdb_get_method_bind(GDExtensionConstStringNamePtr p_classname, GDExtensionConstStringNamePtr p_methodname, GDExtensionInt p_hash) {
 	const StringName classname = *reinterpret_cast<const StringName *>(p_classname);
 	const StringName methodname = *reinterpret_cast<const StringName *>(p_methodname);
-	MethodBind *mb = ClassDB::get_method(classname, methodname);
+	bool exists = false;
+	MethodBind *mb = ClassDB::get_method_with_compatibility(classname, methodname, p_hash, &exists);
+	if (!mb && exists) {
+		ERR_PRINT("Method '" + classname + "." + methodname + "' has changed and no compatibility fallback has been provided. Please open an issue.");
+		return nullptr;
+	}
 	ERR_FAIL_COND_V(!mb, nullptr);
 	if (mb->get_hash() != p_hash) {
 		ERR_PRINT("Hash mismatch for method '" + classname + "." + methodname + "'.");
diff --git a/core/object/class_db.cpp b/core/object/class_db.cpp
index d920ae6ca05..b9cababc908 100644
--- a/core/object/class_db.cpp
+++ b/core/object/class_db.cpp
@@ -556,6 +556,60 @@ MethodBind *ClassDB::get_method(const StringName &p_class, const StringName &p_n
 	return nullptr;
 }
 
+Vector<uint32_t> ClassDB::get_method_compatibility_hashes(const StringName &p_class, const StringName &p_name) {
+	OBJTYPE_RLOCK;
+
+	ClassInfo *type = classes.getptr(p_class);
+
+	while (type) {
+		if (type->method_map_compatibility.has(p_name)) {
+			LocalVector<MethodBind *> *c = type->method_map_compatibility.getptr(p_name);
+			Vector<uint32_t> ret;
+			for (uint32_t i = 0; i < c->size(); i++) {
+				ret.push_back((*c)[i]->get_hash());
+			}
+			return ret;
+		}
+		type = type->inherits_ptr;
+	}
+	return Vector<uint32_t>();
+}
+
+MethodBind *ClassDB::get_method_with_compatibility(const StringName &p_class, const StringName &p_name, uint64_t p_hash, bool *r_method_exists, bool *r_is_deprecated) {
+	OBJTYPE_RLOCK;
+
+	ClassInfo *type = classes.getptr(p_class);
+
+	while (type) {
+		MethodBind **method = type->method_map.getptr(p_name);
+		if (method && *method) {
+			if (r_method_exists) {
+				*r_method_exists = true;
+			}
+			if ((*method)->get_hash() == p_hash) {
+				return *method;
+			}
+		}
+
+		LocalVector<MethodBind *> *compat = type->method_map_compatibility.getptr(p_name);
+		if (compat) {
+			if (r_method_exists) {
+				*r_method_exists = true;
+			}
+			for (uint32_t i = 0; i < compat->size(); i++) {
+				if ((*compat)[i]->get_hash() == p_hash) {
+					if (r_is_deprecated) {
+						*r_is_deprecated = true;
+					}
+					return (*compat)[i];
+				}
+			}
+		}
+		type = type->inherits_ptr;
+	}
+	return nullptr;
+}
+
 void ClassDB::bind_integer_constant(const StringName &p_class, const StringName &p_enum, const StringName &p_name, int64_t p_constant, bool p_is_bitfield) {
 	OBJTYPE_WLOCK;
 
@@ -1268,11 +1322,30 @@ bool ClassDB::has_method(const StringName &p_class, const StringName &p_method,
 }
 
 void ClassDB::bind_method_custom(const StringName &p_class, MethodBind *p_method) {
+	_bind_method_custom(p_class, p_method, false);
+}
+void ClassDB::bind_compatibility_method_custom(const StringName &p_class, MethodBind *p_method) {
+	_bind_method_custom(p_class, p_method, true);
+}
+
+void ClassDB::_bind_compatibility(ClassInfo *type, MethodBind *p_method) {
+	if (!type->method_map_compatibility.has(p_method->get_name())) {
+		type->method_map_compatibility.insert(p_method->get_name(), LocalVector<MethodBind *>());
+	}
+	type->method_map_compatibility[p_method->get_name()].push_back(p_method);
+}
+
+void ClassDB::_bind_method_custom(const StringName &p_class, MethodBind *p_method, bool p_compatibility) {
 	ClassInfo *type = classes.getptr(p_class);
 	if (!type) {
 		ERR_FAIL_MSG("Couldn't bind custom method '" + p_method->get_name() + "' for instance '" + p_class + "'.");
 	}
 
+	if (p_compatibility) {
+		_bind_compatibility(type, p_method);
+		return;
+	}
+
 	if (type->method_map.has(p_method->get_name())) {
 		// overloading not supported
 		ERR_FAIL_MSG("Method already bound '" + p_class + "::" + p_method->get_name() + "'.");
@@ -1285,11 +1358,44 @@ void ClassDB::bind_method_custom(const StringName &p_class, MethodBind *p_method
 	type->method_map[p_method->get_name()] = p_method;
 }
 
+MethodBind *ClassDB::_bind_vararg_method(MethodBind *p_bind, const StringName &p_name, const Vector<Variant> &p_default_args, bool p_compatibility) {
+	MethodBind *bind = p_bind;
+	bind->set_name(p_name);
+	bind->set_default_arguments(p_default_args);
+
+	String instance_type = bind->get_instance_class();
+
+	ClassInfo *type = classes.getptr(instance_type);
+	if (!type) {
+		memdelete(bind);
+		ERR_FAIL_COND_V(!type, nullptr);
+	}
+
+	if (p_compatibility) {
+		_bind_compatibility(type, bind);
+		return bind;
+	}
+
+	if (type->method_map.has(p_name)) {
+		memdelete(bind);
+		// Overloading not supported
+		ERR_FAIL_V_MSG(nullptr, "Method already bound: " + instance_type + "::" + p_name + ".");
+	}
+	type->method_map[p_name] = bind;
 #ifdef DEBUG_METHODS_ENABLED
-MethodBind *ClassDB::bind_methodfi(uint32_t p_flags, MethodBind *p_bind, const MethodDefinition &method_name, const Variant **p_defs, int p_defcount) {
+	// FIXME: <reduz> set_return_type is no longer in MethodBind, so I guess it should be moved to vararg method bind
+	//bind->set_return_type("Variant");
+	type->method_order.push_back(p_name);
+#endif
+
+	return bind;
+}
+
+#ifdef DEBUG_METHODS_ENABLED
+MethodBind *ClassDB::bind_methodfi(uint32_t p_flags, MethodBind *p_bind, bool p_compatibility, const MethodDefinition &method_name, const Variant **p_defs, int p_defcount) {
 	StringName mdname = method_name.name;
 #else
-MethodBind *ClassDB::bind_methodfi(uint32_t p_flags, MethodBind *p_bind, const char *method_name, const Variant **p_defs, int p_defcount) {
+MethodBind *ClassDB::bind_methodfi(uint32_t p_flags, MethodBind *p_bind, bool p_compatibility, const char *method_name, const Variant **p_defs, int p_defcount) {
 	StringName mdname = StaticCString::create(method_name);
 #endif
 
@@ -1301,7 +1407,7 @@ MethodBind *ClassDB::bind_methodfi(uint32_t p_flags, MethodBind *p_bind, const c
 
 #ifdef DEBUG_ENABLED
 
-	ERR_FAIL_COND_V_MSG(has_method(instance_type, mdname), nullptr, "Class " + String(instance_type) + " already has a method " + String(mdname) + ".");
+	ERR_FAIL_COND_V_MSG(!p_compatibility && has_method(instance_type, mdname), nullptr, "Class " + String(instance_type) + " already has a method " + String(mdname) + ".");
 #endif
 
 	ClassInfo *type = classes.getptr(instance_type);
@@ -1310,7 +1416,7 @@ MethodBind *ClassDB::bind_methodfi(uint32_t p_flags, MethodBind *p_bind, const c
 		ERR_FAIL_V_MSG(nullptr, "Couldn't bind method '" + mdname + "' for instance '" + instance_type + "'.");
 	}
 
-	if (type->method_map.has(mdname)) {
+	if (!p_compatibility && type->method_map.has(mdname)) {
 		memdelete(p_bind);
 		// overloading not supported
 		ERR_FAIL_V_MSG(nullptr, "Method already bound '" + instance_type + "::" + mdname + "'.");
@@ -1325,10 +1431,16 @@ MethodBind *ClassDB::bind_methodfi(uint32_t p_flags, MethodBind *p_bind, const c
 
 	p_bind->set_argument_names(method_name.args);
 
-	type->method_order.push_back(mdname);
+	if (!p_compatibility) {
+		type->method_order.push_back(mdname);
+	}
 #endif
 
-	type->method_map[mdname] = p_bind;
+	if (p_compatibility) {
+		_bind_compatibility(type, p_bind);
+	} else {
+		type->method_map[mdname] = p_bind;
+	}
 
 	Vector<Variant> defvals;
 
@@ -1602,7 +1714,13 @@ void ClassDB::cleanup() {
 		for (KeyValue<StringName, MethodBind *> &F : ti.method_map) {
 			memdelete(F.value);
 		}
+		for (KeyValue<StringName, LocalVector<MethodBind *>> &F : ti.method_map_compatibility) {
+			for (uint32_t i = 0; i < F.value.size(); i++) {
+				memdelete(F.value[i]);
+			}
+		}
 	}
+
 	classes.clear();
 	resource_base_extensions.clear();
 	compat_classes.clear();
diff --git a/core/object/class_db.h b/core/object/class_db.h
index 1a5e9235cf1..ce64336a45c 100644
--- a/core/object/class_db.h
+++ b/core/object/class_db.h
@@ -105,6 +105,7 @@ public:
 		ObjectGDExtension *gdextension = nullptr;
 
 		HashMap<StringName, MethodBind *> method_map;
+		HashMap<StringName, LocalVector<MethodBind *>> method_map_compatibility;
 		HashMap<StringName, int64_t> constant_map;
 		struct EnumInfo {
 			List<StringName> constants;
@@ -148,9 +149,9 @@ public:
 	static HashMap<StringName, StringName> compat_classes;
 
 #ifdef DEBUG_METHODS_ENABLED
-	static MethodBind *bind_methodfi(uint32_t p_flags, MethodBind *p_bind, const MethodDefinition &method_name, const Variant **p_defs, int p_defcount);
+	static MethodBind *bind_methodfi(uint32_t p_flags, MethodBind *p_bind, bool p_compatibility, const MethodDefinition &method_name, const Variant **p_defs, int p_defcount);
 #else
-	static MethodBind *bind_methodfi(uint32_t p_flags, MethodBind *p_bind, const char *method_name, const Variant **p_defs, int p_defcount);
+	static MethodBind *bind_methodfi(uint32_t p_flags, MethodBind *p_bind, bool p_compatibility, const char *method_name, const Variant **p_defs, int p_defcount);
 #endif
 
 	static APIType current_api;
@@ -172,6 +173,9 @@ private:
 	// Non-locking variants of get_parent_class and is_parent_class.
 	static StringName _get_parent_class(const StringName &p_class);
 	static bool _is_parent_class(const StringName &p_class, const StringName &p_inherits);
+	static void _bind_compatibility(ClassInfo *type, MethodBind *p_method);
+	static MethodBind *_bind_vararg_method(MethodBind *p_bind, const StringName &p_name, const Vector<Variant> &p_default_args, bool p_compatibility);
+	static void _bind_method_custom(const StringName &p_class, MethodBind *p_method, bool p_compatibility);
 
 public:
 	// DO NOT USE THIS!!!!!! NEEDS TO BE PUBLIC BUT DO NOT USE NO MATTER WHAT!!!
@@ -273,7 +277,7 @@ public:
 		if constexpr (std::is_same<typename member_function_traits<M>::return_type, Object *>::value) {
 			bind->set_return_type_is_raw_object_ptr(true);
 		}
-		return bind_methodfi(METHOD_FLAGS_DEFAULT, bind, p_method_name, sizeof...(p_args) == 0 ? nullptr : (const Variant **)argptrs, sizeof...(p_args));
+		return bind_methodfi(METHOD_FLAGS_DEFAULT, bind, false, p_method_name, sizeof...(p_args) == 0 ? nullptr : (const Variant **)argptrs, sizeof...(p_args));
 	}
 
 	template <class N, class M, typename... VarArgs>
@@ -288,7 +292,36 @@ public:
 		if constexpr (std::is_same<typename member_function_traits<M>::return_type, Object *>::value) {
 			bind->set_return_type_is_raw_object_ptr(true);
 		}
-		return bind_methodfi(METHOD_FLAGS_DEFAULT, bind, p_method_name, sizeof...(p_args) == 0 ? nullptr : (const Variant **)argptrs, sizeof...(p_args));
+		return bind_methodfi(METHOD_FLAGS_DEFAULT, bind, false, p_method_name, sizeof...(p_args) == 0 ? nullptr : (const Variant **)argptrs, sizeof...(p_args));
+	}
+
+	template <class N, class M, typename... VarArgs>
+	static MethodBind *bind_compatibility_method(N p_method_name, M p_method, VarArgs... p_args) {
+		Variant args[sizeof...(p_args) + 1] = { p_args..., Variant() }; // +1 makes sure zero sized arrays are also supported.
+		const Variant *argptrs[sizeof...(p_args) + 1];
+		for (uint32_t i = 0; i < sizeof...(p_args); i++) {
+			argptrs[i] = &args[i];
+		}
+		MethodBind *bind = create_method_bind(p_method);
+		if constexpr (std::is_same<typename member_function_traits<M>::return_type, Object *>::value) {
+			bind->set_return_type_is_raw_object_ptr(true);
+		}
+		return bind_methodfi(METHOD_FLAGS_DEFAULT, bind, true, p_method_name, sizeof...(p_args) == 0 ? nullptr : (const Variant **)argptrs, sizeof...(p_args));
+	}
+
+	template <class N, class M, typename... VarArgs>
+	static MethodBind *bind_compatibility_static_method(const StringName &p_class, N p_method_name, M p_method, VarArgs... p_args) {
+		Variant args[sizeof...(p_args) + 1] = { p_args..., Variant() }; // +1 makes sure zero sized arrays are also supported.
+		const Variant *argptrs[sizeof...(p_args) + 1];
+		for (uint32_t i = 0; i < sizeof...(p_args); i++) {
+			argptrs[i] = &args[i];
+		}
+		MethodBind *bind = create_static_method_bind(p_method);
+		bind->set_instance_class(p_class);
+		if constexpr (std::is_same<typename member_function_traits<M>::return_type, Object *>::value) {
+			bind->set_return_type_is_raw_object_ptr(true);
+		}
+		return bind_methodfi(METHOD_FLAGS_DEFAULT, bind, true, p_method_name, sizeof...(p_args) == 0 ? nullptr : (const Variant **)argptrs, sizeof...(p_args));
 	}
 
 	template <class M>
@@ -298,36 +331,27 @@ public:
 		MethodBind *bind = create_vararg_method_bind(p_method, p_info, p_return_nil_is_variant);
 		ERR_FAIL_COND_V(!bind, nullptr);
 
-		bind->set_name(p_name);
-		bind->set_default_arguments(p_default_args);
 		if constexpr (std::is_same<typename member_function_traits<M>::return_type, Object *>::value) {
 			bind->set_return_type_is_raw_object_ptr(true);
 		}
+		return _bind_vararg_method(bind, p_name, p_default_args, false);
+	}
 
-		String instance_type = bind->get_instance_class();
+	template <class M>
+	static MethodBind *bind_compatibility_vararg_method(uint32_t p_flags, const StringName &p_name, M p_method, const MethodInfo &p_info = MethodInfo(), const Vector<Variant> &p_default_args = Vector<Variant>(), bool p_return_nil_is_variant = true) {
+		GLOBAL_LOCK_FUNCTION;
 
-		ClassInfo *type = classes.getptr(instance_type);
-		if (!type) {
-			memdelete(bind);
-			ERR_FAIL_COND_V(!type, nullptr);
+		MethodBind *bind = create_vararg_method_bind(p_method, p_info, p_return_nil_is_variant);
+		ERR_FAIL_COND_V(!bind, nullptr);
+
+		if constexpr (std::is_same<typename member_function_traits<M>::return_type, Object *>::value) {
+			bind->set_return_type_is_raw_object_ptr(true);
 		}
-
-		if (type->method_map.has(p_name)) {
-			memdelete(bind);
-			// Overloading not supported
-			ERR_FAIL_V_MSG(nullptr, "Method already bound: " + instance_type + "::" + p_name + ".");
-		}
-		type->method_map[p_name] = bind;
-#ifdef DEBUG_METHODS_ENABLED
-		// FIXME: <reduz> set_return_type is no longer in MethodBind, so I guess it should be moved to vararg method bind
-		//bind->set_return_type("Variant");
-		type->method_order.push_back(p_name);
-#endif
-
-		return bind;
+		return _bind_vararg_method(bind, p_name, p_default_args, true);
 	}
 
 	static void bind_method_custom(const StringName &p_class, MethodBind *p_method);
+	static void bind_compatibility_method_custom(const StringName &p_class, MethodBind *p_method);
 
 	static void add_signal(const StringName &p_class, const MethodInfo &p_signal);
 	static bool has_signal(const StringName &p_class, const StringName &p_signal, bool p_no_inheritance = false);
@@ -358,6 +382,8 @@ public:
 	static void get_method_list(const StringName &p_class, List<MethodInfo> *p_methods, bool p_no_inheritance = false, bool p_exclude_from_properties = false);
 	static bool get_method_info(const StringName &p_class, const StringName &p_method, MethodInfo *r_info, bool p_no_inheritance = false, bool p_exclude_from_properties = false);
 	static MethodBind *get_method(const StringName &p_class, const StringName &p_name);
+	static MethodBind *get_method_with_compatibility(const StringName &p_class, const StringName &p_name, uint64_t p_hash, bool *r_method_exists = nullptr, bool *r_is_deprecated = nullptr);
+	static Vector<uint32_t> get_method_compatibility_hashes(const StringName &p_class, const StringName &p_name);
 
 	static void add_virtual_method(const StringName &p_class, const MethodInfo &p_method, bool p_virtual = true, const Vector<String> &p_arg_names = Vector<String>(), bool p_object_core = false);
 	static void get_virtual_methods(const StringName &p_class, List<MethodInfo> *p_methods, bool p_no_inheritance = false);
diff --git a/main/main.cpp b/main/main.cpp
index 6b89d961476..5ba1eac1f3a 100644
--- a/main/main.cpp
+++ b/main/main.cpp
@@ -217,6 +217,8 @@ static bool print_fps = false;
 #ifdef TOOLS_ENABLED
 static bool dump_gdextension_interface = false;
 static bool dump_extension_api = false;
+static bool validate_extension_api = false;
+static String validate_extension_api_file;
 #endif
 bool profile_gpu = false;
 
@@ -458,6 +460,7 @@ void Main::print_help(const char *p_binary) {
 	OS::get_singleton()->print("  --build-solutions                 Build the scripting solutions (e.g. for C# projects). Implies --editor and requires a valid project to edit.\n");
 	OS::get_singleton()->print("  --dump-gdextension-interface      Generate GDExtension header file 'gdextension_interface.h' in the current folder. This file is the base file required to implement a GDExtension.\n");
 	OS::get_singleton()->print("  --dump-extension-api              Generate JSON dump of the Godot API for GDExtension bindings named 'extension_api.json' in the current folder.\n");
+	OS::get_singleton()->print("  --validate-extension-api <path>   Validate an extension API file dumped (with the option above) from a previous version of the engine to ensure API compatibility. If incompatibilities or errors are detected, the return code will be non zero.\n");
 	OS::get_singleton()->print("  --startup-benchmark               Benchmark the startup time and print it to console.\n");
 	OS::get_singleton()->print("  --startup-benchmark-file <path>   Benchmark the startup time and save it to a given file in JSON format.\n");
 #ifdef TESTS_ENABLED
@@ -1133,6 +1136,25 @@ Error Main::setup(const char *execpath, int argc, char *argv[], bool p_second_ph
 			// run the project instead of a cmdline tool.
 			// Needs full refactoring to fix properly.
 			main_args.push_back(I->get());
+		} else if (I->get() == "--validate-extension-api") {
+			// Register as an editor instance to use low-end fallback if relevant.
+			editor = true;
+			cmdline_tool = true;
+			validate_extension_api = true;
+			// Hack. Not needed but otherwise we end up detecting that this should
+			// run the project instead of a cmdline tool.
+			// Needs full refactoring to fix properly.
+			main_args.push_back(I->get());
+
+			if (I->next()) {
+				validate_extension_api_file = I->next()->get();
+
+				N = I->next()->next();
+			} else {
+				OS::get_singleton()->print("Missing file to load argument after --validate-extension-api, aborting.");
+				goto error;
+			}
+
 		} else if (I->get() == "--export-release" || I->get() == "--export-debug" ||
 				I->get() == "--export-pack") { // Export project
 			// Actually handling is done in start().
@@ -2650,6 +2672,12 @@ bool Main::start() {
 		return false;
 	}
 
+	if (validate_extension_api) {
+		bool valid = GDExtensionAPIDump::validate_extension_json_file(validate_extension_api_file) == OK;
+		OS::get_singleton()->set_exit_code(valid ? EXIT_SUCCESS : EXIT_FAILURE);
+		return false;
+	}
+
 #ifndef DISABLE_DEPRECATED
 	if (converting_project) {
 		int ret = ProjectConverter3To4(converter_max_kb_file, converter_max_line_length).convert();