diff --git a/fastboot/device/commands.cpp b/fastboot/device/commands.cpp
index 1e853bf..eb18141 100644
--- a/fastboot/device/commands.cpp
+++ b/fastboot/device/commands.cpp
@@ -43,7 +43,8 @@
 using namespace android::fs_mgr;
 
 bool GetVarHandler(FastbootDevice* device, const std::vector<std::string>& args) {
-    using VariableHandler = std::function<bool(FastbootDevice*, const std::vector<std::string>&)>;
+    using VariableHandler =
+            std::function<bool(FastbootDevice*, const std::vector<std::string>&, std::string*)>;
     const std::unordered_map<std::string, VariableHandler> kVariableMap = {
             {FB_VAR_VERSION, GetVersion},
             {FB_VAR_VERSION_BOOTLOADER, GetBootloaderVersion},
@@ -65,11 +66,15 @@
     // args[0] is command name, args[1] is variable.
     auto found_variable = kVariableMap.find(args[1]);
     if (found_variable == kVariableMap.end()) {
-        return device->WriteStatus(FastbootResult::FAIL, "Unknown variable");
+        return device->WriteFail("Unknown variable");
     }
 
+    std::string message;
     std::vector<std::string> getvar_args(args.begin() + 2, args.end());
-    return found_variable->second(device, getvar_args);
+    if (!found_variable->second(device, getvar_args, &message)) {
+        return device->WriteFail(message);
+    }
+    return device->WriteOkay(message);
 }
 
 bool EraseHandler(FastbootDevice* device, const std::vector<std::string>& args) {
diff --git a/fastboot/device/variables.cpp b/fastboot/device/variables.cpp
index 65cfea3..8eeda98 100644
--- a/fastboot/device/variables.cpp
+++ b/fastboot/device/variables.cpp
@@ -35,141 +35,190 @@
 constexpr int kMaxDownloadSizeDefault = 0x20000000;
 constexpr char kFastbootProtocolVersion[] = "0.4";
 
-bool GetVersion(FastbootDevice* device, const std::vector<std::string>& /* args */) {
-    return device->WriteOkay(kFastbootProtocolVersion);
+bool GetVersion(FastbootDevice* /* device */, const std::vector<std::string>& /* args */,
+                std::string* message) {
+    *message = kFastbootProtocolVersion;
+    return true;
 }
 
-bool GetBootloaderVersion(FastbootDevice* device, const std::vector<std::string>& /* args */) {
-    return device->WriteOkay(android::base::GetProperty("ro.bootloader", ""));
+bool GetBootloaderVersion(FastbootDevice* /* device */, const std::vector<std::string>& /* args */,
+                          std::string* message) {
+    *message = android::base::GetProperty("ro.bootloader", "");
+    return true;
 }
 
-bool GetBasebandVersion(FastbootDevice* device, const std::vector<std::string>& /* args */) {
-    return device->WriteOkay(android::base::GetProperty("ro.build.expect.baseband", ""));
+bool GetBasebandVersion(FastbootDevice* /* device */, const std::vector<std::string>& /* args */,
+                        std::string* message) {
+    *message = android::base::GetProperty("ro.build.expect.baseband", "");
+    return true;
 }
 
-bool GetProduct(FastbootDevice* device, const std::vector<std::string>& /* args */) {
-    return device->WriteOkay(android::base::GetProperty("ro.product.device", ""));
+bool GetProduct(FastbootDevice* /* device */, const std::vector<std::string>& /* args */,
+                std::string* message) {
+    *message = android::base::GetProperty("ro.product.device", "");
+    return true;
 }
 
-bool GetSerial(FastbootDevice* device, const std::vector<std::string>& /* args */) {
-    return device->WriteOkay(android::base::GetProperty("ro.serialno", ""));
+bool GetSerial(FastbootDevice* /* device */, const std::vector<std::string>& /* args */,
+               std::string* message) {
+    *message = android::base::GetProperty("ro.serialno", "");
+    return true;
 }
 
-bool GetSecure(FastbootDevice* device, const std::vector<std::string>& /* args */) {
-    return device->WriteOkay(android::base::GetBoolProperty("ro.secure", "") ? "yes" : "no");
+bool GetSecure(FastbootDevice* /* device */, const std::vector<std::string>& /* args */,
+               std::string* message) {
+    *message = android::base::GetBoolProperty("ro.secure", "") ? "yes" : "no";
+    return true;
 }
 
-bool GetCurrentSlot(FastbootDevice* device, const std::vector<std::string>& /* args */) {
+bool GetCurrentSlot(FastbootDevice* device, const std::vector<std::string>& /* args */,
+                    std::string* message) {
     std::string suffix = device->GetCurrentSlot();
-    std::string slot = suffix.size() == 2 ? suffix.substr(1) : suffix;
-    return device->WriteOkay(slot);
+    *message = suffix.size() == 2 ? suffix.substr(1) : suffix;
+    return true;
 }
 
-bool GetSlotCount(FastbootDevice* device, const std::vector<std::string>& /* args */) {
+bool GetSlotCount(FastbootDevice* device, const std::vector<std::string>& /* args */,
+                  std::string* message) {
     auto boot_control_hal = device->boot_control_hal();
     if (!boot_control_hal) {
-        return "0";
+        *message = "0";
+    } else {
+        *message = std::to_string(boot_control_hal->getNumberSlots());
     }
-    return device->WriteOkay(std::to_string(boot_control_hal->getNumberSlots()));
+    return true;
 }
 
-bool GetSlotSuccessful(FastbootDevice* device, const std::vector<std::string>& args) {
+bool GetSlotSuccessful(FastbootDevice* device, const std::vector<std::string>& args,
+                       std::string* message) {
     if (args.empty()) {
-        return device->WriteFail("Missing argument");
+        *message = "Missing argument";
+        return false;
     }
     Slot slot;
     if (!GetSlotNumber(args[0], &slot)) {
-        return device->WriteFail("Invalid slot");
+        *message = "Invalid slot";
+        return false;
     }
     auto boot_control_hal = device->boot_control_hal();
     if (!boot_control_hal) {
-        return device->WriteFail("Device has no slots");
+        *message = "Device has no slots";
+        return false;
     }
     if (boot_control_hal->isSlotMarkedSuccessful(slot) != BoolResult::TRUE) {
-        return device->WriteOkay("no");
+        *message = "no";
+    } else {
+        *message = "yes";
     }
-    return device->WriteOkay("yes");
+    return true;
 }
 
-bool GetSlotUnbootable(FastbootDevice* device, const std::vector<std::string>& args) {
+bool GetSlotUnbootable(FastbootDevice* device, const std::vector<std::string>& args,
+                       std::string* message) {
     if (args.empty()) {
-        return device->WriteFail("Missing argument");
+        *message = "Missing argument";
+        return false;
     }
     Slot slot;
     if (!GetSlotNumber(args[0], &slot)) {
-        return device->WriteFail("Invalid slot");
+        *message = "Invalid slot";
+        return false;
     }
     auto boot_control_hal = device->boot_control_hal();
     if (!boot_control_hal) {
-        return device->WriteFail("Device has no slots");
+        *message = "Device has no slots";
+        return false;
     }
     if (boot_control_hal->isSlotBootable(slot) != BoolResult::TRUE) {
-        return device->WriteOkay("yes");
+        *message = "yes";
+    } else {
+        *message = "no";
     }
-    return device->WriteOkay("no");
+    return true;
 }
 
-bool GetMaxDownloadSize(FastbootDevice* device, const std::vector<std::string>& /* args */) {
-    return device->WriteOkay(std::to_string(kMaxDownloadSizeDefault));
+bool GetMaxDownloadSize(FastbootDevice* /* device */, const std::vector<std::string>& /* args */,
+                        std::string* message) {
+    *message = std::to_string(kMaxDownloadSizeDefault);
+    return true;
 }
 
-bool GetUnlocked(FastbootDevice* device, const std::vector<std::string>& /* args */) {
-    return device->WriteOkay("yes");
+bool GetUnlocked(FastbootDevice* /* device */, const std::vector<std::string>& /* args */,
+                 std::string* message) {
+    *message = "yes";
+    return true;
 }
 
-bool GetHasSlot(FastbootDevice* device, const std::vector<std::string>& args) {
+bool GetHasSlot(FastbootDevice* device, const std::vector<std::string>& args,
+                std::string* message) {
     if (args.empty()) {
-        return device->WriteFail("Missing argument");
+        *message = "Missing argument";
+        return false;
     }
     std::string slot_suffix = device->GetCurrentSlot();
     if (slot_suffix.empty()) {
-        return device->WriteOkay("no");
+        *message = "no";
+        return true;
     }
     std::string partition_name = args[0] + slot_suffix;
     if (FindPhysicalPartition(partition_name) ||
         LogicalPartitionExists(partition_name, slot_suffix)) {
-        return device->WriteOkay("yes");
+        *message = "yes";
+    } else {
+        *message = "no";
     }
-    return device->WriteOkay("no");
+    return true;
 }
 
-bool GetPartitionSize(FastbootDevice* device, const std::vector<std::string>& args) {
+bool GetPartitionSize(FastbootDevice* device, const std::vector<std::string>& args,
+                      std::string* message) {
     if (args.size() < 1) {
-        return device->WriteFail("Missing argument");
+        *message = "Missing argument";
+        return false;
     }
     // Zero-length partitions cannot be created through device-mapper, so we
     // special case them here.
     bool is_zero_length;
     if (LogicalPartitionExists(args[0], device->GetCurrentSlot(), &is_zero_length) &&
         is_zero_length) {
-        return device->WriteOkay("0");
+        *message = "0";
+        return true;
     }
     // Otherwise, open the partition as normal.
     PartitionHandle handle;
     if (!OpenPartition(device, args[0], &handle)) {
-        return device->WriteFail("Could not open partition");
+        *message = "Could not open partition";
+        return false;
     }
     uint64_t size = get_block_device_size(handle.fd());
-    return device->WriteOkay(android::base::StringPrintf("%" PRIX64, size));
+    *message = android::base::StringPrintf("%" PRIX64, size);
+    return true;
 }
 
-bool GetPartitionIsLogical(FastbootDevice* device, const std::vector<std::string>& args) {
+bool GetPartitionIsLogical(FastbootDevice* device, const std::vector<std::string>& args,
+                           std::string* message) {
     if (args.size() < 1) {
-        return device->WriteFail("Missing argument");
+        *message = "Missing argument";
+        return false;
     }
     // Note: if a partition name is in both the GPT and the super partition, we
     // return "true", to be consistent with prefering to flash logical partitions
     // over physical ones.
     std::string partition_name = args[0];
     if (LogicalPartitionExists(partition_name, device->GetCurrentSlot())) {
-        return device->WriteOkay("yes");
+        *message = "yes";
+        return true;
     }
     if (FindPhysicalPartition(partition_name)) {
-        return device->WriteOkay("no");
+        *message = "no";
+        return true;
     }
-    return device->WriteFail("Partition not found");
+    *message = "Partition not found";
+    return false;
 }
 
-bool GetIsUserspace(FastbootDevice* device, const std::vector<std::string>& /* args */) {
-    return device->WriteOkay("yes");
+bool GetIsUserspace(FastbootDevice* /* device */, const std::vector<std::string>& /* args */,
+                    std::string* message) {
+    *message = "yes";
+    return true;
 }
diff --git a/fastboot/device/variables.h b/fastboot/device/variables.h
index 554a080..b643bbd 100644
--- a/fastboot/device/variables.h
+++ b/fastboot/device/variables.h
@@ -21,19 +21,30 @@
 
 class FastbootDevice;
 
-bool GetVersion(FastbootDevice* device, const std::vector<std::string>& args);
-bool GetBootloaderVersion(FastbootDevice* device, const std::vector<std::string>& args);
-bool GetBasebandVersion(FastbootDevice* device, const std::vector<std::string>& args);
-bool GetProduct(FastbootDevice* device, const std::vector<std::string>& args);
-bool GetSerial(FastbootDevice* device, const std::vector<std::string>& args);
-bool GetSecure(FastbootDevice* device, const std::vector<std::string>& args);
-bool GetCurrentSlot(FastbootDevice* device, const std::vector<std::string>& args);
-bool GetSlotCount(FastbootDevice* device, const std::vector<std::string>& args);
-bool GetSlotSuccessful(FastbootDevice* device, const std::vector<std::string>& args);
-bool GetSlotUnbootable(FastbootDevice* device, const std::vector<std::string>& args);
-bool GetMaxDownloadSize(FastbootDevice* device, const std::vector<std::string>& args);
-bool GetUnlocked(FastbootDevice* device, const std::vector<std::string>& args);
-bool GetHasSlot(FastbootDevice* device, const std::vector<std::string>& args);
-bool GetPartitionSize(FastbootDevice* device, const std::vector<std::string>& args);
-bool GetPartitionIsLogical(FastbootDevice* device, const std::vector<std::string>& args);
-bool GetIsUserspace(FastbootDevice* device, const std::vector<std::string>& args);
+bool GetVersion(FastbootDevice* device, const std::vector<std::string>& args, std::string* message);
+bool GetBootloaderVersion(FastbootDevice* device, const std::vector<std::string>& args,
+                          std::string* message);
+bool GetBasebandVersion(FastbootDevice* device, const std::vector<std::string>& args,
+                        std::string* message);
+bool GetProduct(FastbootDevice* device, const std::vector<std::string>& args, std::string* message);
+bool GetSerial(FastbootDevice* device, const std::vector<std::string>& args, std::string* message);
+bool GetSecure(FastbootDevice* device, const std::vector<std::string>& args, std::string* message);
+bool GetCurrentSlot(FastbootDevice* device, const std::vector<std::string>& args,
+                    std::string* message);
+bool GetSlotCount(FastbootDevice* device, const std::vector<std::string>& args,
+                  std::string* message);
+bool GetSlotSuccessful(FastbootDevice* device, const std::vector<std::string>& args,
+                       std::string* message);
+bool GetSlotUnbootable(FastbootDevice* device, const std::vector<std::string>& args,
+                       std::string* message);
+bool GetMaxDownloadSize(FastbootDevice* device, const std::vector<std::string>& args,
+                        std::string* message);
+bool GetUnlocked(FastbootDevice* device, const std::vector<std::string>& args,
+                 std::string* message);
+bool GetHasSlot(FastbootDevice* device, const std::vector<std::string>& args, std::string* message);
+bool GetPartitionSize(FastbootDevice* device, const std::vector<std::string>& args,
+                      std::string* message);
+bool GetPartitionIsLogical(FastbootDevice* device, const std::vector<std::string>& args,
+                           std::string* message);
+bool GetIsUserspace(FastbootDevice* device, const std::vector<std::string>& args,
+                    std::string* message);