diff --git a/base/cvd/cuttlefish/host/commands/assemble_cvd/assemble_cvd_flags.cpp b/base/cvd/cuttlefish/host/commands/assemble_cvd/assemble_cvd_flags.cpp
index e62f828b4d0..862cd29b0ce 100644
--- a/base/cvd/cuttlefish/host/commands/assemble_cvd/assemble_cvd_flags.cpp
+++ b/base/cvd/cuttlefish/host/commands/assemble_cvd/assemble_cvd_flags.cpp
@@ -497,11 +497,18 @@ DEFINE_string(fuchsia_root_image, CF_DEFAULTS_FUCHSIA_ROOT_IMAGE,
 
 DEFINE_string(
     custom_partition_path, CF_DEFAULTS_CUSTOM_PARTITION_PATH,
-    "Location of custom image that will be passed as a \"custom\" partition"
-    "to rootfs and can be used by /dev/block/by-name/custom. Multiple images "
-    "can be passed, separated by semicolons and can be used as "
-    "/dev/block/by-name/custom_1, /dev/block/by-name/custom_2, etc. Example: "
-    "--custom_partition_path=\"/path/to/custom.img;/path/to/other.img\"");
+    "Location of custom image that will be passed as a custom partition "
+    "to rootfs. Multiple entries can be passed, separated by semicolons. "
+    "Each entry uses key=value pairs separated by commas. "
+    "Supported keys: name (partition label), path (image file path), "
+    "ab (true/false for A/B slots). 'path' is required; 'name' defaults to "
+    "'custom', 'custom_1', etc.; 'ab' defaults to false. "
+    "Example with A/B: "
+    "--custom_partition_path=\"name=oem1,path=./oem1.img,ab=true;"
+    "name=oem2,path=./oem2.img\" "
+    "Example without names (legacy): "
+    "--custom_partition_path=\"/path/to/custom.img;/path/to/other.img\" "
+    "For multi-instance, separate per-instance values with '|'.");
 
 DEFINE_string(
     blank_sdcard_image_mb, CF_DEFAULTS_BLANK_SDCARD_IMAGE_MB,
diff --git a/base/cvd/cuttlefish/host/commands/assemble_cvd/disk/android_composite_disk_config.cc b/base/cvd/cuttlefish/host/commands/assemble_cvd/disk/android_composite_disk_config.cc
index 2c31ea21708..f78f25a8ad0 100644
--- a/base/cvd/cuttlefish/host/commands/assemble_cvd/disk/android_composite_disk_config.cc
+++ b/base/cvd/cuttlefish/host/commands/assemble_cvd/disk/android_composite_disk_config.cc
@@ -26,6 +26,7 @@
 #include <string_view>
 #include <vector>
 
+#include "absl/log/log.h"
 #include "absl/strings/str_cat.h"
 #include "absl/strings/str_split.h"
 
@@ -72,6 +73,63 @@ std::optional<ImagePartition> HibernationImage(
   return FileExists(path) ? std::optional<ImagePartition>(image) : std::nullopt;
 }
 
+struct CustomPartitionSpec {
+  std::string label;
+  std::string path;
+  bool ab_enabled;
+};
+
+Result<CustomPartitionSpec> ParseCustomPartitionSpec(std::string_view spec,
+                                                     int index) {
+  std::string label;
+  std::string path;
+  bool ab_enabled = false;
+
+  // Key=value format: "name=oem,path=./oem.img,ab=true"
+  if (spec.find('=') != std::string_view::npos) {
+    std::vector<std::string_view> kvpairs = absl::StrSplit(spec, ',');
+    for (const auto& kvpair : kvpairs) {
+      std::vector<std::string_view> kv = absl::StrSplit(kvpair, '=');
+      CF_EXPECTF(kv.size() == 2,
+                 "Invalid key=value pair '{}' in custom partition spec '{}'",
+                 kvpair, spec);
+      std::string_view key = kv[0];
+      std::string_view value = kv[1];
+      if (key == "name") {
+        label = std::string(value);
+      } else if (key == "path") {
+        path = std::string(value);
+      } else if (key == "ab") {
+        CF_EXPECTF(value == "true" || value == "false",
+                   "Invalid value '{}' for key 'ab' in custom partition spec "
+                   "'{}'. Expected 'true' or 'false'",
+                   value, spec);
+        ab_enabled = (value == "true");
+      } else {
+        return CF_ERRF("Unknown key '{}' in custom partition spec '{}'. "
+                       "Valid keys are: name, path, ab",
+                       key, spec);
+      }
+    }
+    CF_EXPECTF(!path.empty(),
+               "Missing required 'path' key in custom partition spec '{}'",
+               spec);
+    if (label.empty()) {
+      label = index > 0 ? "custom_" + std::to_string(index) : "custom";
+    }
+  } else {
+    // Legacy format: plain path only
+    label = index > 0 ? "custom_" + std::to_string(index) : "custom";
+    path = std::string(spec);
+  }
+
+  return CustomPartitionSpec{
+      .label = std::move(label),
+      .path = std::move(path),
+      .ab_enabled = ab_enabled,
+  };
+}
+
 }  // namespace
 
 Result<std::vector<ImagePartition>> AndroidCompositeDiskConfig(
@@ -214,10 +272,26 @@ Result<std::vector<ImagePartition>> AndroidCompositeDiskConfig(
     std::vector<std::string_view> custom_partition_paths =
         absl::StrSplit(custom_partition_path, ';');
     for (int i = 0; i < custom_partition_paths.size(); i++) {
-      partitions.push_back(ImagePartition{
-          .label = i > 0 ? "custom_" + std::to_string(i) : "custom",
-          .image_file_path = AbsolutePath(custom_partition_paths[i]),
-      });
+      CustomPartitionSpec spec =
+          CF_EXPECT(ParseCustomPartitionSpec(custom_partition_paths[i], i));
+      if (spec.ab_enabled) {
+        VLOG(1) << "Adding custom partition: " << spec.label + "_a";
+        partitions.push_back(ImagePartition{
+            .label = spec.label + "_a",
+            .image_file_path = AbsolutePath(spec.path),
+        });
+        VLOG(1) << "Adding custom partition: " << spec.label + "_b";
+        partitions.push_back(ImagePartition{
+            .label = spec.label + "_b",
+            .image_file_path = AbsolutePath(spec.path),
+        });
+      } else {
+        VLOG(1) << "Adding custom partition: " << spec.label;
+        partitions.push_back(ImagePartition{
+            .label = spec.label,
+            .image_file_path = AbsolutePath(spec.path),
+        });
+      }
     }
   }
 
diff --git a/base/cvd/cuttlefish/host/commands/assemble_cvd/disk_builder.cpp b/base/cvd/cuttlefish/host/commands/assemble_cvd/disk_builder.cpp
index 6fdb6390210..1637fb7c850 100644
--- a/base/cvd/cuttlefish/host/commands/assemble_cvd/disk_builder.cpp
+++ b/base/cvd/cuttlefish/host/commands/assemble_cvd/disk_builder.cpp
@@ -155,7 +155,7 @@ Result<std::string> DiskBuilder::TextConfig() {
   CF_EXPECT(!partitions_.empty() ^ !entire_disk_.empty(),
             "Specify either partitions or a whole disk");
   for (auto& partition : partitions_) {
-    disk_conf << partition.image_file_path << "\n";
+    disk_conf << partition.label << ":" << partition.image_file_path << "\n";
   }
   if (!entire_disk_.empty()) {
     disk_conf << entire_disk_;
diff --git a/base/cvd/cuttlefish/host/commands/assemble_cvd/disk_image_flags_vectorization.cc b/base/cvd/cuttlefish/host/commands/assemble_cvd/disk_image_flags_vectorization.cc
index 8bc5ee5c2ea..2981373cc19 100644
--- a/base/cvd/cuttlefish/host/commands/assemble_cvd/disk_image_flags_vectorization.cc
+++ b/base/cvd/cuttlefish/host/commands/assemble_cvd/disk_image_flags_vectorization.cc
@@ -84,8 +84,10 @@ Result<void> DiskImageFlagsVectorization(
   std::vector<std::string> fuchsia_root_image =
       absl::StrSplit(FLAGS_fuchsia_root_image, ',');
 
+  // custom_partition_path uses ',' inside key=value entries, so split
+  // on '|' for multi-instance separation instead of ','.
   std::vector<std::string> custom_partition_path =
-      absl::StrSplit(FLAGS_custom_partition_path, ',');
+      absl::StrSplit(FLAGS_custom_partition_path, '|');
 
   std::vector<std::string> blank_sdcard_image_mb =
       absl::StrSplit(FLAGS_blank_sdcard_image_mb, ',');