diff --git a/METADATA b/METADATA
deleted file mode 100644
index d97975c..0000000
--- a/METADATA
+++ /dev/null
@@ -1,3 +0,0 @@
-third_party {
-  license_type: NOTICE
-}
diff --git a/aosp/service_delegate_android_interface.h b/aosp/service_delegate_android_interface.h
index c73c6de..5e139d7 100644
--- a/aosp/service_delegate_android_interface.h
+++ b/aosp/service_delegate_android_interface.h
@@ -17,8 +17,6 @@
 #ifndef UPDATE_ENGINE_AOSP_SERVICE_DELEGATE_ANDROID_INTERFACE_H_
 #define UPDATE_ENGINE_AOSP_SERVICE_DELEGATE_ANDROID_INTERFACE_H_
 
-#include <inttypes.h>
-
 #include <memory>
 #include <string>
 #include <vector>
diff --git a/aosp/update_attempter_android.cc b/aosp/update_attempter_android.cc
index f29383a..89c79f3 100644
--- a/aosp/update_attempter_android.cc
+++ b/aosp/update_attempter_android.cc
@@ -17,6 +17,7 @@
 #include "update_engine/aosp/update_attempter_android.h"
 
 #include <algorithm>
+#include <iterator>
 #include <map>
 #include <memory>
 #include <ostream>
@@ -56,6 +57,7 @@
 #include "update_engine/payload_consumer/payload_verifier.h"
 #include "update_engine/payload_consumer/postinstall_runner_action.h"
 #include "update_engine/update_boot_flags_action.h"
+#include "update_engine/update_metadata.pb.h"
 #include "update_engine/update_status.h"
 #include "update_engine/update_status_utils.h"
 
@@ -546,6 +548,32 @@
   return !(a == b);
 }
 
+bool VerifyPayloadMetadata(Error* error,
+                           std::string_view metadata,
+                           const PayloadMetadata& payload_metadata) {
+  auto payload_verifier = PayloadVerifier::CreateInstanceFromZipPath(
+      constants::kUpdateCertificatesPath);
+  if (!payload_verifier) {
+    return LogAndSetError(error,
+                          __LINE__,
+                          __FILE__,
+                          "Failed to create the payload verifier from " +
+                              std::string(constants::kUpdateCertificatesPath),
+                          ErrorCode::kDownloadManifestParseError);
+  }
+  auto errorcode = payload_metadata.ValidateMetadataSignature(
+      metadata, "", *payload_verifier);
+  if (errorcode != ErrorCode::kSuccess) {
+    return LogAndSetError(error,
+                          __LINE__,
+                          __FILE__,
+                          "Failed to validate metadata signature: " +
+                              utils::ErrorCodeToString(errorcode),
+                          errorcode);
+  }
+  return true;
+}
+
 bool UpdateAttempterAndroid::VerifyPayloadParseManifest(
     const std::string& metadata_filename,
     std::string_view expected_metadata_hash,
@@ -619,27 +647,9 @@
                 << HexEncode(metadata_hash);
     }
   }
+  TEST_AND_RETURN_FALSE(
+      VerifyPayloadMetadata(error, ToStringView(metadata), payload_metadata));
 
-  auto payload_verifier = PayloadVerifier::CreateInstanceFromZipPath(
-      constants::kUpdateCertificatesPath);
-  if (!payload_verifier) {
-    return LogAndSetError(error,
-                          __LINE__,
-                          __FILE__,
-                          "Failed to create the payload verifier from " +
-                              std::string(constants::kUpdateCertificatesPath),
-                          ErrorCode::kDownloadManifestParseError);
-  }
-  errorcode = payload_metadata.ValidateMetadataSignature(
-      metadata, "", *payload_verifier);
-  if (errorcode != ErrorCode::kSuccess) {
-    return LogAndSetError(error,
-                          __LINE__,
-                          __FILE__,
-                          "Failed to validate metadata signature: " +
-                              utils::ErrorCodeToString(errorcode),
-                          errorcode);
-  }
   if (!payload_metadata.GetManifest(metadata, manifest)) {
     return LogAndSetError(error,
                           __LINE__,
@@ -1454,16 +1464,162 @@
   processor_->StartProcessing();
 }
 
+bool ParsePayloadMetadata(Error* error,
+                          std::string_view manifest_bytes,
+                          DeltaArchiveManifest* manifest) {
+  PayloadMetadata payload_metadata;
+  ErrorCode errorcode{};
+  if (payload_metadata.ParsePayloadHeader(manifest_bytes, &errorcode) !=
+      MetadataParseResult::kSuccess) {
+    return LogAndSetError(error,
+                          __LINE__,
+                          __FILE__,
+                          "Failed to parse payload header: " +
+                              utils::ErrorCodeToString(errorcode),
+                          errorcode);
+  }
+  uint64_t metadata_size = payload_metadata.GetMetadataSize() +
+                           payload_metadata.GetMetadataSignatureSize();
+  if (metadata_size < kMaxPayloadHeaderSize ||
+      metadata_size > manifest_bytes.size()) {
+    return LogAndSetError(error,
+                          __LINE__,
+                          __FILE__,
+                          "Invalid metadata size on cached manifest: " +
+                              std::to_string(metadata_size),
+                          ErrorCode::kDownloadManifestParseError);
+  }
+  TEST_AND_RETURN_FALSE(
+      VerifyPayloadMetadata(error, manifest_bytes, payload_metadata));
+
+  if (!payload_metadata.GetManifest(manifest_bytes, manifest)) {
+    return LogAndSetError(error,
+                          __LINE__,
+                          __FILE__,
+                          "Failed to parse manifest. Might need to install "
+                          "OTA first and re-try this API",
+                          ErrorCode::kDownloadManifestParseError);
+  }
+  return true;
+}
+
 bool UpdateAttempterAndroid::TriggerPostinstall(const std::string& partition,
                                                 Error* error) {
-  if (error) {
-    return LogAndSetGenericError(
+  if (processor_->IsRunning()) {
+    return LogAndSetError(error,
+                          __LINE__,
+                          __FILE__,
+                          "Already processing an update, cancel it first.",
+                          ErrorCode::kUpdateProcessing);
+  }
+  bool postinstall_succeeded = false;
+  if (!prefs_->GetBoolean(kPrefsPostInstallSucceeded, &postinstall_succeeded)) {
+    return LogAndSetError(
         error,
         __LINE__,
         __FILE__,
-        __FUNCTION__ + std::string(" is not implemented"));
+        "Postinstall action did not run. "
+        "OTA update must first reach the "
+        "Postinstall phase(which verfies that all partitions can be mounted) "
+        "before calling TriggerPostinstall",
+        ErrorCode::kPostinstallRunnerError);
   }
-  return false;
+  if (!postinstall_succeeded) {
+    return LogAndSetError(
+        error,
+        __LINE__,
+        __FILE__,
+        "Postinstall action did not complete successfully. "
+        "OTA update must first reach the "
+        "Postinstall phase(which verfies that all partitions can be mounted) "
+        "before calling TriggerPostinstall",
+        ErrorCode::kPostinstallRunnerError);
+  }
+
+  InstallPlan install_plan;
+  install_plan.source_slot = GetCurrentSlot();
+  install_plan.target_slot = GetTargetSlot();
+  install_plan.switch_slot_on_reboot = false;
+  install_plan.run_post_install = true;
+  install_plan.download_url =
+      std::string(kPrefsManifestBytes) + ":" + install_plan_.download_url;
+
+  std::string manifest_bytes;
+  // kPrefsManifestBytes is set during DownloadAction
+  if (!prefs_->GetString(kPrefsManifestBytes, &manifest_bytes)) {
+    return LogAndSetError(
+        error,
+        __LINE__,
+        __FILE__,
+        "Cached manifest not found. TriggerPostinstall can only be called "
+        "after OTA get past at least FilesystemVerification stage",
+        ErrorCode::kDownloadStateInitializationError);
+  }
+  DeltaArchiveManifest manifest;
+  TEST_AND_RETURN_FALSE(ParsePayloadMetadata(error, manifest_bytes, &manifest));
+  ErrorCode errorcode{};
+  if (!boot_control_->GetDynamicPartitionControl()->PreparePartitionsForUpdate(
+          GetCurrentSlot(),
+          GetTargetSlot(),
+          manifest,
+          false /* should update */,
+          nullptr,
+          &errorcode)) {
+    return LogAndSetError(error,
+                          __LINE__,
+                          __FILE__,
+                          "Failed to PreparePartitionsForUpdate",
+                          errorcode);
+  }
+  std::vector<PartitionUpdate> partitions;
+  std::copy_if(manifest.partitions().begin(),
+               manifest.partitions().end(),
+               std::back_inserter(partitions),
+               [&partition](const PartitionUpdate& part) {
+                 return part.partition_name() == partition;
+               });
+  if (partitions.empty()) {
+    return LogAndSetError(error,
+                          __LINE__,
+                          __FILE__,
+                          "Partition " + partition + " not found",
+                          ErrorCode::kDownloadStateInitializationError);
+  }
+  // We only want to trigger postinstall for a specific partition,
+  // and since we already checked partitions array is non-empty, reading just
+  // the first partition is enough.
+  if (!partitions[0].has_postinstall_path() ||
+      partitions[0].postinstall_path().empty()) {
+    return LogAndSetError(error,
+                          __LINE__,
+                          __FILE__,
+                          "Partition " + partition +
+                              " does not have a postinstall script defined",
+                          ErrorCode::kDownloadStateInitializationError);
+  }
+  if (!install_plan.ParsePartitions(
+          partitions, boot_control_, manifest.block_size(), &errorcode)) {
+    return LogAndSetError(error,
+                          __LINE__,
+                          __FILE__,
+                          "Failed to parse manifest partitions. Might need "
+                          "to install OTA first and re-try this API",
+                          ErrorCode::kDownloadManifestParseError);
+  }
+  LOG(INFO) << "Trigger postinstall with this install plan: "
+            << install_plan.ToString();
+
+  auto postinstall_runner_action =
+      std::make_unique<PostinstallRunnerAction>(boot_control_, hardware_);
+  postinstall_runner_action->set_delegate(this);
+
+  auto install_plan_action = std::make_unique<InstallPlanAction>(install_plan);
+  BondActions(install_plan_action.get(), postinstall_runner_action.get());
+  processor_->EnqueueAction(std::move(install_plan_action));
+  processor_->EnqueueAction(std::move(postinstall_runner_action));
+  SetStatusAndNotify(UpdateStatus::FINALIZING);
+  ScheduleProcessingStart();
+  return true;
 }
 
 void UpdateAttempterAndroid::OnCleanupProgressUpdate(double progress) {
diff --git a/payload_consumer/filesystem_verifier_action.cc b/payload_consumer/filesystem_verifier_action.cc
index 8c21673..956f90b 100644
--- a/payload_consumer/filesystem_verifier_action.cc
+++ b/payload_consumer/filesystem_verifier_action.cc
@@ -16,7 +16,6 @@
 
 #include "update_engine/payload_consumer/filesystem_verifier_action.h"
 
-#include <errno.h>
 #include <fcntl.h>
 #include <sys/stat.h>
 #include <sys/types.h>
@@ -28,7 +27,6 @@
 #include <memory>
 #include <numeric>
 #include <string>
-#include <utility>
 
 #include <base/bind.h>
 #include <brillo/data_encoding.h>
diff --git a/payload_consumer/payload_metadata.cc b/payload_consumer/payload_metadata.cc
index d2e42f0..649d9be 100644
--- a/payload_consumer/payload_metadata.cc
+++ b/payload_consumer/payload_metadata.cc
@@ -154,7 +154,7 @@
 }
 
 ErrorCode PayloadMetadata::ValidateMetadataSignature(
-    const brillo::Blob& payload,
+    const std::string_view payload,
     const string& metadata_signature,
     const PayloadVerifier& payload_verifier) const {
   if (payload.size() < metadata_size_ + metadata_signature_size_)
diff --git a/payload_consumer/payload_metadata.h b/payload_consumer/payload_metadata.h
index 4d2d5b0..fd24f20 100644
--- a/payload_consumer/payload_metadata.h
+++ b/payload_consumer/payload_metadata.h
@@ -17,14 +17,12 @@
 #ifndef UPDATE_ENGINE_PAYLOAD_CONSUMER_PAYLOAD_METADATA_H_
 #define UPDATE_ENGINE_PAYLOAD_CONSUMER_PAYLOAD_METADATA_H_
 
-#include <inttypes.h>
-
 #include <string>
-#include <vector>
 
 #include <android-base/macros.h>
 #include <brillo/secure_blob.h>
 
+#include "update_engine/common/utils.h"
 #include "update_engine/common/error_code.h"
 #include "update_engine/payload_consumer/payload_verifier.h"
 #include "update_engine/update_metadata.pb.h"
@@ -55,6 +53,12 @@
   // the payload.
   MetadataParseResult ParsePayloadHeader(const brillo::Blob& payload,
                                          ErrorCode* error);
+  MetadataParseResult ParsePayloadHeader(std::string_view payload,
+                                         ErrorCode* error) {
+    return ParsePayloadHeader(reinterpret_cast<const uint8_t*>(payload.data()),
+                              payload.size(),
+                              error);
+  }
   MetadataParseResult ParsePayloadHeader(const unsigned char* payload,
                                          size_t size,
                                          ErrorCode* error);
@@ -69,9 +73,16 @@
   // to the payload server doesn't exploit any vulnerability in the code that
   // parses the protocol buffer.
   ErrorCode ValidateMetadataSignature(
-      const brillo::Blob& payload,
+      std::string_view payload,
       const std::string& metadata_signature,
       const PayloadVerifier& payload_verifier) const;
+  ErrorCode ValidateMetadataSignature(
+      const std::vector<uint8_t>& payload,
+      const std::string& metadata_signature,
+      const PayloadVerifier& payload_verifier) const {
+    return ValidateMetadataSignature(
+        ToStringView(payload), metadata_signature, payload_verifier);
+  }
 
   // Returns the major payload version. If the version was not yet parsed,
   // returns zero.
@@ -93,6 +104,12 @@
   bool GetManifest(const unsigned char* payload,
                    size_t size,
                    DeltaArchiveManifest* out_manifest) const;
+  bool GetManifest(std::string_view payload,
+                   DeltaArchiveManifest* out_manifest) const {
+    return GetManifest(reinterpret_cast<const uint8_t*>(payload.data()),
+                       payload.size(),
+                       out_manifest);
+  }
 
   // Parses a payload file |payload_path| and prepares the metadata properties,
   // manifest and metadata signatures. Can be used as an easy to use utility to
diff --git a/payload_consumer/postinstall_runner_action.cc b/payload_consumer/postinstall_runner_action.cc
index 5a6eeab..da9075a 100644
--- a/payload_consumer/postinstall_runner_action.cc
+++ b/payload_consumer/postinstall_runner_action.cc
@@ -35,7 +35,6 @@
 #include "update_engine/common/action_processor.h"
 #include "update_engine/common/boot_control_interface.h"
 #include "update_engine/common/error_code_utils.h"
-#include "update_engine/common/platform_constants.h"
 #include "update_engine/common/subprocess.h"
 #include "update_engine/common/utils.h"
 
@@ -280,14 +279,20 @@
   // Runs the postinstall script asynchronously to free up the main loop while
   // it's running.
   vector<string> command = {abs_path};
-#ifdef __ANDROID__
   // In Brillo and Android, we pass the slot number and status fd.
   command.push_back(std::to_string(install_plan_.target_slot));
   command.push_back(std::to_string(kPostinstallStatusFd));
-#else
-  // Chrome OS postinstall expects the target rootfs as the first parameter.
-  command.push_back(partition.target_path);
-#endif  // __ANDROID__
+  // If install plan only contains one partition, notify the script. Most likely
+  // we are scheduled by `triggerPostinstall` API. Certain scripts might want
+  // different behaviors when triggered by `triggerPostinstall` API. For
+  // example, call scheduler API to schedule a postinstall run during
+  // applyPayload(), and only run actual postinstall work if scheduled by
+  // external async scheduler.
+  if (install_plan_.partitions.size() == 1 &&
+      !install_plan_.switch_slot_on_reboot &&
+      install_plan_.download_url.starts_with(kPrefsManifestBytes)) {
+    command.push_back("1");
+  }
 
   current_command_ = Subprocess::Get().ExecFlags(
       command,