diff --git a/Gemfile b/Gemfile
index 34b6b06b..3ee5010b 100644
--- a/Gemfile
+++ b/Gemfile
@@ -58,6 +58,11 @@ gem 'parallel', '~> 1.23'
 gem "httparty", "~> 0.21.0"
 gem "addressable", "~> 2.8"
 
+# For advanced batching of many HTTP requests.
+gem "async", "~> 2.6", require: false
+gem "async-http", "~> 0.61.0", require: false
+gem "thread-local", "~> 1.1", require: false
+
 # For debugging.
 gem 'web-console', '~> 4.2', group: :development
 
diff --git a/Gemfile.lock b/Gemfile.lock
index 6dad04d4..3a334dd6 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -82,6 +82,23 @@ GEM
       tzinfo (~> 2.0)
     addressable (2.8.4)
       public_suffix (>= 2.0.2, < 6.0)
+    async (2.6.5)
+      console (~> 1.10)
+      fiber-annotation
+      io-event (~> 1.1)
+      timers (~> 4.1)
+    async-http (0.61.0)
+      async (>= 1.25)
+      async-io (>= 1.28)
+      async-pool (>= 0.2)
+      protocol-http (~> 0.25.0)
+      protocol-http1 (~> 0.16.0)
+      protocol-http2 (~> 0.15.0)
+      traces (>= 0.10.0)
+    async-io (1.37.0)
+      async
+    async-pool (0.4.0)
+      async (>= 1.25)
     babel-source (5.8.35)
     babel-transpiler (0.7.0)
       babel-source (>= 4.0, < 6)
@@ -95,6 +112,9 @@ GEM
     builder (3.2.4)
     concurrent-ruby (1.2.2)
     connection_pool (2.2.5)
+    console (1.23.2)
+      fiber-annotation
+      fiber-local
     crass (1.0.6)
     date (3.3.3)
     devise (4.9.3)
@@ -116,6 +136,8 @@ GEM
     erubi (1.12.0)
     execjs (2.5.2)
     ffi (1.15.5)
+    fiber-annotation (0.2.0)
+    fiber-local (1.0.0)
     globalid (1.2.1)
       activesupport (>= 6.1)
     globalize (6.3.0)
@@ -136,6 +158,7 @@ GEM
     i18n (1.14.1)
       concurrent-ruby (~> 1.0)
     io-console (0.6.0)
+    io-event (1.3.3)
     irb (1.8.3)
       rdoc
       reline (>= 0.3.8)
@@ -181,6 +204,13 @@ GEM
       racc (~> 1.4)
     orm_adapter (0.5.0)
     parallel (1.23.0)
+    protocol-hpack (1.4.2)
+    protocol-http (0.25.0)
+    protocol-http1 (0.16.0)
+      protocol-http (~> 0.22)
+    protocol-http2 (0.15.1)
+      protocol-hpack (~> 1.4)
+      protocol-http (~> 0.18)
     psych (5.1.1.1)
       stringio
     public_suffix (5.0.3)
@@ -287,8 +317,11 @@ GEM
     terser (1.1.17)
       execjs (>= 0.3.0, < 3)
     thor (1.2.2)
+    thread-local (1.1.0)
     tilt (2.2.0)
     timeout (0.4.0)
+    timers (4.3.5)
+    traces (0.11.1)
     tzinfo (2.0.6)
       concurrent-ruby (~> 1.0)
     unf (0.1.4)
@@ -314,6 +347,8 @@ PLATFORMS
 DEPENDENCIES
   RocketAMF!
   addressable (~> 2.8)
+  async (~> 2.6)
+  async-http (~> 0.61.0)
   bootsnap (~> 1.16)
   devise (~> 4.9, >= 4.9.2)
   devise-encryptable (~> 0.2.0)
@@ -344,6 +379,7 @@ DEPENDENCIES
   sprockets (~> 4.2)
   stackprof (~> 0.2.25)
   terser (~> 1.1, >= 1.1.17)
+  thread-local (~> 1.1)
   web-console (~> 4.2)
   will_paginate (~> 4.0)
 
diff --git a/db/migrate/20231110043543_add_manifest_url_to_swf_assets.rb b/db/migrate/20231110043543_add_manifest_url_to_swf_assets.rb
index e357af1b..93afa3b3 100644
--- a/db/migrate/20231110043543_add_manifest_url_to_swf_assets.rb
+++ b/db/migrate/20231110043543_add_manifest_url_to_swf_assets.rb
@@ -1,55 +1,5 @@
 class AddManifestUrlToSwfAssets < ActiveRecord::Migration[7.1]
   def change
     add_column :swf_assets, :manifest_url, :string
-
-    # Okay, this is a big one to run upward! We're going to infer the manifest
-    # for as many assets as we can!
-    reversible do |direction|
-      direction.up do
-        Net::HTTP.start("images.neopets.com", 443, use_ssl: true) do |http|
-          SwfAsset.find_each do |swf_asset|
-            begin
-              manifest_url = infer_manifest_url(http, swf_asset.url)
-            rescue StandardError => error
-              Rails.logger.warn "Could not infer manifest URL for #{swf_asset.id}, skipping: #{error.message}"
-              next
-            end
-
-            Rails.logger.info "#{swf_asset.id}: #{manifest_url}"
-            swf_asset.manifest_url = manifest_url
-            swf_asset.save!
-          end
-        end
-      end
-    end
-  end
-
-  SWF_URL_PATTERN = %r{^(?:https?:)?//images\.neopets\.com/cp/(bio|items)/swf/(.+?)_([a-z0-9]+)\.swf$}
-  def infer_manifest_url(http, swf_url)
-    url_match = swf_url.match(SWF_URL_PATTERN)
-    raise ArgumentError, "not a valid SWF URL: #{swf_url}" if url_match.nil?
-    
-    # Build the potential manifest URLs, from the two structures we know of.
-    type, folders, hash_str = url_match.captures
-    potential_manifest_urls = [
-      "https://images.neopets.com/cp/#{type}/data/#{folders}/manifest.json",
-      "https://images.neopets.com/cp/#{type}/data/#{folders}_#{hash_str}/manifest.json",
-    ]
-
-    # Send a HEAD request to test each manifest URL, without downloading its
-    # content. If it succeeds, we're done!
-    potential_manifest_urls.each do |potential_manifest_url|
-      res = http.head potential_manifest_url
-      if res.is_a? Net::HTTPOK
-        return potential_manifest_url 
-      elsif res.is_a? Net::HTTPNotFound
-        next
-      else
-        raise "unexpected manifest response code: #{res.code}"
-      end
-    end
-
-    # Otherwise, there's no valid manifest URL.
-    raise "none of the common manifest URL patterns returned HTTP 200"
   end
 end
diff --git a/lib/tasks/swf_assets.rake b/lib/tasks/swf_assets.rake
new file mode 100644
index 00000000..45edbad6
--- /dev/null
+++ b/lib/tasks/swf_assets.rake
@@ -0,0 +1,93 @@
+require 'async/barrier'
+require 'async/http/internet/instance'
+
+namespace :swf_assets do
+	desc "Backfill manifest_url for SwfAsset models"
+	task manifests: [:environment] do
+		assets = SwfAsset.where(manifest_url: nil)
+		count = assets.count
+		puts "Found #{count} assets without manifests"
+
+		Async do
+			# Share a pool of persistent connections, rather than reconnecting on
+			# each request. (This library does that automatically!)
+			internet = Async::HTTP::Internet.instance
+
+			# Load the assets in batches, then process each batch in two steps: first
+			# inferring all manifest URLs in the batch, then saving all assets in the
+			# batch. (This makes the update step more efficient, and it also avoids
+			# simultaneous queries across the fibers, which ActiveRecord disallows!)
+			#
+			# We keep track of a shared index `i` here, but we only actually
+			# increment it once each task is *done*, so that the numbers output in
+			# the right order!
+			i = 0
+			assets.find_in_batches(batch_size: 1000) do |batch|
+				# Create a barrier, to let us wait on all the tasks; then under it
+				# create a semaphore, to limit how many tasks run at once.
+				barrier = Async::Barrier.new
+				semaphore = Async::Semaphore.new(100, parent: barrier)
+
+				batch.each do |asset|
+					semaphore.async do |task|
+						manifest_url = nil
+						begin
+							task.with_timeout(5) do
+								manifest_url = infer_manifest_url(asset.url, internet)
+							end
+						rescue StandardError => error
+							i += 1
+							puts "[#{i}/#{count}] ⚠️  Skipping #{asset.id}: #{error.message}"
+							next
+						end
+
+						i += 1
+						puts "[#{i}/#{count}] Manifest for #{asset.id}: #{manifest_url}"
+
+						# Write, but don't yet save, the manifest URL.
+						asset.manifest_url = manifest_url
+					end
+				end
+
+				# Wait for all the above tasks to finish. (Then, all of the assets that
+				# succeeded should have an unsaved `manifest_url` change.)
+				barrier.wait
+
+				# Save all of the assets in the batch. (We do this in a transaction not
+				# for the transactional semantics, but because it's notably faster than
+				# doing a commit between each query, which is what sending the queries
+				# individually would effectively do!)
+				SwfAsset.transaction { batch.each(&:save!) }
+			end
+		end
+	end
+end
+
+SWF_URL_PATTERN = %r{^(?:https?:)?//images\.neopets\.com/cp/(bio|items)/swf/(.+?)_([a-z0-9]+)\.swf$}
+def infer_manifest_url(swf_url, internet)
+  url_match = swf_url.match(SWF_URL_PATTERN)
+  raise ArgumentError, "not a valid SWF URL: #{swf_url}" if url_match.nil?
+  
+  # Build the potential manifest URLs, from the two structures we know of.
+  type, folders, hash_str = url_match.captures
+  potential_manifest_urls = [
+    "https://images.neopets.com/cp/#{type}/data/#{folders}/manifest.json",
+    "https://images.neopets.com/cp/#{type}/data/#{folders}_#{hash_str}/manifest.json",
+  ]
+
+  # Send a HEAD request to test each manifest URL, without downloading its
+  # content. If it succeeds, we're done!
+  potential_manifest_urls.each do |potential_manifest_url|
+    res = internet.head potential_manifest_url
+    if res.ok?
+      return potential_manifest_url 
+    elsif res.status == 404
+      next # Ok, this was not the manifest!
+    else
+      raise "unexpected manifest response code: #{res.status}"
+    end
+  end
+
+  # Otherwise, there's no valid manifest URL.
+  raise "none of the common manifest URL patterns returned HTTP 200"
+end
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