diff --git a/app/jobs/verify_fixity_job.rb b/app/jobs/verify_fixity_job.rb
index 1e42e51..e292785 100644
--- a/app/jobs/verify_fixity_job.rb
+++ b/app/jobs/verify_fixity_job.rb
@@ -25,7 +25,7 @@ def perform(stored_object_id)
   def handle_unexpected_error(fixity_verification_record, err)
     fixity_verification_record.update!(
       status: :failure,
-      error_message: "An unexpected error occurred: #{err.message}"
+      error_message: "An unexpected error occurred: #{err.class.name} -> #{err.message}"
     )
   end
 
diff --git a/config/initializers/check_please.rb b/config/initializers/check_please.rb
index 3a7e9f1..a731818 100644
--- a/config/initializers/check_please.rb
+++ b/config/initializers/check_please.rb
@@ -1,4 +1,12 @@
 # frozen_string_literal: true
 
-# Load CHECK_PLEASE config
-CHECK_PLEASE = Rails.application.config_for(:check_please).deep_symbolize_keys
+CHECK_PLEASE = Rails.application.config_for(:check_please)
+
+def validate_check_please_config!
+  missing_options = [:http_base_url, :ws_url, :auth_token, :http_timeout].reject do |required_config_option|
+    CHECK_PLEASE.key?(required_config_option)
+  end
+  raise "Missing required check_please.yml options: #{missing_options.inspect}" if missing_options.present?
+end
+
+validate_check_please_config!
diff --git a/config/templates/check_please.template.yml b/config/templates/check_please.template.yml
index 06ee634..3e5fc62 100644
--- a/config/templates/check_please.template.yml
+++ b/config/templates/check_please.template.yml
@@ -1,6 +1,10 @@
 development:
+  http_base_url: 'http://dev.example.com'
   ws_url: 'ws://dev.example.com/cable'
   auth_token: 'changethis'
+  http_timeout: 300 # 5 minutes
 test:
+  http_base_url: 'http://test.example.com'
   ws_url: 'ws://test.example.com/cable'
   auth_token: 'changethis'
+  http_timeout: 300 # 5 minutes
diff --git a/lib/atc/aws/fixity_check.rb b/lib/atc/aws/fixity_check.rb
index 1561c8d..0502ee7 100644
--- a/lib/atc/aws/fixity_check.rb
+++ b/lib/atc/aws/fixity_check.rb
@@ -4,6 +4,7 @@ class Atc::Aws::FixityCheck
   def initialize(stored_object, fixity_check_identifier)
     @bucket_name = stored_object.storage_provider.container_name
     @object_path = stored_object.path
+    @expected_object_size = stored_object.source_object.object_size
     @fixity_checksum_algorithm = stored_object.source_object.fixity_checksum_algorithm
     @fixity_check_identifier = fixity_check_identifier
   end
@@ -18,7 +19,9 @@ def fixity_checksum_object_size
     ).perform(
       @fixity_check_identifier, @bucket_name,
       @object_path, @fixity_checksum_algorithm.name.downcase,
-      Atc::Aws::RemoteFixityCheck::HTTP
+      # A synchronous HTTP check is faster than polling for smaller files,
+      # so we'll only use the polling method for larger files.
+      @expected_object_size < 1.gigabyte ? Atc::Aws::RemoteFixityCheck::HTTP : Atc::Aws::RemoteFixityCheck::HTTP_POLLING
     )
     [response['checksum_hexdigest'], response['object_size'], response['error_message']]
   end
diff --git a/lib/atc/aws/remote_fixity_check.rb b/lib/atc/aws/remote_fixity_check.rb
index 6ba4649..3d37932 100644
--- a/lib/atc/aws/remote_fixity_check.rb
+++ b/lib/atc/aws/remote_fixity_check.rb
@@ -7,9 +7,14 @@
 # rubocop:disable Metrics/PerceivedComplexity
 
 class Atc::Aws::RemoteFixityCheck
-  STALLED_FIXITY_CHECK_JOB_TIMEOUT = 10.seconds
+  # This value shouldn't be too high because we want to detect stalled jobs fairly soon after they stall,
+  # but it should be high enough to account for downtime/delays related to CheckPlease app deployments.
+  STALLED_FIXITY_CHECK_JOB_TIMEOUT = 1.minute
+  POLLING_DELAY = 2.seconds
+  MAX_WAIT_TIME_FOR_POLLING_JOB_START = 1.hour
   WEBSOCKET = 'websocket'
   HTTP = 'http'
+  HTTP_POLLING = 'http_polling'
 
   def initialize(http_base_url, ws_url, auth_token)
     @http_base_url = http_base_url
@@ -18,7 +23,7 @@ def initialize(http_base_url, ws_url, auth_token)
   end
 
   def http_client
-    @http_client ||= ::Faraday.new(url: @http_base_url) do |f|
+    @http_client ||= ::Faraday.new(url: @http_base_url, request: { timeout: CHECK_PLEASE['http_timeout'] }) do |f|
       f.request :authorization, 'Bearer', @auth_token
       f.response :json # decode response bodies as JSON
       f.response :raise_error # raise 4xx and 5xx responses as errors
@@ -41,6 +46,8 @@ def perform(job_identifier, bucket_name, object_path, checksum_algorithm_name, m
       perform_websocket(job_identifier, bucket_name, object_path, checksum_algorithm_name)['data']
     when HTTP
       perform_http(bucket_name, object_path, checksum_algorithm_name)
+    when HTTP_POLLING
+      perform_http_polling(bucket_name, object_path, checksum_algorithm_name)
     else
       raise ArgumentError, "Unsupported perform method: #{method}"
     end
@@ -61,7 +68,67 @@ def perform_http(bucket_name, object_path, checksum_algorithm_name)
     JSON.parse(response.body)
   rescue StandardError => e
     {
-      'checksum_hexdigest' => nil, 'object_size' => nil, 'error_message' => "An unexpected error occurred: #{e.message}"
+      'checksum_hexdigest' => nil, 'object_size' => nil,
+      'error_message' => "An unexpected error occurred: #{e.class.name} -> #{e.message}"
+    }
+  end
+
+  def perform_http_polling(bucket_name, object_path, checksum_algorithm_name)
+    payload = {
+      'fixity_check' => {
+        'bucket_name' => bucket_name,
+        'object_path' => object_path,
+        'checksum_algorithm_name' => checksum_algorithm_name
+      }
+    }.to_json
+
+    fixity_check_create_response = http_client.post('/fixity_checks', payload) { |request|
+      request.headers['Content-Type'] = 'application/json'
+    }.body
+
+    if fixity_check_create_response['error_message'].present?
+      # Raise any unexpected error message.  It will be handled elsewhere.
+      raise Atc::Exceptions::AtcError,
+            fixity_check_create_response['error_message']
+    end
+
+    # If we got here, that means that the fixity check request was created successfully.
+    # Now we'll poll and wait for it to complete.
+    last_progress_update_time = nil
+    fixity_check_response = nil
+    loop do
+      sleep POLLING_DELAY
+
+      fixity_check_response = http_client.get("/fixity_checks/#{fixity_check_create_response['id']}") { |request|
+        request.headers['Content-Type'] = 'application/json'
+      }.body
+
+      status = fixity_check_response['status']
+
+      break if ['success', 'error'].include?(status)
+
+      # If the fixity check has a 'pending' status, we'll skip this polling iteration and try again in the next poll
+      # operation.  It is the reponsibility of the CheckPlease app to periodically clean up jobs that have been pending
+      # for too long, so we don't need to worry about that case in this app.
+      next if status == 'pending'
+
+      # If we got here, that means that the job is in progress.  Let's account for the
+      # possibility of the job timing out, if something goes wrong on the CheckPlease app side.
+      last_progress_update_time = Time.zone.parse(fixity_check_response['updated_at'])
+      if job_unresponsive?(last_progress_update_time)
+        raise Atc::Exceptions::RemoteFixityCheckTimeout,
+              'Timed out while waiting for a response.'
+      end
+    rescue Faraday::Error => e
+      # If we run into a network error, log it, but continue looping
+      Rails.logger.info 'Error connecting to CheckPlease app during fixity check polling '\
+                        "loop iteration: #{e.class.name} -> #{e.message}"
+    end
+    fixity_check_response
+  rescue StandardError => e
+    {
+      'checksum_hexdigest' => nil, 'object_size' => nil,
+      'error_message' => "An unexpected error occurred: #{e.class.name} -> #{e.message}"
     }
   end
 
diff --git a/lib/atc/exceptions.rb b/lib/atc/exceptions.rb
index f210aee..f7a5cae 100644
--- a/lib/atc/exceptions.rb
+++ b/lib/atc/exceptions.rb
@@ -7,4 +7,5 @@ class TransferError < AtcError; end
   class ObjectExists < AtcError; end
   class StorageProviderMappingNotFound < AtcError; end
   class RemoteFixityCheckTimeout < AtcError; end
+  class PollingWaitTimeoutError < AtcError; end
 end
diff --git a/lib/tasks/aws.rake b/lib/tasks/aws.rake
index c413a22..6163c94 100644
--- a/lib/tasks/aws.rake
+++ b/lib/tasks/aws.rake
@@ -14,7 +14,7 @@ namespace :atc do
       )
       response = remote_fixity_check.perform(
         job_identifier, bucket_name, object_path, checksum_algorithm_name,
-        Atc::Aws::RemoteFixityCheck::HTTP
+        Atc::Aws::RemoteFixityCheck::HTTP_POLLING
       )
       puts "Response: #{response.inspect}"
     end
diff --git a/lib/tasks/comparison.rake b/lib/tasks/comparison.rake
index 8fbd994..c56c026 100644
--- a/lib/tasks/comparison.rake
+++ b/lib/tasks/comparison.rake
@@ -41,5 +41,25 @@ namespace :atc do
 
       puts "Done!"
     end
+
+    desc 'List multipart info for an S3 object (including part size)'
+    # Example usage: bucket_name=cul-dlstor-digital-testing1 path='test-5gb-file-ubuntu.iso'
+    task list_multipart_info: :environment do
+      bucket_name = ENV['bucket_name']
+      path = ENV['path']
+
+      attributes = S3_CLIENT.get_object_attributes(
+        bucket: bucket_name,
+        key: path,
+        object_attributes: %w[ETag Checksum ObjectParts StorageClass ObjectSize]
+      )
+
+      if attributes.object_parts.blank?
+        puts "Single part file."
+      else
+        puts "Found multipart file (#{attributes.object_parts.total_parts_count} parts) "\
+              "with individual part array data for #{attributes.object_parts.parts.length} parts."
+      end
+    end
   end
 end
diff --git a/spec/atc/aws/remote_fixity_check_spec.rb b/spec/atc/aws/remote_fixity_check_spec.rb
index 011e36b..2589ec6 100644
--- a/spec/atc/aws/remote_fixity_check_spec.rb
+++ b/spec/atc/aws/remote_fixity_check_spec.rb
@@ -139,7 +139,7 @@
         {
           'checksum_hexdigest' => nil,
           'object_size' => nil,
-          'error_message' => 'An unexpected error occurred: oh no!'
+          'error_message' => 'An unexpected error occurred: StandardError -> oh no!'
         }
       )
     end
diff --git a/spec/jobs/verify_fixity_job_spec.rb b/spec/jobs/verify_fixity_job_spec.rb
index 094a343..14e3c05 100644
--- a/spec/jobs/verify_fixity_job_spec.rb
+++ b/spec/jobs/verify_fixity_job_spec.rb
@@ -80,7 +80,9 @@
       expect(aws_fixity_verification_pending).to receive(:update!).and_call_original
       verify_fixity_job.handle_unexpected_error(aws_fixity_verification_pending, StandardError.new('oh no!'))
       expect(aws_fixity_verification_pending.status).to eq('failure')
-      expect(aws_fixity_verification_pending.error_message).to eq('An unexpected error occurred: oh no!')
+      expect(aws_fixity_verification_pending.error_message).to eq(
+        'An unexpected error occurred: StandardError -> oh no!'
+      )
       expect(aws_fixity_verification_pending.changed?).to eq(false) # verify that the record has no unsaved changes
     end
   end