diff --git a/blog_giveaway_demo.rb b/blog_giveaway_demo.rb
new file mode 100644
index 0000000..eaa58d5
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+++ b/blog_giveaway_demo.rb
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+# Proves Post#with_lock (Prism::Orm::Pessimistic, see ClaimsController#create)
+# actually prevents overselling a limited giveaway under real concurrency -
+# not threads inside one process, but several genuinely independent
+# `Prism::Server` OS processes (see blog_giveaway_worker.rb) all pointed at
+# the SAME shared database file, racing to POST /posts/:slug/claims on the
+# same post at (as close to) the same instant as a spin-wait barrier on a
+# shared "go" file can arrange.
+#
+# A post with giveaway_slots: 3 and 8 distinct entrants means at most 3 can
+# ever legitimately win a slot - if the read-count-then-decide in
+# ClaimsController#create were not held under one lock for its entire
+# critical section, several of those 8 requests could all read the same
+# "2 of 3 claimed" snapshot and all decide a slot was still free, oversold.
+require 'fileutils'
+require 'rbconfig'
+require 'net/http'
+require 'tmpdir'
+
+ROOT = __dir__
+SCRATCH_DB_DIR = File.join(ROOT, 'examples/blog/db/giveaway_demo')
+RUBY = RbConfig.ruby
+GIVEAWAY_SLOTS = 3
+# Capped by examples/blog/app.rb's own 'logins/ip' throttle
+# (limit: 5, period: 60) - every entrant below signs in over a real POST
+# /session from this same machine's loopback address, exactly like every
+# other real visitor would, so this demo has to live within the same
+# limit a real attacker would be stopped by rather than special-casing
+# itself around it. 5 entrants racing 3 slots is still more than enough
+# to prove an oversell is impossible (2 must lose).
+ENTRANTS = 5
+
+# Fresh scratch database every run, isolated from examples/blog/db/
+# development.pstore (blog_demo.rb's own data).
+FileUtils.rm_rf(SCRATCH_DB_DIR)
+ENV['PRISM_BLOG_DB_DIR'] = SCRATCH_DB_DIR
+
+$LOAD_PATH.unshift(File.join(ROOT, 'lib'))
+require 'prism'
+require_relative 'examples/blog/app'
+
+def cookie_from(response)
+  (response.get_fields('set-cookie') || []).map { |line| line.split(';').first }.join('; ')
+end
+
+def csrf_token_from(html)
+  html[/name="csrf-token" content="([^"]+)"/, 1]
+end
+
+setup_server = Prism::Server.new(Blog, port: 0).start
+slug = nil
+
+Net::HTTP.start('127.0.0.1', setup_server.port) do |http|
+  # The host: creates the giveaway post. Entrants never author anything,
+  # they only ever race to claim a slot on this one post.
+  new_session = http.get('/session/new')
+  cookie = cookie_from(new_session)
+  token = csrf_token_from(new_session.body)
+  signup_body = URI.encode_www_form(
+    'user[email]' => 'host@example.com', 'user[password]' => 'secret123',
+    'user[password_confirmation]' => 'secret123', 'authenticity_token' => token
+  )
+  signup = http.post('/users', signup_body, 'Content-Type' => 'application/x-www-form-urlencoded', 'Cookie' => cookie)
+  raise "host signup failed (#{signup.code})" unless signup.code == '302'
+  cookie = cookie_from(signup)
+
+  home = http.get('/', 'Cookie' => cookie)
+  token = csrf_token_from(home.body)
+  create_body = URI.encode_www_form(
+    'post[title]' => 'Signed copy giveaway', 'post[body]' => 'Three signed copies, first come first served.',
+    'post[giveaway_slots]' => GIVEAWAY_SLOTS.to_s, 'authenticity_token' => token
+  )
+  created = http.post('/posts', create_body, 'Content-Type' => 'application/x-www-form-urlencoded', 'Cookie' => cookie)
+  raise "post creation failed (#{created.code})" unless created.code == '302'
+  slug = created['location'][%r{/posts/([^/?]+)}, 1]
+  raise 'could not extract slug from Location header' unless slug
+end
+
+# ENTRANTS distinct accounts, created directly against the store rather
+# than through POST /users - each entrant still authenticates over a real
+# POST /session in blog_giveaway_worker.rb (that is the request that has
+# to survive examples/blog's real login throttle), but their signups
+# happening over HTTP too would trip the SEPARATE 'signups/ip' throttle
+# (limit: 3) with only 1 (host) + ENTRANTS of them all coming from this
+# same demo process' loopback address - a constraint about this demo
+# script issuing many signups back to back, not about the giveaway locking
+# this file exists to prove.
+ENTRANTS.times do |i|
+  User.create!(
+    'email' => "entrant#{i}@example.com", 'password' => 'secret123', 'password_confirmation' => 'secret123'
+  )
+end
+
+puts "setup done: post '#{slug}' has #{GIVEAWAY_SLOTS} giveaway slots, #{ENTRANTS} entrants about to race for them"
+
+Dir.mktmpdir('giveaway_demo') do |scratch|
+  go_file = File.join(scratch, 'go')
+  pids_and_outputs = ENTRANTS.times.map do |i|
+    label = "entrant#{i}"
+    out_path = File.join(scratch, "#{label}.out")
+    pid = Process.spawn(
+      { 'PRISM_BLOG_DB_DIR' => SCRATCH_DB_DIR }, RUBY, '-Ilib', 'blog_giveaway_worker.rb',
+      label, "#{label}@example.com", 'secret123', slug, go_file,
+      chdir: ROOT, out: out_path, err: File.join(scratch, "#{label}.err")
+    )
+    [pid, out_path]
+  end
+
+  # Generous: a cold `ruby` boot of a fresh Prism::Server (migrations
+  # re-checked, sign-in, a page load for a CSRF token) per worker, times
+  # ENTRANTS of them all starting around the same moment, comfortably
+  # finishes well under this - every worker should already be parked on
+  # its spin-wait by the time this file appears.
+  sleep 3
+  File.write(go_file, 'go')
+
+  results = pids_and_outputs.map do |pid, out_path|
+    _, status = Process.wait2(pid)
+    raise "worker exited with #{status}, see #{out_path}" unless status.success?
+    File.read(out_path).strip
+  end
+
+  outcomes = results.each_with_object(Hash.new(0)) do |line, tally|
+    _label, outcome = line.split(':')
+    tally[outcome] += 1
+  end
+
+  puts "outcomes across #{ENTRANTS} racing, fully independent server processes: #{outcomes}"
+  raise "expected already_claimed count 0, got #{outcomes['already_claimed']}" unless outcomes['already_claimed'].zero?
+  raise "expected unknown count 0, got #{outcomes['unknown']}" unless outcomes['unknown'].zero?
+  unless outcomes['claimed'] == GIVEAWAY_SLOTS
+    raise "expected exactly #{GIVEAWAY_SLOTS} claimed slots, got #{outcomes['claimed']} - Post#with_lock did not " \
+          'prevent an oversell'
+  end
+  unless outcomes['sold_out'] == ENTRANTS - GIVEAWAY_SLOTS
+    raise "expected #{ENTRANTS - GIVEAWAY_SLOTS} sold_out responses, got #{outcomes['sold_out']}"
+  end
+  puts "exactly #{GIVEAWAY_SLOTS} of #{ENTRANTS} racing requests won a slot, the rest correctly saw sold_out - no oversell"
+end
+
+# Independent confirmation straight from the store, not just trusting the
+# workers' own HTTP responses: a brand new transaction against the same
+# file, opened fresh by THIS process, after every worker process has
+# already exited and released its own file lock.
+post = Post.find(Post.find_by!(slug: slug).id)
+claim_count = Claim.where(post_id: post.id).count
+if claim_count != GIVEAWAY_SLOTS
+  raise "Claim.count on disk is #{claim_count}, expected exactly #{GIVEAWAY_SLOTS}"
+end
+if post.claims_count != GIVEAWAY_SLOTS
+  raise "Post#claims_count is #{post.claims_count}, expected exactly #{GIVEAWAY_SLOTS} (counter cache drifted from the real row count)"
+end
+puts "storage-level check: Claim.where(post_id: ...).count == #{claim_count}, Post#claims_count == #{post.claims_count}, both exactly #{GIVEAWAY_SLOTS}"
+puts 'blog_giveaway_demo done: a hard-capped giveaway held its cap under real cross-process concurrency'