diff --git a/Gemfile b/Gemfile
index 84ba85e..32f0a39 100644
--- a/Gemfile
+++ b/Gemfile
@@ -71,3 +71,5 @@ gem "blind_index"
 gem "sentry-ruby"
 gem "sentry-rails"
 gem "aws-sdk-s3", require: false
+gem "slack-ruby-client"
+gem "slocks", git: "https://github.com/24c02/slocks"
diff --git a/Gemfile.lock b/Gemfile.lock
index ca04bbe..831235f 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -1,3 +1,11 @@
+GIT
+  remote: https://github.com/24c02/slocks
+  revision: 08f4b98858b30e52ea212ae4ebdca1cea7978b93
+  specs:
+    slocks (0.1.1)
+      actionview (>= 6.0)
+      activesupport (>= 6.0)
+
 GEM
   remote: https://rubygems.org/
   specs:
@@ -140,12 +148,19 @@ GEM
       faraday-net_http (>= 2.0, < 3.5)
       json
       logger
+    faraday-mashify (1.0.2)
+      faraday (~> 2.0)
+      hashie
+    faraday-multipart (1.2.0)
+      multipart-post (~> 2.0)
     faraday-net_http (3.4.2)
       net-http (~> 0.5)
     fiddle (1.1.8)
     fugit (1.12.1)
       et-orbi (~> 1.4)
       raabro (~> 1.4)
+    gli (2.22.2)
+      ostruct
     globalid (1.3.0)
       activesupport (>= 6.1)
     hashid-rails (1.4.1)
@@ -201,6 +216,7 @@ GEM
     msgpack (1.8.0)
     multi_xml (0.8.1)
       bigdecimal (>= 3.1, < 5)
+    multipart-post (2.4.1)
     mutex_m (0.3.0)
     net-http (0.9.1)
       uri (>= 0.11.1)
@@ -249,6 +265,7 @@ GEM
     omniauth-oauth2 (1.9.0)
       oauth2 (>= 2.0.2, < 3)
       omniauth (~> 2.0)
+    ostruct (0.6.3)
     parallel (1.27.0)
     parser (3.3.10.1)
       ast (~> 2.4.1)
@@ -400,6 +417,13 @@ GEM
     sentry-ruby (6.3.0)
       bigdecimal
       concurrent-ruby (~> 1.0, >= 1.0.2)
+    slack-ruby-client (3.1.0)
+      faraday (>= 2.0.1)
+      faraday-mashify
+      faraday-multipart
+      gli
+      hashie
+      logger
     snaky_hash (2.0.3)
       hashie (>= 0.1.0, < 6)
       version_gem (>= 1.1.8, < 3)
@@ -506,6 +530,8 @@ DEPENDENCIES
   selenium-webdriver
   sentry-rails
   sentry-ruby
+  slack-ruby-client
+  slocks!
   solid_cable
   solid_cache
   solid_queue
diff --git a/app/controllers/slack/events_controller.rb b/app/controllers/slack/events_controller.rb
new file mode 100644
index 0000000..6a9f378
--- /dev/null
+++ b/app/controllers/slack/events_controller.rb
@@ -0,0 +1,41 @@
+# frozen_string_literal: true
+
+module Slack
+  class EventsController < ActionController::API
+    before_action :verify_slack_signature
+
+    def create
+      # Handle Slack URL verification challenge
+      if params[:type] == "url_verification"
+        return render json: { challenge: params[:challenge] }
+      end
+
+      # Handle event callbacks
+      if params[:type] == "event_callback"
+        event = params[:event]
+
+        # Filter to message events with files in monitored channels
+        if event[:type] == "message" && event[:files].present? && monitored_channel?(event[:channel])
+          ProcessSlackFileUploadJob.perform_later(event.to_unsafe_h)
+        end
+      end
+
+      # Respond immediately (Slack requires < 3s response)
+      head :ok
+    end
+
+    private
+
+    def verify_slack_signature
+      ::Slack::Events::Request.new(request).verify!
+    rescue ::Slack::Events::Request::MissingSigningSecret,
+           ::Slack::Events::Request::InvalidSignature,
+           ::Slack::Events::Request::TimestampExpired
+      render json: { error: "Invalid signature" }, status: :unauthorized
+    end
+
+    def monitored_channel?(channel_id)
+      Rails.application.config.slack.cdn_channels.include?(channel_id)
+    end
+  end
+end
diff --git a/app/jobs/process_slack_file_upload_job.rb b/app/jobs/process_slack_file_upload_job.rb
new file mode 100644
index 0000000..a1bb961
--- /dev/null
+++ b/app/jobs/process_slack_file_upload_job.rb
@@ -0,0 +1,249 @@
+# frozen_string_literal: true
+
+class ProcessSlackFileUploadJob < ApplicationJob
+  include ActionView::Helpers::NumberHelper
+
+  queue_as :default
+
+  class QuotaExceededError < StandardError
+    attr_reader :reason, :details
+
+    def initialize(reason, details = nil)
+      @reason = reason
+      @details = details
+      super(details || reason.to_s)
+    end
+  end
+
+  def perform(event)
+    @channel_id = event["channel"]
+    @message_ts = event["ts"]
+    @slack_user_id = event["user"]
+    @files = event["files"]
+
+    return unless @files.present?
+
+    @slack = SlackService.client
+    @user = nil
+
+    begin
+      @user = find_or_create_user
+
+      add_reaction("beachball")
+      reply_in_thread(pick_flavor_message)
+
+      uploads = process_files
+      notify_success(uploads)
+    rescue QuotaExceededError => e
+      notify_quota_exceeded(e)
+    rescue => e
+      notify_error(e)
+    end
+  end
+
+  private
+
+  def find_or_create_user
+    user = User.find_by(slack_id: @slack_user_id)
+
+    unless user
+      profile = @slack.users_info(user: @slack_user_id).user
+
+      user = User.create!(
+        slack_id: @slack_user_id,
+        email: profile[:profile][:email] || "slack-#{@slack_user_id}@temp.hackclub.com",
+        name: profile[:real_name] || profile[:name] || "Slack User"
+      )
+    end
+
+    user
+  end
+
+  def process_files
+    uploads = []
+
+    @files.each do |file|
+      original_url = file["url_private"]
+      upload = Upload.create_from_url(
+        original_url,
+        user: @user,
+        provenance: :slack,
+        original_url: original_url,
+        authorization: "Bearer #{Rails.application.config.slack.bot_token}"
+      )
+
+      enforce_quota!(upload)
+      uploads << upload
+    end
+
+    uploads
+  end
+
+  def enforce_quota!(upload)
+    quota_service = QuotaService.new(@user)
+    policy = quota_service.current_policy
+
+    if upload.byte_size > policy.max_file_size
+      upload.destroy!
+      raise QuotaExceededError.new(
+        :file_too_large,
+        "File is #{number_to_human_size(upload.byte_size)} but max is #{number_to_human_size(policy.max_file_size)}"
+      )
+    end
+
+    return unless @user.total_storage_bytes > policy.max_total_storage
+
+    upload.destroy!
+    raise QuotaExceededError.new(
+      :storage_exceeded,
+      "You've used #{number_to_human_size(@user.total_storage_bytes)} of your #{number_to_human_size(policy.max_total_storage)} storage"
+    )
+  end
+
+  def notify_success(uploads)
+    remove_reaction("beachball")
+    add_reaction("white_check_mark")
+
+    @slack.chat_postMessage(
+      channel: @channel_id,
+      thread_ts: @message_ts,
+      text: "Yeah! Here's yo' links",
+      **render_slack_template("upload_success", uploads: uploads, slack_user_id: @slack_user_id)
+    )
+  end
+
+  def notify_quota_exceeded(error)
+    remove_reaction("beachball")
+    add_reaction("x")
+
+    error_text = case error.reason
+    when :file_too_large
+      [
+        "_cdnpheus tries to pick up the file but it's too heavy. she strains. she sweats. she gives up._ #{error.details}",
+        "whoa there, that file is THICC. #{error.details} – verify at cdn.hackclub.com for chonkier uploads!",
+        "_cdnpheus attempts to stuff the file into her tiny dinosaur backpack. it does not fit._ #{error.details}",
+        "i tried to eat this file but it's too big and i'm just a small dinosaur :( #{error.details}"
+      ].sample
+    when :storage_exceeded
+      [
+        "_cdnpheus opens her filing cabinet but papers explode everywhere._ you're out of space! #{error.details}",
+        "your storage is fuller than my inbox after i mass-DM'd everyone about my soundcloud. #{error.details}",
+        "no room at the inn! #{error.details} – delete some files or verify at cdn.hackclub.com for more space"
+      ].sample
+    else
+      [
+        "quota exceeded! verify at cdn.hackclub.com to unlock your true potential",
+        "_cdnpheus taps the \"quota exceeded\" sign apologetically_"
+      ].sample
+    end
+
+    reply_in_thread(error_text)
+  end
+
+  def notify_error(error)
+    Rails.logger.error "Slack file upload failed: #{error.message}\n#{error.backtrace.join("\n")}"
+    sentry_event = Sentry.capture_exception(error)
+    sentry_id = sentry_event&.event_id || "unknown"
+
+    return unless @slack
+
+    begin
+      remove_reaction("beachball")
+      add_reaction("x")
+
+      @slack.chat_postMessage(
+        channel: @channel_id,
+        thread_ts: @message_ts,
+        text: "Something went wrong uploading your file",
+        **render_slack_template("upload_error",
+          flavor_message: pick_error_message,
+          error_message: error.message,
+          backtrace: format_backtrace(error.backtrace),
+          sentry_id: sentry_id)
+      )
+    rescue => slack_error
+      Rails.logger.error "Failed to send Slack error notification: #{slack_error.message}"
+    end
+  end
+
+  def add_reaction(emoji)
+    @slack.reactions_add(channel: @channel_id, timestamp: @message_ts, name: emoji)
+  rescue StandardError
+    nil
+  end
+
+  def remove_reaction(emoji)
+    @slack.reactions_remove(channel: @channel_id, timestamp: @message_ts, name: emoji)
+  rescue StandardError
+    nil
+  end
+
+  def reply_in_thread(text)
+    @slack.chat_postMessage(channel: @channel_id, thread_ts: @message_ts, text: text)
+  end
+
+  def render_slack_template(template, locals = {})
+    json = ApplicationController.render(
+      template: "slack/#{template}",
+      formats: [ :slack_message ],
+      locals:
+    )
+    JSON.parse(json, symbolize_names: true)
+  end
+
+  def pick_flavor_message
+    # Collect all possible flavor messages based on file extensions
+    flavor_messages = [ "thanks, i'm gonna sell these to adfly!" ]  # generic fallback
+
+    @files.each do |file|
+      ext = File.extname(file["name"]).delete_prefix(".").downcase
+      case ext
+      when "gif"
+        flavor_messages += [ "_gif_ that file to me and i'll upload it", "_gif_ me all all your files!" ]
+      when "heic"
+        flavor_messages << "What the heic???"
+      when "mov"
+        flavor_messages << "I'll _mov_ that to a permanent link for you"
+      when "html"
+        flavor_messages += [ "Oh, launching a new website?", "uwu, what's this site?", "WooOOAAah hey! Are you serving a site?", "h-t-m-ello :wave:" ]
+      when "rar"
+        flavor_messages += [ ".rawr xD", "i also go \"rar\" sometimes!" ]
+      end
+    end
+
+    flavor_messages.sample
+  end
+
+  def format_backtrace(backtrace)
+    return "" if backtrace.blank?
+
+    Rails.backtrace_cleaner.clean(backtrace).first(3).map do |line|
+      if line =~ /^(.+):(\d+):in\s+'(.+)'$/
+        file, line_num, method_name = $1, $2, $3
+        url = "https://github.com/hackclub/cdn/blob/main/#{file}#L#{line_num}"
+        "<#{url}|#{file}:#{line_num}> in `#{method_name}`"
+      elsif line =~ /^(.+):(\d+)/
+        file, line_num = $1, $2
+        url = "https://github.com/hackclub/cdn/blob/main/#{file}#L#{line_num}"
+        "<#{url}|#{file}:#{line_num}>"
+      else
+        line
+      end
+    end.join("\n")
+  end
+
+  def pick_error_message
+    [
+      "_cdnpheus sneezes and drops the files on the ground before blowing her nose on a blank jpeg._",
+      "_cdnpheus trips and your files slip out of her hands and into an inconveniently placed sewer grate._",
+      "_cdnpheus accidentally slips the files into a folder in her briefcase labeled \"homework\". she starts sweating profusely._",
+      "Hmmm... I'm having trouble thinking right now. Whenever I focus, the only thing that comes to mind is this error",
+      "Aw jeez, this is embarrassing. My database just texted me this",
+      "I just opened my notebook to take a note, but it just says this error all over the pages",
+      "Do you ever try to remember something, but end up thinking about server errors instead? Wait... what were we talking about?",
+      "Super embarrassing, but I just forgot how to upload files.",
+      "i live. i hunger. i. fail to upload your file. i. am. sinister.",
+      "_cdnpheus tries to catch the file but it phases through her claws like a ghost. spooky._"
+    ].sample
+  end
+end
diff --git a/app/models/user.rb b/app/models/user.rb
index 43a933a..7ab3469 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -12,7 +12,7 @@ class User < ApplicationRecord
 
   scope :admins, -> { where(is_admin: true) }
 
-  validates :hca_id, presence: true, uniqueness: true
+  validates :hca_id, uniqueness: true, allow_nil: true
   validates :quota_policy, inclusion: { in: Quota::ADMIN_ASSIGNABLE.map(&:to_s) }, allow_nil: true
   encrypts :hca_access_token
 
diff --git a/app/services/slack_service.rb b/app/services/slack_service.rb
new file mode 100644
index 0000000..155db8c
--- /dev/null
+++ b/app/services/slack_service.rb
@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class SlackService
+  def self.client
+    @client ||= Slack::Web::Client.new(token: Rails.application.config.slack.bot_token)
+  end
+end
diff --git a/app/views/slack/upload_error.slack_message.slocks b/app/views/slack/upload_error.slack_message.slocks
new file mode 100644
index 0000000..f889ab3
--- /dev/null
+++ b/app/views/slack/upload_error.slack_message.slocks
@@ -0,0 +1,5 @@
+section "#{flavor_message}", markdown: true
+
+section ":warning: `#{error_message}`\n```#{backtrace}```", markdown: true
+
+context [mrkdwn_text("sentry ID: `#{sentry_id}` • <https://cdn.hackclub.com|cdn.hackclub.com>")]
diff --git a/app/views/slack/upload_success.slack_message.slocks b/app/views/slack/upload_success.slack_message.slocks
new file mode 100644
index 0000000..10f4771
--- /dev/null
+++ b/app/views/slack/upload_success.slack_message.slocks
@@ -0,0 +1,7 @@
+section "Yeah! Here's yo' links <@#{slack_user_id}>!", markdown: true
+
+uploads.each do |upload|
+  section "#{upload.cdn_url}", markdown: true
+end
+
+context [mrkdwn_text("(manage your files at <https://cdn.hackclub.com|cdn.hackclub.com>!)")]
diff --git a/config/initializers/slack.rb b/config/initializers/slack.rb
new file mode 100644
index 0000000..c5dde74
--- /dev/null
+++ b/config/initializers/slack.rb
@@ -0,0 +1,8 @@
+Rails.application.config.slack = ActiveSupport::OrderedOptions.new
+Rails.application.config.slack.bot_token = ENV.fetch("SLACK_BOT_TOKEN", nil)
+Rails.application.config.slack.signing_secret = ENV.fetch("SLACK_SIGNING_SECRET", nil)
+Rails.application.config.slack.cdn_channels = ENV.fetch("CDN_CHANNELS", "").split(",").map(&:strip)
+
+Slack::Events.configure do |config|
+  config.signing_secret = Rails.application.config.slack.signing_secret
+end
diff --git a/config/routes.rb b/config/routes.rb
index b9341e6..3cc5d31 100644
--- a/config/routes.rb
+++ b/config/routes.rb
@@ -44,6 +44,11 @@ Rails.application.routes.draw do
   # Rescue endpoint to find uploads by original URL
   get "/rescue", to: "external_uploads#rescue", as: :rescue_upload
 
+  # Slack events webhook
+  namespace :slack do
+    post "events", to: "events#create"
+  end
+
   # External upload redirects (must be last to avoid conflicts)
   get "/:id/*filename", to: "external_uploads#show", constraints: { id: /[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}/ }, as: :external_upload
 end