diff --git a/lib/prism/orm/n_plus_one_detector.rb b/lib/prism/orm/n_plus_one_detector.rb
new file mode 100644
index 0000000..58048ba
--- /dev/null
+++ b/lib/prism/orm/n_plus_one_detector.rb
@@ -0,0 +1,115 @@
+require 'set'
+
+module Prism
+  module Orm
+    # Raised by NPlusOneDetector.report! when raise_on_violation? is set and
+    # at least one association crossed the threshold - lets a test suite
+    # turn a silent perf regression into a hard failure instead of a log
+    # line nobody reads.
+    class NPlusOneViolation < StandardError; end
+
+    # Bullet-lite: flags the classic N+1 pattern - the SAME association
+    # lazily loaded (association_cache miss, i.e. NOT pre-populated by
+    # Relation's `includes` preloader) for more than `threshold` DISTINCT
+    # owner records within a single request span. Opt-in and thread-local,
+    # so it costs nothing unless explicitly enabled (development/test) and
+    # stays correct under concurrent requests running on separate threads.
+    #
+    # Hooked from two places: Associable's lazy reader fallbacks call
+    # #record_lazy_load right before running the query a cache miss would
+    # otherwise trigger silently; Application#dispatch calls #reset! at the
+    # start of each request's instrumented span and #report! right after it
+    # finishes, matching Rails' request-scoped Bullet middleware without
+    # needing an actual middleware layer (this framework's `process_action.
+    # prism` instrumentation event is emitted only AFTER the whole action
+    # already ran - see Instrumentation::Bus#instrument - so there is no
+    # "before" hook to subscribe to; the reset/report pair is called
+    # directly instead).
+    module NPlusOneDetector
+      class << self
+        def enable!(threshold: 2, io: $stderr, raise_on_violation: false)
+          @threshold = threshold
+          @io = io
+          @raise_on_violation = raise_on_violation
+          @enabled = true
+        end
+
+        def disable!
+          @enabled = false
+        end
+
+        def enabled?
+          !!@enabled
+        end
+
+        def threshold
+          @threshold || 2
+        end
+
+        def raise_on_violation?
+          !!@raise_on_violation
+        end
+
+        def io
+          @io || $stderr
+        end
+
+        # Called by Associable's lazy reader fallbacks. `owner` is the
+        # record the association is being read off of (`self` in the
+        # reader); distinct owners are what count towards the threshold, so
+        # reading the same association twice on the SAME instance (a plain
+        # cache-warm re-read once #record_lazy_load has already fired once
+        # this request) does not inflate the count further.
+        def record_lazy_load(owner, association_name)
+          return unless enabled?
+          tracker[[owner.class.name, association_name]] << owner_key(owner)
+        end
+
+        # Resets the thread-local tracker - called once per request, before
+        # any association reader can possibly fire.
+        def reset!
+          Thread.current[:prism_n_plus_one_tracker] = nil
+        end
+
+        # Called once per request, after the whole action ran. Emits one
+        # line per association whose lazy loads crossed `threshold`, then
+        # clears the tracker so a violation is never reported twice.
+        def report!(context = {})
+          return unless enabled?
+          violations.each do |(class_name, association_name), count|
+            emit(class_name, association_name, count, context)
+          end
+          reset!
+        end
+
+        private
+
+        def owner_key(owner)
+          owner.respond_to?(:id) && owner.id ? owner.id : owner.object_id
+        end
+
+        def emit(class_name, association_name, count, context)
+          message = "N+1 query detected: #{class_name}##{association_name} lazily loaded for " \
+                    "#{count} separate #{class_name} records#{context_suffix(context)} - " \
+                    "add .includes(:#{association_name}) to the query that loaded them"
+          io.puts(message)
+          raise NPlusOneViolation, message if raise_on_violation?
+        end
+
+        def context_suffix(context)
+          context.empty? ? '' : " (#{context.map { |k, v| "#{k}=#{v.inspect}" }.join(' ')})"
+        end
+
+        def violations
+          tracker.each_with_object({}) do |(key, owner_keys), acc|
+            acc[key] = owner_keys.size if owner_keys.size >= threshold
+          end
+        end
+
+        def tracker
+          Thread.current[:prism_n_plus_one_tracker] ||= Hash.new { |h, k| h[k] = Set.new }
+        end
+      end
+    end
+  end
+end