diff --git a/app/jobs/scheduled/summaries_backfill.rb b/app/jobs/scheduled/summaries_backfill.rb
index 10dcc719..f6114d24 100644
--- a/app/jobs/scheduled/summaries_backfill.rb
+++ b/app/jobs/scheduled/summaries_backfill.rb
@@ -57,7 +57,7 @@ module ::Jobs
                           ais.target_type = 'Topic' AND
                           ais.summary_type = '#{summary_type}'
         SQL
-        .where("topics.created_at > current_timestamp - INTERVAL '#{max_age_days.to_i} DAY'")
+        .where("topics.last_posted_at > current_timestamp - INTERVAL '#{max_age_days.to_i} DAY'")
         .where(
           <<~SQL, # (1..1) gets stored ad (1..2).
           ais.id IS NULL OR (
diff --git a/lib/summarization/fold_content.rb b/lib/summarization/fold_content.rb
index 831d311e..7cacc8c0 100644
--- a/lib/summarization/fold_content.rb
+++ b/lib/summarization/fold_content.rb
@@ -58,8 +58,9 @@ module DiscourseAi
           if summary
             @existing_summary = summary
 
-            if existing_summary.original_content_sha != latest_sha
-              @existing_summary.mark_as_outdated
+            if summary.original_content_sha != latest_sha ||
+                 content_to_summarize.any? { |cts| cts[:last_version_at] > summary.updated_at }
+              summary.mark_as_outdated
             end
           end
         end
diff --git a/lib/summarization/strategies/chat_messages.rb b/lib/summarization/strategies/chat_messages.rb
index 2f073d71..20163464 100644
--- a/lib/summarization/strategies/chat_messages.rb
+++ b/lib/summarization/strategies/chat_messages.rb
@@ -23,8 +23,8 @@ module DiscourseAi
             .where("chat_messages.created_at > ?", since.hours.ago)
             .includes(:user)
             .order(created_at: :asc)
-            .pluck(:id, :username_lower, :message)
-            .map { { id: _1, poster: _2, text: _3 } }
+            .pluck(:id, :username_lower, :message, :updated_at)
+            .map { { id: _1, poster: _2, text: _3, last_version_at: _4 } }
         end
 
         def summary_extension_prompt(summary, contents)
diff --git a/lib/summarization/strategies/hot_topic_gists.rb b/lib/summarization/strategies/hot_topic_gists.rb
index b3e88876..24835817 100644
--- a/lib/summarization/strategies/hot_topic_gists.rb
+++ b/lib/summarization/strategies/hot_topic_gists.rb
@@ -49,16 +49,16 @@ module DiscourseAi
               .joins(:user)
               .where("post_number IN (?)", recent_hot_posts << op_post_number)
               .order(:post_number)
-              .pluck(:post_number, :raw, :username)
+              .pluck(:post_number, :raw, :username, :last_version_at)
 
-          posts_data.reduce([]) do |memo, (pn, raw, username)|
+          posts_data.reduce([]) do |memo, (pn, raw, username, last_version_at)|
             raw_text = raw
 
             if pn == 1 && target.topic_embed&.embed_content_cache.present?
               raw_text = target.topic_embed&.embed_content_cache
             end
 
-            memo << { poster: username, id: pn, text: raw_text }
+            memo << { poster: username, id: pn, text: raw_text, last_version_at: last_version_at }
           end
         end
 
diff --git a/lib/summarization/strategies/topic_summary.rb b/lib/summarization/strategies/topic_summary.rb
index e97c391a..4dfca68c 100644
--- a/lib/summarization/strategies/topic_summary.rb
+++ b/lib/summarization/strategies/topic_summary.rb
@@ -18,16 +18,17 @@ module DiscourseAi
               :post_number,
               :raw,
               :username,
+              :last_version_at,
             )
 
-          posts_data.reduce([]) do |memo, (pn, raw, username)|
+          posts_data.reduce([]) do |memo, (pn, raw, username, last_version_at)|
             raw_text = raw
 
             if pn == 1 && target.topic_embed&.embed_content_cache.present?
               raw_text = target.topic_embed&.embed_content_cache
             end
 
-            memo << { poster: username, id: pn, text: raw_text }
+            memo << { poster: username, id: pn, text: raw_text, last_version_at: last_version_at }
           end
         end
 
diff --git a/spec/jobs/scheduled/summaries_backfill_spec.rb b/spec/jobs/scheduled/summaries_backfill_spec.rb
index 401f4d20..ccf23b06 100644
--- a/spec/jobs/scheduled/summaries_backfill_spec.rb
+++ b/spec/jobs/scheduled/summaries_backfill_spec.rb
@@ -1,7 +1,9 @@
 # frozen_string_literal: true
 
 RSpec.describe Jobs::SummariesBackfill do
-  fab!(:topic) { Fabricate(:topic, word_count: 200, highest_post_number: 2) }
+  fab!(:topic) do
+    Fabricate(:topic, word_count: 200, highest_post_number: 2, last_posted_at: 2.hours.ago)
+  end
   let(:limit) { 24 } # guarantee two summaries per batch
   let(:intervals) { 12 } # budget is split into intervals. Job runs every five minutes.
 
@@ -73,7 +75,7 @@ RSpec.describe Jobs::SummariesBackfill do
 
     it "respects max age setting" do
       SiteSetting.ai_summary_backfill_topic_max_age_days = 1
-      topic.update!(created_at: 2.days.ago)
+      topic.update!(last_posted_at: 2.days.ago)
 
       expect(subject.backfill_candidates(type)).to be_empty
     end
@@ -112,14 +114,14 @@ RSpec.describe Jobs::SummariesBackfill do
     end
 
     it "updates the highest_target_number if the summary turned to be up to date" do
+      og_highest_post_number = topic.highest_post_number
       existing_summary =
         Fabricate(
           :ai_summary,
           target: topic,
           updated_at: 3.hours.ago,
-          highest_target_number: topic.highest_post_number,
+          highest_target_number: og_highest_post_number,
         )
-      og_highest_post_number = topic.highest_post_number
       topic.update!(highest_post_number: og_highest_post_number + 1)
 
       # No prepared responses here. We don't perform a completion call.
diff --git a/spec/lib/modules/summarization/fold_content_spec.rb b/spec/lib/modules/summarization/fold_content_spec.rb
index e1972bf6..40bf9cd1 100644
--- a/spec/lib/modules/summarization/fold_content_spec.rb
+++ b/spec/lib/modules/summarization/fold_content_spec.rb
@@ -3,15 +3,15 @@
 RSpec.describe DiscourseAi::Summarization::FoldContent do
   subject(:summarizer) { DiscourseAi::Summarization.topic_summary(topic) }
 
+  let!(:llm_model) { assign_fake_provider_to(:ai_summarization_model) }
+
+  fab!(:topic) { Fabricate(:topic, highest_post_number: 2) }
+  fab!(:post_1) { Fabricate(:post, topic: topic, post_number: 1, raw: "This is a text") }
+
+  before { SiteSetting.ai_summarization_enabled = true }
+
   describe "#summarize" do
-    let!(:llm_model) { assign_fake_provider_to(:ai_summarization_model) }
-
-    fab!(:topic) { Fabricate(:topic, highest_post_number: 2) }
-    fab!(:post_1) { Fabricate(:post, topic: topic, post_number: 1, raw: "This is a text") }
-
     before do
-      SiteSetting.ai_summarization_enabled = true
-
       # Make sure each content fits in a single chunk.
       # 700 is the number of tokens reserved for the prompt.
       model_tokens =
@@ -52,4 +52,38 @@ RSpec.describe DiscourseAi::Summarization::FoldContent do
       end
     end
   end
+
+  describe "#existing_summary" do
+    context "when a summary already exists" do
+      fab!(:ai_summary) do
+        Fabricate(
+          :ai_summary,
+          target: topic,
+          highest_target_number: topic.highest_post_number,
+          original_content_sha: AiSummary.build_sha("1"),
+        )
+      end
+
+      it "doesn't mark it as outdated" do
+        expect(summarizer.existing_summary.outdated).to eq(false)
+      end
+
+      context "when it's outdated because there are new targets" do
+        before { Fabricate(:post, topic: topic, post_number: 2, raw: "This is a text") }
+
+        it "marks it as outdated" do
+          expect(summarizer.existing_summary.outdated).to eq(true)
+        end
+      end
+
+      context "when it's outdated because existing content changes" do
+        it "marks it as outdated" do
+          ai_summary.update!(updated_at: 20.minutes.ago)
+          post_1.update!(last_version_at: 5.minutes.ago)
+
+          expect(summarizer.existing_summary.outdated).to eq(true)
+        end
+      end
+    end
+  end
 end