diff --git a/engine/lib/tasks/coplan_summaries.rake b/engine/lib/tasks/coplan_summaries.rake
new file mode 100644
index 00000000..c22ac864
--- /dev/null
+++ b/engine/lib/tasks/coplan_summaries.rake
@@ -0,0 +1,51 @@
+namespace :coplan do
+  namespace :summaries do
+    # One-time backfill of AI summaries for plans created before the
+    # summary infra (COPLAN-24, #118), which only fires on new
+    # PlanVersion creates. Enqueues SummarizePlanJob for every plan
+    # missing a summary; the job's sha-claim debounce makes this safe
+    # to re-run.
+    #
+    # Throttled with a sleep between batches to avoid spiking OpenAI
+    # cost/rate limits. Tune via env:
+    #
+    #   BATCH_SIZE=25 INTERVAL=5 bin/rails coplan:summaries:backfill
+    #
+    # Plans that already have a summary are excluded by the query;
+    # plans with no current_plan_version (no content) are skipped.
+    desc "Backfill AI summaries for plans missing one (COPLAN-31)"
+    task backfill: :environment do
+      batch_size = Integer(ENV["BATCH_SIZE"].presence || 25)
+      interval   = Float(ENV["INTERVAL"].presence || 5)
+
+      # A prior SummarizePlanJob may have claimed the current content
+      # sha (summary_content_sha256) but then hit CoPlan::Ai::Error or
+      # got a blank response — leaving the plan summary-less with the
+      # sha already claimed. Re-enqueuing alone would no-op forever at
+      # the job's claim_sha guard, so those plans could never be
+      # backfilled. Clear the stale claims (only for summary-less
+      # plans; already-summarized plans keep theirs) so the enqueued
+      # jobs can re-claim and retry.
+      reset = CoPlan::Plan.where(summary: nil)
+                          .where.not(summary_content_sha256: nil)
+                          .update_all(summary_content_sha256: nil)
+
+      enqueued = 0
+      skipped  = 0
+
+      CoPlan::Plan.where(summary: nil).find_each(batch_size: batch_size) do |plan|
+        if plan.current_plan_version_id.nil?
+          skipped += 1
+          next
+        end
+
+        CoPlan::SummarizePlanJob.perform_later(plan_id: plan.id)
+        enqueued += 1
+
+        sleep(interval) if interval.positive? && (enqueued % batch_size).zero?
+      end
+
+      puts "coplan:summaries:backfill — enqueued=#{enqueued} skipped=#{skipped} reset_claims=#{reset} (skipped = no current_plan_version; already-summarized excluded by query)"
+    end
+  end
+end
diff --git a/spec/lib/tasks/coplan_summaries_spec.rb b/spec/lib/tasks/coplan_summaries_spec.rb
new file mode 100644
index 00000000..1e6d5ade
--- /dev/null
+++ b/spec/lib/tasks/coplan_summaries_spec.rb
@@ -0,0 +1,82 @@
+require "rails_helper"
+require "rake"
+
+RSpec.describe "coplan:summaries:backfill", type: :task do
+  include ActiveJob::TestHelper
+
+  subject(:run_task) { Rake::Task["coplan:summaries:backfill"].tap(&:reenable).invoke }
+
+  around do |example|
+    previous_application = Rake.application
+    Rake.application = Rake::Application.new
+    Rake::Task.define_task(:environment)
+    load CoPlan::Engine.root.join("lib/tasks/coplan_summaries.rake").to_s
+    example.run
+  ensure
+    Rake.application = previous_application
+  end
+
+  before { clear_enqueued_jobs }
+
+  it "enqueues SummarizePlanJob for summary-less plans with a current version" do
+    plan = create(:plan)
+
+    expect { run_task }.to have_enqueued_job(CoPlan::SummarizePlanJob).with(plan_id: plan.id)
+  end
+
+  it "skips plans that already have a summary" do
+    create(:plan).update_columns(summary: "Already summarized.")
+
+    expect { run_task }.not_to have_enqueued_job(CoPlan::SummarizePlanJob)
+  end
+
+  it "skips plans with no current_plan_version" do
+    create(:plan).update_columns(current_plan_version_id: nil)
+
+    expect { run_task }.not_to have_enqueued_job(CoPlan::SummarizePlanJob)
+  end
+
+  it "enqueues only the eligible plans when mixed" do
+    eligible = create(:plan)
+    create(:plan).update_columns(summary: "Has one.")
+    create(:plan).update_columns(current_plan_version_id: nil)
+
+    # Plan creation enqueues its own SummarizePlanJob (PlanVersion
+    # after_create_commit); clear those so we observe only the task's.
+    clear_enqueued_jobs
+    run_task
+
+    expect(enqueued_jobs.map { |job| job[:args].first["plan_id"] }).to eq([eligible.id])
+  end
+
+  it "clears a stale sha claim on a summary-less plan so the retried job isn't debounced away" do
+    plan = create(:plan)
+    # Simulate a prior job that claimed the sha but failed/blanked:
+    # summary stays nil while the sha is already claimed.
+    plan.update_columns(summary_content_sha256: plan.current_plan_version.content_sha256)
+
+    expect { run_task }.to have_enqueued_job(CoPlan::SummarizePlanJob).with(plan_id: plan.id)
+    expect(plan.reload.summary_content_sha256).to be_nil
+  end
+
+  it "leaves an already-summarized plan's sha claim intact" do
+    plan = create(:plan)
+    sha = plan.current_plan_version.content_sha256
+    plan.update_columns(summary: "Done.", summary_content_sha256: sha)
+
+    run_task
+
+    expect(plan.reload.summary_content_sha256).to eq(sha)
+  end
+
+  it "falls back to defaults when BATCH_SIZE/INTERVAL are blank" do
+    plan = create(:plan)
+    ENV["BATCH_SIZE"] = ""
+    ENV["INTERVAL"] = ""
+
+    expect { run_task }.to have_enqueued_job(CoPlan::SummarizePlanJob).with(plan_id: plan.id)
+  ensure
+    ENV.delete("BATCH_SIZE")
+    ENV.delete("INTERVAL")
+  end
+end