diff --git a/db/migrate/20260601202711_add_summary_to_coplan_plans.co_plan.rb b/db/migrate/20260601202711_add_summary_to_coplan_plans.co_plan.rb
new file mode 100644
index 00000000..67d28e1f
--- /dev/null
+++ b/db/migrate/20260601202711_add_summary_to_coplan_plans.co_plan.rb
@@ -0,0 +1,15 @@
+# This migration comes from co_plan (originally 20260601152600)
+class AddSummaryToCoplanPlans < ActiveRecord::Migration[8.1]
+  def change
+    change_table :coplan_plans do |t|
+      t.text :summary
+      t.datetime :summary_generated_at
+      # SHA256 of the PlanVersion content the summary was generated from.
+      # Used by SummarizePlanJob to debounce regeneration: if the plan's
+      # current content sha hasn't changed since the last summary, the job
+      # no-ops. This lets us fire the job from every PlanVersion#after_create_commit
+      # without re-calling the AI on rapid back-to-back edits.
+      t.string :summary_content_sha256, limit: 64
+    end
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index 33574be1..1a0c1ccf 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[8.1].define(version: 2026_06_01_202009) do
+ActiveRecord::Schema[8.1].define(version: 2026_06_01_202711) do
   create_table "active_admin_comments", id: { type: :string, limit: 36 }, charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.bigint "author_id"
     t.string "author_type"
@@ -226,6 +226,9 @@
     t.string "plan_type_id", limit: 36
     t.text "search_text", size: :medium
     t.string "status", default: "brainstorm", null: false
+    t.text "summary"
+    t.string "summary_content_sha256", limit: 64
+    t.datetime "summary_generated_at"
     t.string "title", null: false
     t.datetime "updated_at", null: false
     t.index ["created_by_user_id"], name: "index_coplan_plans_on_created_by_user_id"
diff --git a/engine/app/jobs/coplan/summarize_plan_job.rb b/engine/app/jobs/coplan/summarize_plan_job.rb
new file mode 100644
index 00000000..f2d1c68f
--- /dev/null
+++ b/engine/app/jobs/coplan/summarize_plan_job.rb
@@ -0,0 +1,70 @@
+module CoPlan
+  # Regenerates a plan's AI-generated summary.
+  #
+  # Enqueued from PlanVersion#after_create_commit. Debounced via
+  # `plan.summary_content_sha256`: each job atomically "claims" the
+  # current content sha before calling the AI. Two workers that wake
+  # up against the same plan-and-sha will race on the claim — only one
+  # wins, the other no-ops. Without the atomic claim, both would pass
+  # a naive pre-check, both call the AI, and waste a full AI request.
+  #
+  # AI errors are discarded rather than retried — a stale summary is
+  # fine, and the next PlanVersion will trigger another attempt. The
+  # claim survives the failure, which is intentional: we'd rather skip
+  # this revision than retry a broken prompt in a loop.
+  class SummarizePlanJob < ApplicationJob
+    PROMPT_PATH = CoPlan::Engine.root.join("prompts", "summarize.md").freeze
+
+    queue_as :default
+
+    discard_on CoPlan::Ai::Error
+
+    def perform(plan_id:)
+      plan = Plan.find_by(id: plan_id)
+      return unless plan
+
+      current_sha = plan.current_plan_version&.content_sha256
+      return if current_sha.blank?
+
+      return unless claim_sha(plan, current_sha)
+
+      summary = generate_summary(plan)
+      return if summary.blank?
+
+      persist_summary(plan, summary, current_sha)
+    end
+
+    private
+
+    # Atomic claim: set summary_content_sha256 = current_sha only if it
+    # isn't already current_sha. Returns true if THIS job won the claim.
+    #
+    # Using a single conditional UPDATE (one round-trip, atomic at the
+    # DB) means concurrent workers can't both pass the check and both
+    # call the AI — exactly one row update succeeds per sha.
+    def claim_sha(plan, current_sha)
+      claimed = Plan.where(id: plan.id)
+                    .where("summary_content_sha256 IS NULL OR summary_content_sha256 != ?", current_sha)
+                    .update_all(summary_content_sha256: current_sha)
+      claimed.positive?
+    end
+
+    def generate_summary(plan)
+      content = plan.current_content
+      return nil if content.blank?
+
+      CoPlan::Ai.call(
+        system_prompt: File.read(PROMPT_PATH),
+        user_content: content
+      ).to_s.strip.presence
+    end
+
+    # Write the summary only if our claim is still current — if a newer
+    # version landed mid-flight, a fresher job has already re-claimed
+    # the sha and we'd be stomping its work.
+    def persist_summary(plan, summary, expected_sha)
+      Plan.where(id: plan.id, summary_content_sha256: expected_sha)
+          .update_all(summary: summary, summary_generated_at: Time.current)
+    end
+  end
+end
diff --git a/engine/app/models/coplan/plan_version.rb b/engine/app/models/coplan/plan_version.rb
index d84c9116..11215d1f 100644
--- a/engine/app/models/coplan/plan_version.rb
+++ b/engine/app/models/coplan/plan_version.rb
@@ -23,6 +23,7 @@ def history_kind
 
     after_create_commit :extract_references
     after_create_commit :broadcast_history_update
+    after_create_commit :enqueue_summary_regeneration
 
     private
 
@@ -64,5 +65,13 @@ def broadcast_references_update
     def compute_sha256
       self.content_sha256 = Digest::SHA256.hexdigest(content_markdown)
     end
+
+    # Fire SummarizePlanJob after every new version. The job is debounced
+    # against `plan.summary_content_sha256`, so rapid back-to-back versions
+    # collapse to a single AI call. The `wait` further reduces wasted calls
+    # during a burst of edits (e.g. a session commit followed by another).
+    def enqueue_summary_regeneration
+      SummarizePlanJob.set(wait: 10.seconds).perform_later(plan_id: plan_id)
+    end
   end
 end
diff --git a/engine/app/services/coplan/ai.rb b/engine/app/services/coplan/ai.rb
new file mode 100644
index 00000000..4ce9d6a8
--- /dev/null
+++ b/engine/app/services/coplan/ai.rb
@@ -0,0 +1,23 @@
+module CoPlan
+  # Provider-agnostic facade for AI calls where the caller doesn't care
+  # which underlying provider runs the prompt. Use this from any place
+  # that just wants "an AI" (e.g. SummarizePlanJob).
+  #
+  # Provider-specific jobs that need to pin a model or provider per call
+  # (e.g. AutomatedReviewJob, where each reviewer is configured with its
+  # own provider+model) should keep calling AiProviders::OpenAi /
+  # AiProviders::Anthropic directly.
+  #
+  # The provider chosen here is an implementation detail; swap it without
+  # touching callers. Raises CoPlan::Ai::Error on provider failure so
+  # callers can `discard_on` without knowing which provider is in use.
+  module Ai
+    class Error < StandardError; end
+
+    def self.call(system_prompt:, user_content:)
+      AiProviders::OpenAi.call(system_prompt: system_prompt, user_content: user_content)
+    rescue AiProviders::OpenAi::Error => e
+      raise Error, e.message
+    end
+  end
+end
diff --git a/engine/db/migrate/20260601152600_add_summary_to_coplan_plans.rb b/engine/db/migrate/20260601152600_add_summary_to_coplan_plans.rb
new file mode 100644
index 00000000..52477f5d
--- /dev/null
+++ b/engine/db/migrate/20260601152600_add_summary_to_coplan_plans.rb
@@ -0,0 +1,14 @@
+class AddSummaryToCoplanPlans < ActiveRecord::Migration[8.1]
+  def change
+    change_table :coplan_plans do |t|
+      t.text :summary
+      t.datetime :summary_generated_at
+      # SHA256 of the PlanVersion content the summary was generated from.
+      # Used by SummarizePlanJob to debounce regeneration: if the plan's
+      # current content sha hasn't changed since the last summary, the job
+      # no-ops. This lets us fire the job from every PlanVersion#after_create_commit
+      # without re-calling the AI on rapid back-to-back edits.
+      t.string :summary_content_sha256, limit: 64
+    end
+  end
+end
diff --git a/engine/prompts/summarize.md b/engine/prompts/summarize.md
new file mode 100644
index 00000000..069ef3b8
--- /dev/null
+++ b/engine/prompts/summarize.md
@@ -0,0 +1,7 @@
+You are summarizing engineering planning documents.
+
+Write a 1-2 sentence summary of the plan below. Be concrete — what is the
+plan proposing, and (if applicable) why. Skip filler like "This plan
+discusses..." or "The author proposes...". Start with the substance.
+
+Plain prose only. No Markdown, no lists, no headings. Maximum ~280 characters.
diff --git a/spec/jobs/summarize_plan_job_spec.rb b/spec/jobs/summarize_plan_job_spec.rb
new file mode 100644
index 00000000..ae296e5d
--- /dev/null
+++ b/spec/jobs/summarize_plan_job_spec.rb
@@ -0,0 +1,160 @@
+require "rails_helper"
+
+RSpec.describe CoPlan::SummarizePlanJob, type: :job do
+  include ActiveJob::TestHelper
+
+  let(:plan) { create(:plan) }
+  let(:current_sha) { plan.current_plan_version.content_sha256 }
+
+  before do
+    allow(CoPlan::Ai).to receive(:call).and_return("Fresh summary.")
+  end
+
+  describe "#perform" do
+    it "passes the summarize prompt and plan content to CoPlan::Ai" do
+      described_class.perform_now(plan_id: plan.id)
+
+      expect(CoPlan::Ai).to have_received(:call).with(
+        system_prompt: File.read(CoPlan::SummarizePlanJob::PROMPT_PATH),
+        user_content: plan.current_content
+      )
+    end
+
+    it "updates summary, generated_at, and sha when content has changed" do
+      freeze_time do
+        expect {
+          described_class.perform_now(plan_id: plan.id)
+        }.to change { plan.reload.summary }.from(nil).to("Fresh summary.")
+          .and change { plan.reload.summary_content_sha256 }.from(nil).to(current_sha)
+
+        expect(plan.reload.summary_generated_at).to eq(Time.current)
+      end
+    end
+
+    it "strips whitespace from the AI response before persisting" do
+      allow(CoPlan::Ai).to receive(:call).and_return("  trimmed\n\n")
+
+      described_class.perform_now(plan_id: plan.id)
+
+      expect(plan.reload.summary).to eq("trimmed")
+    end
+
+    it "no-ops when summary_content_sha256 already matches current content" do
+      plan.update!(
+        summary: "Existing.",
+        summary_generated_at: 1.hour.ago,
+        summary_content_sha256: current_sha
+      )
+
+      described_class.perform_now(plan_id: plan.id)
+
+      expect(CoPlan::Ai).not_to have_received(:call)
+      expect(plan.reload.summary).to eq("Existing.")
+    end
+
+    it "regenerates when a new PlanVersion changes the content sha" do
+      plan.update!(summary: "Old.", summary_content_sha256: current_sha, summary_generated_at: 1.hour.ago)
+      new_version = create(:plan_version, plan: plan, revision: plan.current_revision + 1,
+                                          content_markdown: "# New content\n\nDifferent text.")
+      plan.update!(current_plan_version: new_version, current_revision: new_version.revision)
+
+      described_class.perform_now(plan_id: plan.id)
+
+      expect(plan.reload.summary).to eq("Fresh summary.")
+      expect(plan.summary_content_sha256).to eq(new_version.content_sha256)
+    end
+
+    it "does not update when the AI returns blank" do
+      allow(CoPlan::Ai).to receive(:call).and_return("   \n")
+
+      expect {
+        described_class.perform_now(plan_id: plan.id)
+      }.not_to change { plan.reload.summary }
+    end
+
+    it "no-ops when the plan has no current version" do
+      plan.update_columns(current_plan_version_id: nil, current_revision: 0)
+
+      described_class.perform_now(plan_id: plan.id)
+
+      expect(CoPlan::Ai).not_to have_received(:call)
+    end
+
+    it "no-ops when the plan has been deleted" do
+      missing_id = SecureRandom.uuid
+
+      expect {
+        described_class.perform_now(plan_id: missing_id)
+      }.not_to raise_error
+      expect(CoPlan::Ai).not_to have_received(:call)
+    end
+
+    # Concurrency guard: two workers waking up against the same plan-and-sha
+    # must collapse to a single AI call. Without the atomic claim, both jobs
+    # would pass a naive pre-check and both burn an AI request.
+    it "claims the sha before calling AI so a concurrent worker skips" do
+      call_count = 0
+      allow(CoPlan::Ai).to receive(:call) do
+        call_count += 1
+        # Simulate a second worker firing while we're mid-AI-call. With the
+        # claim already taken, this inner perform should no-op.
+        described_class.perform_now(plan_id: plan.id)
+        "Fresh summary."
+      end
+
+      described_class.perform_now(plan_id: plan.id)
+
+      expect(call_count).to eq(1)
+      expect(plan.reload.summary).to eq("Fresh summary.")
+    end
+
+    # Race-condition guard: a slow job started against revision N must
+    # NOT overwrite a fresher summary already persisted for revision N+1.
+    it "does not overwrite a fresher summary when a newer version landed mid-flight" do
+      stale_sha = current_sha
+
+      # Simulate "a newer version landed while the AI was thinking" by
+      # mutating the plan's current_plan_version between the AI call and
+      # the persist step.
+      allow(CoPlan::Ai).to receive(:call) do
+        newer = create(:plan_version, plan: plan, revision: plan.current_revision + 1,
+                                       content_markdown: "# Fresher\n\nNewer body.")
+        plan.update!(current_plan_version: newer, current_revision: newer.revision,
+                     summary: "Fresher summary persisted by the newer job.",
+                     summary_content_sha256: newer.content_sha256,
+                     summary_generated_at: Time.current)
+        "Stale summary from the slow job."
+      end
+
+      described_class.perform_now(plan_id: plan.id)
+
+      expect(plan.reload.summary).to eq("Fresher summary persisted by the newer job.")
+      expect(plan.summary_content_sha256).not_to eq(stale_sha)
+    end
+
+    it "discards on AI errors instead of retrying" do
+      allow(CoPlan::Ai).to receive(:call).and_raise(CoPlan::Ai::Error, "boom")
+
+      expect {
+        perform_enqueued_jobs { described_class.perform_later(plan_id: plan.id) }
+      }.not_to raise_error
+    end
+
+    it "enqueues on the default queue" do
+      existing_plan_id = plan.id
+      expect {
+        described_class.perform_later(plan_id: existing_plan_id)
+      }.to have_enqueued_job(described_class).on_queue("default").with(plan_id: existing_plan_id)
+    end
+  end
+
+  describe "PlanVersion after_create_commit hook" do
+    it "enqueues SummarizePlanJob when a new version is created" do
+      existing_plan = create(:plan)
+
+      expect {
+        create(:plan_version, plan: existing_plan, revision: existing_plan.current_revision + 1)
+      }.to have_enqueued_job(described_class).with(plan_id: existing_plan.id)
+    end
+  end
+end
diff --git a/spec/services/ai_spec.rb b/spec/services/ai_spec.rb
new file mode 100644
index 00000000..2d6b257d
--- /dev/null
+++ b/spec/services/ai_spec.rb
@@ -0,0 +1,26 @@
+require "rails_helper"
+
+RSpec.describe CoPlan::Ai do
+  describe ".call" do
+    it "delegates to AiProviders::OpenAi and returns its response" do
+      allow(CoPlan::AiProviders::OpenAi).to receive(:call).and_return("ai output")
+
+      result = described_class.call(system_prompt: "sys", user_content: "body")
+
+      expect(result).to eq("ai output")
+      expect(CoPlan::AiProviders::OpenAi).to have_received(:call).with(
+        system_prompt: "sys",
+        user_content: "body"
+      )
+    end
+
+    it "wraps provider errors in CoPlan::Ai::Error so callers don't know the provider" do
+      allow(CoPlan::AiProviders::OpenAi).to receive(:call)
+        .and_raise(CoPlan::AiProviders::OpenAi::Error, "rate limited")
+
+      expect {
+        described_class.call(system_prompt: "sys", user_content: "body")
+      }.to raise_error(CoPlan::Ai::Error, "rate limited")
+    end
+  end
+end