diff --git a/Gemfile b/Gemfile
index 054f8860..5a0ec0ed 100644
--- a/Gemfile
+++ b/Gemfile
@@ -61,6 +61,9 @@ group :development, :test do
   # See https://guides.rubyonrails.org/debugging_rails_applications.html#debugging-with-the-debug-gem
   gem "debug", platforms: %i[ mri windows ], require: "debug/prelude"
 
+  # Generate realistic, reproducible local seed data
+  gem "faker"
+
   # Audits gems for known security defects (use config/bundler-audit.yml to ignore issues)
   gem "bundler-audit", require: false
 
diff --git a/Gemfile.lock b/Gemfile.lock
index dc1b40d9..6cd855cf 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -165,6 +165,8 @@ GEM
     factory_bot_rails (6.5.1)
       factory_bot (~> 6.5)
       railties (>= 6.1.0)
+    faker (3.8.0)
+      i18n (>= 1.8.11, < 2)
     faraday (2.14.1)
       faraday-net_http (>= 2.0, < 3.5)
       json
@@ -537,6 +539,7 @@ DEPENDENCIES
   debug
   diffy
   factory_bot_rails
+  faker
   image_processing (~> 1.2)
   importmap-rails
   jbuilder
@@ -612,6 +615,7 @@ CHECKSUMS
   event_stream_parser (1.0.0) sha256=a2683bab70126286f8184dc88f7968ffc4028f813161fb073ec90d171f7de3c8
   factory_bot (6.5.6) sha256=12beb373214dccc086a7a63763d6718c49769d5606f0501e0a4442676917e077
   factory_bot_rails (6.5.1) sha256=d3cc4851eae4dea8a665ec4a4516895045e710554d2b5ac9e68b94d351bc6d68
+  faker (3.8.0) sha256=c147b308df73a90f27a4fc84f18d4c22ef0ad9c2a64b2b61c86fd0ca71753efc
   faraday (2.14.1) sha256=a43cceedc1e39d188f4d2cdd360a8aaa6a11da0c407052e426ba8d3fb42ef61c
   faraday-mashify (1.0.2) sha256=ebdc93b3f807af9a4b18c3d967fc7bcda01e297aed9cc2015ffe8db6e7add2e9
   faraday-multipart (1.2.0) sha256=7d89a949693714176f612323ca13746a2ded204031a6ba528adee788694ef757
diff --git a/db/seeds.rb b/db/seeds.rb
index 4dd3442f..f133f618 100644
--- a/db/seeds.rb
+++ b/db/seeds.rb
@@ -1,141 +1,6 @@
-puts "Seeding users..."
-hampton = CoPlan::User.find_or_create_by!(email: "hampton@squareup.com") do |u|
-  u.external_id = "hampton@squareup.com"
-  u.name = "Hampton Lintorn-Catlin"
-  u.username = "hampton"
-  u.admin = true
-  u.avatar_url = "https://avatars.githubusercontent.com/u/111?s=80"
-  u.title = "Staff Engineer"
-  u.team = "Developer Tools"
+if Rails.env.local?
+  require_relative "seeds/development"
+  CoPlan::DevelopmentSeed.call
+else
+  warn "Development seed data is only available in development and test environments."
 end
-hampton.update!(avatar_url: "https://avatars.githubusercontent.com/u/111?s=80") if hampton.avatar_url.blank?
-
-puts "Seeding plans..."
-if CoPlan::Plan.count == 0
-  plan = CoPlan::Plans::Create.call(
-    title: "Q3 Product Roadmap",
-    content: "# Q3 Product Roadmap\n\n## Goals\n\n- Launch new dashboard\n- Improve API performance\n- Add team collaboration features\n\n## Timeline\n\n### Month 1\n- Design reviews\n- Technical planning\n\n### Month 2\n- Core implementation\n- Testing\n\n### Month 3\n- Beta launch\n- Feedback collection\n",
-    user: hampton
-  )
-  plan.update!(status: "considering")
-end
-
-puts "Seeding comments..."
-if CoPlan::CommentThread.count == 0
-  plan = CoPlan::Plan.first
-  if plan&.current_plan_version
-    reviewer = CoPlan::User.find_or_create_by!(email: "reviewer@squareup.com") do |u|
-      u.external_id = "reviewer@squareup.com"
-      u.name = "Plan Reviewer"
-      u.title = "Senior Engineer"
-      u.team = "Platform"
-    end
-
-    thread = CoPlan::CommentThread.create!(
-      plan: plan,
-      plan_version: plan.current_plan_version,
-      start_line: 5,
-      end_line: 8,
-      created_by_user: reviewer
-    )
-    thread.comments.create!(
-      author_type: "human",
-      author_id: reviewer.id,
-      body_markdown: "I think the timeline for Month 1 is too aggressive. Can we break this into smaller milestones?"
-    )
-
-    general_thread = CoPlan::CommentThread.create!(
-      plan: plan,
-      plan_version: plan.current_plan_version,
-      created_by_user: hampton
-    )
-    general_thread.comments.create!(
-      author_type: "human",
-      author_id: hampton.id,
-      body_markdown: "Overall this is looking good. Let's move forward with the **beta launch** plan."
-    )
-  end
-end
-
-puts "Seeding API tokens..."
-if CoPlan::ApiToken.count == 0
-  raw_token = "dev-api-token-#{SecureRandom.hex(8)}"
-  CoPlan::ApiToken.create!(
-    user: hampton,
-    name: "Development Agent",
-    token_digest: Digest::SHA256.hexdigest(raw_token),
-    token_prefix: raw_token[0, 8]
-  )
-  puts "  Created API token: #{raw_token}"
-  puts "  (Save this — it won't be shown again)"
-end
-
-puts "Seeding plan types..."
-# Icons come from the built-in named set (CoPlan::PlansHelper::PLAN_TYPE_ICONS)
-# so each install can brand its document types.
-general = CoPlan::PlanType.find_or_create_by!(name: "General") do |pt|
-  pt.description = "General-purpose plan"
-end
-general.update!(icon: "file-text") if general.icon.blank?
-CoPlan::PlanType.find_or_create_by!(name: "RFC") do |pt|
-  pt.description = "Request for Comments — propose a significant change for team review"
-  pt.default_tags = ["rfc"]
-end.tap { |pt| pt.update!(icon: "scroll") if pt.icon.blank? }
-CoPlan::PlanType.find_or_create_by!(name: "Design Doc") do |pt|
-  pt.description = "Technical design document for a new system or feature"
-  pt.default_tags = ["design"]
-end.tap { |pt| pt.update!(icon: "compass") if pt.icon.blank? }
-CoPlan::PlanType.find_or_create_by!(name: "ADR") do |pt|
-  pt.description = "Architecture Decision Record — document a key technical decision"
-  pt.default_tags = ["adr"]
-end.tap { |pt| pt.update!(icon: "scale") if pt.icon.blank? }
-
-# Backfill any plans without a plan type
-CoPlan::Plan.where(plan_type_id: nil).update_all(plan_type_id: general.id)
-
-puts "Seeding tags on plans..."
-CoPlan::Plan.includes(:tags, :plan_type).find_each do |p|
-  if p.tags.empty? && p.plan_type&.default_tags&.any?
-    p.tag_names = p.plan_type.default_tags
-  end
-end
-
-# Add some demo plans with tags for local development
-if CoPlan::Plan.count < 3
-  rfc_type = CoPlan::PlanType.find_by(name: "RFC")
-  design_type = CoPlan::PlanType.find_by(name: "Design Doc")
-
-  api_plan = CoPlan::Plans::Create.call(
-    title: "API Rate Limiting Strategy",
-    content: "# API Rate Limiting Strategy\n\n## Problem\n\nOur API endpoints have no rate limiting, leading to occasional abuse.\n\n## Proposal\n\nImplement token-bucket rate limiting at the gateway level.\n\n## Alternatives Considered\n\n- Per-IP limiting\n- API key quotas\n",
-    user: hampton,
-    plan_type_id: rfc_type&.id
-  )
-  api_plan.update!(status: "considering")
-  api_plan.tag_names = ["api", "infrastructure", "security"]
-
-  auth_plan = CoPlan::Plans::Create.call(
-    title: "Authentication System Redesign",
-    content: "# Authentication System Redesign\n\n## Goals\n\n- Migrate from session-based to token-based auth\n- Support SSO providers\n- Improve security posture\n\n## Architecture\n\nOIDC-based flow with JWT access tokens.\n",
-    user: hampton,
-    plan_type_id: design_type&.id
-  )
-  auth_plan.update!(status: "developing")
-  auth_plan.tag_names = ["security", "infrastructure", "design"]
-end
-
-# Tag the original Q3 roadmap if it has no tags
-q3 = CoPlan::Plan.find_by(title: "Q3 Product Roadmap")
-q3.tag_names = ["roadmap", "product"] if q3 && q3.tags.empty?
-
-puts "Seeding folders..."
-if CoPlan::Folder.count == 0
-  product = CoPlan::Folder.find_or_create_by_path!("Product/Q3", created_by_user: hampton)
-  infra = CoPlan::Folder.find_or_create_by_path!("Infrastructure", created_by_user: hampton)
-
-  q3&.update!(folder: product) if q3&.folder_id.nil?
-  api_seed = CoPlan::Plan.find_by(title: "API Rate Limiting Strategy")
-  api_seed&.update!(folder: infra) if api_seed&.folder_id.nil?
-end
-
-puts "Done! #{CoPlan::User.count} users, #{CoPlan::Plan.count} plans, #{CoPlan::Folder.count} folders, #{CoPlan::CommentThread.count} threads, #{CoPlan::Comment.count} comments, #{CoPlan::ApiToken.count} API tokens."
diff --git a/db/seeds/development.rb b/db/seeds/development.rb
new file mode 100644
index 00000000..b14d7642
--- /dev/null
+++ b/db/seeds/development.rb
@@ -0,0 +1,264 @@
+require "faker"
+
+module CoPlan
+  # A deterministic, idempotent dataset for exercising CoPlan's development UI.
+  # Existing seed documents keep local content edits; missing records are restored
+  # on the next run and seed-owned metadata remains consistent.
+  module DevelopmentSeed
+    module_function
+
+    USERS = [
+      { key: "alex", locale: :en, admin: true },
+      { key: "priya", locale: :en },
+      { key: "mateo", locale: :es },
+      { key: "aiko", locale: :ja },
+      { key: "noura", locale: :ar },
+      { key: "sam", locale: :en }
+    ].freeze
+
+    PLAN_TYPES = [
+      { name: "General", icon: "file-text", description: "A flexible document for notes and proposals", default_tags: [] },
+      { name: "RFC", icon: "scroll", description: "A request for comments on a significant change", default_tags: [ "rfc" ] },
+      { name: "Design Doc", icon: "compass", description: "Technical design for a system or feature", default_tags: [ "design" ] },
+      { name: "ADR", icon: "scale", description: "A durable architecture decision record", default_tags: [ "architecture" ] },
+      { name: "Product Brief", icon: "lightbulb", description: "Product context, goals, and measures of success", default_tags: [ "product" ] },
+      { name: "Runbook", icon: "wrench", description: "Operational diagnosis and recovery steps", default_tags: [ "operations" ] },
+      { name: "Research Note", icon: "flask", description: "Findings, evidence, and open questions", default_tags: [ "research" ] },
+      { name: "Roadmap", icon: "map", description: "Sequenced outcomes and milestones", default_tags: [ "roadmap" ] }
+    ].freeze
+
+    DOCUMENTS = [
+      {
+        key: "one-line-decision", author: "alex", type: "ADR", title: "Use UUIDv7 identifiers",
+        tags: %w[architecture database], visibility: "published", folder: "Engineering/Architecture decisions", sections: 0
+      },
+      {
+        key: "api-gateway", author: "alex", type: "RFC", title: "RFC: Consolidating edge authentication in the API gateway",
+        tags: %w[api security infrastructure], visibility: "published", folder: "Engineering/Active projects", fixture: :flowchart
+      },
+      {
+        key: "mobile-checkout", author: "mateo", type: "Design Doc", title: "Mobile checkout: resilient state transitions when the network disappears",
+        tags: %w[mobile reliability payments], visibility: "published", folder: "Product/Mobile", fixture: :state_diagram
+      },
+      {
+        key: "spanish-brief", author: "mateo", type: "Product Brief", title: "Mejoras para la experiencia de incorporación",
+        tags: %w[onboarding product localization], visibility: "published", folder: "Product/Discovery", fixture: :spanish
+      },
+      {
+        key: "japanese-roadmap", author: "aiko", type: "Roadmap", title: "信頼性向上ロードマップ — 2027年前半",
+        tags: %w[reliability roadmap], visibility: "published", folder: "Operations/Reliability", fixture: :japanese
+      },
+      {
+        key: "arabic-research", author: "noura", type: "Research Note", title: "بحث: تقليل مخاطر سرقة الجلسات",
+        tags: %w[security research authentication], visibility: "published", folder: "Research/Security", fixture: :arabic
+      },
+      {
+        key: "incident-runbook", author: "aiko", type: "Runbook", title: "Payments API latency incident runbook",
+        tags: %w[operations payments on-call], visibility: "published", folder: "Operations/Runbooks"
+      },
+      {
+        key: "experiment-results", author: "sam", type: "Research Note", title: "Search ranking experiment #42",
+        tags: %w[search experimentation data], visibility: "published", folder: "Research/Experiments", fixture: :table
+      },
+      {
+        key: "draft-notes", author: "priya", type: "General", title: "Untitled thoughts on activation",
+        tags: %w[product draft], visibility: "draft", folder: "Personal notes", sections: 1
+      },
+      {
+        key: "archived-proposal", author: "alex", type: "RFC", title: "Retired proposal: weekly XML exports",
+        tags: %w[archive integrations], visibility: "published", archived: true, folder: "Engineering/Archive", sections: 1
+      },
+      {
+        key: "emoji-title", author: "priya", type: "Product Brief", title: "Faster feedback loops ⚡",
+        tags: %w[collaboration product], visibility: "published", folder: "Product/Discovery"
+      },
+      {
+        key: "long-title", author: "noura", type: "Design Doc",
+        title: "Designing a privacy-preserving audit trail for delegated administrative actions across regional data boundaries",
+        tags: %w[security privacy compliance], visibility: "published", folder: "Engineering/Architecture decisions", fixture: :sequence_diagram
+      },
+      {
+        key: "long-mobile-toc", author: "priya", type: "Product Brief", title: "The complete guide to launching shared workspaces across web, iOS, and Android",
+        tags: %w[collaboration mobile launch], visibility: "published", folder: "Product/Launches/Shared workspace", long: true
+      }
+    ].freeze
+
+    CONTENT_FIXTURES = {
+      flowchart: <<~MARKDOWN,
+        ```mermaid
+        flowchart LR
+          Client --> Gateway
+          Gateway --> API
+          API --> DB[(Database)]
+        ```
+      MARKDOWN
+      state_diagram: <<~MARKDOWN,
+        ```mermaid
+        stateDiagram-v2
+          [*] --> Editing
+          Editing --> Submitting
+          Submitting --> Confirmed
+          Confirmed --> [*]
+        ```
+      MARKDOWN
+      sequence_diagram: <<~MARKDOWN,
+        ```mermaid
+        sequenceDiagram
+          participant A as Administrator
+          participant S as Service
+          A->>S: Delegated action
+          S-->>A: Receipt ID
+        ```
+      MARKDOWN
+      table: <<~MARKDOWN,
+        | Metric | Control | Treatment |
+        | --- | ---: | ---: |
+        | Success | 61.2% | 62.9% |
+        | Errors | 18.4% | 17.7% |
+      MARKDOWN
+      spanish: "## Problema\n\nLas personas nuevas necesitan saber qué paso completar.\n\n## Resultado\n\nUna lista breve muestra el siguiente paso.",
+      japanese: "## 目標\n\n障害の影響を小さくし、復旧までの時間を短縮します。\n\n## 次のステップ\n\n復旧手順を自動で検証します。",
+      arabic: "## الملخص\n\nتقارن هذه المذكرة بين الجلسات قصيرة العمر وتدوير الرموز.\n\n## الخطوة التالية\n\nتشغيل تجربة محكومة لقياس الأمان."
+    }.freeze
+
+    LONG_DOCUMENT_SECTIONS = [
+      "Executive summary", "Customer problem", "Audience", "Principles", "Goals", "Non-goals",
+      "Jobs to be done", "Information architecture", "Web navigation", "iOS navigation", "Android navigation",
+      "Small-screen table of contents", "Workspace creation", "Invitations", "Roles and permissions",
+      "Document placement", "Folder behavior", "Search", "Activity", "Notifications", "Empty states",
+      "Loading and offline states", "Accessibility", "Localization", "Analytics", "Privacy", "Performance budgets",
+      "Migration", "Rollout", "Support readiness", "Risks", "Open questions", "Decision log", "Appendix: terminology"
+    ].freeze
+
+    def call
+      puts "Seeding reusable development data..."
+      with_reproducible_faker do
+        users = seed_users
+        plan_types = seed_plan_types
+        plans = seed_documents(users, plan_types)
+        seed_shared_library_examples(users, plans)
+      end
+
+      puts "Done! #{User.count} users, #{Plan.count} documents, #{Folder.count} folders, #{Tag.count} tags, and #{PlanType.count} document types."
+    end
+
+    def seed_users
+      USERS.index_with do |attributes|
+        user = User.find_or_initialize_by(external_id: "seed:#{attributes.fetch(:key)}")
+        Faker::Config.locale = attributes.fetch(:locale)
+        name = Faker::Name.name
+        Faker::Config.locale = :en
+        user.assign_attributes(
+          name: name,
+          username: attributes.fetch(:key),
+          email: Faker::Internet.unique.email(name: name),
+          title: Faker::Job.title,
+          team: Faker::Company.industry,
+          admin: attributes.fetch(:admin, false),
+          metadata: user.metadata.to_h.merge("development_seed" => true)
+        )
+        user.save!
+        user.library
+        user
+      end.transform_keys { |attributes| attributes.fetch(:key) }
+    end
+
+    def seed_plan_types
+      PLAN_TYPES.index_with do |attributes|
+        PlanType.find_or_initialize_by(name: attributes.fetch(:name)).tap do |plan_type|
+          plan_type.assign_attributes(attributes)
+          plan_type.save!
+        end
+      end.transform_keys { |attributes| attributes.fetch(:name) }
+    end
+
+    def seed_documents(users, plan_types)
+      DOCUMENTS.index_with do |definition|
+        author = users.fetch(definition.fetch(:author))
+        plan = find_seed_document(definition.fetch(:key), author)
+
+        unless plan
+          plan = Plans::Create.call(
+            title: definition.fetch(:title),
+            content: document_content(definition),
+            user: author,
+            plan_type_id: plan_types.fetch(definition.fetch(:type)).id,
+            visibility: definition.fetch(:visibility)
+          )
+          plan.update!(
+            archived_at: definition[:archived] ? Time.current : nil,
+            metadata: plan.metadata.to_h.merge("development_seed_key" => definition.fetch(:key))
+          )
+        end
+
+        plan.tag_names = definition.fetch(:tags)
+        place(plan, definition.fetch(:folder), author)
+        plan
+      end.transform_keys { |definition| definition.fetch(:key) }
+    end
+
+    def find_seed_document(key, author)
+      author.created_plans.detect { |plan| plan.metadata.to_h["development_seed_key"] == key }
+    end
+
+    def place(plan, path, user)
+      folder = Folder.find_or_create_by_path!(path, library: user.library, created_by_user: user)
+      result = Plans::Place.call(plan: plan, folder: folder, actor: user)
+      raise result.error unless result.success?
+    end
+
+    def seed_shared_library_examples(users, plans)
+      # Demonstrate that a published document can sit on someone else's shelf
+      # without changing the author's organization.
+      place(plans.fetch("api-gateway"), "Reading list/Security", users.fetch("noura"))
+      place(plans.fetch("experiment-results"), "Research to discuss", users.fetch("alex"))
+    end
+
+    def document_content(definition)
+      return long_document_content if definition[:long]
+
+      fixture = CONTENT_FIXTURES[definition[:fixture]]
+      parts = [ "# #{definition.fetch(:title)}" ]
+
+      if %i[spanish japanese arabic].include?(definition[:fixture])
+        parts << fixture
+        return parts.join("\n\n")
+      end
+
+      parts << Faker::Lorem.paragraph(sentence_count: 3)
+      definition.fetch(:sections, 3).times do
+        heading = Faker::Lorem.words(number: 3).join(" ").capitalize
+        parts << "## #{heading}\n\n#{Faker::Lorem.paragraph(sentence_count: 3)}"
+      end
+      parts << fixture if fixture
+      parts.join("\n\n")
+    end
+
+    def long_document_content
+      sections = LONG_DOCUMENT_SECTIONS.map do |heading|
+        "## #{heading}\n\n#{Faker::Lorem.paragraph(sentence_count: 3)}"
+      end.join("\n\n")
+
+      <<~MARKDOWN
+        # The complete guide to launching shared workspaces
+
+        #{Faker::Lorem.paragraph(sentence_count: 3)}
+
+        #{sections}
+      MARKDOWN
+    end
+
+    def with_reproducible_faker
+      previous_random = Faker::Config.random
+      previous_locale = Faker::Config.locale
+      Faker::Config.random = Random.new(12_345)
+      Faker::Config.locale = :en
+      Faker::UniqueGenerator.clear
+      yield
+    ensure
+      Faker::Config.random = previous_random
+      Faker::Config.locale = previous_locale
+      Faker::UniqueGenerator.clear
+    end
+  end
+end
diff --git a/spec/lib/development_seed_spec.rb b/spec/lib/development_seed_spec.rb
new file mode 100644
index 00000000..593e8ba6
--- /dev/null
+++ b/spec/lib/development_seed_spec.rb
@@ -0,0 +1,57 @@
+require "rails_helper"
+require Rails.root.join("db/seeds/development")
+
+RSpec.describe CoPlan::DevelopmentSeed do
+  describe ".call" do
+    it "creates a varied, idempotent development dataset" do
+      described_class.call
+
+      counts = [
+        CoPlan::User.count,
+        CoPlan::Plan.count,
+        CoPlan::Folder.count,
+        CoPlan::Tag.count,
+        CoPlan::PlanType.count,
+        CoPlan::PlanPlacement.count
+      ]
+      generated_users = CoPlan::User.order(:external_id).pluck(:name, :email, :title, :team)
+
+      described_class.call
+
+      expect([
+        CoPlan::User.count,
+        CoPlan::Plan.count,
+        CoPlan::Folder.count,
+        CoPlan::Tag.count,
+        CoPlan::PlanType.count,
+        CoPlan::PlanPlacement.count
+      ]).to eq(counts)
+      expect(CoPlan::User.order(:external_id).pluck(:name, :email, :title, :team)).to eq(generated_users)
+
+      seeded_plans = CoPlan::Plan.select { |plan| plan.metadata.to_h.key?("development_seed_key") }
+      expect(seeded_plans.size).to be >= 12
+      expect(seeded_plans.map(&:visibility)).to include("draft", "published")
+      expect(seeded_plans).to include(be_archived)
+      expect(seeded_plans.map { |plan| plan.plan_type.name }.uniq.size).to be >= 7
+      expect(seeded_plans.flat_map(&:tag_names).uniq.size).to be >= 12
+      expect(seeded_plans.map(&:title)).to include(a_string_matching(/信頼性/), a_string_matching(/تقليل/))
+
+      long_document = seeded_plans.find { |plan| plan.metadata["development_seed_key"] == "long-mobile-toc" }
+      expect(long_document.current_content.scan(/^## /).size).to be >= 30
+      expect(seeded_plans.count { |plan| plan.current_content.include?("```mermaid") }).to be >= 3
+      expect(CoPlan::Folder.where.not(parent_id: nil)).to exist
+    end
+
+    it "preserves edits to existing seed document content and titles" do
+      described_class.call
+      plan = CoPlan::Plan.find { |candidate| candidate.metadata.to_h["development_seed_key"] == "api-gateway" }
+      plan.update!(title: "Locally edited title")
+      plan.current_plan_version.update!(content_markdown: "# Locally edited content", content_sha256: nil)
+
+      described_class.call
+
+      expect(plan.reload.title).to eq("Locally edited title")
+      expect(plan.current_content).to eq("# Locally edited content")
+    end
+  end
+end