feat: add Heroku preboot context to strong_migrations destructive-operation messages

config/initializers/strong_migrations.rb

@@ -4,3 +4,19 @@
 # https://github.com/ankane/strong_migrations
 StrongMigrations.start_after = 20260101000000 # rubocop:disable Style/NumericLiterals
 StrongMigrations.target_postgresql_version = 16
+
+# With preboot, the release phase runs before old web dynos stop, so old code
+# serves against the new schema for ~3 minutes. Say so where the migration
+# author will see it, next to the gem's `safety_assured` advice.
+preboot_note = <<~TXT
+  Heroku preboot: this runs in the release phase before old web dynos stop,
+  so old code serves against the new schema for ~3 minutes after the deploy.
+  See AGENTS.md ("Migrations and Heroku preboot"). If the migration cannot
+  be made forward-compatible, disable preboot for this deploy.
+TXT
+
+%i[change_column change_column_with_not_null change_column_default
+   change_column_null rename_column rename_table rename_schema
+   rename_enum_value execute].each do |key|
+  StrongMigrations.error_messages[key] = "#{StrongMigrations.error_messages.fetch(key)}\n\n#{preboot_note}"
+end

spec/config/strong_migrations_config_spec.rb

@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe 'StrongMigrations configuration' do # rubocop:disable RSpec/DescribeClass
+  describe 'destructive-operation messages' do
+    preboot_keys = %i[change_column change_column_with_not_null change_column_default
+                      change_column_null rename_column rename_table rename_schema
+                      rename_enum_value execute]
+
+    it 'appends the Heroku preboot note after the gem default' do
+      message = StrongMigrations.error_messages[:rename_column]
+
+      expect(message).to start_with("Renaming a column that's in use will cause errors")
+      expect(message).to end_with("disable preboot for this deploy.\n")
+    end
+
+    it 'carries the note exactly once on each destructive-operation message' do
+      counts = preboot_keys.index_with do |key|
+        StrongMigrations.error_messages.fetch(key).scan('disable preboot for this deploy').size
+      end
+
+      expect(counts.values).to all(eq(1))
+    end
+
+    it 'leaves messages for other checks unchanged' do
+      expect(StrongMigrations.error_messages[:add_index]).not_to include('Heroku preboot')
+    end
+  end
+end