From b582714965b96b735489c71077b569e986c31a8a Mon Sep 17 00:00:00 2001
From: Claude <noreply@anthropic.com>
Date: Mon, 7 Sep 2026 11:59:41 +0000
Subject: [PATCH] Pin the Heroku build's NodeJS version to what the app
 requires
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

The `package.json` generated for Heroku carried only `cacheDirectories`.
Heroku's NodeJS buildpack reads that file — the project root's, not the Ember
application's — to decide which NodeJS to build with,
and falls back to the current LTS release when it names no `engines.node`.

The requirement belongs to the Ember application:
the `ember-cli >= 6.8` blueprint needs NodeJS `>= 20.19.0`,
and nothing carried that up to the file Heroku actually consults,
so `rake ember:compile` could run on a NodeJS the application rejects.

Read `engines.node` from each configured application and write the one they
agree on. Applications that disagree are reported and left unpinned rather
than resolved: one build serves all of them, and picking a single answer would
pin the build to a version the others reject.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014rAfkAVGVifbaEoTVeT66u
---
 CHANGELOG.md                                  |  5 ++
 lib/ember_cli/app.rb                          | 12 +++++
 lib/ember_cli/path_set.rb                     |  4 ++
 .../ember/heroku/heroku_generator.rb          | 20 ++++++++
 .../ember/heroku/templates/package.json.erb   |  5 ++
 .../ember/heroku/heroku_generator_spec.rb     | 48 +++++++++++++++++++
 spec/lib/ember_cli/app_spec.rb                | 36 ++++++++++++++
 7 files changed, 130 insertions(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index b7860353..43049c96 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,11 @@ main
   natively since `5.0`, and every Rails version this gem supports is `>= 5.2`.
   To upgrade, remove `rails_12factor` from your project's `Gemfile`
 * Replace `EmberCli::App#yarn_enabled?` with `EmberCli::App#yarn?`
+* Pin the NodeJS version in the `package.json` that `rails generate
+  ember:heroku` writes, from the `engines.node` the Ember applications
+  declare. Applications that declare different versions are left unpinned
+* Add `EmberCli::App#node_engine`, reading `engines.node` from the Ember
+  application's `package.json`
 
 0.13.2
 ------
diff --git a/lib/ember_cli/app.rb b/lib/ember_cli/app.rb
index 0349f3a2..9745d06a 100644
--- a/lib/ember_cli/app.rb
+++ b/lib/ember_cli/app.rb
@@ -1,3 +1,5 @@
+require "json"
+
 require "html_page/renderer"
 require "ember_cli/path_set"
 require "ember_cli/shell"
@@ -99,6 +101,16 @@ def bower?
       paths.bower_json.exist?
     end
 
+    def node_engine
+      package_json = paths.package_json
+
+      if package_json.exist?
+        JSON.parse(package_json.read).dig("engines", "node")
+      end
+    rescue JSON::ParserError
+      nil
+    end
+
     def to_rack
       deploy.to_rack
     end
diff --git a/lib/ember_cli/path_set.rb b/lib/ember_cli/path_set.rb
index b14a2c5b..e3936880 100644
--- a/lib/ember_cli/path_set.rb
+++ b/lib/ember_cli/path_set.rb
@@ -35,6 +35,10 @@ def gemfile
       @gemfile ||= root.join("Gemfile")
     end
 
+    def package_json
+      @package_json ||= root.join("package.json")
+    end
+
     def bower_json
       root.join("bower.json")
     end
diff --git a/lib/generators/ember/heroku/heroku_generator.rb b/lib/generators/ember/heroku/heroku_generator.rb
index 36717e35..9378096e 100644
--- a/lib/generators/ember/heroku/heroku_generator.rb
+++ b/lib/generators/ember/heroku/heroku_generator.rb
@@ -16,6 +16,26 @@ def identify_as_yarn_project
 
     private
 
+    def node_engine
+      return @node_engine if defined?(@node_engine)
+
+      declared = apps.map(&:node_engine).compact.uniq
+
+      @node_engine =
+        if declared.size > 1
+          say_status(
+            :conflict,
+            "Ember applications declare different `engines.node` " \
+            "(#{declared.join(", ")}); pin one in package.json by hand",
+            :red,
+          )
+
+          nil
+        else
+          declared.first
+        end
+    end
+
     def cache_directories
       all_cached_directories.map do |cached_directory|
         cached_directory.relative_path_from(Rails.root).to_s
diff --git a/lib/generators/ember/heroku/templates/package.json.erb b/lib/generators/ember/heroku/templates/package.json.erb
index a029a9cb..f9845a0c 100644
--- a/lib/generators/ember/heroku/templates/package.json.erb
+++ b/lib/generators/ember/heroku/templates/package.json.erb
@@ -4,5 +4,10 @@
     "bower": "*"
   },
   <% end %>
+  <% if node_engine %>
+  "engines": {
+    "node": <%= node_engine.to_json %>
+  },
+  <% end %>
   "cacheDirectories": <%= cache_directories.to_json %>
 }
diff --git a/spec/generators/ember/heroku/heroku_generator_spec.rb b/spec/generators/ember/heroku/heroku_generator_spec.rb
index 51d86442..0b07423f 100644
--- a/spec/generators/ember/heroku/heroku_generator_spec.rb
+++ b/spec/generators/ember/heroku/heroku_generator_spec.rb
@@ -82,6 +82,54 @@ def gemfile_contents
       end
     end
 
+    describe "engines" do
+      it "pins the NodeJS version the Ember applications declare" do
+        setup_destination
+        configure_applications(node_engine: ">= 20.19.0")
+
+        run_generator
+
+        expect(package_json.fetch("engines")).to eq("node" => ">= 20.19.0")
+      end
+
+      it "omits engines when no Ember application declares one" do
+        setup_destination
+        configure_applications(node_engine: nil)
+
+        run_generator
+
+        expect(package_json.keys).not_to include("engines")
+      end
+
+      it "omits engines when the Ember applications disagree" do
+        setup_destination
+        configure_applications(
+          { node_engine: ">= 20.19.0" },
+          { node_engine: ">= 22.0.0" },
+        )
+
+        run_generator
+
+        expect(package_json.keys).not_to include("engines")
+      end
+
+      def configure_applications(*attributes)
+        apps = attributes.map do |app_attributes|
+          instance_double(
+            EmberCli::App,
+            {
+              bower?: false,
+              cached_directories: [],
+              yarn?: false,
+            }.merge(app_attributes),
+          )
+        end
+
+        allow(EmberCli).to receive(:apps).
+          and_return(apps.map.with_index { |app, i| ["app-#{i}", app] }.to_h)
+      end
+    end
+
     def depend_on_bower(bower_enabled)
       allow_any_instance_of(EmberCli::App).
         to receive(:bower?).and_return(bower_enabled)
diff --git a/spec/lib/ember_cli/app_spec.rb b/spec/lib/ember_cli/app_spec.rb
index 3d906ab6..0ead0291 100644
--- a/spec/lib/ember_cli/app_spec.rb
+++ b/spec/lib/ember_cli/app_spec.rb
@@ -93,6 +93,42 @@
     end
   end
 
+  describe "#node_engine" do
+    it "reads engines.node from the application's package.json" do
+      stub_package_json('{"engines":{"node":">= 20.19.0"}}')
+      app = EmberCli::App.new("with node engine")
+
+      expect(app.node_engine).to eq ">= 20.19.0"
+    end
+
+    it "returns nil when the package.json declares no engines" do
+      stub_package_json('{"name":"frontend"}')
+      app = EmberCli::App.new("without engines")
+
+      expect(app.node_engine).to be_nil
+    end
+
+    it "returns nil when the package.json is absent" do
+      stub_paths(package_json: double("Pathname", exist?: false))
+      app = EmberCli::App.new("without package json")
+
+      expect(app.node_engine).to be_nil
+    end
+
+    it "returns nil when the package.json is malformed" do
+      stub_package_json("{")
+      app = EmberCli::App.new("with malformed package json")
+
+      expect(app.node_engine).to be_nil
+    end
+
+    def stub_package_json(contents)
+      stub_paths(
+        package_json: double("Pathname", exist?: true, read: contents),
+      )
+    end
+  end
+
   describe "#compile" do
     it "exits with exit status of 0" do
       passed = EmberCli["my-app"].compile