Add changelog entry and tests for stderr log output

- Add changelog entry for PR #869 documenting the bug fix
- Add tests verifying [Shakapacker] log messages go to stderr, not stdout
- Tests ensure stdout remains clean when using --json flag for valid JSON output

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: justin808

CHANGELOG.md

@@ -14,6 +14,7 @@ Changes since the last non-beta release.
 ### Fixed
 
 - **Fixed NODE_ENV=test causing DefinePlugin warnings**. [PR #870](https://github.com/shakacode/shakapacker/pull/870) by [justin808](https://github.com/justin808). When RAILS_ENV=test, Shakapacker now sets NODE_ENV=development instead of NODE_ENV=test. This prevents webpack/rspack DefinePlugin conflicts since these bundlers only recognize "development" and "production" as valid NODE_ENV values.
+- **Fixed `--json` flag output being corrupted by log messages**. [PR #869](https://github.com/shakacode/shakapacker/pull/869) by [justin808](https://github.com/justin808). `[Shakapacker]` log messages are now written to stderr instead of stdout, allowing `bin/shakapacker --profile --json` to produce valid JSON output that can be piped to tools like `webpack-bundle-analyzer`. Resolves [#868](https://github.com/shakacode/shakapacker/issues/868).
 
 ## [v9.5.0] - January 7, 2026
 

spec/shakapacker/webpack_runner_spec.rb

@@ -206,8 +206,66 @@
     end
   end
 
+  describe "stdout/stderr separation for JSON output" do
+    it "does not write [Shakapacker] log messages to stdout" do
+      Dir.chdir(test_app_path) do
+        klass = Shakapacker::WebpackRunner
+        instance = klass.new(["--json"])
+
+        allow(klass).to receive(:new).and_return(instance)
+        allow(Shakapacker::Utils::Manager).to receive(:error_unless_package_manager_is_obvious!)
+
+        allow(instance).to receive(:system) do |*args|
+          system("true")
+          true
+        end
+
+        stdout_output, stderr_output = capture_stdout_and_stderr { klass.run(["--json"]) }
+
+        # Stdout should NOT contain [Shakapacker] log messages
+        expect(stdout_output).not_to match(/\[Shakapacker\]/)
+
+        # Stderr SHOULD contain [Shakapacker] log messages
+        expect(stderr_output).to match(/\[Shakapacker\]/)
+      end
+    end
+
+    it "keeps stdout clean for valid JSON output when using --json flag" do
+      Dir.chdir(test_app_path) do
+        klass = Shakapacker::WebpackRunner
+        instance = klass.new(["--profile", "--json"])
+
+        allow(klass).to receive(:new).and_return(instance)
+        allow(Shakapacker::Utils::Manager).to receive(:error_unless_package_manager_is_obvious!)
+
+        allow(instance).to receive(:system) do |*args|
+          system("true")
+          true
+        end
+
+        stdout_output, = capture_stdout_and_stderr { klass.run(["--profile", "--json"]) }
+
+        # Stdout should be empty (no log messages polluting it)
+        # The actual JSON would come from webpack itself, not shakapacker
+        expect(stdout_output).to be_empty
+      end
+    end
+  end
+
   private
 
+    def capture_stdout_and_stderr
+      old_stdout = $stdout
+      old_stderr = $stderr
+      $stdout = StringIO.new
+      $stderr = StringIO.new
+      yield
+      [$stdout.string, $stderr.string]
+    ensure
+      $stdout = old_stdout
+      $stderr = old_stderr
+    end
+
     def capture_stderr
       old_stderr = $stderr
       $stderr = StringIO.new