feat: add pnpm setup step for GitHub Actions workflows

src/awscdk/base.ts

@@ -2,7 +2,7 @@ import { Component, TextFile, awscdk } from 'projen';
 import { PROJEN_MARKER } from 'projen/lib/common';
 import { NodePackageManager } from 'projen/lib/javascript';
 import { PipelineEngine } from '../engine';
-import { AwsAssumeRoleStep, PipelineStep, ProjenScriptStep, SimpleCommandStep, StepSequence } from '../steps';
+import { AwsAssumeRoleStep, PipelineStep, ProjenScriptStep, SimpleCommandStep, StepSequence, PnpmSetupStep } from '../steps';
 import { VersioningConfig, VersioningSetup } from '../versioning';
 
 /**
@@ -251,6 +251,14 @@ export abstract class CDKPipeline extends Component {
 
   protected provideInstallStep(): PipelineStep {
     const seq = new StepSequence(this.project, this.baseOptions.preInstallSteps ?? []);
+
+    // Detect and add pnpm setup if needed (GitHub only)
+    if (this.app.package.packageManager === NodePackageManager.PNPM && this.engineType() === PipelineEngine.GITHUB) {
+      seq.addSteps(new PnpmSetupStep(this.project, {
+        version: (this.app.package as any).pnpmVersion,
+      }));
+    }
+
     if (this.baseOptions.preInstallCommands) {
       seq.addSteps(new SimpleCommandStep(this.project, this.baseOptions.preInstallCommands));
     }
@@ -365,6 +373,9 @@ export abstract class CDKPipeline extends Component {
       case NodePackageManager.NPM:
         commands.push(`npm install ${packageName}`);
         break;
+      case NodePackageManager.PNPM:
+        commands.push(`pnpm add ${packageName}`);
+        break;
       default:
         throw new Error('No install scripts for packageManager: ' + this.app.package.packageManager);
     }

src/steps/index.ts

@@ -2,4 +2,5 @@ export * from './step';
 export * from './artifact-steps';
 export * from './aws-assume-role.step';
 export * from './registries';
-export * from './amplify-deploy.step';
+export * from './amplify-deploy.step';
+export * from './package-manager-setup.step';

src/steps/package-manager-setup.step.ts

@@ -0,0 +1,41 @@
+import { Project } from 'projen';
+import { GithubStepConfig, PipelineStep } from './step';
+
+/**
+ * Options for the PnpmSetupStep.
+ */
+export interface PnpmSetupStepOptions {
+  /**
+   * The version of pnpm to install.
+   * If not provided, defaults to '9'.
+   */
+  readonly version?: string;
+}
+
+/**
+ * Step to setup pnpm using the pnpm/action-setup GitHub Action.
+ *
+ * This step is automatically injected when a project uses pnpm as its package manager.
+ * It ensures pnpm is available in the GitHub Actions workflow environment before
+ * running any install commands.
+ */
+export class PnpmSetupStep extends PipelineStep {
+
+  constructor(project: Project, private options: PnpmSetupStepOptions = {}) {
+    super(project);
+  }
+
+  public toGithub(): GithubStepConfig {
+    return {
+      env: {},
+      needs: [],
+      steps: [{
+        name: 'Setup pnpm',
+        uses: 'pnpm/action-setup@v4',
+        with: {
+          version: this.options.version ?? '9',
+        },
+      }],
+    };
+  }
+}