fix(USE-003): emit staged mode preview summary in upload_artifact handler (#26313)

Author: Copilot

actions/setup/js/upload_artifact.cjs

@@ -565,7 +565,20 @@ async function main(config = {}) {
         };
       }
     } else {
-      core.info(`Staged mode: skipping artifact upload for slot ${i}`);
+      // Emit a staged mode preview summary.
+      let summaryContent = "## 🎭 Staged Mode: Upload Artifact Preview\n\n";
+      summaryContent += "The following artifact upload would be performed if staged mode was disabled:\n\n";
+      summaryContent += `**Artifact Name:** ${artifactName}\n\n`;
+      summaryContent += `**Files (${files.length}):**\n\n`;
+      for (const file of files) {
+        summaryContent += `- \`${file}\`\n`;
+      }
+      summaryContent += `\n**Total Size:** ${totalSize} bytes\n\n`;
+      summaryContent += `**Retention:** ${retentionDays} days\n\n`;
+      summaryContent += `> ℹ️ Upload skipped (staged mode active)\n\n`;
+
+      await core.summary.addRaw(summaryContent).write();
+      core.info("📝 Upload artifact preview written to step summary");
     }
 
     // Set step outputs so downstream jobs can reference the tmp ID.

actions/setup/js/upload_artifact.test.cjs

@@ -398,6 +398,20 @@ describe("upload_artifact.cjs", () => {
       expect(results[0].success).toBe(true);
       expect(mockArtifactClient.uploadArtifact).not.toHaveBeenCalled();
     });
+
+    it("emits a staged mode preview summary when staged mode is active", async () => {
+      process.env.GH_AW_SAFE_OUTPUTS_STAGED = "true";
+      writeStaging("output.json");
+
+      await runHandler(buildConfig(), [{ type: "upload_artifact", path: "output.json" }]);
+
+      expect(mockCore.summary.addRaw).toHaveBeenCalled();
+      const summaryContent = mockCore.summary.addRaw.mock.calls[0][0];
+      expect(summaryContent).toContain("🎭 Staged Mode");
+      expect(summaryContent).toContain("Upload Artifact Preview");
+      expect(summaryContent).toContain("output.json");
+      expect(mockCore.summary.write).toHaveBeenCalled();
+    });
   });
 
   describe("resolver file", () => {