diff --git a/README.md b/README.md
index a4e6b91..c84ce23 100644
--- a/README.md
+++ b/README.md
@@ -40,10 +40,12 @@
 
 This extension requires:
 
--   **[PostScript Language](https://marketplace.visualstudio.com/items?itemName=mxschmitt.postscript)** extension for syntax highlighting
+-   **[PostScript Language](https://marketplace.visualstudio.com/items?itemName=mxschmitt.postscript)** extension — required for the preview button to appear (it registers `.ps`/`.eps` files as PostScript, which this extension's preview button and activation depend on; it also provides syntax highlighting). Installed automatically alongside this extension.
 -   **GhostScript** (provides `ps2pdf`)
 -   **Poppler** (provides `pdftocairo` and `pdfinfo`)
 
+> **No preview button?** If the preview icon does not appear in the editor title bar, your `.ps`/`.eps` file is likely opening as *Plain Text* (check the language indicator in the status bar). Make sure the [PostScript Language](https://marketplace.visualstudio.com/items?itemName=mxschmitt.postscript) extension is installed and enabled, then reload VS Code.
+
 ### macOS
 
 ```bash
diff --git a/package.json b/package.json
index ba9235c..2e68c00 100644
--- a/package.json
+++ b/package.json
@@ -28,6 +28,9 @@
 	"activationEvents": [
 		"onLanguage:postscript"
 	],
+	"extensionDependencies": [
+		"mxschmitt.postscript"
+	],
 	"main": "./out/extension.js",
 	"contributes": {
 		"commands": [
diff --git a/src/test/runTest.ts b/src/test/runTest.ts
index 81ceb16..ca41f5c 100644
--- a/src/test/runTest.ts
+++ b/src/test/runTest.ts
@@ -1,5 +1,56 @@
+import * as cp from "child_process";
+import * as fs from "fs";
 import * as path from "path";
-import { runTests } from "@vscode/test-electron";
+import {
+    downloadAndUnzipVSCode,
+    resolveCliArgsFromVSCodeExecutablePath,
+    runTests,
+} from "@vscode/test-electron";
+
+const POSTSCRIPT_LANGUAGE_VSIX_URL =
+    "https://marketplace.visualstudio.com/_apis/public/gallery/publishers/mxschmitt/vsextensions/postscript/latest/vspackage";
+
+/**
+ * This extension declares an extension dependency on mxschmitt.postscript
+ * (it registers the `postscript` language for .ps/.eps files), so the
+ * extension under test cannot activate unless that dependency is present in
+ * the test instance. Install it from a downloaded VSIX file: installing by id
+ * can fail in the test CLI when the marketplace signature verification
+ * tooling is unavailable ("Signature verification failed with 'ENOENT'").
+ */
+async function installPostscriptLanguageExtension(
+    vscodeExecutablePath: string
+) {
+    const vsixPath = path.resolve(
+        __dirname,
+        "../../.vscode-test/mxschmitt.postscript.vsix"
+    );
+    const response = await fetch(POSTSCRIPT_LANGUAGE_VSIX_URL);
+    if (!response.ok) {
+        throw new Error(
+            `Failed to download PostScript Language extension: HTTP ${response.status}`
+        );
+    }
+    fs.mkdirSync(path.dirname(vsixPath), { recursive: true });
+    fs.writeFileSync(vsixPath, Buffer.from(await response.arrayBuffer()));
+
+    const [cliPath, ...cliArgs] =
+        resolveCliArgsFromVSCodeExecutablePath(vscodeExecutablePath);
+    const result = cp.spawnSync(
+        cliPath,
+        [...cliArgs, "--install-extension", vsixPath],
+        {
+            encoding: "utf-8",
+            stdio: "inherit",
+            shell: process.platform === "win32",
+        }
+    );
+    if (result.status !== 0) {
+        throw new Error(
+            `Failed to install PostScript Language extension (exit code ${result.status})`
+        );
+    }
+}
 
 async function main() {
     try {
@@ -11,14 +62,20 @@ async function main() {
         // Passed to --extensionTestsPath
         const extensionTestsPath = path.resolve(__dirname, "./suite/index");
 
-        // Download VS Code, unzip it and run the integration test
+        // Download VS Code and install the PostScript Language dependency
+        // into the test instance before running the tests.
+        const vscodeExecutablePath = await downloadAndUnzipVSCode();
+        await installPostscriptLanguageExtension(vscodeExecutablePath);
+
+        // Run the integration test
         await runTests({
+            vscodeExecutablePath,
             extensionDevelopmentPath,
             extensionTestsPath,
             launchArgs: [extensionDevelopmentPath],
         });
     } catch (err) {
-        console.error("Failed to run tests");
+        console.error("Failed to run tests", err);
         process.exit(1);
     }
 }