diff --git a/packages/@apphosting/common/package.json b/packages/@apphosting/common/package.json
index 25ae288e..ed144d7b 100644
--- a/packages/@apphosting/common/package.json
+++ b/packages/@apphosting/common/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@apphosting/common",
-  "version": "0.0.5",
+  "version": "0.0.6",
   "description": "Shared library code for App Hosting framework adapters",
   "author": {
     "name": "Firebase",
@@ -17,6 +17,7 @@
   },
   "scripts": {
     "build": "tsc",
+    "test": "ts-mocha -p tsconfig.json 'src/**/*.spec.ts' 'src/*.spec.ts'",
     "localregistry:start": "npx verdaccio --config ../publish-dev/verdaccio-config.yaml",
     "localregistry:publish": "(npm view --registry=http://localhost:4873 @apphosting/common && npm unpublish --@apphosting:registry=http://localhost:4873 --force); npm publish --@apphosting:registry=http://localhost:4873"
   },
diff --git a/packages/@apphosting/common/src/index.spec.ts b/packages/@apphosting/common/src/index.spec.ts
new file mode 100644
index 00000000..27bdbd57
--- /dev/null
+++ b/packages/@apphosting/common/src/index.spec.ts
@@ -0,0 +1,30 @@
+import assert from "assert";
+import fs from "fs";
+import path from "path";
+import os from "os";
+import { updateOrCreateGitignore } from "./index";
+
+describe("update or create .gitignore", () => {
+  let tmpDir: string;
+  beforeEach(() => {
+    tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), "test-gitignore"));
+  });
+
+  afterEach(() => {
+    fs.rmSync(tmpDir, { recursive: true, force: true });
+  });
+
+  it(".gitignore file exists and is correctly updated with missing paths", () => {
+    fs.writeFileSync(path.join(tmpDir, ".gitignore"), "existingpath/");
+
+    updateOrCreateGitignore(tmpDir, ["existingpath/", "newpath/"]);
+
+    const gitignoreContent = fs.readFileSync(path.join(tmpDir, ".gitignore"), "utf-8");
+    assert.equal(`existingpath/\nnewpath/`, gitignoreContent);
+  });
+  it(".gitignore file does not exist and is created", () => {
+    updateOrCreateGitignore(tmpDir, ["chickenpath/", "newpath/"]);
+    const gitignoreContent = fs.readFileSync(path.join(tmpDir, ".gitignore"), "utf-8");
+    assert.equal(`chickenpath/\nnewpath/`, gitignoreContent);
+  });
+});
diff --git a/packages/@apphosting/common/src/index.ts b/packages/@apphosting/common/src/index.ts
index 6319a0cc..f16d32db 100644
--- a/packages/@apphosting/common/src/index.ts
+++ b/packages/@apphosting/common/src/index.ts
@@ -1,4 +1,6 @@
 import { spawn } from "child_process";
+import * as fs from "node:fs";
+import * as path from "node:path";
 
 // Output bundle metadata specifications to be written to bundle.yaml
 export interface OutputBundleConfig {
@@ -139,3 +141,26 @@ export function getBuildOptions(): BuildOptions {
     projectDirectory: process.cwd(),
   };
 }
+
+/**
+ * Updates or creates a .gitignore file with the given entries in the given path
+ */
+export function updateOrCreateGitignore(dirPath: string, entries: string[]) {
+  const gitignorePath = path.join(dirPath, ".gitignore");
+
+  if (!fs.existsSync(gitignorePath)) {
+    console.log(`creating ${gitignorePath} with entries: ${entries.join("\n")}`);
+    fs.writeFileSync(gitignorePath, entries.join("\n"));
+    return;
+  }
+
+  let content = fs.readFileSync(gitignorePath, "utf-8");
+  for (const entry of entries) {
+    if (!content.split("\n").includes(entry)) {
+      console.log(`adding ${entry} to ${gitignorePath}`);
+      content += `\n${entry}`;
+    }
+  }
+
+  fs.writeFileSync(gitignorePath, content);
+}