diff --git a/README.md b/README.md
index b1e14e0..8a45af2 100644
--- a/README.md
+++ b/README.md
@@ -36,6 +36,12 @@ npx @typeofweb/ignore-monorepo-buildstep
 
 ![Ignore Build Step settings in Vercel](./docs/vercel_settings.png)
 
+By default, `ignore-monorepo-buildstep` will compare `HEAD^` and `HEAD`. You can override this and use an env variable exposed by Vercel:
+
+```
+npx @typeofweb/ignore-monorepo-buildstep $VERCEL_GIT_PREVIOUS_SHA
+```
+
 ### Result
 
 When any changes are introduced to `packages/common`, both apps `a` and `b` will be built:
diff --git a/src/git.ts b/src/git.ts
new file mode 100644
index 0000000..d1aeb5b
--- /dev/null
+++ b/src/git.ts
@@ -0,0 +1,24 @@
+import { execFile } from "node:child_process";
+import { promisify } from "node:util";
+const execFileAsync = promisify(execFile);
+
+export const compare = ({
+	from,
+	to,
+	paths,
+	pathsToIgnore,
+}: {
+	from: string;
+	to: string;
+	paths: string[];
+	pathsToIgnore: string[];
+}) => {
+	return execFileAsync(`git`, [
+		`diff`,
+		from,
+		to,
+		`--quiet`,
+		...paths,
+		...pathsToIgnore.map((path) => `:^${path}`),
+	]);
+};
diff --git a/src/index.ts b/src/index.ts
index 39093e4..683ea69 100755
--- a/src/index.ts
+++ b/src/index.ts
@@ -2,18 +2,19 @@
 
 import { existsSync } from "node:fs";
 import Path from "node:path";
-import { execFile } from "node:child_process";
-import { promisify } from "node:util";
 import { promiseErrorToSettled } from "./utils.js";
 import {
 	readWorkspaceDirs,
 	readWorkspaceSettings,
 	resolveWorkspaceDeps,
 } from "./pnpmWorkspace.js";
-const execFileAsync = promisify(execFile);
+import { compare } from "./git.js";
 
 const cwd = process.cwd();
 
+const [_node, _bin, gitFromPointer = "HEAD^", gitToPointer = "HEAD"] =
+	process.argv;
+
 const rootDir = cwd
 	.split(Path.sep)
 	.map((_, idx) => Path.join(cwd, "../".repeat(idx)))
@@ -41,28 +42,30 @@ const result = await Promise.all([
 	...workspaceDepsRelativePaths.map(async (path) => {
 		return {
 			result: await promiseErrorToSettled(
-				execFileAsync(`git`, [`diff`, `HEAD^`, `HEAD`, `--quiet`, path]),
+				compare({
+					from: gitFromPointer,
+					to: gitToPointer,
+					paths: [path],
+					pathsToIgnore: [],
+				}),
 			),
 			path,
 		};
 	}),
 	(async () => {
-		const pathsToIgnore = (await readWorkspaceDirs({ rootDir, cwd }))
-			.map((path) => Path.relative(cwd, path))
-			.map((path) => `:^${path}`);
+		const pathsToIgnore = (await readWorkspaceDirs({ rootDir, cwd })).map(
+			(path) => Path.relative(cwd, path),
+		);
 		const relativeRoot = Path.relative(cwd, rootDir);
 
 		return {
 			result: await promiseErrorToSettled(
-				execFileAsync(`git`, [
-					`diff`,
-					`HEAD^`,
-					`HEAD`,
-					`--quiet`,
-					`--`,
-					relativeRoot,
-					...pathsToIgnore,
-				]),
+				compare({
+					from: gitFromPointer,
+					to: gitToPointer,
+					paths: [relativeRoot],
+					pathsToIgnore,
+				}),
 			),
 			path: relativeRoot,
 		};