diff --git a/README.md b/README.md
index d871a0f..b9fae6d 100644
--- a/README.md
+++ b/README.md
@@ -67,6 +67,36 @@ jobs:
 +       build-script: "ci"
 ```
 
+#### Clean up state between builds
+
+For repositories or custom monorepo setups where files are modified in ways that are not reset by `npm ci && npm run build`, it may be necessary to define a custom "clean" script. This script resets any file modifications after the upstream (`target`) build ends and your PR code (`HEAD`) is checked out, but before installation of npm dependencies for `HEAD`:
+
+```diff
+name: Compressed Size
+on: [pull_request]
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - uses: preactjs/compressed-size-action@v2
+      with:
+        repo-token: "${{ secrets.GITHUB_TOKEN }}"
++       clean-script: "clean"
+```
+
+```jsonc
+// package.json
+{
+  "scripts": {
+    // example - a simple nested node_modules setup:
+    "postinstall": "cd packages && npm i",
+    // between the two builds, we need to delete the inner node_modules:
+    "clean": "rm -rf packages/node_modules"
+  }
+}
+```
+
 ### Customizing the list of files
 
 `compressed-size-action` defaults to tracking the size of all JavaScript files within `dist/` directories - anywhere in your repository, not just at the root. You can change the list of files to be tracked and reported using the `pattern` and `exclude` options, both of which are [minimatch patterns](https://github.com/motemen/minimatch-cheat-sheet/blob/master/README.md):
diff --git a/action.yml b/action.yml
index 202ec7c..25e0b6f 100644
--- a/action.yml
+++ b/action.yml
@@ -9,6 +9,8 @@ inputs:
     description: 'The GITHUB_TOKEN secret'
     required: false
     default: ${{ github.token }}
+  clean-script:
+    description: 'An npm-script that cleans/resets state between branch builds'
   build-script:
     description: 'The npm-script to run that builds your project'
     default: 'build'
diff --git a/src/index.js b/src/index.js
index 4ae925d..ea7544b 100644
--- a/src/index.js
+++ b/src/index.js
@@ -90,6 +90,13 @@ async function run(octokit, context, token) {
 	}
 	endGroup();
 
+	const cleanScript = getInput('clean-script');
+	if (cleanScript) {
+		startGroup(`[base] Cleanup via ${npm} run ${cleanScript}`);
+		await exec(`${npm} run ${cleanScript}`);
+		endGroup();
+	}
+
 	startGroup(`[base] Install Dependencies`);
 
 	yarnLock = await fileExists(path.resolve(cwd, 'yarn.lock'));