diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index a3137d1..849b090 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -141,6 +141,61 @@ jobs:
           issue-number: ${{ github.event.pull_request.number }}
           token: ${{ secrets.GITHUB_TOKEN }}
 
+  test-job-with-custom-base-ref:
+    runs-on: ubuntu-latest
+    name: Test custom base ref
+    if: github.event_name == 'pull_request'
+    steps:
+      - uses: actions/checkout@v7
+        with:
+          fetch-depth: 0
+
+      - uses: actions/setup-java@v5
+        with:
+          distribution: 'temurin'
+          java-version-file: .java-version
+
+      - name: Downgrade dependency version to see the diff
+        run: sed -i -E 's/androidx.paging:paging-common-ktx:[[:digit:]]+.[[:digit:]]+.[[:digit:]]+/androidx.paging:paging-common-ktx:3.2.0/g' testproject/build.gradle
+
+      - uses: gradle/actions/setup-gradle@v6
+
+      # Ref resolved in the local repository only
+      - id: dependency-diff-local-ref
+        uses: ./
+        with:
+          configuration: runtimeClasspath
+          build-root-directory: testproject
+          project: ""
+          base-ref: HEAD^1
+          debug: true
+
+      - uses: actions/upload-artifact@v7
+        with:
+          name: file-diff-test-base-ref-local
+          path: ${{ steps.dependency-diff-local-ref.outputs.file-diff }}
+          if-no-files-found: error
+
+      # The action restores the checked out commit, which reverts the downgrade above
+      - name: Downgrade dependency version again to see the diff
+        run: sed -i -E 's/androidx.paging:paging-common-ktx:[[:digit:]]+.[[:digit:]]+.[[:digit:]]+/androidx.paging:paging-common-ktx:3.2.0/g' testproject/build.gradle
+
+      # Sha fetched from origin
+      - id: dependency-diff-sha
+        uses: ./
+        with:
+          configuration: runtimeClasspath
+          build-root-directory: testproject
+          project: ""
+          base-ref: ${{ github.event.pull_request.base.sha }}
+          debug: true
+
+      - uses: actions/upload-artifact@v7
+        with:
+          name: file-diff-test-base-ref-sha
+          path: ${{ steps.dependency-diff-sha.outputs.file-diff }}
+          if-no-files-found: error
+
   test-on-different-os:
     strategy:
       fail-fast: false
diff --git a/Readme.md b/Readme.md
index 63460c4..87cdcaa 100644
--- a/Readme.md
+++ b/Readme.md
@@ -64,6 +64,7 @@ All inputs with their default values:
         configuration: 'releaseRuntimeClasspath'
         project: 'app'
         build-root-directory: .
+        base-ref: ''
         additional-gradle-arguments: ''
         lib-version: 'latest'
 ```
@@ -75,9 +76,33 @@ Dependency diff for root projects can be configured using `project: ''`.
  For Android projects use the one that has `com.android.application` plugin applied.
 - **`build-root-directory`** - Relative path to folder containing gradle wrapper. 
 Example usage: `build-root-directory: library`
+- **`base-ref`** - The ref currently checked out revision is compared against. 
+Accepts anything git can resolve to a commit - a branch name, a tag, a commit sha or an expression like `HEAD^1`. 
+Refs available in the local repository are used as is, everything else is fetched from `origin` first. 
+When left empty (the default) the action falls back to `github.base_ref`, i.e. the branch the pull request targets. 
+See [Stacked pull requests](#stacked-pull-requests).
 - **`additional-gradle-arguments`** - Additional arguments passed to internal Gradle invocation. Example: `"--no-configuration-cache"` or `"--stacktrace"`  
 - **`lib-version`** - Overrides [dependency-tree-diff](https://github.com/JakeWharton/dependency-tree-diff) dependency version. Example: `"1.2.1"`, `"1.1.0"`, `"latest"`
 
+### Stacked pull requests
+
+For [stacked pull requests](https://docs.github.com/en/pull-requests/reference/stacked-pull-requests) `github.base_ref` points at the base of the whole stack (e.g. `main`), 
+while the merge commit CI checks out is built on top of the parent pull request. 
+Both sides of the comparison then contain a different version of `main`, and dependency changes that landed there in the meantime get reported as if they were introduced by the pull request.
+
+Comparing against the first parent of the checked out merge commit puts the same `main` on both sides:
+
+```yml
+    - uses: actions/checkout@v4
+      with:
+        fetch-depth: 2
+
+    - id: dependency-diff
+      uses: usefulness/dependency-tree-diff-action@v2
+      with:
+        base-ref: HEAD^1
+```
+
 <details><summary></summary>
 <p>
 
diff --git a/action.yml b/action.yml
index 2f08fbe..e251563 100644
--- a/action.yml
+++ b/action.yml
@@ -13,6 +13,10 @@ inputs:
     description: 'Build root directory'
     required: false
     default: '.'
+  base-ref:
+    description: 'Git ref (branch name, tag or commit sha) the currently checked out revision is compared against. Defaults to `github.base_ref`'
+    required: false
+    default: ''
   lib-version:
     description: 'Dependency diff library version'
     required: true
@@ -36,7 +40,7 @@ outputs:
     description: "Path to a file containing dependencies dump from currently checked out ref"
     value: ${{ steps.diff-generator-unix.outputs.file-dependencies-head || steps.diff-generator-windows.outputs.file-dependencies-head }}
   file-dependencies-base:
-    description: "Path to a file containing dependencies dump from `github.base_ref`"
+    description: "Path to a file containing dependencies dump from the base ref"
     value: ${{ steps.diff-generator-unix.outputs.file-dependencies-base || steps.diff-generator-windows.outputs.file-dependencies-base }}
 branding:
   color: 'red'
@@ -49,7 +53,7 @@ runs:
       env:
         INPUT_PROJECT: ${{ inputs.project }}
         INPUT_CONFIGURATION: ${{ inputs.configuration }}
-        INPUT_BASEREF: ${{ github.base_ref }}
+        INPUT_BASEREF: ${{ inputs.base-ref || github.base_ref }}
         INPUT_BUILD_ROOT_DIR: ${{ inputs.build-root-directory }}
         INPUT_VERSION: ${{ inputs.lib-version }}
         INPUT_ADDITIONAL_GRADLE_ARGUMENTS: ${{ inputs.additional-gradle-arguments }}
@@ -63,7 +67,7 @@ runs:
       env:
         INPUT_PROJECT: ${{ inputs.project }}
         INPUT_CONFIGURATION: ${{ inputs.configuration }}
-        INPUT_BASEREF: ${{ github.base_ref }}
+        INPUT_BASEREF: ${{ inputs.base-ref || github.base_ref }}
         INPUT_BUILD_ROOT_DIR: ${{ inputs.build-root-directory }}
         INPUT_VERSION: ${{ inputs.lib-version }}
         INPUT_ADDITIONAL_GRADLE_ARGUMENTS: ${{ inputs.additional-gradle-arguments }}
diff --git a/entrypoint.ps1 b/entrypoint.ps1
index c85bb15..478f263 100644
--- a/entrypoint.ps1
+++ b/entrypoint.ps1
@@ -11,6 +11,11 @@ param(
 
 $ErrorActionPreference = "Stop"
 
+if ([string]::IsNullOrWhiteSpace($InputBaseRef)) {
+    Write-Host "::error::There is no ref to compare against. The action falls back to ``github.base_ref``, which is only set for pull request events - pass the ``base-ref`` input explicitly for other triggers."
+    exit 1
+}
+
 Set-Location $InputBuildRootDir
 
 $headers = @{
@@ -41,8 +46,23 @@ $currentHead = git rev-parse HEAD
 
 $cmd = "./gradlew.bat $InputAdditionalGradleArguments ${InputProject}:dependencies --configuration $InputConfiguration"
 Invoke-Expression $cmd | Out-File -FilePath "dependency-tree-diff_dependencies-head.txt" -Encoding UTF8
-git fetch --force origin "${InputBaseRef}:${InputBaseRef}" --no-tags
-git switch --force $InputBaseRef
+# Prefer the ref as published by `origin`, but fall back to the local repository,
+# so refs that only exist locally (`HEAD^1`, a not yet pushed commit) keep working.
+try {
+    git fetch --force origin $InputBaseRef --no-tags
+    $fetched = $LASTEXITCODE -eq 0
+} catch {
+    $fetched = $false
+}
+
+if ($fetched) {
+    $baseRef = "FETCH_HEAD"
+} else {
+    Write-Host "Could not fetch '$InputBaseRef' from origin, resolving it in the local repository instead"
+    $baseRef = $InputBaseRef
+}
+
+git switch --force --detach $baseRef
 Invoke-Expression $cmd | Out-File -FilePath "dependency-tree-diff_dependencies-base.txt" -Encoding UTF8
 java -jar dependency-tree-diff.jar dependency-tree-diff_dependencies-base.txt dependency-tree-diff_dependencies-head.txt | Out-File -FilePath "dependency-tree-diff_output.txt" -Encoding UTF8
 
diff --git a/entrypoint.sh b/entrypoint.sh
index 578c222..be06c08 100755
--- a/entrypoint.sh
+++ b/entrypoint.sh
@@ -1,5 +1,10 @@
 #!/bin/bash -el
 
+if [ -z "$INPUT_BASEREF" ]; then
+  echo "::error::There is no ref to compare against. The action falls back to \`github.base_ref\`, which is only set for pull request events - pass the \`base-ref\` input explicitly for other triggers."
+  exit 1
+fi
+
 cd "$INPUT_BUILD_ROOT_DIR"
 
 if [ "$INPUT_VERSION" == "latest" ]; then
@@ -28,8 +33,17 @@ chmod +x dependency-tree-diff.jar
 current_head=$(git rev-parse HEAD)
 
 ./gradlew $INPUT_ADDITIONAL_GRADLE_ARGUMENTS "$INPUT_PROJECT":dependencies --configuration "$INPUT_CONFIGURATION" > dependency-tree-diff_dependencies-head.txt
-git fetch --force origin "$INPUT_BASEREF":"$INPUT_BASEREF" --no-tags
-git switch --force "$INPUT_BASEREF"
+
+# Prefer the ref as published by `origin`, but fall back to the local repository,
+# so refs that only exist locally (`HEAD^1`, a not yet pushed commit) keep working.
+if git fetch --force origin "$INPUT_BASEREF" --no-tags; then
+  base_ref="FETCH_HEAD"
+else
+  echo "Could not fetch '$INPUT_BASEREF' from origin, resolving it in the local repository instead"
+  base_ref="$INPUT_BASEREF"
+fi
+
+git switch --force --detach "$base_ref"
 ./gradlew $INPUT_ADDITIONAL_GRADLE_ARGUMENTS "$INPUT_PROJECT":dependencies --configuration "$INPUT_CONFIGURATION" > dependency-tree-diff_dependencies-base.txt
 java -jar dependency-tree-diff.jar dependency-tree-diff_dependencies-base.txt dependency-tree-diff_dependencies-head.txt > dependency-tree-diff_output.txt