try git-based

action.yml

@@ -3,6 +3,9 @@ description: 📃 List all features in a monorepo in a GitHub Action
 inputs:
   path:
     default: .
+  ref:
+    default: ${{ github.ref }}
+  base-ref: {}
 outputs:
   features:
     value: ${{ steps.main.outputs.features }}
@@ -11,24 +14,9 @@ outputs:
 runs:
   using: composite
   steps:
-    - id: paths-filter
-      uses: dorny/paths-filter@v2
-      with:
-        list-files: json
-        filters: |
-          src: 
-            - ${{ inputs.path }}/src/*/**
-          test: 
-            - ${{ inputs.path }}/test/*/**
-    - run: echo "$PATHS_FILTER_OUTPUTS" | jq
-      shell: bash
-      env:
-        PATHS_FILTER_OUTPUTS: ${{ toJSON(steps.paths-filter.outputs) }}
     - id: main
       run: '"$GITHUB_ACTION_PATH/mainw"'
       shell: bash
       env:
         INPUT_PATH: ${{ inputs.path }}
-        # These are ALREADY JSON-encoded to JSON strings, so DON'T double-encode them.
-        PATHS_FILTER_OUTPUTS_SRC_FILES: ${{ steps.paths-filter.outputs.src_files }}
-        PATHS_FILTER_OUTPUTS_TEST_FILES: ${{ steps.paths-filter.outputs.test_files }}
+        GITHUB_EVENT: ${{ toJSON(github.event) }}

main.ts

@@ -3,9 +3,11 @@ import * as core from "npm:@actions/core"
 import { readFile, writeFile } from "node:fs/promises"
 import process from "node:process"
 import { glob } from "npm:glob"
+import { $ } from "npm:zx"
 
 const path = core.getInput("path")
 process.chdir(path)
+$.cwd = process.cwd()
 
 const features = await Promise.all(
   (await glob("src/*/devcontainer-feature.json")).map(f =>
@@ -15,23 +17,25 @@ const features = await Promise.all(
 )
 core.setOutput("features", JSON.stringify(features))
 
-// These are the CHANGED files.
-// https://github.com/dorny/paths-filter
-const srcFiles = JSON.parse(process.env.PATHS_FILTER_OUTPUTS_SRC_FILES)
-const testFiles = JSON.parse(process.env.PATHS_FILTER_OUTPUTS_TEST_FILES)
-console.dir(srcFiles)
-console.dir(testFiles)
-
-const changedFeatureIds = [
-  // These paths are from BEFORE the 'process.chdir()'
-  ...srcFiles.map(x => x.match(/src\/(.*?)\//)[1]).filter(x => x),
-  ...testFiles.map(x => x.match(/test\/(.*?)\//)[1]).filter(x => x),
-]
-const changedFeatures = (await Promise.all(
-  changedFeatureIds.map(id =>
-    readFile(`src/${id}/devcontainer-feature.json`, "utf8")
-      .then(x => JSON.parse(x))
-      .catch(() => {})
-  )
-)).filter(x => x)
+const ref = core.getInput("ref")
+const event = JSON.parse(process.env.GITHUB_EVENT)
+let changedFeatures;
+if (event.pull_request) {
+  const baseRef = event.pull_request.base.ref
+  const changedFiles = (await $`git diff --name-only ${baseRef} ${ref}`).toString().split(/\r?\n/g)
+  const changedIds = changedFiles.map(x => /src\/(.*?)\//.match(x)?.[1]).filter(x => x)
+  changedFeatures = (await Promise.all(
+    changedIds.map(x => readFile(`src/${id}/devcontainer-feature.json`, "utf8")
+      .catch(() => {}))
+  ))
+    .filter(x => x)
+    .map(x => {
+      try {
+        return JSON.parse(x)
+      } catch {}
+    })
+    .filter(x => x)
+} else {
+  changedFeatures = []
+}
 core.setOutput("changed-features", JSON.stringify(changedFeatures))