diff --git a/.github/scripts/find_changed_modules.gsh b/.github/scripts/find_changed_modules.gsh
new file mode 100644
index 0000000..fe9762a
--- /dev/null
+++ b/.github/scripts/find_changed_modules.gsh
@@ -0,0 +1,119 @@
+import (
+	"encoding/json"
+	"fmt"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"sort"
+)
+
+func formulaDir(module, changedPath string) string {
+	dir := filepath.dir(changedPath)
+	for dir.hasPrefix(module + "/") {
+		matches := filepath.glob(filepath.join(dir, "*_llar.gox"))!
+		if matches.len > 0 {
+			return dir
+		}
+		dir = filepath.dir(dir)
+	}
+	return ""
+}
+
+func hasFormula(module string) (bool, error) {
+	found := false
+	err := filepath.walkDir(module, (_, entry, err) => {
+		if err != nil {
+			return err
+		}
+		if !entry.isDir && entry.name.hasSuffix("_llar.gox") {
+			found = true
+			return fs.SkipAll
+		}
+		return nil
+	})
+	if os.isNotExist(err) {
+		return false, nil
+	}
+	return found, err
+}
+
+baseSHA := $BASE_SHA
+defaultBranch := $DEFAULT_BRANCH
+eventName := $EVENT_NAME
+headSHA := $HEAD_SHA
+refName := $REF_NAME
+
+var diffBase string
+if eventName == "pull_request" {
+	capout => { git "merge-base", baseSHA, headSHA }
+	lastErr!
+	diffBase = output.trimSpace
+} else if refName == defaultBranch {
+	diffBase = baseSHA
+} else {
+	capout => { git "merge-base", "origin/"+defaultBranch, headSHA }
+	lastErr!
+	diffBase = output.trimSpace
+}
+
+capout => {
+	git "diff", "--name-only", "--diff-filter=ACDMRT", "-z", diffBase, headSHA
+}
+lastErr!
+
+var modules map[string]bool = {}
+var formulaDirs map[string]map[string]bool = {}
+for changedPath in output.split("\x00") {
+	parts := changedPath.splitN("/", 3)
+	if parts.len < 3 {
+		continue
+	}
+	module := parts[0] + "/" + parts[1]
+	_, err := os.stat(filepath.join(module, "versions.json"))
+	if err != nil {
+		if !os.isNotExist(err) {
+			panic err
+		}
+		has := hasFormula(module)!
+		if has {
+			panic fmt.Sprintf("module %s is missing versions.json", module)
+		}
+		continue
+	}
+	modules[module] = true
+
+	dir := formulaDir(module, changedPath)
+	if dir != "" {
+		dirs := formulaDirs[module]
+		if dirs == nil {
+			dirs = {}
+			formulaDirs[module] = dirs
+		}
+		dirs[dir] = true
+	}
+}
+
+changedModules := []string([])
+changedModules <- [module for module, _ in modules]...
+sort.strings changedModules
+
+for module in changedModules {
+	dirs := formulaDirs[module]
+	if len(dirs) > 1 {
+		changedDirs := [dir for dir, _ in dirs]
+		sort.strings changedDirs
+		panic fmt.Sprintf("module %s changes multiple Formula directories: %s; llar test cannot validate multiple fromVer ranges yet", module, changedDirs.join(", "))
+	}
+}
+
+// TODO: When changed modules depend on each other, test only the leaf modules
+// in their dependency graph.
+modulesJSON := json.marshal(changedModules)!
+
+outputFile := os.openFile($GITHUB_OUTPUT, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0644)!
+defer outputFile.close()
+fmt.fprintf! outputFile, "modules=%s\nhas_modules=%t\n", modulesJSON, changedModules.len > 0
+
+summaryFile := os.openFile($GITHUB_STEP_SUMMARY, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0644)!
+defer summaryFile.close()
+fmt.fprintf! summaryFile, "Changed modules: %s\n", modulesJSON
diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index dad4618..7b1f1fc 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -25,38 +25,23 @@ jobs:
         with:
           fetch-depth: 0
 
+      - name: Set up Go
+        uses: actions/setup-go@v7
+        with:
+          go-version: 1.24.0
+          cache: false
+
       - name: Find changed modules
         id: modules
-        shell: bash
         env:
           BASE_SHA: ${{ github.event.pull_request.base.sha || github.event.before }}
+          DEFAULT_BRANCH: ${{ github.event.repository.default_branch }}
+          EVENT_NAME: ${{ github.event_name }}
           HEAD_SHA: ${{ github.event.pull_request.head.sha || github.event.after }}
+          REF_NAME: ${{ github.ref_name }}
         run: |
-          changed_modules=()
-
-          changed_paths=(git diff --name-only --diff-filter=ACMRT -z "$BASE_SHA" "$HEAD_SHA")
-          if ! git cat-file -e "${BASE_SHA}^{commit}" 2>/dev/null; then
-            # A first branch push has no previous commit. For example, GitHub
-            # supplies an all-zero `before` SHA when creating a new branch.
-            changed_paths=(git ls-tree -r -z --name-only "$HEAD_SHA")
-          fi
-
-          while IFS= read -r -d '' path; do
-            IFS=/ read -r owner repo _ <<< "$path"
-            module="$owner/$repo"
-            if [[ -f "$module/versions.json" ]]; then
-              changed_modules+=("$module")
-            fi
-          done < <("${changed_paths[@]}")
-
-          modules_json="$({ printf '%s\n' "${changed_modules[@]}" | sort -u; } | jq -Rsc 'split("\n") | map(select(length > 0))')"
-          echo "modules=$modules_json" >> "$GITHUB_OUTPUT"
-          if [[ "$modules_json" == "[]" ]]; then
-            echo "has_modules=false" >> "$GITHUB_OUTPUT"
-          else
-            echo "has_modules=true" >> "$GITHUB_OUTPUT"
-          fi
-          echo "Changed modules: $modules_json" >> "$GITHUB_STEP_SUMMARY"
+          go install -tags=linknamefix -ldflags="-checklinkname=0" github.com/goplus/ixgo/cmd/ixgo@v1.1.1
+          "$(go env GOPATH)/bin/ixgo" run .github/scripts/find_changed_modules.gsh
 
   test:
     name: ${{ matrix.module }} (${{ matrix.goos }}-${{ matrix.goarch }})