diff --git a/tools/compliance/projectmetadata/projectmetadata.go b/tools/compliance/projectmetadata/projectmetadata.go
index 1861b47..f9ddbae 100644
--- a/tools/compliance/projectmetadata/projectmetadata.go
+++ b/tools/compliance/projectmetadata/projectmetadata.go
@@ -118,6 +118,7 @@
 // a `ProjectMetadata`, pm (can be nil even without error), or a non-nil `err`.
 type projectIndex struct {
 	project string
+	path    string
 	pm      *ProjectMetadata
 	err     error
 	done    chan struct{}
@@ -230,6 +231,19 @@
 	return result, nil
 }
 
+// AllMetadataFiles returns the sorted list of all METADATA files read thus far.
+func (ix *Index) AllMetadataFiles() []string {
+	files := []string(nil)
+	ix.projects.Range(func(key, value any) bool {
+		pi := value.(*projectIndex)
+		if pi.path != "" {
+			files = append(files, pi.path)
+		}
+		return true
+	})
+	return files
+}
+
 // readMetadataFile tries to read and parse a METADATA file at `path` for `project`.
 func (ix *Index) readMetadataFile(pi *projectIndex, path string) {
 	f, err := ix.rootFS.Open(path)
@@ -250,9 +264,24 @@
 	pm := &ProjectMetadata{project: pi.project}
 	err = uo.Unmarshal(data, &pm.proto)
 	if err != nil {
-		pi.err = fmt.Errorf("error in project %q metadata %q: %w", pi.project, path, err)
+		pi.err = fmt.Errorf(`error in project %q METADATA %q: %v
+
+METADATA and METADATA.android files must parse as text protobufs
+defined by
+   build/soong/compliance/project_metadata_proto/project_metadata.proto
+
+* unknown fields don't matter
+* check invalid ENUM names
+* check quoting
+* check unescaped nested quotes
+* check the comment marker for protobuf is '#' not '//'
+
+if importing a library that uses a different sort of METADATA file, add
+a METADATA.android file beside it to parse instead
+`, pi.project, path, err)
 		return
 	}
 
+	pi.path = path
 	pi.pm = pm
 }