diff --git a/backup/archive.go b/backup/archive.go
index 6c2b054..b78f704 100644
--- a/backup/archive.go
+++ b/backup/archive.go
@@ -163,6 +163,10 @@ func BuildArchive(ctx context.Context, w io.Writer, in BuildInput) (*Manifest, [
 	}
 
 	// Extra fixed entries, in a stable (sorted) order for reproducibility.
+	// Failure contract (cms parity — the cssManifestsFile block in
+	// buildBackupArchive): a failed Create skips the entry silently, a failed
+	// Write appends a warning. Extra entries are regenerable extras; a problem
+	// with one must never abort the archive.
 	if len(in.Extra) > 0 {
 		names := make([]string, 0, len(in.Extra))
 		for name := range in.Extra {
@@ -172,10 +176,10 @@ func BuildArchive(ctx context.Context, w io.Writer, in BuildInput) (*Manifest, [
 		for _, name := range names {
 			entry, err := zipWriter.Create(name)
 			if err != nil {
-				return nil, warnings, fmt.Errorf("failed to create %s in zip: %w", name, err)
+				continue
 			}
 			if _, err := entry.Write(in.Extra[name]); err != nil {
-				return nil, warnings, fmt.Errorf("failed to write %s: %w", name, err)
+				warnings = append(warnings, fmt.Sprintf("Failed to write %s: %v", name, err))
 			}
 		}
 	}
diff --git a/backup/archive_test.go b/backup/archive_test.go
index 60aecda..a4e6812 100644
--- a/backup/archive_test.go
+++ b/backup/archive_test.go
@@ -9,6 +9,7 @@ import (
 	"io"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 )
 
@@ -95,3 +96,76 @@ func TestBuildArchiveRoundTrip(t *testing.T) {
 		t.Fatal("media file missing or corrupted in archive")
 	}
 }
+
+// TestBuildArchiveExtraEntries locks the happy path of the Extra contract: extra
+// entries land in the archive with their exact contents. The failure half of the
+// contract (Create failure → skip silently, Write failure → warning, never a
+// fatal error — cms cssManifestsFile parity) is documented at the Extra block in
+// BuildArchive; zip.Writer failures cannot be injected deterministically without
+// error-injection infrastructure, so the failure path is covered for section
+// files below where the failure is controllable.
+func TestBuildArchiveExtraEntries(t *testing.T) {
+	var buf bytes.Buffer
+	_, warnings, err := BuildArchive(context.Background(), &buf, BuildInput{
+		Source:   Source{SiteTitle: "Test Site"},
+		Dump:     []byte("dump"),
+		Password: "correct horse",
+		Extra: map[string][]byte{
+			"state/css-manifests.json": []byte(`{"plugins":{}}`),
+		},
+	})
+	if err != nil {
+		t.Fatalf("BuildArchive: %v", err)
+	}
+	if len(warnings) != 0 {
+		t.Fatalf("unexpected warnings: %v", warnings)
+	}
+	if got := string(readZipEntry(t, buf.Bytes(), "state/css-manifests.json")); got != `{"plugins":{}}` {
+		t.Fatalf("extra entry content = %q", got)
+	}
+}
+
+// TestBuildArchiveUnreadableFileWarnsAndContinues proves the warning-not-error
+// contract: a file that cannot be opened inside an existing section dir appends
+// a warning and the archive still completes with every readable file present.
+func TestBuildArchiveUnreadableFileWarnsAndContinues(t *testing.T) {
+	if os.Getuid() == 0 {
+		t.Skip("running as root: chmod 0 files remain readable")
+	}
+	dir := t.TempDir()
+	mediaDir := filepath.Join(dir, "media")
+	if err := os.MkdirAll(mediaDir, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(mediaDir, "good.txt"), []byte("ok"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	badPath := filepath.Join(mediaDir, "bad.txt")
+	if err := os.WriteFile(badPath, []byte("nope"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Chmod(badPath, 0o000); err != nil {
+		t.Fatal(err)
+	}
+
+	var buf bytes.Buffer
+	manifest, warnings, err := BuildArchive(context.Background(), &buf, BuildInput{
+		Source:   Source{SiteTitle: "Test Site"},
+		Dump:     []byte("dump"),
+		Password: "correct horse",
+		Sections: []Section{{Prefix: "media/", Dir: mediaDir}},
+	})
+	if err != nil {
+		t.Fatalf("unreadable file must not abort the archive, got error: %v", err)
+	}
+	if len(warnings) != 1 || !strings.Contains(warnings[0], "bad.txt") {
+		t.Fatalf("want exactly one warning mentioning bad.txt, got: %v", warnings)
+	}
+	// Both files were collected (open fails later, at write time) — cms parity.
+	if manifest.MediaCount != 2 {
+		t.Fatalf("media count = %d, want 2", manifest.MediaCount)
+	}
+	if got := string(readZipEntry(t, buf.Bytes(), "media/good.txt")); got != "ok" {
+		t.Fatalf("good.txt content = %q", got)
+	}
+}