diff --git a/cmd/vintner/download.go b/cmd/vintner/download.go
index 57ec428..0985d10 100644
--- a/cmd/vintner/download.go
+++ b/cmd/vintner/download.go
@@ -45,6 +45,17 @@ func runDownload(args []string) int {
 	}
 	packages := fs.Args()
 
+	for _, a := range archsFlag {
+		if !validArchitectures[a] {
+			fmt.Fprintf(os.Stderr, "vintner download: invalid --architecture %q (expected one of x86, x64, arm, arm64, host)\n", a)
+			return 2
+		}
+	}
+	if *hostArch != "" && !validHostArchs[*hostArch] {
+		fmt.Fprintf(os.Stderr, "vintner download: invalid --host-arch %q (expected one of x86, x64, arm64)\n", *hostArch)
+		return 2
+	}
+
 	opts := &download.Options{
 		Package:         packages,
 		Ignore:          []string(ignoreFlag),
@@ -267,6 +278,9 @@ func printPackageList(headerKey string, pkgs []*download.Package, language strin
 	}
 }
 
+var validArchitectures = map[string]bool{"x86": true, "x64": true, "arm": true, "arm64": true, "host": true}
+var validHostArchs = map[string]bool{"x86": true, "x64": true, "arm64": true}
+
 func detectHostArch() string {
 	if runtime.GOARCH == "arm64" {
 		return "arm64"
diff --git a/internal/download/fetch.go b/internal/download/fetch.go
index 34c072f..ec8b127 100644
--- a/internal/download/fetch.go
+++ b/internal/download/fetch.go
@@ -84,6 +84,9 @@ func FetchPayloads(selected []*Package, cacheDir string, allowHashMismatch bool)
 func fetchOnePayloadWithRetries(payload Payload, dest, fileID string, allowHashMismatch bool) (int64, error) {
 	var lastErr error
 	for attempt := 0; attempt < maxDownloadAttempts; attempt++ {
+		if attempt > 0 {
+			time.Sleep(retryBackoff(attempt))
+		}
 		n, err := tryDownloadPayload(payload, dest, fileID, allowHashMismatch)
 		if err == nil {
 			return n, nil
@@ -94,6 +97,17 @@ func fetchOnePayloadWithRetries(payload Payload, dest, fileID string, allowHashM
 	return 0, fmt.Errorf("giving up on %s after %d attempts: %w", fileID, maxDownloadAttempts, lastErr)
 }
 
+// retryBackoff gives a transient failure (network blip, momentary rate
+// limiting) a little room to clear before hammering the same URL again:
+// 1s, 2s, 4s, 8s, capped at 10s.
+func retryBackoff(attempt int) time.Duration {
+	d := time.Second << uint(attempt-1)
+	if d > 10*time.Second {
+		d = 10 * time.Second
+	}
+	return d
+}
+
 func tryDownloadPayload(payload Payload, dest, fileID string, allowHashMismatch bool) (int64, error) {
 	if fi, err := os.Stat(dest); err == nil && fi.Mode().IsRegular() {
 		if payload.SHA256 != "" {
diff --git a/internal/download/manifest.go b/internal/download/manifest.go
index 9858d38..9027b4b 100644
--- a/internal/download/manifest.go
+++ b/internal/download/manifest.go
@@ -198,6 +198,9 @@ const maxManifestAttempts = 5
 func httpGet(url string) ([]byte, error) {
 	var lastErr error
 	for attempt := 0; attempt < maxManifestAttempts; attempt++ {
+		if attempt > 0 {
+			time.Sleep(retryBackoff(attempt))
+		}
 		data, err := tryHTTPGet(url)
 		if err == nil {
 			return data, nil
diff --git a/internal/download/select.go b/internal/download/select.go
index 2f75435..a4a63f4 100644
--- a/internal/download/select.go
+++ b/internal/download/select.go
@@ -15,8 +15,7 @@ var reSDKVersion = regexp.MustCompile(`^\d+\.\d+\.\d+`)
 // default) explicitly chose to include/exclude the component.
 type TriState = *bool
 
-func on() TriState  { v := true; return &v }
-func off() TriState { v := false; return &v }
+func on() TriState { v := true; return &v }
 
 // Options holds every flag that feeds package selection and download.
 type Options struct {
@@ -303,6 +302,7 @@ func selectSDK(opts *Options, idx Index) error {
 			}
 		}
 		if !found {
+			sort.Strings(versions)
 			return fmt.Errorf("WinSDK version %s not found (available: %s)", opts.SDKVersion, strings.Join(versions, ", "))
 		}
 	}
@@ -347,7 +347,14 @@ func collectDependencyClosure(idx Index, included map[string]bool, target string
 	included[key] = true
 
 	ret := []*Package{p}
-	for target, dep := range p.Dependencies() {
+	deps := p.Dependencies()
+	targets := make([]string, 0, len(deps))
+	for target := range deps {
+		targets = append(targets, target)
+	}
+	sort.Strings(targets)
+	for _, target := range targets {
+		dep := deps[target]
 		id := target
 		if dep.TargetID != "" {
 			id = dep.TargetID
diff --git a/internal/wrapper/run.go b/internal/wrapper/run.go
index aa82871..33f40fa 100644
--- a/internal/wrapper/run.go
+++ b/internal/wrapper/run.go
@@ -310,4 +310,11 @@ func pumpLines(r io.Reader, w *os.File, filter lineFilter) {
 		}
 		fmt.Fprintln(w, line)
 	}
+	// bufio.Scanner silently stops (dropping the rest of the stream) once a
+	// single line exceeds its 16MB buffer - surface that rather than letting
+	// build output vanish without explanation (heavily templated C++ error
+	// messages are the realistic way to hit this).
+	if err := scanner.Err(); err != nil {
+		fmt.Fprintf(w, "vintner: output truncated: %v\n", err)
+	}
 }