diff --git a/internal/cmd/serverless/pack.go b/internal/cmd/serverless/pack.go
index 35b2656..b36c67d 100644
--- a/internal/cmd/serverless/pack.go
+++ b/internal/cmd/serverless/pack.go
@@ -20,10 +20,15 @@ import (
 // memory and base64 expands it by ~4/3.
 const maxPackEntryBytes int64 = 10 << 20 // 10 MiB
 
-// maxPackTotalBytes bounds the archive as a whole. The per-file cap alone does
-// not: a virtualenv is thousands of small files and would sail past it.
+// maxPackTotalBytes bounds uncompressed file bytes held in memory while packing.
+// The per-file cap alone does not: a virtualenv is thousands of small files
+// and would sail past it.
 const maxPackTotalBytes int64 = 25 << 20 // 25 MiB
 
+// maxArchiveBytes is SourceUploadCreate.declaredByteLength (maximum 10485760),
+// which is the zip after compression, not the uncompressed tree.
+const maxArchiveBytes int64 = 10 << 20 // 10 MiB
+
 // runwareIgnoreFile is the project's own exclude list, and the only one read.
 //
 // .gitignore is deliberately NOT consulted. What a project keeps out of version
@@ -490,6 +495,12 @@ func writeArchive(root string, files []packedFile) ([]byte, error) {
 	if err := zw.Close(); err != nil {
 		return nil, fmt.Errorf("close zip: %w", err)
 	}
+	if int64(buf.Len()) > maxArchiveBytes {
+		return nil, fmt.Errorf(
+			"the packed archive is %s; the maximum is %s.\n%s\nExclude what the app does not need with a %s file",
+			humanBytes(int64(buf.Len())), humanBytes(maxArchiveBytes), largestFilesSummary(files), runwareIgnoreFile,
+		)
+	}
 	return buf.Bytes(), nil
 }
 
diff --git a/internal/cmd/serverless/pack_test.go b/internal/cmd/serverless/pack_test.go
index 834c0a2..92c3a12 100644
--- a/internal/cmd/serverless/pack_test.go
+++ b/internal/cmd/serverless/pack_test.go
@@ -3,6 +3,7 @@ package serverless
 import (
 	"archive/zip"
 	"bytes"
+	"crypto/rand"
 	"io"
 	"os"
 	"path/filepath"
@@ -531,6 +532,33 @@ func TestPackDirectory_TotalTooLarge(t *testing.T) {
 	}
 }
 
+func TestWriteArchive_RejectsZipOverAPILimit(t *testing.T) {
+	dir := t.TempDir()
+	// Uncompressed total stays under maxPackTotalBytes; Deflate cannot shrink
+	// random bytes enough to fit declaredByteLength.
+	const chunk = 6 << 20
+	for _, name := range []string{"a.bin", "b.bin"} {
+		buf := make([]byte, chunk)
+		if _, err := rand.Read(buf); err != nil {
+			t.Fatal(err)
+		}
+		if err := os.WriteFile(filepath.Join(dir, name), buf, 0o600); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	_, err := writeArchive(dir, []packedFile{
+		{rel: "a.bin", size: chunk},
+		{rel: "b.bin", size: chunk},
+	})
+	if err == nil {
+		t.Fatal("expected an error for a zip over declaredByteLength")
+	}
+	if !strings.Contains(err.Error(), "packed archive") || !strings.Contains(err.Error(), humanBytes(maxArchiveBytes)) {
+		t.Errorf("error %q does not name the zip cap", err)
+	}
+}
+
 // TestPackDirectory_ModelFileAlwaysPackedFromExcludedDirectory is the reviewer's
 // case: the exemption for the model file has to survive an ignore rule that
 // matches one of its ANCESTORS. The walk meets the directory first, and the