diff --git a/cmd/ndjson.go b/cmd/ndjson.go
new file mode 100644
index 0000000..fa3e829
--- /dev/null
+++ b/cmd/ndjson.go
@@ -0,0 +1,159 @@
+package cmd
+
+import (
+	"bufio"
+	"encoding/base64"
+	"encoding/json"
+	"fmt"
+	"os"
+	"slices"
+	"strings"
+	"time"
+	"unicode/utf8"
+)
+
+// Entry is the in-memory representation of a stored key-value pair.
+type Entry struct {
+	Key       string
+	Value     []byte
+	ExpiresAt uint64 // Unix timestamp; 0 = never expires
+}
+
+// jsonEntry is the NDJSON on-disk format.
+type jsonEntry struct {
+	Key       string `json:"key"`
+	Value     string `json:"value"`
+	Encoding  string `json:"encoding,omitempty"`
+	ExpiresAt *int64 `json:"expires_at,omitempty"`
+}
+
+// readStoreFile reads all non-expired entries from an NDJSON file.
+// Returns empty slice (not error) if file does not exist.
+func readStoreFile(path string) ([]Entry, error) {
+	f, err := os.Open(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, nil
+		}
+		return nil, err
+	}
+	defer f.Close()
+
+	now := uint64(time.Now().Unix())
+	var entries []Entry
+	scanner := bufio.NewScanner(f)
+	scanner.Buffer(make([]byte, 0, 64*1024), 8*1024*1024)
+	lineNo := 0
+	for scanner.Scan() {
+		lineNo++
+		line := scanner.Bytes()
+		if len(line) == 0 {
+			continue
+		}
+		var je jsonEntry
+		if err := json.Unmarshal(line, &je); err != nil {
+			return nil, fmt.Errorf("line %d: %w", lineNo, err)
+		}
+		entry, err := decodeJsonEntry(je)
+		if err != nil {
+			return nil, fmt.Errorf("line %d: %w", lineNo, err)
+		}
+		// Skip expired entries
+		if entry.ExpiresAt > 0 && entry.ExpiresAt <= now {
+			continue
+		}
+		entries = append(entries, entry)
+	}
+	return entries, scanner.Err()
+}
+
+// writeStoreFile atomically writes entries to an NDJSON file, sorted by key.
+// Expired entries are excluded. Empty entry list writes an empty file.
+func writeStoreFile(path string, entries []Entry) error {
+	// Sort by key for deterministic output
+	slices.SortFunc(entries, func(a, b Entry) int {
+		return strings.Compare(a.Key, b.Key)
+	})
+
+	tmp := path + ".tmp"
+	f, err := os.Create(tmp)
+	if err != nil {
+		return err
+	}
+	defer func() {
+		f.Close()
+		os.Remove(tmp) // clean up on failure; no-op after successful rename
+	}()
+
+	w := bufio.NewWriter(f)
+	now := uint64(time.Now().Unix())
+	for _, e := range entries {
+		if e.ExpiresAt > 0 && e.ExpiresAt <= now {
+			continue
+		}
+		je := encodeJsonEntry(e)
+		data, err := json.Marshal(je)
+		if err != nil {
+			return fmt.Errorf("key %q: %w", e.Key, err)
+		}
+		w.Write(data)
+		w.WriteByte('\n')
+	}
+	if err := w.Flush(); err != nil {
+		return err
+	}
+	if err := f.Sync(); err != nil {
+		return err
+	}
+	if err := f.Close(); err != nil {
+		return err
+	}
+	return os.Rename(tmp, path)
+}
+
+func decodeJsonEntry(je jsonEntry) (Entry, error) {
+	var value []byte
+	switch je.Encoding {
+	case "", "text":
+		value = []byte(je.Value)
+	case "base64":
+		var err error
+		value, err = base64.StdEncoding.DecodeString(je.Value)
+		if err != nil {
+			return Entry{}, fmt.Errorf("decode base64 for %q: %w", je.Key, err)
+		}
+	default:
+		return Entry{}, fmt.Errorf("unsupported encoding %q for %q", je.Encoding, je.Key)
+	}
+	var expiresAt uint64
+	if je.ExpiresAt != nil {
+		expiresAt = uint64(*je.ExpiresAt)
+	}
+	return Entry{Key: je.Key, Value: value, ExpiresAt: expiresAt}, nil
+}
+
+func encodeJsonEntry(e Entry) jsonEntry {
+	je := jsonEntry{Key: e.Key}
+	if utf8.Valid(e.Value) {
+		je.Value = string(e.Value)
+		je.Encoding = "text"
+	} else {
+		je.Value = base64.StdEncoding.EncodeToString(e.Value)
+		je.Encoding = "base64"
+	}
+	if e.ExpiresAt > 0 {
+		ts := int64(e.ExpiresAt)
+		je.ExpiresAt = &ts
+	}
+	return je
+}
+
+// findEntry returns the index of the entry with the given key, or -1.
+func findEntry(entries []Entry, key string) int {
+	for i, e := range entries {
+		if e.Key == key {
+			return i
+		}
+	}
+	return -1
+}
diff --git a/cmd/ndjson_test.go b/cmd/ndjson_test.go
new file mode 100644
index 0000000..3deb593
--- /dev/null
+++ b/cmd/ndjson_test.go
@@ -0,0 +1,117 @@
+package cmd
+
+import (
+	"path/filepath"
+	"testing"
+	"time"
+)
+
+func TestReadWriteRoundtrip(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "test.ndjson")
+
+	entries := []Entry{
+		{Key: "alpha", Value: []byte("hello")},
+		{Key: "beta", Value: []byte("world"), ExpiresAt: uint64(time.Now().Add(time.Hour).Unix())},
+		{Key: "gamma", Value: []byte{0xff, 0xfe}}, // binary
+	}
+
+	if err := writeStoreFile(path, entries); err != nil {
+		t.Fatal(err)
+	}
+
+	got, err := readStoreFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if len(got) != len(entries) {
+		t.Fatalf("got %d entries, want %d", len(got), len(entries))
+	}
+	for i := range entries {
+		if got[i].Key != entries[i].Key {
+			t.Errorf("entry %d: key = %q, want %q", i, got[i].Key, entries[i].Key)
+		}
+		if string(got[i].Value) != string(entries[i].Value) {
+			t.Errorf("entry %d: value mismatch", i)
+		}
+	}
+}
+
+func TestReadStoreFileSkipsExpired(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "test.ndjson")
+
+	entries := []Entry{
+		{Key: "alive", Value: []byte("yes")},
+		{Key: "dead", Value: []byte("no"), ExpiresAt: 1}, // expired long ago
+	}
+
+	if err := writeStoreFile(path, entries); err != nil {
+		t.Fatal(err)
+	}
+
+	got, err := readStoreFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if len(got) != 1 || got[0].Key != "alive" {
+		t.Fatalf("expected only 'alive', got %v", got)
+	}
+}
+
+func TestReadStoreFileNotExist(t *testing.T) {
+	got, err := readStoreFile("/nonexistent/path.ndjson")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(got) != 0 {
+		t.Fatalf("expected empty, got %d entries", len(got))
+	}
+}
+
+func TestWriteStoreFileSortsKeys(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "test.ndjson")
+
+	entries := []Entry{
+		{Key: "charlie", Value: []byte("3")},
+		{Key: "alpha", Value: []byte("1")},
+		{Key: "bravo", Value: []byte("2")},
+	}
+
+	if err := writeStoreFile(path, entries); err != nil {
+		t.Fatal(err)
+	}
+
+	got, err := readStoreFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if got[0].Key != "alpha" || got[1].Key != "bravo" || got[2].Key != "charlie" {
+		t.Fatalf("entries not sorted: %v", got)
+	}
+}
+
+func TestWriteStoreFileAtomic(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "test.ndjson")
+
+	// Write initial data
+	if err := writeStoreFile(path, []Entry{{Key: "a", Value: []byte("1")}}); err != nil {
+		t.Fatal(err)
+	}
+
+	// Overwrite — should not leave .tmp files
+	if err := writeStoreFile(path, []Entry{{Key: "b", Value: []byte("2")}}); err != nil {
+		t.Fatal(err)
+	}
+
+	// Check no .tmp file remains
+	matches, _ := filepath.Glob(filepath.Join(dir, "*.tmp"))
+	if len(matches) > 0 {
+		t.Fatalf("leftover tmp files: %v", matches)
+	}
+}