diff --git a/internal/parser/parser_test.go b/internal/parser/parser_test.go
new file mode 100644
index 0000000..55abf8c
--- /dev/null
+++ b/internal/parser/parser_test.go
@@ -0,0 +1,59 @@
+package parser
+
+import (
+	"testing"
+
+	"github.com/yourname/reddit-scraper/internal/fetcher"
+)
+
+func TestParseJSONPost_Valid(t *testing.T) {
+	j := fetcher.JSONPost{
+		ID:          "abc123",
+		Subreddit:   "golang",
+		Title:       "An interesting post",
+		Author:      "alice",
+		CreatedUTC:  1620000000,
+		Score:       10,
+		NumComments: 2,
+		Selftext:    "Hello world",
+		URL:         "https://reddit.com/r/golang/comments/abc123",
+		Permalink:   "/r/golang/comments/abc123",
+	}
+	p, err := ParseJSONPost(j)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if p.ID != j.ID {
+		t.Fatalf("id mismatch: %s != %s", p.ID, j.ID)
+	}
+	if p.Author != "alice" {
+		t.Fatalf("author mismatch: %s", p.Author)
+	}
+}
+
+func TestParseJSONPost_DeletedAuthor(t *testing.T) {
+	j := fetcher.JSONPost{
+		ID:         "d1",
+		Subreddit:  "test",
+		Title:      "post",
+		Author:     "",
+		CreatedUTC: 1600000000,
+	}
+	p, err := ParseJSONPost(j)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if p.Author != "[deleted]" {
+		t.Fatalf("expected [deleted], got %s", p.Author)
+	}
+}
+
+func TestParseJSONPost_MissingFields(t *testing.T) {
+	j := fetcher.JSONPost{
+		ID: "",
+	}
+	_, err := ParseJSONPost(j)
+	if err == nil {
+		t.Fatalf("expected error for missing id")
+	}
+}
diff --git a/internal/storage/dedup.go b/internal/storage/dedup.go
new file mode 100644
index 0000000..41fb3f0
--- /dev/null
+++ b/internal/storage/dedup.go
@@ -0,0 +1,76 @@
+package storage
+
+import (
+	"bufio"
+	"os"
+	"sync"
+)
+
+// DedupIndex keeps an in-memory set of seen IDs and persists them to an append-only file.
+type DedupIndex struct {
+	path string
+	mu   sync.Mutex
+	set  map[string]struct{}
+	f    *os.File
+}
+
+// LoadDedup loads existing IDs from path and prepares to append new ones.
+func LoadDedup(path string) (*DedupIndex, error) {
+	idx := &DedupIndex{path: path, set: make(map[string]struct{})}
+	// Ensure file exists
+	f, err := os.OpenFile(path, os.O_CREATE|os.O_RDWR|os.O_APPEND, 0o644)
+	if err != nil {
+		return nil, err
+	}
+	idx.f = f
+	// Read existing lines
+	sc := bufio.NewScanner(f)
+	for sc.Scan() {
+		idx.set[sc.Text()] = struct{}{}
+	}
+	// ignore scanner error for now
+	return idx, nil
+}
+
+// Seen reports whether id was seen before.
+func (d *DedupIndex) Seen(id string) bool {
+	d.mu.Lock()
+	defer d.mu.Unlock()
+	_, ok := d.set[id]
+	return ok
+}
+
+// Add marks id as seen and appends to underlying file.
+func (d *DedupIndex) Add(id string) error {
+	d.mu.Lock()
+	defer d.mu.Unlock()
+	if _, ok := d.set[id]; ok {
+		return nil
+	}
+	if d.f == nil {
+		// should not happen if loaded properly
+		f, err := os.OpenFile(d.path, os.O_CREATE|os.O_RDWR|os.O_APPEND, 0o644)
+		if err != nil {
+			return err
+		}
+		d.f = f
+	}
+	if _, err := d.f.WriteString(id + "\n"); err != nil {
+		return err
+	}
+	if err := d.f.Sync(); err != nil {
+		// best-effort; ignore
+	}
+	d.set[id] = struct{}{}
+	return nil
+}
+
+// Close closes the underlying file handle.
+func (d *DedupIndex) Close() error {
+	d.mu.Lock()
+	defer d.mu.Unlock()
+	if d.f != nil {
+		return d.f.Close()
+	}
+	return nil
+}
diff --git a/internal/storage/jsonl_writer.go b/internal/storage/jsonl_writer.go
new file mode 100644
index 0000000..46a52ac
--- /dev/null
+++ b/internal/storage/jsonl_writer.go
@@ -0,0 +1,81 @@
+package storage
+
+import (
+	"bufio"
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"time"
+)
+
+// JSONLWriter writes records as line-delimited JSON and rotates files by size.
+type JSONLWriter struct {
+	keyword  string
+	outDir   string
+	file     *os.File
+	writer   *bufio.Writer
+	maxBytes int64
+	written  int64
+	part     int
+}
+
+// NewJSONLWriter creates a writer for a keyword in outDir. maxBytes triggers rotation.
+func NewJSONLWriter(keyword, outDir string, maxBytes int64) (*JSONLWriter, error) {
+	w := &JSONLWriter{keyword: keyword, outDir: outDir, maxBytes: maxBytes}
+	if err := w.rotate(); err != nil {
+		return nil, err
+	}
+	return w, nil
+}
+
+func (w *JSONLWriter) rotate() error {
+	if w.writer != nil {
+		w.writer.Flush()
+	}
+	if w.file != nil {
+		w.file.Close()
+	}
+	w.part++
+	ts := time.Now().UTC().Format("20060102T150405Z")
+	name := fmt.Sprintf("%s_%s_part%d.jsonl", w.keyword, ts, w.part)
+	path := filepath.Join(w.outDir, name)
+	f, err := os.Create(path)
+	if err != nil {
+		return err
+	}
+	w.file = f
+	w.writer = bufio.NewWriterSize(f, 16*1024)
+	w.written = 0
+	return nil
+}
+
+// Write marshals v to JSON and writes it as a newline-delimited record.
+func (w *JSONLWriter) Write(v interface{}) error {
+	b, err := json.Marshal(v)
+	if err != nil {
+		return err
+	}
+	n, err := w.writer.Write(append(b, '\n'))
+	if err != nil {
+		return err
+	}
+	w.written += int64(n)
+	if w.maxBytes > 0 && w.written >= w.maxBytes {
+		return w.rotate()
+	}
+	return nil
+}
+
+// Close flushes and closes the underlying file.
+func (w *JSONLWriter) Close() error {
+	if w.writer != nil {
+		if err := w.writer.Flush(); err != nil {
+			return err
+		}
+	}
+	if w.file != nil {
+		return w.file.Close()
+	}
+	return nil
+}