diff --git a/src/lib/__tests__/ytdlp.test.ts b/src/lib/__tests__/ytdlp.test.ts
new file mode 100644
index 0000000..dfa4530
--- /dev/null
+++ b/src/lib/__tests__/ytdlp.test.ts
@@ -0,0 +1,63 @@
+import { buildYtdlpArgs } from '../ytdlp'
+
+const base = {
+  url: 'https://www.youtube.com/watch?v=dQw4w9WgXcQ',
+  uuid: 'abc-123',
+  format: 'mp4',
+  quality: 'best',
+  subtitles: false,
+  extraArgs: null,
+}
+
+describe('buildYtdlpArgs', () => {
+  it('always includes --no-playlist', () => {
+    expect(buildYtdlpArgs(base)).toContain('--no-playlist')
+  })
+
+  it('sets output path with uuid and %(ext)s template', () => {
+    const args = buildYtdlpArgs(base)
+    const idx = args.indexOf('-o')
+    expect(idx).toBeGreaterThan(-1)
+    expect(args[idx + 1]).toContain('abc-123')
+    expect(args[idx + 1]).toContain('%(ext)s')
+  })
+
+  it('adds format filter when quality is not "best"', () => {
+    const args = buildYtdlpArgs({ ...base, quality: '1080p' })
+    expect(args).toContain('-f')
+  })
+
+  it('does not add -f flag when quality is "best"', () => {
+    expect(buildYtdlpArgs(base)).not.toContain('-f')
+  })
+
+  it('adds subtitle flags when subtitles is true', () => {
+    const args = buildYtdlpArgs({ ...base, subtitles: true })
+    expect(args).toContain('--write-sub')
+    expect(args).toContain('--sub-lang')
+  })
+
+  it('does not add subtitle flags when subtitles is false', () => {
+    expect(buildYtdlpArgs(base)).not.toContain('--write-sub')
+  })
+
+  it('appends extra args from a JSON array string', () => {
+    const args = buildYtdlpArgs({
+      ...base,
+      extraArgs: '["--sponsorblock-remove","all"]',
+    })
+    expect(args).toContain('--sponsorblock-remove')
+    expect(args).toContain('all')
+  })
+
+  it('ignores malformed extraArgs JSON', () => {
+    expect(() =>
+      buildYtdlpArgs({ ...base, extraArgs: 'not-json' })
+    ).not.toThrow()
+  })
+
+  it('url is the last argument', () => {
+    const args = buildYtdlpArgs(base)
+    expect(args[args.length - 1]).toBe(base.url)
+  })
+})
diff --git a/src/lib/ytdlp.ts b/src/lib/ytdlp.ts
new file mode 100644
index 0000000..ab56be8
--- /dev/null
+++ b/src/lib/ytdlp.ts
@@ -0,0 +1,39 @@
+import path from 'path'
+import { config } from '../../config/app.config'
+
+export type YtdlpParams = {
+  url: string
+  uuid: string
+  format: string
+  quality: string
+  subtitles: boolean
+  extraArgs: string | null
+}
+
+export function buildYtdlpArgs(params: YtdlpParams): string[] {
+  const { url, uuid, format, quality, subtitles, extraArgs } = params
+  const outputTemplate = path.join(config.STORAGE_PATH, `${uuid}.%(ext)s`)
+
+  const args: string[] = ['--no-playlist', '-o', outputTemplate]
+
+  if (quality !== 'best') {
+    const height = quality.replace('p', '')
+    args.push('-f', `${format}[height<=?${height}]+bestaudio/best[height<=?${height}]`)
+  }
+
+  if (subtitles) {
+    args.push('--write-sub', '--sub-lang', 'fr,en')
+  }
+
+  if (extraArgs) {
+    try {
+      const extra = JSON.parse(extraArgs) as string[]
+      args.push(...extra)
+    } catch {
+      // malformed extraArgs — skip silently
+    }
+  }
+
+  args.push(url)
+  return args
+}