diff --git a/src/__tests__/chart-document.test.ts b/src/__tests__/chart-document.test.ts
new file mode 100644
index 0000000..9f773a0
--- /dev/null
+++ b/src/__tests__/chart-document.test.ts
@@ -0,0 +1,142 @@
+/**
+ * Round-trip tests for writeChartFolder.
+ *
+ * Tests go through parseChartAndIni on the folder output: build a
+ * ParsedChart (+ optional assets), write to a file list, re-parse,
+ * and assert on the resulting ParsedChart. No assertions about file
+ * order, raw ini text contents, or which filename the writer chose —
+ * those are implementation details of the orchestrator.
+ */
+
+import { describe, expect, it } from 'vitest'
+
+import { createEmptyChart } from '../chart/create-chart'
+import { writeChartFolder } from '../chart/chart-document'
+import { parseChartAndIni, type ParsedChart } from '../chart/parse-chart-and-ini'
+
+function roundTrip(
+	chart: ParsedChart,
+	assets: { fileName: string; data: Uint8Array }[] = [],
+): { parsedChart: ParsedChart; files: { fileName: string; data: Uint8Array }[] } {
+	const files = writeChartFolder({ parsedChart: chart, assets })
+	const result = parseChartAndIni(files)
+	if (!result.parsedChart) {
+		throw new Error(`round-trip produced no parsedChart: ${JSON.stringify(result.chartFolderIssues)}`)
+	}
+	return { parsedChart: result.parsedChart, files }
+}
+
+describe('writeChartFolder round-trip: format selection', () => {
+	it('round-trips a .chart document to format="chart"', () => {
+		const chart = createEmptyChart({ format: 'chart' })
+		chart.metadata.name = 'Test Song'
+		const re = roundTrip(chart).parsedChart
+		expect(re.format).toBe('chart')
+	})
+
+	it('round-trips a .mid document to format="mid"', () => {
+		const chart = createEmptyChart({ format: 'mid' })
+		chart.metadata.name = 'Test Song'
+		const re = roundTrip(chart).parsedChart
+		expect(re.format).toBe('mid')
+	})
+})
+
+describe('writeChartFolder round-trip: ini content', () => {
+	it('preserves basic string metadata', () => {
+		const chart = createEmptyChart({ format: 'chart' })
+		chart.metadata.name = 'My Song'
+		chart.metadata.artist = 'My Artist'
+		const re = roundTrip(chart).parsedChart
+		expect(re.metadata.name).toBe('My Song')
+		expect(re.metadata.artist).toBe('My Artist')
+	})
+
+	it('preserves extraIniFields for unknown ini keys', () => {
+		const chart = createEmptyChart({ format: 'chart' })
+		chart.metadata.extraIniFields = { custom_field: 'custom_value' }
+		const re = roundTrip(chart).parsedChart
+		expect(re.metadata.extraIniFields).toMatchObject({ custom_field: 'custom_value' })
+	})
+
+	it('does not round-trip chart_offset via the ini path (it belongs in [Song])', () => {
+		// chart_offset is a [Song]-only field. Writing a .chart document with
+		// chart_offset must round-trip it intact, and the ini must NOT carry a
+		// competing value that would confuse the parse.
+		const chart = createEmptyChart({ format: 'chart' })
+		chart.metadata.chart_offset = 250
+		const re = roundTrip(chart).parsedChart
+		expect(re.metadata.chart_offset).toBe(250)
+	})
+})
+
+describe('writeChartFolder round-trip: assets passthrough', () => {
+	it('passes audio/image assets through verbatim', () => {
+		const chart = createEmptyChart({ format: 'chart' })
+		const ogg = new Uint8Array([0x4f, 0x67, 0x67, 0x53])
+		const png = new Uint8Array([0x89, 0x50, 0x4e, 0x47])
+		const { files } = roundTrip(chart, [
+			{ fileName: 'song.ogg', data: ogg },
+			{ fileName: 'album.png', data: png },
+		])
+		const oggFile = files.find(f => f.fileName === 'song.ogg')
+		const pngFile = files.find(f => f.fileName === 'album.png')
+		expect(oggFile?.data).toBe(ogg)
+		expect(pngFile?.data).toBe(png)
+	})
+})
+
+describe('writeChartFolder round-trip: complete folder', () => {
+	it('.chart folder preserves metadata and chart identity', () => {
+		const chart = createEmptyChart({ format: 'chart', resolution: 480 })
+		chart.metadata.name = 'Round Trip'
+		chart.metadata.artist = 'Tester'
+		chart.metadata.pro_drums = true
+		const re = roundTrip(chart).parsedChart
+		expect(re.format).toBe('chart')
+		expect(re.resolution).toBe(480)
+		expect(re.metadata.name).toBe('Round Trip')
+		expect(re.metadata.artist).toBe('Tester')
+		expect(re.metadata.pro_drums).toBe(true)
+	})
+
+	it('.mid folder preserves metadata and passes assets through', () => {
+		const chart = createEmptyChart({ format: 'mid', resolution: 480 })
+		chart.metadata.name = 'Midi Song'
+		const ogg = new Uint8Array([0x4f, 0x67, 0x67, 0x53, 0x00])
+		const { parsedChart, files } = roundTrip(chart, [{ fileName: 'song.ogg', data: ogg }])
+		expect(parsedChart.format).toBe('mid')
+		expect(parsedChart.metadata.name).toBe('Midi Song')
+		expect(files.find(f => f.fileName === 'song.ogg')?.data).toBe(ogg)
+	})
+
+	it('overwrites stale notes.chart / notes.mid / song.ini entries in assets', () => {
+		// Common caller pattern: pass the full file list from parseChartAndIni as
+		// `assets` without filtering. The writer should emit its own fresh chart
+		// + ini and drop the stale ones — no duplicates, no cross-format leaks.
+		const chart = createEmptyChart({ format: 'chart' })
+		chart.metadata.name = 'Fresh'
+		const staleChart = new TextEncoder().encode('[Song]\r\n{\r\n  Name = "Stale"\r\n}\r\n')
+		const staleMid = new Uint8Array([0x4d, 0x54, 0x68, 0x64]) // 'MThd'
+		const staleIni = new TextEncoder().encode('[song]\r\nname = Stale\r\n')
+		const cover = new Uint8Array([0x89, 0x50, 0x4e, 0x47])
+		const { parsedChart, files } = roundTrip(chart, [
+			{ fileName: 'notes.chart', data: staleChart },
+			{ fileName: 'NOTES.MID', data: staleMid }, // case-insensitive
+			{ fileName: 'song.ini', data: staleIni },
+			{ fileName: 'album.png', data: cover },
+		])
+		// Exactly one of each chart file survives — the freshly-serialized one.
+		const chartFiles = files.filter(f => /^notes\.(chart|mid)$/i.test(f.fileName))
+		const iniFiles = files.filter(f => f.fileName.toLowerCase() === 'song.ini')
+		expect(chartFiles).toHaveLength(1)
+		expect(chartFiles[0].fileName).toBe('notes.chart')
+		expect(chartFiles[0].data).not.toBe(staleChart)
+		expect(iniFiles).toHaveLength(1)
+		expect(iniFiles[0].data).not.toBe(staleIni)
+		// Non-chart assets untouched.
+		expect(files.find(f => f.fileName === 'album.png')?.data).toBe(cover)
+		// Re-parse sees the fresh metadata, not the stale.
+		expect(parsedChart.metadata.name).toBe('Fresh')
+	})
+})
diff --git a/src/chart/chart-document.ts b/src/chart/chart-document.ts
new file mode 100644
index 0000000..c4e3953
--- /dev/null
+++ b/src/chart/chart-document.ts
@@ -0,0 +1,75 @@
+/**
+ * `ChartDocument` bundles a {@link ParsedChart} with the non-chart files
+ * (audio, album art, videos, any unrecognized files) from the source folder.
+ *
+ * This is the counterpart shape to `parseChartAndIni`'s input: in → raw file
+ * list, out → `ChartDocument`; writing the reverse: `ChartDocument` → raw
+ * file list via `writeChartFolder`.
+ *
+ * Metadata lives on `parsedChart.metadata` (the consolidated shape from
+ * `parseChartAndIni`) — there is no separate `metadata` field on
+ * `ChartDocument`, by design.
+ */
+
+import { writeChartFile } from './chart-writer'
+import { writeMidiFile } from './midi-writer'
+import type { ParsedChart } from './parse-chart-and-ini'
+import { writeIniFile } from '../ini/ini-writer'
+import type { File } from '../interfaces'
+
+export interface ChartDocument {
+	/** The parsed chart data. `parsedChart.metadata` carries ini fields. */
+	parsedChart: ParsedChart
+	/** Non-chart / non-ini files from the source folder — passed through verbatim on write. */
+	assets: File[]
+}
+
+/**
+ * Serialize a {@link ChartDocument} back to a flat list of files suitable for
+ * writing to disk or packaging into a zip/sng.
+ *
+ * Output:
+ *   - `notes.chart` or `notes.mid`, depending on `parsedChart.format`
+ *   - `song.ini` (from `parsedChart.metadata`)
+ *   - every entry in `doc.assets` that isn't already a chart or ini file, in order
+ *
+ * If `doc.assets` contains its own `notes.chart` / `notes.mid` / `song.ini`
+ * entries (e.g. because the caller kept the full file list from
+ * `parseChartAndIni` as-is), those entries are dropped — the fresh
+ * serialized versions win. Comparisons are case-insensitive to match the
+ * parser's file lookup.
+ */
+const CHART_FILE_NAMES = new Set(['notes.chart', 'notes.mid', 'song.ini'])
+
+function isChartLikeFile(fileName: string): boolean {
+	return CHART_FILE_NAMES.has(fileName.toLowerCase())
+}
+
+export function writeChartFolder(doc: ChartDocument): File[] {
+	const encoder = new TextEncoder()
+	const out: File[] = []
+
+	if (doc.parsedChart.format === 'chart') {
+		out.push({
+			fileName: 'notes.chart',
+			data: encoder.encode(writeChartFile(doc.parsedChart)),
+		})
+	} else {
+		out.push({
+			fileName: 'notes.mid',
+			data: writeMidiFile(doc.parsedChart),
+		})
+	}
+
+	out.push({
+		fileName: 'song.ini',
+		data: encoder.encode(writeIniFile(doc.parsedChart.metadata)),
+	})
+
+	for (const asset of doc.assets) {
+		if (isChartLikeFile(asset.fileName)) continue
+		out.push(asset)
+	}
+
+	return out
+}
diff --git a/src/chart/index.ts b/src/chart/index.ts
index f7761fe..c6f9c59 100644
--- a/src/chart/index.ts
+++ b/src/chart/index.ts
@@ -1,3 +1,4 @@
+export * from './chart-document'
 export * from './chart-scanner'
 export * from './chart-writer'
 export * from './create-chart'
diff --git a/src/index.ts b/src/index.ts
index c10c441..f87587a 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -12,8 +12,8 @@ import { scanVideo } from './video'
 export * from './interfaces'
 export * from './chart/note-parsing-interfaces'
 export { parseChartFile } from './chart/notes-parser'
-export { parseChartAndIni, createEmptyChart } from './chart'
-export type { ParsedChart, ParseChartAndIniResult } from './chart'
+export { parseChartAndIni, createEmptyChart, writeChartFolder } from './chart'
+export type { ParsedChart, ParseChartAndIniResult, ChartDocument } from './chart'
 export { scanIni } from './ini'
 export type { IniMetadata } from './ini'
 export { calculateTrackHash } from './chart/track-hasher'