diff --git a/src/core/project.ts b/src/core/project.ts
new file mode 100644
index 0000000..1fdbe91
--- /dev/null
+++ b/src/core/project.ts
@@ -0,0 +1,59 @@
+import type { PierInput } from './types';
+
+/** Project file schema (spec §8). Bump only with a migration. */
+export const SCHEMA_VERSION = 1;
+
+export interface ProjectInfo {
+  name: string;
+  bridge: string;
+  client: string;
+  author: string;
+  crs: string;
+  memo: string;
+}
+
+export interface ProjectPier {
+  id: string;
+  /** Input as read from the spreadsheet. The 3D model is not stored; it is regenerated on open (spec §8). */
+  input: PierInput;
+}
+
+export interface ProjectFile {
+  schemaVersion: number;
+  project: ProjectInfo;
+  source: { fileName: string; sheets: string[] };
+  piers: ProjectPier[];
+  history: { version: string; note: string; at: string }[];
+}
+
+/** Serialises a project to the pretty-printed JSON stored in <교량명>_교각설계.pzr.json. */
+export function serializeProject(p: ProjectFile): string {
+  return `${JSON.stringify(p, null, 2)}\n`;
+}
+
+/**
+ * Parses and checks a project file. Throws with a message that names the first problem, so a damaged file is
+ * refused instead of being half-loaded.
+ */
+export function parseProject(text: string): ProjectFile {
+  let raw: unknown;
+  try {
+    raw = JSON.parse(text);
+  } catch {
+    throw new Error('프로젝트 파일이 JSON으로 읽히지 않습니다.');
+  }
+  const obj = raw as Partial<ProjectFile>;
+  if (!obj || typeof obj !== 'object') throw new Error('프로젝트 파일 형식이 아닙니다.');
+  if (obj.schemaVersion !== SCHEMA_VERSION) {
+    throw new Error(`지원하지 않는 파일 버전입니다 (schemaVersion ${String(obj.schemaVersion)}, 지원: ${SCHEMA_VERSION}).`);
+  }
+  if (!Array.isArray(obj.piers)) throw new Error('piers 목록이 없습니다.');
+  obj.piers.forEach((p, i) => {
+    if (!p || typeof p.id !== 'string' || !p.input) throw new Error(`교각 ${i + 1}: id 또는 input이 없습니다.`);
+    const input = p.input;
+    if (typeof input.name !== 'string' || !Array.isArray(input.ips) || !Array.isArray(input.bearings)) {
+      throw new Error(`교각 ${i + 1}(${String(input.name)}): 입력 형식이 맞지 않습니다.`);
+    }
+  });
+  return obj as ProjectFile;
+}
diff --git a/tests/project.test.ts b/tests/project.test.ts
new file mode 100644
index 0000000..3715474
--- /dev/null
+++ b/tests/project.test.ts
@@ -0,0 +1,49 @@
+import { describe, expect, it } from 'vitest';
+import { buildPier } from '../src/core/geometry';
+import { parseProject, serializeProject, SCHEMA_VERSION, type ProjectFile } from '../src/core/project';
+import { loadP1 } from './helpers';
+
+function sampleProject(): ProjectFile {
+  const p1 = loadP1();
+  // 20 piers: P1 plus 19 variants with different names and coping widths, so the file has a realistic size.
+  const piers = Array.from({ length: 20 }, (_, i) => {
+    const input = i === 0 ? p1 : { ...p1, name: `P${i + 1}`, copingWidthMm: 3100 + i * 10 };
+    return { id: `pier-${i + 1}`, input };
+  });
+  return {
+    schemaVersion: SCHEMA_VERSION,
+    project: { name: '운곡교 교각', bridge: '운곡교', client: '', author: '', crs: 'KATEC', memo: '' },
+    source: { fileName: '운곡교P1입력.xlsx', sheets: ['교각', 'IP', '받침'] },
+    piers,
+    history: [{ version: '0.1.00', note: '최초 저장', at: '2026-10-22T00:00:00Z' }],
+  };
+}
+
+describe('project file round trip (spec §8, plan B3)', () => {
+  it('save → open gives the same inputs, without the source workbook', () => {
+    const original = sampleProject();
+    const text = serializeProject(original);
+    const reopened = parseProject(text);
+    expect(reopened).toEqual(original);
+  });
+
+  it('regenerated models match the originals after reopening (20 piers)', () => {
+    const original = sampleProject();
+    const reopened = parseProject(serializeProject(original));
+    original.piers.forEach((p, i) => {
+      expect(buildPier(reopened.piers[i].input)).toEqual(buildPier(p.input));
+    });
+  });
+
+  it('refuses another schema version instead of loading it', () => {
+    const text = serializeProject({ ...sampleProject(), schemaVersion: 99 });
+    expect(() => parseProject(text)).toThrow('지원하지 않는 파일 버전');
+  });
+
+  it('refuses a damaged file and names the broken pier', () => {
+    const project = sampleProject();
+    const broken = { ...project, piers: [project.piers[0], { id: 'pier-2', input: null }] };
+    expect(() => parseProject(JSON.stringify(broken))).toThrow('교각 2');
+    expect(() => parseProject('{ not json')).toThrow('JSON');
+  });
+});