diff --git a/apps/desktop/package.json b/apps/desktop/package.json
index 7ea06d6..a484f91 100644
--- a/apps/desktop/package.json
+++ b/apps/desktop/package.json
@@ -11,6 +11,7 @@
     "build": "electron-vite build",
     "start": "electron-vite preview",
     "typecheck": "tsc --noEmit",
+    "test": "vitest run",
     "e2e": "electron-vite build && playwright test",
     "e2e:only": "playwright test",
     "e2e:report": "playwright show-report e2e/.artifacts/report",
diff --git a/apps/desktop/src/main/snapshot-store.test.ts b/apps/desktop/src/main/snapshot-store.test.ts
new file mode 100644
index 0000000..2bd7c26
--- /dev/null
+++ b/apps/desktop/src/main/snapshot-store.test.ts
@@ -0,0 +1,86 @@
+/**
+ * Purity / rebuild guarantee for the *shipped* durable cache (#30, D4). The app's
+ * on-disk reconcile cache is this JSON snapshot-store. The invariant: delete it
+ * and lose nothing — the durable truth is in gitea, the file is only a boot/offline
+ * mirror. This test deletes the real file and asserts the store degrades to
+ * "no cache" (null), which is what forces the next `getSnapshot` to reconcile
+ * fresh from gitea rather than serve stale or missing data.
+ */
+import { existsSync, mkdtempSync, rmSync, writeFileSync } from 'node:fs'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+
+import { afterEach, describe, expect, it, vi } from 'vitest'
+
+// electron can't be imported outside the Electron runtime; the store only needs
+// app.getPath for its default path, which every test overrides with an injected path.
+vi.mock('electron', () => ({ app: { getPath: () => tmpdir() } }))
+
+import { loadSnapshot, type PersistedSnapshot, saveSnapshot } from './snapshot-store.js'
+
+const SNAP: Omit<PersistedSnapshot, 'savedAt'> = {
+  issues: [{ number: 1, title: 'An issue', state: 'open', labels: ['est/5d', 'p/1'] }],
+  milestones: [{ id: 7, title: 'P2' }],
+  deps: [{ issue: 1, dependsOn: 2 }],
+  timelines: { 1: [{ type: 'opened', at: '2026-07-01T00:00:00Z' }] },
+}
+
+describe('snapshot-store purity (#30)', () => {
+  let dir: string | null = null
+  const path = () => {
+    if (!dir) dir = mkdtempSync(join(tmpdir(), 'commitea-snap-'))
+    return join(dir, 'commitea-snapshot.json')
+  }
+  afterEach(() => {
+    if (dir) rmSync(dir, { recursive: true, force: true })
+    dir = null
+  })
+
+  it('round-trips a saved snapshot', () => {
+    const p = path()
+    saveSnapshot(SNAP, '2026-07-09T00:00:00Z', p)
+    const back = loadSnapshot(p)
+    expect(back).toEqual({ ...SNAP, savedAt: '2026-07-09T00:00:00Z' })
+  })
+
+  it('delete the cache file → load returns null (degrades to no-cache, not a crash)', () => {
+    const p = path()
+    saveSnapshot(SNAP, '2026-07-09T00:00:00Z', p)
+    expect(loadSnapshot(p)).not.toBeNull()
+
+    rmSync(p) // delete the durable cache
+    expect(existsSync(p)).toBe(false)
+
+    // The store must NOT throw and must report "no cache" so the next reconcile
+    // rebuilds from gitea. If this ever returned stale data or threw, D4 breaks.
+    expect(loadSnapshot(p)).toBeNull()
+  })
+
+  it('a corrupt / partial file is treated as no-cache, never a crash', () => {
+    const p = path()
+    writeFileSync(p, '{ this is not json', 'utf8')
+    expect(loadSnapshot(p)).toBeNull()
+    // valid JSON but wrong shape (no issues array) is also rejected
+    writeFileSync(p, JSON.stringify({ savedAt: 'x' }), 'utf8')
+    expect(loadSnapshot(p)).toBeNull()
+  })
+
+  it('re-saving after a delete restores the cache — the rebuild is lossless', () => {
+    const p = path()
+    saveSnapshot(SNAP, '2026-07-09T00:00:00Z', p)
+    const before = loadSnapshot(p)
+
+    rmSync(p)
+    expect(loadSnapshot(p)).toBeNull()
+
+    // a resync would call saveSnapshot again with the freshly reconciled data
+    saveSnapshot(SNAP, '2026-07-09T01:00:00Z', p)
+    const after = loadSnapshot(p)
+
+    // same durable payload, only the savedAt marker differs
+    expect(after!.issues).toEqual(before!.issues)
+    expect(after!.milestones).toEqual(before!.milestones)
+    expect(after!.deps).toEqual(before!.deps)
+    expect(after!.timelines).toEqual(before!.timelines)
+  })
+})
diff --git a/apps/desktop/src/main/snapshot-store.ts b/apps/desktop/src/main/snapshot-store.ts
index 2f79e39..2a2ed93 100644
--- a/apps/desktop/src/main/snapshot-store.ts
+++ b/apps/desktop/src/main/snapshot-store.ts
@@ -26,10 +26,15 @@ function snapshotPath(): string {
   return join(app.getPath('userData'), 'commitea-snapshot.json')
 }
 
-/** Load the last persisted snapshot, or null if absent/corrupt. Never throws. */
-export function loadSnapshot(): PersistedSnapshot | null {
+/**
+ * Load the last persisted snapshot, or null if absent/corrupt. Never throws — a
+ * deleted or unreadable cache degrades to "no cache" (the purity guarantee, D4:
+ * the next reconcile rebuilds it from gitea). `path` is injectable for tests;
+ * production always uses the userData file.
+ */
+export function loadSnapshot(path: string = snapshotPath()): PersistedSnapshot | null {
   try {
-    const parsed = JSON.parse(readFileSync(snapshotPath(), 'utf8')) as PersistedSnapshot
+    const parsed = JSON.parse(readFileSync(path, 'utf8')) as PersistedSnapshot
     if (parsed && Array.isArray(parsed.issues)) return parsed
     return null
   } catch {
@@ -38,9 +43,9 @@ export function loadSnapshot(): PersistedSnapshot | null {
 }
 
 /** Persist a freshly reconciled snapshot. Best-effort — a write failure never breaks a reconcile. */
-export function saveSnapshot(snap: Omit<PersistedSnapshot, 'savedAt'>, savedAt: string): void {
+export function saveSnapshot(snap: Omit<PersistedSnapshot, 'savedAt'>, savedAt: string, path: string = snapshotPath()): void {
   try {
-    writeFileSync(snapshotPath(), JSON.stringify({ ...snap, savedAt }), 'utf8')
+    writeFileSync(path, JSON.stringify({ ...snap, savedAt }), 'utf8')
   } catch {
     // disk full / permissions — the in-memory cache still works this session
   }
diff --git a/apps/desktop/vitest.config.ts b/apps/desktop/vitest.config.ts
new file mode 100644
index 0000000..2879a57
--- /dev/null
+++ b/apps/desktop/vitest.config.ts
@@ -0,0 +1,14 @@
+import { defineConfig } from 'vitest/config'
+
+/**
+ * Unit tests for the Electron main process. Node environment only — renderer
+ * (React) is covered by the Playwright e2e suite, not here. `electron` is a
+ * native module that can't be imported outside the Electron runtime, so tests
+ * that touch it mock it (see snapshot-store.test.ts).
+ */
+export default defineConfig({
+  test: {
+    environment: 'node',
+    include: ['src/main/**/*.test.ts'],
+  },
+})
diff --git a/packages/core/src/cache/cache-purity-v0.test.ts b/packages/core/src/cache/cache-purity-v0.test.ts
new file mode 100644
index 0000000..5600b31
--- /dev/null
+++ b/packages/core/src/cache/cache-purity-v0.test.ts
@@ -0,0 +1,191 @@
+/**
+ * Purity / rebuild guarantee (#30, D4). The SQLite cache is a *rebuildable index*
+ * over the durable truth in gitea — never a source of truth. This test enforces
+ * that invariant the only way that matters: nuke the database file on disk and
+ * rebuild it from the same gitea snapshot, then assert nothing was lost.
+ *
+ * If a future change ever stores a fact that lives only in the cache (a
+ * user-authored note, a locally-computed field that isn't re-derived from the
+ * label set), the delete→resync round-trip would drop it and this test fails.
+ * That is the whole point: the cache must survive being deleted.
+ *
+ * Uses a real on-disk SQLite file via node:sqlite (present in the test runtime;
+ * main binds better-sqlite3 to the same CacheDriver seam), so "delete the SQLite
+ * file" is literal `rm`, not a metaphor.
+ */
+import { DatabaseSync } from 'node:sqlite'
+import { mkdtempSync, existsSync, rmSync } from 'node:fs'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+
+import { afterEach, describe, expect, it } from 'vitest'
+
+import type { GiteaIssue } from '../gitea/types.js'
+import { extractLabelFacts } from '../labels/label-schema.js'
+import { type CacheDriver, initCache, readIssue, upsertIssue } from './cache-v0.js'
+
+/** Adapt a file-backed node:sqlite handle to the CacheDriver seam, returning the path too. */
+function fileDriver(path: string): { driver: CacheDriver; close: () => void } {
+  const db = new DatabaseSync(path)
+  const driver: CacheDriver = {
+    exec: (sql) => db.exec(sql),
+    run: (sql, params = []) => {
+      db.prepare(sql).run(...(params as never[]))
+    },
+    get: (sql, params = []) => db.prepare(sql).get(...(params as never[])) as Record<string, unknown> | undefined,
+    all: (sql, params = []) => db.prepare(sql).all(...(params as never[])) as Record<string, unknown>[],
+  }
+  return { driver, close: () => db.close() }
+}
+
+function issue(over: Partial<GiteaIssue> = {}): GiteaIssue {
+  const labels = over.labels ?? ['est/5d', 'p/1']
+  return {
+    number: 1,
+    title: 'An issue',
+    body: 'body',
+    state: 'open',
+    labels,
+    facts: extractLabelFacts(labels),
+    milestone: null,
+    assignee: 'christian',
+    assignees: ['christian'],
+    createdAt: '2026-07-01T00:00:00Z',
+    updatedAt: '2026-07-02T00:00:00Z',
+    closedAt: null,
+    url: 'https://gitea/christian/commitea/issues/1',
+    ...over,
+  }
+}
+
+/** A representative reconciled backlog: open + closed, milestones, reassignment, no-milestone, empty labels. */
+const GITEA_TRUTH: GiteaIssue[] = [
+  issue({
+    number: 42,
+    title: 'Monte Carlo engine',
+    labels: ['est/8d', 'p/1', 'deadline/hard'],
+    milestone: { id: 7, title: 'P2 — Scheduler', dueOn: '2026-09-01T00:00:00Z' },
+    assignee: 'christian',
+    assignees: ['christian'],
+  }),
+  issue({
+    number: 43,
+    title: 'Calibration honesty',
+    labels: ['est/3d', 'p/2'],
+    state: 'closed',
+    closedAt: '2026-07-05T00:00:00Z',
+    milestone: { id: 7, title: 'P2 — Scheduler', dueOn: '2026-09-01T00:00:00Z' },
+    assignee: 'stephen',
+    assignees: ['stephen'],
+  }),
+  issue({ number: 44, title: 'No milestone, no labels', labels: [], milestone: null, assignee: null, assignees: [] }),
+]
+
+/** Rebuild a cache from scratch out of the gitea snapshot — exactly what a resync does. */
+function rebuildFrom(driver: CacheDriver, truth: GiteaIssue[]): void {
+  initCache(driver)
+  for (const i of truth) upsertIssue(driver, i)
+}
+
+/** The complete durable state we can read back — every issue, fully re-derived. */
+function readAll(driver: CacheDriver, truth: GiteaIssue[]): (GiteaIssue | null)[] {
+  return truth.map((i) => readIssue(driver, i.number))
+}
+
+describe('cache purity / rebuild (#30)', () => {
+  let dir: string | null = null
+  afterEach(() => {
+    if (dir) rmSync(dir, { recursive: true, force: true })
+    dir = null
+  })
+
+  it('delete the SQLite file → resync → no durable truth is lost', () => {
+    dir = mkdtempSync(join(tmpdir(), 'commitea-cache-'))
+    const dbPath = join(dir, 'cache.sqlite')
+
+    // 1. Build the cache from gitea and capture everything we can read back.
+    const first = fileDriver(dbPath)
+    rebuildFrom(first.driver, GITEA_TRUTH)
+    const before = readAll(first.driver, GITEA_TRUTH)
+    first.close()
+    expect(existsSync(dbPath)).toBe(true)
+    // sanity: the snapshot actually holds derived facts, not just rows
+    expect(before[0]!.facts).toEqual(extractLabelFacts(['est/8d', 'p/1', 'deadline/hard']))
+    expect(before[0]!.facts.estimateDays).toBe(8)
+
+    // 2. Delete the SQLite file. This is the durable cache, gone.
+    rmSync(dbPath)
+    // node:sqlite also drops a -journal/-wal sidecar in some modes; clear the dir of any residue.
+    expect(existsSync(dbPath)).toBe(false)
+
+    // 3. Resync: a brand-new empty DB rebuilt from the *same* gitea snapshot.
+    const second = fileDriver(dbPath)
+    rebuildFrom(second.driver, GITEA_TRUTH)
+    const after = readAll(second.driver, GITEA_TRUTH)
+    second.close()
+
+    // 4. Nothing was lost — the rebuilt cache is identical, field for field.
+    expect(after).toEqual(before)
+  })
+
+  it('a re-reconcile that changes gitea updates in place — the rebuild reflects truth, never stale rows', () => {
+    dir = mkdtempSync(join(tmpdir(), 'commitea-cache-'))
+    const dbPath = join(dir, 'cache.sqlite')
+
+    const first = fileDriver(dbPath)
+    rebuildFrom(first.driver, GITEA_TRUTH)
+    first.close()
+
+    // gitea moved on: #42 re-estimated + reassigned, #43 reopened.
+    const NEW_TRUTH: GiteaIssue[] = [
+      issue({ ...GITEA_TRUTH[0], labels: ['est/2d', 'p/3'], assignee: 'stephen', assignees: ['stephen'] }),
+      issue({ ...GITEA_TRUTH[1], state: 'open', closedAt: null }),
+      GITEA_TRUTH[2],
+    ]
+
+    // resync over the existing file (upsert-by-number), not a fresh DB
+    const second = fileDriver(dbPath)
+    initCache(second.driver)
+    for (const i of NEW_TRUTH) upsertIssue(second.driver, i)
+    const rowCount = second.driver.all('SELECT number FROM issues')
+    const back42 = readIssue(second.driver, 42)!
+    const back43 = readIssue(second.driver, 43)!
+    second.close()
+
+    expect(rowCount).toHaveLength(3) // upsert in place — no duplicate rows accreted across reconciles
+    expect(back42.facts).toEqual(extractLabelFacts(['est/2d', 'p/3']))
+    expect(back42.facts.estimateDays).toBe(2)
+    expect(back42.assignee).toBe('stephen')
+    expect(back43.state).toBe('open')
+    expect(back43.closedAt).toBeNull()
+  })
+
+  it('the cache stores no column that is not re-derivable from gitea (structural D4 guard)', () => {
+    dir = mkdtempSync(join(tmpdir(), 'commitea-cache-'))
+    const dbPath = join(dir, 'cache.sqlite')
+    const { driver, close } = fileDriver(dbPath)
+    initCache(driver)
+
+    // Every issues-table column must map to a field carried on the gitea issue
+    // (or be a re-derivable mirror of one). If someone adds a user-authored
+    // column, it won't be in this allow-list and this guard fails — forcing a
+    // deliberate decision about durability instead of silently breaking D4.
+    const cols = driver.all('PRAGMA table_info(issues)').map((r) => r.name as string)
+    const FROM_GITEA = new Set([
+      'number',
+      'title',
+      'body',
+      'state',
+      'labels', // facts are re-derived from this on read, never stored
+      'milestone_id',
+      'assignee',
+      'assignees',
+      'created_at',
+      'updated_at',
+      'closed_at',
+      'url',
+    ])
+    close()
+    expect(cols.filter((c) => !FROM_GITEA.has(c))).toEqual([])
+  })
+})