diff --git a/src/save/checksum.test.ts b/src/save/checksum.test.ts
new file mode 100644
index 0000000..20f53d4
--- /dev/null
+++ b/src/save/checksum.test.ts
@@ -0,0 +1,35 @@
+import { describe, it, expect } from 'vitest';
+import { crc32hex, canonicalJSON } from './checksum';
+
+// Tests for the pure-function save core: deterministic CRC-32 + canonical JSON.
+// Both functions are load-bearing for envelope checksums (see envelope.test.ts).
+
+describe('crc32hex', () => {
+  it('is deterministic — same input always returns same output', () => {
+    expect(crc32hex('hello')).toBe(crc32hex('hello'));
+  });
+
+  it('returns 8-char lowercase hex', () => {
+    expect(crc32hex('hello')).toMatch(/^[0-9a-f]{8}$/);
+  });
+
+  it('differs for different inputs', () => {
+    expect(crc32hex('hello')).not.toBe(crc32hex('world'));
+  });
+});
+
+describe('canonicalJSON', () => {
+  it('produces byte-identical output for objects with same keys in any order', () => {
+    expect(canonicalJSON({ b: 1, a: 2 })).toBe(canonicalJSON({ a: 2, b: 1 }));
+  });
+
+  it('sorts nested object keys recursively', () => {
+    expect(canonicalJSON({ b: { z: 1, a: 2 }, a: 1 })).toBe(
+      canonicalJSON({ a: 1, b: { a: 2, z: 1 } }),
+    );
+  });
+
+  it('does NOT sort arrays — order is meaningful', () => {
+    expect(canonicalJSON([3, 1, 2])).toBe('[3,1,2]');
+  });
+});
diff --git a/src/save/envelope.test.ts b/src/save/envelope.test.ts
new file mode 100644
index 0000000..52b7504
--- /dev/null
+++ b/src/save/envelope.test.ts
@@ -0,0 +1,86 @@
+import { describe, it, expect } from 'vitest';
+import {
+  wrap,
+  unwrap,
+  SaveCorruptError,
+  SaveEnvelopeSchema,
+  type SaveEnvelope,
+} from './envelope';
+
+// Tests for the SaveEnvelope wrap/unwrap pair. The envelope is the load-bearing
+// shape from CLAUDE.md: `{schemaVersion, payload, checksum}`. Tampering or
+// lossy-storage corruption is detected via CRC-32 mismatch on unwrap.
+
+describe('wrap', () => {
+  it('returns an envelope with schemaVersion, payload, and 8-char hex checksum', () => {
+    const env = wrap({ foo: 'bar' }, 1);
+    expect(env.schemaVersion).toBe(1);
+    expect(env.payload).toEqual({ foo: 'bar' });
+    expect(env.checksum).toMatch(/^[0-9a-f]{8}$/);
+  });
+});
+
+describe('unwrap', () => {
+  it('round-trips several payload shapes', () => {
+    const shapes: unknown[] = [
+      { foo: 'bar' },
+      { nested: { a: 1, b: { c: [1, 2, 3] } } },
+      { garden: { tiles: [{ id: 'tile-1' }] }, plants: [] },
+      [1, 2, 3],
+      { empty: {} },
+    ];
+    for (const p of shapes) {
+      expect(unwrap(wrap(p, 1))).toEqual(p);
+    }
+  });
+
+  it('throws SaveCorruptError when checksum is tampered', () => {
+    const env = wrap({ x: 1 }, 1);
+    const tampered: SaveEnvelope<unknown> = { ...env, checksum: 'deadbeef' };
+    let caught: unknown = null;
+    try {
+      unwrap(tampered);
+    } catch (e) {
+      caught = e;
+    }
+    expect(caught).toBeInstanceOf(SaveCorruptError);
+    const err = caught as SaveCorruptError;
+    expect(err.expected).toBe('deadbeef');
+    expect(err.actual).toBe(env.checksum);
+  });
+
+  it('throws SaveCorruptError when payload is tampered (checksum mismatch)', () => {
+    const env = wrap({ x: 1 }, 1);
+    const tampered: SaveEnvelope<unknown> = { ...env, payload: { x: 2 } };
+    expect(() => unwrap(tampered)).toThrow(SaveCorruptError);
+  });
+});
+
+describe('SaveEnvelopeSchema', () => {
+  it('accepts a valid envelope', () => {
+    const env = wrap({ foo: 'bar' }, 1);
+    expect(SaveEnvelopeSchema.safeParse(env).success).toBe(true);
+  });
+
+  it('accepts schemaVersion 0 (synthetic v0 per CONTEXT D-05)', () => {
+    const env = { schemaVersion: 0, payload: {}, checksum: '00000000' };
+    expect(SaveEnvelopeSchema.safeParse(env).success).toBe(true);
+  });
+
+  it('rejects malformed envelopes (missing keys)', () => {
+    const noChecksum = { schemaVersion: 1, payload: {} };
+    const noVersion = { payload: {}, checksum: '00000000' };
+    expect(SaveEnvelopeSchema.safeParse(noChecksum).success).toBe(false);
+    expect(SaveEnvelopeSchema.safeParse(noVersion).success).toBe(false);
+  });
+
+  it('rejects malformed envelopes (non-hex checksum)', () => {
+    const bad = { schemaVersion: 1, payload: {}, checksum: 'NOT-HEX!' };
+    expect(SaveEnvelopeSchema.safeParse(bad).success).toBe(false);
+  });
+
+  it('rejects negative schemaVersion', () => {
+    const bad = { schemaVersion: -1, payload: {}, checksum: '00000000' };
+    expect(SaveEnvelopeSchema.safeParse(bad).success).toBe(false);
+  });
+});
diff --git a/src/save/migrations.test.ts b/src/save/migrations.test.ts
new file mode 100644
index 0000000..8e77867
--- /dev/null
+++ b/src/save/migrations.test.ts
@@ -0,0 +1,64 @@
+import { describe, it, expect, vi } from 'vitest';
+import { migrate, CURRENT_SCHEMA_VERSION, migrations } from './migrations';
+
+// Tests for the forward-only migration registry. The synthetic v0 → v1
+// migration (CONTEXT D-05) is the load-bearing one — Phase 4's real
+// migrate_v1_to_v2 will follow the exact same shape.
+
+describe('CURRENT_SCHEMA_VERSION', () => {
+  it('is 1 in Phase 1 (sanity)', () => {
+    expect(CURRENT_SCHEMA_VERSION).toBe(1);
+  });
+});
+
+describe('migrate (synthetic v0 → v1 per CONTEXT D-04 + D-05)', () => {
+  it('synthetic v0 payload migrates to v1 shape', () => {
+    const v0 = { garden: [{ id: 'tile-1' }, { id: 'tile-2' }] };
+    const result = migrate(v0, 0);
+    expect(result.toVersion).toBe(1);
+    expect(result.payload).toMatchObject({
+      garden: { tiles: [{ id: 'tile-1' }, { id: 'tile-2' }] },
+      plants: [],
+      harvestedFragmentIds: [],
+      lastTickAt: expect.any(Number),
+      settings: {
+        musicVolume: expect.any(Number),
+        ambientVolume: expect.any(Number),
+        sfxVolume: expect.any(Number),
+      },
+    });
+  });
+
+  it('migrating from v1 is a no-op (returns payload unchanged at toVersion 1)', () => {
+    const v1 = {
+      garden: { tiles: [] },
+      plants: [],
+      harvestedFragmentIds: [],
+      lastTickAt: 1234567890,
+      settings: { musicVolume: 0.7, ambientVolume: 0.5, sfxVolume: 0.8 },
+    };
+    const result = migrate(v1, 1);
+    expect(result.toVersion).toBe(1);
+    expect(result.payload).toEqual(v1);
+  });
+
+  it('throws when fromVersion is in the future (no migration registered)', () => {
+    expect(() => migrate({}, 99)).toThrow();
+  });
+
+  it('throws when fromVersion is negative', () => {
+    expect(() => migrate({}, -1)).toThrow();
+  });
+
+  it('invokes migrations[1] exactly once when migrating v0 → v1', () => {
+    const original = migrations[1];
+    const spy = vi.fn(original);
+    migrations[1] = spy;
+    try {
+      migrate({ garden: [] }, 0);
+      expect(spy).toHaveBeenCalledTimes(1);
+    } finally {
+      migrations[1] = original;
+    }
+  });
+});