diff --git a/src/transactions/builder.ts b/src/transactions/builder.ts
new file mode 100644
index 0000000..05badb8
--- /dev/null
+++ b/src/transactions/builder.ts
@@ -0,0 +1,22 @@
+import crypto from 'crypto';
+
+export type Memo =
+  | { type: 'MEMO_TEXT'; value: string }
+  | { type: 'MEMO_HASH'; value: Buffer };
+
+export function encodeMemo(input: string | Record<string, unknown>): Memo {
+  if (typeof input === 'string') {
+    const bytes = Buffer.byteLength(input, 'utf8');
+    if (bytes <= 28) return { type: 'MEMO_TEXT', value: input };
+    throw new Error('Memo text exceeds 28 bytes');
+  }
+
+  const json = JSON.stringify(input);
+  const b64 = Buffer.from(json, 'utf8').toString('base64');
+  if (Buffer.byteLength(b64, 'utf8') <= 28) {
+    return { type: 'MEMO_TEXT', value: b64 };
+  }
+
+  const hash = crypto.createHash('sha256').update(b64, 'utf8').digest();
+  return { type: 'MEMO_HASH', value: hash };
+}
diff --git a/src/transactions/index.ts b/src/transactions/index.ts
index 04d4874..47b091c 100644
--- a/src/transactions/index.ts
+++ b/src/transactions/index.ts
@@ -1,2 +1,5 @@
 // eslint-disable-next-line @typescript-eslint/no-explicit-any
 export function buildMultisigTransaction(..._args: unknown[]): unknown { return undefined; }
+
+export { encodeMemo } from './builder';
+export { fetchSequenceNumber } from './sequence';
diff --git a/src/transactions/sequence.ts b/src/transactions/sequence.ts
new file mode 100644
index 0000000..b578fdf
--- /dev/null
+++ b/src/transactions/sequence.ts
@@ -0,0 +1,25 @@
+import { Server } from '@stellar/stellar-sdk';
+import { TESTNET_HORIZON_URL } from '../utils/constants';
+import { AccountNotFoundError } from '../utils/errors';
+
+const cache = new Map<string, { sequence: string; ts: number }>();
+const TTL = 5000; // 5 seconds
+
+export async function fetchSequenceNumber(accountId: string): Promise<string> {
+  const now = Date.now();
+  const cached = cache.get(accountId);
+  if (cached && now - cached.ts < TTL) return cached.sequence;
+
+  const server = new Server(TESTNET_HORIZON_URL);
+  try {
+    const res: any = await server.accounts().accountId(accountId).call();
+    const sequence = String(res.sequence);
+    cache.set(accountId, { sequence, ts: now });
+    return sequence;
+  } catch (err: any) {
+    if (err && err.response && err.response.status === 404) {
+      throw new AccountNotFoundError(accountId);
+    }
+    throw err;
+  }
+}
diff --git a/tests/unit/transactions/builder.test.ts b/tests/unit/transactions/builder.test.ts
new file mode 100644
index 0000000..ad0325e
--- /dev/null
+++ b/tests/unit/transactions/builder.test.ts
@@ -0,0 +1,29 @@
+import { encodeMemo } from '../../../src/transactions';
+
+describe('encodeMemo', () => {
+  it('encodes short string as MEMO_TEXT', () => {
+    const memo = encodeMemo('hello');
+    expect(memo).toEqual({ type: 'MEMO_TEXT', value: 'hello' });
+  });
+
+  it('throws for string longer than 28 bytes', () => {
+    const long = 'a'.repeat(29);
+    expect(() => encodeMemo(long)).toThrow();
+  });
+
+  it('encodes small object as base64 MEMO_TEXT', () => {
+    const obj = { a: 1 };
+    const memo = encodeMemo(obj);
+    expect(memo.type).toBe('MEMO_TEXT');
+    expect(typeof (memo as any).value).toBe('string');
+    expect((memo as any).value.length).toBeGreaterThan(0);
+  });
+
+  it('hashes large object to MEMO_HASH', () => {
+    const obj = { data: 'x'.repeat(200) };
+    const memo = encodeMemo(obj);
+    expect(memo.type).toBe('MEMO_HASH');
+    expect(Buffer.isBuffer((memo as any).value)).toBe(true);
+    expect(((memo as any).value as Buffer).length).toBe(32);
+  });
+});
diff --git a/tests/unit/transactions/sequence.test.ts b/tests/unit/transactions/sequence.test.ts
new file mode 100644
index 0000000..f93b4da
--- /dev/null
+++ b/tests/unit/transactions/sequence.test.ts
@@ -0,0 +1,45 @@
+import { fetchSequenceNumber } from '../../../src/transactions';
+import { Server } from '@stellar/stellar-sdk';
+import { AccountNotFoundError } from '../../../src/utils/errors';
+
+jest.mock('@stellar/stellar-sdk');
+
+describe('fetchSequenceNumber', () => {
+  const accountId = 'GABC123';
+
+  beforeEach(() => {
+    jest.clearAllMocks();
+  });
+
+  it('returns sequence from horizon', async () => {
+    const mockCall = jest.fn().mockResolvedValue({ sequence: '42' });
+    // @ts-ignore
+    Server.mockImplementation(() => ({ accounts: () => ({ accountId: () => ({ call: mockCall }) }) }));
+
+    const seq = await fetchSequenceNumber(accountId);
+    expect(seq).toBe('42');
+    expect(mockCall).toHaveBeenCalled();
+  });
+
+  it('caches result for 5s', async () => {
+    const mockCall = jest.fn().mockResolvedValue({ sequence: '100' });
+    // @ts-ignore
+    Server.mockImplementation(() => ({ accounts: () => ({ accountId: () => ({ call: mockCall }) }) }));
+
+    const a = await fetchSequenceNumber(accountId);
+    const b = await fetchSequenceNumber(accountId);
+    expect(a).toBe('100');
+    expect(b).toBe('100');
+    expect(mockCall).toHaveBeenCalledTimes(1);
+  });
+
+  it('throws AccountNotFoundError on 404', async () => {
+    const err: any = new Error('not found');
+    err.response = { status: 404 };
+    const mockCall = jest.fn().mockRejectedValue(err);
+    // @ts-ignore
+    Server.mockImplementation(() => ({ accounts: () => ({ accountId: () => ({ call: mockCall }) }) }));
+
+    await expect(fetchSequenceNumber(accountId)).rejects.toBeInstanceOf(AccountNotFoundError);
+  });
+});