diff --git a/.github/workflows/sdk_tests.yml b/.github/workflows/sdk_tests.yml
new file mode 100644
index 0000000..7c00c54
--- /dev/null
+++ b/.github/workflows/sdk_tests.yml
@@ -0,0 +1,29 @@
+name: SDK Tests
+
+on:
+  push:
+    branches: ["main"]
+  pull_request:
+    branches: ["main"]
+
+jobs:
+  sdk-tests:
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: zk/sdk
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: 20
+
+      - name: Install dependencies
+        run: npm install
+
+      - name: Run SDK tests
+        run: npm test
diff --git a/gateway-contract/contracts/core_contract/core.md b/gateway-contract/contracts/core_contract/core.md
index e69de29..1ec2f04 100644
--- a/gateway-contract/contracts/core_contract/core.md
+++ b/gateway-contract/contracts/core_contract/core.md
@@ -0,0 +1,12 @@
+# Core Contract Notes
+
+## SMT Root Sequencing Requirement
+
+`register_resolver` enforces strict root sequencing:
+
+- `public_signals.old_root` must exactly equal the current on-chain SMT root.
+- A successful `register_resolver` updates the on-chain root to `public_signals.new_root`.
+- Any later call reusing the pre-update root is rejected as stale.
+
+This replay protection prevents re-submitting proofs against an already-consumed root. In tests, the stale replay path is asserted to panic with `Error(Contract, #4)` (`StaleRoot`).
+
diff --git a/gateway-contract/contracts/core_contract/src/test.rs b/gateway-contract/contracts/core_contract/src/test.rs
index 2252621..c854a76 100644
--- a/gateway-contract/contracts/core_contract/src/test.rs
+++ b/gateway-contract/contracts/core_contract/src/test.rs
@@ -414,6 +414,43 @@ fn test_register_resolver_stale_root_fails() {
     client.register_resolver(&caller, &hash, &dummy_proof(&env), &signals);
 }
 
+#[test]
+#[should_panic(expected = "Error(Contract, #4)")]
+fn test_register_resolver_stale_root_after_first_registration() {
+    let env = Env::default();
+    env.mock_all_auths();
+    let (_, client, initial_root) = setup_with_root(&env);
+
+    let caller = Address::generate(&env);
+    let first_hash = commitment(&env, 70);
+    let first_new_root = BytesN::from_array(&env, &[71u8; 32]);
+
+    // First registration succeeds and advances SMT root.
+    client.register_resolver(
+        &caller,
+        &first_hash,
+        &dummy_proof(&env),
+        &PublicSignals {
+            old_root: initial_root.clone(),
+            new_root: first_new_root,
+        },
+    );
+
+    let second_hash = commitment(&env, 72);
+    let second_new_root = BytesN::from_array(&env, &[73u8; 32]);
+
+    // Replay with the original old_root must fail as stale.
+    client.register_resolver(
+        &caller,
+        &second_hash,
+        &dummy_proof(&env),
+        &PublicSignals {
+            old_root: initial_root,
+            new_root: second_new_root,
+        },
+    );
+}
+
 #[test]
 #[should_panic(expected = "Error(Contract, #6)")]
 fn test_resolve_stellar_no_address_linked_when_not_set() {
diff --git a/gateway-contract/contracts/escrow_contract/src/events.rs b/gateway-contract/contracts/escrow_contract/src/events.rs
index 7c97835..42952fe 100644
--- a/gateway-contract/contracts/escrow_contract/src/events.rs
+++ b/gateway-contract/contracts/escrow_contract/src/events.rs
@@ -165,4 +165,19 @@ impl Events {
         }
         .publish(env);
     }
+
+    /// Emits a DEPOSIT event with topics (symbol!("DEPOSIT"), commitment)
+    /// and data (owner, amount, new_balance).
+    pub fn deposit(
+        env: &Env,
+        commitment: BytesN<32>,
+        owner: Address,
+        amount: i128,
+        new_balance: i128,
+    ) {
+        env.events().publish(
+            (symbol_short!("DEPOSIT"), commitment),
+            (owner, amount, new_balance),
+        );
+    }
 }
diff --git a/gateway-contract/contracts/escrow_contract/src/lib.rs b/gateway-contract/contracts/escrow_contract/src/lib.rs
index d37b7da..e5b1805 100644
--- a/gateway-contract/contracts/escrow_contract/src/lib.rs
+++ b/gateway-contract/contracts/escrow_contract/src/lib.rs
@@ -103,6 +103,39 @@ impl EscrowContract {
         Events::vault_crt(&env, commitment, token, owner);
     }
 
+    /// Deposits tokens into an active vault.
+    ///
+    /// The vault owner must authorize the call. Funds are transferred from the
+    /// owner to this contract and reflected in the vault's tracked balance.
+    ///
+    /// ### Errors
+    /// - `InvalidAmount`: If `amount <= 0`.
+    /// - `VaultNotFound`: If the vault does not exist.
+    /// - `VaultInactive`: If the vault has been cancelled/inactivated.
+    pub fn deposit(env: Env, commitment: BytesN<32>, amount: i128) {
+        if amount <= 0 {
+            panic_with_error!(&env, EscrowError::InvalidAmount);
+        }
+
+        let config = read_vault_config(&env, &commitment)
+            .unwrap_or_else(|| panic_with_error!(&env, EscrowError::VaultNotFound));
+        config.owner.require_auth();
+
+        let mut state = read_vault_state(&env, &commitment)
+            .unwrap_or_else(|| panic_with_error!(&env, EscrowError::VaultNotFound));
+        if !state.is_active {
+            panic_with_error!(&env, EscrowError::VaultInactive);
+        }
+
+        let token_client = token::Client::new(&env, &config.token);
+        token_client.transfer(&config.owner, &env.current_contract_address(), &amount);
+
+        state.balance += amount;
+        write_vault_state(&env, &commitment, &state);
+
+        Events::deposit(&env, commitment, config.owner, amount, state.balance);
+    }
+
     /// Schedules a payment from one vault to another.
     ///
     /// Funds are reserved in the source vault immediately upon scheduling.
diff --git a/gateway-contract/contracts/escrow_contract/src/test.rs b/gateway-contract/contracts/escrow_contract/src/test.rs
index 74cc779..931baa7 100644
--- a/gateway-contract/contracts/escrow_contract/src/test.rs
+++ b/gateway-contract/contracts/escrow_contract/src/test.rs
@@ -85,6 +85,102 @@ fn read_vault(env: &Env, contract_id: &Address, id: &BytesN<32>) -> VaultState {
     })
 }
 
+#[test]
+fn test_deposit_success() {
+    let env = Env::default();
+    env.mock_all_auths();
+    let (contract_id, client, token, _token_admin, from, _to) = setup_test(&env);
+
+    let owner = Address::generate(&env);
+    create_vault(&env, &contract_id, &from, &owner, &token, 0);
+
+    let token_admin_client = StellarAssetClient::new(&env, &token);
+    token_admin_client.mint(&owner, &500);
+
+    client.deposit(&from, &200);
+
+    env.as_contract(&contract_id, || {
+        let state: VaultState = env
+            .storage()
+            .persistent()
+            .get(&DataKey::VaultState(from.clone()))
+            .unwrap();
+        assert_eq!(state.balance, 200);
+    });
+
+    let token_client = TokenClient::new(&env, &token);
+    assert_eq!(token_client.balance(&owner), 300);
+    assert_eq!(token_client.balance(&contract_id), 200);
+
+    let deposit_events = env
+        .events()
+        .all()
+        .iter()
+        .filter(|(event_contract, _, _)| event_contract == &contract_id)
+        .count();
+    assert_eq!(deposit_events, 1);
+}
+
+#[test]
+fn test_deposit_zero_amount_rejected() {
+    let env = Env::default();
+    env.mock_all_auths();
+    let (contract_id, client, token, _token_admin, from, _to) = setup_test(&env);
+
+    create_vault(
+        &env,
+        &contract_id,
+        &from,
+        &Address::generate(&env),
+        &token,
+        0,
+    );
+
+    let result = client.try_deposit(&from, &0);
+    assert!(matches!(
+        result,
+        Err(Ok(err)) if err == Error::from_contract_error(EscrowError::InvalidAmount as u32)
+    ));
+}
+
+#[test]
+#[should_panic]
+fn test_deposit_non_owner_panics() {
+    let env = Env::default();
+    let (contract_id, client, token, _token_admin, from, _to) = setup_test(&env);
+
+    let owner = Address::generate(&env);
+    create_vault(&env, &contract_id, &from, &owner, &token, 0);
+
+    client.deposit(&from, &10);
+}
+
+#[test]
+fn test_deposit_inactive_vault_rejected() {
+    let env = Env::default();
+    env.mock_all_auths();
+    let (contract_id, client, token, _token_admin, from, _to) = setup_test(&env);
+
+    let owner = Address::generate(&env);
+    create_vault(&env, &contract_id, &from, &owner, &token, 0);
+
+    env.as_contract(&contract_id, || {
+        let state = VaultState {
+            balance: 0,
+            is_active: false,
+        };
+        env.storage()
+            .persistent()
+            .set(&DataKey::VaultState(from.clone()), &state);
+    });
+
+    let result = client.try_deposit(&from, &10);
+    assert!(matches!(
+        result,
+        Err(Ok(err)) if err == Error::from_contract_error(EscrowError::VaultInactive as u32)
+    ));
+}
+
 #[test]
 fn test_schedule_payment_success() {
     let env = Env::default();
diff --git a/zk/sdk/jest.config.ts b/zk/sdk/jest.config.ts
new file mode 100644
index 0000000..29f71fa
--- /dev/null
+++ b/zk/sdk/jest.config.ts
@@ -0,0 +1,9 @@
+const config = {
+  preset: "ts-jest",
+  testEnvironment: "node",
+  roots: ["<rootDir>/src"],
+  testMatch: ["**/__tests__/**/*.test.ts"],
+  clearMocks: true,
+};
+
+export default config;
diff --git a/zk/sdk/package.json b/zk/sdk/package.json
index de9ffaa..9a84938 100644
--- a/zk/sdk/package.json
+++ b/zk/sdk/package.json
@@ -6,4 +6,4 @@
   "dependencies": {
     "circomlibjs": "^0.1.7"
   }
-}
\ No newline at end of file
+}
diff --git a/zk/sdk/src/__tests__/smoke.test.ts b/zk/sdk/src/__tests__/smoke.test.ts
new file mode 100644
index 0000000..f0ccb3a
--- /dev/null
+++ b/zk/sdk/src/__tests__/smoke.test.ts
@@ -0,0 +1,11 @@
+import * as sdk from "../index";
+
+declare const describe: (name: string, fn: () => void) => void;
+declare const it: (name: string, fn: () => void) => void;
+declare const expect: (value: unknown) => { toBeDefined: () => void };
+
+describe("zk sdk smoke", () => {
+  it("imports index without throwing", () => {
+    expect(sdk).toBeDefined();
+  });
+});