diff --git a/src/session.ts b/src/session.ts
index bd078c7..6fd28bf 100644
--- a/src/session.ts
+++ b/src/session.ts
@@ -45,14 +45,14 @@ import type { SignerAdapter } from "./signer.js";
 
 /**
  * Session key authorization domain separator (matches Rust `PQTX_SESSION_DOMAIN`).
- * `b"PQTX_SESSION_V1\0"` — 16 bytes.
+ * `b"PQTX_SESSION_V2\0"` — 16 bytes.
  */
 export const PQTX_SESSION_DOMAIN = new Uint8Array([
   0x50, 0x51, 0x54, 0x58, // PQTX
   0x5f, 0x53, 0x45, 0x53, // _SES
   0x53, 0x49, 0x4f, 0x4e, // SION
-  0x5f, 0x56, 0x31, 0x00, // _V1\0
-]); // b"PQTX_SESSION_V1\0"
+  0x5f, 0x56, 0x32, 0x00, // _V2\0
+]); // b"PQTX_SESSION_V2\0"
 
 /** Maximum session public-key size accepted by Shell Chain nodes. */
 export const MAX_SESSION_PUBKEY_BYTES = 4 * 1024;
@@ -84,7 +84,7 @@ export interface SessionKeyConfig {
 /**
  * Compute the session key authorization hash that the root key must sign.
  *
- * `auth_hash = blake3(PQTX_SESSION_V1\0(16B) || session_pubkey || session_algo(1B) || target(32B|zero) || value_cap(32B BE) || expiry_block(8B BE) || chain_id(8B BE))`
+ * `auth_hash = blake3(PQTX_SESSION_V2\0(16B) || session_pubkey || session_algo(1B) || target_present(1B) || target(32B|zero) || value_cap(32B BE) || expiry_block(8B BE) || chain_id(8B BE))`
  *
  * This hash binds the session key to a specific chain, expiry, value cap,
  * and optional target, preventing unauthorized delegation.
@@ -106,9 +106,12 @@ export function computeSessionAuthHash(
     throw new RangeError(`sessionAlgoId must fit in one byte, got ${sessionAlgoId}`);
   }
 
-  // Target: 32 bytes (address) or 32 zero bytes (no restriction).
+  // Target: explicit presence byte plus 32 bytes. The presence byte keeps an
+  // unrestricted authorization distinct from one restricted to the zero address.
+  const targetPresent = new Uint8Array([0]);
   const targetBytes = new Uint8Array(32);
   if (config.target !== null && config.target !== undefined) {
+    targetPresent[0] = 1;
     const addrBytes = shellAddressToBytes(config.target);
     targetBytes.set(addrBytes.slice(0, 32));
   }
@@ -141,6 +144,7 @@ export function computeSessionAuthHash(
     PQTX_SESSION_DOMAIN,
     sessionPubkey,
     new Uint8Array([sessionAlgoId]),
+    targetPresent,
     targetBytes,
     valueCapBytes,
     expiryBytes,
diff --git a/src/types.ts b/src/types.ts
index 35d8209..d3a3129 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -174,7 +174,7 @@ export const AA_SESSION_KEY_GAS_SURCHARGE = 20_000;
  *
  * 1. `session_pubkey` and `session_algo` are authorized by the root key's
  *    `root_signature` over
- *    `auth_hash = blake3(PQTX_SESSION_V1\0 (16B) || session_pubkey || session_algo(1B) || target(32B|zero) || value_cap(32B BE) || expiry_block(8B BE) || chain_id(8B BE))`.
+ *    `auth_hash = blake3(PQTX_SESSION_V2\0 (16B) || session_pubkey || session_algo(1B) || target_present(1B) || target(32B|zero) || value_cap(32B BE) || expiry_block(8B BE) || chain_id(8B BE))`.
  * 2. The transaction is signed by `session_pubkey` via `session_signature`.
  * 3. `expiry_block` must be > current block at validation time.
  * 4. Σ(inner_call.value) ≤ `value_cap`.
@@ -206,7 +206,7 @@ export interface SessionAuth {
   expiry_block: number;
   /**
    * Root account's PQ signature over the session key authorization hash.
-   * `auth_hash = blake3(PQTX_SESSION_V1\0 (16B) || session_pubkey || session_algo(1B) || target(32B|zero) || value_cap || expiry_block || chain_id)`.
+   * `auth_hash = blake3(PQTX_SESSION_V2\0 (16B) || session_pubkey || session_algo(1B) || target_present(1B) || target(32B|zero) || value_cap || expiry_block || chain_id)`.
    */
   root_signature: number[];
   /** Session key's PQ signature over the tx `sender_signing_hash()`. */
diff --git a/tests/session.test.mjs b/tests/session.test.mjs
index 8fe4d50..0108752 100644
--- a/tests/session.test.mjs
+++ b/tests/session.test.mjs
@@ -88,15 +88,46 @@ test("session-2: computeSessionAuthHash produces 32-byte BLAKE3 output", () => {
   // Different expiry must produce different hash
   const hashDifferentExpiry = computeSessionAuthHash(session.publicKey, session.algoId, { ...config, expiryBlock: 2000 });
   assert.notDeepEqual(hash, hashDifferentExpiry, "different expiryBlock must produce different hash");
+
+  const zeroTarget = `0x${"00".repeat(32)}`;
+  const hashZeroTarget = computeSessionAuthHash(session.publicKey, session.algoId, {
+    ...config,
+    target: zeroTarget,
+  });
+  assert.notDeepEqual(hash, hashZeroTarget, "unrestricted and zero-address targets must differ");
 });
 
 test("session-2b: PQTX_SESSION_DOMAIN is 16 bytes matching spec", () => {
   assert.equal(PQTX_SESSION_DOMAIN.length, 16, "domain must be 16 bytes");
-  // b"PQTX_SESSION_V1\0"
-  const expected = Buffer.from("PQTX_SESSION_V1\0");
+  // b"PQTX_SESSION_V2\0"
+  const expected = Buffer.from("PQTX_SESSION_V2\0");
   assert.deepEqual(Buffer.from(PQTX_SESSION_DOMAIN), expected, "domain bytes mismatch");
 });
 
+test("session-2c: target-presence hashes match Shell Chain vectors", () => {
+  const sessionPubkey = new Uint8Array(32).fill(0x11);
+  const base = {
+    chainId: 1337n,
+    expiryBlock: 500,
+    valueCap: 100n,
+    target: null,
+  };
+  const unrestricted = computeSessionAuthHash(sessionPubkey, 1, base);
+  const zeroTarget = computeSessionAuthHash(sessionPubkey, 1, {
+    ...base,
+    target: `0x${"00".repeat(32)}`,
+  });
+
+  assert.equal(
+    Buffer.from(unrestricted).toString("hex"),
+    "3fceca0ef7542e4933a956618d2f94b663fb72be319e015a09f960409ed8e4f7",
+  );
+  assert.equal(
+    Buffer.from(zeroTarget).toString("hex"),
+    "43faa3987b61c88a5b0f758024f09fa549b10ec62979a7ba5c4ce6ff5ee65088",
+  );
+});
+
 // ── Vector 3: createSessionAuth produces valid structure ─────────────────────
 
 test("session-3: createSessionAuth returns well-formed SessionAuth", async () => {