diff --git a/creator-keys/tests/key_supply.rs b/creator-keys/tests/key_supply.rs
index cb41e06..ae84451 100644
--- a/creator-keys/tests/key_supply.rs
+++ b/creator-keys/tests/key_supply.rs
@@ -86,8 +86,12 @@ fn test_get_total_key_supply_increments_after_three_sequential_buys() {
     let buyer2 = Address::generate(&env);
     let buyer3 = Address::generate(&env);
     client.register_creator(
-        &creator,
-        &String::from_str(&env, "alice"),
+        &creator_keys::RegisterCreatorParams {
+            creator: creator.clone(),
+            handle: String::from_str(&env, "alice"),
+        },
+        &None,
+        &None,
         &None,
         &None,
         &None,
diff --git a/creator-keys/tests/key_transfer_sender_balance_decrement.rs b/creator-keys/tests/key_transfer_sender_balance_decrement.rs
new file mode 100644
index 0000000..544a986
--- /dev/null
+++ b/creator-keys/tests/key_transfer_sender_balance_decrement.rs
@@ -0,0 +1,377 @@
+//! Issue #419 — Unit tests for key transfer sender balance decrement.
+//!
+//! Verifies that `transfer_keys` decrements the sender's key balance by exactly
+//! the transferred amount and increments the recipient's balance by the same
+//! amount, while leaving total creator supply unchanged. Every test computes an
+//! explicit before/after `delta = before - after` (or `after - before` for the
+//! recipient) and asserts `delta == amount` so off-by-one regressions in any
+//! direction are caught immediately.
+//!
+//! Acceptance criteria from issue #419:
+//!   1. Sender balance decremented by exactly the transfer amount.
+//!   2. Recipient balance incremented by exactly the transfer amount.
+//!   3. Total supply unchanged after transfer.
+//!
+//! ## Why relative deltas instead of absolute post-conditions
+//!
+//! `tests/transfer_keys.rs` already covers absolute post-conditions
+//! (`sender_balance == before - 1` etc.). Those tests catch the easy case
+//! where the decrement is zero or doubled, but a subtle off-by-one or partial
+//! commit (e.g. `sender - amount + 1`) would still pass against hard-coded
+//! constants if the test was written against an incorrect expectation. The
+//! delta-based assertions in this file pin the contract against the actual
+//! pre-state stored in the ledger, so any deviation — even by one key — is
+//! flagged. Future contributors: please keep both perspectives.
+
+mod contract_test_env;
+
+use contract_test_env::{
+    register_creator_keys, register_test_creator, set_pricing_and_fees, test_env_with_auths,
+};
+use creator_keys::CreatorKeysContractClient;
+use soroban_sdk::testutils::Address as _;
+use soroban_sdk::{Address, Env};
+
+/// Fixed key price used by all tests in this file, matching the existing
+/// `transfer_keys.rs` helper setup.
+const KEY_PRICE: i128 = 100;
+
+/// Helper: register a creator with the standard 90/10 fee split and pricing.
+fn setup_pricing(client: &CreatorKeysContractClient<'_>, env: &Env) {
+    set_pricing_and_fees(env, client, KEY_PRICE, 9_000, 1_000);
+}
+
+/// Helper: buy `count` keys for `buyer` under `creator`.
+fn buy_keys(
+    client: &CreatorKeysContractClient<'_>,
+    creator: &Address,
+    buyer: &Address,
+    count: u32,
+) {
+    for _ in 0..count {
+        client.buy_key(creator, buyer, &KEY_PRICE, &None);
+    }
+}
+
+/// Asserts the canonical exact-delta invariant for any successful transfer:
+/// the sender loses exactly `amount` and the recipient gains exactly `amount`,
+/// while `get_total_key_supply` is unchanged.
+//
+// The eight-argument signature is intentional: capturing the pre-state of both
+// parties plus total supply surfaces all four inputs the contract mutates, and
+// lets the assertion be a single self-contained call. Slimming any further would
+// hide the exact-delta invariant behind an opaque struct.
+//
+// Read-only helper, so once -7 limits on the lint are still acceptable here.
+#[allow(clippy::too_many_arguments)]
+fn assert_sender_recipient_exact_delta(
+    client: &CreatorKeysContractClient<'_>,
+    creator: &Address,
+    sender: &Address,
+    recipient: &Address,
+    amount: u32,
+    sender_before: u32,
+    recipient_before: u32,
+    supply_before: u32,
+) {
+    let sender_after = client.get_key_balance(creator, sender);
+    let recipient_after = client.get_key_balance(creator, recipient);
+    let supply_after = client.get_total_key_supply(creator);
+
+    let sender_delta = sender_before - sender_after;
+    let recipient_delta = recipient_after - recipient_before;
+
+    assert_eq!(
+        sender_delta, amount,
+        "sender balance must be decremented by exactly the transfer amount"
+    );
+    assert_eq!(
+        recipient_delta, amount,
+        "recipient balance must be incremented by exactly the transfer amount"
+    );
+    assert_eq!(
+        sender_delta, recipient_delta,
+        "sender decrement must equal recipient increment (each transfer is a 1:1 move)"
+    );
+    assert_eq!(
+        supply_after, supply_before,
+        "total creator supply must be unchanged after a peer-to-peer transfer"
+    );
+}
+
+/// Single-key transfer: sender has multiple keys, leaves two behind,
+/// recipient starts with zero. Delta on both sides must be exactly 1.
+#[test]
+fn test_transfer_keys_sender_balance_decremented_by_exact_amount_single() {
+    let env = test_env_with_auths();
+    let (client, _) = register_creator_keys(&env);
+    setup_pricing(&client, &env);
+
+    let creator = register_test_creator(&env, &client, "alice");
+    let sender = Address::generate(&env);
+    let recipient = Address::generate(&env);
+
+    // Sender starts with 5 keys; recipient starts with 0.
+    buy_keys(&client, &creator, &sender, 5);
+
+    let sender_before = client.get_key_balance(&creator, &sender);
+    let recipient_before = client.get_key_balance(&creator, &recipient);
+    let supply_before = client.get_total_key_supply(&creator);
+    assert_eq!(sender_before, 5);
+    assert_eq!(recipient_before, 0);
+    assert_eq!(supply_before, 5);
+
+    let amount: u32 = 1;
+    client.transfer_keys(&creator, &sender, &recipient, &amount);
+
+    assert_sender_recipient_exact_delta(
+        &client,
+        &creator,
+        &sender,
+        &recipient,
+        amount,
+        sender_before,
+        recipient_before,
+        supply_before,
+    );
+
+    // Sanity-check the absolute post-conditions as well.
+    assert_eq!(client.get_key_balance(&creator, &sender), 4);
+    assert_eq!(client.get_key_balance(&creator, &recipient), 1);
+    assert_eq!(client.get_total_key_supply(&creator), 5);
+}
+
+/// Multi-key transfer: send 5 of 10 to a recipient that already has 3. The
+/// sender must lose exactly 5 and the recipient must gain exactly 5; total
+/// supply is untouched.
+#[test]
+fn test_transfer_keys_sender_balance_decremented_by_exact_amount_multi() {
+    let env = test_env_with_auths();
+    let (client, _) = register_creator_keys(&env);
+    setup_pricing(&client, &env);
+
+    let creator = register_test_creator(&env, &client, "multi");
+    let sender = Address::generate(&env);
+    let recipient = Address::generate(&env);
+
+    buy_keys(&client, &creator, &sender, 10);
+    buy_keys(&client, &creator, &recipient, 3);
+
+    let sender_before = client.get_key_balance(&creator, &sender);
+    let recipient_before = client.get_key_balance(&creator, &recipient);
+    let supply_before = client.get_total_key_supply(&creator);
+    assert_eq!(sender_before, 10);
+    assert_eq!(recipient_before, 3);
+    assert_eq!(supply_before, 13);
+
+    let amount: u32 = 5;
+    client.transfer_keys(&creator, &sender, &recipient, &amount);
+
+    assert_sender_recipient_exact_delta(
+        &client,
+        &creator,
+        &sender,
+        &recipient,
+        amount,
+        sender_before,
+        recipient_before,
+        supply_before,
+    );
+
+    assert_eq!(client.get_key_balance(&creator, &sender), 5);
+    assert_eq!(client.get_key_balance(&creator, &recipient), 8);
+    assert_eq!(client.get_total_key_supply(&creator), 13);
+}
+
+/// Full-drain transfer: sender transfers every key they own. The sender
+/// balance must be zero and the delta on the recipient side equals the
+/// (now-zeroed) sender's prior balance.
+#[test]
+fn test_transfer_keys_sender_zeroed_out_exact_decrement() {
+    let env = test_env_with_auths();
+    let (client, _) = register_creator_keys(&env);
+    setup_pricing(&client, &env);
+
+    let creator = register_test_creator(&env, &client, "zero");
+    let sender = Address::generate(&env);
+    let recipient = Address::generate(&env);
+
+    buy_keys(&client, &creator, &sender, 4);
+
+    let sender_before = client.get_key_balance(&creator, &sender);
+    let recipient_before = client.get_key_balance(&creator, &recipient);
+    let supply_before = client.get_total_key_supply(&creator);
+
+    let amount = sender_before; // transfer every key the sender holds
+    client.transfer_keys(&creator, &sender, &recipient, &amount);
+
+    assert_eq!(
+        client.get_key_balance(&creator, &sender),
+        0,
+        "sender must be fully decremented to zero"
+    );
+    assert_eq!(
+        client.get_key_balance(&creator, &recipient),
+        recipient_before + amount,
+        "recipient must receive exactly the transferred amount"
+    );
+    assert_eq!(
+        client.get_total_key_supply(&creator),
+        supply_before,
+        "supply must remain unchanged after a draining transfer"
+    );
+}
+
+/// Multiple sequential transfers must accumulate on the sender side. The total
+/// decrement across three calls (2 + 3 + 1) must equal 6, regardless of
+/// intermediate recipient balances.
+#[test]
+fn test_transfer_keys_sender_decrement_accumulates_across_calls() {
+    let env = test_env_with_auths();
+    let (client, _) = register_creator_keys(&env);
+    setup_pricing(&client, &env);
+
+    let creator = register_test_creator(&env, &client, "accum");
+    let sender = Address::generate(&env);
+    let recipient_a = Address::generate(&env);
+    let recipient_b = Address::generate(&env);
+
+    buy_keys(&client, &creator, &sender, 10);
+
+    let sender_before = client.get_key_balance(&creator, &sender);
+    let supply_before = client.get_total_key_supply(&creator);
+    assert_eq!(sender_before, 10);
+
+    client.transfer_keys(&creator, &sender, &recipient_a, &2);
+    client.transfer_keys(&creator, &sender, &recipient_b, &3);
+    client.transfer_keys(&creator, &sender, &recipient_a, &1);
+
+    let sender_after = client.get_key_balance(&creator, &sender);
+    let total_transferred: u32 = 2 + 3 + 1;
+
+    assert_eq!(
+        sender_before - sender_after,
+        total_transferred,
+        "sender decrement must equal the sum of transfer amounts across calls"
+    );
+    assert_eq!(
+        client.get_total_key_supply(&creator),
+        supply_before,
+        "supply must remain unchanged across sequential transfer_keys calls"
+    );
+    assert_eq!(
+        client.get_key_balance(&creator, &recipient_a),
+        3,
+        "recipient A receives exactly 2 + 1 = 3 keys"
+    );
+    assert_eq!(
+        client.get_key_balance(&creator, &recipient_b),
+        3,
+        "recipient B receives exactly 3 keys"
+    );
+}
+
+/// Sender and recipient balances must be isolated across creators. A transfer
+/// under creator A must not influence sender/recipient balances under creator B.
+#[test]
+fn test_transfer_keys_sender_recipient_deltas_isolated_per_creator() {
+    let env = test_env_with_auths();
+    let (client, _) = register_creator_keys(&env);
+    setup_pricing(&client, &env);
+
+    let creator_a = register_test_creator(&env, &client, "alpha");
+    let creator_b = register_test_creator(&env, &client, "beta");
+    let sender = Address::generate(&env);
+    let recipient = Address::generate(&env);
+
+    buy_keys(&client, &creator_a, &sender, 7);
+    buy_keys(&client, &creator_b, &sender, 6);
+
+    let sender_a_before = client.get_key_balance(&creator_a, &sender);
+    let sender_b_before = client.get_key_balance(&creator_b, &sender);
+    let recipient_a_before = client.get_key_balance(&creator_a, &recipient);
+    let recipient_b_before = client.get_key_balance(&creator_b, &recipient);
+    let supply_a_before = client.get_total_key_supply(&creator_a);
+    let supply_b_before = client.get_total_key_supply(&creator_b);
+
+    let amount: u32 = 4;
+    client.transfer_keys(&creator_a, &sender, &recipient, &amount);
+
+    let sender_a_after = client.get_key_balance(&creator_a, &sender);
+    let sender_b_after = client.get_key_balance(&creator_b, &sender);
+    let recipient_a_after = client.get_key_balance(&creator_a, &recipient);
+    let recipient_b_after = client.get_key_balance(&creator_b, &recipient);
+
+    assert_eq!(
+        sender_a_before - sender_a_after,
+        amount,
+        "creator A sender must lose exactly the transferred amount"
+    );
+    assert_eq!(
+        sender_b_before, sender_b_after,
+        "creator B sender balance must be untouched"
+    );
+    assert_eq!(
+        recipient_a_after - recipient_a_before,
+        amount,
+        "creator A recipient must gain exactly the transferred amount"
+    );
+    assert_eq!(
+        recipient_b_before, recipient_b_after,
+        "creator B recipient balance must be untouched"
+    );
+    assert_eq!(
+        client.get_total_key_supply(&creator_a),
+        supply_a_before,
+        "creator A total supply must be unchanged"
+    );
+    assert_eq!(
+        client.get_total_key_supply(&creator_b),
+        supply_b_before,
+        "creator B total supply must be unchanged"
+    );
+}
+
+/// A transferKeys call that reverts (insufficient balance) must not change any
+/// sender or recipient balance, and must leave total supply unchanged.
+#[test]
+fn test_transfer_keys_reverted_call_does_not_decrement_sender() {
+    let env = test_env_with_auths();
+    let (client, _) = register_creator_keys(&env);
+    setup_pricing(&client, &env);
+
+    let creator = register_test_creator(&env, &client, "rejected");
+    let sender = Address::generate(&env);
+    let recipient = Address::generate(&env);
+
+    buy_keys(&client, &creator, &sender, 3);
+
+    let sender_before = client.get_key_balance(&creator, &sender);
+    let recipient_before = client.get_key_balance(&creator, &recipient);
+    let supply_before = client.get_total_key_supply(&creator);
+    assert_eq!(sender_before, 3);
+    assert_eq!(recipient_before, 0);
+
+    // Attempting to transfer 10 keys when sender only holds 3 must revert.
+    let result = client.try_transfer_keys(&creator, &sender, &recipient, &10u32);
+    assert!(
+        result.is_err(),
+        "transfer with insufficient balance must revert"
+    );
+
+    assert_eq!(
+        client.get_key_balance(&creator, &sender),
+        sender_before,
+        "sender balance must be unchanged on a rejected transfer"
+    );
+    assert_eq!(
+        client.get_key_balance(&creator, &recipient),
+        recipient_before,
+        "recipient balance must be unchanged on a rejected transfer"
+    );
+    assert_eq!(
+        client.get_total_key_supply(&creator),
+        supply_before,
+        "total supply must remain unchanged after a reverted transfer"
+    );
+}
diff --git a/docs/key-transfer-authorization.md b/docs/key-transfer-authorization.md
index b7cb3e9..d4bc798 100644
--- a/docs/key-transfer-authorization.md
+++ b/docs/key-transfer-authorization.md
@@ -166,6 +166,6 @@ This mirrors the `sell` event pattern (`(Symbol("sell"), creator, seller)` / `su
 | **Who authorizes?** | Only `from` (sender). No `to` (recipient) approval required. |
 | **Auth mechanism** | `from.require_auth()` — Soroban signature verification. |
 | **Fee?** | None. No bonding curve interaction. No fee math. |
-| **Self-transfer?** | Rejected with `ContractError::ZeroAddress` when `from == to`. |
+| **Self-transfer?** | Rejected with `ContractError::SelfTransfer` when `from == to`. |
 | **State changes** | `KeyBalance` decremented for `from`, incremented for `to`. Supply unchanged. |
 | **Access level** | Key holder (`from`). |