diff --git a/Cargo.lock b/Cargo.lock
index 67c4b696c..f3063ba77 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -7704,6 +7704,7 @@ name = "ping_core"
 version = "0.1.0"
 dependencies = [
  "lee_core",
+ "risc0-zkvm",
  "serde",
 ]
 
diff --git a/integration_tests/tests/cross_zone_ping.rs b/integration_tests/tests/cross_zone_ping.rs
index 527a0135b..35aa213ee 100644
--- a/integration_tests/tests/cross_zone_ping.rs
+++ b/integration_tests/tests/cross_zone_ping.rs
@@ -22,7 +22,9 @@ use integration_tests::{
 };
 use lee::{AccountId, PublicTransaction, public_transaction::Message};
 use lee_core::program::ProgramId;
-use ping_core::{ReceiverInstruction, SenderInstruction, ping_record_pda};
+use ping_core::{
+    ReceiverInstruction, SenderInstruction, ping_record_pda, sender_config_account_id,
+};
 use sequencer_core::config::{CrossZoneConfig, CrossZonePeer, CrossZoneRoute};
 use sequencer_service_rpc::{RpcClient as _, SequencerClient};
 use tokio::test;
@@ -104,7 +106,6 @@ fn build_ping_tx(target_zone: [u8; 32], receiver_id: ProgramId) -> LeeTransactio
     let payload: Vec<u8> = words.iter().flat_map(|word| word.to_le_bytes()).collect();
 
     let send = SenderInstruction::Send {
-        outbox_program_id: outbox_id,
         target_zone,
         target_program_id: receiver_id,
         target_accounts: vec![ping_record_pda(receiver_id).into_value()],
@@ -112,15 +113,11 @@ fn build_ping_tx(target_zone: [u8; 32], receiver_id: ProgramId) -> LeeTransactio
         ordinal,
     };
 
-    let outbox_account = outbox_pda(
-        outbox_id,
-        programs::ping_sender().id(),
-        &target_zone,
-        ordinal,
-    );
+    let sender_id = programs::ping_sender().id();
+    let outbox_account = outbox_pda(outbox_id, sender_id, &target_zone, ordinal);
     let message = Message::try_new(
-        programs::ping_sender().id(),
-        vec![outbox_account],
+        sender_id,
+        vec![sender_config_account_id(sender_id), outbox_account],
         vec![],
         send,
     )
diff --git a/integration_tests/tests/cross_zone_state_machine.rs b/integration_tests/tests/cross_zone_state_machine.rs
index b484f12f0..968fc38a9 100644
--- a/integration_tests/tests/cross_zone_state_machine.rs
+++ b/integration_tests/tests/cross_zone_state_machine.rs
@@ -22,7 +22,9 @@ use lee::{
     public_transaction::{Message, WitnessSet},
 };
 use lee_core::account::Account;
-use ping_core::{ReceiverInstruction, ping_record_pda};
+use ping_core::{
+    ReceiverInstruction, outbox_bytes, ping_record_pda, read_outbox, sender_config_account_id,
+};
 
 const INITIAL_BALANCE: u128 = 100;
 const LOCK_AMOUNT: u128 = 30;
@@ -94,6 +96,43 @@ fn seed_wrapped_config(state: &mut V03State) {
     )]);
 }
 
+/// Seeds the ping-sender config account pinning the real outbox, matching what
+/// genesis seeds for a real zone.
+fn seed_ping_sender_config(state: &mut V03State) {
+    let sender_id = programs::ping_sender().id();
+    *state = std::mem::replace(state, V03State::new()).with_public_accounts([(
+        sender_config_account_id(sender_id),
+        Account {
+            program_owner: sender_id,
+            data: outbox_bytes(programs::cross_zone_outbox().id())
+                .to_vec()
+                .try_into()
+                .expect("outbox id fits in account data"),
+            ..Default::default()
+        },
+    )]);
+}
+
+/// A `ping_sender::Send` carrying `payload` to `target_zone`, over the accounts
+/// given rather than the correct ones, so tests can vary them.
+fn send_tx(accounts: Vec<AccountId>, target_zone: [u8; 32], ordinal: u32) -> PublicTransaction {
+    let receiver_id = programs::ping_receiver().id();
+    let words = risc0_zkvm::serde::to_vec(&ReceiverInstruction::Record {
+        payload: b"ping".to_vec(),
+    })
+    .expect("serialize ping instruction");
+    let send = ping_core::SenderInstruction::Send {
+        target_zone,
+        target_program_id: receiver_id,
+        target_accounts: vec![ping_record_pda(receiver_id).into_value()],
+        payload: words.iter().flat_map(|word| word.to_le_bytes()).collect(),
+        ordinal,
+    };
+    let message = Message::try_new(programs::ping_sender().id(), accounts, vec![], send)
+        .expect("build ping_sender message");
+    PublicTransaction::new(message, WitnessSet::from_raw_parts(vec![]))
+}
+
 /// The wrapped-token `Mint` the bridge forwards, serialized as the cross-zone
 /// payload (risc0 words, little-endian bytes).
 fn mint_payload() -> Vec<u8> {
@@ -423,6 +462,7 @@ fn two_emitters_share_an_ordinal_without_colliding() {
             ..Default::default()
         },
     )]);
+    seed_ping_sender_config(&mut state);
 
     let lock_slot = outbox_pda(outbox_id, bridge_lock_id, &zone_b, ordinal);
     let send_slot = outbox_pda(outbox_id, sender_id, &zone_b, ordinal);
@@ -436,23 +476,12 @@ fn two_emitters_share_an_ordinal_without_colliding() {
         .expect("the lock executes");
     state.apply_state_diff(diff);
 
-    let words = risc0_zkvm::serde::to_vec(&ReceiverInstruction::Record {
-        payload: b"ping".to_vec(),
-    })
-    .expect("serialize ping instruction");
-    let send = ping_core::SenderInstruction::Send {
-        outbox_program_id: outbox_id,
-        target_zone: zone_b,
-        target_program_id: receiver_id,
-        target_accounts: vec![ping_record_pda(receiver_id).into_value()],
-        payload: words.iter().flat_map(|word| word.to_le_bytes()).collect(),
+    let send = send_tx(
+        vec![sender_config_account_id(sender_id), send_slot],
+        zone_b,
         ordinal,
-    };
-    let message = Message::try_new(sender_id, vec![send_slot], vec![], send)
-        .expect("build ping_sender message");
-    let send_tx = PublicTransaction::new(message, WitnessSet::from_raw_parts(vec![]));
-
-    let send_diff = ValidatedStateDiff::from_public_transaction(&send_tx, &state, 2, 0)
+    );
+    let send_diff = ValidatedStateDiff::from_public_transaction(&send, &state, 2, 0)
         .expect("the send executes into its own slot, not the lock's");
 
     let record = OutboxRecord::from_bytes(
@@ -472,6 +501,135 @@ fn two_emitters_share_an_ordinal_without_colliding() {
     assert_eq!(lock_record.emitter, bridge_lock_id);
 }
 
+/// A caller can no longer aim an emission at a program of their own and still
+/// succeed, leaving no record of it. With the program no longer an instruction
+/// field, the account is the only way left to try.
+#[test]
+fn a_send_into_a_foreign_outbox_slot_is_rejected() {
+    let sender_id = programs::ping_sender().id();
+    let zone_b = [2_u8; 32];
+    let ordinal = 0;
+
+    let mut state = base_state();
+    seed_ping_sender_config(&mut state);
+
+    // A slot under some other program, which is what the caller would have to
+    // pass to reach it.
+    let foreign_slot = outbox_pda([3; 8], sender_id, &zone_b, ordinal);
+    let send = send_tx(
+        vec![sender_config_account_id(sender_id), foreign_slot],
+        zone_b,
+        ordinal,
+    );
+
+    // Refused inside the pinned outbox, not by the sender: the chained call goes
+    // there whatever account the caller passes, which is the point.
+    let Err(err) = ValidatedStateDiff::from_public_transaction(&send, &state, 1, 0) else {
+        panic!("a send into a slot outside the pinned outbox must not execute");
+    };
+    assert!(
+        format!("{err:?}").contains("Account must be the outbox PDA"),
+        "rejected for the wrong reason: {err:?}"
+    );
+}
+
+/// An emitter with no pin cannot fall back to a caller-named outbox: it stops
+/// emitting. The state a zone reaches by skipping the genesis init.
+#[test]
+fn a_send_before_the_pin_is_set_is_rejected() {
+    let sender_id = programs::ping_sender().id();
+    let outbox_id = programs::cross_zone_outbox().id();
+    let zone_b = [2_u8; 32];
+    let ordinal = 0;
+
+    let state = base_state();
+    let slot = outbox_pda(outbox_id, sender_id, &zone_b, ordinal);
+    let send = send_tx(
+        vec![sender_config_account_id(sender_id), slot],
+        zone_b,
+        ordinal,
+    );
+
+    let Err(err) = ValidatedStateDiff::from_public_transaction(&send, &state, 1, 0) else {
+        panic!("a send with no outbox pinned must not execute");
+    };
+    assert!(
+        format!("{err:?}").contains("config account holds an outbox program id"),
+        "rejected for the wrong reason: {err:?}"
+    );
+}
+
+/// The config is read by address, so substituting another account for it fails
+/// rather than pinning the outbox to whatever that account happens to hold.
+#[test]
+fn a_send_with_a_substituted_config_account_is_rejected() {
+    let sender_id = programs::ping_sender().id();
+    let outbox_id = programs::cross_zone_outbox().id();
+    let zone_b = [2_u8; 32];
+    let ordinal = 0;
+
+    let mut state = base_state();
+    seed_ping_sender_config(&mut state);
+
+    let slot = outbox_pda(outbox_id, sender_id, &zone_b, ordinal);
+    let send = send_tx(vec![ping_record_pda(sender_id), slot], zone_b, ordinal);
+
+    let Err(err) = ValidatedStateDiff::from_public_transaction(&send, &state, 1, 0) else {
+        panic!("a send over a substituted config account must not execute");
+    };
+    assert!(
+        format!("{err:?}").contains("must be the ping-sender config PDA"),
+        "rejected for the wrong reason: {err:?}"
+    );
+}
+
+/// Written once: an identical re-init has to succeed, since genesis is replayed
+/// during multi-sequencer reconstruction, while one naming a different outbox has
+/// to fail, or anyone could redirect every emission on the zone after genesis.
+#[test]
+fn the_outbox_pin_is_written_once_and_replayable() {
+    let sender_id = programs::ping_sender().id();
+    let config_id = sender_config_account_id(sender_id);
+
+    // Unsigned and nonce-free, as genesis builds it: the config PDA has no signer.
+    let init = |outbox: lee_core::program::ProgramId| {
+        let message = Message::try_new(
+            sender_id,
+            vec![config_id],
+            vec![],
+            ping_core::SenderInstruction::InitConfig {
+                outbox_program_id: outbox,
+            },
+        )
+        .expect("build InitConfig message");
+        PublicTransaction::new(message, WitnessSet::from_raw_parts(vec![]))
+    };
+
+    let mut state = base_state();
+    let outbox_id = programs::cross_zone_outbox().id();
+
+    let first = init(outbox_id);
+    let diff = ValidatedStateDiff::from_public_transaction(&first, &state, 1, 0)
+        .expect("the first init claims the config PDA");
+    state.apply_state_diff(diff);
+    assert_eq!(
+        read_outbox(&state.get_account_by_id(config_id).data.into_inner()),
+        Some(outbox_id),
+        "the config pins the outbox after genesis"
+    );
+
+    ValidatedStateDiff::from_public_transaction(&init(outbox_id), &state, 2, 0)
+        .expect("replaying the identical init is a no-op, not a failure");
+
+    let Err(err) = ValidatedStateDiff::from_public_transaction(&init([3; 8]), &state, 3, 0) else {
+        panic!("a re-init naming a different outbox must not execute");
+    };
+    assert!(
+        format!("{err:?}").contains("already pins a different outbox"),
+        "rejected for the wrong reason: {err:?}"
+    );
+}
+
 /// Drives a hand-built `cross_zone_inbox::Dispatch` (as the watcher would inject)
 /// and asserts it chains into `wrapped_token::Mint`, crediting the recipient.
 #[test]
diff --git a/integration_tests/tests/cross_zone_verified.rs b/integration_tests/tests/cross_zone_verified.rs
index 59e3cce50..577239d05 100644
--- a/integration_tests/tests/cross_zone_verified.rs
+++ b/integration_tests/tests/cross_zone_verified.rs
@@ -21,7 +21,9 @@ use integration_tests::{
 };
 use lee::{AccountId, PublicTransaction, public_transaction::Message};
 use lee_core::program::ProgramId;
-use ping_core::{ReceiverInstruction, SenderInstruction, ping_record_pda};
+use ping_core::{
+    ReceiverInstruction, SenderInstruction, ping_record_pda, sender_config_account_id,
+};
 use sequencer_core::config::{CrossZoneConfig, CrossZonePeer, CrossZoneRoute};
 use sequencer_service_rpc::RpcClient as _;
 use tokio::test;
@@ -109,7 +111,6 @@ fn build_ping_tx(target_zone: [u8; 32], receiver_id: ProgramId) -> LeeTransactio
     let payload: Vec<u8> = words.iter().flat_map(|word| word.to_le_bytes()).collect();
 
     let send = SenderInstruction::Send {
-        outbox_program_id: outbox_id,
         target_zone,
         target_program_id: receiver_id,
         target_accounts: vec![ping_record_pda(receiver_id).into_value()],
@@ -117,15 +118,11 @@ fn build_ping_tx(target_zone: [u8; 32], receiver_id: ProgramId) -> LeeTransactio
         ordinal,
     };
 
-    let outbox_account = outbox_pda(
-        outbox_id,
-        programs::ping_sender().id(),
-        &target_zone,
-        ordinal,
-    );
+    let sender_id = programs::ping_sender().id();
+    let outbox_account = outbox_pda(outbox_id, sender_id, &target_zone, ordinal);
     let message = Message::try_new(
-        programs::ping_sender().id(),
-        vec![outbox_account],
+        sender_id,
+        vec![sender_config_account_id(sender_id), outbox_account],
         vec![],
         send,
     )
diff --git a/integration_tests/tests/cross_zone_watcher_restart.rs b/integration_tests/tests/cross_zone_watcher_restart.rs
index 73028f56f..f91d59c97 100644
--- a/integration_tests/tests/cross_zone_watcher_restart.rs
+++ b/integration_tests/tests/cross_zone_watcher_restart.rs
@@ -25,7 +25,9 @@ use integration_tests::{
 };
 use lee::{AccountId, PublicTransaction, public_transaction::Message};
 use lee_core::program::ProgramId;
-use ping_core::{ReceiverInstruction, SenderInstruction, ping_record_pda};
+use ping_core::{
+    ReceiverInstruction, SenderInstruction, ping_record_pda, sender_config_account_id,
+};
 use sequencer_core::config::{CrossZoneConfig, CrossZonePeer, CrossZoneRoute};
 use sequencer_service_rpc::{RpcClient as _, SequencerClient};
 use tokio::test;
@@ -190,7 +192,6 @@ fn build_ping_tx(target_zone: [u8; 32], receiver_id: ProgramId) -> LeeTransactio
     let payload: Vec<u8> = words.iter().flat_map(|word| word.to_le_bytes()).collect();
 
     let send = SenderInstruction::Send {
-        outbox_program_id: outbox_id,
         target_zone,
         target_program_id: receiver_id,
         target_accounts: vec![ping_record_pda(receiver_id).into_value()],
@@ -198,15 +199,11 @@ fn build_ping_tx(target_zone: [u8; 32], receiver_id: ProgramId) -> LeeTransactio
         ordinal,
     };
 
-    let outbox_account = outbox_pda(
-        outbox_id,
-        programs::ping_sender().id(),
-        &target_zone,
-        ordinal,
-    );
+    let sender_id = programs::ping_sender().id();
+    let outbox_account = outbox_pda(outbox_id, sender_id, &target_zone, ordinal);
     let message = Message::try_new(
-        programs::ping_sender().id(),
-        vec![outbox_account],
+        sender_id,
+        vec![sender_config_account_id(sender_id), outbox_account],
         vec![],
         send,
     )
diff --git a/lez/cross_zone/src/lib.rs b/lez/cross_zone/src/lib.rs
index 7ea3fdf0f..d0eafd6fd 100644
--- a/lez/cross_zone/src/lib.rs
+++ b/lez/cross_zone/src/lib.rs
@@ -64,13 +64,18 @@ pub fn is_sequencer_only_program(program_id: ProgramId) -> bool {
 #[must_use]
 pub fn extract_emission(program_id: ProgramId, instruction_data: &[u32]) -> Option<Emission> {
     if program_id == programs::ping_sender().id() {
-        let ping_core::SenderInstruction::Send {
+        // Not every transaction to an emitter emits: `InitConfig` is one of its
+        // instructions, so a non-`Send` decode is an ordinary non-emitting tx.
+        let Ok(ping_core::SenderInstruction::Send {
             target_zone,
             target_program_id,
             target_accounts,
             payload,
             ..
-        } = risc0_zkvm::serde::from_slice(instruction_data).ok()?;
+        }) = risc0_zkvm::serde::from_slice(instruction_data)
+        else {
+            return None;
+        };
         Some(Emission {
             target_zone,
             target_program_id,
@@ -215,6 +220,20 @@ pub fn build_wrapped_token_init_config_tx() -> lee::PublicTransaction {
     )
 }
 
+/// The genesis transaction that pins the outbox `ping_sender` chains into,
+/// without importing the outbox id into the guest.
+#[must_use]
+pub fn build_ping_sender_init_config_tx() -> lee::PublicTransaction {
+    let ping_sender_id = programs::ping_sender().id();
+    genesis_public_tx(
+        ping_sender_id,
+        vec![ping_core::sender_config_account_id(ping_sender_id)],
+        ping_core::SenderInstruction::InitConfig {
+            outbox_program_id: programs::cross_zone_outbox().id(),
+        },
+    )
+}
+
 /// Builds an unsigned, sequencer-origin genesis transaction invoking `instruction`
 /// on `program_id` over `account_ids`.
 fn genesis_public_tx<I: Serialize>(
diff --git a/lez/indexer/core/src/cross_zone_verifier.rs b/lez/indexer/core/src/cross_zone_verifier.rs
index 1cc48ee17..03d742a55 100644
--- a/lez/indexer/core/src/cross_zone_verifier.rs
+++ b/lez/indexer/core/src/cross_zone_verifier.rs
@@ -746,7 +746,6 @@ mod tests {
     fn emission(payload: &[u8]) -> LeeTransaction {
         let receiver_id = programs::ping_receiver().id();
         let send = SenderInstruction::Send {
-            outbox_program_id: programs::cross_zone_outbox().id(),
             target_zone: SELF_ZONE,
             target_program_id: receiver_id,
             target_accounts: vec![ping_record_pda(receiver_id).into_value()],
diff --git a/lez/programs/ping_core/Cargo.toml b/lez/programs/ping_core/Cargo.toml
index 29870630f..93b0e430d 100644
--- a/lez/programs/ping_core/Cargo.toml
+++ b/lez/programs/ping_core/Cargo.toml
@@ -10,3 +10,6 @@ workspace = true
 [dependencies]
 lee_core.workspace = true
 serde = { workspace = true, features = ["alloc"] }
+
+[dev-dependencies]
+risc0-zkvm.workspace = true
diff --git a/lez/programs/ping_core/src/lib.rs b/lez/programs/ping_core/src/lib.rs
index 80b272479..cc2214bbb 100644
--- a/lez/programs/ping_core/src/lib.rs
+++ b/lez/programs/ping_core/src/lib.rs
@@ -5,6 +5,7 @@ use lee_core::{
 use serde::{Deserialize, Serialize};
 
 const PING_RECORD_SEED: [u8; 32] = *b"/LEZ/v0.3/PingRecord/0000000000/";
+const SENDER_CONFIG_SEED: [u8; 32] = *b"/LEZ/v0.3/PingSenderCfg/0000000/";
 
 /// Instruction delivered to `ping_receiver` by the inbox: record the payload.
 #[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
@@ -12,17 +13,29 @@ pub enum ReceiverInstruction {
     Record { payload: Vec<u8> },
 }
 
-/// Instruction to `ping_sender`: forwarded verbatim into `cross_zone_outbox::Instruction::Emit`.
+/// Instruction to `ping_sender`. `Send`'s emission fields are forwarded verbatim
+/// into `cross_zone_outbox::Instruction::Emit`.
+///
+/// Variants are append-only. risc0 serde encodes the variant as a bare leading
+/// tag word, so inserting one ahead of `Send` shifts every existing encoding.
 #[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
 pub enum SenderInstruction {
+    /// Emit a cross-zone message through the pinned outbox.
+    ///
+    /// Required accounts (2): the sender config PDA, then the outbox PDA.
     Send {
-        outbox_program_id: ProgramId,
         target_zone: [u8; 32],
         target_program_id: ProgramId,
         target_accounts: Vec<[u8; 32]>,
         payload: Vec<u8>,
         ordinal: u32,
     },
+    /// Pins the outbox program, written once into a default config PDA at
+    /// genesis. A re-run naming a different outbox is refused; an identical one
+    /// is a no-op, which is what genesis replay does.
+    ///
+    /// Required accounts (1): the sender config PDA.
+    InitConfig { outbox_program_id: ProgramId },
 }
 
 /// The account a `ping_receiver` records the latest delivered payload into.
@@ -36,3 +49,65 @@ pub fn ping_record_pda(receiver_id: ProgramId) -> AccountId {
 pub const fn ping_record_seed() -> PdaSeed {
     PdaSeed::new(PING_RECORD_SEED)
 }
+
+/// PDA holding the outbox program id, seeded at genesis so the guest can pin the
+/// program it chains into without importing the outbox image id.
+#[must_use]
+pub fn sender_config_account_id(sender_id: ProgramId) -> AccountId {
+    AccountId::for_public_pda(&sender_id, &sender_config_seed())
+}
+
+#[must_use]
+pub const fn sender_config_seed() -> PdaSeed {
+    PdaSeed::new(SENDER_CONFIG_SEED)
+}
+
+/// Encodes the pinned outbox program id for the config account's data.
+#[must_use]
+pub fn outbox_bytes(outbox_program_id: ProgramId) -> [u8; 32] {
+    let mut bytes = [0_u8; 32];
+    for (word, chunk) in outbox_program_id.iter().zip(bytes.chunks_exact_mut(4)) {
+        chunk.copy_from_slice(&word.to_le_bytes());
+    }
+    bytes
+}
+
+/// Decodes the pinned outbox program id from the config account's data.
+#[must_use]
+pub fn read_outbox(data: &[u8]) -> Option<ProgramId> {
+    if data.len() < 32 {
+        return None;
+    }
+    let mut outbox_program_id = [0_u32; 8];
+    for (word, chunk) in outbox_program_id.iter_mut().zip(data[..32].chunks_exact(4)) {
+        *word = u32::from_le_bytes(chunk.try_into().unwrap_or_else(|_| unreachable!()));
+    }
+    Some(outbox_program_id)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// `extract_emission` decodes `Send` off peer transactions, so its tag word is
+    /// wire format: a variant inserted ahead of it would silently shift every
+    /// existing encoding.
+    #[test]
+    fn send_is_the_first_variant() {
+        let send = SenderInstruction::Send {
+            target_zone: [7; 32],
+            target_program_id: [1; 8],
+            target_accounts: vec![],
+            payload: vec![],
+            ordinal: 0,
+        };
+        let words = risc0_zkvm::serde::to_vec(&send).expect("Send serializes");
+        assert_eq!(words[0], 0);
+    }
+
+    #[test]
+    fn outbox_id_round_trips() {
+        let outbox: ProgramId = [9; 8];
+        assert_eq!(read_outbox(&outbox_bytes(outbox)), Some(outbox));
+    }
+}
diff --git a/lez/programs/ping_sender/src/main.rs b/lez/programs/ping_sender/src/main.rs
index d0ad04f56..7ae0ea038 100644
--- a/lez/programs/ping_sender/src/main.rs
+++ b/lez/programs/ping_sender/src/main.rs
@@ -1,9 +1,14 @@
 use cross_zone_outbox_core::Instruction as OutboxInstruction;
 use lee_core::{
-    account::AccountWithMetadata,
-    program::{AccountPostState, ChainedCall, ProgramInput, ProgramOutput, read_lee_inputs},
+    account::{Account, AccountWithMetadata},
+    program::{
+        AccountPostState, ChainedCall, Claim, ProgramId, ProgramInput, ProgramOutput,
+        read_lee_inputs,
+    },
+};
+use ping_core::{
+    SenderInstruction, outbox_bytes, read_outbox, sender_config_account_id, sender_config_seed,
 };
-use ping_core::SenderInstruction;
 
 fn main() {
     let (
@@ -21,19 +26,63 @@ fn main() {
         "ping_sender is only invoked as a top-level user transaction"
     );
 
-    let SenderInstruction::Send {
-        outbox_program_id,
-        target_zone,
-        target_program_id,
-        target_accounts,
-        payload,
-        ordinal,
-    } = instruction;
+    match instruction {
+        SenderInstruction::Send {
+            target_zone,
+            target_program_id,
+            target_accounts,
+            payload,
+            ordinal,
+        } => send(
+            self_program_id,
+            caller_program_id,
+            pre_states,
+            instruction_words,
+            target_zone,
+            target_program_id,
+            target_accounts,
+            payload,
+            ordinal,
+        ),
+        SenderInstruction::InitConfig { outbox_program_id } => init_config(
+            self_program_id,
+            caller_program_id,
+            pre_states,
+            instruction_words,
+            outbox_program_id,
+        ),
+    }
+}
 
-    // The single account is the outbox PDA the chained call writes into; the
-    // outbox claims it, so ping_sender forwards it unchanged.
-    let [outbox] =
-        <[AccountWithMetadata; 1]>::try_from(pre_states).expect("Send requires exactly 1 account");
+#[expect(
+    clippy::too_many_arguments,
+    reason = "the emission fields are passed through verbatim"
+)]
+fn send(
+    self_program_id: ProgramId,
+    caller_program_id: Option<ProgramId>,
+    pre_states: Vec<AccountWithMetadata>,
+    instruction_words: Vec<u32>,
+    target_zone: [u8; 32],
+    target_program_id: ProgramId,
+    target_accounts: Vec<[u8; 32]>,
+    payload: Vec<u8>,
+    ordinal: u32,
+) {
+    // pre_states: [config PDA, outbox PDA]. The outbox claims its own slot, so
+    // ping_sender forwards it unchanged.
+    let [config, outbox] = <[AccountWithMetadata; 2]>::try_from(pre_states)
+        .expect("Send requires the config and outbox accounts");
+
+    // Pinned rather than caller-named: chaining elsewhere would let an emission
+    // skip the real outbox and leave no record of itself.
+    assert_eq!(
+        config.account_id,
+        sender_config_account_id(self_program_id),
+        "first account must be the ping-sender config PDA"
+    );
+    let outbox_program_id = read_outbox(&config.account.data.clone().into_inner())
+        .expect("config account holds an outbox program id");
 
     let call = ChainedCall::new(
         outbox_program_id,
@@ -47,13 +96,65 @@ fn main() {
         },
     );
 
+    let config_post = AccountPostState::new(config.account.clone());
+
     ProgramOutput::new(
         self_program_id,
         caller_program_id,
         instruction_words,
-        vec![outbox.clone()],
-        vec![AccountPostState::new(outbox.account)],
+        vec![config, outbox.clone()],
+        vec![config_post, AccountPostState::new(outbox.account)],
     )
     .with_chained_calls(vec![call])
     .write();
 }
+
+/// Writes the outbox program id into the config PDA exactly once at genesis.
+fn init_config(
+    self_program_id: ProgramId,
+    caller_program_id: Option<ProgramId>,
+    pre_states: Vec<AccountWithMetadata>,
+    instruction_words: Vec<u32>,
+    outbox_program_id: ProgramId,
+) {
+    // pre_states: [config PDA].
+    let [config] = <[AccountWithMetadata; 1]>::try_from(pre_states)
+        .expect("InitConfig requires the config account");
+    assert_eq!(
+        config.account_id,
+        sender_config_account_id(self_program_id),
+        "account must be the ping-sender config PDA"
+    );
+    // Init-once, idempotent under genesis replay: a `default` config is a first
+    // init; an already-owned one must already pin exactly this outbox, since
+    // genesis is replayed onto seeded state during multi-sequencer reconstruction.
+    // `new_claimed_if_default` alone would not stop a later self-owned rewrite.
+    if config.account != Account::default() {
+        assert_eq!(
+            config.account.program_owner, self_program_id,
+            "ping-sender config PDA is owned by another program"
+        );
+        assert_eq!(
+            config.account.data.clone().into_inner(),
+            outbox_bytes(outbox_program_id).to_vec(),
+            "ping-sender config already pins a different outbox"
+        );
+    }
+
+    let mut config_account = config.account.clone();
+    config_account.data = outbox_bytes(outbox_program_id)
+        .to_vec()
+        .try_into()
+        .expect("outbox id fits in account data");
+    let config_post =
+        AccountPostState::new_claimed_if_default(config_account, Claim::Pda(sender_config_seed()));
+
+    ProgramOutput::new(
+        self_program_id,
+        caller_program_id,
+        instruction_words,
+        vec![config],
+        vec![config_post],
+    )
+    .write();
+}
diff --git a/lez/sequencer/core/src/cross_zone_watcher.rs b/lez/sequencer/core/src/cross_zone_watcher.rs
index 0ddc4b6d3..96c254fc2 100644
--- a/lez/sequencer/core/src/cross_zone_watcher.rs
+++ b/lez/sequencer/core/src/cross_zone_watcher.rs
@@ -707,7 +707,6 @@ mod tests {
     fn emission_to(target_program_id: lee_core::program::ProgramId) -> LeeTransaction {
         let receiver_id = programs::ping_receiver().id();
         let send = SenderInstruction::Send {
-            outbox_program_id: programs::cross_zone_outbox().id(),
             target_zone: SELF_ZONE,
             target_program_id,
             target_accounts: vec![ping_record_pda(receiver_id).into_value()],
diff --git a/lez/sequencer/core/src/lib.rs b/lez/sequencer/core/src/lib.rs
index b9269e980..7cf5b260c 100644
--- a/lez/sequencer/core/src/lib.rs
+++ b/lez/sequencer/core/src/lib.rs
@@ -1522,13 +1522,15 @@ fn build_genesis_state(config: &SequencerConfig) -> (lee::V03State, Vec<LeeTrans
     );
 
     // Config txs seed the config accounts by transaction, so every node
-    // reconstructs them by replaying the genesis block. The wrapped-token minter is
-    // initialized on every zone (wrapped_token is a builtin), since its InitConfig
-    // is user-callable and a config PDA left default would be claimable by anyone as
-    // the first initializer (a minter hijack). The inbox allowlist is initialized
-    // only on receiving zones; the inbox is sequencer-only, so its default config
-    // PDA is not user-claimable, merely unused until the zone receives.
+    // reconstructs them by replaying the genesis block. The wrapped-token minter and
+    // the ping-sender outbox pin are initialized on every zone: both are builtins
+    // with a user-callable InitConfig, so a config PDA left default is claimable by
+    // the first initializer, hijacking the minter or repointing the emitter's
+    // outbox. The inbox allowlist is initialized only on receiving zones; the inbox
+    // is sequencer-only, so its default config PDA is not user-claimable, merely
+    // unused until the zone receives.
     let wrapped_token_config_tx = std::iter::once(cross_zone::build_wrapped_token_init_config_tx());
+    let ping_sender_config_tx = std::iter::once(cross_zone::build_ping_sender_init_config_tx());
     let inbox_config_tx = config.cross_zone.as_ref().map(|cross_zone| {
         let self_zone = *config.bedrock_config.channel_id.as_ref();
         cross_zone::build_inbox_init_config_tx(self_zone, cross_zone)
@@ -1549,6 +1551,7 @@ fn build_genesis_state(config: &SequencerConfig) -> (lee::V03State, Vec<LeeTrans
     });
 
     let genesis_txs = wrapped_token_config_tx
+        .chain(ping_sender_config_tx)
         .chain(inbox_config_tx)
         .chain(supply_txs)
         .chain(std::iter::once(clock_invocation(0)))
diff --git a/tools/cross_zone_chat/src/main.rs b/tools/cross_zone_chat/src/main.rs
index e482a0afa..13b97e5db 100644
--- a/tools/cross_zone_chat/src/main.rs
+++ b/tools/cross_zone_chat/src/main.rs
@@ -61,7 +61,9 @@ use lee::{
     public_transaction::{Message, WitnessSet},
 };
 use log::{info, warn};
-use ping_core::{ReceiverInstruction, SenderInstruction, ping_record_pda};
+use ping_core::{
+    ReceiverInstruction, SenderInstruction, ping_record_pda, sender_config_account_id,
+};
 use sequencer_service_rpc::{RpcClient as _, SequencerClient, SequencerClientBuilder};
 use serde::{Deserialize, Serialize};
 use test_fixtures::{
@@ -522,7 +524,9 @@ fn decode_inbox_text(instruction_data: &[u32]) -> Option<String> {
 fn decode_send_ordinal(instruction_data: &[u32]) -> Option<u32> {
     let instruction: SenderInstruction =
         risc0_zkvm::serde::from_slice::<SenderInstruction, u32>(instruction_data).ok()?;
-    let SenderInstruction::Send { ordinal, .. } = instruction;
+    let SenderInstruction::Send { ordinal, .. } = instruction else {
+        return None;
+    };
     Some(ordinal)
 }
 
@@ -554,7 +558,6 @@ fn build_send_tx(other_zone: ZoneId, ordinal: u32, text: &str) -> LeeTransaction
     let payload: Vec<u8> = words.iter().flat_map(|word| word.to_le_bytes()).collect();
 
     let send = SenderInstruction::Send {
-        outbox_program_id: outbox_id,
         target_zone: other_zone,
         target_program_id: receiver_id,
         target_accounts: vec![ping_record_pda(receiver_id).into_value()],
@@ -562,15 +565,11 @@ fn build_send_tx(other_zone: ZoneId, ordinal: u32, text: &str) -> LeeTransaction
         ordinal,
     };
 
-    let outbox_account = outbox_pda(
-        outbox_id,
-        programs::ping_sender().id(),
-        &other_zone,
-        ordinal,
-    );
+    let sender_id = programs::ping_sender().id();
+    let outbox_account = outbox_pda(outbox_id, sender_id, &other_zone, ordinal);
     let message = Message::try_new(
-        programs::ping_sender().id(),
-        vec![outbox_account],
+        sender_id,
+        vec![sender_config_account_id(sender_id), outbox_account],
         vec![],
         send,
     )