diff --git a/engine/Cargo.toml b/engine/Cargo.toml
index a2c58377..59e623c8 100644
--- a/engine/Cargo.toml
+++ b/engine/Cargo.toml
@@ -51,6 +51,7 @@ magicblock-engine = { path = ".", features = ["testkit"] }
 nucleus = { workspace = true, features = ["testkit"] }
 v42-calculator-interface = { workspace = true, features = ["builder"] }
 
+solana-instruction = { workspace = true }
 solana-instruction-error = { workspace = true }
 solana-packet = { workspace = true }
 solana-signer = { workspace = true }
diff --git a/engine/src/accessor.rs b/engine/src/accessor.rs
index 7b56f03d..e00f0ae7 100644
--- a/engine/src/accessor.rs
+++ b/engine/src/accessor.rs
@@ -1,11 +1,11 @@
 //! Account- and transaction-scoped operation facades.
 
-use std::{sync::atomic::Ordering, time::Duration};
+use std::{collections::BTreeSet, sync::atomic::Ordering, time::Duration};
 
 use keeper::{ExecutionRecord, TransactionView};
 use magic_root_interface::MagicRootInstruction;
 use processor::{SequencerMessage, Simulation, SimulatorMessage};
-use solana_account::OwnedAccount;
+use solana_account::{AccountMode, OwnedAccount};
 use solana_instruction::Instruction;
 use solana_pubkey::Pubkey;
 use solana_transaction::TransactionResult;
@@ -35,18 +35,58 @@ pub struct TransactionAccessor<'a> {
 impl AccountAccessor<'_> {
     /// Creates the account by patching in every field and finalizing it,
     /// optionally running follow-up `actions` once it is finalized.
+    ///
+    /// Missing writable accounts named by `actions` (ER-only PDAs such as an
+    /// auction tree) are asserted with [`MagicRootInstruction::Prepare`] in the
+    /// same transaction, so a follow-up action can initialize them (for example
+    /// `create_ephemeral`). `Prepare` materializes the placeholder only if the
+    /// account still does not exist at execution time, so concurrent creates
+    /// that share such an account converge on the first materialization instead
+    /// of failing each other.
     pub async fn create(
         &self,
         acc: impl Into<OwnedAccount>,
         actions: Option<Vec<Instruction>>,
     ) -> Result<()> {
-        let mut instructions = MagicRootInstruction::compose_account(self.pubkey, acc.into())?;
+        let mut instructions = Vec::new();
+        if let Some(actions) = actions.as_ref() {
+            for pubkey in self.missing_writable_action_accounts(actions) {
+                instructions.push(MagicRootInstruction::Prepare.compose(pubkey)?);
+            }
+        }
+        instructions.extend(MagicRootInstruction::compose_account(
+            self.pubkey,
+            acc.into(),
+        )?);
         if let Some(actions) = actions {
             instructions.push(MagicRootInstruction::PostFinalize(actions).compose(self.pubkey)?);
         }
         self.execute(instructions).await
     }
 
+    /// Writable action accounts that are absent or only a slot-0 Placeholder.
+    fn missing_writable_action_accounts(&self, actions: &[Instruction]) -> Vec<Pubkey> {
+        let accounts = self.engine.accounts();
+        let loader = accounts.loader();
+        let mut seen = BTreeSet::new();
+        let mut missing = Vec::new();
+        for action in actions {
+            for meta in &action.accounts {
+                if !meta.is_writable || meta.pubkey == self.pubkey || !seen.insert(meta.pubkey) {
+                    continue;
+                }
+                let needs_prepare =
+                    loader.load(&meta.pubkey).ok().flatten().is_none_or(|account| {
+                        account.is(AccountMode::Placeholder) && account.slot() == 0
+                    });
+                if needs_prepare {
+                    missing.push(meta.pubkey);
+                }
+            }
+        }
+        missing
+    }
+
     /// Updates the account by patching in every field of `account`
     pub async fn update(&self, acc: impl Into<OwnedAccount>) -> Result<()> {
         let instructions = MagicRootInstruction::compose_account(self.pubkey, acc.into())?;
diff --git a/engine/tests/accounts.rs b/engine/tests/accounts.rs
index 28e0e739..8f29567a 100644
--- a/engine/tests/accounts.rs
+++ b/engine/tests/accounts.rs
@@ -12,6 +12,7 @@ use keeper::testkit::{
 };
 use magic_root_interface::MagicRootInstruction;
 use solana_account::{AccountBuilder, AccountMode, OwnedAccount, ReadableAccount};
+use solana_instruction::AccountMeta;
 use solana_instruction_error::InstructionError;
 use solana_pubkey::Pubkey;
 use solana_system_interface::MAX_PERMITTED_DATA_LENGTH;
@@ -458,3 +459,122 @@ async fn create_runs_post_finalize_actions() {
 
     te.close().await;
 }
+
+// A writable action account that is never initialized stays a Placeholder.
+// PostFinalize must reject that and roll back, not commit an empty Ephemeral.
+#[tokio::test(flavor = "multi_thread")]
+async fn create_rejects_uninitialized_writable_action_accounts() {
+    let te = TestEngine::new().await;
+
+    let source = store_v42(&te, 0, AccountMode::Delegated);
+    let ok_key = Pubkey::new_unique();
+    let missing = Pubkey::new_unique();
+    let mut benign = transfer(source, ok_key, 1);
+    benign.accounts.push(AccountMeta::new(missing, false));
+    let acc = v42_builder(0, AccountMode::Delegated);
+    let result = te.account(ok_key).create(acc, Some(vec![benign])).await;
+    assert!(
+        result.is_err(),
+        "untouched writable placeholder must fail PostFinalize"
+    );
+    assert!(
+        te.get_account(ok_key).is_none(),
+        "create rolls back when a writable action account is left uninitialized"
+    );
+    assert!(
+        te.get_account(missing).is_none(),
+        "the prepared placeholder rolls back with the transaction"
+    );
+
+    te.close().await;
+}
+
+// Concurrent creates of different accounts can name the same missing writable
+// in their actions (the auction-tree pattern). Prepare materializes it only if
+// it still does not exist at execution time, so no create fails another one:
+// each fails solely its own PostFinalize check (the action never initializes
+// the shared writable), never with InvalidArgument or AlreadyProcessed.
+#[tokio::test(flavor = "multi_thread")]
+async fn concurrent_creates_share_one_missing_writable() {
+    let te = TestEngine::new().await;
+
+    let source = store_v42(&te, 0, AccountMode::Delegated);
+    let shared = Pubkey::new_unique();
+    let engine = Engine::clone(&te);
+    let mut handles = Vec::new();
+    for _ in 0..8 {
+        let engine = engine.clone();
+        let target = Pubkey::new_unique();
+        let mut action = transfer(source, target, 1);
+        action.accounts.push(AccountMeta::new(shared, false));
+        handles.push(tokio::spawn(async move {
+            let result = engine
+                .account(target)
+                .create(v42_builder(0, AccountMode::Delegated), Some(vec![action]))
+                .await;
+            (target, result)
+        }));
+    }
+    for handle in handles {
+        let (target, result) = handle.await.expect("create task");
+        let error = result.expect_err("untouched shared writable fails PostFinalize");
+        assert!(
+            matches!(
+                error,
+                EngineError::TransactionExecution(TransactionError::InstructionError(
+                    _,
+                    InstructionError::Immutable,
+                ))
+            ),
+            "each create fails only its own PostFinalize check: {error:?}"
+        );
+        assert!(
+            te.get_account(target).is_none(),
+            "failed create commits nothing"
+        );
+    }
+    assert!(
+        te.get_account(shared).is_none(),
+        "rolled-back creates leave no shared placeholder behind"
+    );
+
+    te.close().await;
+}
+
+// Prepare materializes a missing account as an empty placeholder and leaves any
+// existing account untouched — the execution-time idempotence the concurrent
+// create path relies on.
+#[tokio::test(flavor = "multi_thread")]
+async fn prepare_creates_missing_and_noops_on_existing() {
+    let te = TestEngine::new().await;
+
+    let fresh = Pubkey::new_unique();
+    te.execute(&[MagicRootInstruction::Prepare.compose(fresh).unwrap()])
+        .await
+        .expect("prepare materializes a missing account");
+    let placeholder = te.get_account(fresh).expect("placeholder committed");
+    assert!(placeholder.is(AccountMode::Placeholder));
+    assert_eq!(placeholder.slot(), 1);
+
+    let existing = store_v42(&te, 7, AccountMode::Delegated);
+    let before = te.get_account(existing).expect("existing account");
+    let other = Pubkey::new_unique();
+    te.execute(&[
+        MagicRootInstruction::Prepare.compose(existing).unwrap(),
+        MagicRootInstruction::Prepare.compose(other).unwrap(),
+    ])
+    .await
+    .expect("prepare of an existing account is a no-op");
+    let unchanged = te.get_account(existing).expect("existing account remains");
+    assert!(unchanged.is(AccountMode::Delegated), "mode untouched");
+    assert_eq!(unchanged.slot(), before.slot(), "slot untouched");
+    assert_eq!(unchanged.data(), before.data(), "data untouched");
+    assert!(
+        te.get_account(other)
+            .expect("second prepare committed")
+            .is(AccountMode::Placeholder),
+        "prepare still materializes the missing account in the same transaction"
+    );
+
+    te.close().await;
+}
diff --git a/programs/magic-root-interface/src/lib.rs b/programs/magic-root-interface/src/lib.rs
index 6d0d2811..aaf0c5ce 100644
--- a/programs/magic-root-interface/src/lib.rs
+++ b/programs/magic-root-interface/src/lib.rs
@@ -21,6 +21,12 @@ pub enum MagicRootInstruction {
     /// (e.g. initializing a freshly created account); each is invoked via CPI
     /// against the accounts it declares.
     PostFinalize(Vec<Instruction>),
+    /// Materialize the target as an empty system-owned placeholder if it still
+    /// does not exist at execution time; a target that already exists is left
+    /// untouched. This makes the "create the shared writables my actions need"
+    /// step idempotent, so concurrent creates that name the same missing
+    /// account all converge on whichever transaction materializes it first.
+    Prepare,
 }
 
 impl MagicRootInstruction {
diff --git a/programs/magic-root-program/Cargo.toml b/programs/magic-root-program/Cargo.toml
index 9df75161..a8a77a24 100644
--- a/programs/magic-root-program/Cargo.toml
+++ b/programs/magic-root-program/Cargo.toml
@@ -19,6 +19,7 @@ solana-account = { workspace = true }
 solana-instruction = { workspace = true }
 solana-instruction-error = { workspace = true }
 solana-program-runtime = { workspace = true }
+solana-sdk-ids = { workspace = true }
 solana-svm-log-collector = { workspace = true }
 solana-transaction-context = { workspace = true }
 
diff --git a/programs/magic-root-program/README.md b/programs/magic-root-program/README.md
index ca2abe63..95afefba 100644
--- a/programs/magic-root-program/README.md
+++ b/programs/magic-root-program/README.md
@@ -35,6 +35,12 @@ authorization. The SVM's separate top-level-only privilege rule is unchanged.
   placed immediately after the target's `Finalize` by internal composers. It
   rejects any immutable instruction account marked writable and any action that
   targets MagicRoot itself.
+- `Prepare` materializes the target as an empty system-owned placeholder at
+  slot 1 when the target still does not exist at execution time (loaded as a
+  slot-0 placeholder); any existing target is left untouched. The check runs
+  under the transaction's account locks, so concurrent transactions asserting
+  the same missing account converge on the first materialization instead of
+  failing each other.
 
 After authority and caller checks pass, MagicRoot does not determine whether a
 complete account image is stale. Callers must supply current state; slot and
diff --git a/programs/magic-root-program/src/account.rs b/programs/magic-root-program/src/account.rs
index 325bee70..5bcebbe3 100644
--- a/programs/magic-root-program/src/account.rs
+++ b/programs/magic-root-program/src/account.rs
@@ -70,6 +70,35 @@ pub(crate) fn finalize(
     Ok(())
 }
 
+/// Materializes the target as an empty system-owned placeholder when it does
+/// not exist yet; a target that already exists in any form is left untouched.
+///
+/// The existence check runs at execution time under the transaction's account
+/// locks, so concurrent transactions asserting the same missing account cannot
+/// race: the first one materializes it and the rest no-op.
+pub(crate) fn prepare(
+    ctx: &InvokeContext<'_, '_>,
+    target: IndexOfAccount,
+) -> Result<(), InstructionError> {
+    let mut account = ctx.transaction_context.accounts().try_borrow_mut(target)?;
+    if !account.is(AccountMode::Placeholder) || account.slot() != 0 {
+        ic_msg!(ctx, "MagicRoot: prepare skipped, target exists");
+        return Ok(());
+    }
+    let patches = [
+        AccountFieldPatch::Owner(solana_sdk_ids::system_program::id()),
+        AccountFieldPatch::Slot(1),
+    ];
+    for patch in patches {
+        if let Err(error) = patch.apply(&mut account) {
+            ic_msg!(ctx, "MagicRoot: {}", error);
+            return Err(InstructionError::InvalidArgument);
+        }
+    }
+    ic_msg!(ctx, "MagicRoot: prepared placeholder");
+    Ok(())
+}
+
 /// Marks the target account closed for removal from storage.
 pub(crate) fn delete(
     ctx: &mut InvokeContext<'_, '_>,
diff --git a/programs/magic-root-program/src/processor.rs b/programs/magic-root-program/src/processor.rs
index ebb29fd4..e70b207d 100644
--- a/programs/magic-root-program/src/processor.rs
+++ b/programs/magic-root-program/src/processor.rs
@@ -71,6 +71,7 @@ fn dispatch(
         MagicRootInstruction::Finalize(flags) => account::finalize(ctx, target, flags),
         MagicRootInstruction::Delete => account::delete(ctx, target),
         MagicRootInstruction::PostFinalize(actions) => post_finalize::process(ctx, actions),
+        MagicRootInstruction::Prepare => account::prepare(ctx, target),
     }
 }