diff --git a/examples/folio/app/shared/src/commonMain/kotlin/app/folio/feature/ledger/AddTransactionViewModel.kt b/examples/folio/app/shared/src/commonMain/kotlin/app/folio/feature/ledger/AddTransactionViewModel.kt
index fbb160d..20337c2 100644
--- a/examples/folio/app/shared/src/commonMain/kotlin/app/folio/feature/ledger/AddTransactionViewModel.kt
+++ b/examples/folio/app/shared/src/commonMain/kotlin/app/folio/feature/ledger/AddTransactionViewModel.kt
@@ -3,6 +3,7 @@ package app.folio.feature.ledger
 import androidx.lifecycle.ViewModel
 import androidx.lifecycle.viewModelScope
 import app.folio.core.data.Account
+import app.folio.core.data.MAX_TRANSACTION_AMOUNT_CENTS
 import app.folio.core.data.Repository
 import app.folio.core.data.TxnType
 import app.folio.navigation.Navigator
@@ -87,6 +88,10 @@ class AddTransactionViewModel(
             form.update { it.copy(error = "Amount must be greater than zero") }
             return
         }
+        if (cents > MAX_TRANSACTION_AMOUNT_CENTS) {
+            form.update { it.copy(error = "Amount is too large (max \$1,000,000.00)") }
+            return
+        }
         viewModelScope.launch {
             try {
                 repository.createTransaction(accountId, s.type, cents, s.note)
diff --git a/examples/folio/core/src/commonMain/kotlin/app/folio/core/data/Repository.kt b/examples/folio/core/src/commonMain/kotlin/app/folio/core/data/Repository.kt
index 020407e..ea8d8e0 100644
--- a/examples/folio/core/src/commonMain/kotlin/app/folio/core/data/Repository.kt
+++ b/examples/folio/core/src/commonMain/kotlin/app/folio/core/data/Repository.kt
@@ -6,6 +6,8 @@ import dev.zacsweers.metro.Inject
 import dev.zacsweers.metro.SingleIn
 import kotlinx.coroutines.flow.StateFlow
 
+const val MAX_TRANSACTION_AMOUNT_CENTS = 100_000_000L
+
 @SingleIn(AppScope::class)
 @Inject
 class Repository(private val store: LedgerStore) {
@@ -27,6 +29,7 @@ class Repository(private val store: LedgerStore) {
 
     suspend fun createTransaction(accountId: String, type: TxnType, amount: Long, note: String): Transaction {
         require(amount > 0) { "Amount must be greater than zero" }
+        require(amount <= MAX_TRANSACTION_AMOUNT_CENTS) { "Amount is too large (max \$1,000,000.00)" }
         requireNotNull(getAccount(accountId)) { "Account not found" }
         val txn = Transaction(
             id = Platform.makeId(),
diff --git a/examples/folio/core/src/commonTest/kotlin/app/folio/core/data/RepositoryTest.kt b/examples/folio/core/src/commonTest/kotlin/app/folio/core/data/RepositoryTest.kt
new file mode 100644
index 0000000..65f9d81
--- /dev/null
+++ b/examples/folio/core/src/commonTest/kotlin/app/folio/core/data/RepositoryTest.kt
@@ -0,0 +1,66 @@
+package app.folio.core.data
+
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.test.runTest
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertFailsWith
+import kotlin.test.assertTrue
+
+private class FakeLedgerStore : LedgerStore {
+    override val accounts = MutableStateFlow<List<Account>>(emptyList())
+    override val transactions = MutableStateFlow<List<Transaction>>(emptyList())
+    override val session = MutableStateFlow<Session?>(null)
+
+    override suspend fun accountExistsByName(name: String): Boolean =
+        accounts.value.any { it.name == name }
+
+    override suspend fun insertAccount(id: String, name: String, createdAt: Long) {
+        accounts.value = accounts.value + Account(id, name, createdAt)
+    }
+
+    override suspend fun insertTxn(
+        id: String,
+        accountId: String,
+        type: TxnType,
+        amount: Long,
+        note: String,
+        createdAt: Long,
+    ) {
+        transactions.value = transactions.value + Transaction(id, accountId, type, amount, note, createdAt)
+    }
+
+    override suspend fun upsertSession(user: String, loggedInAt: Long) {
+        session.value = Session(user, loggedInAt)
+    }
+
+    override suspend fun clearSession() {
+        session.value = null
+    }
+}
+
+class RepositoryTest {
+    @Test
+    fun rejectsAmountAboveOneMillionDollars() = runTest {
+        val repository = Repository(FakeLedgerStore())
+        val account = repository.createAccount("Checking")
+
+        assertFailsWith<IllegalArgumentException> {
+            repository.createTransaction(account.id, TxnType.credit, 100_000_001L, "")
+        }
+        assertFailsWith<IllegalArgumentException> {
+            repository.createTransaction(account.id, TxnType.credit, Long.MAX_VALUE, "")
+        }
+        assertTrue(repository.transactions.value.isEmpty())
+    }
+
+    @Test
+    fun acceptsAmountAtOneMillionDollars() = runTest {
+        val repository = Repository(FakeLedgerStore())
+        val account = repository.createAccount("Checking")
+
+        repository.createTransaction(account.id, TxnType.credit, 100_000_000L, "rent")
+
+        assertEquals(listOf(100_000_000L), repository.transactions.value.map { it.amount })
+    }
+}