fix: app arg packing issue when there are exactly 15 args

Author: neilcampbell

src/transactions/method-call.ts

@@ -17,7 +17,7 @@ import { calculateExtraProgramPages } from '../util'
 import { AppCreateParams, AppDeleteParams, AppMethodCallParams, AppUpdateParams } from './app-call'
 import { buildTransactionCommonData } from './common'
 
-const ARGS_TUPLE_PACKING_THRESHOLD = 14 // 14+ args trigger tuple packing, excluding the method selector
+const ARGS_TUPLE_PACKING_THRESHOLD = 15 // ARC-4 allows 15 ABI args (slots 1-15) before tuple packing is needed
 
 /** Parameters to define an ABI method call create transaction. */
 export type AppCreateMethodCall = Expand<AppMethodCall<AppCreateParams>>
@@ -339,8 +339,8 @@ function encodeMethodArguments(method: ABIMethod, args: (ABIValue | undefined)[]
     throw new Error('Mismatch in length of non-transaction arguments')
   }
 
-  // Apply ARC-4 tuple packing for methods with more than 14 arguments
-  // 14 instead of 15 in the ARC-4 because the first argument (method selector) is added separately
+  // Apply ARC-4 tuple packing for methods with more than 15 ABI arguments
+  // Algorand allows 16 app args total; slot 0 is the method selector, leaving 15 for ABI args
   if (abiTypes.length > ARGS_TUPLE_PACKING_THRESHOLD) {
     encodedArgs.push(...encodeArgsWithTuplePacking(abiTypes, abiValues))
   } else {
@@ -372,14 +372,16 @@ function encodeArgsIndividually(abiTypes: ABIType[], abiValues: ABIValue[]): Uin
 function encodeArgsWithTuplePacking(abiTypes: ABIType[], abiValues: ABIValue[]): Uint8Array[] {
   const encodedArgs: Uint8Array[] = []
 
-  // Encode first 14 arguments individually
-  const first14AbiTypes = abiTypes.slice(0, ARGS_TUPLE_PACKING_THRESHOLD)
-  const first14AbiValues = abiValues.slice(0, ARGS_TUPLE_PACKING_THRESHOLD)
-  encodedArgs.push(...encodeArgsIndividually(first14AbiTypes, first14AbiValues))
+  // When packing is needed (> 15 args), we split at 14 to leave one slot for the packed tuple
+  // This gives us: 1 (selector) + 14 (individual) + 1 (packed tuple) = 16 total app args
+  const splitAt = ARGS_TUPLE_PACKING_THRESHOLD - 1
+  const firstAbiTypes = abiTypes.slice(0, splitAt)
+  const firstAbiValues = abiValues.slice(0, splitAt)
+  encodedArgs.push(...encodeArgsIndividually(firstAbiTypes, firstAbiValues))
 
-  // Pack remaining arguments into tuple at position 15
-  const remainingAbiTypes = abiTypes.slice(ARGS_TUPLE_PACKING_THRESHOLD)
-  const remainingAbiValues = abiValues.slice(ARGS_TUPLE_PACKING_THRESHOLD)
+  // Pack remaining arguments into a tuple
+  const remainingAbiTypes = abiTypes.slice(splitAt)
+  const remainingAbiValues = abiValues.slice(splitAt)
 
   if (remainingAbiTypes.length > 0) {
     const tupleType = new ABITupleType(remainingAbiTypes)

src/types/composer.spec.ts

@@ -1,4 +1,4 @@
-import { ABIMethod } from '@algorandfoundation/algokit-abi'
+import { ABIMethod, ABITupleType, ABIType } from '@algorandfoundation/algokit-abi'
 import { Address } from '@algorandfoundation/algokit-common'
 import { Transaction, TransactionType } from '@algorandfoundation/algokit-transact'
 import { beforeEach, describe, expect, test, vi } from 'vitest'
@@ -424,4 +424,49 @@ describe('TransactionComposer', () => {
       expect(resetSpy).toHaveBeenCalled()
     })
   })
+
+  describe('ARC-4 tuple packing', () => {
+    const uint8ArrayType = ABIType.from('uint8[]')
+    const singleArray = uint8ArrayType.encode([1])
+    const twoArrays = new ABITupleType([uint8ArrayType, uint8ArrayType]).encode([[1], [1]])
+    const threeArrays = new ABITupleType([uint8ArrayType, uint8ArrayType, uint8ArrayType]).encode([[1], [1], [1]])
+
+    const testCases: { numAbiArgs: number; expectedTxnArgs: number; expectedLastArg: Uint8Array }[] = [
+      { numAbiArgs: 1, expectedTxnArgs: 2, expectedLastArg: singleArray },
+      { numAbiArgs: 13, expectedTxnArgs: 14, expectedLastArg: singleArray },
+      { numAbiArgs: 14, expectedTxnArgs: 15, expectedLastArg: singleArray },
+      { numAbiArgs: 15, expectedTxnArgs: 16, expectedLastArg: singleArray },
+      { numAbiArgs: 16, expectedTxnArgs: 16, expectedLastArg: twoArrays },
+      { numAbiArgs: 17, expectedTxnArgs: 16, expectedLastArg: threeArrays },
+    ]
+
+    test.each(testCases)(
+      'should handle $numAbiArgs ABI args correctly (expecting $expectedTxnArgs txn args)',
+      async ({ numAbiArgs, expectedTxnArgs, expectedLastArg }) => {
+        const { algorand, context } = fixture
+        const sender = context.testAccount
+
+        // Build method signature with the specified number of uint8[] args
+        const argsSignature = Array(numAbiArgs).fill('uint8[]').join(',')
+        const method = ABIMethod.fromSignature(`args${numAbiArgs}(${argsSignature})void`)
+
+        const composer = algorand.newGroup({ populateAppCallResources: false, coverAppCallInnerTransactionFees: false })
+
+        composer.addAppCallMethodCall({
+          appId: 1234n,
+          method,
+          sender,
+          args: Array(numAbiArgs).fill([1]), // Each arg is [1] (a uint8 array with value 1)
+        })
+
+        const built = await composer.build()
+        const txn = built.transactions[0].txn
+
+        const args = txn.appCall?.args ?? []
+        expect(args.length).toBe(expectedTxnArgs)
+        expect(args[0]).toEqual(method.getSelector())
+        expect(args[args.length - 1]).toEqual(expectedLastArg)
+      },
+    )
+  })
 })