SimulateTransactionResponse

public struct SimulateTransactionResponse : Decodable, Sendable

Response from simulating a Soroban contract invocation.

Contains all information needed to understand what will happen when a transaction executes:

• Return values from contract function calls
• Resource requirements (CPU instructions, memory, I/O)
• Ledger footprint (which ledger entries will be read/written)
• Authorization requirements for multi-party transactions
• Events emitted during simulation
• Required fees and resource costs

Use this response to:

• Get results from read-only contract calls without submitting a transaction
• Determine resource limits before submitting a write transaction
• Check if ledger entries need restoration before invocation
• Validate that a transaction will succeed before submission

Before submitting a write transaction, you must:

1. Simulate the transaction
2. Use transactionData and minResourceFee from the simulation
3. Add these to your transaction
4. Sign and submit

Example:

let simResponse = await server.simulateTransaction(simulateTxRequest: request)
switch simResponse {
case .success(let simulation):
    // Check for errors
    if let error = simulation.error {
        print("Simulation failed: \(error)")
        return
    }

    // Check if restoration is needed
    if let restore = simulation.restorePreamble {
        print("Must restore footprint first")
        // Submit RestoreFootprint operation
    }

    // Get return value for read calls
    if let result = simulation.results?.first?.returnValue {
        print("Contract returned: \(result)")
    }

    // For write calls, use simulation data
    transaction.setSorobanTransactionData(simulation.transactionData!)
    transaction.addResourceFee(simulation.minResourceFee!)
case .failure(let error):
    print("RPC error: \(error)")
}

See also:

• [SorobanServer.simulateTransaction] for invoking simulation
• Stellar developer docs