Message mode

As previously mentioned, messages sent via the send() function utilize the mode parameter of the SendParameters structure. The mode is an Int value, which is combined from base modes and optional flags, which are also Int values.

It’s possible to use raw Int values and manually provide them for the mode, but for your convenience there is a set of constants you may use to easily construct the compound mode. Take a look at the following tables for more information on base modes and optional flags.

Note that there are other message-sending functions — they do not use the SendParameters struct, but accept the mode as one of their parameters.

Base modes

Mode value	Constant name	Description
$0$	Since Tact 1.6 SendDefaultMode	Ordinary message (default).
$64$	SendRemainingValue	Carries all the remaining value of the inbound message in addition to the value initially indicated in the new message.
$128$	Use with caution SendRemainingBalance	Carries all the remaining balance of the current smart contract instead of the value originally indicated in the message.
$1024$	Since Tact 1.5 SendOnlyEstimateFee	Doesn’t send the message, only estimates the forward fees if the message-sending function computes these.

The base mode SendRemainingValue does not take previous actions into account, i.e., it doesn’t recalculate the remaining value of the incoming message based on previously sent messages or actions performed during the action phase.

Unlike SendRemainingValue, the base mode SendRemainingBalance always calculates the current value of the contract balance, which can help solve problems with complex outbound message processing.

However, be very careful when using SendRemainingBalance, because it works with the balance of the entire contract, and any mistake with it can lead to a total loss of funds.

Optional flags

Flag value	Constant name	Description
$+1$	SendPayGasSeparately	Deprecated since Tact 1.6.5 Use SendPayFwdFeesSeparately instead.
$+1$	SendPayFwdFeesSeparately	Pay forward fees separately from the message value.
$+2$	SendIgnoreErrors	Ignore any errors arising while processing this message during the action phase.
$+16$	SendBounceIfActionFail	Bounce the transaction in case of any errors during the action phase. Has no effect if flag $+2$, SendIgnoreErrors, is used.
$+32$	SendDestroyIfZero	The current account (contract) will be destroyed if its resulting balance is zero. This flag is often used with mode $128$, SendRemainingBalance.

Combining modes with flags

To create the Int value for the mode field of SendParameters, you simply combine a base mode with optional flags using the bitwise OR operation.

For example, if you want to send a regular message and pay transfer fees separately, use the default mode $0$ and add flag $+1$ to obtain mode = 1, which is equivalent to using the constant SendPayFwdFeesSeparately.

Alternatively, if you want to send the entire contract balance and destroy it immediately, use mode $128$ and add flag $+32$ to get mode = 160, which is equivalent to SendRemainingBalance | SendDestroyIfZero.

Here’s how the latter example would look in code:

let to: Address = address("...");
let value: Int = ton("1");
send(SendParameters{
    to: to,
    value: value,
    mode: SendRemainingBalance | SendDestroyIfZero,
    body: "Hello, World!".asComment(),
});

Note that there can be only one base mode, but the number of optional flags may vary: you can use all, none, or only some of them.

Caution

While adding (+) base modes together with optional flags is possible, it is discouraged due to the possibility of obtaining incorrect values. Instead, use the bitwise OR |, as it is specifically designed for correctly combining flags and performing bit manipulations of the mode.

Functions with implicit mode

Some message-sending functions do not allow setting a mode by passing an argument. This is because their internal logic requires a specific fixed set of modes to be used instead:

• emit() sends a message with the SendDefaultMode ($0$).
• self.reply(), self.notify(), and self.forward() all use the SendRemainingValue mode unless the self.storageReserve constant is overridden to be greater than $0$, in which case they attempt to use the SendRemainingBalance mode.