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Forward fees

This page explains how smart contracts handle message forwarding fees and value transfers between contracts.

Overview​

When a smart contract sends a query to another smart contract, it must pay for:

  • Sending the internal message (message forwarding fees)
  • Processing the message (gas fees)
  • Sending back the answer, if required (message forwarding fees)

Standard message handling​

note

In most cases, the sender:

  1. Attaches a small amount of Toncoin (typically one Toncoin) to the internal message
  2. Sets the "bounce" flag (sends a bounceable internal message)

The receiver then returns the unused portion of the received value with the answer, deducting message forwarding fees. This is typically done using SENDRAWMSG with mode = 64.

Message bouncing​

Automatic bouncing​

If the receiver cannot parse the received message and terminates with a non-zero exit code (for example, due to an unhandled cell deserialization exception), the message automatically bounces back to its sender. The bounced message:

  • Has its "bounce" flag cleared
  • Has its "bounced" flag set
  • Contains 32 bits 0xffffffff followed by 256 bits from the original message

Handling bounced messages​

Always check the "bounced" flag of incoming internal messages before parsing the op field. This prevents processing a bounced message as a new query.

If the "bounced" flag is set:

  • You can identify the failed query by deserializing op and query_id
  • Take appropriate action based on the failure
  • Alternatively, terminate with zero exit code to ignore bounced messages
note

The "bounced" flag cannot be forged because it's rewritten during sending. If a message has the "bounced" flag set, it's definitely a result of a bounced message from the receiver.

Error handling​

If the receiver successfully parses the incoming query but:

  • The requested method op is not supported
  • Another error condition is met

Send a response with op equal to 0xffffffff or another appropriate value using SENDRAWMSG with mode = 64.

Value transfer with confirmation​

Some operations require both value transfer and confirmation. For example, the validator elections smart contract receives:

  • An election participation request
  • The stake as the attached value

Implementation​

  1. The sender attaches exactly one Toncoin to the intended value
  2. If an error occurs (e.g., stake not accepted):
    • Return the full received amount (minus processing fees)
    • Include an error message using SENDRAWMSG with mode = 64
  3. On success:
    • Create a confirmation message
    • Return exactly one Toncoin (minus message transferring fees)
    • Use SENDRAWMSG with mode = 1
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