Does calling require a portion of the canister cycle?

Developers Motoko
1

I am currently developing a program that involves a process where A canister calls B canister , and I have noticed that it occupies a portion of A’s cycle. Do you have any idea what this might be about?

2

The docs AI is useful for these sorts of questions. Enter the ICP ecosystem

Inter-Canister Call Charging

Inter-Canister Call Charging

Inter-canister calls (also called Xnet calls) are charged to the sending canister, not the receiving one. Here’s how the costs break down:

Cost Components

Component 13-node subnet 34-node subnet
Base fee per call (request + response overhead) 260,000 cycles 680,000 cycles
Per byte transmitted 1,000 cycles/byte 2,615 cycles/byte
Update message execution (base fee on target) 5,000,000 cycles 13,076,923 cycles
Per 1B Wasm instructions executed 1,000,000,000 cycles 2,615,384,615 cycles

Who Pays What

  • Sending canister pays the Xnet call overhead (base + per-byte transmission costs).
  • Receiving canister pays for executing the message (update message base fee + instruction costs).
  • Ingress messages (user → canister) are charged to the receiving canister instead.

Reply Cost Reservation

When a canister makes an inter-canister call, it prepays the maximum-size reply cost upfront. If the actual reply is smaller than the maximum, the difference is refunded. This is handled at the protocol level via ic0.call_perform, which sets aside MAX_CYCLES_PER_RESPONSE cycles for response processing.

Attaching Cycles to a Call

Some canister methods require cycles to be explicitly attached as a per-request fee (e.g., the exchange rate canister charges 1 billion cycles per request). You can do this in:

Motoko:

await (with cycles = 500_000_000) target.someMethod();

Rust:

Call::unbounded_wait(principal, "someMethod")
    .with_cycles(500_000_000u128)
    .await
    .expect("call failed");

Programmatic Cost Lookup

Rather than hardcoding costs, use the ic0.cost_call system API to get the current cost at runtime — it accounts for method name size, payload size, and the current cost schedule.

Query Calls Are Free

Query calls are not charged — they run on a single node, bypass consensus, and incur no cycle cost.