ckBTC skill: CLI examples use created_at_time = null — recommend setting timestamp for deduplication

Problem

The ckBTC skill's CLI examples (in the Deploy & Test / Using icp to Interact with ckBTC Directly section and Verify It Works section) all use created_at_time = null in icrc1_transfer calls:

icp canister call mxzaz-hqaaa-aaaar-qaada-cai icrc1_transfer \
  '(record {
    ...
    created_at_time = null;
  })' -e ic

Setting created_at_time = null disables ICRC-1 transaction deduplication. If a developer copy-pastes this command and runs it twice (a common accident), both transfers execute independently and the amount is sent twice.

This is a correctness issue for anyone using the skill as a reference for CLI-level testing or scripting.

Expected behavior

The ICRC-1 spec deduplication window is 24 hours. When created_at_time is set to the current nanosecond timestamp and the exact same transfer args are submitted again within that window, the ledger returns Duplicate { duplicate_of: block_index } instead of executing again.

The developer docs ledger guide states:

Always set created_at_time to enable deduplication. Without it, two identical transfers submitted within 24 hours both execute.

Suggested fix

Replace created_at_time = null in CLI examples with the current nanosecond timestamp. A portable bash one-liner (works on macOS and Linux):

# Set timestamp once
export NOW_NS=$(python3 -c "import time; print(int(time.time() * 1e9))")

icp canister call mxzaz-hqaaa-aaaar-qaada-cai icrc1_transfer \
  "(record {
    to = record { owner = principal \"$RECIPIENT\"; subaccount = null };
    amount = 100_000;
    fee = opt 10;
    memo = null;
    from_subaccount = null;
    created_at_time = opt ($NOW_NS : nat64);
  })" -e ic

Alternatively, add a note explaining that null disables deduplication and is only appropriate for one-off manual calls where double-submission is not a concern.

Affected files

• skills/ckbtc/SKILL.md — CLI examples in Deploy & Test and Verify It Works sections

Related

Surfaced while writing the ICP developer docs Bitcoin integration guide.

Open skills/ckbtc/SKILL.md and review the CLI examples in the Deploy & Test / Using icp to Interact with ckBTC Directly and Verify It Works sections. Update the created_at_time guidance and examples so the deduplication behavior is clear, then check that the commands remain consistent and portable.