Use Case

I'm building an automated market maker bot for CAT tokens on Dexie. The bot creates 30-60 offers at a time, each sized to specific tiers (e.g. 2.4, 1.2, 0.6, 0.24 XCH). To do this efficiently, I pre-split coins to exact sizes so each offer locks exactly one coin.

The problem is that make_offer doesn't let me specify which coin to use. The wallet's internal coin selector picks automatically, which means:

1. I can't control which coin gets locked — the wallet might pick my reserve coin or a coin from the wrong tier
2. I have to poll before/after each offer to detect which coin disappeared (adds ~1-5s per offer)
3. Coin reuse can occur when creating offers in rapid succession — the wallet picks the same coin twice before it's marked as locked, creating overlapping offers

Current Workaround

Before each offer I snapshot spendable coins via get_xch_coins/get_cat_coins, create the offer, then poll until a coin disappears from the spendable set. This works but is slow and fragile.

Proposed Change

Add an optional coin_ids field to the MakeOffer RPC request:

pub struct MakeOffer {
    pub requested_assets: Vec<OfferAmount>,
    pub offered_assets: Vec<OfferAmount>,
    pub fee: Amount,
    #[serde(default)]
    pub receive_address: Option<String>,
    #[serde(default)]
    pub expires_at_second: Option<u64>,
    #[serde(default = "yes")]
    pub auto_import: bool,
    #[serde(default)]
    pub coin_ids: Option<Vec<String>>,  // <-- NEW: optional specific coins to use
}

When coin_ids is provided, pass them through to select_spends() (or prepare_spends_for_selection()) so the wallet uses exactly those coins instead of auto-selecting. When omitted, behaviour is unchanged.

Why This Should Be Straightforward

The internal plumbing already exists — combine() and split() in coin_management.rs both accept selected_coin_ids: Vec<Bytes32>. The make_offer flow just needs the same pattern: accept optional coin IDs, and if provided, use them instead of calling the auto-selector.

Benefits

• Deterministic offer creation — the caller knows exactly which coin is locked without polling
• Faster batch creation — no need for before/after snapshots between offers
• No coin reuse risk — the caller manages which coins are available
• Enables market maker bots — essential for any automated trading system creating many offers

This would be a huge quality-of-life improvement for anyone building trading bots or automated offer systems on Sage. Happy to help test if a branch is available.