diff --git a/src/legacy/legacy-cli.ts b/src/legacy/legacy-cli.ts
index 80f9994..c4354e7 100644
--- a/src/legacy/legacy-cli.ts
+++ b/src/legacy/legacy-cli.ts
@@ -1511,15 +1511,19 @@ const COMMAND_HELP: Record<string, CommandHelp> = {
   },
   'invoice-pay': {
     usage: 'invoice-pay <id-or-prefix> [--amount <value>] [--target-index <n>]',
-    description: 'Pay an invoice. By default pays the remaining amount for the first target. For multi-target invoices, specify --target-index.',
+    description: 'Pay an invoice. By default pays the remaining amount for the first target. Use --amount for partial or over-payment; the coin is implicit in the invoice.',
     flags: [
-      { flag: '--amount <value>', description: 'Amount to pay in smallest units (positive integer). Defaults to remaining amount.' },
+      { flag: '--amount <value>', description: 'Amount to pay in HUMAN units of the invoice\'s coin (e.g. "50" = 50 UCT, "0.5" = 0.5 UCT). Defaults to remaining amount needed to cover the target.' },
       { flag: '--target-index <n>', description: 'Target index for multi-target invoices (0-based)', default: '0' },
     ],
     examples: [
-      'npm run cli -- invoice-pay a1b2c3d4',
-      'npm run cli -- invoice-pay a1b2c3d4 --amount 500000',
-      'npm run cli -- invoice-pay a1b2c3d4 --target-index 1 --amount 250000',
+      'sphere invoice pay a1b2c3d4',
+      'sphere invoice pay a1b2c3d4 --amount 0.5',
+      'sphere invoice pay a1b2c3d4 --target-index 1 --amount 25',
+    ],
+    notes: [
+      '--amount is in human units (whole coins, fractional allowed) — matches `payments send` and `invoice create --asset`.',
+      'NFT targets reject --amount (use full-pay form without --amount).',
     ],
   },
   'invoice-return': {
@@ -4793,11 +4797,39 @@ async function main(): Promise<void> {
 
         const payParamsMut: Record<string, unknown> = { targetIndex };
         if (amountIdx2 !== -1 && args[amountIdx2 + 1]) {
-          const rawAmount = args[amountIdx2 + 1];
-          if (!/^[1-9][0-9]*$/.test(rawAmount!)) {
-            failWithHelp('invoice-pay', `invalid amount "${rawAmount}" — must be a positive integer in smallest units (no decimals, no leading zeros)`);
+          const rawAmount = args[amountIdx2 + 1]!;
+          // Issue #36 — canonical UX: `--amount <value>` is in HUMAN units
+          // (e.g. "50" = 50 whole UCT, "0.5" = 0.5 UCT), matching
+          // `payments send 50 UCT` and `invoice create --asset 50 UCT`.
+          // The coin is implicit in the invoice's target/asset entry.
+          // Convert to smallest-unit integer here before handing off; the
+          // SDK's PayInvoiceParams.amount is a raw atom-count string.
+          const invoiceRef = sphere.accounting!.getInvoice(invoiceId);
+          if (!invoiceRef) {
+            failWithHelp('invoice-pay', `invoice ${invoiceId} not found in memory`);
+          }
+          const target = invoiceRef.terms.targets[targetIndex];
+          if (!target) {
+            failWithHelp('invoice-pay', `--target-index ${targetIndex} out of range (invoice has ${invoiceRef.terms.targets.length} target(s))`);
+          }
+          // assetIndex is currently CLI-fixed to 0 (see "Out of scope" in #36).
+          const asset = target.assets[0];
+          if (!asset || !asset.coin) {
+            failWithHelp('invoice-pay', '--amount only applies to coin-asset targets, not NFT');
+          }
+          const [coinSymbol] = asset.coin;
+          const { decimals } = resolveCoin(coinSymbol);
+          let smallest: bigint;
+          try {
+            smallest = toSmallestUnit(rawAmount, decimals);
+          } catch (err) {
+            const msg = err instanceof Error ? err.message : String(err);
+            failWithHelp('invoice-pay', `invalid amount "${rawAmount}" — ${msg}`);
+          }
+          if (smallest <= 0n) {
+            failWithHelp('invoice-pay', `invalid amount "${rawAmount}" — must be positive`);
           }
-          payParamsMut['amount'] = rawAmount;
+          payParamsMut['amount'] = smallest.toString();
         }
         const payParams = payParamsMut as unknown as PayInvoiceParams;