feat: add configurable routing fee leeway for Loop Out off-chain payments

Other/swap_out-loop.md

@@ -91,6 +91,10 @@ target_local_ratio = 50
 workers = 1
 # Timeout in seconds for individual route prepay probes (default: 15)
 probe_timeout = 15
+# Percentage leeway added on top of probed routing fee for max off-chain fee limit (default: 100 for +100% / 2x headroom)
+fee_leeway_pct = 100
+# Base satoshis buffer added to fee leeway to absorb base fees and small fluctuations (default: 500)
+fee_leeway_base_sats = 500
 ```
 
 ---
@@ -138,7 +142,8 @@ python3 Other/swap_out-loop.py --history --csv
 | `-w`, `--workers` | `int` | Config / `1` | Number of concurrent worker threads for route probing (recommended: `1` or `2`). |
 | `--probe-timeout` | `int` | Config / `15` | Timeout in seconds for individual route prepay probes. |
 | `--skip-prepay-probe` | `flag` | `False` | Skip active prepay probing and rely on queryroutes theoretical fees (not recommended). |
-| `--max-routing-fee`| `int` | Auto buffer | Upper limit on off-chain routing fees in satoshis. |
+| `--max-routing-fee`| `int` | Leeway buffer | Upper limit on off-chain routing fees in satoshis. Set to 0 to omit fee limit (uses Loop daemon default). |
+| `--fee-leeway-pct` | `float`| Config / `100%` | Percentage leeway added on top of probed routing fee for max off-chain fee budget (e.g. 100 = 2x headroom). |
 | `--dest-addr` | `str` | LND wallet | Custom destination address for swept on-chain funds. |
 | `--dry-run` | `flag` | `False` | Simulates candidate selection, live quotes, and route probes without executing. |
 | `--auto-approve` | `flag` | `False` | Automatically executes the top-ranked candidate without interactive prompt. |

Other/swap_out-loop.py

@@ -913,6 +913,41 @@ def interactive_menu_select(candidates: List[Dict[str, Any]]) -> Optional[Dict[s
             return None
 
 
+def calculate_max_routing_fee_budget(
+    probed_routing_fee: int,
+    config: Any,
+    explicit_max_routing_fee: Optional[int] = None,
+    explicit_leeway_pct: Optional[float] = None,
+) -> int:
+    """
+    Calculates the max off-chain routing fee budget to pass to loop out.
+    - If explicit_max_routing_fee is 0, returns 0 (unbounded / loop daemon default).
+    - If explicit_max_routing_fee > 0, returns that exact satoshi cap.
+    - Otherwise applies percentage leeway (default 100% / 2.0x) plus base satoshi buffer (default 500).
+    """
+    if explicit_max_routing_fee is not None and explicit_max_routing_fee == 0:
+        return 0
+    if explicit_max_routing_fee is not None and explicit_max_routing_fee > 0:
+        return explicit_max_routing_fee
+
+    leeway_pct = (
+        explicit_leeway_pct
+        if explicit_leeway_pct is not None
+        else (
+            config.getfloat("loop", "fee_leeway_pct", fallback=100.0)
+            if hasattr(config, "getfloat")
+            else 100.0
+        )
+    )
+    base_buffer = (
+        config.getint("loop", "fee_leeway_base_sats", fallback=500)
+        if hasattr(config, "getint")
+        else 500
+    )
+    multiplier = 1.0 + (max(0.0, leeway_pct) / 100.0)
+    return int(probed_routing_fee * multiplier) + base_buffer
+
+
 def execute_loop_out(
     config: Any,
     channel_id: str,
@@ -927,9 +962,9 @@ def execute_loop_out(
     label = f"Loop-Out: {alias} ({channel_id})" if alias else f"Loop-Out: {channel_id}"
     if dry_run:
         fake_swap_id = "dry-run-swap-" + binascii.hexlify(os.urandom(16)).decode()
-        print_color(f"\n[DRY RUN] Simulated litloop out execution for channel {channel_id}:", Colors.WARNING, bold=True)
+        fee_arg = f" --max_swap_routing_fee {max_routing_fee}" if max_routing_fee > 0 else ""
         print_color(
-            f'  Command: litloop out --amt {amt} --channel {channel_id} --conf_target {conf_target} --label "{label}" --force',
+            f'  Command: litloop out --amt {amt} --channel {channel_id} --conf_target {conf_target} --label "{label}" --force{fee_arg}',
             Colors.WARNING,
         )
         return {"success": True, "swap_id": fake_swap_id, "dry_run": True}
@@ -1097,8 +1132,14 @@ def parse_arguments() -> argparse.Namespace:
     parser.add_argument(
         "--max-routing-fee",
         type=int,
-        default=0,
-        help="Maximum off-chain swap routing fee in satoshis.",
+        default=None,
+        help="Maximum off-chain swap routing fee in satoshis. If 0, no fee limit is enforced (uses loop daemon default).",
+    )
+    parser.add_argument(
+        "--fee-leeway-pct",
+        type=float,
+        default=None,
+        help="Percentage leeway added on top of probed routing fee for max off-chain fee limit (default from config or 100%%).",
     )
     parser.add_argument(
         "--dest-addr",
@@ -1351,8 +1392,25 @@ def main():
         print_color("Operation cancelled. No swap initiated.", Colors.WARNING)
         sys.exit(0)
 
-    # Max routing fee budget
-    max_rf = args.max_routing_fee or int(selected["routing_fee"] * 1.5) + 50
+    # Max routing fee budget with configurable leeway
+    max_rf = calculate_max_routing_fee_budget(
+        probed_routing_fee=selected["routing_fee"],
+        config=config,
+        explicit_max_routing_fee=args.max_routing_fee,
+        explicit_leeway_pct=args.fee_leeway_pct,
+    )
+
+    if max_rf > 0:
+        buffer_sats = max_rf - selected["routing_fee"]
+        print_color(
+            f"Routing Fee Budget: {max_rf:,} sats (Probed: {selected['routing_fee']:,} sats + {buffer_sats:,} sat leeway)",
+            Colors.OKCYAN,
+        )
+    else:
+        print_color(
+            "Routing Fee Budget: Unbounded (using Loop daemon default limit)",
+            Colors.OKCYAN,
+        )
 
     # Execute
     res = execute_loop_out(

config.ini.example

@@ -116,6 +116,10 @@ target_local_ratio = 50
 workers = 1
 # Timeout in seconds for individual route prepay probes (default: 15)
 probe_timeout = 15
+# Percentage leeway added on top of probed routing fee for max off-chain fee limit (default: 100 for +100% / 2x headroom)
+fee_leeway_pct = 100
+# Base satoshis buffer added to fee leeway to absorb base fees and small fluctuations (default: 500)
+fee_leeway_base_sats = 500
 # Loop daemon connection parameters (used if loop_command is 'loop' instead of 'litloop')
 # rpcserver = localhost:8443
 # tlscertpath = ~/.lit/tls.cert

tests/Other/test_swap_out_loop.py

@@ -515,3 +515,60 @@ def test_evaluate_single_candidate_success():
         assert res["service_fee"] == 2000
         assert res["routing_fee"] == 25
         assert res["total_cost"] == 2195
+
+
+def test_calculate_max_routing_fee_budget_default():
+    """Verify default fee leeway calculation (100% leeway / 2.0x + 500 sats buffer)."""
+    budget = swap_out_loop.calculate_max_routing_fee_budget(
+        probed_routing_fee=20_000,
+        config={},
+    )
+    # 20,000 * 2.0 + 500 = 40,500
+    assert budget == 40_500
+
+
+def test_calculate_max_routing_fee_budget_explicit_zero_unbounded():
+    """Verify that explicit 0 returns 0 (unbounded / loop daemon default)."""
+    budget = swap_out_loop.calculate_max_routing_fee_budget(
+        probed_routing_fee=20_000,
+        config={},
+        explicit_max_routing_fee=0,
+    )
+    assert budget == 0
+
+
+def test_calculate_max_routing_fee_budget_explicit_positive():
+    """Verify that explicit positive fee overrides all leeway calculations."""
+    budget = swap_out_loop.calculate_max_routing_fee_budget(
+        probed_routing_fee=20_000,
+        config={},
+        explicit_max_routing_fee=35_000,
+    )
+    assert budget == 35_000
+
+
+def test_calculate_max_routing_fee_budget_custom_config():
+    """Verify custom config settings for fee_leeway_pct and fee_leeway_base_sats."""
+    import configparser
+    cfg = configparser.ConfigParser()
+    cfg.add_section("loop")
+    cfg.set("loop", "fee_leeway_pct", "50")
+    cfg.set("loop", "fee_leeway_base_sats", "1000")
+
+    budget = swap_out_loop.calculate_max_routing_fee_budget(
+        probed_routing_fee=20_000,
+        config=cfg,
+    )
+    # 20,000 * 1.5 + 1000 = 31,000
+    assert budget == 31_000
+
+
+def test_calculate_max_routing_fee_budget_cli_leeway_override():
+    """Verify that explicit_leeway_pct overrides config."""
+    budget = swap_out_loop.calculate_max_routing_fee_budget(
+        probed_routing_fee=20_000,
+        config={},
+        explicit_leeway_pct=200.0,
+    )
+    # 20,000 * 3.0 + 500 = 60,500
+    assert budget == 60_500