diff --git a/electrum/submarine_swaps.py b/electrum/submarine_swaps.py
index 30a07ec29..dbe2fc99d 100644
--- a/electrum/submarine_swaps.py
+++ b/electrum/submarine_swaps.py
@@ -11,7 +11,7 @@ from .crypto import sha256, hash_160
 from .ecc import ECPrivkey
 from .bitcoin import (script_to_p2wsh, opcodes, p2wsh_nested_script, push_script,
                       is_segwit_address, construct_witness)
-from .transaction import PartialTxInput, PartialTxOutput, PartialTransaction
+from .transaction import PartialTxInput, PartialTxOutput, PartialTransaction, Transaction, TxInput
 from .transaction import script_GetOp, match_script_against_template, OPPushDataGeneric, OPPushDataPubkey
 from .util import log_exceptions
 from .lnutil import REDEEM_AFTER_DOUBLE_SPENT_DELAY, ln_dummy_address
@@ -246,7 +246,7 @@ class SwapManager(Logger):
         assert self.lnwatcher
         privkey = os.urandom(32)
         pubkey = ECPrivkey(privkey).get_public_key_bytes(compressed=True)
-        lnaddr, invoice = await self.lnworker.create_invoice(
+        lnaddr, invoice = self.lnworker.create_invoice(
             amount_msat=lightning_amount_sat * 1000,
             message='swap',
             expiry=3600 * 24,
@@ -535,9 +535,12 @@ class SwapManager(Logger):
             send_amount += self.get_claim_fee()
         return send_amount
 
-    def get_swap_by_tx(self, tx):
+    def get_swap_by_tx(self, tx: Transaction) -> Optional[SwapData]:
         # determine if tx is spending from a swap
         txin = tx.inputs()[0]
+        return self.get_swap_by_claim_txin(txin)
+
+    def get_swap_by_claim_txin(self, txin: TxInput) -> Optional[SwapData]:
         for key, swap in self.swaps.items():
             if txin.prevout.txid.hex() == swap.funding_txid:
                 return swap
@@ -549,10 +552,29 @@ class SwapManager(Logger):
                 return True
         return False
 
-    def sign_tx(self, tx, swap):
+    def add_txin_info(self, txin: PartialTxInput) -> None:
+        """Add some info to a claim txin.
+        note: even without signing, this is useful for tx size estimation.
+        """
+        swap = self.get_swap_by_claim_txin(txin)
+        if not swap:
+            return
+        preimage = swap.preimage if swap.is_reverse else 0
+        witness_script = swap.redeem_script
+        txin.script_type = 'p2wsh'
+        txin.num_sig = 1  # hack so that txin not considered "is_complete"
+        txin.script_sig = b''
+        txin.witness_script = witness_script
+        sig_dummy = b'\x00' * 71  # DER-encoded ECDSA sig, with low S and low R
+        witness = [sig_dummy, preimage, witness_script]
+        txin.witness_sizehint = len(bytes.fromhex(construct_witness(witness)))
+
+    def sign_tx(self, tx: PartialTransaction, swap: SwapData) -> None:
         preimage = swap.preimage if swap.is_reverse else 0
         witness_script = swap.redeem_script
         txin = tx.inputs()[0]
+        assert len(tx.inputs()) == 1, f"expected 1 input for swap claim tx. found {len(tx.inputs())}"
+        assert txin.prevout.txid.hex() == swap.funding_txid
         txin.script_type = 'p2wsh'
         txin.script_sig = b''
         txin.witness_script = witness_script
diff --git a/electrum/transaction.py b/electrum/transaction.py
index 57cb89995..7e4d206ce 100644
--- a/electrum/transaction.py
+++ b/electrum/transaction.py
@@ -712,6 +712,8 @@ class Transaction:
         if not txin.is_segwit():
             return construct_witness([])
 
+        if estimate_size and txin.witness_sizehint is not None:
+            return '00' * txin.witness_sizehint
         if _type in ('address', 'unknown') and estimate_size:
             _type = cls.guess_txintype_from_address(txin.address)
         pubkeys, sig_list = cls.get_siglist(txin, estimate_size=estimate_size)
@@ -1213,6 +1215,7 @@ class PartialTxInput(TxInput, PSBTSection):
         self.spent_txid = None  # type: Optional[str]  # txid of the spender
         self._is_p2sh_segwit = None  # type: Optional[bool]  # None means unknown
         self._is_native_segwit = None  # type: Optional[bool]  # None means unknown
+        self.witness_sizehint = None  # type: Optional[int]  # byte size of serialized complete witness, for tx size est
 
     @property
     def utxo(self):
diff --git a/electrum/wallet.py b/electrum/wallet.py
index 5c7278e17..88f4eb676 100644
--- a/electrum/wallet.py
+++ b/electrum/wallet.py
@@ -1929,10 +1929,13 @@ class Abstract_Wallet(AddressSynchronizer, ABC):
         address = self.get_txin_address(txin)
         # note: we add input utxos regardless of is_mine
         self._add_input_utxo_info(txin, ignore_network_issues=ignore_network_issues, address=address)
-        if not self.is_mine(address):
+        is_mine = self.is_mine(address)
+        if not is_mine:
             is_mine = self._learn_derivation_path_for_address_from_txinout(txin, address)
-            if not is_mine:
-                return
+        if not is_mine:
+            if self.lnworker:
+                self.lnworker.swap_manager.add_txin_info(txin)
+            return
         # set script_type first, as later checks might rely on it:
         txin.script_type = self.get_txin_type(address)
         txin.num_sig = self.m if isinstance(self, Multisig_Wallet) else 1