diff --git a/lightningd/pay.c b/lightningd/pay.c
index e4d83726b..8043b04cb 100644
--- a/lightningd/pay.c
+++ b/lightningd/pay.c
@@ -280,11 +280,11 @@ void payment_succeeded(struct lightningd *ld, struct htlc_out *hout,
 	struct wallet_payment *payment;
 
 	wallet_payment_set_status(ld->wallet, &hout->payment_hash,
-				  /* FIXME: Set partid! */ 0,
+				  hout->partid,
 				  PAYMENT_COMPLETE, rval);
 	payment = wallet_payment_by_hash(tmpctx, ld->wallet,
 					 &hout->payment_hash,
-					 /* FIXME: Set partid! */ 0);
+					 hout->partid);
 	assert(payment);
 
 	tell_waiters_success(ld, &hout->payment_hash, payment);
@@ -982,7 +982,7 @@ send_payment(struct lightningd *ld,
 				final_tlv,
 				route[i].amount,
 				base_expiry + route[i].delay,
-				route[i].amount, payment_secret);
+				total_msat, payment_secret);
 	if (!onion) {
 		return command_fail(cmd, PAY_DESTINATION_PERM_FAIL,
 				    "Destination does not support"
@@ -1094,6 +1094,7 @@ static struct command_result *json_sendonion(struct command *cmd,
 	struct lightningd *ld = cmd->ld;
 	const char *label;
 	struct secret *path_secrets;
+	u64 *partid;
 
 	if (!param(cmd, buffer, params,
 		   p_req("onion", param_bin_from_hex, &onion),
@@ -1101,6 +1102,7 @@ static struct command_result *json_sendonion(struct command *cmd,
 		   p_req("payment_hash", param_sha256, &payment_hash),
 		   p_opt("label", param_escaped_string, &label),
 		   p_opt("shared_secrets", param_secrets_array, &path_secrets),
+		   p_opt_def("partid", param_u64, &partid, 0),
 		   NULL))
 		return command_param_failed();
 
@@ -1112,7 +1114,7 @@ static struct command_result *json_sendonion(struct command *cmd,
 				    "with failcode=%d",
 				    failcode);
 
-	return send_payment_core(ld, cmd, payment_hash, /* FIXME: Set partid! */0,
+	return send_payment_core(ld, cmd, payment_hash, *partid,
 				 first_hop, AMOUNT_MSAT(0), AMOUNT_MSAT(0),
 				 label, NULL, &packet, NULL, NULL, NULL,
 				 path_secrets);
@@ -1163,6 +1165,7 @@ static struct command_result *json_sendpay(struct command *cmd,
 	struct route_hop *route;
 	struct amount_msat *msat;
 	const char *b11str, *label = NULL;
+	u64 *partid;
 	struct secret *payment_secret;
 
 	/* For generating help, give new-style. */
@@ -1175,6 +1178,7 @@ static struct command_result *json_sendpay(struct command *cmd,
 			   p_opt("bolt11", param_string, &b11str),
 			   p_opt("payment_secret", param_secret,
 				 &payment_secret),
+			   p_opt_def("partid", param_u64, &partid, 0),
 			   NULL))
 			return command_param_failed();
 	} else if (params->type == JSMN_ARRAY) {
@@ -1186,6 +1190,7 @@ static struct command_result *json_sendpay(struct command *cmd,
 			   p_opt("bolt11", param_string, &b11str),
 			   p_opt("payment_secret", param_secret,
 				 &payment_secret),
+			   p_opt_def("partid", param_u64, &partid, 0),
 			   NULL))
 			return command_param_failed();
 	} else {
@@ -1199,6 +1204,7 @@ static struct command_result *json_sendpay(struct command *cmd,
 			   p_opt("bolt11", param_string, &b11str),
 			   p_opt("payment_secret", param_secret,
 				 &payment_secret),
+			   p_opt_def("partid", param_u64, &partid, 0),
 			   NULL))
 			return command_param_failed();
 
@@ -1265,20 +1271,21 @@ static struct command_result *json_sendpay(struct command *cmd,
 	 * requesting. The final hop amount is what we actually give, which can
 	 * be from the msatoshi to twice msatoshi. */
 
-	/* if not: msatoshi <= finalhop.amount <= 2 * msatoshi, fail. */
+	if (*partid && !msat)
+		return command_fail(cmd, JSONRPC2_INVALID_PARAMS,
+				    "Must specify msatoshi with partid");
+
+	/* if not: finalhop.amount <= 2 * msatoshi, fail. */
 	if (msat) {
 		struct amount_msat limit = route[routetok->size-1].amount;
 
-		if (amount_msat_less(*msat, limit))
+		if (!amount_msat_add(&limit, limit, limit))
 			return command_fail(cmd, JSONRPC2_INVALID_PARAMS,
-					    "msatoshi %s less than final %s",
-					    type_to_string(tmpctx,
-							   struct amount_msat,
-							   msat),
+					    "Unbelievable final amount %s",
 					    type_to_string(tmpctx,
 							   struct amount_msat,
 							   &route[routetok->size-1].amount));
-		limit.millisatoshis *= 2; /* Raw: sanity check */
+
 		if (amount_msat_greater(*msat, limit))
 			return command_fail(cmd, JSONRPC2_INVALID_PARAMS,
 					    "msatoshi %s more than twice final %s",
@@ -1299,10 +1306,9 @@ static struct command_result *json_sendpay(struct command *cmd,
 	}
 #endif
 
-	return send_payment(cmd->ld, cmd, rhash, /* FIXME: Set partid! */ 0,
+	return send_payment(cmd->ld, cmd, rhash, *partid,
 			    route,
-			    msat ? *msat : route[routetok->size-1].amount,
-			    /* FIXME: Set total_msat! */
+			    route[routetok->size-1].amount,
 			    msat ? *msat : route[routetok->size-1].amount,
 			    label, b11str, payment_secret);
 }
diff --git a/tests/test_pay.py b/tests/test_pay.py
index cd20ed7c8..009a7cb4f 100644
--- a/tests/test_pay.py
+++ b/tests/test_pay.py
@@ -2567,3 +2567,61 @@ def test_sendonion_rpc(node_factory):
     except RpcError as e:
         assert(e.error['code'] == 204)
         assert(e.error['data']['raw_message'] == "400f00000000000003e80000006c")
+
+
+@unittest.skipIf(not DEVELOPER, "needs DEVELOPER=1")
+def test_partial_payment(node_factory, bitcoind, executor):
+    # We want to test two payments at the same time, before we send commit
+    l1, l2, l3, l4 = node_factory.get_nodes(4, [{}] + [{'disconnect': ['=WIRE_UPDATE_ADD_HTLC-nocommit']}] * 2 + [{}])
+
+    # Two routes to l4: one via l2, and one via l3.
+    l1.rpc.connect(l2.info['id'], 'localhost', l2.port)
+    l1.fund_channel(l2, 100000)
+    l1.rpc.connect(l3.info['id'], 'localhost', l3.port)
+    l1.fund_channel(l3, 100000)
+    l2.rpc.connect(l4.info['id'], 'localhost', l4.port)
+    scid24 = l2.fund_channel(l4, 100000)
+    l3.rpc.connect(l4.info['id'], 'localhost', l4.port)
+    scid34 = l3.fund_channel(l4, 100000)
+    bitcoind.generate_block(5)
+
+    # Wait until l1 knows about all channels.
+    wait_for(lambda: len(l1.rpc.listchannels()['channels']) == 8)
+
+    inv = l4.rpc.invoice(1000, 'inv', 'inv')
+    paysecret = l4.rpc.decodepay(inv['bolt11'])['payment_secret']
+
+    # Separate routes for each part of the payment.
+    r134 = l1.rpc.getroute(l4.info['id'], 500, 1, exclude=[scid24 + '/0', scid24 + '/1'])['route']
+    r124 = l1.rpc.getroute(l4.info['id'], 500, 1, exclude=[scid34 + '/0', scid34 + '/1'])['route']
+
+    # These can happen in parallel.
+    l1.rpc.call('sendpay', [r134, inv['payment_hash'], None, 1000, inv['bolt11'], paysecret, 1])
+
+    # Can't mix non-parallel payment!
+    with pytest.raises(RpcError, match=r'Already have parallel payment in progress'):
+        l1.rpc.call('sendpay', {'route': r124,
+                                'payment_hash': inv['payment_hash'],
+                                'msatoshi': 1000,
+                                'payment_secret': paysecret})
+
+    # It will not allow a parallel with different msatoshi!
+    with pytest.raises(RpcError, match=r'msatoshi was previously 1000msat, now 999msat'):
+        l1.rpc.call('sendpay', [r124, inv['payment_hash'], None, 999, inv['bolt11'], paysecret, 2])
+
+    # This will work fine.
+    l1.rpc.call('sendpay', [r124, inv['payment_hash'], None, 1000, inv['bolt11'], paysecret, 2])
+
+    # Any more would exceed total payment
+    with pytest.raises(RpcError, match=r'Already have 1000msat of 1000msat payments in progress'):
+        l1.rpc.call('sendpay', [r124, inv['payment_hash'], None, 1000, inv['bolt11'], paysecret, 3])
+
+    # But repeat is a NOOP.
+    l1.rpc.call('sendpay', [r124, inv['payment_hash'], None, 1000, inv['bolt11'], paysecret, 1])
+    l1.rpc.call('sendpay', [r134, inv['payment_hash'], None, 1000, inv['bolt11'], paysecret, 2])
+
+    # Now continue, payments will fail because receiver doesn't do MPP.
+    l2.rpc.dev_reenable_commit(l4.info['id'])
+    l3.rpc.dev_reenable_commit(l4.info['id'])
+
+    # FIXME: waitsendpay needs a 'partid' field.