pay: accept invstring as first parameter name.

diff --git a/common/json_param.h b/common/json_param.h
index a000c71b..7b25715a 100644
--- a/common/json_param.h
+++ b/common/json_param.h
@@ -89,7 +89,7 @@ enum param_style {
  * Add a required parameter.
  */
 #define p_req_depr(name, depr_start, depr_end, cbx, arg)     \
-	      name"",                                        \
+	      name,                                          \
 	      PARAM_REQUIRED,                                \
 	      (depr_start), (depr_end),			     \
 	      (param_cbx)(cbx),				     \
@@ -99,7 +99,10 @@ enum param_style {
 			       (const jsmntok_t *)NULL,      \
 			       (arg)) == (struct command_result *)NULL)
 
-#define p_req(name, cbx, arg) p_req_depr(name, NULL, NULL, (cbx), (arg))
+#define p_req(name, cbx, arg) p_req_depr(name"", NULL, NULL, (cbx), (arg))
+
+/* name is not a literal: I promise its lifetime is infinite though! */
+#define p_req_var(name, cbx, arg) p_req_depr(name, NULL, NULL, (cbx), (arg))
 
 /*
  * Add an optional parameter.  *arg is set to NULL if it isn't found.
diff --git a/contrib/msggen/msggen/schema.json b/contrib/msggen/msggen/schema.json
index 8c929073..7841b7cb 100644
--- a/contrib/msggen/msggen/schema.json
+++ b/contrib/msggen/msggen/schema.json
@@ -30449,6 +30449,8 @@
           "bolt11": {
             "type": "string",
             "description": [
+              "NOTE: `invstring` is an alternate name for this parameter, since v26.06, for xpay compatibility.",
+              "",
               "Bolt11 or bolt12 invoice (such as one received from lightningd-fetchinvoice(7)). For a bolt11 invoice, if it does not contain an amount, *amount_msat* is required, otherwise if it is specified it must be *null*."
             ]
           },
diff --git a/doc/schemas/pay.json b/doc/schemas/pay.json
index 5e89a6ca..8d023c5b 100644
--- a/doc/schemas/pay.json
+++ b/doc/schemas/pay.json
@@ -21,6 +21,8 @@
       "bolt11": {
         "type": "string",
         "description": [
+          "NOTE: `invstring` is an alternate name for this parameter, since v26.06, for xpay compatibility.",
+          "",
           "Bolt11 or bolt12 invoice (such as one received from lightningd-fetchinvoice(7)). For a bolt11 invoice, if it does not contain an amount, *amount_msat* is required, otherwise if it is specified it must be *null*."
         ]
       },
diff --git a/plugins/pay.c b/plugins/pay.c
index 64bc4879..5f338ed7 100644
--- a/plugins/pay.c
+++ b/plugins/pay.c
@@ -1274,6 +1274,7 @@ static struct command_result *json_pay(struct command *cmd,
 	struct out_req *req;
 	struct route_exclusion **exclusions;
 	bool *dev_use_shadow;
+	bool use_invstring;
 
 	/* If any of the modifiers need to add params to the JSON-RPC call we
 	 * would add them to the `param()` call below, and have them be
@@ -1285,9 +1286,10 @@ static struct command_result *json_pay(struct command *cmd,
 	 * deployed by Lightning implementations.
 	 */
 	/* FIXME: Typo in spec for CLTV in descripton!  But it breaks our spelling check, so we omit it above */
+	/* We accept invstring, too (and use it in the help message) */
+	use_invstring = (!params || (params->type == JSMN_OBJECT && json_get_member(buf, params, "invstring")));
 	if (!param_check(cmd, buf, params,
-		   /* FIXME: parameter should be invstring now */
-		   p_req("bolt11", param_invstring, &b11str),
+		   p_req_var(use_invstring ? "invstring" : "bolt11", param_invstring, &b11str),
 		   p_opt("amount_msat", param_msat, &msat),
 		   p_opt("label", param_string, &label),
 		   p_opt_def("riskfactor", param_millionths,
diff --git a/tests/test_pay.py b/tests/test_pay.py
index 81e2b1e7..e33df239 100644
--- a/tests/test_pay.py
+++ b/tests/test_pay.py
@@ -84,6 +84,14 @@ def test_pay(node_factory):
     assert apys_1[0]['routed_in_msat'] == apys_2[0]['routed_out_msat']
 
 
+def test_pay_invstring(node_factory):
+    l1, l2 = node_factory.line_graph(2)
+
+    l1.rpc.check_request_schemas = False
+    inv = l2.rpc.invoice(123000, 'test_pay_invstring', 'description')['bolt11']
+    l1.rpc.call('pay', {'invstring': inv})
+
+
 def test_pay_amounts(node_factory):
     l1, l2 = node_factory.line_graph(2)
     inv = l2.rpc.invoice(Millisatoshi("123sat"), 'test_pay_amounts', 'description')['bolt11']
diff --git a/tests/test_plugin.py b/tests/test_plugin.py
index b4e215f2..b75df23b 100644
--- a/tests/test_plugin.py
+++ b/tests/test_plugin.py
@@ -441,7 +441,7 @@ def test_pay_plugin(node_factory):
         l1.rpc.call('pay')
 
     # Make sure usage messages are present.
-    msg = 'pay bolt11 [amount_msat] [label] [riskfactor] [maxfeepercent] '\
+    msg = 'pay invstring [amount_msat] [label] [riskfactor] [maxfeepercent] '\
           '[retry_for] [maxdelay] [exemptfee] [localinvreqid] [exclude] '\
           '[maxfee] [description] [partial_msat]'
     # We run with --developer: