diff --git a/common/bolt11.h b/common/bolt11.h
index e802fc801d4a..b75ad55ef430 100644
--- a/common/bolt11.h
+++ b/common/bolt11.h
@@ -9,7 +9,7 @@
 #include <secp256k1_recovery.h>
 
 /* We only have 10 bits for the field length, meaning < 640 bytes */
-#define BOLT11_FIELD_BYTE_LIMIT ((1 << 10) * 5 / 8)
+#define BOLT11_FIELD_BYTE_LIMIT (((1 << 10) * 5 / 8) - 1)
 
 /* BOLT #11:
  * * `c` (24): `data_length` variable.
diff --git a/lightningd/invoice.c b/lightningd/invoice.c
index 31362ab6c7ef..97e7cc9d6eec 100644
--- a/lightningd/invoice.c
+++ b/lightningd/invoice.c
@@ -1137,8 +1137,8 @@ static struct command_result *json_invoice(struct command *cmd,
 
 	if (strlen(desc_val) > BOLT11_FIELD_BYTE_LIMIT && !*hashonly) {
 		return command_fail(cmd, JSONRPC2_INVALID_PARAMS,
-				    "Descriptions greater than %d bytes "
-				    "not yet supported "
+				    "Description greater than %d bytes "
+				    "invalid "
 				    "(description length %zu)",
 				    BOLT11_FIELD_BYTE_LIMIT,
 				    strlen(desc_val));
diff --git a/plugins/keysend.c b/plugins/keysend.c
index 6104bf607ccd..d1257e213c8a 100644
--- a/plugins/keysend.c
+++ b/plugins/keysend.c
@@ -4,6 +4,7 @@
 #include <ccan/asort/asort.h>
 #include <ccan/cast/cast.h>
 #include <ccan/tal/str/str.h>
+#include <common/bolt11.h>
 #include <common/gossmap.h>
 #include <common/json_param.h>
 #include <common/json_stream.h>
@@ -563,7 +564,7 @@ static struct command_result *htlc_accepted_call(struct command *cmd,
 					   (const char *)desc_field->value);
 		json_add_string(req->js, "description", desc);
 		/* Don't exceed max possible desc length! */
-		if (strlen(desc) > 1023)
+		if (strlen(desc) > BOLT11_FIELD_BYTE_LIMIT)
 			json_add_bool(req->js, "deschashonly", true);
 	} else {
 		json_add_string(req->js, "description", "keysend");
diff --git a/tests/test_invoices.py b/tests/test_invoices.py
index 1e0260415de4..68df91c40e18 100644
--- a/tests/test_invoices.py
+++ b/tests/test_invoices.py
@@ -937,3 +937,27 @@ def test_invoice_botched_migration(node_factory, chainparams):
     assert ([(i['created_index'], i['label']) for i in l1.rpc.listinvoices()["invoices"]]
             == [(1, "made_after_bad_migration"), (2, "label1")])
     assert l1.rpc.invoice(100, "test", "test")["created_index"] == 3
+
+
+def test_invoice_maxdesc(node_factory, chainparams):
+    l1, l2 = node_factory.line_graph(2)
+
+    # BOLT #11:
+    #
+    # Note that the maximum length of a Tagged Field's `data` is constricted
+    # by the maximum value of `data_length`. This is 1023 x 5 bits, or 639
+    # bytes.
+    maxdesc = "x" * 639
+
+    # This should fail!
+    with pytest.raises(RpcError, match=r'Description greater than 639 bytes invalid \(description length 641\)'):
+        l1.rpc.invoice(123000, 'test_invoice_maxdesc', maxdesc + 'xx')
+
+    # This should also fail, but used to produce
+    # lnbcrt1230n1p5dm097sp545trjl795r3mm86mk4ln5jpjvnh04x8aryl3qadjt99vspu646zspp52hf43ln8vg0564ljwccs8d84xc70ls8n7wdmp75ygp7ll8rprqzsdqq0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rc0pu8s7rcxqyjw5qcqp99qxpqysgqr6l8swzm6jc42ehy4v7s83jrggtwa9ua39cvy46c46tmqwn97mn43ycww7e9cf4w5ws8lxnef2k3m5nfa5c34nz54jaxhzc5e72q0ccq26n9fx
+    with pytest.raises(RpcError, match=r'Description greater than 639 bytes invalid \(description length 640\)'):
+        l1.rpc.invoice(123000, 'test_invoice_maxdesc2', maxdesc + 'x')
+
+    # This should succeed.
+    inv = l1.rpc.invoice(123000, 'test_invoice_maxdesc3', maxdesc)
+    assert l1.rpc.decode(inv['bolt11'])['description'] == maxdesc
diff --git a/tests/test_pay.py b/tests/test_pay.py
index ab78f76a701a..72d5d20e2201 100644
--- a/tests/test_pay.py
+++ b/tests/test_pay.py
@@ -11,6 +11,7 @@
     tu64_encode
 )
 import copy
+import json
 import os
 import pytest
 import random
@@ -3697,6 +3698,30 @@ def test_keysend_maxfee(node_factory):
     assert len(l3.rpc.listinvoices()['invoices']) == 1
 
 
+@pytest.mark.parametrize("tlv_payload_length", [638, 639, 640, 641, 1022, 1023, 1024])
+def test_keysend_description_size_limit(node_factory, tlv_payload_length):
+    """
+    Test keysend description handling near BOLT11 field size limits.
+
+    Checks boundary conditions where the payload length is just below,
+    exactly at, and just above the maximum allowed tagged-field size.
+
+    See common/bolt11.h: BOLT11_FIELD_BYTE_LIMIT.
+    """
+    l1, l2 = node_factory.line_graph(2, wait_for_announce=True)
+    amt = 10000
+    prefix = 'keysend: {"message": ""}'
+    base_len = len(prefix)
+
+    # Prep TLV payload with test length
+    body_len = tlv_payload_length - base_len
+    tlv_payload = json.dumps({"message": "a" * body_len}).encode().hex()
+
+    # Send keysend payment with test payload
+    l1.rpc.keysend(l2.info["id"], amt, extratlvs={7629169: tlv_payload})
+    assert len(l2.rpc.listinvoices()["invoices"]) == 1
+
+
 def test_invalid_onion_channel_update(node_factory):
     '''
     Some onion failures "should" send a `channel_update`.