diff --git a/modules/client/sync.cc b/modules/client/sync.cc
index 018fc6686..efaf84366 100644
--- a/modules/client/sync.cc
+++ b/modules/client/sync.cc
@@ -442,11 +442,32 @@ try
 				int64_t(data.range.second)
 		};
 
+		char buf[48];
+		const string_view &next_batch_token
+		{
+			// The polylog phased since token. We pack two numbers separted by a '_'
+			// character which cannot be urlencoded atm. The first is the usual
+			// since token integer, which is negative for phased initial sync. The
+			// second part is the next_batch upper-bound integer which is a snapshot
+			// of the server's sequence number when the phased sync started.
+			data.phased?
+				fmt::sprintf
+				{
+					buf, "%ld_%lu", next_batch, data.range.second
+				}:
+
+			// The normal integer since token.
+				fmt::sprintf
+				{
+					buf, "%ld", next_batch
+				}
+		};
+
 		json::stack::member
 		{
 			*data.out, "next_batch", json::value
 			{
-				lex_cast(next_batch), json::STRING
+				next_batch_token ,json::STRING
 			}
 		};
 	}
diff --git a/modules/client/sync/args.h b/modules/client/sync/args.h
index 99320a7bf..9ef300534 100644
--- a/modules/client/sync/args.h
+++ b/modules/client/sync/args.h
@@ -26,10 +26,15 @@ struct ircd::m::sync::args
 		request.query["filter"]
 	};
 
+	std::pair<string_view, string_view> since_token
+	{
+		split(request.query.get("since", "0"_sv), '_')
+	};
+
 	uint64_t since
 	{
 		// 6.2.1 A point in time to continue a sync from.
-		request.query.get<uint64_t>("since", 0)
+		lex_cast<uint64_t>(since_token.first)
 	};
 
 	uint64_t next_batch
@@ -41,7 +46,7 @@ struct ircd::m::sync::args
 		// time. But that would be nice. Many sync modules do not support this
 		// because the results of repeated calls for range may become empty
 		// after a while.
-		request.query.get<uint64_t>("next_batch", -1)
+		uint64_t(lex_cast<int64_t>(request.query.get("next_batch", since_token.second?: "-1"_sv)))
 	};
 
 	steady_point timesout{[this]