diff --git a/matrix/sync.cc b/matrix/sync.cc
index 507ea2352..d3da3a1c7 100644
--- a/matrix/sync.cc
+++ b/matrix/sync.cc
@@ -135,13 +135,26 @@ ircd::m::sync::make_since(const mutable_buffer &buf,
                           const int64_t &val,
                           const string_view &flags)
 {
-	return fmt::sprintf
+	const string_view &prefix
+	{
+		val?
+			"ctor_"_sv:
+			""_sv
+	};
+
+	const string_view &snapshot
 	{
-		buf, "ctor_%lu%s%s",
-		val,
 		flags?
 			"_0_"_sv:
-			flags,
+			flags
+	};
+
+	return fmt::sprintf
+	{
+		buf, "%s%lu%s%s",
+		prefix,
+		val,
+		snapshot,
 		flags,
 	};
 }
diff --git a/modules/client/sync.cc b/modules/client/sync.cc
index 9e5fb0840..5c5c2e15a 100644
--- a/modules/client/sync.cc
+++ b/modules/client/sync.cc
@@ -14,7 +14,7 @@ namespace ircd::m::sync
 	struct response;
 
 	static const_buffer flush(data &, resource::response::chunked &, const const_buffer &);
-	static void empty_response(data &, const uint64_t &next_batch);
+	static bool empty_response(data &, const uint64_t &next_batch);
 	static bool linear_handle(data &);
 	static bool polylog_handle(data &);
 	static bool longpoll_handle(data &);
@@ -27,6 +27,7 @@ namespace ircd::m::sync
 	extern conf::item<bool> longpoll_enable;
 	extern conf::item<bool> polylog_phased;
 	extern conf::item<bool> polylog_only;
+	extern conf::item<bool> MSC2855;
 
 	extern resource::method method_get;
 	extern const string_view description;
@@ -150,6 +151,13 @@ ircd::m::sync::longpoll_enable
 	{ "default",  true                               },
 };
 
+decltype(ircd::m::sync::MSC2855)
+ircd::m::sync::MSC2855
+{
+	{ "name",     "ircd.client.sync.MSC2855.enable" },
+	{ "default",  true                              },
+};
+
 //
 // GET sync
 //
@@ -202,15 +210,20 @@ ircd::m::sync::handle_get(client &client,
 	// event which doesn't exist yet. There is no reason for the since parameter
 	// to be greater than that, unless it's a negative integer and phased
 	// sync is enabled
-	if(!polylog_phased || !phased_range)
-		if(range.first > range.second)
-			throw m::NOT_FOUND
-			{
-				"Since parameter '%lu' is too far in the future."
-				" Cannot be greater than '%lu'.",
-				range.first,
-				range.second
-			};
+	const bool invalid_since
+	{
+		(!polylog_phased || !phased_range)
+		&& (range.first > range.second)
+	};
+
+	if(unlikely(invalid_since && !MSC2855))
+		throw m::NOT_FOUND
+		{
+			"Since parameter '%lu' is too far in the future."
+			" Cannot be greater than '%lu'.",
+			range.first,
+			range.second
+		};
 
 	// Query and cache the device ID for the access token of this request on
 	// the stack here for this sync.
@@ -346,36 +359,38 @@ ircd::m::sync::handle_get(client &client,
 	// The return value from the operation will be false if no output was
 	// generated by the sync operation, indicating we should finally send an
 	// empty response.
-	const bool complete
+	bool complete
 	{
-		should_linear?
-			linear_handle(data):
-
-		should_polylog?
-			polylog_handle(data):
-
 		false
+		|| invalid_since
 	};
 
-	if(complete)
-		return std::move(response);
+	if(!complete && should_polylog)
+		complete = polylog_handle(data);
 
-	if(longpoll_handle(data))
-		return std::move(response);
+	if(!complete && should_linear)
+		complete = linear_handle(data);
 
-	const auto &next_batch
-	{
-		polylog_only?
-			data.range.first:
+	if(!complete)
+		complete = longpoll_handle(data);
 
-		data.range.second
-	};
+	if(!complete || invalid_since)
+		complete = empty_response(data, uint64_t
+		{
+			invalid_since?
+				0UL:
 
-	empty_response(data, next_batch);
+			polylog_only?
+				data.range.first:
+
+			data.range.second
+		});
+
+	assert(complete);
 	return std::move(response);
 }
 
-void
+bool
 ircd::m::sync::empty_response(data &data,
                               const uint64_t &next_batch)
 {
@@ -404,12 +419,30 @@ ircd::m::sync::empty_response(data &data,
 		}
 	};
 
-	log::debug
+	const auto &reason
 	{
-		log, "request %s timeout @%lu",
-		loghead(data),
-		next_batch
+		next_batch?
+			"timeout"_sv:
+			"MSC2855 clear cache & reload"_sv
 	};
+
+	const auto &level
+	{
+		next_batch?
+			log::DEBUG:
+			log::WARNING
+	};
+
+	log::logf
+	{
+		log, level,
+		"request %s %s @%lu",
+		loghead(data),
+		reason,
+		next_batch,
+	};
+
+	return true;
 }
 
 ircd::const_buffer