diff --git a/src/sfizz/Synth.cpp b/src/sfizz/Synth.cpp
index c2f73319..48295c40 100644
--- a/src/sfizz/Synth.cpp
+++ b/src/sfizz/Synth.cpp
@@ -250,6 +250,7 @@ void Synth::Impl::clear()
     changedCCsThisCycle_.clear();
     keyLabels_.clear();
     keySlots_.clear();
+    swSlots_.clear();
     keyswitchLabels_.clear();
     globalOpcodes_.clear();
     masterOpcodes_.clear();
@@ -745,6 +746,18 @@ void Synth::Impl::finalizeSfzLoad()
         for (unsigned key = loKey; key <= hiKey; ++key)
             keySlots_.set(key);
     }
+    // cache the set of keyswitches assigned
+    for (const RegionPtr& regionPtr : regions_) {
+        if (absl::optional<uint8_t> sw = regionPtr->lastKeyswitch) {
+            swSlots_.set(*sw);
+        }
+        else if (absl::optional<Range<uint8_t>> swRange = regionPtr->lastKeyswitchRange) {
+            unsigned loKey = swRange->getStart();
+            unsigned hiKey = swRange->getEnd();
+            for (unsigned key = loKey; key <= hiKey; ++key)
+                swSlots_.set(key);
+        }
+    }
 }
 
 bool Synth::loadScalaFile(const fs::path& path)
diff --git a/src/sfizz/SynthMessaging.cpp b/src/sfizz/SynthMessaging.cpp
index f8672cca..c441807e 100644
--- a/src/sfizz/SynthMessaging.cpp
+++ b/src/sfizz/SynthMessaging.cpp
@@ -46,6 +46,14 @@ void sfz::Synth::dispatchMessage(Client& client, int delay, const char* path, co
 
         //----------------------------------------------------------------------
 
+        MATCH("/sw/slots", "") {
+            const BitArray<128>& switches = impl.swSlots_;
+            sfizz_blob_t blob { switches.data(), static_cast<uint32_t>(switches.byte_size()) };
+            client.receive<'b'>(delay, path, &blob);
+        } break;
+
+        //----------------------------------------------------------------------
+
         MATCH("/cc/slots", "") {
             const BitArray<config::numCCs>& ccs = impl.currentUsedCCs_;
             sfizz_blob_t blob { ccs.data(), static_cast<uint32_t>(ccs.byte_size()) };
diff --git a/src/sfizz/SynthPrivate.h b/src/sfizz/SynthPrivate.h
index aaf6f72a..5c8b4479 100644
--- a/src/sfizz/SynthPrivate.h
+++ b/src/sfizz/SynthPrivate.h
@@ -217,6 +217,7 @@ struct Synth::Impl final: public Parser::Listener {
     std::map<int, size_t> ccLabelsMap_;
     std::vector<NoteNamePair> keyLabels_;
     BitArray<128> keySlots_;
+    BitArray<128> swSlots_;
     std::vector<NoteNamePair> keyswitchLabels_;
 
     // Set as sw_default if present in the file