diff --git a/mistral/bitstream.cc b/mistral/bitstream.cc
index b46a9f6b..3f088035 100644
--- a/mistral/bitstream.cc
+++ b/mistral/bitstream.cc
@@ -60,7 +60,15 @@ struct MistralBitgen
 
     void options()
     {
-        if (!ctx->setting<bool>("compress_rbf", false)) {
+        // Cyclone V bitstreams may be emitted compressed or uncompressed.
+        // We default to COMPRESSED. An uncompressed bitstream still
+        // configures the device (the FPGA manager reports "operating"),
+        // but on the socfpga FPGA-manager load path (verified on a
+        // DE10-Nano / MiSTer) it leaves user IO non-functional; the
+        // compressed encoding matches Quartus output and works on
+        // silicon. OPT_B carries the compression-format flag in the
+        // header and must track the chosen encoding.
+        if (ctx->setting<bool>("uncompressed_rbf", false)) {
             cv->opt_b_set(CycloneV::COMPRESSION_DIS, true);
             cv->opt_r_set(CycloneV::OPT_B, 0xffffff40adffffffULL);
         } else
diff --git a/mistral/constids.inc b/mistral/constids.inc
index 754a96b3..f46cd1a7 100644
--- a/mistral/constids.inc
+++ b/mistral/constids.inc
@@ -109,6 +109,7 @@ X(CFG_DBITS)
 X(clkout)
 X(clkout1)
 X(compress_rbf)
+X(uncompressed_rbf)
 X(oscena)
 X(placer)
 X(router)
diff --git a/mistral/main.cc b/mistral/main.cc
index d6d9333c..d40f1a81 100644
--- a/mistral/main.cc
+++ b/mistral/main.cc
@@ -49,7 +49,10 @@ po::options_description MistralCommandHandler::getArchOptions()
     specific.add_options()("device", po::value<std::string>(), "device name (e.g. 5CSEBA6U23I7)");
     specific.add_options()("qsf", po::value<std::string>(), "path to QSF constraints file");
     specific.add_options()("rbf", po::value<std::string>(), "RBF bitstream to write");
-    specific.add_options()("compress-rbf", "generate compressed bitstream");
+    specific.add_options()("uncompressed-rbf",
+                           "emit an uncompressed bitstream (default is compressed; uncompressed "
+                           "configures the device but may leave IO non-functional on some loaders)");
+    specific.add_options()("compress-rbf", "deprecated, no-op: compressed output is now the default");
 
     return specific;
 }
@@ -79,8 +82,11 @@ std::unique_ptr<Context> MistralCommandHandler::createContext(dict<std::string,
     }
     chipArgs.device = vm["device"].as<std::string>();
     auto ctx = std::unique_ptr<Context>(new Context(chipArgs));
+    if (vm.count("uncompressed-rbf"))
+        ctx->settings[id_uncompressed_rbf] = Property::State::S1;
     if (vm.count("compress-rbf"))
-        ctx->settings[id_compress_rbf] = Property::State::S1;
+        log_warning("--compress-rbf is deprecated and has no effect; compressed output is now the "
+                    "default (pass --uncompressed-rbf to opt out).\n");
     return ctx;
 }