diff --git a/difftest/default.nix b/difftest/default.nix
index 6d6282958..44541b05a 100644
--- a/difftest/default.nix
+++ b/difftest/default.nix
@@ -48,6 +48,10 @@ let
       VERILATED_LIB_DIR = "${verilated}/lib";
       SPIKE_LIB_DIR = "${libspike}/lib";
       SPIKE_INTERFACES_LIB_DIR = "${libspike_interfaces}/lib";
+      SPIKE_ISA_STRING =
+        "rv32gc" +
+        (builtins.concatStringsSep "_" elaborateConfig.parameter.extensions)
+        + "_Zvl${toString elaborateConfig.parameter.vLen}b";
       DESIGN_VLEN = elaborateConfig.parameter.vLen;
       DESIGN_DLEN = elaborateConfig.parameter.dLen;
     };
diff --git a/difftest/test_common/src/lib.rs b/difftest/test_common/src/lib.rs
index 5d8ac346b..e53d269cb 100644
--- a/difftest/test_common/src/lib.rs
+++ b/difftest/test_common/src/lib.rs
@@ -32,7 +32,7 @@ pub struct CommonArgs {
   pub dlen: u32,
 
   /// ISA config
-  #[arg(long, default_value = "rv32gcv")]
+  #[arg(long, default_value = env!("SPIKE_ISA_STRING"))]
   pub set: String,
 }