diff --git a/bin/gstack-diff-scope b/bin/gstack-diff-scope
index bf1b4af84..38450e6b1 100755
--- a/bin/gstack-diff-scope
+++ b/bin/gstack-diff-scope
@@ -4,7 +4,15 @@
 # Or:    gstack-diff-scope main           → prints SCOPE_*=... lines
 set -euo pipefail
 
-BASE="${1:-main}"
+# Detect the repo's default branch when no arg is given (#703-class
+# platform-agnostic rule): origin/HEAD -> origin/main -> origin/master -> main.
+_default_base() {
+  git symbolic-ref refs/remotes/origin/HEAD 2>/dev/null | sed 's|refs/remotes/origin/||' && return
+  git rev-parse --verify -q origin/main >/dev/null 2>&1 && { echo "main"; return; }
+  git rev-parse --verify -q origin/master >/dev/null 2>&1 && { echo "master"; return; }
+  echo "main"
+}
+BASE="${1:-$(_default_base)}"
 
 # Get changed file list
 FILES=$(git diff "${BASE}...HEAD" --name-only 2>/dev/null || git diff "${BASE}" --name-only 2>/dev/null || echo "")
diff --git a/bin/gstack-next-version b/bin/gstack-next-version
index 455dd72f2..6a291d96e 100755
--- a/bin/gstack-next-version
+++ b/bin/gstack-next-version
@@ -405,7 +405,16 @@ function parseArgs(argv: string[]): { base: string; bump: Bump; current: string;
     else if (a === "-h" || a === "--help") help = true;
   }
   if (help) return { base: "", bump: "micro", current: "", excludePR: null, help: true };
-  if (!base) base = "main";
+  if (!base) {
+    // Detect the default branch instead of assuming main (local-only repos
+    // on trunk/master work like GitHub repos on main).
+    try {
+      const head = execFileSync("git", ["symbolic-ref", "refs/remotes/origin/HEAD"], { encoding: "utf-8", stdio: ["ignore", "pipe", "ignore"] }).trim();
+      base = head.replace("refs/remotes/origin/", "") || "main";
+    } catch {
+      base = "main";
+    }
+  }
   if (!bump) {
     console.error("Error: --bump is required (major|minor|patch|micro)");
     process.exit(2);