diff --git a/src/tests/mir_stageb_string_utils_skip_ws.rs b/src/tests/mir_stageb_string_utils_skip_ws.rs
new file mode 100644
index 00000000..43de1124
--- /dev/null
+++ b/src/tests/mir_stageb_string_utils_skip_ws.rs
@@ -0,0 +1,142 @@
+/*!
+ * Phase 25.1 - ParserStringUtilsBox.skip_ws Void < 0 TypeError 再現テスト
+ *
+ * 目的: stageb_program_min_using_only.hako で発生する
+ *       「Void < Integer(0)」TypeError を Rust テストで再現する
+ *
+ * エラー詳細:
+ * - 関数: ParserStringUtilsBox.skip_ws/2
+ * - 条件: loop(j < n) の Compare で Void < 0 が発生
+ * - 原因: j または n が Void になるパスが存在
+ */
+
+#[cfg(test)]
+mod tests {
+    use crate::ast::ASTNode;
+    use crate::backend::VM;
+    use crate::mir::{MirCompiler, MirVerifier};
+    use crate::parser::NyashParser;
+
+    /// 最小再現ケース: ParserStringUtilsBox + Main
+    fn make_minimal_src() -> String {
+        // ParserStringUtilsBox 本体を include
+        let utils =
+            include_str!("../../lang/src/compiler/parser/scan/parser_string_utils_box.hako");
+
+        // 最小 Main: stageb_program_min_using_only と同等の呼び出し
+        let main = r#"
+using lang.compiler.parser.scan.parser_string_utils_box as ParserStringUtilsBox
+
+static box Main {
+  main(args) {
+    // stageb_program_min_using_only と同じ入力
+    local src = "using \"foo/bar.hako\" as Foo\n"
+
+    // skip_ws を直接呼び出し（Void < 0 を誘発）
+    local pos = ParserStringUtilsBox.skip_ws(src, 0)
+
+    print("skip_ws result: " + ("" + pos))
+    return 0
+  }
+}
+"#;
+
+        format!("{}\n{}", utils, main)
+    }
+
+    #[test]
+    fn mir_stageb_string_utils_skip_ws_compile() {
+        // Enable Stage-3 parser for 'local' keyword
+        std::env::set_var("NYASH_PARSER_STAGE3", "1");
+        std::env::set_var("HAKO_PARSER_STAGE3", "1");
+
+        let src = make_minimal_src();
+        let ast: ASTNode = NyashParser::parse_from_string(&src).expect("parse should succeed");
+
+        let mut mc = MirCompiler::with_options(true); // Stage-3 enabled
+        let cr = mc
+            .compile(ast)
+            .expect("ParserStringUtilsBox compile should succeed");
+
+        // MIR 検証
+        let mut verifier = MirVerifier::new();
+        if let Err(errors) = verifier.verify_module(&cr.module) {
+            eprintln!("[mir-verify] Verification failed:");
+            for e in &errors {
+                eprintln!("  - {}", e);
+            }
+            panic!("MIR verification failed for ParserStringUtilsBox.skip_ws");
+        }
+
+        println!("✅ MIR compile and verify succeeded");
+    }
+
+    #[test]
+    fn mir_stageb_string_utils_skip_ws_exec_reproduce_void_lt_zero() {
+        // Enable Stage-3 parser
+        std::env::set_var("NYASH_PARSER_STAGE3", "1");
+        std::env::set_var("HAKO_PARSER_STAGE3", "1");
+
+        let src = make_minimal_src();
+        let ast: ASTNode = NyashParser::parse_from_string(&src).expect("parse ok");
+
+        let mut mc = MirCompiler::with_options(true); // Stage-3 enabled
+        let cr = mc.compile(ast).expect("compile ok");
+
+        // MIR 検証
+        let mut verifier = MirVerifier::new();
+        verifier
+            .verify_module(&cr.module)
+            .expect("MIR verify should pass");
+
+        // VM 実行（ここで Void < 0 エラーが出るはず）
+        let mut vm = VM::new();
+        let exec_result = vm.execute_module(&cr.module);
+
+        // 現状は TypeError が出ることを確認（再現用）
+        if let Err(e) = exec_result {
+            let err_msg = format!("{:?}", e);
+            println!("❌ Expected error reproduced: {}", err_msg);
+
+            // Void < Integer(0) エラーを検出
+            assert!(
+                err_msg.contains("Void") || err_msg.contains("TypeError"),
+                "Expected Void < Integer(0) TypeError, got: {}",
+                err_msg
+            );
+        } else {
+            panic!("Expected Void < 0 TypeError, but execution succeeded!");
+        }
+    }
+
+    #[test]
+    #[ignore] // このテストは修正後にパスするはず
+    fn mir_stageb_string_utils_skip_ws_exec_success() {
+        // Enable Stage-3 parser
+        std::env::set_var("NYASH_PARSER_STAGE3", "1");
+        std::env::set_var("HAKO_PARSER_STAGE3", "1");
+
+        let src = make_minimal_src();
+        let ast: ASTNode = NyashParser::parse_from_string(&src).expect("parse ok");
+
+        let mut mc = MirCompiler::with_options(true); // Stage-3 enabled
+        let cr = mc.compile(ast).expect("compile ok");
+
+        // MIR 検証
+        let mut verifier = MirVerifier::new();
+        verifier
+            .verify_module(&cr.module)
+            .expect("MIR verify should pass");
+
+        // VM 実行（修正後は成功するはず）
+        let mut vm = VM::new();
+        let exec_result = vm.execute_module(&cr.module);
+
+        assert!(
+            exec_result.is_ok(),
+            "skip_ws should succeed after Void < 0 fix"
+        );
+
+        println!("✅ skip_ws execution succeeded (bug fixed!)");
+    }
+}
diff --git a/src/tests/mod.rs b/src/tests/mod.rs
index 594289d4..53d22c18 100644
--- a/src/tests/mod.rs
+++ b/src/tests/mod.rs
@@ -19,6 +19,7 @@ pub mod mir_stage1_using_resolver_verify;
 pub mod stage1_cli_entry_ssa_smoke;
 pub mod mir_stageb_like_args_length;
 pub mod mir_stageb_loop_break_continue;
+pub mod mir_stageb_string_utils_skip_ws; // Phase 25.1: skip_ws Void < 0 TypeError 再現
 pub mod mir_value_kind; // Phase 26-A-5: ValueId型安全化統合テスト
 pub mod nyash_abi_basic;
 pub mod parser_static_box_members;