diff --git a/codeflash/languages/java/support.py b/codeflash/languages/java/support.py
index 7115d2225..500308391 100644
--- a/codeflash/languages/java/support.py
+++ b/codeflash/languages/java/support.py
@@ -546,6 +546,21 @@ def ensure_runtime_environment(self, project_root: Path) -> bool:
         if self._language_version is None:
             self._detect_java_version()
 
+        if self._language_version is not None:
+            try:
+                major = int(self._language_version)
+                if major < 11:
+                    logger.error(
+                        "Java %s detected, but codeflash requires JDK 11 or later. "
+                        "The codeflash-runtime JAR and --add-opens flags are incompatible with JDK %s. "
+                        "Please install JDK 11+ and ensure it is on your PATH.",
+                        self._language_version,
+                        self._language_version,
+                    )
+                    return False
+            except ValueError:
+                pass
+
         self._test_framework = config.test_framework
 
         return True
@@ -568,9 +583,9 @@ def _detect_java_version(self) -> None:
                     end = line.find('"', start + 1)
                     if start != -1 and end != -1:
                         full_version = line[start + 1 : end]
-                        # Use major version only: "17.0.2" -> "17", "1.8.0_292" -> "8"
-                        major = full_version.split(".")[0]
-                        self._language_version = "8" if major == "1" else major
+                        # Use major version only: "17.0.2" -> "17". JDK 8 and earlier (reported as
+                        # "1.x.y") are unsupported — the downstream minimum-version check rejects them.
+                        self._language_version = full_version.split(".")[0]
                         return
         except Exception:
             pass
diff --git a/tests/test_languages/test_java/test_support.py b/tests/test_languages/test_java/test_support.py
index b5cba5ab4..3f991f7a4 100644
--- a/tests/test_languages/test_java/test_support.py
+++ b/tests/test_languages/test_java/test_support.py
@@ -134,3 +134,74 @@ def test_discover_functions_from_fixture(self, support, java_fixture_path: Path)
         source = calculator_file.read_text(encoding="utf-8")
         functions = support.discover_functions(source, calculator_file)
         assert len(functions) > 0
+
+
+class TestJdkVersionCheck:
+    def test_jdk8_rejected(self, tmp_path: Path) -> None:
+        from unittest.mock import MagicMock, patch
+
+        support = get_java_support()
+
+        mock_config = MagicMock()
+        mock_config.java_version = "8"
+        mock_config.test_framework = "junit5"
+
+        with patch("codeflash.languages.java.support.detect_java_project", return_value=mock_config):
+            result = support.ensure_runtime_environment(tmp_path)
+        assert result is False
+
+    def test_jdk11_accepted(self, tmp_path: Path) -> None:
+        from unittest.mock import MagicMock, patch
+
+        support = get_java_support()
+
+        mock_config = MagicMock()
+        mock_config.java_version = "11"
+        mock_config.test_framework = "junit5"
+
+        with patch("codeflash.languages.java.support.detect_java_project", return_value=mock_config):
+            result = support.ensure_runtime_environment(tmp_path)
+        assert result is True
+
+    def test_jdk21_accepted(self, tmp_path: Path) -> None:
+        from unittest.mock import MagicMock, patch
+
+        support = get_java_support()
+
+        mock_config = MagicMock()
+        mock_config.java_version = "21"
+        mock_config.test_framework = "junit5"
+
+        with patch("codeflash.languages.java.support.detect_java_project", return_value=mock_config):
+            result = support.ensure_runtime_environment(tmp_path)
+        assert result is True
+
+    def test_detect_java_version_legacy_jdk8_format(self) -> None:
+        from unittest.mock import MagicMock, patch
+
+        support = get_java_support()
+        support._language_version = None
+
+        mock_result = MagicMock()
+        mock_result.stderr = 'openjdk version "1.8.0_292"\n'
+        mock_result.stdout = ""
+
+        with patch("subprocess.run", return_value=mock_result):
+            support._detect_java_version()
+
+        assert support._language_version == "1"
+
+    def test_detect_java_version_modern_format(self) -> None:
+        from unittest.mock import MagicMock, patch
+
+        support = get_java_support()
+        support._language_version = None
+
+        mock_result = MagicMock()
+        mock_result.stderr = 'openjdk version "17.0.2"\n'
+        mock_result.stdout = ""
+
+        with patch("subprocess.run", return_value=mock_result):
+            support._detect_java_version()
+
+        assert support._language_version == "17"