fix: parse single-line fenced LLM replies in analyzer

src/lab_harness/analysis/analyzer.py

@@ -143,8 +143,9 @@ def generate_script_with_ai(
 
         # Strip markdown fences if present
         if script.startswith("```"):
-            first_nl = script.index("\n")
-            script = script[first_nl + 1 :]
+            # Drop the opening fence line; a single-line reply has no newline to split on.
+            first_nl = script.find("\n")
+            script = script[first_nl + 1 :] if first_nl != -1 else script[3:]
             if script.endswith("```"):
                 script = script[:-3]
             script = script.strip()

tests/test_analyzer_ai_paths.py

@@ -78,6 +78,16 @@ def test_markdown_fences_are_stripped(self, llm_on, tmp_path: Path, raw: str):
         script = Analyzer().generate_script_with_ai(tmp_path / "x.csv", "IV")
         assert script == "import numpy\nprint(1)"
 
+    @pytest.mark.parametrize(
+        ("raw", "expected"),
+        [("```print(1)```", "print(1)"), ("```", ""), ("```python\n```", "")],
+        ids=["single-line", "bare", "empty-block"],
+    )
+    def test_fences_without_newline_do_not_raise(self, llm_on, tmp_path: Path, raw: str, expected: str):
+        """A fenced reply with no newline after the opening fence must not raise ValueError."""
+        llm_on.return_value = _router(raw)
+        assert Analyzer().generate_script_with_ai(tmp_path / "x.csv", "IV") == expected
+
 
 class TestInterpretResults:
     def test_data_path_preview_in_context(self, llm_on, tmp_path: Path):