diff --git a/src/llm/providers/reasoning.py b/src/llm/providers/reasoning.py
index cbb8b1270..752132292 100644
--- a/src/llm/providers/reasoning.py
+++ b/src/llm/providers/reasoning.py
@@ -54,13 +54,15 @@ def _is_json_document(content: str) -> bool:
     """Whether ``content`` is one complete JSON object or array.
 
     Reasoning followed by a closing tag is never valid JSON, so a structured
-    answer that carries the tag inside a string value is left whole.
+    answer that carries the tag inside a string value is left whole. Input
+    nested too deeply to parse is not treated as JSON rather than failing the
+    request.
     """
     stripped = content.strip()
     if not stripped.startswith(("{", "[")):
         return False
     try:
         json.loads(stripped)
-    except ValueError:
+    except (ValueError, RecursionError):
         return False
     return True
diff --git a/tests/test_reasoning.py b/tests/test_reasoning.py
index b2ade433b..3cbee1ef7 100644
--- a/tests/test_reasoning.py
+++ b/tests/test_reasoning.py
@@ -66,6 +66,13 @@ def test_strip_reasoning_prefilled(content: str, expected: str) -> None:
     assert strip_reasoning(content, prefilled=True) == expected
 
 
+def test_strip_reasoning_prefilled_survives_deeply_nested_json() -> None:
+    # Deep enough for json.loads to raise RecursionError on every supported Python.
+    depth = 100_000
+    content = "[" * depth + '"</think>"' + "]" * depth
+    assert strip_reasoning(content, prefilled=True) == '"' + "]" * depth
+
+
 def _openai_provider(content: str, **kwargs: Any) -> OpenAIProvider:
     provider = OpenAIProvider(api_key="test", **kwargs)
     message = SimpleNamespace(content=content, tool_calls=None)