diff --git a/src/tool_parsing.py b/src/tool_parsing.py
index cd9ecd1..b31e114 100644
--- a/src/tool_parsing.py
+++ b/src/tool_parsing.py
@@ -69,6 +69,8 @@ _TOOL_CODE_RE = re.compile(
 # fullwidth (U+FF5C) and ascii '|' in any count.
 _DSML_PIPES = r"[｜|]+"
 def _normalize_dsml(text: str) -> str:
+    if not isinstance(text, str):
+        return ""
     if "DSML" not in text:
         return text
     t = text
diff --git a/tests/test_tool_parsing_nonstring.py b/tests/test_tool_parsing_nonstring.py
new file mode 100644
index 0000000..7bd1975
--- /dev/null
+++ b/tests/test_tool_parsing_nonstring.py
@@ -0,0 +1,19 @@
+"""Regression: tool-block parsing must tolerate a non-string input.
+
+`_normalize_dsml` did `if "DSML" not in text` (TypeError on None) and the public
+`parse_tool_blocks`/`strip_tool_blocks` then ran regexes on it. Coercing a
+non-string to "" in `_normalize_dsml` makes the whole chain safe.
+"""
+import src.agent_tools  # noqa: F401  (break agent_tools<->tool_parsing import cycle)
+from src.tool_parsing import _normalize_dsml, parse_tool_blocks, strip_tool_blocks
+
+
+def test_non_string_does_not_crash():
+    assert _normalize_dsml(None) == ""
+    assert parse_tool_blocks(None) == []
+    assert strip_tool_blocks(None) == ""
+
+
+def test_plain_text_passes_through():
+    assert strip_tool_blocks("hello world") == "hello world"
+    assert parse_tool_blocks("no tools here") == []