diff --git a/headroom/proxy/output_steering.py b/headroom/proxy/output_steering.py
index 2d86a86bc..7a39dae09 100644
--- a/headroom/proxy/output_steering.py
+++ b/headroom/proxy/output_steering.py
@@ -36,8 +36,13 @@ def apply_verbosity_steering(body: dict[str, Any], level: int) -> bool:
         return True
     if isinstance(system, list):
         for block in system:
-            if isinstance(block, dict) and block.get("text", "").startswith(_STEERING_SENTINEL):
-                if block["text"] == text:
+            # Guard the text is a string before ``startswith``: a malformed
+            # client block (``{"type": "text", "text": null}``) would otherwise
+            # raise ``AttributeError`` here and 500 the request. The OpenAI chat
+            # sibling below already guards this exact case.
+            block_text = block.get("text") if isinstance(block, dict) else None
+            if isinstance(block_text, str) and block_text.startswith(_STEERING_SENTINEL):
+                if block_text == text:
                     return False
                 block["text"] = text
                 return True
diff --git a/tests/test_output_steering.py b/tests/test_output_steering.py
index 8b94f66c8..39a88a6b8 100644
--- a/tests/test_output_steering.py
+++ b/tests/test_output_steering.py
@@ -35,6 +35,23 @@ def test_anthropic_steering_preserves_cached_prefix_block() -> None:
     assert body["system"][1] == {"type": "text", "text": steering_text(2)}
 
 
+def test_anthropic_steering_tolerates_non_string_system_block_text() -> None:
+    # A malformed client block ({"type": "text", "text": null}) must not crash
+    # `.startswith` and 500 the request; steering is still appended. The OpenAI
+    # chat sibling already guards this exact case.
+    body = {
+        "system": [
+            {"type": "text", "text": None},
+            {"type": "text", "text": "Real system prompt."},
+        ]
+    }
+
+    assert apply_verbosity_steering(body, 2) is True
+    # The malformed block is left as-is and a steering block is appended.
+    assert body["system"][0] == {"type": "text", "text": None}
+    assert body["system"][-1] == {"type": "text", "text": steering_text(2)}
+
+
 def test_openai_responses_steering_is_idempotent() -> None:
     body = {"instructions": "System."}