diff --git a/headroom/transforms/smart_crusher.py b/headroom/transforms/smart_crusher.py
index 4dde88736..6ec0e4443 100644
--- a/headroom/transforms/smart_crusher.py
+++ b/headroom/transforms/smart_crusher.py
@@ -1202,7 +1202,12 @@ class SmartCrusher(Transform):
             if msg.get("role") == "assistant" and msg.get("tool_calls"):
                 for tc in msg.get("tool_calls", []):
                     if isinstance(tc, dict):
-                        func = tc.get("function", {})
+                        # `tc.get("function", {})` returns None for an explicit
+                        # {"function": null} (the default only applies to a
+                        # missing key), and `.get` on None raises AttributeError,
+                        # crashing apply(). Use the null-safe form the sibling
+                        # `_build_tool_name_index` already uses (line ~118).
+                        func = tc.get("function") or {}
                         args = func.get("arguments", "")
                         if isinstance(args, str) and args:
                             context_parts.append(args)
diff --git a/tests/test_transforms/test_smart_crusher_bugs.py b/tests/test_transforms/test_smart_crusher_bugs.py
index 616806d99..0981f2cba 100644
--- a/tests/test_transforms/test_smart_crusher_bugs.py
+++ b/tests/test_transforms/test_smart_crusher_bugs.py
@@ -163,6 +163,26 @@ class TestLosslessOnlyMode:
         assert json.loads(out.compressed) == rows
 
 
+def test_extract_context_survives_null_function_tool_call() -> None:
+    # A tool_call with an explicit {"function": null} must not crash context
+    # extraction: `dict.get("function", {})` returns None for a present-but-null
+    # key, and `.get` on None raises AttributeError inside apply().
+    crusher = _make_crusher()
+    messages = [
+        {
+            "role": "assistant",
+            "tool_calls": [
+                {"id": "1", "type": "function", "function": None},
+                {"id": "2", "type": "function", "function": {"arguments": "keep-me"}},
+            ],
+        },
+    ]
+
+    ctx = crusher._extract_context_from_messages(messages)
+
+    assert "keep-me" in ctx
+
+
 # Stage 3c.1 lockstep bug-fix tests previously lived here; they probed
 # Python helpers (`_percentile_linear`, `_detect_sequential_pattern`,
 # `_detect_rare_status_values`, `_compute_k_split`) that were removed