diff --git a/aai_coding/harness.py b/aai_coding/harness.py
index ef76dc1..522b32a 100644
--- a/aai_coding/harness.py
+++ b/aai_coding/harness.py
@@ -7,7 +7,11 @@
 
 NO_TRUNCATE = 'Output pipe truncates below 20 lines. Drop the pipe or keep >=20: truncation is decided before the output exists, so keep enough to diagnose surprises.'
 NO_STDERR_MERGE = 'Do not merge stderr into stdout with 2>&1. Run the command bare: the harness pushes large output to a file by itself, and stderr kept separate makes a crash unmissable.'
-_TRUNC = re.compile(r'\|\s*(tail|head)\s+(-n\s*)?-?([1-9]|1[0-9])\b')
+# Two ways a head/tail pipe truncates below 20: an explicit count under 20, or no count at all -
+# bare `head`/`tail` default to 10, so a countless pipe cuts exactly as hard as the `-10` we reject.
+_TRUNC = re.compile(r'\|\s*(?:tail|head)'
+                    r'(?:\s+(?:-n\s*)?-?(?:[1-9]|1[0-9])\b'  # explicit: -5, -n 5, -19
+                    r'|(?=\s*(?:$|[|;&\n])))')                # bare: defaults to 10
 _MERGE = re.compile(r'2>\s*&\s*1')
 
 
diff --git a/tests/test_harness.py b/tests/test_harness.py
index 3884200..7a3c8b4 100644
--- a/tests/test_harness.py
+++ b/tests/test_harness.py
@@ -3,7 +3,18 @@
 import pytest
 from shutil import which
 
-from aai_coding.harness import claude_air, claude_drop_sentinel, claude_slop, synthetic_resume
+from aai_coding.harness import bash_guard_msg, claude_air, claude_drop_sentinel, claude_slop, synthetic_resume
+
+
+def test_bash_guard_bare_head_tail():
+    "A countless head/tail keeps 10 lines, the same cut as the -10 form the guard already rejects"
+    assert bash_guard_msg('tar tzf x.tgz | head')
+    assert bash_guard_msg('ls ~ | tail')
+    assert bash_guard_msg('cat big.log | head | wc -l')
+    assert bash_guard_msg('ls | head; echo done')
+    assert bash_guard_msg('ls | header') is None           # merely starts with those four letters
+    assert bash_guard_msg('cat f | head -c 200') is None   # bytes, not lines
+    assert bash_guard_msg('git status | head -30') is None
 
 
 def test_synthetic_resume(tmp_path):