diff --git a/app/burnin/stages.py b/app/burnin/stages.py
index 13ca8e7..beca66f 100644
--- a/app/burnin/stages.py
+++ b/app/burnin/stages.py
@@ -24,6 +24,25 @@ class _BadblocksResult(TypedDict):
     output: str
     aborted: bool
 
+
+def _build_badblocks_cmd(devname: str) -> str:
+    """Construct the wrapped badblocks command for a given device.
+
+    Wraps badblocks under `sh -c 'echo PID:$$; exec ...'` so we can
+    capture the remote PID for out-of-band kill -9 (asyncssh's signal
+    channel is ignored by sshd). Geometry (-b -c -p) is operator-tunable
+    via Settings → Burn-in; defaults match the Spearfoot disk-burnin.sh
+    recommendation for large HDDs.
+    """
+    return (
+        f"sh -c 'echo PID:$$; exec badblocks "
+        f"-wsv "
+        f"-b {settings.surface_validate_block_size} "
+        f"-c {settings.surface_validate_block_buffer} "
+        f"-p {settings.surface_validate_passes} "
+        f"/dev/{devname}'"
+    )
+
 from . import kill
 from ._common import (
     POLL_INTERVAL,
@@ -404,20 +423,7 @@ async def _stage_surface_validate_ssh(job_id: int, devname: str, drive_id: int)
             # we need the PID to issue an out-of-band `kill -9` over a fresh
             # session when we want to abort.
             #
-            # Block geometry is operator-tunable (Settings → Burn-in):
-            #   -b N   block size in bytes  (settings.surface_validate_block_size)
-            #   -c N   blocks held per IO   (settings.surface_validate_block_buffer)
-            #   -p N   pass count           (settings.surface_validate_passes)
-            # Defaults preserve original behavior (-b 4096 -c 64 -p 1).
-            bb_args = (
-                f"-wsv "
-                f"-b {settings.surface_validate_block_size} "
-                f"-c {settings.surface_validate_block_buffer} "
-                f"-p {settings.surface_validate_passes}"
-            )
-            cmd = (
-                f"sh -c 'echo PID:$$; exec badblocks {bb_args} /dev/{devname}'"
-            )
+            cmd = _build_badblocks_cmd(devname)
             async with conn.create_process(cmd) as proc:
                 import re as _re
 
diff --git a/tests/test_badblocks_cmd.py b/tests/test_badblocks_cmd.py
new file mode 100644
index 0000000..993a578
--- /dev/null
+++ b/tests/test_badblocks_cmd.py
@@ -0,0 +1,77 @@
+"""Verifies the Spearfoot tunables (block_size, block_buffer, passes)
+actually thread through to the badblocks command line.
+
+These three settings are exposed in Settings → Burn-in. Without a test,
+nothing catches if a future refactor drops one of the flags or reads
+from the wrong attribute. The defaults match the Spearfoot disk-burnin.sh
+community script; non-defaults can roughly halve runtime on multi-TB
+drives at the cost of more RAM.
+
+Run inside the container image so app deps are present.
+"""
+
+from __future__ import annotations
+
+import unittest
+
+from app.burnin.stages import _build_badblocks_cmd
+from app.config import settings
+
+
+class TestBadblocksCmd(unittest.TestCase):
+
+    def setUp(self):
+        # Snapshot defaults so each test can mutate freely without
+        # polluting siblings or the running process.
+        self._snap = (
+            settings.surface_validate_block_size,
+            settings.surface_validate_block_buffer,
+            settings.surface_validate_passes,
+        )
+
+    def tearDown(self):
+        (
+            settings.surface_validate_block_size,
+            settings.surface_validate_block_buffer,
+            settings.surface_validate_passes,
+        ) = self._snap
+
+    def test_defaults_match_spearfoot(self):
+        """Out of the box: -b 4096 -c 64 -p 1 — matches the
+        disk-burnin.sh community script's recommendation for HDDs."""
+        cmd = _build_badblocks_cmd("sda")
+        self.assertIn("-b 4096", cmd)
+        self.assertIn("-c 64", cmd)
+        self.assertIn("-p 1", cmd)
+        self.assertIn("/dev/sda", cmd)
+        # Destructive write+verify mode must always be present — anything
+        # else (read-only, non-destructive) defeats the purpose of burn-in.
+        self.assertIn("-wsv", cmd)
+
+    def test_tunables_propagate_to_cmd(self):
+        """Operator-set values (e.g. for paranoid 3-pass burn-in on a
+        suspect drive, or 8 KiB blocks for faster scan on a 24 TB HDD)
+        must end up in the shell command."""
+        settings.surface_validate_block_size = 8192
+        settings.surface_validate_block_buffer = 128
+        settings.surface_validate_passes = 3
+        cmd = _build_badblocks_cmd("sdb")
+        self.assertIn("-b 8192", cmd)
+        self.assertIn("-c 128", cmd)
+        self.assertIn("-p 3", cmd)
+        self.assertNotIn("-b 4096", cmd)  # no leak from defaults
+        self.assertNotIn("-c 64", cmd)
+        self.assertIn("/dev/sdb", cmd)
+
+    def test_pid_capture_wrapper_intact(self):
+        """The `sh -c 'echo PID:$$; exec ...'` wrapper is what makes
+        out-of-band kill -9 work over a fresh SSH session — asyncssh's
+        signal channel is silently ignored by sshd. If a future refactor
+        drops the wrapper, a cancel won't actually stop the test."""
+        cmd = _build_badblocks_cmd("sda")
+        self.assertTrue(cmd.startswith("sh -c 'echo PID:$$; exec badblocks"))
+        self.assertTrue(cmd.endswith("'"))
+
+
+if __name__ == "__main__":
+    unittest.main()