diff --git a/plugins/magpie-setup/skills/isolated-setup-doctor/scripts/README.md b/plugins/magpie-setup/skills/isolated-setup-doctor/scripts/README.md
index c422fca4e..9c6faf693 100644
--- a/plugins/magpie-setup/skills/isolated-setup-doctor/scripts/README.md
+++ b/plugins/magpie-setup/skills/isolated-setup-doctor/scripts/README.md
@@ -25,6 +25,11 @@ entry, add a probe for it:
 1. **A script here** — short, deterministic, side-effect-free, printing
    the `PROBE:` line above. One failure mode per script: a probe that
    conflates two restrictions makes the report ambiguous when it fails.
+   Hold a command's output in a variable, not a file: `/tmp` is read-only
+   inside the sandbox, and a failed redirection skips the command and
+   exits 1, which a probe reading exit codes takes for the command's own
+   answer. Probe 1 once passed every agent that way. Add a test under
+   [`../tests/`](../tests/) that runs the script against a fake binary.
 2. **An interpretation table in `SKILL.md`** — three to five rows
    mapping result strings to ✓ / ✗ / ⊘ / ⚠.
 3. **A remediation link** to the catalogue section that fixes it. Link
diff --git a/plugins/magpie-setup/skills/isolated-setup-doctor/scripts/probe-1-ssh-agent.sh b/plugins/magpie-setup/skills/isolated-setup-doctor/scripts/probe-1-ssh-agent.sh
index 9d28bccca..5600982ea 100755
--- a/plugins/magpie-setup/skills/isolated-setup-doctor/scripts/probe-1-ssh-agent.sh
+++ b/plugins/magpie-setup/skills/isolated-setup-doctor/scripts/probe-1-ssh-agent.sh
@@ -22,11 +22,13 @@ elif [ ! -S "$SSH_AUTH_SOCK" ]; then
   echo "PROBE: ssh-agent → ✗ (socket file at SSH_AUTH_SOCK not stat-able from inside sandbox)"
   echo "       SSH_AUTH_SOCK=$SSH_AUTH_SOCK"
 else
-  ssh-add -l > /tmp/ssh-add.out 2>&1; rc=$?
+  # Held in a variable, not a file: /tmp is read-only inside the sandbox, and a
+  # failed redirection never runs ssh-add but exits 1, the "no identities" code.
+  out=$(ssh-add -l 2>&1); rc=$?
   case "$rc" in
-    0) echo "PROBE: ssh-agent → ✓ ($(wc -l < /tmp/ssh-add.out | tr -d ' ') identities listed)" ;;
+    0) echo "PROBE: ssh-agent → ✓ ($(printf '%s\n' "$out" | wc -l | tr -d ' ') identities listed)" ;;
     1) echo "PROBE: ssh-agent → ✓ (agent reachable, no identities configured)" ;;
-    2) echo "PROBE: ssh-agent → ✗ (agent unreachable: $(head -1 /tmp/ssh-add.out))" ;;
-    *) echo "PROBE: ssh-agent → ⚠ (unexpected rc=$rc: $(head -1 /tmp/ssh-add.out))" ;;
+    2) echo "PROBE: ssh-agent → ✗ (agent unreachable: $(printf '%s\n' "$out" | head -1))" ;;
+    *) echo "PROBE: ssh-agent → ⚠ (unexpected rc=$rc: $(printf '%s\n' "$out" | head -1))" ;;
   esac
 fi
diff --git a/plugins/magpie-setup/skills/isolated-setup-doctor/tests/test_probe_1_ssh_agent.py b/plugins/magpie-setup/skills/isolated-setup-doctor/tests/test_probe_1_ssh_agent.py
new file mode 100644
index 000000000..6acdc70ea
--- /dev/null
+++ b/plugins/magpie-setup/skills/isolated-setup-doctor/tests/test_probe_1_ssh_agent.py
@@ -0,0 +1,98 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+"""Probe 1 reports what `ssh-add -l` returned, and writes no file.
+
+The probe used to capture `ssh-add -l` into `/tmp/ssh-add.out`. Inside
+the sandbox `/tmp` is read-only, so the redirection failed, `ssh-add`
+never ran, and the redirection's own exit code of 1 was read as "agent
+reachable, no identities": a pass for an agent the probe never asked.
+"""
+
+from __future__ import annotations
+
+import os
+import socket
+import stat
+import subprocess
+import tempfile
+import time
+import unittest
+from pathlib import Path
+
+PROBE = Path(__file__).resolve().parents[1] / "scripts" / "probe-1-ssh-agent.sh"
+OLD_OUTPUT = Path("/tmp/ssh-add.out")
+
+
+class ProbeOneSshAgent(unittest.TestCase):
+    def setUp(self) -> None:
+        self._tmp = tempfile.TemporaryDirectory()
+        self.tmp = Path(self._tmp.name)
+        self.bin = self.tmp / "bin"
+        self.bin.mkdir()
+        self.addCleanup(self._tmp.cleanup)
+        # The probe only checks that SSH_AUTH_SOCK is a socket; the fake
+        # ssh-add never connects to it. A sandbox may refuse bind(2), so fall
+        # back to the session's own agent socket.
+        self.sock_path = self.tmp / "agent.sock"
+        sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
+        self.addCleanup(sock.close)
+        try:
+            sock.bind(str(self.sock_path))
+        except OSError:
+            existing = os.environ.get("SSH_AUTH_SOCK", "")
+            if not (existing and Path(existing).is_socket()):
+                self.skipTest("cannot bind a unix socket and no SSH_AUTH_SOCK to reuse")
+            self.sock_path = Path(existing)
+
+    def fake_ssh_add(self, output: str, rc: int) -> None:
+        script = self.bin / "ssh-add"
+        script.write_text(f"#!/bin/sh\nprintf '{output}'\nexit {rc}\n")
+        script.chmod(script.stat().st_mode | stat.S_IXUSR)
+
+    def run_probe(self) -> str:
+        env = {
+            "PATH": f"{self.bin}{os.pathsep}{os.environ.get('PATH', '')}",
+            "SSH_AUTH_SOCK": str(self.sock_path),
+            "TMPDIR": str(self.tmp),
+        }
+        result = subprocess.run(
+            ["bash", str(PROBE)], env=env, capture_output=True, text=True, check=True
+        )
+        return result.stdout.strip()
+
+    def test_counts_listed_identities(self) -> None:
+        self.fake_ssh_add("4096 SHA256:aaa key-a (RSA)\n4096 SHA256:bbb key-b (RSA)\n", 0)
+        self.assertEqual(self.run_probe(), "PROBE: ssh-agent → ✓ (2 identities listed)")
+
+    def test_reports_unreachable_agent_with_its_message(self) -> None:
+        self.fake_ssh_add("Could not open a connection to your authentication agent.\n", 2)
+        self.assertEqual(
+            self.run_probe(),
+            "PROBE: ssh-agent → ✗ (agent unreachable: "
+            "Could not open a connection to your authentication agent.)",
+        )
+
+    def test_writes_nothing_under_tmp(self) -> None:
+        self.fake_ssh_add("4096 SHA256:aaa key-a (RSA)\n", 0)
+        started = time.time()
+        self.run_probe()
+        if OLD_OUTPUT.exists():
+            self.assertLess(OLD_OUTPUT.stat().st_mtime, started)
+
+
+if __name__ == "__main__":
+    unittest.main()