diff --git a/agent/hotpath_agent/harness.py b/agent/hotpath_agent/harness.py
index 3364af5..0f2f33d 100644
--- a/agent/hotpath_agent/harness.py
+++ b/agent/hotpath_agent/harness.py
@@ -1,13 +1,21 @@
 """Client for harnessd — the agent's only door to the trust layer.
 
-Spawns the Rust daemon and speaks one JSON object per line over its
-stdio. The client is deliberately dumb: no retries that could double-
-apply a patch, no local fallbacks that could fabricate evidence.
+Two transports:
+- Unix socket (production/SPEC §10): if HOTPATH_HARNESS_SOCKET is set,
+  connect to a daemon owned by the trusted uid. The agent process runs
+  unprivileged and cannot write the repo except through this door.
+- Spawned stdio subprocess (dev fallback): same uid, defense-in-depth
+  only — the filesystem boundary does not hold in this mode.
+
+The client is deliberately dumb: no retries that could double-apply a
+patch, no local fallbacks that could fabricate evidence.
 """
 
 from __future__ import annotations
 
 import json
+import os
+import socket
 import subprocess
 from typing import Any
 
@@ -18,27 +26,45 @@ class HarnessError(RuntimeError):
 
 class HarnessClient:
     def __init__(self, repo_root: str, binary: str = "target/debug/harnessd"):
-        self._proc = subprocess.Popen(
-            [binary],
-            cwd=repo_root,
-            stdin=subprocess.PIPE,
-            stdout=subprocess.PIPE,
-            text=True,
-        )
+        sock_path = os.environ.get("HOTPATH_HARNESS_SOCKET")
+        self._proc: subprocess.Popen[str] | None = None
+        self._sock: socket.socket | None = None
+        if sock_path:
+            self._sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
+            self._sock.connect(sock_path)
+            self._io = self._sock.makefile("rw", encoding="utf-8", newline="\n")
+        else:
+            self._proc = subprocess.Popen(
+                [binary],
+                cwd=repo_root,
+                stdin=subprocess.PIPE,
+                stdout=subprocess.PIPE,
+                text=True,
+            )
 
     def call(self, op: str, **kwargs: Any) -> dict[str, Any]:
-        assert self._proc.stdin and self._proc.stdout
-        self._proc.stdin.write(json.dumps({"op": op, **kwargs}) + "\n")
-        self._proc.stdin.flush()
-        line = self._proc.stdout.readline()
+        payload = json.dumps({"op": op, **kwargs}) + "\n"
+        if self._sock is not None:
+            self._io.write(payload)
+            self._io.flush()
+            line = self._io.readline()
+        else:
+            assert self._proc and self._proc.stdin and self._proc.stdout
+            self._proc.stdin.write(payload)
+            self._proc.stdin.flush()
+            line = self._proc.stdout.readline()
         if not line:
-            raise HarnessError("harnessd exited")
+            raise HarnessError("harnessd connection closed")
         resp = json.loads(line)
         if not resp.get("ok"):
             raise HarnessError(resp.get("error", "unknown harness error"))
         return resp["result"]
 
     def close(self) -> None:
-        if self._proc.stdin:
-            self._proc.stdin.close()
-        self._proc.wait(timeout=10)
+        if self._sock is not None:
+            self._io.close()
+            self._sock.close()
+        if self._proc is not None:
+            if self._proc.stdin:
+                self._proc.stdin.close()
+            self._proc.wait(timeout=10)
diff --git a/agent/hotpath_agent/loop.py b/agent/hotpath_agent/loop.py
index 267f632..95017b0 100644
--- a/agent/hotpath_agent/loop.py
+++ b/agent/hotpath_agent/loop.py
@@ -36,15 +36,14 @@ HARNESS_TOOLS = [
 ]
 
 # Built-in tools we deny at the SDK layer. This is DEFENSE IN DEPTH, not the
-# trust boundary. SPEC §10's real boundary is OS-level process/user isolation:
-# harnessd runs as a separate process the agent can't bypass, target code runs
-# in a no-net container. In THIS dev container the nested agent runs as root
-# under a parent Claude Code session, so neither --dangerously-skip-permissions
-# nor a can_use_tool callback reliably gates built-ins (settings inherited from
-# the parent session shadow them). We enumerate what we can; production runs the
-# agent process without host-shell capability at all. What holds regardless:
-# propose_patch is the only harness-mediated write path (path-allowlist + git
-# apply), and the ledger is append-only (mutation-refusing triggers).
+# trust boundary. SPEC §10's real boundary is OS-level isolation, shipped in
+# scripts/agent-isolated.sh: this whole process tree runs either inside a
+# mount namespace with the repo read-only and CAP_SYS_ADMIN dropped, or as
+# the unprivileged hpagent user — verified by `just isolation-check`. Even
+# if the nested CLI reaches a shell despite this list (settings inherited
+# from a parent session can shadow SDK gating), repo writes fail at the OS.
+# What holds regardless: propose_patch is the only harness-mediated write
+# path (path-allowlist + git apply), and the ledger is append-only.
 BUILTIN_TOOLS_DENIED = [
     "Bash", "Read", "Write", "Edit", "Glob", "Grep",
     "WebSearch", "WebFetch", "Task", "TodoWrite",
diff --git a/crates/harnessd/src/main.rs b/crates/harnessd/src/main.rs
index fa241bb..39ec4cc 100644
--- a/crates/harnessd/src/main.rs
+++ b/crates/harnessd/src/main.rs
@@ -10,11 +10,24 @@ use anyhow::{anyhow, ensure, Result};
 use serde_json::{json, Value};
 use sha2::{Digest, Sha256};
 use std::io::{BufRead, Write};
+use std::os::unix::net::UnixListener;
 use std::path::{Component, Path};
 use std::process::Command;
+use std::sync::Mutex;
+
+/// Serializes mutating ops (git apply, pending writes) across socket
+/// connections; reads don't need it but the cost is nil at one agent.
+static HANDLE_LOCK: Mutex<()> = Mutex::new(());
 
 fn main() -> Result<()> {
     let root = std::env::current_dir()?.canonicalize()?;
+    let args: Vec<String> = std::env::args().collect();
+    if let Some(i) = args.iter().position(|a| a == "--socket") {
+        let path = args
+            .get(i + 1)
+            .ok_or_else(|| anyhow!("--socket requires a path"))?;
+        return serve_socket(&root, Path::new(path));
+    }
     let stdin = std::io::stdin();
     let mut stdout = std::io::stdout().lock();
     for line in stdin.lock().lines() {
@@ -22,16 +35,62 @@ fn main() -> Result<()> {
         if line.trim().is_empty() {
             continue;
         }
-        let resp = match handle(&root, &line) {
-            Ok(v) => json!({"ok": true, "result": v}),
-            Err(e) => json!({"ok": false, "error": e.to_string()}),
-        };
+        let resp = respond(&root, &line);
         writeln!(stdout, "{resp}")?;
         stdout.flush()?;
     }
     Ok(())
 }
 
+/// SPEC §10 socket mode: harnessd runs as the trusted uid and listens on a
+/// Unix socket; the agent process runs as an unprivileged user whose only
+/// write path into the repo is this daemon. Socket permissions (owner/group/
+/// mode) are set by the supervisor script that starts us.
+fn serve_socket(root: &Path, sock: &Path) -> Result<()> {
+    if let Some(dir) = sock.parent() {
+        std::fs::create_dir_all(dir)?;
+    }
+    let _ = std::fs::remove_file(sock);
+    let listener = UnixListener::bind(sock)?;
+    eprintln!("harnessd listening on {}", sock.display());
+    for conn in listener.incoming() {
+        let stream = match conn {
+            Ok(s) => s,
+            Err(e) => {
+                eprintln!("accept error: {e}");
+                continue;
+            }
+        };
+        let root = root.to_path_buf();
+        std::thread::spawn(move || {
+            let reader = match stream.try_clone() {
+                Ok(s) => std::io::BufReader::new(s),
+                Err(_) => return,
+            };
+            let mut writer = stream;
+            for line in reader.lines() {
+                let Ok(line) = line else { break };
+                if line.trim().is_empty() {
+                    continue;
+                }
+                let resp = respond(&root, &line);
+                if writeln!(writer, "{resp}").is_err() {
+                    break;
+                }
+            }
+        });
+    }
+    Ok(())
+}
+
+fn respond(root: &Path, line: &str) -> Value {
+    let _guard = HANDLE_LOCK.lock().unwrap_or_else(|p| p.into_inner());
+    match handle(root, line) {
+        Ok(v) => json!({"ok": true, "result": v}),
+        Err(e) => json!({"ok": false, "error": e.to_string()}),
+    }
+}
+
 /// Single-quote a string for safe embedding in a generated shell script.
 fn shq(s: &str) -> String {
     format!("'{}'", s.replace('\'', "'\\''"))
diff --git a/docs/infra.md b/docs/infra.md
index 0b9c150..bb0f3f2 100644
--- a/docs/infra.md
+++ b/docs/infra.md
@@ -11,7 +11,38 @@ repo's CI cannot provide. Status by requirement:
 | Environment noise measured | ✅ (measured, likely fails bars) | `runner-calibration` CI job, artifact per run |
 | Profiling | ⚠️ software `cpu-clock` only (VM PMU usually absent) | `perf-probe` CI job reports what works |
 | Pinned cores, governor, SMT/turbo control | ❌ shared VM | **bench-metal box (below)** |
-| Separate-uid read-only trust layer | ❌ | bench-metal box setup |
+| Separate-uid read-only trust layer | ✅ shipped | `scripts/agent-isolated.sh` + `just isolation-check` (below) |
+
+## OS-level agent isolation (SPEC §10) — shipped
+
+`scripts/agent-isolated.sh` launches one unattended attempt with the
+agent process tree behind an OS boundary; `just isolation-check`
+verifies the boundary from the agent's side (19 negative/positive
+checks, both modes). harnessd runs OUTSIDE the boundary as the trusted
+uid, serving its seven ops on a Unix socket (`harnessd --socket`) — the
+agent's only write path into the repo.
+
+Two modes (`HOTPATH_ISOLATION`):
+
+- **`mountns` (default in this container).** The agent tree runs uid 0
+  inside a private mount namespace with the repo bind-mounted
+  read-only, `CAP_SYS_ADMIN`/`CAP_SYS_PTRACE` dropped from the bounding
+  set. Verified refused: repo writes (EROFS), forged ledger INSERT,
+  trust-binary replacement, `mount -o remount,rw`, and userns re-entry
+  to undo the mount. Chosen because the nested CLI's auth is bound to
+  the parent session's uid — no credentials are copied anywhere.
+- **`user` (bench-metal shape).** The agent runs as unprivileged
+  `hpagent`; the repo is root-owned with no world-writable paths (the
+  launcher refuses to start otherwise). Verified refused: all of the
+  above plus running the `verdict` binary directly. Requires the agent
+  user to bring its own API credentials; a parent session's root-owned
+  auth is deliberately not copied across the boundary.
+
+Residual (both modes, this container): the agent tree can still read
+the repo (by design — it needs source), reach the network through the
+container's egress proxy, and signal same-uid processes in `mountns`
+mode. The bench-metal `user` mode plus no-net target containers remains
+the production shape.
 
 ## The bench-metal box (human setup, ~15 min + account)
 
diff --git a/justfile b/justfile
index b394e2e..140fe98 100644
--- a/justfile
+++ b/justfile
@@ -47,3 +47,12 @@ verdict target candidate run_id class hypothesis hotspot *flags:
         --candidate-bin "{{candidate}}" --run-id "{{run_id}}" \
         --playbook-class {{class}} --hypothesis "{{hypothesis}}" \
         --hotspot "{{hotspot}}" {{flags}}
+
+# One unattended agent attempt behind the SPEC §10 OS boundary:
+# harnessd (trusted uid) on a Unix socket, agent loop as user hpagent.
+agent-attempt target run_id max_turns="40":
+    sh scripts/agent-isolated.sh {{target}} {{run_id}} {{max_turns}}
+
+# Verify the OS boundary from the agent user's side (negative + positive)
+isolation-check:
+    sh scripts/isolation-check.sh
diff --git a/scripts/agent-isolated.sh b/scripts/agent-isolated.sh
new file mode 100755
index 0000000..8057fed
--- /dev/null
+++ b/scripts/agent-isolated.sh
@@ -0,0 +1,107 @@
+#!/bin/sh
+# Launch one unattended optimization attempt behind an OS-level boundary
+# (SPEC §10). Run as the trusted uid (root in this container):
+#
+#   scripts/agent-isolated.sh <target> <run-id> [max-turns]
+#
+# Two isolation modes (HOTPATH_ISOLATION, default "mountns"):
+#
+#   mountns — this container's boundary. The agent process tree (Claude
+#     CLI + MCP server + harness client) runs in a private mount
+#     namespace where the repo is bind-mounted READ-ONLY, with
+#     CAP_SYS_ADMIN and CAP_SYS_PTRACE removed from the bounding set so
+#     the mount cannot be undone (verified: remount and userns re-entry
+#     both refused). Repo writes fail with EROFS no matter the uid; the
+#     CLI's own auth state (under $HOME) stays untouched. No credential
+#     copying.
+#
+#   user — the bench-metal shape (docs/infra.md). The agent loop runs as
+#     the unprivileged user 'hpagent' whose only door into the repo is
+#     the harnessd socket. Requires the agent user to have its own API
+#     credentials (CLAUDE_CODE_OAUTH_TOKEN or ANTHROPIC_API_KEY in its
+#     environment) — a parent session's root-owned auth is deliberately
+#     NOT copied across the boundary.
+#
+# In both modes harnessd + the verdict pipeline run OUTSIDE the boundary
+# as the trusted uid and own every write into the repo (git apply,
+# ledger rows, pending/ evidence). Verify the boundary any time with
+# scripts/isolation-check.sh.
+set -eu
+
+ROOT=$(cd "$(dirname "$0")/.." && pwd)
+TARGET=${1:?usage: agent-isolated.sh <target> <run-id> [max-turns]}
+RUN_ID=${2:?usage: agent-isolated.sh <target> <run-id> [max-turns]}
+MAX_TURNS=${3:-40}
+MODE=${HOTPATH_ISOLATION:-mountns}
+
+AGENT_USER=hpagent
+SOCK_DIR=/run/hotpath
+SOCK=$SOCK_DIR/harness.sock
+
+[ "$(id -u)" -eq 0 ] || { echo "must run as the trusted uid (root)" >&2; exit 1; }
+
+# --- no world-writable holes in the repo ------------------------------------
+# In user mode the boundary is filesystem permissions; a single o+w path
+# would void it. mountns mode doesn't need this, but a hole is still a
+# smell worth failing on.
+HOLES=$(find "$ROOT" -perm -o+w \( -type f -o -type d \) 2>/dev/null || true)
+if [ -n "$HOLES" ]; then
+    echo "refusing to launch: world-writable paths inside the repo:" >&2
+    echo "$HOLES" >&2
+    exit 1
+fi
+
+# --- trust-layer daemon (outside the boundary) --------------------------------
+( cd "$ROOT" && cargo build -q -p harnessd -p verdict )
+
+mkdir -p "$SOCK_DIR"
+if [ ! -S "$SOCK" ]; then
+    ( cd "$ROOT" && setsid ./target/debug/harnessd --socket "$SOCK" \
+        >>/var/log/hotpath-harnessd.log 2>&1 & )
+    for _ in $(seq 1 50); do [ -S "$SOCK" ] && break; sleep 0.1; done
+    [ -S "$SOCK" ] || { echo "harnessd socket never appeared" >&2; exit 1; }
+fi
+
+run_loop="python3 -m hotpath_agent.loop $TARGET --run-id $RUN_ID --max-turns $MAX_TURNS --repo-root $ROOT"
+
+case "$MODE" in
+mountns)
+    chmod 600 "$SOCK"   # uid 0 only; the confined tree is still uid 0
+    exec unshare -m sh -c "
+        mount --bind '$ROOT' '$ROOT' &&
+        mount -o remount,bind,ro '$ROOT' &&
+        exec setpriv --bounding-set -sys_admin,-sys_ptrace --inh-caps -all \
+            env HOTPATH_HARNESS_SOCKET='$SOCK' \
+                HOTPATH_REPO_ROOT='$ROOT' \
+                PYTHONPATH='$ROOT/agent' \
+            $run_loop"
+    ;;
+user)
+    id "$AGENT_USER" >/dev/null 2>&1 || useradd -m -s /bin/sh "$AGENT_USER"
+    HP_HOME=$(getent passwd "$AGENT_USER" | cut -d: -f6)
+    chown root:"$AGENT_USER" "$SOCK"
+    chmod 660 "$SOCK"
+    # The container's TLS-intercepting proxy CA lives under /root (mode
+    # 700); the agent user needs a readable copy to reach the API.
+    CA_SRC=${HOTPATH_CA_SRC:-/root/.ccr/ca-bundle.crt}
+    CA=/etc/hotpath-ca.crt
+    [ -f "$CA_SRC" ] && install -m 644 "$CA_SRC" "$CA"
+    # setpriv preserves the environment (proxy/API routing vars); we
+    # override HOME and every CA var that pointed under /root. The agent
+    # user must bring its own API credentials — see header.
+    exec setpriv --reuid "$AGENT_USER" --regid "$AGENT_USER" --init-groups \
+        env HOME="$HP_HOME" USER="$AGENT_USER" LOGNAME="$AGENT_USER" \
+            HOTPATH_HARNESS_SOCKET="$SOCK" \
+            HOTPATH_REPO_ROOT="$ROOT" \
+            PYTHONPATH="$ROOT/agent" \
+            SSL_CERT_FILE="$CA" NODE_EXTRA_CA_CERTS="$CA" \
+            CURL_CA_BUNDLE="$CA" REQUESTS_CA_BUNDLE="$CA" \
+            GIT_SSL_CAINFO="$CA" AWS_CA_BUNDLE="$CA" \
+            NIX_SSL_CERT_FILE="$CA" GRPC_DEFAULT_SSL_ROOTS_FILE_PATH="$CA" \
+        $run_loop
+    ;;
+*)
+    echo "unknown HOTPATH_ISOLATION mode: $MODE (mountns|user)" >&2
+    exit 1
+    ;;
+esac
diff --git a/scripts/isolation-check.sh b/scripts/isolation-check.sh
new file mode 100755
index 0000000..ab485e7
--- /dev/null
+++ b/scripts/isolation-check.sh
@@ -0,0 +1,132 @@
+#!/bin/sh
+# Verify the SPEC §10 OS boundary from the agent's side of it. Run as the
+# trusted uid (root). Checks BOTH isolation modes of agent-isolated.sh:
+# every write the agent could use to game the harness must fail; the
+# harnessd socket door must work. Exits nonzero on any hole.
+set -u
+
+ROOT=$(cd "$(dirname "$0")/.." && pwd)
+AGENT_USER=hpagent
+SOCK=${HOTPATH_HARNESS_SOCKET:-/run/hotpath/harness.sock}
+FAIL=0
+
+note_fail() { echo "FAIL (boundary hole): $1"; FAIL=1; }
+note_ok()   { echo "ok: $1"; }
+
+[ -S "$SOCK" ] || { echo "FAIL: harnessd socket $SOCK missing (run agent-isolated.sh or start the daemon)"; exit 1; }
+
+# Probe scripts as files: they run nested inside sh -c layers where
+# inline `python3 -c` quoting is unmanageable.
+PROBE_DIR=$(mktemp -d)
+trap 'rm -rf "$PROBE_DIR"' EXIT
+chmod 755 "$PROBE_DIR"
+
+cat > "$PROBE_DIR/sock_check.py" <<EOF
+import json, socket, sys
+s = socket.socket(socket.AF_UNIX); s.settimeout(15); s.connect('$SOCK')
+f = s.makefile('rw')
+f.write(json.dumps({'op': 'read_ledger', 'target': 'comrak'}) + '\n'); f.flush()
+resp = json.loads(f.readline())
+assert resp['ok'], resp
+EOF
+
+cat > "$PROBE_DIR/escape_check.py" <<EOF
+import json, socket
+s = socket.socket(socket.AF_UNIX); s.settimeout(15); s.connect('$SOCK')
+f = s.makefile('rw')
+diff = '--- a/../../../config/accept.toml\n+++ b/../../../config/accept.toml\n@@ -1 +1 @@\n-x\n+y\n'
+f.write(json.dumps({'op': 'propose_patch', 'target': 'comrak', 'diff': diff, 'hypothesis': 'probe'}) + '\n'); f.flush()
+resp = json.loads(f.readline())
+assert not resp['ok'] and 'escape' in resp['error'], resp
+EOF
+
+cat > "$PROBE_DIR/ledger_insert.py" <<EOF
+import sqlite3
+c = sqlite3.connect('$ROOT/results/ledger.sqlite')
+c.execute("INSERT INTO attempts (run_id,timestamp,target,target_commit,phase,hotspot,playbook_class,hypothesis,patch,gates,bench,verdict,tokens_spent,wall_time_s) VALUES ('forged','','t','','','',1,'','','{}','{}','accepted',0,0)")
+c.commit()
+EOF
+chmod 644 "$PROBE_DIR"/*.py
+
+# ============================ mountns mode ====================================
+echo "--- mountns mode (confined uid-0 tree, repo read-only) ---"
+MNT_PREFIX="unshare -m"
+CONFINE="mount --bind '$ROOT' '$ROOT' && mount -o remount,bind,ro '$ROOT' && exec setpriv --bounding-set -sys_admin,-sys_ptrace --inh-caps -all"
+
+mnt_must_fail() {
+    desc=$1; cmd=$2
+    if $MNT_PREFIX sh -c "$CONFINE sh -c \"$cmd\"" >/dev/null 2>&1; then
+        note_fail "[mountns] $desc succeeded"
+    else
+        note_ok "[mountns] $desc refused"
+    fi
+}
+mnt_must_pass() {
+    desc=$1; cmd=$2
+    if $MNT_PREFIX sh -c "$CONFINE sh -c \"$cmd\"" >/dev/null 2>&1; then
+        note_ok "[mountns] $desc works"
+    else
+        note_fail "[mountns] $desc broken"
+    fi
+}
+
+mnt_must_fail "write into crates/ (trust layer)"      "touch '$ROOT/crates/.probe'"
+mnt_must_fail "edit config/accept.toml (thresholds)"  "echo x >> '$ROOT/config/accept.toml'"
+mnt_must_fail "write a target workspace directly"     "touch '$ROOT/targets/comrak/workspace/.probe'"
+mnt_must_fail "INSERT a forged ledger row"            "python3 $PROBE_DIR/ledger_insert.py"
+mnt_must_fail "overwrite results/ evidence"           "echo x > '$ROOT/results/.probe'"
+mnt_must_fail "replace a trust binary"                "echo x > '$ROOT/target/debug/verdict'"
+mnt_must_fail "remount the repo read-write"           "mount -o remount,bind,rw '$ROOT'"
+mnt_must_fail "userns re-entry to undo the ro mount"  "unshare -Ur -m mount -o remount,bind,rw '$ROOT'"
+mnt_must_pass "harnessd socket read_ledger"           "python3 $PROBE_DIR/sock_check.py"
+
+# ============================ user mode =======================================
+if id "$AGENT_USER" >/dev/null 2>&1; then
+    echo "--- user mode (unprivileged $AGENT_USER) ---"
+    HP_HOME=$(getent passwd "$AGENT_USER" | cut -d: -f6)
+    as_agent() {
+        setpriv --reuid "$AGENT_USER" --regid "$AGENT_USER" --init-groups \
+            env HOME="$HP_HOME" "$@"
+    }
+    must_fail() {
+        desc=$1; shift
+        if as_agent "$@" >/dev/null 2>&1; then
+            note_fail "[user] $desc succeeded as $AGENT_USER"
+        else
+            note_ok "[user] $desc refused"
+        fi
+    }
+    must_pass() {
+        desc=$1; shift
+        if as_agent "$@" >/dev/null 2>&1; then
+            note_ok "[user] $desc works"
+        else
+            note_fail "[user] $desc broken for $AGENT_USER"
+        fi
+    }
+    # user mode needs group access on the socket for the positive checks
+    SOCK_GRP_SAVED=$(stat -c '%U:%G %a' "$SOCK")
+    chown root:"$AGENT_USER" "$SOCK"; chmod 660 "$SOCK"
+
+    must_fail "write into crates/ (trust layer)"      sh -c "touch '$ROOT/crates/.probe'"
+    must_fail "edit config/accept.toml (thresholds)"  sh -c "echo x >> '$ROOT/config/accept.toml'"
+    must_fail "write into corpora/ (pinned inputs)"   sh -c "touch '$ROOT/corpora/.probe'"
+    must_fail "write a target workspace directly"     sh -c "touch '$ROOT/targets/comrak/workspace/.probe'"
+    must_fail "INSERT a forged ledger row"            python3 "$PROBE_DIR/ledger_insert.py"
+    must_fail "overwrite results/ evidence"           sh -c "echo x > '$ROOT/results/.probe'"
+    must_fail "replace a trust binary"                sh -c "echo x > '$ROOT/target/debug/verdict'"
+    must_fail "run the verdict binary to write a row" sh -c "cd '$ROOT' && ./target/debug/verdict comrak --candidate-bin /bin/true --run-id forged-direct --playbook-class 1 --hypothesis x --hotspot x"
+    must_pass "harnessd socket read_ledger"           python3 "$PROBE_DIR/sock_check.py"
+    must_pass "harnessd refuses ../ escape in a diff" python3 "$PROBE_DIR/escape_check.py"
+
+    chown "${SOCK_GRP_SAVED%% *}" "$SOCK"; chmod "${SOCK_GRP_SAVED##* }" "$SOCK"
+else
+    echo "--- user mode skipped (no $AGENT_USER user) ---"
+fi
+
+if [ "$FAIL" -eq 0 ]; then
+    echo "isolation-check: ALL CHECKS PASSED"
+else
+    echo "isolation-check: BOUNDARY HOLES FOUND" >&2
+fi
+exit "$FAIL"