diff --git a/.github/scripts/claude-lane-install-isolation.test.cjs b/.github/scripts/claude-lane-install-isolation.test.cjs
index 02bab1fb..7efeed83 100644
--- a/.github/scripts/claude-lane-install-isolation.test.cjs
+++ b/.github/scripts/claude-lane-install-isolation.test.cjs
@@ -5,7 +5,10 @@
 // lane without the isolation the other has.
 
 const assert = require("node:assert/strict");
+const { spawnSync } = require("node:child_process");
+const crypto = require("node:crypto");
 const fs = require("node:fs");
+const os = require("node:os");
 const path = require("node:path");
 const test = require("node:test");
 
@@ -76,3 +79,100 @@ test("every apt-get call ends before its step limit", () => {
     );
   }
 });
+
+// Without the helper, bubblewrap gets --disable-userns and dies on a
+// read-only /proc/sys. sandbox-runtime looks for it only under these static
+// global npm roots and `npm root -g` (src/sandbox/generate-seccomp-filter.ts,
+// buildGlobalNpmPaths, v0.0.81), so a helper anywhere else is never used.
+const helperRoots = [
+  "/usr/lib/node_modules",
+  "/usr/local/lib/node_modules",
+  "/opt/homebrew/lib/node_modules",
+];
+const helperSuffix =
+  "@anthropic-ai/sandbox-runtime/vendor/seccomp/x64/apply-seccomp";
+
+const runHelperInstall = (tarballSha256) => {
+  const run = installStep("pr-review.yml").run;
+  const fragment = run.slice(run.indexOf('case "$RUNNER_ARCH" in'));
+  assert.ok(fragment.startsWith("case"), "the step installs the helper");
+  const dir = fs.mkdtempSync(path.join(os.tmpdir(), "seccomp-helper-"));
+  const root = path.join(dir, "root");
+  const stubs = path.join(dir, "stubs");
+  const packageDir = path.join(
+    dir,
+    "pkg",
+    "package",
+    "vendor",
+    "seccomp",
+    "x64",
+  );
+  fs.mkdirSync(packageDir, { recursive: true });
+  fs.mkdirSync(stubs);
+  fs.writeFileSync(path.join(packageDir, "apply-seccomp"), "helper-bytes\n");
+  const tarball = path.join(dir, "package.tgz");
+  spawnSync("tar", ["-czf", tarball, "-C", path.join(dir, "pkg"), "package"]);
+  const sha256 =
+    tarballSha256 ??
+    crypto.createHash("sha256").update(fs.readFileSync(tarball)).digest("hex");
+  fs.writeFileSync(
+    path.join(stubs, "curl"),
+    '#!/bin/bash\nwhile [ $# -gt 1 ]; do [ "$1" = --output ] && out="$2"; shift; done\necho "$1" >> "$STUB_LOG"\ncp "$FAKE_TARBALL" "$out"\n',
+    { mode: 0o755 },
+  );
+  fs.writeFileSync(
+    path.join(stubs, "sudo"),
+    '#!/bin/bash\necho "sudo $*" >> "$STUB_LOG"\nfor arg; do src="$dest"; dest="$arg"; done\nmkdir -p "$(dirname "$FAKE_ROOT$dest")"\ncp "$src" "$FAKE_ROOT$dest"\n',
+    { mode: 0o755 },
+  );
+  const log = path.join(dir, "stub.log");
+  const result = spawnSync("bash", ["-eo", "pipefail", "-c", fragment], {
+    encoding: "utf8",
+    env: {
+      ...process.env,
+      PATH: `${stubs}:${process.env.PATH}`,
+      RUNNER_ARCH: "X64",
+      RUNNER_TEMP: dir,
+      SANDBOX_RUNTIME_VERSION: "0.0.81",
+      SANDBOX_RUNTIME_SHA256: sha256,
+      FAKE_TARBALL: tarball,
+      FAKE_ROOT: root,
+      STUB_LOG: log,
+    },
+  });
+  const stubLog = fs.existsSync(log) ? fs.readFileSync(log, "utf8") : "";
+  return { result, root, stubLog, dir };
+};
+
+test("the helper lands root-owned where sandbox-runtime looks for it", (t) => {
+  const { result, root, stubLog, dir } = runHelperInstall();
+  t.after(() => fs.rmSync(dir, { recursive: true, force: true }));
+  assert.equal(result.status, 0, result.stderr);
+  const installed = helperRoots
+    .map((base) => path.join(base, helperSuffix))
+    .filter((target) => fs.existsSync(path.join(root, target)));
+  assert.equal(installed.length, 1, "one helper under a searched root");
+  assert.equal(
+    fs.readFileSync(path.join(root, installed[0]), "utf8"),
+    "helper-bytes\n",
+  );
+  assert.match(stubLog, /^sudo install -D -o root -g root -m 0755 -- /mu);
+  assert.match(
+    stubLog,
+    /^https:\/\/registry\.npmjs\.org\/@anthropic-ai\/sandbox-runtime\/-\/sandbox-runtime-0\.0\.81\.tgz$/mu,
+  );
+});
+
+// A missing helper only restores the behavior without it, so a bad tarball
+// warns and lets the lane run instead of stopping it.
+test("a tarball that does not match the pinned SHA-256 installs nothing and warns", (t) => {
+  const { result, root, stubLog, dir } = runHelperInstall("0".repeat(64));
+  t.after(() => fs.rmSync(dir, { recursive: true, force: true }));
+  assert.equal(result.status, 0, result.stderr);
+  assert.match(
+    result.stdout,
+    /^::warning::.*does not match its pinned SHA-256; the seccomp helper is not installed/mu,
+  );
+  assert.equal(fs.existsSync(root), false, "nothing installed");
+  assert.doesNotMatch(stubLog, /^sudo /mu);
+});
diff --git a/.github/workflows/pr-review-security.yml b/.github/workflows/pr-review-security.yml
index 431063be..73a4bb63 100644
--- a/.github/workflows/pr-review-security.yml
+++ b/.github/workflows/pr-review-security.yml
@@ -664,12 +664,25 @@ jobs:
       # Linux's, and a runner without apt-get must already have bwrap. The
       # AppArmor sysctl is lifted only on GitHub-hosted runners, which are
       # discarded after the job, so no reused host keeps the change.
+      # The step also installs sandbox-runtime's apply-seccomp helper. Without
+      # it, Claude Code passes bubblewrap --disable-userns, which writes
+      # /proc/sys/user/max_user_namespaces and dies where Docker mounts
+      # /proc/sys read-only, so every Bash call fails. Claude Code looks for
+      # the helper under the global npm roots, /usr/local/lib/node_modules
+      # among them (sandbox-runtime src/sandbox/generate-seccomp-filter.ts,
+      # buildGlobalNpmPaths). The worker image has no npm, so the helper is
+      # taken from the package tarball after a SHA-256 check. That install is
+      # best-effort: on any failure nothing is installed, a warning names the
+      # cause, and Claude Code runs as it would without the helper.
       - name: Install subprocess isolation
         id: isolate
         if: steps.scope.outputs.review != 'false' && runner.os == 'Linux'
-        timeout-minutes: 1
+        timeout-minutes: 2
         env:
           RUNNER_HOSTING: ${{ runner.environment }}
+          SANDBOX_RUNTIME_VERSION: 0.0.81
+          # SHA-256 of that version's npm tarball; change both together.
+          SANDBOX_RUNTIME_SHA256: 0e0b8249062047f7b592c7995f8ec2f251f3bbebdc0402db1ee23af09983953f
         run: |
           if command -v apt-get >/dev/null &&
             ! { command -v bwrap && command -v socat; } >/dev/null; then
@@ -699,6 +712,33 @@ jobs:
             echo "::error::bubblewrap or socat is not installed; Claude Code cannot scrub its subprocess environment"
             exit 1
           fi
+          case "$RUNNER_ARCH" in
+            X64) arch=x64 ;;
+            ARM64) arch=arm64 ;;
+            *) arch="" ;;
+          esac
+          url="https://registry.npmjs.org/@anthropic-ai/sandbox-runtime/-/sandbox-runtime-$SANDBOX_RUNTIME_VERSION.tgz"
+          helper="/usr/local/lib/node_modules/@anthropic-ai/sandbox-runtime/vendor/seccomp/$arch/apply-seccomp"
+          member="package/vendor/seccomp/$arch/apply-seccomp"
+          work="$(mktemp -d "$RUNNER_TEMP/sandbox-runtime.XXXXXX")"
+          tarball="$work/package.tgz"
+          skip="the seccomp helper is not installed, so Claude Code runs without it"
+          if [ -z "$arch" ]; then
+            echo "::warning::sandbox-runtime ships no apply-seccomp helper for runner architecture $RUNNER_ARCH; $skip"
+          elif ! curl -q --fail --silent --show-error --location \
+            --proto '=https' --proto-redir '=https' \
+            --connect-timeout 10 --max-time 15 --retry 2 --retry-all-errors \
+            --output "$tarball" "$url"; then
+            echo "::warning::the sandbox-runtime $SANDBOX_RUNTIME_VERSION tarball did not download from the npm registry; $skip"
+          elif ! printf '%s  %s\n' "$SANDBOX_RUNTIME_SHA256" "$tarball" | sha256sum -c --quiet -; then
+            echo "::warning::the sandbox-runtime $SANDBOX_RUNTIME_VERSION tarball does not match its pinned SHA-256; $skip"
+          elif ! tar -xzf "$tarball" -C "$work" -- "$member" ||
+            ! sudo install -D -o root -g root -m 0755 -- "$work/$member" "$helper"; then
+            echo "::warning::apply-seccomp from sandbox-runtime $SANDBOX_RUNTIME_VERSION did not extract or install; $skip"
+          else
+            echo "installed apply-seccomp from sandbox-runtime $SANDBOX_RUNTIME_VERSION at $helper"
+          fi
+          rm -rf -- "${work:?}"
 
       # Installed bubblewrap can still fail to create a namespace (unprivileged
       # user namespaces off), and then every Bash call the model makes fails
diff --git a/.github/workflows/pr-review.yml b/.github/workflows/pr-review.yml
index 706e84d1..077218a2 100644
--- a/.github/workflows/pr-review.yml
+++ b/.github/workflows/pr-review.yml
@@ -804,12 +804,25 @@ jobs:
       # Linux's, and a runner without apt-get must already have bwrap. The
       # AppArmor sysctl is lifted only on GitHub-hosted runners, which are
       # discarded after the job, so no reused host keeps the change.
+      # The step also installs sandbox-runtime's apply-seccomp helper. Without
+      # it, Claude Code passes bubblewrap --disable-userns, which writes
+      # /proc/sys/user/max_user_namespaces and dies where Docker mounts
+      # /proc/sys read-only, so every Bash call fails. Claude Code looks for
+      # the helper under the global npm roots, /usr/local/lib/node_modules
+      # among them (sandbox-runtime src/sandbox/generate-seccomp-filter.ts,
+      # buildGlobalNpmPaths). The worker image has no npm, so the helper is
+      # taken from the package tarball after a SHA-256 check. That install is
+      # best-effort: on any failure nothing is installed, a warning names the
+      # cause, and Claude Code runs as it would without the helper.
       - name: Install subprocess isolation
         id: isolate
         if: steps.scope.outputs.review != 'false' && runner.os == 'Linux'
-        timeout-minutes: 1
+        timeout-minutes: 2
         env:
           RUNNER_HOSTING: ${{ runner.environment }}
+          SANDBOX_RUNTIME_VERSION: 0.0.81
+          # SHA-256 of that version's npm tarball; change both together.
+          SANDBOX_RUNTIME_SHA256: 0e0b8249062047f7b592c7995f8ec2f251f3bbebdc0402db1ee23af09983953f
         run: |
           if command -v apt-get >/dev/null &&
             ! { command -v bwrap && command -v socat; } >/dev/null; then
@@ -839,6 +852,33 @@ jobs:
             echo "::error::bubblewrap or socat is not installed; Claude Code cannot scrub its subprocess environment"
             exit 1
           fi
+          case "$RUNNER_ARCH" in
+            X64) arch=x64 ;;
+            ARM64) arch=arm64 ;;
+            *) arch="" ;;
+          esac
+          url="https://registry.npmjs.org/@anthropic-ai/sandbox-runtime/-/sandbox-runtime-$SANDBOX_RUNTIME_VERSION.tgz"
+          helper="/usr/local/lib/node_modules/@anthropic-ai/sandbox-runtime/vendor/seccomp/$arch/apply-seccomp"
+          member="package/vendor/seccomp/$arch/apply-seccomp"
+          work="$(mktemp -d "$RUNNER_TEMP/sandbox-runtime.XXXXXX")"
+          tarball="$work/package.tgz"
+          skip="the seccomp helper is not installed, so Claude Code runs without it"
+          if [ -z "$arch" ]; then
+            echo "::warning::sandbox-runtime ships no apply-seccomp helper for runner architecture $RUNNER_ARCH; $skip"
+          elif ! curl -q --fail --silent --show-error --location \
+            --proto '=https' --proto-redir '=https' \
+            --connect-timeout 10 --max-time 15 --retry 2 --retry-all-errors \
+            --output "$tarball" "$url"; then
+            echo "::warning::the sandbox-runtime $SANDBOX_RUNTIME_VERSION tarball did not download from the npm registry; $skip"
+          elif ! printf '%s  %s\n' "$SANDBOX_RUNTIME_SHA256" "$tarball" | sha256sum -c --quiet -; then
+            echo "::warning::the sandbox-runtime $SANDBOX_RUNTIME_VERSION tarball does not match its pinned SHA-256; $skip"
+          elif ! tar -xzf "$tarball" -C "$work" -- "$member" ||
+            ! sudo install -D -o root -g root -m 0755 -- "$work/$member" "$helper"; then
+            echo "::warning::apply-seccomp from sandbox-runtime $SANDBOX_RUNTIME_VERSION did not extract or install; $skip"
+          else
+            echo "installed apply-seccomp from sandbox-runtime $SANDBOX_RUNTIME_VERSION at $helper"
+          fi
+          rm -rf -- "${work:?}"
 
       # Installed bubblewrap can still fail to create a namespace (unprivileged
       # user namespaces off), and then every Bash call the model makes fails