diff --git a/docs/testing.md b/docs/testing.md
index 42c5cb24c..3f1b2d68b 100644
--- a/docs/testing.md
+++ b/docs/testing.md
@@ -383,7 +383,11 @@ to the CLI entrypoint being verified. The command builds and checks source-owned
 ELF fixtures before testing the read-only Hopper setup plan, approved
 installation, scoped doctor, real CLI/MCP analysis, rejection of a modified
 launcher, idempotent setup without requesting a reinstall, and owned child
-cleanup. It requires the repository's pinned development toolchain and an
+cleanup. Before installation and after teardown, a clean-runner process scan
+also rejects surviving Hopper demo helpers or Xvfb groups, including orphans
+whose launcher exited. This scan observes processes without signaling them;
+the dedicated runner must have no unrelated Hopper/Xvfb sessions. It requires
+the repository's pinned development toolchain and an
 existing host C compiler, `file`, `nm`, and `strings`. Use `verify:hopper:linux`
 instead when Hopper is already installed.
 
diff --git a/scripts/lib/clean-linux-hopper-runtime.d.mts b/scripts/lib/clean-linux-hopper-runtime.d.mts
new file mode 100644
index 000000000..c860c876d
--- /dev/null
+++ b/scripts/lib/clean-linux-hopper-runtime.d.mts
@@ -0,0 +1 @@
+export function assertCleanLinuxHopperRuntime(): Promise<void>;
diff --git a/scripts/lib/clean-linux-hopper-runtime.mjs b/scripts/lib/clean-linux-hopper-runtime.mjs
new file mode 100644
index 000000000..2fc8fbfad
--- /dev/null
+++ b/scripts/lib/clean-linux-hopper-runtime.mjs
@@ -0,0 +1,15 @@
+import { execFile } from "node:child_process";
+import { promisify } from "node:util";
+
+const execFileAsync = promisify(execFile);
+
+/**
+ * Observe leftover demo processes on a dedicated clean Linux runner, including
+ * groups reparented after their launcher exits. This is not ownership proof and
+ * never authorizes signaling; unrelated runtime processes also block this lane.
+ */
+export const assertCleanLinuxHopperRuntime = async () => {
+  const { stdout } = await execFileAsync("ps", ["-ax", "-o", "command="]);
+  if (/\/opt\/hopper\/bin\/Hopper|\bXvfb\b|hopper-demo-x11\.py/u.test(stdout))
+    throw new Error("Clean Linux runner retained a Hopper demo process");
+};
diff --git a/scripts/verify-linux-setup-e2e.mjs b/scripts/verify-linux-setup-e2e.mjs
index a2b79dd60..e08e8b160 100644
--- a/scripts/verify-linux-setup-e2e.mjs
+++ b/scripts/verify-linux-setup-e2e.mjs
@@ -12,6 +12,7 @@ import { tmpdir } from "node:os";
 import { promisify } from "node:util";
 
 import { completeVerifierRun, createVerifierRun } from "./lib/verifier-run.mjs";
+import { assertCleanLinuxHopperRuntime } from "./lib/clean-linux-hopper-runtime.mjs";
 
 const execFileAsync = promisify(execFile);
 const verifierRun = createVerifierRun();
@@ -19,6 +20,7 @@ const command = process.env.REA_VERIFY_SERVER_COMMAND;
 if (process.platform !== "linux" || process.getuid?.() !== 0)
   throw new Error("Linux setup verification requires a root Linux runner");
 if (!command) throw new Error("REA_VERIFY_SERVER_COMMAND is required");
+await assertCleanLinuxHopperRuntime();
 try {
   await access("/opt/hopper/bin/Hopper");
   throw new Error("Linux setup verification requires a clean runner");
@@ -110,6 +112,7 @@ if (
   repeated.appliedActions.length !== 0
 )
   throw new Error("a second setup was not idempotent");
+await assertCleanLinuxHopperRuntime();
 const completedVerifierRun = await completeVerifierRun(verifierRun);
 if (
   completedVerifierRun.process_lineage.status !== "verified" ||
diff --git a/tests/process-global/cleanLinuxHopperRuntime.test.ts b/tests/process-global/cleanLinuxHopperRuntime.test.ts
new file mode 100644
index 000000000..af16911fb
--- /dev/null
+++ b/tests/process-global/cleanLinuxHopperRuntime.test.ts
@@ -0,0 +1,89 @@
+import { execFile } from "node:child_process";
+import { randomUUID } from "node:crypto";
+import { promisify } from "node:util";
+
+import { expect, it } from "vitest";
+
+import { assertCleanLinuxHopperRuntime } from "../../scripts/lib/clean-linux-hopper-runtime.mjs";
+import { completeVerifierRun } from "../../scripts/lib/verifier-run.mjs";
+import { cleanupOwnedProcessGroup } from "../../src/process/ProcessOwnership.js";
+
+const execFileAsync = promisify(execFile);
+
+it.runIf(process.platform === "linux")(
+  "rejects a reparented demo process without signaling it",
+  async () => {
+    const runId = randomUUID();
+    const childCode =
+      "process.title = 'Xvfb'; setInterval(() => {}, 1000); process.send('ready');";
+    const parentCode = `
+      const { spawn } = require('node:child_process');
+      const child = spawn(process.execPath, ['-e', ${JSON.stringify(childCode)}], {
+        detached: true, stdio: ['ignore', 'ignore', 'ignore', 'ipc']
+      });
+      child.once('error', () => process.exit(1));
+      child.once('message', () => {
+        process.stdout.write(JSON.stringify({ pid: child.pid, parentPid: process.pid }));
+        child.disconnect();
+        child.unref();
+      });
+    `;
+    const parent = execFileAsync(process.execPath, ["-e", parentCode], {
+      env: { ...process.env, REA_PROCESS_RUN_ID: runId },
+      timeout: 10_000,
+    });
+    const parentPid = parent.child.pid;
+    if (parentPid === undefined)
+      throw new Error("Fixture parent did not spawn");
+    let orphanPid: number | undefined;
+    try {
+      const { stdout } = await parent;
+      const { pid } = JSON.parse(stdout) as { pid: number };
+      orphanPid = pid;
+      const { stdout: coordinates } = await execFileAsync("ps", [
+        "-p",
+        String(pid),
+        "-o",
+        "ppid=,pgid=",
+      ]);
+      const [currentParent, group] = coordinates
+        .trim()
+        .split(/\s+/u)
+        .map(Number);
+      expect(currentParent).not.toBe(parentPid);
+      expect(group).toBe(pid);
+
+      const lineage = await completeVerifierRun({
+        run_id: runId,
+        verifier_pid: process.pid,
+        parent_pid: process.ppid,
+      });
+      expect(lineage.process_lineage.status).toBe("verified");
+      if (lineage.process_lineage.status === "verified")
+        expect(
+          lineage.process_lineage.descendants.some((p) => p.pid === pid),
+        ).toBe(false);
+
+      await expect(assertCleanLinuxHopperRuntime()).rejects.toThrow(
+        "Clean Linux runner retained a Hopper demo process",
+      );
+      // Observation must not terminate the fixture, even though it resembles Xvfb.
+      const { stdout: alive } = await execFileAsync("ps", [
+        "-p",
+        String(pid),
+        "-o",
+        "stat=",
+      ]);
+      expect(alive.trim()).not.toMatch(/^Z/u);
+    } finally {
+      if (orphanPid !== undefined) {
+        const cleanup = await cleanupOwnedProcessGroup({
+          runId,
+          leaderPid: orphanPid,
+          processGroupId: orphanPid,
+        });
+        expect(cleanup.cleaned, JSON.stringify(cleanup)).toBe(true);
+      }
+    }
+  },
+);