diff --git a/src/cli.ts b/src/cli.ts
index 616baa2..9d5cf13 100644
--- a/src/cli.ts
+++ b/src/cli.ts
@@ -1473,6 +1473,7 @@ async function handleDeadSession(
   await spawnDaemon({
     name: session.name, command: meta.command, args: meta.args, displayCommand: meta.displayCommand, cwd: meta.cwd, tags: meta.tags,
     ...(meta.displayName ? { displayName: meta.displayName } : {}),
+    scrubEnv: RESTART_SCRUBBED_ENV,
   });
   console.log(`Session "${session.name}" restarted.`);
   doAttach(session.name);
@@ -2999,6 +3000,18 @@ function statefulAgentReason(meta: SessionMetadata): string | null {
   return null;
 }
 
+/** Bus-identity env vars stripped from an operator-initiated restart's re-exec.
+ *  `pty restart` (and the dead-session "Restart? [Y/n]" path) re-run a stored
+ *  command under the RESTARTER's shell env. If that shell belongs to a different
+ *  convoy agent, its ST_AGENT/ST_ROOT leak into the re-exec'd session and it
+ *  comes back under the wrong bus identity — the cos-restart incident, where
+ *  restarting cos from smalltalk's shell brought cos back as smalltalk-claude
+ *  (exit 129). Scrubbing them means a restarted session never inherits the
+ *  restarter's identity; the blessed way to restart an agent with a correct
+ *  identity is its supervisor (convoy). Fresh `pty run` is unaffected — a
+ *  convoy-launched create legitimately inherits its own identity. */
+const RESTART_SCRUBBED_ENV = ["ST_AGENT", "ST_ROOT"];
+
 async function cmdRestart(
   name: string,
   yes = false,
@@ -3062,6 +3075,7 @@ async function cmdRestart(
   await spawnDaemon({
     name, command: meta.command, args: meta.args, displayCommand: meta.displayCommand, cwd: meta.cwd, tags: restartTags,
     ...(meta.displayName ? { displayName: meta.displayName } : {}),
+    scrubEnv: RESTART_SCRUBBED_ENV,
   });
   console.log(`Session "${name}" restarted.`);
 
diff --git a/src/spawn.ts b/src/spawn.ts
index efee970..a9de599 100644
--- a/src/spawn.ts
+++ b/src/spawn.ts
@@ -84,6 +84,15 @@ export interface SpawnDaemonOptions {
    *  handler still surfaces immediate failures within milliseconds, so
    *  this only governs the "alive but slow" case. */
   startTimeoutMs?: number;
+  /** Env var names to DELETE from the daemon's inherited environment before it
+   *  spawns — and therefore before the session child inherits it. Used by the
+   *  operator-initiated restart paths to strip the *restarter's* ambient
+   *  bus-identity vars (ST_AGENT/ST_ROOT), so a session re-exec'd from a
+   *  different shell can't come back under that shell's identity. See the
+   *  cos-restart incident. Applied on the spawnViaNode path; the CLI-fallback
+   *  path can't express it (a bundled consumer that hits the fallback also
+   *  isn't the operator-restart context this guards). */
+  scrubEnv?: string[];
 }
 
 /** Default time we wait for a daemon's Unix socket to appear after
@@ -151,6 +160,11 @@ async function spawnViaNode(options: SpawnDaemonOptions, serverModule: string):
   // when its spawner is gone. Off by default — opt in via
   // `bindToSpawnerLifetime` when the caller owns the daemon's lifetime.
   const env: Record<string, string> = { ...process.env, PTY_SERVER_CONFIG: config };
+  // Strip caller-requested vars (e.g. the restarter's leaked bus identity)
+  // before the daemon — and thus the session child — can inherit them.
+  if (options.scrubEnv) {
+    for (const key of options.scrubEnv) delete env[key];
+  }
   if (options.bindToSpawnerLifetime) env.PTY_SPAWNER_PID = String(process.pid);
   const child = spawn(launcherCmd, [...launcherArgs, serverModule], {
     detached: true,
diff --git a/tests/restart-env-scrub.test.ts b/tests/restart-env-scrub.test.ts
new file mode 100644
index 0000000..4ec0644
--- /dev/null
+++ b/tests/restart-env-scrub.test.ts
@@ -0,0 +1,91 @@
+import { describe, it, expect, afterAll } from "vitest";
+import * as fs from "node:fs";
+import * as os from "node:os";
+import * as path from "node:path";
+import { fileURLToPath } from "node:url";
+import { spawnSync } from "node:child_process";
+
+const __dirname = path.dirname(fileURLToPath(import.meta.url));
+const nodeBin = process.execPath;
+const cliPath = path.join(__dirname, "..", "dist", "cli.js");
+const testRoot = fs.mkdtempSync(path.join(os.tmpdir(), "pty-envscrub-"));
+const bgPids: number[] = [];
+afterAll(() => {
+  for (const pid of bgPids) { try { process.kill(pid, "SIGKILL"); } catch {} }
+  fs.rmSync(testRoot, { recursive: true, force: true, maxRetries: 3, retryDelay: 100 });
+});
+
+interface RunOpts { env?: Record<string, string>; unset?: string[]; timeout?: number }
+
+// PTY_SESSION set => restart takes its "already inside a session, not attaching"
+// branch and returns instead of hanging on a non-TTY attach.
+function runCli(dir: string, args: string[], opts: RunOpts = {}) {
+  const env: Record<string, string> = {
+    ...(process.env as Record<string, string>),
+    PTY_SESSION_DIR: dir, PTY_ROOT_LEGACY_SILENT: "1", PTY_SESSION: "outer",
+    ...(opts.env ?? {}),
+  };
+  for (const k of opts.unset ?? []) delete env[k];
+  return spawnSync(nodeBin, [cliPath, ...args], { env, encoding: "utf8", timeout: opts.timeout ?? 15000 });
+}
+
+/** A command that records the ST_AGENT/ST_ROOT it was actually launched with,
+ *  then stays alive. `-` default => "UNSET" when the var is absent. Re-runs on
+ *  every (re)start, so the file always reflects the current child's env. */
+function recorderCmd(outFile: string): string[] {
+  return ["sh", "-c", `printf '%s|%s' "\${ST_AGENT-UNSET}" "\${ST_ROOT-UNSET}" > "${outFile}"; exec sleep 300`];
+}
+
+function createSession(dir: string, name: string, outFile: string, opts: RunOpts): void {
+  const r = runCli(dir, ["run", "-d", "--id", name, "--", ...recorderCmd(outFile)], opts);
+  expect(r.status).toBe(0);
+  try {
+    bgPids.push(Number(fs.readFileSync(path.join(dir, `${name}.pid`), "utf8").trim()));
+  } catch {}
+}
+
+const sleepSync = (ms: number) => Atomics.wait(new Int32Array(new SharedArrayBuffer(4)), 0, 0, ms);
+
+function waitForContent(p: string, timeoutMs = 4000): string {
+  const start = Date.now();
+  while (Date.now() - start < timeoutMs) {
+    try {
+      const s = fs.readFileSync(p, "utf8");
+      if (s.length > 0) return s;
+    } catch {}
+    sleepSync(50);
+  }
+  try { return fs.readFileSync(p, "utf8"); } catch { return ""; }
+}
+
+describe("restart scrubs the restarter's bus-identity env", () => {
+  it("does NOT leak the restarter's ST_AGENT/ST_ROOT into the re-exec'd session", () => {
+    const dir = fs.mkdtempSync(path.join(testRoot, "d-"));
+    const outFile = path.join(dir, "child.env");
+    // Create with no ambient identity so the first launch records UNSET|UNSET.
+    createSession(dir, "s", outFile, { unset: ["ST_AGENT", "ST_ROOT"] });
+    expect(waitForContent(outFile)).toBe("UNSET|UNSET");
+
+    // Restart from a DIFFERENT agent's shell: its identity must not leak in.
+    fs.rmSync(outFile, { force: true });
+    const r = runCli(dir, ["restart", "-y", "s"], {
+      env: { ST_AGENT: "smalltalk-claude", ST_ROOT: "/leaked/convoy" },
+    });
+    expect(r.status).toBe(0);
+    expect(r.stdout).toContain("restarted");
+
+    const recorded = waitForContent(outFile);
+    expect(recorded).toBe("UNSET|UNSET");
+    expect(recorded).not.toContain("smalltalk-claude");
+    expect(recorded).not.toContain("/leaked/convoy");
+  }, 25000);
+
+  it("still inherits the creator's ST_AGENT on a fresh `pty run` (create path unaffected)", () => {
+    const dir = fs.mkdtempSync(path.join(testRoot, "d-"));
+    const outFile = path.join(dir, "child.env");
+    createSession(dir, "fresh", outFile, {
+      env: { ST_AGENT: "creator-abc", ST_ROOT: "/creator/convoy" },
+    });
+    expect(waitForContent(outFile)).toBe("creator-abc|/creator/convoy");
+  }, 20000);
+});