diff --git a/src/down-self-sever.test.ts b/src/down-self-sever.test.ts
new file mode 100644
index 0000000..c81c0a7
--- /dev/null
+++ b/src/down-self-sever.test.ts
@@ -0,0 +1,126 @@
+// SELF-SEVER GUARD (Nathan mandate, convoy incident 2026-07-22) — `convoy down` typed from INSIDE a
+// session it would kill severs the caller mid-command. An agent tearing down the network it lives in is
+// the anti-pattern that can turn a restart into a fleet-wide outage. The guard: refuse unless --force when
+// the caller's identity (ST_AGENT — convoy bakes it into every session's env) names a to-be-killed session.
+//
+// Two layers of proof: pure unit tests for the decision (`selfSeverSession`), and a process-level test that
+// runs the real `bin/convoy down` from within a member's identity and asserts it refuses + leaves the agent
+// alive, that --force overrides, and that a non-member identity is never falsely refused. Scoped to a
+// throwaway XDG_STATE_HOME. Lives in the vitest gate (test.yml), not the hermetic nix flake check.
+
+import { afterEach, describe, expect, it } from "vitest";
+import { spawnSync } from "node:child_process";
+import { mkdirSync, mkdtempSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { dirname, join } from "node:path";
+import { fileURLToPath } from "node:url";
+import { selfSeverSession } from "./up.ts";
+import { PtyHost, processAlive, spawnFromPtyFile, type SupervisedSession } from "./host.ts";
+
+// ── pure ────────────────────────────────────────────────────────────────────────────────────────────
+const sess = (name: string, tags: Record<string, string>): SupervisedSession => ({ name, cwd: null, command: "", args: [], status: "running" as never, pid: null, exitedAt: null, exitCode: null, tags });
+// Test resolver — reads a plain "busId" tag (the real `busIdOf` reads ST_AGENT out of the pty.toml).
+const byTag = (s: SupervisedSession): string | null => s.tags["busId"] ?? null;
+
+describe("selfSeverSession — is `convoy down` about to kill the session it is run from?", () => {
+  const agents = [sess("s.alpha", { busId: "net.alpha" }), sess("s.beta", { busId: "net.beta" })];
+
+  it("ACCEPTANCE: the caller's ST_AGENT matches a to-be-killed session → returns THAT session", () => {
+    expect(selfSeverSession(agents, "net.beta", byTag)?.name).toBe("s.beta");
+  });
+
+  it("a null/empty identity (a plain human terminal) is NEVER a self-sever", () => {
+    expect(selfSeverSession(agents, null, byTag)).toBeNull();
+    expect(selfSeverSession(agents, undefined, byTag)).toBeNull();
+    expect(selfSeverSession(agents, "", byTag)).toBeNull();
+  });
+
+  it("an identity that matches NO killed session (running elsewhere) is not a self-sever", () => {
+    expect(selfSeverSession(agents, "net.gamma", byTag)).toBeNull();
+  });
+
+  it("matches on the EXACT bus id, never a prefix/substring", () => {
+    expect(selfSeverSession(agents, "net.alph", byTag)).toBeNull();
+    expect(selfSeverSession(agents, "net.alpha.ding", byTag)).toBeNull();
+  });
+});
+
+// ── process-level ─────────────────────────────────────────────────────────────────────────────────────
+const repoRoot = join(dirname(fileURLToPath(import.meta.url)), "..");
+const bin = join(repoRoot, "bin", "convoy");
+
+let home = "";
+let net = "";
+const savedPtyRoot = process.env["PTY_ROOT"];
+
+function baseEnv(): NodeJS.ProcessEnv {
+  return { ...process.env, XDG_STATE_HOME: home, ST_ROOT: "", PTY_ROOT: "" };
+}
+
+function freshNet(): void {
+  home = mkdtempSync(join(tmpdir(), "cvy-sever-"));
+  net = join(home, "convoy", "default");
+  mkdirSync(join(net, "catalog"), { recursive: true });
+  mkdirSync(join(net, "smalltalk"), { recursive: true });
+}
+
+/** Stand up one live agent whose bus id (its manifest ST_AGENT) is `id`, and return its pid. */
+async function spawnAgent(id: string): Promise<number> {
+  const workspace = join(net, "agents", id);
+  mkdirSync(join(workspace, ".convoy"), { recursive: true });
+  writeFileSync(
+    join(workspace, ".convoy", "pty.toml"),
+    `prefix = "${id}"\n\n[sessions.claude]\nid = "${id}"\ncommand = "exec sleep 2000000"\n\n[sessions.claude.tags]\nstrategy = "permanent"\nrole = "agent"\n\n[sessions.claude.env]\nST_AGENT = "${id}"\n`,
+  );
+  const { spawned, failed } = await spawnFromPtyFile(workspace, net);
+  if (failed.length > 0 || spawned.length === 0) throw new Error(`spawn ${id} failed: ${JSON.stringify({ spawned, failed })}`);
+  const s = (await new PtyHost(net).sessions()).find((x) => x.name === id);
+  if (!s?.pid) throw new Error(`no pid for ${id}`);
+  return s.pid;
+}
+
+/** Run the real `convoy down` with ST_AGENT set to `asIdentity`. */
+function down(asIdentity: string, ...extra: string[]): { status: number | null; stderr: string } {
+  const r = spawnSync(process.execPath, [bin, "down", net, ...extra], { env: { ...baseEnv(), ST_AGENT: asIdentity }, encoding: "utf8" });
+  return { status: r.status, stderr: r.stderr };
+}
+
+afterEach(() => {
+  try {
+    spawnSync(process.execPath, [bin, "down", net, "--force"], { env: baseEnv() });
+  } catch {
+    /* ignore */
+  }
+  if (home) rmSync(home, { recursive: true, force: true });
+  if (savedPtyRoot === undefined) delete process.env["PTY_ROOT"];
+  else process.env["PTY_ROOT"] = savedPtyRoot;
+});
+
+describe("convoy down — the self-sever guard (real CLI)", () => {
+  it("REFUSES when run from INSIDE a session it would kill, and leaves that agent ALIVE", async () => {
+    freshNet();
+    const pid = await spawnAgent("victim-alpha");
+    const r = down("victim-alpha");
+    expect(r.status, `should refuse (rc=1)\nstderr:\n${r.stderr}`).toBe(1);
+    expect(r.stderr).toMatch(/refusing|INSIDE/);
+    expect(processAlive(pid), "the caller's own agent must NOT be killed by a refused down").toBe(true);
+  }, 45000);
+
+  it("--force overrides the guard — a deliberate 'take me down too' tears it down", async () => {
+    freshNet();
+    const pid = await spawnAgent("victim-beta");
+    const r = down("victim-beta", "--force");
+    expect(r.status, `--force should succeed\nstderr:\n${r.stderr}`).toBe(0);
+    await new Promise((res) => setTimeout(res, 400));
+    expect(processAlive(pid), "--force must actually tear the agent down").toBe(false);
+  }, 45000);
+
+  it("does NOT refuse a caller whose identity is not a member (a plain terminal / another host)", async () => {
+    freshNet();
+    const pid = await spawnAgent("victim-gamma");
+    const r = down("some-outsider-not-in-this-net");
+    expect(r.status, `a non-member caller should proceed (rc=0)\nstderr:\n${r.stderr}`).toBe(0);
+    await new Promise((res) => setTimeout(res, 400));
+    expect(processAlive(pid), "an ordinary down from outside must still tear the agent down").toBe(false);
+  }, 45000);
+});
diff --git a/src/up.ts b/src/up.ts
index f73944c..b026940 100644
--- a/src/up.ts
+++ b/src/up.ts
@@ -667,6 +667,21 @@ export interface DownOptions {
   force?: boolean;
 }
 
+/** Would `convoy down` kill the session the CALLER is running in? A `convoy down` typed at an agent's OWN
+ *  REPL (or a script in its shell) tears down the network it lives in — severing the session mid-command,
+ *  the exact anti-pattern behind the 2026-07-22 outage. The caller's identity is its `ST_AGENT` (convoy
+ *  bakes it into every session's env); a self-sever is when that id matches a to-be-killed session's bus
+ *  id. A null/empty `selfId` (a plain human terminal) is never a self-sever. Returns the matched session
+ *  (so the warning can name it) or null. `resolve` is injectable for tests. Pure. */
+export function selfSeverSession(
+  agents: readonly SupervisedSession[],
+  selfId: string | null | undefined,
+  resolve: (s: SupervisedSession) => string | null = busIdOf,
+): SupervisedSession | null {
+  if (!selfId) return null;
+  return agents.find((s) => resolve(s) === selfId) ?? null;
+}
+
 /** `convoy down [<network>]` — explicit teardown; the ONLY path that kills sessions. Mirror of the
  *  Nomad model: stopping `convoy up` DETACHES (agents keep running), `convoy down` TEARS DOWN. Scope
  *  is convoy's own agents (sessions spawned from a pty.toml — the `ptyfile.session` tag), so it never
@@ -704,6 +719,21 @@ export async function down(opts: DownOptions): Promise<number> {
     return 0;
   }
 
+  // SELF-SEVER GUARD — `convoy down` typed from INSIDE a session it would kill severs the caller
+  // mid-command (an agent tearing down the network it lives in — the shape that turned the 2026-07-22
+  // restart into a fleet-wide outage). The caller's identity is ST_AGENT (convoy bakes it into every
+  // session's env); if it names a session in the kill list, refuse unless --force (a deliberate "take me
+  // down too"). The read-only dry-run above still previews from within — only the real teardown is gated.
+  const self = selfSeverSession(agents, process.env["ST_AGENT"]);
+  if (self !== null && opts.force !== true) {
+    process.stderr.write(
+      `convoy down: refusing — you are running INSIDE ${logicalId(self)} (${self.name}), a session this would kill.\n` +
+        `Tearing down your own network from within severs THIS session mid-command. Run \`convoy down\` from OUTSIDE the\n` +
+        `network (a plain terminal / another host), or \`convoy down --force\` to take yourself down too.\n`,
+    );
+    return 1;
+  }
+
   // Real teardown — a live `convoy up`/app host would RESPAWN gone permanent sessions (reconcile:
   // `if (!gone(s)) continue`), fighting the kill. Refuse unless --force, and point at the owner.
   const owner = lock.liveOwner();