diff --git a/src/index.ts b/src/index.ts
index 4c89a70..8ee966b 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1,4 +1,3 @@
-import path from "node:path";
 import { resolveWriteLease, WriteLeaseNotHeldError } from "./process-write-lease.js";
 import { buildApp } from "./app.js";
 import { loadConfig } from "./config.js";
@@ -9,7 +8,7 @@ import type { CultNetOperationServer } from "cultnet-ts";
 import { createHeimdallRuntimePulse, buildHeimdallHealthDetail, publishHeimdallVerseState } from "./verse-state.js";
 import {
   buildPresenceStatement,
-  nextPublisherSequence,
+  PublisherSequence,
   readRuntimeBundleFacts,
   type RuntimeBundleFacts,
 } from "./runtime-presence.js";
@@ -76,6 +75,7 @@ async function publishVerseState(): Promise<void> {
  * work that can only ever return the same answer or fail.
  */
 let runtimeBundleFacts: RuntimeBundleFacts | undefined;
+const presenceSequence = new PublisherSequence();
 
 async function publishHealthState(): Promise<void> {
   const pulse = createHeimdallRuntimePulse(config);
@@ -111,9 +111,7 @@ async function publishHealthState(): Promise<void> {
     // refuses the pair, and Idunn needs the warming statement to promote it.
     writeLeaseSha256: null,
     observedAtUnixMillis,
-    publisherSequence: await nextPublisherSequence(
-      path.join(config.dataRoot, "runtime-presence-sequence"),
-    ),
+    publisherSequence: presenceSequence.next(observedAtUnixMillis),
     providerHealthIdentityPath: config.providerHealthIdentityPath,
   });
 
diff --git a/src/provider-health-identity.ts b/src/provider-health-identity.ts
index 847d39c..ad4903f 100644
--- a/src/provider-health-identity.ts
+++ b/src/provider-health-identity.ts
@@ -128,6 +128,14 @@ export async function openProviderHealthIdentity(
 ): Promise<PrivateIdentity> {
   const descriptor = passedDescriptor(PRESENCE_IDENTITY_FD_NAME, env);
   if (descriptor === undefined) {
+    if (env.GAMECULT_IDUNN_RUNTIME_BUNDLE) {
+      // Inside an Idunn generation a self-enrolled key is one Idunn does not
+      // trust, and its private store would land in a state root the next
+      // generation must share. The descriptor is the only identity there.
+      throw new Error(
+        `Idunn runtime bundle is set but the ${PRESENCE_IDENTITY_FD_NAME} descriptor was not passed; refusing to self-enroll`
+      );
+    }
     return openOrEnrollProviderHealthIdentity(path);
   }
 
diff --git a/src/runtime-presence.ts b/src/runtime-presence.ts
index 8cf0615..e3ed0b1 100644
--- a/src/runtime-presence.ts
+++ b/src/runtime-presence.ts
@@ -20,7 +20,6 @@
 import path from "node:path";
 import { createHash, createPrivateKey, type KeyObject } from "node:crypto";
 import { readFileSync } from "node:fs";
-import { readFile, writeFile, rename } from "node:fs/promises";
 
 import { decode } from "@msgpack/msgpack";
 import {
@@ -201,35 +200,25 @@ export function openActivationSigner(env: NodeJS.ProcessEnv = process.env): KeyO
 }
 
 /**
- * The publisher sequence must strictly increase for the life of a signer
- * identity, because Idunn treats a repeat or a regression as a replay and
- * refuses the observation.
+ * The publisher sequence a process stamps on its presence statements.
  *
- * An in-process counter is therefore not enough: a restart would reset it and
- * every statement afterwards would be refused until the count climbed past the
- * old high-water mark. It lives beside the target's state instead, and is
- * advanced before use.
+ * Odin compares `publisher_sequence` only against stored presence of the same
+ * activation, and Idunn mints a fresh activation for every start, continuity
+ * restarts included. So the sequence need only strictly increase within this
+ * process; nothing has to survive a restart, and nothing is written to the
+ * state root. Muninn derives its sequence the same way: the observation clock,
+ * never below the previous value plus one.
  */
-export async function nextPublisherSequence(sequencePath: string): Promise<number> {
-  let current = 0;
-  try {
-    const raw = (await readFile(sequencePath, "utf8")).trim();
-    const parsed = Number(raw);
-    if (Number.isSafeInteger(parsed) && parsed > 0) {
-      current = parsed;
-    }
-  } catch (error) {
-    if ((error as NodeJS.ErrnoException).code !== "ENOENT") {
-      throw error;
+export class PublisherSequence {
+  private last = 0;
+
+  next(observedAtUnixMillis: number): number {
+    if (!Number.isSafeInteger(observedAtUnixMillis)) {
+      throw new Error("observedAtUnixMillis is not a safe integer");
     }
+    this.last = Math.max(observedAtUnixMillis, this.last + 1);
+    return this.last;
   }
-  const next = current + 1;
-  // Written through a temporary file: a torn write here would look like a
-  // regression and cost the target every future observation.
-  const temporary = `${sequencePath}.next`;
-  await writeFile(temporary, `${next}\n`, { mode: 0o600 });
-  await rename(temporary, sequencePath);
-  return next;
 }
 
 export interface PresenceStatementInputs {
diff --git a/tests/fixtures/presence-descriptors.ts b/tests/fixtures/presence-descriptors.ts
new file mode 100644
index 0000000..0448330
--- /dev/null
+++ b/tests/fixtures/presence-descriptors.ts
@@ -0,0 +1,72 @@
+/**
+ * What Idunn hands a generation for runtime presence: a parent-only identity
+ * store and an activation seed, passed as descriptors named in LISTEN_FDNAMES.
+ * Every key here is generated for the test or a canary byte pattern.
+ */
+import { closeSync, openSync } from "node:fs";
+import { chmod, mkdir, writeFile } from "node:fs/promises";
+import path from "node:path";
+
+import { encode } from "@msgpack/msgpack";
+import { SingleFileMessagePackBackingStore } from "@gamecult/cultcache-ts";
+import { IDUNN_RUNTIME_ACTIVATION_CREDENTIAL_NAME } from "cultnet-ts";
+
+import { openOrEnrollProviderHealthIdentity } from "../../src/provider-health-identity.js";
+import type { RuntimeBundleFacts } from "../../src/runtime-presence.js";
+
+export const bundleFacts: RuntimeBundleFacts = {
+  target: "heimdall",
+  planId: "plan-1",
+  incarnationId: "incarnation-1",
+  sealedReleaseId: "release-1",
+  runtimeId: "runtime-1",
+  runtimeInstanceId: "instance-1",
+  expectedProjectionSha256: "sha256-projection",
+  activationWitnessSha256: "sha256-witness",
+  activationSignerIdentityId: "activation-signer",
+  stateSchemaGeneration: null,
+  stateContractSha256: null,
+  capabilities: [],
+  healthContract: "health-contract",
+};
+
+/** Write both stores into `dir`, readable by any uid (the children run as others). */
+export async function writePresenceFixtures(dir: string): Promise<void> {
+  await mkdir(dir, { recursive: true });
+  await chmod(dir, 0o755);
+  const enrolled = path.join(dir, "enrolled.cc");
+  await openOrEnrollProviderHealthIdentity(enrolled);
+  const [record] = await new SingleFileMessagePackBackingStore(enrolled).pullAll();
+  // Re-wrapped the way cultcache-rs writes a store, which is how Idunn writes it.
+  await writeFile(
+    path.join(dir, "runtime-presence-identity.cc"),
+    encode([
+      [
+        "gamecult-provider-health-identity",
+        "gamecult.provider_health_identity.private.v1",
+        record!.payload,
+        "2026-09-06T00:00:00.000Z",
+        "gamecult.provider_health_identity.private.v1",
+      ],
+    ]),
+    { mode: 0o644 }
+  );
+  await writeFile(path.join(dir, "activation-seed"), Buffer.alloc(32, 7), { mode: 0o644 });
+}
+
+/** Open both descriptors and name them the way systemd's LISTEN_FDNAMES does. */
+export function openPresenceDescriptors(dir: string): { env: NodeJS.ProcessEnv; close: () => void } {
+  const fds = [
+    [openSync(path.join(dir, "runtime-presence-identity.cc"), "r"), "gamecult-runtime-presence-identity"],
+    [openSync(path.join(dir, "activation-seed"), "r"), IDUNN_RUNTIME_ACTIVATION_CREDENTIAL_NAME],
+  ] as const;
+  const names: string[] = [];
+  for (const [fd, name] of fds) {
+    while (names.length < fd - 3) names.push(`pad-${names.length}`);
+    names[fd - 3] = name;
+  }
+  return {
+    env: { LISTEN_FDNAMES: names.join(":") },
+    close: () => fds.forEach(([fd]) => closeSync(fd)),
+  };
+}
diff --git a/tests/fixtures/state-root-generation-child.ts b/tests/fixtures/state-root-generation-child.ts
new file mode 100644
index 0000000..0e53ade
--- /dev/null
+++ b/tests/fixtures/state-root-generation-child.ts
@@ -0,0 +1,58 @@
+/**
+ * One Idunn generation, as a process: it runs as the uid and gid its parent
+ * gave it, under UMask=0007, and does what Heimdall does to a state root on a
+ * pulse. The parent test spawns two of these under different uids that share
+ * a group.
+ *
+ * argv: <role: first|next> <stateRoot> <fixtureDir>
+ */
+import { readFile, writeFile } from "node:fs/promises";
+import path from "node:path";
+
+import { loadConfig } from "../../src/config.js";
+import { PublisherSequence, buildPresenceStatement } from "../../src/runtime-presence.js";
+import { bundleFacts, openPresenceDescriptors } from "./presence-descriptors.js";
+import { createHeimdallRuntimePulse, publishHeimdallVerseState } from "../../src/verse-state.js";
+
+const [role, stateRoot, fixtureDir] = process.argv.slice(2) as [string, string, string];
+process.umask(0o007);
+
+const config = loadConfig({
+  GC_ACCESS_BASE_URL: "https://heimdall.example.test",
+  GC_ACCESS_DATA_ROOT: stateRoot,
+});
+
+async function publishPresence(): Promise<void> {
+  const descriptors = openPresenceDescriptors(fixtureDir);
+  try {
+    const observedAtUnixMillis = Date.now();
+    await buildPresenceStatement({
+      bundle: bundleFacts,
+      state: "warming",
+      detail: "generation probe",
+      boundEndpoint: null,
+      writeLeaseSha256: null,
+      observedAtUnixMillis,
+      publisherSequence: new PublisherSequence().next(observedAtUnixMillis),
+      providerHealthIdentityPath: path.join(stateRoot, "provider-health-identity.cc"),
+      env: descriptors.env,
+    });
+  } finally {
+    descriptors.close();
+  }
+}
+
+if (role === "next") {
+  // Generation N+1 reads what N left before it replaces it.
+  const previous = await readFile(config.cultCachePath);
+  if (previous.length === 0) throw new Error("previous generation's snapshot is empty");
+}
+
+await publishHeimdallVerseState(config, createHeimdallRuntimePulse(config));
+await publishPresence();
+
+if (role === "first") {
+  // The 0600 counter the 2026-09-07 generation left behind. Nothing reads it
+  // now; it stays as the stale fixture the next generation must live beside.
+  await writeFile(path.join(stateRoot, "runtime-presence-sequence"), "41\n", { mode: 0o600 });
+}
diff --git a/tests/presence-identity-descriptor.test.ts b/tests/presence-identity-descriptor.test.ts
index 97c5233..731d16e 100644
--- a/tests/presence-identity-descriptor.test.ts
+++ b/tests/presence-identity-descriptor.test.ts
@@ -1,4 +1,4 @@
-import { mkdtemp, readFile, writeFile } from "node:fs/promises";
+import { access, mkdtemp, readFile, writeFile } from "node:fs/promises";
 import { openSync, closeSync } from "node:fs";
 import os from "node:os";
 import path from "node:path";
@@ -112,3 +112,29 @@ describe("presence identity descriptor selection", () => {
     });
   });
 });
+
+describe("an Idunn generation without the presence identity descriptor", () => {
+  it("refuses, naming the descriptor, and enrolls nothing", async () => {
+    const dir = await mkdtemp(path.join(os.tmpdir(), "heimdall-nodescriptor-"));
+    const identityPath = path.join(dir, "provider-health-identity.cc");
+
+    await expect(
+      openProviderHealthIdentity(identityPath, {
+        GAMECULT_IDUNN_RUNTIME_BUNDLE: "/run/idunn/bundle",
+        LISTEN_FDNAMES: "some-other-descriptor",
+      })
+    ).rejects.toThrow("gamecult-runtime-presence-identity");
+
+    await expect(access(identityPath)).rejects.toMatchObject({ code: "ENOENT" });
+  });
+
+  it("still self-enrolls outside Idunn", async () => {
+    if (!machineIdAvailable) return;
+    const dir = await mkdtemp(path.join(os.tmpdir(), "heimdall-outside-idunn-"));
+    const identityPath = path.join(dir, "provider-health-identity.cc");
+
+    await openProviderHealthIdentity(identityPath, {});
+
+    await expect(access(identityPath)).resolves.toBeUndefined();
+  });
+});
diff --git a/tests/publisher-sequence.test.ts b/tests/publisher-sequence.test.ts
new file mode 100644
index 0000000..9a4f147
--- /dev/null
+++ b/tests/publisher-sequence.test.ts
@@ -0,0 +1,87 @@
+import { mkdtemp, readdir, readFile, writeFile } from "node:fs/promises";
+import os from "node:os";
+import path from "node:path";
+
+import { describe, expect, it } from "vitest";
+
+import { loadConfig } from "../src/config.js";
+import { PublisherSequence, buildPresenceStatement } from "../src/runtime-presence.js";
+import { bundleFacts, openPresenceDescriptors, writePresenceFixtures } from "./fixtures/presence-descriptors.js";
+
+const machineIdAvailable = await readFile("/etc/machine-id", "utf8").then(
+  () => true,
+  () => false
+);
+
+describe("PublisherSequence", () => {
+  it("starts at the observed millis", () => {
+    expect(new PublisherSequence().next(1_700_000_000_000)).toBe(1_700_000_000_000);
+  });
+
+  it("strictly increases for equal, earlier and later observations", () => {
+    const sequence = new PublisherSequence();
+    const first = sequence.next(1_700_000_000_000);
+    const equal = sequence.next(1_700_000_000_000);
+    const earlier = sequence.next(1_699_999_990_000);
+    const later = sequence.next(1_700_000_050_000);
+    expect([first, equal, earlier, later]).toEqual([
+      1_700_000_000_000,
+      1_700_000_000_001,
+      1_700_000_000_002,
+      1_700_000_050_000,
+    ]);
+  });
+
+  it("never falls below the observed millis once the clock moves ahead", () => {
+    const sequence = new PublisherSequence();
+    sequence.next(1_700_000_000_000);
+    sequence.next(1_700_000_000_000);
+    sequence.next(1_700_000_000_000);
+    expect(sequence.next(1_700_000_000_002)).toBe(1_700_000_000_003);
+    expect(sequence.next(1_700_000_000_010)).toBe(1_700_000_000_010);
+  });
+
+  it("refuses a value that is not a safe integer and keeps its place", () => {
+    const sequence = new PublisherSequence();
+    expect(sequence.next(10)).toBe(10);
+    for (const bad of [Number.NaN, Number.POSITIVE_INFINITY, 1.5, 2 ** 60]) {
+      expect(() => sequence.next(bad)).toThrow("observedAtUnixMillis");
+    }
+    expect(sequence.next(5)).toBe(11);
+  });
+});
+
+describe.skipIf(!machineIdAvailable)("presence publication and the state root", () => {
+  it("touches no path under dataRoot", async () => {
+    const dir = await mkdtemp(path.join(os.tmpdir(), "heimdall-nodataroot-"));
+    await writePresenceFixtures(path.join(dir, "fixtures"));
+    // A regular file as dataRoot: opening anything beneath it fails with
+    // ENOTDIR for every uid, root included.
+    const dataRoot = path.join(dir, "not-a-directory");
+    await writeFile(dataRoot, "");
+    const config = loadConfig({
+      GC_ACCESS_BASE_URL: "https://heimdall.example.test",
+      GC_ACCESS_DATA_ROOT: dataRoot,
+    });
+
+    const descriptors = openPresenceDescriptors(path.join(dir, "fixtures"));
+    try {
+      const observedAtUnixMillis = Date.parse("2026-10-23T12:00:00.000Z");
+      const statement = await buildPresenceStatement({
+        bundle: bundleFacts,
+        state: "warming",
+        detail: "probe",
+        boundEndpoint: null,
+        writeLeaseSha256: null,
+        observedAtUnixMillis,
+        publisherSequence: new PublisherSequence().next(observedAtUnixMillis),
+        providerHealthIdentityPath: config.providerHealthIdentityPath,
+        env: descriptors.env,
+      });
+      expect(statement.target).toBe("heimdall");
+    } finally {
+      descriptors.close();
+    }
+    expect((await readdir(dir)).sort()).toEqual(["fixtures", "not-a-directory"]);
+  });
+});
diff --git a/tests/state-root-generations.test.ts b/tests/state-root-generations.test.ts
new file mode 100644
index 0000000..7632f14
--- /dev/null
+++ b/tests/state-root-generations.test.ts
@@ -0,0 +1,81 @@
+import { execFile } from "node:child_process";
+import { chmod, chown, mkdir, mkdtemp, readdir, stat } from "node:fs/promises";
+import os from "node:os";
+import path from "node:path";
+import { promisify } from "node:util";
+
+import { describe, expect, it } from "vitest";
+
+import { writePresenceFixtures } from "./fixtures/presence-descriptors.js";
+
+const run = promisify(execFile);
+
+const STATE_GID = 4242;
+const GENERATION_N = 61184;
+const GENERATION_N_PLUS_1 = 61185;
+
+// Creating uids and groups needs root; the stopgap's node container runs as root.
+const isRoot = process.getuid?.() === 0;
+
+async function walk(dir: string): Promise<string[]> {
+  const found: string[] = [];
+  for (const entry of await readdir(dir, { withFileTypes: true })) {
+    const full = path.join(dir, entry.name);
+    found.push(full);
+    if (entry.isDirectory()) found.push(...(await walk(full)));
+  }
+  return found;
+}
+
+/**
+ * Idunn gives every generation a fresh DynamicUser uid and one shared group,
+ * UMask=0007, and a 2770 setgid state root. Generation N+1 runs beside or after
+ * N and must be able to read and replace everything N left there.
+ */
+describe.skipIf(!isRoot)("Idunn state root across generations", () => {
+  it("generation N+1, a different uid in the same group, reads and replaces generation N's state", async () => {
+    const base = await mkdtemp(path.join(os.tmpdir(), "heimdall-generations-"));
+    await chmod(base, 0o755);
+
+    // The fixtures stand in for what Idunn passes as descriptors.
+    const fixtures = path.join(base, "fixtures");
+    await writePresenceFixtures(fixtures);
+
+    const stateRoot = path.join(base, "state");
+    await mkdir(stateRoot);
+    await chown(stateRoot, 0, STATE_GID);
+    await chmod(stateRoot, 0o2770);
+
+    const child = path.join(import.meta.dirname, "fixtures", "state-root-generation-child.ts");
+    const generation = (role: "first" | "next", uid: number) =>
+      run(process.execPath, ["--import", "tsx", child, role, stateRoot, fixtures], {
+        uid,
+        gid: STATE_GID,
+        cwd: path.resolve(import.meta.dirname, ".."),
+        env: { PATH: process.env.PATH ?? "", HOME: fixtures },
+      });
+
+    await generation("first", GENERATION_N);
+
+    const left = await walk(stateRoot);
+    expect(left.map((entry) => path.relative(stateRoot, entry)).sort()).toEqual([
+      "cultcache",
+      path.join("cultcache", "heimdall.service.cc"),
+      "runtime-presence-sequence",
+    ]);
+
+    await generation("next", GENERATION_N_PLUS_1);
+
+    // Generation N+1 replaced the snapshot, and no owner-only file is left
+    // behind for generation N+2: everything but the stale 0600 fixture is
+    // readable and writable by the state group.
+    const snapshot = await stat(path.join(stateRoot, "cultcache", "heimdall.service.cc"));
+    expect(snapshot.uid).toBe(GENERATION_N_PLUS_1);
+    expect(snapshot.gid).toBe(STATE_GID);
+    for (const entry of await walk(stateRoot)) {
+      if (path.basename(entry) === "runtime-presence-sequence") continue;
+      expect((await stat(entry)).mode & 0o060, path.relative(stateRoot, entry)).toBe(0o060);
+    }
+    expect((await readdir(stateRoot)).sort()).toEqual(["cultcache", "runtime-presence-sequence"]);
+  }, 60_000);
+});