diff --git a/src/core/user-identity.ts b/src/core/user-identity.ts
new file mode 100644
index 00000000..f2ef3159
--- /dev/null
+++ b/src/core/user-identity.ts
@@ -0,0 +1,161 @@
+/**
+ * Persistent user-principal identity: a single keypair scoped to this machine account rather than any one (harness, cwd) bridge slot -- the root issuer agent-comms#160 introduces, so a user principal can mint capability tokens to the devices it owns, independent of which bridge process happens to be running. See identity-store.ts for the per-slot device identity every bridge already has; this is deliberately a separate, shared identity, not a variant of it.
+ *
+ * Stored at ~/.agent-comms/user-identity.json (mode 0600), sibling to but distinct from any identity-<harness>--<cwd>.json slot file. No lock file: unlike a bridge identity, this key is never itself a live mesh peer that two concurrent holders would collide over on the wire, so the only race that matters is which process's key wins at first creation -- handled below by an exclusive ("wx") file create rather than identity-store.ts's PID-probed lock, which exists for a concern (a live peer-identity collision) this key never has.
+ */
+
+import * as fs from "node:fs";
+import * as os from "node:os";
+import * as path from "node:path";
+import {
+  generateIdentity,
+  certifyKeyPair,
+  getCertificateFingerprint,
+  deriveDeviceId,
+  CERTIFICATE_VALIDITY_MS,
+} from "./identity.js";
+import type { PeerIdentity } from "./identity.js";
+
+/** Renewal margin is one twelfth of the certificate's total validity period -- matching identity-store.ts's own renewal margin so both identities age out on the same schedule. */
+const RENEWAL_MARGIN_FRACTION = 12;
+const RENEWAL_MARGIN_MS = CERTIFICATE_VALIDITY_MS / RENEWAL_MARGIN_FRACTION;
+
+export interface UserIdentityOptions {
+  /** Directory override for tests -- defaults to ~/.agent-comms, the same base directory identity-store.ts's own per-slot files live in. */
+  dir?: string;
+}
+
+interface StoredUserIdentity {
+  privateKey: string;
+  certificate: string;
+  expiresAt: string;
+}
+
+function isStoredUserIdentity(value: unknown): value is StoredUserIdentity {
+  if (typeof value !== "object" || value === null) return false;
+  if (
+    !("privateKey" in value) ||
+    !("certificate" in value) ||
+    !("expiresAt" in value)
+  )
+    return false;
+  return (
+    typeof value.privateKey === "string" &&
+    typeof value.certificate === "string" &&
+    typeof value.expiresAt === "string"
+  );
+}
+
+/** Narrows a caught value to Node's own errno-carrying Error subtype, so a specific error code (e.g. ENOENT, EEXIST) can be checked without an `as` assertion. */
+function isErrnoException(value: unknown): value is NodeJS.ErrnoException {
+  return value instanceof Error && "code" in value;
+}
+
+function userIdentityFile(options?: Readonly<UserIdentityOptions>): string {
+  const dir = options?.dir ?? path.join(os.homedir(), ".agent-comms");
+  return path.join(dir, "user-identity.json");
+}
+
+function toPeerIdentity(stored: Readonly<StoredUserIdentity>): PeerIdentity {
+  return {
+    privateKey: stored.privateKey,
+    certificate: stored.certificate,
+    fingerprint: getCertificateFingerprint(stored.certificate),
+    deviceId: deriveDeviceId(stored.privateKey),
+  };
+}
+
+function persistedRecord(identity: Readonly<PeerIdentity>): StoredUserIdentity {
+  return {
+    privateKey: identity.privateKey,
+    certificate: identity.certificate,
+    expiresAt: new Date(Date.now() + CERTIFICATE_VALIDITY_MS).toISOString(),
+  };
+}
+
+function serializeRecord(stored: Readonly<StoredUserIdentity>): string {
+  return `${JSON.stringify(stored, null, 2)}\n`;
+}
+
+/** Reads the file's raw contents, or undefined if it does not exist yet. Any other filesystem error (permissions, a directory in its place) is surfaced rather than silently treated as "absent", per this codebase's fail-loudly convention. */
+function readRawFile(file: string): string | undefined {
+  try {
+    return fs.readFileSync(file, "utf-8");
+  } catch (err) {
+    if (isErrnoException(err) && err.code === "ENOENT") return undefined;
+    throw err;
+  }
+}
+
+function parseStoredUserIdentity(raw: string): StoredUserIdentity | undefined {
+  let parsed: unknown;
+  try {
+    parsed = JSON.parse(raw);
+  } catch {
+    return undefined;
+  }
+  return isStoredUserIdentity(parsed) ? parsed : undefined;
+}
+
+/**
+ * Renews a stored identity nearing certificate expiry by re-certifying its existing key pair (preserving device-id) rather than replacing it -- see identity.ts's own certifyKeyPair doc comment for why generating a fresh key pair here would be wrong. Returns the existing identity unchanged when it is not yet near expiry.
+ */
+function renewIfNeeded(
+  file: string,
+  stored: Readonly<StoredUserIdentity>,
+): PeerIdentity {
+  const expiresAt = Date.parse(stored.expiresAt);
+  const needsRenewal =
+    Number.isNaN(expiresAt) || Date.now() > expiresAt - RENEWAL_MARGIN_MS;
+  if (!needsRenewal) return toPeerIdentity(stored);
+
+  const renewed = certifyKeyPair(stored.privateKey);
+  fs.writeFileSync(file, serializeRecord(persistedRecord(renewed)), {
+    encoding: "utf-8",
+    mode: 0o600,
+  });
+  return renewed;
+}
+
+/**
+ * Generates a fresh identity and persists it. When `exclusive` is true, the write uses Node's "wx" flag so a concurrent caller that already created the file loses the race outright (EEXIST) rather than silently clobbering whatever the winner just wrote -- the loser then re-reads the winner's own file instead. `exclusive` is false only when the file is known to hold no genuine key material worth protecting (it was corrupt), so there is nothing to race over and a plain overwrite is correct.
+ */
+function createUserIdentity(file: string, exclusive: boolean): PeerIdentity {
+  const identity = generateIdentity();
+  try {
+    fs.writeFileSync(file, serializeRecord(persistedRecord(identity)), {
+      encoding: "utf-8",
+      mode: 0o600,
+      flag: exclusive ? "wx" : "w",
+    });
+    return identity;
+  } catch (err) {
+    if (exclusive && isErrnoException(err) && err.code === "EEXIST") {
+      const raw = readRawFile(file);
+      const stored =
+        raw === undefined ? undefined : parseStoredUserIdentity(raw);
+      if (stored !== undefined) return renewIfNeeded(file, stored);
+    }
+    throw err;
+  }
+}
+
+/**
+ * Loads the persisted user-principal identity, creating it on first use. A record nearing certificate expiry is renewed in place; a missing, corrupt, or unparseable file is treated as absent and regenerated.
+ *
+ * Creation races against a concurrent caller (another bridge process starting at the same moment) by attempting an exclusive ("wx") file create: whichever process's create wins persists its own freshly generated identity, and the loser detects EEXIST and re-reads the winner's file instead of overwriting it. A corrupt file skips the exclusive path entirely -- there is no genuine key material in it worth protecting from a race, so it is overwritten directly, the same treatment identity-store.ts's own loadStoredIdentity/createIdentity pairing gives a corrupt slot file.
+ */
+export function loadOrCreateUserIdentity(
+  options?: Readonly<UserIdentityOptions>,
+): PeerIdentity {
+  const file = userIdentityFile(options);
+  fs.mkdirSync(path.dirname(file), { recursive: true, mode: 0o700 });
+
+  const raw = readRawFile(file);
+  if (raw === undefined) return createUserIdentity(file, true);
+
+  const stored = parseStoredUserIdentity(raw);
+  if (stored === undefined) return createUserIdentity(file, false);
+
+  return renewIfNeeded(file, stored);
+}
diff --git a/src/test/user-identity.test.ts b/src/test/user-identity.test.ts
new file mode 100644
index 00000000..0b9f8d78
--- /dev/null
+++ b/src/test/user-identity.test.ts
@@ -0,0 +1,127 @@
+/**
+ * Unit tests for the persistent user-principal identity (core/user-identity).
+ */
+
+import * as fs from "node:fs";
+import type * as FsModule from "node:fs";
+import { tmpdir } from "node:os";
+import path from "node:path";
+import { test, expect, vi, afterEach } from "vitest";
+import { loadOrCreateUserIdentity } from "../core/user-identity.js";
+import { CERTIFICATE_VALIDITY_MS } from "../core/identity.js";
+
+// node:fs's writeFileSync is wrapped (not replaced) so every test gets the real filesystem by default; only the one race test below overrides it, via mockImplementationOnce, to simulate a concurrent writer winning the exclusive create -- vi.spyOn cannot target an ESM named export directly ("Module namespace is not configurable"), so the wrap has to happen at vi.mock time instead.
+vi.mock("node:fs", async (importOriginal) => {
+  const actual = await importOriginal<typeof FsModule>();
+  return { ...actual, writeFileSync: vi.fn(actual.writeFileSync) };
+});
+
+function tempDir(): string {
+  return fs.mkdtempSync(path.join(tmpdir(), "agent-comms-user-identity-test-"));
+}
+
+function identityFile(dir: string): string {
+  return path.join(dir, "user-identity.json");
+}
+
+afterEach(() => {
+  vi.restoreAllMocks();
+});
+
+test("loadOrCreateUserIdentity persists and reloads the same key material", () => {
+  const dir = tempDir();
+  const first = loadOrCreateUserIdentity({ dir });
+
+  const reloaded = loadOrCreateUserIdentity({ dir });
+  expect(reloaded.fingerprint).toBe(first.fingerprint);
+  expect(reloaded.privateKey).toBe(first.privateKey);
+  expect(reloaded.certificate).toBe(first.certificate);
+  expect(reloaded.deviceId).toEqual(first.deviceId);
+});
+
+// Mask isolating the permission bits from a stat mode's file-type bits.
+const PERMISSION_BITS_MASK = 0o777;
+// Expected owner-only read/write permission bits for a persisted identity file.
+const OWNER_ONLY_RW_PERMISSIONS = 0o600;
+
+test("the identity file is created with owner-only permissions", () => {
+  const dir = tempDir();
+  loadOrCreateUserIdentity({ dir });
+
+  const mode = fs.statSync(identityFile(dir)).mode & PERMISSION_BITS_MASK;
+  expect(mode).toBe(OWNER_ONLY_RW_PERMISSIONS);
+});
+
+test("two different directories never share an identity", () => {
+  const a = loadOrCreateUserIdentity({ dir: tempDir() });
+  const b = loadOrCreateUserIdentity({ dir: tempDir() });
+
+  expect(a.fingerprint).not.toBe(b.fingerprint);
+  expect(a.deviceId).not.toEqual(b.deviceId);
+});
+
+// Offset (in milliseconds) from now used to write a stored identity's expiresAt just inside the renewal window, without it having already expired outright.
+const NEAR_EXPIRY_OFFSET_MS = 1000;
+
+test("a near-expiry identity is renewed without rotating the device-id", () => {
+  const dir = tempDir();
+  const original = loadOrCreateUserIdentity({ dir });
+
+  const file = identityFile(dir);
+  const stored = JSON.parse(fs.readFileSync(file, "utf-8")) as {
+    expiresAt: string;
+  };
+  stored.expiresAt = new Date(Date.now() + NEAR_EXPIRY_OFFSET_MS).toISOString();
+  fs.writeFileSync(file, JSON.stringify(stored));
+
+  const renewed = loadOrCreateUserIdentity({ dir });
+  expect(renewed.deviceId).toEqual(original.deviceId);
+  expect(renewed.privateKey).toBe(original.privateKey);
+  expect(renewed.fingerprint).not.toBe(original.fingerprint);
+});
+
+test("a corrupt identity file is regenerated", () => {
+  const dir = tempDir();
+  loadOrCreateUserIdentity({ dir });
+  fs.writeFileSync(identityFile(dir), "{not json");
+
+  const regenerated = loadOrCreateUserIdentity({ dir });
+  expect(regenerated.fingerprint).toMatch(/^[0-9A-F]{2}(:[0-9A-F]{2})+$/);
+});
+
+test("losing the creation race re-reads the winner's identity instead of overwriting it", () => {
+  const dir = tempDir();
+  const file = identityFile(dir);
+  const winner = loadOrCreateUserIdentity({ dir: tempDir() });
+  const realWriteFileSync = fs.writeFileSync;
+
+  // Simulate a second process winning the exclusive ("wx") create right before this process's own write lands: when this process attempts its create, first write the "winner's" identity for real (as the concurrent process would have -- the recursive call below falls through to the real writeFileSync once this queued override is consumed), then fail this call with EEXIST, exactly as Node's own "wx" flag would for a file that now exists. expiresAt is set a full validity period out, matching persistedRecord's own real behaviour, so this test exercises only the race-handling branch, not renewal too.
+  const writeSpy = vi.mocked(fs.writeFileSync).mockImplementationOnce(() => {
+    realWriteFileSync(
+      file,
+      `${JSON.stringify(
+        {
+          privateKey: winner.privateKey,
+          certificate: winner.certificate,
+          expiresAt: new Date(
+            Date.now() + CERTIFICATE_VALIDITY_MS,
+          ).toISOString(),
+        },
+        null,
+        2,
+      )}\n`,
+      { encoding: "utf-8", mode: 0o600 },
+    );
+    const err = new Error(
+      "EEXIST: file already exists",
+    ) as NodeJS.ErrnoException;
+    err.code = "EEXIST";
+    throw err;
+  });
+
+  const loser = loadOrCreateUserIdentity({ dir });
+
+  expect(writeSpy).toHaveBeenCalled();
+  expect(loser.fingerprint).toBe(winner.fingerprint);
+  expect(loser.deviceId).toEqual(winner.deviceId);
+});
diff --git a/src/test/user-principal-mints-device-grant.test.ts b/src/test/user-principal-mints-device-grant.test.ts
new file mode 100644
index 00000000..a4177990
--- /dev/null
+++ b/src/test/user-principal-mints-device-grant.test.ts
@@ -0,0 +1,86 @@
+/**
+ * The user principal (agent-comms#160) must be a genuine issuer: it must be able to mint a capability token bearing a device it owns, and that token must verify with the principal's own device-id as rootIssuer, exactly as any other issuer's root grant already does (see create-room-mints-owner-grant.test.ts for the room-owner equivalent). This is the concrete proof the issue asks for -- a persisted identity object alone would not demonstrate it can actually issue.
+ *
+ * The capability/scope minted here (`room:member`, scope kind `group`) is a syntactically valid probe only, borrowed from the one capability string already proven to mint and verify elsewhere in this codebase -- the real device-to-user membership grant shape (what capability and scope a device actually presents to prove "the user principal admitted me") is agent-comms#161's own scope, deliberately not decided here.
+ */
+
+import * as fs from "node:fs";
+import { tmpdir } from "node:os";
+import path from "node:path";
+import { test, expect } from "vitest";
+import { deviceIdToHex } from "wire-mesh-core/domain/device-id";
+import {
+  mintCapabilityToken,
+  verifyCapabilityToken,
+} from "wire-mesh-core/domain/tokens";
+import { createSystemClock } from "wire-mesh-core/adapters/system-clock";
+import { loadOrCreateUserIdentity } from "../core/user-identity.js";
+import { generateIdentity } from "../core/identity.js";
+import { toIdentityPort } from "../core/wire-mesh-identity.js";
+import { randomId } from "../core/random-id.js";
+
+/** The probe grant's expiry window. */
+const TOKEN_TTL_MS = 60_000;
+
+function tempDir(): string {
+  return fs.mkdtempSync(
+    path.join(tmpdir(), "agent-comms-user-principal-mint-test-"),
+  );
+}
+
+test("the user principal mints a capability token bearing a device it owns", async () => {
+  const userIdentity = await toIdentityPort(
+    loadOrCreateUserIdentity({ dir: tempDir() }),
+  );
+  const device = await toIdentityPort(generateIdentity());
+  const clock = createSystemClock();
+
+  const verdict = await mintCapabilityToken({
+    identity: userIdentity,
+    clock,
+    tokenId: randomId(),
+    bearer: device.deviceId,
+    capability: "room:member",
+    scope: { kind: "group", path: deviceIdToHex(userIdentity.deviceId) },
+    expires: clock.now() + TOKEN_TTL_MS,
+    delegationsRemaining: 0,
+  });
+
+  expect(
+    verdict.ok,
+    `expected the user principal's mint to succeed, got ${JSON.stringify(verdict)}`,
+  ).toBe(true);
+  if (!verdict.ok) return;
+
+  // Any IdentityPort supplies verification-only crypto primitives -- verifyCapabilityToken never trusts the caller's own identity, only the token's self-certifying issuer-key, so a throwaway identity works here exactly as well as the principal's real one.
+  const verifierIdentity = await toIdentityPort(generateIdentity());
+  const result = await verifyCapabilityToken(verdict.token, {
+    identity: verifierIdentity,
+    clock,
+    revocation: { entriesFor: async () => [] },
+    expectedBearer: device.deviceId,
+  });
+
+  expect(
+    result.ok,
+    `expected the principal's grant to verify, got ${JSON.stringify(result)}`,
+  ).toBe(true);
+  if (!result.ok) return;
+  expect(result.claims.capability).toBe("room:member");
+  expect(result.claims.scope).toEqual({
+    kind: "group",
+    path: deviceIdToHex(userIdentity.deviceId),
+  });
+  expect(result.rootIssuer).toEqual(userIdentity.deviceId);
+  expect(deviceIdToHex(result.rootIssuer)).toBe(
+    deviceIdToHex(userIdentity.deviceId),
+  );
+});
+
+test("the user principal's own device-id is stable across separate loads of the same directory", async () => {
+  const dir = tempDir();
+  const first = await toIdentityPort(loadOrCreateUserIdentity({ dir }));
+  const second = await toIdentityPort(loadOrCreateUserIdentity({ dir }));
+
+  expect(deviceIdToHex(second.deviceId)).toBe(deviceIdToHex(first.deviceId));
+});