diff --git a/hypaware-core/smoke/flows/remote_oidc_login.js b/hypaware-core/smoke/flows/remote_oidc_login.js
index d2d4b3c4..78f257a4 100644
--- a/hypaware-core/smoke/flows/remote_oidc_login.js
+++ b/hypaware-core/smoke/flows/remote_oidc_login.js
@@ -165,7 +165,9 @@ function startStubServer() {
     state.validJwt = `jwt-${state.jwtSeq}`
     return state.validJwt
   }
-  const FUTURE = '2999-01-01T00:00:00Z'
+  // The server sends `expires_at` as a Unix epoch-second (the JWT `exp`), so the
+  // stub mirrors that wire form; the client normalizes it to ISO on the way in.
+  const FUTURE = Math.floor(Date.parse('2999-01-01T00:00:00Z') / 1000)
 
   const server = http.createServer((req, res) => {
     const url = new URL(req.url ?? '/', 'http://127.0.0.1')
diff --git a/llp/0059-oidc-login-client.design.md b/llp/0059-oidc-login-client.design.md
index 4c3cd533..a26291b4 100644
--- a/llp/0059-oidc-login-client.design.md
+++ b/llp/0059-oidc-login-client.design.md
@@ -54,8 +54,10 @@ the client sent; `error` is a provider error or a resolution error (`access_deni
 - `{ "grant_type": "refresh_token", "refresh_token": "..." }`
   → `200 { access_jwt, expires_at, org }` or `401 { error: "invalid_grant" }`
 
-Note the response field is `access_jwt` (not `access_token`) and `expires_at` is an ISO
-timestamp. This is hypaware-server's own token, verified by its own resource path; there
+Note the response field is `access_jwt` (not `access_token`) and `expires_at` is a Unix
+epoch-second (the JWT `exp`, matching the server's bootstrap/refresh identity endpoints);
+the client normalizes it to an ISO string on the way in, so the stored session stays
+ISO-based. This is hypaware-server's own token, verified by its own resource path; there
 is no external JWKS on the client.
 
 ## Modules to add (under `src/core/remote/`)
diff --git a/src/core/remote/identity_client.js b/src/core/remote/identity_client.js
index cf79ed14..a3933595 100644
--- a/src/core/remote/identity_client.js
+++ b/src/core/remote/identity_client.js
@@ -9,8 +9,11 @@ import { Attr, getLogger } from '../observability/index.js'
  * speaks the two token grants the server exposes (LLP 0059 §the-server-
  * contract): `authorization_code` (browser login) and `refresh_token` (silent
  * refresh). The response field is `access_jwt` (not `access_token`) and
- * `expires_at` is an ISO timestamp; the JWT is hypaware-server's own
- * credential, so there is no external JWKS trust on the client.
+ * `expires_at` is a Unix epoch-second (the JWT `exp`, matching the server's
+ * bootstrap/refresh identity endpoints); this client normalizes it to an ISO
+ * string internally, so the stored session and its refresh-decision stay
+ * ISO-based. The JWT is hypaware-server's own credential, so there is no
+ * external JWKS trust on the client.
  *
  * @import { OidcSession, RefreshedAccess } from '../../../src/core/remote/types.js'
  */
@@ -88,7 +91,7 @@ export async function exchangeCode({ identityBase, code, codeVerifier, fetchImpl
   return {
     refreshToken: str(json.refresh_token, 'refresh_token'),
     accessJwt: str(json.access_jwt, 'access_jwt'),
-    expiresAt: isoTimestamp(json.expires_at, 'expires_at'),
+    expiresAt: expiryTimestamp(json.expires_at, 'expires_at'),
     org: str(json.org, 'org'),
   }
 }
@@ -105,7 +108,7 @@ export async function refreshSession({ identityBase, refreshToken, fetchImpl })
   const json = await postToken({ identityBase, body, fetchImpl, operation: 'remote.refresh' })
   return {
     accessJwt: str(json.access_jwt, 'access_jwt'),
-    expiresAt: isoTimestamp(json.expires_at, 'expires_at'),
+    expiresAt: expiryTimestamp(json.expires_at, 'expires_at'),
     // The refresh grant only has to re-mint the access JWT; `org` is fixed for
     // the life of the refresh token. Treat it as optional here and let the
     // caller keep the org it already stored, so a server that omits it on
@@ -225,15 +228,24 @@ function str(v, field) {
 }
 
 /**
- * A non-empty string that also parses as a date. The stdio proxy refreshes
- * whenever the stored expiry is unparseable, so accepting a non-date
- * `expires_at` (e.g. epoch-seconds-as-string) would make every forwarded
- * message a fresh refresh that re-stores the same bad value and never
- * self-corrects. Fail the refresh loudly at parse time instead.
+ * Normalize the server's `expires_at` to an ISO string. The wire value is a
+ * Unix epoch-second (the JWT `exp`); we convert it to ISO so the rest of the
+ * client (storage, `isFresh`) keeps its ISO-based, millisecond comparison.
+ * An ISO string is also accepted so older mocks/servers still parse. The stdio
+ * proxy refreshes whenever the stored expiry is unparseable, so a value that is
+ * neither a finite epoch-second nor a parseable date (e.g. epoch-seconds-as-a-
+ * string) would make every forwarded message a fresh refresh that re-stores the
+ * same bad value and never self-corrects. Fail loudly at parse time instead.
  *
  * @param {unknown} v @param {string} field @returns {string}
  */
-function isoTimestamp(v, field) {
+function expiryTimestamp(v, field) {
+  if (typeof v === 'number') {
+    if (!Number.isFinite(v) || v <= 0) {
+      throw new Error(`identity response field '${field}' is not a valid timestamp`)
+    }
+    return new Date(v * 1000).toISOString()
+  }
   const s = str(v, field)
   if (Number.isNaN(Date.parse(s))) {
     throw new Error(`identity response field '${field}' is not a valid timestamp`)
diff --git a/test/core/remote-identity-client.test.js b/test/core/remote-identity-client.test.js
index 3b6d8d83..f0b2e2ac 100644
--- a/test/core/remote-identity-client.test.js
+++ b/test/core/remote-identity-client.test.js
@@ -44,6 +44,29 @@ test('exchangeCode posts the authorization_code grant and maps the response', as
   assert.deepEqual(JSON.parse(calls[0].init.body), { grant_type: 'authorization_code', code: 'auth-code', code_verifier: 'verifier-1' })
 })
 
+test('exchangeCode accepts the server epoch-second expires_at and normalizes it to ISO', async () => {
+  // The server sends `expires_at` as a Unix epoch-second (the JWT `exp`); the
+  // client stores an ISO string, so it must convert rather than reject.
+  const epoch = 1814400000
+  const { fetchImpl } = stubFetch({
+    body: { session_id: 'sess-1', refresh_token: 'rt-1', access_jwt: 'jwt-1', expires_at: epoch, org: 'acme' },
+  })
+  const session = await exchangeCode({
+    identityBase: 'https://hyp.internal/v1/identity',
+    code: 'auth-code',
+    codeVerifier: 'verifier-1',
+    fetchImpl,
+  })
+  assert.equal(session.expiresAt, new Date(epoch * 1000).toISOString())
+})
+
+test('refreshSession accepts the server epoch-second expires_at and normalizes it to ISO', async () => {
+  const epoch = 1814400000
+  const { fetchImpl } = stubFetch({ body: { access_jwt: 'jwt-2', expires_at: epoch, org: 'acme' } })
+  const refreshed = await refreshSession({ identityBase: 'https://hyp.internal/v1/identity', refreshToken: 'rt-1', fetchImpl })
+  assert.equal(refreshed.expiresAt, new Date(epoch * 1000).toISOString())
+})
+
 test('refreshSession posts the refresh_token grant and maps the response', async () => {
   const { fetchImpl, calls } = stubFetch({
     body: { access_jwt: 'jwt-2', expires_at: '2026-06-29T13:00:00Z', org: 'acme' },
@@ -152,7 +175,10 @@ test('a 2xx with an empty body fails as transient, not a misleading missing-fiel
   )
 })
 
-test('a non-date expires_at is rejected at refresh time, not stored to loop forever', async () => {
+test('a non-date expires_at string is rejected at refresh time, not stored to loop forever', async () => {
+  // A numeric epoch as a *string* is still rejected (only a JSON number is the
+  // epoch-second wire form); this prevents storing an unparseable value that
+  // would make every forwarded message refresh forever.
   const { fetchImpl } = stubFetch({ body: { access_jwt: 'jwt', expires_at: '1719600000', org: 'acme' } })
   await assert.rejects(
     () => refreshSession({ identityBase: 'https://hyp.internal/v1/identity', refreshToken: 'rt', fetchImpl }),