diff --git a/hermes_cli/dashboard_auth/cookies.py b/hermes_cli/dashboard_auth/cookies.py
new file mode 100644
index 00000000000..29e6cf799b7
--- /dev/null
+++ b/hermes_cli/dashboard_auth/cookies.py
@@ -0,0 +1,120 @@
+"""Cookie helpers for dashboard auth.
+
+Three cookies in play:
+  - hermes_session_at: the OAuth access token
+                       (HttpOnly, lifetime = token TTL)
+  - hermes_session_rt: the OAuth refresh token
+                       (HttpOnly, lifetime = 30 days)
+  - hermes_session_pkce: short-lived PKCE state + CSRF nonce + provider
+                         hint (HttpOnly, lifetime = 10 minutes)
+
+All three are ``SameSite=Lax`` (browser will send on cross-site GET
+top-level navigation, which we need for the IDP redirect back to
+``/auth/callback``) and ``Path=/``. ``Secure`` is set ONLY when the
+dashboard was reached over HTTPS — detected via the request URL scheme,
+which honours ``X-Forwarded-Proto`` upstream of Fly's TLS terminator
+when uvicorn is configured with ``proxy_headers=True``. Loopback dev
+traffic is always HTTP so ``Secure`` would lock the cookies out of
+the browser.
+"""
+from __future__ import annotations
+
+from typing import Optional, Tuple
+
+from fastapi import Request
+from fastapi.responses import Response
+
+SESSION_AT_COOKIE = "hermes_session_at"
+SESSION_RT_COOKIE = "hermes_session_rt"
+PKCE_COOKIE = "hermes_session_pkce"
+
+# 30 days — matches Portal's REFRESH_TOKEN_TTL_SECONDS
+_RT_MAX_AGE = 30 * 24 * 60 * 60
+_PKCE_MAX_AGE = 10 * 60
+
+
+def _common_attrs(use_https: bool) -> dict:
+    attrs: dict = {
+        "httponly": True,
+        "samesite": "lax",
+        "path": "/",
+    }
+    if use_https:
+        attrs["secure"] = True
+    return attrs
+
+
+def set_session_cookies(
+    response: Response,
+    *,
+    access_token: str,
+    refresh_token: str,
+    access_token_expires_in: int,
+    use_https: bool,
+) -> None:
+    """Set both session cookies on the response.
+
+    ``access_token_expires_in`` is in seconds. Use the provider's reported
+    TTL for the access token. The refresh token cookie always lives 30
+    days regardless of the underlying provider's refresh TTL.
+    """
+    response.set_cookie(
+        SESSION_AT_COOKIE, access_token,
+        max_age=access_token_expires_in,
+        **_common_attrs(use_https),
+    )
+    response.set_cookie(
+        SESSION_RT_COOKIE, refresh_token,
+        max_age=_RT_MAX_AGE,
+        **_common_attrs(use_https),
+    )
+
+
+def clear_session_cookies(response: Response) -> None:
+    """Emit Max-Age=0 deletions for both session cookies."""
+    # Path must match the set-path for the delete to apply.
+    response.set_cookie(
+        SESSION_AT_COOKIE, "", max_age=0,
+        path="/", httponly=True, samesite="lax",
+    )
+    response.set_cookie(
+        SESSION_RT_COOKIE, "", max_age=0,
+        path="/", httponly=True, samesite="lax",
+    )
+
+
+def set_pkce_cookie(response: Response, *, payload: str, use_https: bool) -> None:
+    response.set_cookie(
+        PKCE_COOKIE, payload,
+        max_age=_PKCE_MAX_AGE,
+        **_common_attrs(use_https),
+    )
+
+
+def clear_pkce_cookie(response: Response) -> None:
+    response.set_cookie(
+        PKCE_COOKIE, "", max_age=0,
+        path="/", httponly=True, samesite="lax",
+    )
+
+
+def read_session_cookies(request: Request) -> Tuple[Optional[str], Optional[str]]:
+    """Returns (access_token, refresh_token), either may be None."""
+    at = request.cookies.get(SESSION_AT_COOKIE)
+    rt = request.cookies.get(SESSION_RT_COOKIE)
+    return at, rt
+
+
+def read_pkce_cookie(request: Request) -> Optional[str]:
+    return request.cookies.get(PKCE_COOKIE)
+
+
+def detect_https(request: Request) -> bool:
+    """Decide whether to set the ``Secure`` cookie flag.
+
+    Reads ``request.url.scheme`` — under uvicorn's ``proxy_headers=True``
+    (which start_server enables when the gate is active), this honours
+    ``X-Forwarded-Proto`` from Fly's TLS terminator. Loopback traffic is
+    always HTTP so this returns False there.
+    """
+    return request.url.scheme == "https"
diff --git a/tests/hermes_cli/test_dashboard_auth_cookies.py b/tests/hermes_cli/test_dashboard_auth_cookies.py
new file mode 100644
index 00000000000..c13662d156d
--- /dev/null
+++ b/tests/hermes_cli/test_dashboard_auth_cookies.py
@@ -0,0 +1,158 @@
+"""Tests for the dashboard-auth cookie helpers."""
+from __future__ import annotations
+
+import pytest
+from fastapi import FastAPI
+from fastapi.responses import Response
+from fastapi.testclient import TestClient
+from starlette.requests import Request
+
+from hermes_cli.dashboard_auth.cookies import (
+    PKCE_COOKIE,
+    SESSION_AT_COOKIE,
+    SESSION_RT_COOKIE,
+    clear_pkce_cookie,
+    clear_session_cookies,
+    read_pkce_cookie,
+    read_session_cookies,
+    set_pkce_cookie,
+    set_session_cookies,
+)
+
+
+def _build_app(use_https: bool = True):
+    app = FastAPI()
+
+    @app.get("/set")
+    def set_endpoint():
+        r = Response("ok")
+        set_session_cookies(
+            r, access_token="AT", refresh_token="RT",
+            access_token_expires_in=3600, use_https=use_https,
+        )
+        return r
+
+    @app.get("/set-pkce")
+    def set_pkce():
+        r = Response("ok")
+        set_pkce_cookie(r, payload="provider=stub;state=s;verifier=v",
+                        use_https=use_https)
+        return r
+
+    @app.get("/clear")
+    def clear():
+        r = Response("ok")
+        clear_session_cookies(r)
+        clear_pkce_cookie(r)
+        return r
+
+    return app
+
+
+def test_session_cookies_are_httponly_samesite_lax_secure_in_https():
+    client = TestClient(_build_app(use_https=True))
+    r = client.get("/set")
+    cookies = r.headers.get_list("set-cookie")
+    at = next(c for c in cookies if c.startswith(f"{SESSION_AT_COOKIE}="))
+    rt = next(c for c in cookies if c.startswith(f"{SESSION_RT_COOKIE}="))
+    for c in (at, rt):
+        assert "HttpOnly" in c
+        assert "samesite=lax" in c.lower()
+        assert "Secure" in c
+        assert "Path=/" in c
+
+
+def test_session_cookies_omit_secure_when_http():
+    client = TestClient(_build_app(use_https=False))
+    r = client.get("/set")
+    for c in r.headers.get_list("set-cookie"):
+        if c.startswith(f"{SESSION_AT_COOKIE}=") or c.startswith(f"{SESSION_RT_COOKIE}="):
+            assert "Secure" not in c, f"Cookie unexpectedly Secure: {c}"
+
+
+def test_session_cookies_have_30day_rt_and_token_ttl_at():
+    client = TestClient(_build_app(use_https=True))
+    r = client.get("/set")
+    cookies = r.headers.get_list("set-cookie")
+    at = next(c for c in cookies if c.startswith(f"{SESSION_AT_COOKIE}="))
+    rt = next(c for c in cookies if c.startswith(f"{SESSION_RT_COOKIE}="))
+    assert "Max-Age=3600" in at
+    assert "Max-Age=2592000" in rt  # 30 days = 30 * 86400
+
+
+def test_clear_session_cookies_emits_expired_at_and_rt():
+    client = TestClient(_build_app())
+    r = client.get("/clear")
+    cookies = r.headers.get_list("set-cookie")
+    assert any(
+        c.startswith(f"{SESSION_AT_COOKIE}=") and "Max-Age=0" in c
+        for c in cookies
+    )
+    assert any(
+        c.startswith(f"{SESSION_RT_COOKIE}=") and "Max-Age=0" in c
+        for c in cookies
+    )
+
+
+def test_pkce_cookie_short_ttl_and_path_root():
+    client = TestClient(_build_app(use_https=True))
+    r = client.get("/set-pkce")
+    pkce = next(
+        c for c in r.headers.get_list("set-cookie")
+        if c.startswith(f"{PKCE_COOKIE}=")
+    )
+    assert "HttpOnly" in pkce
+    assert "Max-Age=600" in pkce  # 10 minutes
+    assert "Path=/" in pkce
+    assert "Secure" in pkce
+
+
+def test_read_session_cookies_from_request():
+    scope = {
+        "type": "http",
+        "method": "GET",
+        "path": "/",
+        "headers": [(
+            b"cookie",
+            f"{SESSION_AT_COOKIE}=at_value; {SESSION_RT_COOKIE}=rt_value".encode(),
+        )],
+    }
+    req = Request(scope)
+    at, rt = read_session_cookies(req)
+    assert at == "at_value"
+    assert rt == "rt_value"
+
+
+def test_read_session_cookies_missing_returns_none():
+    req = Request({"type": "http", "method": "GET", "path": "/", "headers": []})
+    assert read_session_cookies(req) == (None, None)
+
+
+def test_read_pkce_cookie_round_trip():
+    scope = {
+        "type": "http",
+        "method": "GET",
+        "path": "/",
+        "headers": [(b"cookie", f"{PKCE_COOKIE}=state=s;verifier=v".encode())],
+    }
+    req = Request(scope)
+    assert read_pkce_cookie(req) == "state=s"  # NB: cookie value stops at ';'
+
+
+def test_detect_https_via_scheme():
+    """``detect_https`` reads from request.url.scheme.
+
+    Under uvicorn proxy_headers=True the scheme is rewritten from
+    ``X-Forwarded-Proto``; that's an integration concern, not unit.
+    """
+    from hermes_cli.dashboard_auth.cookies import detect_https
+    http_req = Request({
+        "type": "http", "method": "GET", "path": "/", "scheme": "http",
+        "headers": [], "server": ("x", 80),
+    })
+    https_req = Request({
+        "type": "http", "method": "GET", "path": "/", "scheme": "https",
+        "headers": [], "server": ("x", 443),
+    })
+    assert detect_https(http_req) is False
+    assert detect_https(https_req) is True