diff --git a/services/hermes/oauth2-proxy.yaml b/services/hermes/oauth2-proxy.yaml
index 5cd9d01f..c32d0c88 100644
--- a/services/hermes/oauth2-proxy.yaml
+++ b/services/hermes/oauth2-proxy.yaml
@@ -82,7 +82,12 @@ spec:
           args:
             - --provider=oidc
             - --config=/vault/secrets/oidc-config
-            - --redirect-url=https://triage.bstein.dev/oauth2/callback
+            # Host-less on purpose: oauth2-proxy then builds the callback from
+            # the (trusted-proxy) request host, so a login started on the legacy
+            # host completes there. Pinning a host would set the __Host- CSRF
+            # cookie on one origin and land the callback on the other, which
+            # fails as "unable to find a valid CSRF token".
+            - --redirect-url=/oauth2/callback
             - --oidc-issuer-url=https://sso.bstein.dev/realms/atlas
             - --user-id-claim=sub
             - --code-challenge-method=S256
@@ -181,7 +186,8 @@ spec:
           args:
             - --provider=oidc
             - --config=/vault/secrets/oidc-config
-            - --redirect-url=https://chat.bstein.dev/oauth2/callback
+            # Host-less on purpose; see the triage proxy above.
+            - --redirect-url=/oauth2/callback
             - --oidc-issuer-url=https://sso.bstein.dev/realms/atlas
             - --user-id-claim=sub
             - --code-challenge-method=S256
diff --git a/testing/tests/test_hermes_public_host_continuity.py b/testing/tests/test_hermes_public_host_continuity.py
index eb62c696..ff66629f 100644
--- a/testing/tests/test_hermes_public_host_continuity.py
+++ b/testing/tests/test_hermes_public_host_continuity.py
@@ -133,6 +133,29 @@ def test_ensure_script_registers_legacy_and_renamed_origins_together(
         assert f"https://{legacy}" in call.group(0)
 
 
+@pytest.mark.parametrize(
+    "deployment", ["oauth2-proxy-hermes-chat", "oauth2-proxy-hermes-triage"]
+)
+def test_multi_host_proxies_derive_their_callback_from_the_request(deployment: str):
+    """A pinned callback host breaks logins started on the other hostname.
+
+    oauth2-proxy returns ``--redirect-url`` verbatim when it carries a host, so
+    a login started on the legacy host would send the browser to the canonical
+    host's callback. The CSRF cookie uses the ``__Host-`` prefix and cannot
+    cross origins, so the callback fails to find it. Leaving the URL host-less
+    makes oauth2-proxy build it from the request host instead.
+    """
+    spec = _named(HERMES / "oauth2-proxy.yaml", "Deployment", deployment)
+    args = spec["spec"]["template"]["spec"]["containers"][0]["args"]
+    redirect = next(a for a in args if a.startswith("--redirect-url="))
+    value = redirect.split("=", 1)[1]
+    assert value.startswith("/"), f"{deployment} pins a callback host: {value}"
+
+    # Derivation only trusts X-Forwarded-Host behind a declared reverse proxy.
+    assert "--reverse-proxy=true" in args
+    assert any(a.startswith("--trusted-proxy-ip=") for a in args)
+
+
 def test_ensure_job_is_rerun_whenever_the_script_changes():
     """The Job is immutable, so a stale name silently skips the rerun."""
     job = _named(