diff --git a/.github/workflows/validate-scaffold.yaml b/.github/workflows/validate-scaffold.yaml
index a3350d3..fc85afa 100644
--- a/.github/workflows/validate-scaffold.yaml
+++ b/.github/workflows/validate-scaffold.yaml
@@ -59,6 +59,12 @@ jobs:
         # renders the explicit local/prod profiles selected through KSail.
         run: ./scripts/validate-observability-option.sh
 
+      - name: 🔐 Validate SSO deep-link redirects
+        # Every host behind oauth2-proxy finishes its login on its own host and
+        # returns the user to the page they asked for: Dex must accept exactly
+        # those callbacks, with no fixed redirect_url or per-route redirect header.
+        run: ./scripts/tests/test-oauth2-proxy-sso-redirects.sh
+
       - name: 🏷️ Validate default-off recommended workload labels
         # Existing instances keep their default paths. Exercise the explicit
         # profiles and the exact immutable shared-policy contract.
diff --git a/k8s/bases/apps/homepage/http-route.yaml b/k8s/bases/apps/homepage/http-route.yaml
index e66189a..0a1cbe2 100644
--- a/k8s/bases/apps/homepage/http-route.yaml
+++ b/k8s/bases/apps/homepage/http-route.yaml
@@ -12,11 +12,6 @@ spec:
     - ${domain}
   rules:
     - filters:
-        - type: RequestHeaderModifier
-          requestHeaderModifier:
-            set:
-              - name: X-Auth-Request-Redirect
-                value: https://${domain}/
         - type: ResponseHeaderModifier
           responseHeaderModifier:
             set:
diff --git a/k8s/bases/infrastructure/controllers/cilium/http-route.yaml b/k8s/bases/infrastructure/controllers/cilium/http-route.yaml
index bb03afc..765d450 100644
--- a/k8s/bases/infrastructure/controllers/cilium/http-route.yaml
+++ b/k8s/bases/infrastructure/controllers/cilium/http-route.yaml
@@ -20,11 +20,6 @@ spec:
     - hubble.${domain}
   rules:
     - filters:
-        - type: RequestHeaderModifier
-          requestHeaderModifier:
-            set:
-              - name: X-Auth-Request-Redirect
-                value: https://hubble.${domain}/
         - type: ResponseHeaderModifier
           responseHeaderModifier:
             set:
diff --git a/k8s/bases/infrastructure/controllers/coroot/http-route.yaml b/k8s/bases/infrastructure/controllers/coroot/http-route.yaml
index 1f6d638..b9be755 100644
--- a/k8s/bases/infrastructure/controllers/coroot/http-route.yaml
+++ b/k8s/bases/infrastructure/controllers/coroot/http-route.yaml
@@ -24,11 +24,6 @@ spec:
     - observability.${domain}
   rules:
     - filters:
-        - type: RequestHeaderModifier
-          requestHeaderModifier:
-            set:
-              - name: X-Auth-Request-Redirect
-                value: https://observability.${domain}/
         - type: ResponseHeaderModifier
           responseHeaderModifier:
             set:
diff --git a/k8s/bases/infrastructure/controllers/dex/helm-release.yaml b/k8s/bases/infrastructure/controllers/dex/helm-release.yaml
index ccd0f28..c6c10d2 100644
--- a/k8s/bases/infrastructure/controllers/dex/helm-release.yaml
+++ b/k8s/bases/infrastructure/controllers/dex/helm-release.yaml
@@ -70,6 +70,20 @@ spec:
             - "https://budget.${domain}/oidc/callback"
             - "https://vault.${domain}/ui/vault/auth/oidc/oidc/callback"
             - "https://grafana.${domain}/login/generic_oauth"
+            # oauth2-proxy's callback on every host it fronts, so it can finish a
+            # login on the host the user started from and return them to the page
+            # they asked for. Dex matches callbacks exactly, so each host needs its
+            # own entry: scripts/tests/test-oauth2-proxy-sso-redirects.sh fails
+            # when an HTTPRoute puts a host behind oauth2-proxy without one, or
+            # when an entry outlives its route in every profile and provider. These
+            # stay after the entries above: the observability-coroot components
+            # patch this list by index.
+            - "https://${domain}/oauth2/callback"
+            - "https://alertmanager.${domain}/oauth2/callback"
+            - "https://hubble.${domain}/oauth2/callback"
+            - "https://observability.${domain}/oauth2/callback"
+            - "https://opencost.${domain}/oauth2/callback"
+            - "https://prometheus.${domain}/oauth2/callback"
           trustedPeers:
             - kubectl
         - name: kubectl
diff --git a/k8s/bases/infrastructure/controllers/kube-prometheus-stack/http-route-alertmanager.yaml b/k8s/bases/infrastructure/controllers/kube-prometheus-stack/http-route-alertmanager.yaml
index 91609e4..7e41b51 100644
--- a/k8s/bases/infrastructure/controllers/kube-prometheus-stack/http-route-alertmanager.yaml
+++ b/k8s/bases/infrastructure/controllers/kube-prometheus-stack/http-route-alertmanager.yaml
@@ -19,11 +19,6 @@ spec:
     - alertmanager.${domain}
   rules:
     - filters:
-        - type: RequestHeaderModifier
-          requestHeaderModifier:
-            set:
-              - name: X-Auth-Request-Redirect
-                value: https://alertmanager.${domain}/
         - type: ResponseHeaderModifier
           responseHeaderModifier:
             set:
diff --git a/k8s/bases/infrastructure/controllers/kube-prometheus-stack/http-route-prometheus.yaml b/k8s/bases/infrastructure/controllers/kube-prometheus-stack/http-route-prometheus.yaml
index 4817652..97c8856 100644
--- a/k8s/bases/infrastructure/controllers/kube-prometheus-stack/http-route-prometheus.yaml
+++ b/k8s/bases/infrastructure/controllers/kube-prometheus-stack/http-route-prometheus.yaml
@@ -19,11 +19,6 @@ spec:
     - prometheus.${domain}
   rules:
     - filters:
-        - type: RequestHeaderModifier
-          requestHeaderModifier:
-            set:
-              - name: X-Auth-Request-Redirect
-                value: https://prometheus.${domain}/
         - type: ResponseHeaderModifier
           responseHeaderModifier:
             set:
diff --git a/k8s/bases/infrastructure/controllers/oauth2-proxy/helm-release.yaml b/k8s/bases/infrastructure/controllers/oauth2-proxy/helm-release.yaml
index 20d0ac4..39de94a 100644
--- a/k8s/bases/infrastructure/controllers/oauth2-proxy/helm-release.yaml
+++ b/k8s/bases/infrastructure/controllers/oauth2-proxy/helm-release.yaml
@@ -81,7 +81,15 @@ spec:
         email_domains = ["*"]
         cookie_domains=[".${domain}"]
         whitelist_domains = ["${domain}", ".${domain}"]
-        redirect_url = "https://oauth2-proxy.${domain}/oauth2/callback"
+        # No redirect_url, deliberately: oauth2-proxy then finishes each login at
+        # /oauth2/callback on the host the user started from (https, because
+        # cookie_secure is set), and returns them to the page they asked for as a
+        # path on that same host. A fixed callback host would bring every login
+        # back through one host, where that path belongs to no app. Dex accepts
+        # these callbacks because every fronted host is registered on its
+        # public-client, which scripts/tests/test-oauth2-proxy-sso-redirects.sh
+        # enforces.
+        cookie_secure = true
         upstreams = [ "http://auth-proxy.oauth2-proxy.svc.cluster.local:80" ]
         # upstream_timeout — max time to await upstream response headers
         # (default 30s). Raised to 90s so KEDA scale-to-zero apps behind this
diff --git a/k8s/bases/infrastructure/controllers/opencost/http-route.yaml b/k8s/bases/infrastructure/controllers/opencost/http-route.yaml
index 4e98b15..f96a04c 100644
--- a/k8s/bases/infrastructure/controllers/opencost/http-route.yaml
+++ b/k8s/bases/infrastructure/controllers/opencost/http-route.yaml
@@ -21,11 +21,6 @@ spec:
     - opencost.${domain}
   rules:
     - filters:
-        - type: RequestHeaderModifier
-          requestHeaderModifier:
-            set:
-              - name: X-Auth-Request-Redirect
-                value: https://opencost.${domain}/
         - type: ResponseHeaderModifier
           responseHeaderModifier:
             set:
diff --git a/k8s/components/observability-coroot/remove-kps-ui/kustomization.yaml b/k8s/components/observability-coroot/remove-kps-ui/kustomization.yaml
index 8b26ab4..5dd1ec4 100644
--- a/k8s/components/observability-coroot/remove-kps-ui/kustomization.yaml
+++ b/k8s/components/observability-coroot/remove-kps-ui/kustomization.yaml
@@ -79,7 +79,19 @@ patches:
       kind: HelmRelease
       name: dex
       namespace: dex
+    # Prometheus and Alertmanager oauth2-proxy login callbacks first (highest index
+    # first, after remove-opencost-references took OpenCost's), then Grafana's own.
     patch: |-
+      - op: test
+        path: /spec/values/config/staticClients/0/redirectURIs/9
+        value: "https://prometheus.${domain}/oauth2/callback"
+      - op: remove
+        path: /spec/values/config/staticClients/0/redirectURIs/9
+      - op: test
+        path: /spec/values/config/staticClients/0/redirectURIs/6
+        value: "https://alertmanager.${domain}/oauth2/callback"
+      - op: remove
+        path: /spec/values/config/staticClients/0/redirectURIs/6
       - op: test
         path: /spec/values/config/staticClients/0/redirectURIs/4
         value: "https://grafana.${domain}/login/generic_oauth"
diff --git a/k8s/components/observability-coroot/remove-opencost-references/kustomization.yaml b/k8s/components/observability-coroot/remove-opencost-references/kustomization.yaml
index 1035a74..1e95522 100644
--- a/k8s/components/observability-coroot/remove-opencost-references/kustomization.yaml
+++ b/k8s/components/observability-coroot/remove-opencost-references/kustomization.yaml
@@ -334,3 +334,17 @@ patches:
         value: opencost
       - op: remove
         path: /spec/match/namespaceSelector/matchExpressions/0/values/14
+  # OpenCost's oauth2-proxy login callback on Dex's public-client, which this profile no
+  # longer routes. remove-kps-ui then removes later entries by index, so it must run after.
+  - target:
+      group: helm.toolkit.fluxcd.io
+      version: v2
+      kind: HelmRelease
+      name: dex
+      namespace: dex
+    patch: |-
+      - op: test
+        path: /spec/values/config/staticClients/0/redirectURIs/9
+        value: "https://opencost.${domain}/oauth2/callback"
+      - op: remove
+        path: /spec/values/config/staticClients/0/redirectURIs/9
diff --git a/scripts/tests/test-oauth2-proxy-sso-redirects.sh b/scripts/tests/test-oauth2-proxy-sso-redirects.sh
new file mode 100755
index 0000000..9230f2b
--- /dev/null
+++ b/scripts/tests/test-oauth2-proxy-sso-redirects.sh
@@ -0,0 +1,206 @@
+#!/usr/bin/env bash
+# Guards the oauth2-proxy login round-trip on every host it fronts.
+#
+# oauth2-proxy finishes a login at /oauth2/callback on the host the user started
+# from, then returns them to the page they asked for as a path on that host. So
+# oauth2-proxy must not be given a fixed redirect_url, and no route may replace
+# that page with a fixed X-Auth-Request-Redirect. Dex only accepts a
+# callback it has registered exactly, so every host an HTTPRoute sends to
+# oauth2-proxy needs `https://<host>/oauth2/callback` on Dex's public-client.
+# Without it, logging in on that host stops at Dex with "Unregistered
+# redirect_uri". The reverse also holds: an entry whose route is gone is a
+# callback Dex keeps accepting for no reason, so it must be removed with the
+# route.
+#
+# Hosts are read from the rendered provider overlays, the trees Flux applies,
+# not from raw files: a route a provider does not include needs no callback,
+# and a route only a provider patch adds still does.
+#
+# Ported from devantler-tech/platform (#3169), where the fix was verified live.
+set -euo pipefail
+
+script_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+repo_root="$(cd "${script_dir}/../.." && pwd)"
+readonly repo_root
+
+readonly providers=(hetzner docker)
+# Each observability profile is its own set of layers (scripts/validate-observability-option.sh),
+# and the profiles put different hosts behind oauth2-proxy, so every profile is rendered.
+readonly profiles=(default coroot)
+layers_of() {
+  case "$1" in
+    default) echo infrastructure/controllers infrastructure apps ;;
+    coroot) echo infrastructure-controllers-coroot infrastructure-coroot apps-coroot ;;
+  esac
+}
+readonly dex_file='k8s/bases/infrastructure/controllers/dex/helm-release.yaml'
+
+workdir="$(mktemp -d)"
+trap 'rm -rf "${workdir}"' EXIT
+
+fail() {
+  printf '::error::%s\n' "$1" >&2
+  exit 1
+}
+
+# Every HTTPRoute that sends traffic to the oauth2-proxy Service. A backendRef
+# with no namespace means the route's own namespace, and one with no kind means
+# a core Service, per the Gateway API defaults. `$ns` is a yq variable, not a
+# shell one.
+# shellcheck disable=SC2016
+readonly fronted_routes='
+  select(.kind == "HTTPRoute")
+  | .metadata.namespace as $ns
+  | select(
+      [ .spec.rules[]?.backendRefs[]?
+        | select(
+            (.group // "") == ""
+            and (.kind // "Service") == "Service"
+            and .name == "oauth2-proxy"
+            and (.namespace // $ns) == "oauth2-proxy")
+      ] | length > 0)'
+
+hosts_of() {
+  yq eval -N "${fronted_routes} | .spec.hostnames[]" "$1" | sort -u
+}
+
+# --- Self-test the route selector ------------------------------------------
+# A selector that matches nothing would make every check below pass over an
+# empty set, so prove it picks exactly the routes it should on a fixture first.
+cat >"${workdir}/fixture.yaml" <<'EOF'
+apiVersion: gateway.networking.k8s.io/v1
+kind: HTTPRoute
+metadata: {name: fronted, namespace: app}
+spec:
+  hostnames: [fronted.example]
+  rules:
+    - backendRefs: [{name: oauth2-proxy, namespace: oauth2-proxy, port: 80}]
+---
+apiVersion: gateway.networking.k8s.io/v1
+kind: HTTPRoute
+metadata: {name: same-namespace, namespace: oauth2-proxy}
+spec:
+  hostnames: [same-namespace.example]
+  rules:
+    - backendRefs: [{name: oauth2-proxy, port: 80}]
+---
+apiVersion: gateway.networking.k8s.io/v1
+kind: HTTPRoute
+metadata: {name: direct, namespace: app}
+spec:
+  hostnames: [direct.example]
+  rules:
+    - backendRefs: [{name: app, port: 80}]
+---
+apiVersion: gateway.networking.k8s.io/v1
+kind: HTTPRoute
+metadata: {name: other-namespace, namespace: app}
+spec:
+  hostnames: [other-namespace.example]
+  rules:
+    - backendRefs: [{name: oauth2-proxy, port: 80}]
+EOF
+fixture_hosts="$(hosts_of "${workdir}/fixture.yaml" | tr '\n' ' ')"
+[[ "${fixture_hosts}" == 'fronted.example same-namespace.example ' ]] ||
+  fail "the oauth2-proxy route selector picked '${fixture_hosts}' from its fixture instead of 'fronted.example same-namespace.example'; fix the selector before trusting this test"
+
+# --- Render every layer Flux reconciles --------------------------------------
+all_hosts="${workdir}/all-hosts.txt"
+: >"${all_hosts}"
+
+profile_providers=()
+for profile in "${profiles[@]}"; do
+  for p in "${providers[@]}"; do
+    profile_providers+=("${p}:${profile}")
+  done
+done
+
+for profile_provider in "${profile_providers[@]}"; do
+  profile="${profile_provider#*:}"
+  provider="${profile_provider%%:*} (${profile} profile)"
+  rendered="${workdir}/${profile_provider/:/-}.yaml"
+  : >"${rendered}"
+  read -ra layers <<<"$(layers_of "${profile}")"
+  for layer in "${layers[@]}"; do
+    dir="${repo_root}/k8s/providers/${profile_provider%%:*}/${layer}"
+    # Check kubectl's own status: in a { kubectl; printf; } group the status is
+    # printf's, so a layer that failed to build would be read as an empty one.
+    kubectl kustomize "${dir}" >>"${rendered}" 2>"${workdir}/render.err" ||
+      fail "k8s/providers/${profile_provider%%:*}/${layer} failed to build: $(tail -5 "${workdir}/render.err")"
+    printf '\n---\n' >>"${rendered}"
+  done
+
+  # A route that names no hostname inherits the listener's, so this test cannot
+  # tell which callback it needs. Refuse it rather than skip it.
+  unnamed="$(yq eval -N "${fronted_routes} | select((.spec.hostnames // []) | length == 0) | .metadata.namespace + \"/\" + .metadata.name" "${rendered}")"
+  [[ -z "${unnamed}" ]] ||
+    fail "${provider}: these HTTPRoutes send traffic to oauth2-proxy without naming a hostname, so their login callback cannot be checked: ${unnamed//$'\n'/, }"
+
+  # oauth2-proxy prefers an X-Auth-Request-Redirect header over the page the
+  # user asked for, so a route that sets one sends every login to that one URL.
+  fixed="$(yq eval -N "${fronted_routes} | select([.spec.rules[]?.filters[]? | select(.type == \"RequestHeaderModifier\") | ((.requestHeaderModifier.set // []) + (.requestHeaderModifier.add // []))[] | select(.name | downcase == \"x-auth-request-redirect\")] | length > 0) | .metadata.namespace + \"/\" + .metadata.name" "${rendered}")"
+  [[ -z "${fixed}" ]] ||
+    fail "${provider}: these HTTPRoutes send every login through oauth2-proxy to one fixed URL, dropping the page the user asked for. Remove their X-Auth-Request-Redirect header: ${fixed//$'\n'/, }"
+
+  hosts="$(hosts_of "${rendered}")"
+  if [[ "${profile_provider}" == hetzner:* && -z "${hosts}" ]]; then
+    fail 'the production overlays put no host behind oauth2-proxy; either the SSO wiring changed shape or this test stopped seeing it'
+  fi
+
+  # oauth2-proxy's own route is an HTTPRoute in the rendered tree here, so the
+  # selector above already sees its host. Should the chart render that route
+  # instead (gatewayApi values), read its hostnames from the HelmRelease too.
+  own_hosts="$(yq eval -N 'select(.kind == "HelmRelease" and .metadata.name == "oauth2-proxy" and .metadata.namespace == "oauth2-proxy") | .spec.values.gatewayApi.hostnames[]?' "${rendered}")"
+  grep -qx "oauth2-proxy\.[^.]*.*" <<<"$(printf '%s\n%s\n' "${hosts}" "${own_hosts}")" ||
+    fail "${provider}: no route puts oauth2-proxy's own host behind it; its callback host can no longer be derived"
+
+  # oauth2-proxy must finish each login on the host it started from. A fixed
+  # redirect_url brings every login back through one host, where the path
+  # oauth2-proxy returns the user to belongs to no app. The callback it derives
+  # instead is https — the scheme Dex registers — only while cookie_secure is set.
+  o2p="$(yq eval -N 'select(.kind == "HelmRelease" and .metadata.name == "oauth2-proxy" and .metadata.namespace == "oauth2-proxy") | .spec.values' "${rendered}")"
+  o2p_config="$(yq eval -N '.config.configFile // ""' - <<<"${o2p}")"
+  [[ -n "${o2p_config}" ]] ||
+    fail "${provider}: the oauth2-proxy HelmRelease has no config.configFile; its login settings moved, so this test is checking nothing"
+  if grep -Eq '^[[:space:]]*redirect_url[[:space:]]*=' <<<"${o2p_config}" ||
+    grep -Eq 'redirect[-_]url' <<<"$(yq eval -N '.extraArgs // ""' - <<<"${o2p}")"; then
+    fail "${provider}: oauth2-proxy sets a fixed redirect_url, so every login finishes on one host and returns the user to a path on a host that serves no app. Remove it from k8s/bases/infrastructure/controllers/oauth2-proxy/helm-release.yaml; oauth2-proxy then finishes each login on the host it started from"
+  fi
+  grep -Eq '^[[:space:]]*cookie_secure[[:space:]]*=[[:space:]]*true[[:space:]]*$' <<<"${o2p_config}" ||
+    fail "${provider}: oauth2-proxy must set cookie_secure = true: it is what makes the login callback it derives https, the only scheme Dex has registered"
+
+  registered="$(yq eval -N 'select(.kind == "HelmRelease" and .metadata.name == "dex" and .metadata.namespace == "dex") | .spec.values.config.staticClients[] | select(.id == "public-client") | .redirectURIs[]' "${rendered}")"
+  [[ -n "${registered}" ]] ||
+    fail "${provider}: no Dex public-client redirectURIs were rendered; the Dex client moved or was renamed, so this test is checking nothing"
+
+  missing=()
+  while IFS= read -r host; do
+    [[ -n "${host}" ]] || continue
+    grep -qxF -- "https://${host}/oauth2/callback" <<<"${registered}" ||
+      missing+=("https://${host}/oauth2/callback")
+  done < <(printf '%s\n%s\n' "${hosts}" "${own_hosts}" | sort -u)
+
+  if ((${#missing[@]} > 0)); then
+    fail "${provider}: these hosts are behind oauth2-proxy but Dex does not accept their login callback, so signing in there fails with 'Unregistered redirect_uri'. Add each to the public-client redirectURIs in ${dex_file}: ${missing[*]}"
+  fi
+
+  printf '%s\n%s\n' "${hosts}" "${own_hosts}" >>"${all_hosts}"
+  printf '%s\n' "${registered}" >"${workdir}/${profile_provider/:/-}-registered.txt"
+done
+
+# --- No callback outlives its route ----------------------------------------
+# The Dex client is shared by both providers, so an entry is live while any
+# provider still fronts its host.
+sort -u -o "${all_hosts}" "${all_hosts}"
+stale=()
+while IFS= read -r uri; do
+  [[ "${uri}" =~ ^https://([^/]+)/oauth2/callback$ ]] || continue
+  grep -qxF -- "${BASH_REMATCH[1]}" "${all_hosts}" || stale+=("${uri}")
+done < <(sort -u "${workdir}"/*-registered.txt)
+
+if ((${#stale[@]} > 0)); then
+  fail "Dex still accepts an oauth2-proxy login callback on hosts no HTTPRoute sends to oauth2-proxy. Remove each from the public-client redirectURIs in ${dex_file}: ${stale[*]}"
+fi
+
+printf 'PASS: a login through oauth2-proxy returns to the page it started from on all %d hosts it fronts, and Dex accepts no other oauth2-proxy callback\n' \
+  "$(grep -c . "${all_hosts}")"
diff --git a/scripts/validate-observability-option.sh b/scripts/validate-observability-option.sh
index e03acee..64be86b 100755
--- a/scripts/validate-observability-option.sh
+++ b/scripts/validate-observability-option.sh
@@ -797,10 +797,12 @@ assert_coroot_sso_controller_contract() {
         select(.spec.hostnames == ["observability.${domain}"]) |
         select(.spec.rules | length == 1) |
         select(.spec.rules[0].backendRefs == [{"name":"oauth2-proxy","namespace":"oauth2-proxy","port":80}]) |
-        select(any(.spec.rules[0].filters[]?;
-          .type == "RequestHeaderModifier" and
-          any(.requestHeaderModifier.set[]?;
-            .name == "X-Auth-Request-Redirect" and .value == "https://observability.${domain}/"))) |
+        # No fixed X-Auth-Request-Redirect: oauth2-proxy returns the user to the
+        # page they asked for (scripts/tests/test-oauth2-proxy-sso-redirects.sh).
+        select(all(.spec.rules[0].filters[]?;
+          .type != "RequestHeaderModifier" or
+          ([((.requestHeaderModifier.set // []) + (.requestHeaderModifier.add // []))[] |
+            select(.name | ascii_downcase == "x-auth-request-redirect")] | length == 0))) |
         select(any(.spec.rules[0].filters[]?;
           .type == "ResponseHeaderModifier" and
           any(.responseHeaderModifier.set[]?;