diff --git a/complement/tests/internal/dockerutil/files.go b/complement/tests/internal/dockerutil/files.go
new file mode 100644
index 0000000000..62d2c557af
--- /dev/null
+++ b/complement/tests/internal/dockerutil/files.go
@@ -0,0 +1,54 @@
+package dockerutil
+
+import (
+	"archive/tar"
+	"bytes"
+	"fmt"
+	"testing"
+
+	"github.com/docker/docker/api/types/container"
+	"github.com/docker/docker/client"
+)
+
+// Write `data` as a file into a container at the given `path`.
+//
+// Internally, produces an uncompressed single-file tape archive (tar) that is sent to the docker
+// daemon to be unpacked into the container filesystem.
+func WriteFileIntoContainer(t *testing.T, docker *client.Client, containerID string, path string, data []byte) error {
+	// Create a fake/virtual tar file in memory that we can copy to the container
+	// via https://stackoverflow.com/a/52131297/796832
+	var buf bytes.Buffer
+	tw := tar.NewWriter(&buf)
+	err := tw.WriteHeader(&tar.Header{
+		Name: path,
+		Mode: 0777,
+		Size: int64(len(data)),
+	})
+	if err != nil {
+		return fmt.Errorf("WriteIntoContainer: failed to write tarball header for %s: %v", path, err)
+	}
+	_, err = tw.Write([]byte(data))
+	if err != nil {
+		return fmt.Errorf("WriteIntoContainer: failed to write tarball data for %s: %w", path, err)
+	}
+
+	err = tw.Close()
+	if err != nil {
+		return fmt.Errorf("WriteIntoContainer: failed to close tarball writer for %s: %w", path, err)
+	}
+
+	// Put our new fake file in the container volume
+	err = docker.CopyToContainer(
+		t.Context(),
+		containerID,
+		"/",
+		&buf,
+		container.CopyToContainerOptions{
+			AllowOverwriteDirWithFile: false,
+		},
+	)
+	if err != nil {
+		return fmt.Errorf("WriteIntoContainer: failed to copy: %s", err)
+	}
+	return nil
+}
diff --git a/complement/tests/oidc_test.go b/complement/tests/oidc_test.go
new file mode 100644
index 0000000000..bddb382058
--- /dev/null
+++ b/complement/tests/oidc_test.go
@@ -0,0 +1,117 @@
+// This file is licensed under the Affero General Public License (AGPL) version 3.
+//
+// Copyright (C) 2026 Element Creations Ltd
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as
+// published by the Free Software Foundation, either version 3 of the
+// License, or (at your option) any later version.
+//
+// See the GNU Affero General Public License for more details:
+// <https://www.gnu.org/licenses/agpl-3.0.html>.
+
+package synapse_tests
+
+import (
+	"net/http"
+	"net/url"
+	"strings"
+	"testing"
+
+	dockerClient "github.com/docker/docker/client"
+	"github.com/element-hq/synapse/tests/internal/dockerutil"
+	"github.com/matrix-org/complement"
+	"github.com/matrix-org/complement/client"
+	"github.com/matrix-org/complement/match"
+	"github.com/matrix-org/complement/must"
+)
+
+const OIDC_HOMESERVER_CONFIG string = `
+oidc_providers:
+ - idp_id: "test_provider"
+   idp_name: "Test OIDC Provider"
+   issuer: "https://example.invalid"
+   client_id: "test_client_id"
+   client_secret: "test_secret"
+   scopes: ["openid"]
+   discover: true
+   user_mapping_provider:
+     module: "synapse.handlers.oidc.JinjaOidcMappingProvider"
+     config:
+       display_name_template: "{{ user.given_name }}"
+       email_template: "{{ user.email }}"
+`
+
+// Test that Synapse still starts up when configured with an OIDC provider that is unavailable.
+//
+// This is a regression test: Synapse previously would fail to start up
+// at all if the OIDC provider was down on startup.
+// https://github.com/element-hq/synapse/issues/8088
+//
+// Now instead of failing to start, Synapse will produce a 503 response on the
+// `/_matrix/client/v3/login/sso/redirect/oidc-test_provider` endpoint.
+func TestOIDCProviderUnavailable(t *testing.T) {
+	// Deploy a single homeserver
+	deployment := complement.Deploy(t, 1)
+	defer deployment.Destroy(t)
+
+	// Get Docker client to manipulate container
+	dc, err := dockerClient.NewClientWithOpts(
+		dockerClient.FromEnv,
+		dockerClient.WithAPIVersionNegotiation(),
+	)
+	must.NotError(t, "failed creating docker client", err)
+
+	// Configure the OIDC Provider by writing a config fragment
+	err = dockerutil.WriteFileIntoContainer(
+		t,
+		dc,
+		deployment.ContainerID(t, "hs1"),
+		"/conf/homeserver.d/oidc_provider.yaml",
+		[]byte(OIDC_HOMESERVER_CONFIG),
+	)
+	if err != nil {
+		t.Fatalf("Failed to write updated config to container: %v", err)
+	}
+
+	// Restart the homeserver to apply the new config
+	deployment.StopServer(t, "hs1")
+	// Careful: port number changes here
+	deployment.StartServer(t, "hs1")
+	// Must get after the restart so the port number is correct
+	unauthedClient := deployment.UnauthenticatedClient(t, "hs1")
+
+	// Test that trying to log in with an OIDC provider that is down
+	// causes an HTML error page to be shown to the user.
+	// (This replaces the redirect that would happen if the provider was
+	// up.)
+	//
+	// More importantly, implicitly tests that Synapse can start up
+	// and answer requests even though an OIDC provider is down.
+	t.Run("/login/sso/redirect shows HTML error", func(t *testing.T) {
+		// Build a request to the /redirect/ endpoint, that would normally be navigated to
+		// by the user's browser in order to start the login flow.
+		queryParams := url.Values{}
+		queryParams.Add("redirectUrl", "http://redirect.invalid/redirect")
+		res := unauthedClient.Do(t, "GET", []string{"_matrix", "client", "v3", "login", "sso", "redirect", "oidc-test_provider"},
+			client.WithQueries(queryParams),
+		)
+
+		body := must.MatchResponse(t, res, match.HTTPResponse{
+			// Should get a 503
+			StatusCode: http.StatusServiceUnavailable,
+
+			Headers: map[string]string{
+				// Should get an HTML page explaining the problem to the user
+				"Content-Type": "text/html; charset=utf-8",
+			},
+		})
+
+		bodyText := string(body)
+
+		// The HTML page contains phrases from the template we expect
+		if !strings.Contains(bodyText, "login provider is unavailable right now") {
+			t.Fatalf("Keyword not found in HTML error page, got %s", bodyText)
+		}
+	})
+}