diff --git a/public/lib/classes/router/hook_callbacks.php b/public/lib/classes/router/hook_callbacks.php
index 44bf7c2ae21..933cf8f30e0 100644
--- a/public/lib/classes/router/hook_callbacks.php
+++ b/public/lib/classes/router/hook_callbacks.php
@@ -45,5 +45,26 @@ class hook_callbacks {
             route_loader_interface::class,
             \DI\get(route_loader::class),
         );
+
+        // Until Moodle 6.0 we will allow the router to be unconfigured for backwards compatibility.
+        // To support legacy shims like /course/tags.php we need a custom route resolver which tries both.
+        // The following definition can be removed in Moodle 6.0.
+        // - \Slim\Interfaces\RouteResolverInterface
+        // - \Slim\Interfaces\RouteCollectorInterface
+        // - \Psr\Http\Message\ResponseFactoryInterface
+        // Along with the route_resolver class itself.
+        // TODO MDL-87637 Remove in Moodle 6.0.
+        $hook->add_definition(
+            \Slim\Interfaces\RouteResolverInterface::class,
+            \DI\get(route_resolver::class),
+        );
+        $hook->add_definition(
+            \Slim\Interfaces\RouteCollectorInterface::class,
+            \DI\get(\Slim\Routing\RouteCollector::class),
+        );
+        $hook->add_definition(
+            \Psr\Http\Message\ResponseFactoryInterface::class,
+            \DI\factory([\Slim\Factory\AppFactory::class, 'determineResponseFactory']),
+        );
     }
 }
diff --git a/public/lib/classes/router/route_resolver.php b/public/lib/classes/router/route_resolver.php
new file mode 100644
index 00000000000..55d63380680
--- /dev/null
+++ b/public/lib/classes/router/route_resolver.php
@@ -0,0 +1,78 @@
+<?php
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+namespace core\router;
+
+use Slim\Routing\RouteResolver;
+use Slim\Routing\RoutingResults;
+
+/**
+ * Route Resolver that supports routing via r.php.
+ *
+ * Note: This is a temporary shim to support legacy shims until Moodle 6.0.
+ * After that, this class and its references can be removed.
+ * See MDL-87625 for more details.
+ * This class and it's DI definitions will be removed in Moodle 6.0.
+ *
+ * @package    core
+ * @copyright  Andrew Lyons <andrew@nicols.co.uk>
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+class route_resolver extends RouteResolver {
+    #[\Override]
+    public function computeRoutingResults(string $uri, string $method): RoutingResults {
+        $originalresult = parent::computeRoutingResults($uri, $method);
+        if ($originalresult->getRouteStatus() === RoutingResults::FOUND) {
+            return $originalresult;
+        }
+
+        // In some situations, like hardcoded shims, the request will look like /course/tags.php.
+        // When the router is not correctly configured (that ism it still has /r.php in the basepath),
+        // we need to adjust the URI to include /r.php so that the routing can work.
+        // The basepath includes the wwwroot and /r.php. We need to remove the /r.php from that to leave just the wwwroot.
+        // Then we can replae that with the basepath including /r.php.
+        $basepath = \core\di::get(\core\router::class)->basepath;
+
+        if (str_ends_with($basepath, '/r.php')) {
+            if (str_starts_with($uri, $basepath)) {
+                // The router is in legacy mode and the requested URI already includes /r.php.
+                return $originalresult;
+            } else {
+                // The router is in legacy mode but the requested URI does not include /r.php, so add it.
+                $updateduri = $basepath . substr($uri, strlen($basepath) - strlen('/r.php'));
+            }
+        } else {
+            $suffixedbasepath = $basepath . '/r.php';
+            if (str_starts_with($uri, $suffixedbasepath)) {
+                // The router is correctly configured but the requested URI includes /r.php, so remove it.
+                $updateduri = $basepath . substr($uri, strlen($suffixedbasepath));
+            } else {
+                // The router is correctly configured and the requested URI does not include /r.php.
+                return $originalresult;
+            }
+        }
+
+        // Try again with the updated URI.
+        $result = parent::computeRoutingResults($updateduri, $method);
+
+        if ($result->getRouteStatus() === RoutingResults::FOUND) {
+            return $result;
+        }
+
+        // Still not found, return the original result.
+        return $originalresult;
+    }
+}
diff --git a/public/lib/tests/router/route_resolver_test.php b/public/lib/tests/router/route_resolver_test.php
new file mode 100644
index 00000000000..71dffabd016
--- /dev/null
+++ b/public/lib/tests/router/route_resolver_test.php
@@ -0,0 +1,149 @@
+<?php
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+namespace core\router;
+
+/**
+ * Tests for the route resolver that supports routing with and without the r.php prefix.
+ *
+ * @package    core
+ * @category   test
+ * @copyright  Andrew Lyons <andrew@nicols.co.uk>
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+#[\PHPUnit\Framework\Attributes\CoversClass(route_resolver::class)]
+final class route_resolver_test extends \core\tests\router\route_testcase {
+    /**
+     * Ensure that the computation of routing results can add or remove /r.php as needed.
+     */
+    #[\PHPUnit\Framework\Attributes\DataProvider('compute_routing_results_provider')]
+    public function test_compute_routing_results(
+        string $basepath,
+        string $path,
+        string $requestedpath,
+        int $expectedresult,
+    ): void {
+        $this->add_route_to_route_loader(
+            self::class,
+            'method_under_test',
+            '',
+        );
+
+        $router = $this->get_router($basepath);
+        $result = $router->get_app()->getRouteResolver()->computeRoutingResults(
+            $requestedpath,
+            'GET',
+        );
+
+        $this->assertEquals($expectedresult, $result->getRouteStatus());
+    }
+
+    /**
+     * A test route handler.
+     */
+    #[\core\router\route(path: '/method_under_test')]
+    public function method_under_test(): void {
+        // Do nothing.
+    }
+
+    /**
+     * Data provider for the routing provider.
+     *
+     * @return \Generator<string, array<int|string>, mixed, void>
+     */
+    public static function compute_routing_results_provider(): \Generator {
+        $prefixes = ['/moodle', ''];
+
+        foreach ($prefixes as $prefix) {
+            yield "Router in legacy mode, request without r.php, wwwroot is '{$prefix}'" => [
+                "{$prefix}/r.php",
+                '/method_under_test',
+                "{$prefix}/method_under_test",
+                \Slim\Routing\RoutingResults::FOUND,
+            ];
+
+            yield "Router in legacy mode, request with r.php, wwwroot is '{$prefix}'" => [
+                "{$prefix}/r.php",
+                '/method_under_test',
+                "{$prefix}/r.php/method_under_test",
+                \Slim\Routing\RoutingResults::FOUND,
+            ];
+
+            yield "Router in normal mode, request without r.php, wwwroot is '{$prefix}'" => [
+                "{$prefix}",
+                '/method_under_test',
+                "{$prefix}/method_under_test",
+                \Slim\Routing\RoutingResults::FOUND,
+            ];
+
+            yield "Router in normal mode, request with r.php, wwwroot is '{$prefix}'" => [
+                "{$prefix}",
+                '/method_under_test',
+                "{$prefix}/r.php/method_under_test",
+                \Slim\Routing\RoutingResults::FOUND,
+            ];
+
+            yield "Router in legacy mode, request unrelated path, wwwroot is '{$prefix}'" => [
+                "{$prefix}/r.php",
+                '/method_under_test',
+                "{$prefix}/otherpath",
+                \Slim\Routing\RoutingResults::NOT_FOUND,
+            ];
+
+            yield "Router in normal mode, request unrelated path, wwwroot is '{$prefix}'" => [
+                "{$prefix}",
+                '/method_under_test',
+                "{$prefix}/otherpath",
+                \Slim\Routing\RoutingResults::NOT_FOUND,
+            ];
+
+            yield "Router in legacy mode, request root path, wwwroot is '{$prefix}'" => [
+                "{$prefix}/r.php",
+                '/method_under_test',
+                "{$prefix}/",
+                \Slim\Routing\RoutingResults::NOT_FOUND,
+            ];
+
+            yield "Router in normal mode, request root path, wwwroot is '{$prefix}'" => [
+                "{$prefix}",
+                '/method_under_test',
+                "{$prefix}/",
+                \Slim\Routing\RoutingResults::NOT_FOUND,
+            ];
+
+            yield "Router in legacy mode, request r.php only, wwwroot is '{$prefix}'" => [
+                "{$prefix}/r.php",
+                '/method_under_test',
+                "{$prefix}/r.php",
+                \Slim\Routing\RoutingResults::NOT_FOUND,
+            ];
+
+            yield "Router in normal mode, request r.php only, wwwroot is '{$prefix}'" => [
+                "{$prefix}",
+                '/method_under_test',
+                "{$prefix}/r.php",
+                \Slim\Routing\RoutingResults::NOT_FOUND,
+            ];
+
+            yield "Router in legacy mode, request empty path, wwwroot is '{$prefix}'" => [
+                "{$prefix}/r.php",
+                '/method_under_test',
+                "{$prefix}",
+                \Slim\Routing\RoutingResults::NOT_FOUND,
+            ];
+        }
+    }
+}