diff --git a/etc/skel/install-media-stack.sh b/etc/skel/install-media-stack.sh
index 213aba82..bf0abe97 100755
--- a/etc/skel/install-media-stack.sh
+++ b/etc/skel/install-media-stack.sh
@@ -1541,6 +1541,15 @@ servarr_auth_payload_write() {
 	MEDIA_STACK_AUTH_USERNAME="$MEDIA_STACK_AUTH_USERNAME" MEDIA_STACK_AUTH_PASSWORD="$password" php -r '$input = stream_get_contents(STDIN); $data = json_decode($input, true); if (!is_array($data)) { fwrite(STDERR, "Invalid Servarr host-config JSON\n"); exit(1); } $data["id"] = isset($data["id"]) ? $data["id"] : 1; $data["authenticationMethod"] = "forms"; $data["authenticationRequired"] = "enabled"; $data["username"] = getenv("MEDIA_STACK_AUTH_USERNAME"); $data["password"] = getenv("MEDIA_STACK_AUTH_PASSWORD"); $data["passwordConfirmation"] = getenv("MEDIA_STACK_AUTH_PASSWORD"); if (isset($data["updateMechanism"]) && $data["updateMechanism"] === "script" && (!isset($data["updateScriptPath"]) || $data["updateScriptPath"] === "")) { $data["updateMechanism"] = "builtIn"; } echo json_encode($data, JSON_UNESCAPED_SLASHES);' <"$source_json" >"$target_json"
 }
 
+# The configure step writes UrlBase before seeding, so it is live on the seed start:
+# unprefixed API paths answer an empty 307, which curl -f accepts. Build the host-config
+# URL under the UrlBase so the JSON fetch and the PUT reach the API.
+servarr_host_config_url() {
+	local config_file="$1" seed_port="$2" api_version="$3" url_base
+	url_base=$(sed -n -E 's|.*<UrlBase>([^<]*)</UrlBase>.*|\1|p' "$config_file" 2>/dev/null | head -n 1 || true)
+	printf 'http://127.0.0.1:%s%s/api/%s/config/host' "$seed_port" "${url_base%/}" "$api_version"
+}
+
 servarr_auth_seed() {
 	local app="$1" install_name="$2" dll="$3" desired_port="$4" api_version="$5" password="$6" extra_args="${7:-}"
 	local datadir="$HOME/.config/${app}"
@@ -1559,7 +1568,7 @@ servarr_auth_seed() {
 
 	seed_port="$desired_port"
 	session="${app}-auth-seed"
-	base_url="http://127.0.0.1:${seed_port}/api/${api_version}/config/host"
+	base_url=$(servarr_host_config_url "$config_file" "$seed_port" "$api_version")
 	response_json=$(mktemp)
 	payload_json=$(mktemp)
 	curl_config=$(mktemp)
diff --git a/scripts/lib/tests/development/installMediaStackScriptTest.php b/scripts/lib/tests/development/installMediaStackScriptTest.php
index d5cafb8f..2dfc808f 100644
--- a/scripts/lib/tests/development/installMediaStackScriptTest.php
+++ b/scripts/lib/tests/development/installMediaStackScriptTest.php
@@ -225,11 +225,34 @@ public function testMediaStackCredentialsUseDelugeStylePhpCsprngAndOwnerOnlyFile
         ], $this->script);
     }
 
+    public function testServarrHostConfigUrlUsesUrlBase(): void
+    {
+        // Servarr answers unprefixed API paths with an empty 307 once UrlBase is set (Refs #858).
+        $script = implode("\n", array(
+            '#!/usr/bin/env bash', 'set -euo pipefail', 'dir=$(mktemp -d)',
+            $this->pmssExtractShellFunctions($this->script, array('servarr_host_config_url')),
+            'probe() { printf "%s\n" "<Config>" "$1" "</Config>" >"$dir/config.xml"; servarr_host_config_url "$dir/config.xml" 17878 "$2"; printf "\n"; }',
+            'probe "  <UrlBase>/public-bob/radarr</UrlBase>" v3',
+            'probe "  <UrlBase>/public-bob/prowlarr/</UrlBase>" v1',
+            'probe "  <UrlBase></UrlBase>" v3',
+            'probe "  <UrlBase />" v3',
+            'servarr_host_config_url "$dir/missing.xml" 17878 v3; printf "\n"',
+            'rm -rf "$dir"', '',
+        ));
+        $this->assertSame(implode("\n", array(
+            'http://127.0.0.1:17878/public-bob/radarr/api/v3/config/host',
+            'http://127.0.0.1:17878/public-bob/prowlarr/api/v1/config/host',
+            'http://127.0.0.1:17878/api/v3/config/host',
+            'http://127.0.0.1:17878/api/v3/config/host',
+            'http://127.0.0.1:17878/api/v3/config/host',
+        )), $this->pmssRunShellHarness($script));
+    }
+
     public function testServarrAuthSeedingUsesLocalApiAndFailsClosed(): void
     {
         $this->assertStringContainsAllStrings([
             'servarr_auth_seed() {',
-            'base_url="http://127.0.0.1:${seed_port}/api/${api_version}/config/host"',
+            'base_url=$(servarr_host_config_url "$config_file" "$seed_port" "$api_version")',
             '$data["authenticationMethod"] = "forms"',
             '$data["authenticationRequired"] = "enabled"',
             'servarr_credentials_mark_existing_unknown() {',
@@ -374,6 +397,7 @@ public function testServarrAuthSeedAuthenticatesProbeReadAndWrite(): void
             'servarr_api_key_curl_config_write',
             'servarr_api_key_curl_config_wait',
             'servarr_auth_payload_write',
+            'servarr_host_config_url',
             'servarr_auth_seed',
         ));
         $script = implode("\n", array(