diff --git a/packages/core/src/presets/preset.ts b/packages/core/src/presets/preset.ts
index 8bee77fe..225c3b36 100644
--- a/packages/core/src/presets/preset.ts
+++ b/packages/core/src/presets/preset.ts
@@ -35,7 +35,7 @@ export const baseOptions = (
   name: string,
   resources: Resource[],
   timeout: number = Env.DEFAULT_TIMEOUT,
-  baseUrls?: string[]
+  baseUrls?: readonly string[]
 ): Option[] => {
   const urlOption: Option = {
     id: 'url',
@@ -55,7 +55,7 @@ export const baseOptions = (
         label: baseUrls[0],
         value: 'undefined',
       },
-      ...baseUrls.splice(1).map((url) => ({
+      ...baseUrls.slice(1).map((url) => ({
         label: url,
         value: url,
       })),
diff --git a/packages/core/src/utils/env.ts b/packages/core/src/utils/env.ts
index 927b87c9..b1bb2433 100644
--- a/packages/core/src/utils/env.ts
+++ b/packages/core/src/utils/env.ts
@@ -92,7 +92,7 @@ const namedRegexes = makeValidator((x) => {
 const removeTrailingSlash = (x: string) =>
   x.endsWith('/') ? x.slice(0, -1) : x;
 
-const presetUrls = makeExactValidator<string[]>((x) => {
+const presetUrls = makeExactValidator<readonly string[]>((x) => {
   if (typeof x !== 'string') {
     throw new EnvError('Preset URLs must be a string or an array of strings');
   }
@@ -111,10 +111,10 @@ const presetUrls = makeExactValidator<string[]>((x) => {
         'Preset URLs must be an array of URLs or a single URL'
       );
     }
-    return urls.map(removeTrailingSlash);
+    return Object.freeze(urls.map(removeTrailingSlash));
   } catch (e) {
     if (typeof x === 'string' && validateUrl(x)) {
-      return [removeTrailingSlash(x)];
+      return Object.freeze([removeTrailingSlash(x)]);
     }
     throw new EnvError('Preset URLs must be an array of URLs or a single URL');
   }