diff --git a/packages/ui/src/components/config-editor.tsx b/packages/ui/src/components/config-editor.tsx
index 129b8f7..1865067 100644
--- a/packages/ui/src/components/config-editor.tsx
+++ b/packages/ui/src/components/config-editor.tsx
@@ -1,6 +1,7 @@
 import type React from "react";
 import { useEffect, useState } from "react";
 import { type ZodType, z } from "zod";
+import { MAX_DOWNLOAD_THREADS, MIN_DOWNLOAD_THREADS } from "@/lib/config";
 import { useSettingsStore } from "@/models/settings";
 import type { LauncherConfig } from "@/types";
 import { Button } from "./ui/button";
@@ -22,7 +23,10 @@ const launcherConfigSchema: ZodType<LauncherConfig> = z.object({
   javaPath: z.string(),
   width: z.number(),
   height: z.number(),
-  downloadThreads: z.number(),
+  downloadThreads: z
+    .number()
+    .min(MIN_DOWNLOAD_THREADS)
+    .max(MAX_DOWNLOAD_THREADS),
   customBackgroundPath: z.string().nullable(),
   enableGpuAcceleration: z.boolean(),
   enableVisualEffects: z.boolean(),
diff --git a/packages/ui/src/lib/config.ts b/packages/ui/src/lib/config.ts
new file mode 100644
index 0000000..ef24327
--- /dev/null
+++ b/packages/ui/src/lib/config.ts
@@ -0,0 +1,2 @@
+export const MIN_DOWNLOAD_THREADS = 1;
+export const MAX_DOWNLOAD_THREADS = 64;
diff --git a/packages/ui/src/pages/settings.tsx b/packages/ui/src/pages/settings.tsx
index 9387e23..208c102 100644
--- a/packages/ui/src/pages/settings.tsx
+++ b/packages/ui/src/pages/settings.tsx
@@ -31,6 +31,7 @@ import {
 import { Spinner } from "@/components/ui/spinner";
 import { Switch } from "@/components/ui/switch";
 import { Tabs, TabsContent, TabsList, TabsTrigger } from "@/components/ui/tabs";
+import { MAX_DOWNLOAD_THREADS, MIN_DOWNLOAD_THREADS } from "@/lib/config";
 import { useJavaStore } from "@/models/java";
 import { useSettingsStore } from "@/models/settings";
 
@@ -168,8 +169,8 @@ export function SettingsPage() {
                       onBlur={() => {
                         settings.save();
                       }}
-                      min={1}
-                      max={64}
+                      min={MIN_DOWNLOAD_THREADS}
+                      max={MAX_DOWNLOAD_THREADS}
                     />
                   </Field>
                 </FieldSet>
diff --git a/src-tauri/src/core/config.rs b/src-tauri/src/core/config.rs
index d1f306a..417b774 100644
--- a/src-tauri/src/core/config.rs
+++ b/src-tauri/src/core/config.rs
@@ -5,6 +5,9 @@ use std::sync::Mutex;
 use tauri::{AppHandle, Manager};
 use ts_rs::TS;
 
+pub const MIN_DOWNLOAD_THREADS: u32 = 1;
+pub const MAX_DOWNLOAD_THREADS: u32 = 64;
+
 #[derive(Debug, Clone, Serialize, Deserialize, TS)]
 #[serde(rename_all = "camelCase")]
 #[ts(export, export_to = "config.ts")]
@@ -85,7 +88,8 @@ pub struct LauncherConfig {
     pub java_path: String,
     pub width: u32,
     pub height: u32,
-    pub download_threads: u32, // concurrent download threads (1-128)
+    /// Concurrent download threads. Clamped via MIN_DOWNLOAD_THREADS/MAX_DOWNLOAD_THREADS.
+    pub download_threads: u32,
     pub custom_background_path: Option<String>,
     pub enable_gpu_acceleration: bool,
     pub enable_visual_effects: bool,
@@ -109,7 +113,7 @@ impl Default for LauncherConfig {
             java_path: "java".to_string(),
             width: 854,
             height: 480,
-            download_threads: 32,
+            download_threads: 8,
             custom_background_path: None,
             enable_gpu_acceleration: false,
             enable_visual_effects: true,
@@ -125,6 +129,14 @@ impl Default for LauncherConfig {
     }
 }
 
+impl LauncherConfig {
+    pub fn sanitize(&mut self) {
+        self.download_threads =
+            self.download_threads
+                .clamp(MIN_DOWNLOAD_THREADS, MAX_DOWNLOAD_THREADS);
+    }
+}
+
 pub struct ConfigState {
     pub config: Mutex<LauncherConfig>,
     pub file_path: PathBuf,
@@ -137,7 +149,9 @@ impl ConfigState {
 
         let config = if config_path.exists() {
             let content = fs::read_to_string(&config_path).unwrap_or_default();
-            serde_json::from_str(&content).unwrap_or_default()
+            let mut config: LauncherConfig = serde_json::from_str(&content).unwrap_or_default();
+            config.sanitize();
+            config
         } else {
             LauncherConfig::default()
         };
@@ -150,7 +164,8 @@ impl ConfigState {
 
     pub fn save(&self) -> Result<(), String> {
         let config = self.config.lock().unwrap();
-        let content = serde_json::to_string_pretty(&*config).map_err(|e| e.to_string())?;
+        let content = serde_json::to_string_pretty(&*config)
+            .map_err(|e| format!("Failed to serialize config: {}", e))?;
         fs::create_dir_all(self.file_path.parent().unwrap()).map_err(|e| e.to_string())?;
         fs::write(&self.file_path, content).map_err(|e| e.to_string())?;
         Ok(())
diff --git a/src-tauri/src/main.rs b/src-tauri/src/main.rs
index 952d250..fec8f11 100644
--- a/src-tauri/src/main.rs
+++ b/src-tauri/src/main.rs
@@ -1646,11 +1646,15 @@ async fn save_raw_config(
     content: String,
 ) -> Result<(), String> {
     // Validate JSON
-    let new_config: core::config::LauncherConfig =
+    let mut new_config: core::config::LauncherConfig =
         serde_json::from_str(&content).map_err(|e| format!("Invalid JSON: {}", e))?;
+    new_config.sanitize();
+
+    let normalized_content = serde_json::to_string_pretty(&new_config)
+        .map_err(|e| format!("Failed to serialize config: {}", e))?;
 
     // Save to file
-    tokio::fs::write(&state.file_path, &content)
+    tokio::fs::write(&state.file_path, &normalized_content)
         .await
         .map_err(|e| e.to_string())?;