diff --git a/package.json b/package.json
index 5912f7cc8..f1a5eb274 100644
--- a/package.json
+++ b/package.json
@@ -8,7 +8,7 @@
     "deploy": "npm run setup-kv && wrangler deploy",
     "dev": "wrangler dev",
     "start": "wrangler dev",
-    "test": "vitest",
+    "test": "vitest run",
     "setup-kv": "node scripts/setup-kv.cjs",
     "build:node": "node_modules/.bin/esbuild src/platforms/node-server.js --bundle --platform=node --target=node18 --format=cjs --outfile=dist/node-server.cjs",
     "dev:node": "npm run build:node && node dist/node-server.cjs"
diff --git a/src/builders/ClashConfigBuilder.js b/src/builders/ClashConfigBuilder.js
index d4222dbfe..7f520e628 100644
--- a/src/builders/ClashConfigBuilder.js
+++ b/src/builders/ClashConfigBuilder.js
@@ -6,6 +6,7 @@ import { addProxyWithDedup } from './helpers/proxyHelpers.js';
 import { buildSelectorMembers, buildNodeSelectMembers, buildCustomRuleMembers, uniqueNames } from './helpers/groupBuilder.js';
 import { emitClashRules, sanitizeClashProxyGroups } from './helpers/clashConfigUtils.js';
 import { normalizeGroupName, findGroupIndexByName } from './helpers/groupNameUtils.js';
+import { InvalidConfigError } from '../services/errors.js';
 
 /**
  * Check if the client supports MRS (Meta Rule Set) format
@@ -600,11 +601,7 @@ export class ClashConfigBuilder extends BaseConfigBuilder {
                     newGroup.use = newGroup.use.filter(p => allProviderNames.has(p));
                 }
 
-                // Add group if:
-                // 1. Has valid proxies or use, OR
-                // 2. Is url-test/fallback type (will be filled by validateProxyGroups)
-                const isAutoFillableType = newGroup.type === 'url-test' || newGroup.type === 'fallback';
-                if ((newGroup.proxies?.length > 0) || (newGroup.use?.length > 0) || isAutoFillableType) {
+                if ((newGroup.proxies?.length > 0) || (newGroup.use?.length > 0) || newGroup.type) {
                     this.config['proxy-groups'].push(newGroup);
                 }
             }
@@ -612,25 +609,27 @@ export class ClashConfigBuilder extends BaseConfigBuilder {
     }
 
     /**
-     * Validate proxy groups before final output
-     * Ensures url-test/fallback groups have proxies, fills empty ones with all nodes
+     * Reject invalid proxy groups before final output.
+     * Why: empty groups make Clash reject the whole config, so we should fail fast
+     * instead of masking the upstream merge/parsing problem.
      */
     validateProxyGroups() {
-        const proxyList = this.getProxyList();
-        const providerNames = this.getAllProviderNames();
-
         (this.config['proxy-groups'] || []).forEach(group => {
-            // For url-test/fallback groups, ensure they have proxies or providers
-            if ((group.type === 'url-test' || group.type === 'fallback') &&
-                (!group.proxies || group.proxies.length === 0) &&
-                (!group.use || group.use.length === 0)) {
-                // Fill with all available proxies
-                group.proxies = [...proxyList];
-                // Also use all providers if available
-                if (providerNames.length > 0) {
-                    group.use = [...providerNames];
-                }
+            const requiresMembers = group?.type === 'url-test' || group?.type === 'fallback';
+            if (!requiresMembers) {
+                return;
+            }
+
+            const hasProxyRefs = Array.isArray(group.proxies) && group.proxies.length > 0;
+            const hasProviderRefs = Array.isArray(group.use) && group.use.length > 0;
+            if (hasProxyRefs || hasProviderRefs) {
+                return;
             }
+
+            const groupName = group?.name || '(unnamed group)';
+            throw new InvalidConfigError(
+                `Invalid proxy group "${groupName}": type "${group.type}" requires at least one proxy or provider reference`
+            );
         });
     }
 
@@ -657,10 +656,8 @@ export class ClashConfigBuilder extends BaseConfigBuilder {
             };
         }
 
-        // Validate proxy groups: fill empty url-test/fallback groups with all proxies
-        this.validateProxyGroups();
-
         sanitizeClashProxyGroups(this.config);
+        this.validateProxyGroups();
 
         this.config.rules = [
             ...ruleResults,
diff --git a/src/services/errors.js b/src/services/errors.js
index 8288603a0..1b27c692e 100644
--- a/src/services/errors.js
+++ b/src/services/errors.js
@@ -19,3 +19,10 @@ export class InvalidPayloadError extends ServiceError {
         this.name = 'InvalidPayloadError';
     }
 }
+
+export class InvalidConfigError extends ServiceError {
+    constructor(message = 'Invalid config') {
+        super(message, 400);
+        this.name = 'InvalidConfigError';
+    }
+}
diff --git a/test/proxy-groups-override.test.js b/test/proxy-groups-override.test.js
index d90d3bfa0..3a3eafc02 100644
--- a/test/proxy-groups-override.test.js
+++ b/test/proxy-groups-override.test.js
@@ -300,7 +300,7 @@ proxy-groups:
             expect(autoGroups[0].url).toBe('http://custom.test/204');
         });
 
-        it('should fill empty url-test proxies with all available nodes', async () => {
+        it('should reject empty url-test groups instead of silently filling them', async () => {
             const inputWithEmptyProxies = `
 proxies:
   - name: Node-A
@@ -321,15 +321,9 @@ proxy-groups:
     proxies: []
 `;
             const builder = new ClashConfigBuilder(inputWithEmptyProxies, 'minimal', [], null, 'zh-CN', 'test-agent');
-            const yamlText = await builder.build();
-            const config = yaml.load(yamlText);
-
-            const emptyGroup = config['proxy-groups'].find(g => g.name === 'Empty Test Group');
-            expect(emptyGroup).toBeDefined();
-            // Empty group should be filled with all available proxies
-            expect(emptyGroup.proxies.length).toBeGreaterThan(0);
-            expect(emptyGroup.proxies).toContain('Node-A');
-            expect(emptyGroup.proxies).toContain('Node-B');
+            await expect(builder.build()).rejects.toThrow(
+                'Invalid proxy group "Empty Test Group": type "url-test" requires at least one proxy or provider reference'
+            );
         });
 
         it('should filter out invalid proxy references from user groups', async () => {
diff --git a/test/worker.test.js b/test/worker.test.js
index 5f097b20c..e90ce6f35 100644
--- a/test/worker.test.js
+++ b/test/worker.test.js
@@ -77,6 +77,29 @@ describe('Worker', () => {
         expect(text).toContain('proxies:');
     });
 
+    it('GET /clash rejects empty url-test proxy groups with a diagnostic error', async () => {
+        const app = createTestApp();
+        const config = `
+proxies:
+  - name: Node-A
+    type: ss
+    server: a.example.com
+    port: 443
+    cipher: aes-128-gcm
+    password: test
+proxy-groups:
+  - name: Empty Test Group
+    type: url-test
+    proxies: []
+`;
+        const res = await app.request(`http://localhost/clash?config=${encodeURIComponent(config)}`);
+
+        expect(res.status).toBe(400);
+        const text = await res.text();
+        expect(text).toContain('Invalid proxy group "Empty Test Group"');
+        expect(text).toContain('requires at least one proxy or provider reference');
+    });
+
     it('GET /shorten-v2 returns short code', async () => {
         const url = 'http://example.com';
         const kvMock = {