diff --git a/ui/server/services/pilotdeckConfig.js b/ui/server/services/pilotdeckConfig.js
index 430dcbbd..77152d54 100644
--- a/ui/server/services/pilotdeckConfig.js
+++ b/ui/server/services/pilotdeckConfig.js
@@ -42,6 +42,13 @@ function normalizeString(value) {
   return typeof value === 'string' ? value.trim() : '';
 }
 
+function describeModelRef(value) {
+  if (value === undefined) return 'undefined';
+  if (value === null) return 'null';
+  if (typeof value === 'object') return JSON.stringify(value);
+  return String(value);
+}
+
 function deepMerge(base, override) {
   if (!isRecord(base)) return clone(override);
   const output = clone(base);
@@ -181,6 +188,10 @@ function validateProvider(id, provider, errors) {
 }
 
 function validateModelRef(config, ref, label, errors) {
+  if (ref !== undefined && ref !== null && typeof ref !== 'string') {
+    errors.push(`${label} must be a provider/model string; got ${describeModelRef(ref)}`);
+    return;
+  }
   const modelRef = normalizeString(ref);
   if (!modelRef) return;
   if (!resolveModel(config, modelRef, { allowMissing: true })) {
@@ -462,8 +473,6 @@ export function syncAgentModelWithRouter(config) {
   if (!agentRef) return config;
   const slash = agentRef.indexOf('/');
   if (slash <= 0 || slash >= agentRef.length - 1) return config;
-  const providerId = agentRef.slice(0, slash);
-  const modelId = agentRef.slice(slash + 1);
 
   if (!isRecord(config.router)) return config;
   if (!isRecord(config.router.scenarios)) return config;
@@ -473,9 +482,7 @@ export function syncAgentModelWithRouter(config) {
     ? currentDefault.trim()
     : (isRecord(currentDefault) ? normalizeString(currentDefault.id) : '');
   if (currentId === agentRef) return config;
-  config.router.scenarios.default = typeof currentDefault === 'string'
-    ? agentRef
-    : { id: agentRef, provider: providerId, model: modelId };
+  config.router.scenarios.default = agentRef;
   return config;
 }
 
diff --git a/ui/server/services/pilotdeckConfig.router-model-ref.test.js b/ui/server/services/pilotdeckConfig.router-model-ref.test.js
new file mode 100644
index 00000000..11607f30
--- /dev/null
+++ b/ui/server/services/pilotdeckConfig.router-model-ref.test.js
@@ -0,0 +1,45 @@
+import { describe, expect, it } from 'vitest';
+import { syncAgentModelWithRouter, validatePilotDeckConfig } from './pilotdeckConfig.js';
+
+function configWithRouter(defaultRef) {
+  return {
+    schemaVersion: 1,
+    agent: { model: 'main/gpt-test' },
+    model: {
+      providers: {
+        main: {
+          protocol: 'openai',
+          url: 'https://example.com/v1',
+          apiKey: 'sk-test',
+          models: {
+            'gpt-test': {},
+          },
+        },
+      },
+    },
+    router: {
+      scenarios: {
+        default: defaultRef,
+      },
+    },
+  };
+}
+
+describe('router model references', () => {
+  it('rejects object-shaped router model refs', () => {
+    const validation = validatePilotDeckConfig(
+      configWithRouter({ id: 'main/gpt-test', provider: 'main', model: 'gpt-test' }),
+    );
+
+    expect(validation.valid).toBe(false);
+    expect(validation.errors[0]).toContain('router.scenarios.default must be a provider/model string');
+  });
+
+  it('syncs router.scenarios.default as a provider/model string', () => {
+    const config = configWithRouter({ id: 'old/model', provider: 'old', model: 'model' });
+
+    syncAgentModelWithRouter(config);
+
+    expect(config.router.scenarios.default).toBe('main/gpt-test');
+  });
+});