diff --git a/src/__tests__/costTracker.test.ts b/src/__tests__/costTracker.test.ts
index fffda47..3ad031b 100644
--- a/src/__tests__/costTracker.test.ts
+++ b/src/__tests__/costTracker.test.ts
@@ -30,15 +30,15 @@ const localStorageMock = (() => {
 vi.stubGlobal('localStorage', localStorageMock);
 
 describe('calculateCostJpy', () => {
-  it('gpt-5-nano のコストを正しく計算する', () => {
+  it('gpt-5.4-nano のコストを正しく計算する', () => {
     // input: 1000 tokens × 10/1M = 0.01, output: 500 tokens × 40/1M = 0.02
-    const cost = calculateCostJpy('gpt-5-nano', 1000, 500);
+    const cost = calculateCostJpy('gpt-5.4-nano', 1000, 500);
     expect(cost).toBeCloseTo(0.03, 4);
   });
 
-  it('gpt-5-mini のコストを正しく計算する', () => {
+  it('gpt-5.4-mini のコストを正しく計算する', () => {
     // input: 1000 × 50/1M = 0.05, output: 500 × 200/1M = 0.10
-    const cost = calculateCostJpy('gpt-5-mini', 1000, 500);
+    const cost = calculateCostJpy('gpt-5.4-mini', 1000, 500);
     expect(cost).toBeCloseTo(0.15, 4);
   });
 
@@ -54,14 +54,14 @@ describe('addUsage / getSessionTotal / getCycleTotal', () => {
 
   it('addUsage でセッション・サイクル累計が増加する', () => {
     const before = getSessionTotal();
-    const cost = addUsage({ modelId: 'gpt-5-nano', promptTokens: 1000, completionTokens: 500 });
+    const cost = addUsage({ modelId: 'gpt-5.4-nano', promptTokens: 1000, completionTokens: 500 });
     expect(cost).toBeGreaterThan(0);
     expect(getSessionTotal()).toBeGreaterThan(before);
     expect(getCycleTotal()).toBeGreaterThan(0);
   });
 
   it('resetCycle でサイクル累計のみリセットされる', () => {
-    addUsage({ modelId: 'gpt-5-nano', promptTokens: 1000, completionTokens: 500 });
+    addUsage({ modelId: 'gpt-5.4-nano', promptTokens: 1000, completionTokens: 500 });
     const sessionBefore = getSessionTotal();
     resetCycle();
     expect(getCycleTotal()).toBe(0);
@@ -83,9 +83,9 @@ describe('checkBudget', () => {
   });
 
   it('サイクル累計が ¥5 超で warn を返す', () => {
-    // サイクル累計を ¥5 超にする: gpt-5-mini × 大量トークン
+    // サイクル累計を ¥5 超にする: gpt-5.4-mini × 大量トークン
     for (let i = 0; i < 50; i++) {
-      addUsage({ modelId: 'gpt-5-mini', promptTokens: 5000, completionTokens: 2000 });
+      addUsage({ modelId: 'gpt-5.4-mini', promptTokens: 5000, completionTokens: 2000 });
     }
     const warning = checkBudget(0.1, true);
     expect(warning).not.toBeNull();
diff --git a/src/__tests__/modelRouter.test.ts b/src/__tests__/modelRouter.test.ts
index 3553848..e830270 100644
--- a/src/__tests__/modelRouter.test.ts
+++ b/src/__tests__/modelRouter.test.ts
@@ -19,43 +19,43 @@ const baseForm: BrainstormForm = {
 describe('selectModel', () => {
   it('手動選択（auto以外）はそのまま返す', () => {
     const input: ModelRouterInput = { taskType: 'generate', depth: 1 };
-    const result = selectModel('gpt-4.1-mini', input, true);
-    expect(result.modelId).toBe('gpt-4.1-mini');
+    const result = selectModel('gpt-5.4-mini', input, true);
+    expect(result.modelId).toBe('gpt-5.4-mini');
     expect(result.reason).toBe('手動選択');
   });
 
-  it('Free mode + auto → 常に gpt-5-nano', () => {
+  it('Free mode + auto → 常に gpt-5.4-nano', () => {
     const input: ModelRouterInput = { taskType: 'generate', depth: 4, form: baseForm };
     const result = selectModel(AUTO_MODEL_ID, input, false);
-    expect(result.modelId).toBe('gpt-5-nano');
+    expect(result.modelId).toBe('gpt-5.4-nano');
   });
 
-  it('Pro + depth 1 + シンプル入力 → gpt-5-nano', () => {
+  it('Pro + depth 1 + シンプル入力 → gpt-5.4-nano', () => {
     const input: ModelRouterInput = { taskType: 'generate', depth: 1, form: baseForm };
     const result = selectModel(AUTO_MODEL_ID, input, true);
-    expect(result.modelId).toBe('gpt-5-nano');
+    expect(result.modelId).toBe('gpt-5.4-nano');
   });
 
-  it('Pro + depth 2 + シンプル入力 → gpt-5-nano', () => {
+  it('Pro + depth 2 + シンプル入力 → gpt-5.4-nano', () => {
     const input: ModelRouterInput = { taskType: 'generate', depth: 2, form: baseForm };
     const result = selectModel(AUTO_MODEL_ID, input, true);
-    expect(result.modelId).toBe('gpt-5-nano');
+    expect(result.modelId).toBe('gpt-5.4-nano');
   });
 
-  it('Pro + depth 3 → gpt-5-mini', () => {
+  it('Pro + depth 3 → gpt-5.4-mini', () => {
     const input: ModelRouterInput = { taskType: 'generate', depth: 3, form: baseForm };
     const result = selectModel(AUTO_MODEL_ID, input, true);
-    expect(result.modelId).toBe('gpt-5-mini');
+    expect(result.modelId).toBe('gpt-5.4-mini');
     expect(result.reason).toContain('深度3');
   });
 
-  it('Pro + depth 4 → gpt-5-mini', () => {
+  it('Pro + depth 4 → gpt-5.4-mini', () => {
     const input: ModelRouterInput = { taskType: 'generate', depth: 4, form: baseForm };
     const result = selectModel(AUTO_MODEL_ID, input, true);
-    expect(result.modelId).toBe('gpt-5-mini');
+    expect(result.modelId).toBe('gpt-5.4-mini');
   });
 
-  it('Pro + 課題3件 + 長い目標 → gpt-5-mini', () => {
+  it('Pro + 課題3件 + 長い目標 → gpt-5.4-mini', () => {
     const form: BrainstormForm = {
       ...baseForm,
       teamGoals:
@@ -68,32 +68,32 @@ describe('selectModel', () => {
     };
     const input: ModelRouterInput = { taskType: 'generate', depth: 2, form };
     const result = selectModel(AUTO_MODEL_ID, input, true);
-    expect(result.modelId).toBe('gpt-5-mini');
+    expect(result.modelId).toBe('gpt-5.4-mini');
     expect(result.reason).toContain('課題');
   });
 
-  it('Pro + 競合データあり → gpt-5-mini', () => {
+  it('Pro + 競合データあり → gpt-5.4-mini', () => {
     const form: BrainstormForm = {
       ...baseForm,
       competitors: [{ name: '競合A', url: 'https://example.com', note: '' }],
     };
     const input: ModelRouterInput = { taskType: 'generate', depth: 1, form };
     const result = selectModel(AUTO_MODEL_ID, input, true);
-    expect(result.modelId).toBe('gpt-5-mini');
+    expect(result.modelId).toBe('gpt-5.4-mini');
     expect(result.reason).toContain('競合');
   });
 
-  it('Pro + KPIデータあり → gpt-5-mini', () => {
+  it('Pro + KPIデータあり → gpt-5.4-mini', () => {
     const form: BrainstormForm = {
       ...baseForm,
       kpis: [{ label: '成約率', value: '15%' }],
     };
     const input: ModelRouterInput = { taskType: 'generate', depth: 1, form };
     const result = selectModel(AUTO_MODEL_ID, input, true);
-    expect(result.modelId).toBe('gpt-5-mini');
+    expect(result.modelId).toBe('gpt-5.4-mini');
   });
 
-  it('Pro + トークン2000超 → gpt-5-mini', () => {
+  it('Pro + トークン2000超 → gpt-5.4-mini', () => {
     const longText = 'あ'.repeat(4100); // 4100文字 / 2 = 2050トークン
     const input: ModelRouterInput = {
       taskType: 'deepDive',
@@ -101,7 +101,7 @@ describe('selectModel', () => {
       messages: [{ role: 'user', content: longText }],
     };
     const result = selectModel(AUTO_MODEL_ID, input, true);
-    expect(result.modelId).toBe('gpt-5-mini');
+    expect(result.modelId).toBe('gpt-5.4-mini');
     expect(result.reason).toContain('トークン');
   });
 });
diff --git a/src/components/layout/HeaderBar.stories.tsx b/src/components/layout/HeaderBar.stories.tsx
index 9bb83d0..834666c 100644
--- a/src/components/layout/HeaderBar.stories.tsx
+++ b/src/components/layout/HeaderBar.stories.tsx
@@ -34,7 +34,7 @@ type Story = StoryObj<typeof meta>;
 export const FreeMode: Story = {
   args: {
     proMode: false,
-    modelLabel: 'Auto',
+    modelLabel: '5.4 Nano',
     connStatus: { status: 'idle', msg: '' },
     isDark: false,
     lastUsedModel: null,
@@ -46,22 +46,22 @@ export const FreeMode: Story = {
 export const ProMode: Story = {
   args: {
     proMode: true,
-    modelLabel: 'Auto',
+    modelLabel: '5.4 Nano',
     connStatus: { status: 'ok', msg: '接続成功' },
     isDark: false,
-    lastUsedModel: 'gpt-5-mini',
+    lastUsedModel: 'gpt-5.4-mini',
     freeRemaining: null,
   },
 };
 
-/** Auto → 5-nano 表示 */
-export const WithAutoResolved: Story = {
+/** 5.4 Nano 表示 */
+export const WithNanoModel: Story = {
   args: {
     proMode: true,
-    modelLabel: 'Auto',
+    modelLabel: '5.4 Nano',
     connStatus: { status: 'ok', msg: '接続成功' },
     isDark: true,
-    lastUsedModel: 'gpt-5-nano',
+    lastUsedModel: 'gpt-5.4-nano',
     freeRemaining: null,
   },
 };
diff --git a/src/components/layout/HeaderBar.tsx b/src/components/layout/HeaderBar.tsx
index 5156929..5b33f95 100644
--- a/src/components/layout/HeaderBar.tsx
+++ b/src/components/layout/HeaderBar.tsx
@@ -121,12 +121,9 @@ export const HeaderBar: React.FC<HeaderBarProps> = ({
         )}
         <div
           className={`flex items-center gap-1.5 px-2.5 py-1 rounded-full text-xs border ${T.cardFlat} ${T.t2}`}
-          title={`使用モデル: ${modelLabel}${lastUsedModel ? ` → ${lastUsedModel}` : ''}`}
+          title={`使用モデル: ${modelLabel}${lastUsedModel && lastUsedModel !== modelLabel ? `（最終: ${lastUsedModel}）` : ''}`}
         >
           <span className={T.accentTxt}>◆</span> {modelLabel}
-          {modelLabel === 'Auto' && lastUsedModel && (
-            <span className={`${T.t3} text-[10px]`}>→ {lastUsedModel.replace('gpt-', '')}</span>
-          )}
           {!proMode &&
             freeRemaining &&
             (() => {
diff --git a/src/components/modals/HelpModal.tsx b/src/components/modals/HelpModal.tsx
index 0151c26..f9a41a0 100644
--- a/src/components/modals/HelpModal.tsx
+++ b/src/components/modals/HelpModal.tsx
@@ -136,9 +136,8 @@ export const HelpModal: React.FC<HelpModalProps> = ({ onClose }) => {
         {/* Cost note */}
         <div className="p-2.5 rounded-lg bg-amber-50 dark:bg-amber-900/15 border border-amber-200 dark:border-amber-700/40">
           <p className="text-xs text-amber-700 dark:text-amber-300">
-            💡 <strong>コストの目安:</strong>{' '}
-            gpt-4.1-nanoなら1回あたり約$0.001〜0.005（〜0.5〜0.8円）。
-            gpt-5-nanoでも1回あたり数円程度です。
+            💡 <strong>コストの目安:</strong> gpt-5.4-nanoなら1回あたり約0.5〜1円程度です。
+            gpt-5.4-miniでも1回あたり数円程度です。
           </p>
         </div>
 
diff --git a/src/components/modals/SettingsModal.stories.tsx b/src/components/modals/SettingsModal.stories.tsx
index b1619ea..5e9a3ae 100644
--- a/src/components/modals/SettingsModal.stories.tsx
+++ b/src/components/modals/SettingsModal.stories.tsx
@@ -25,10 +25,10 @@ const meta = {
 export default meta;
 type Story = StoryObj<typeof meta>;
 
-/** APIキーなし、Autoモデル、残り回数表示 */
+/** APIキーなし、デフォルトモデル、残り回数表示 */
 export const FreeMode: Story = {
   args: {
-    modelId: 'auto',
+    modelId: 'gpt-5.4-nano',
     connStatus: { status: 'idle', msg: '' },
     apiKey: '',
     sessionCost: 0,
@@ -40,11 +40,11 @@ export const FreeMode: Story = {
 /** APIキーあり、コスト表示 */
 export const ProMode: Story = {
   args: {
-    modelId: 'auto',
+    modelId: 'gpt-5.4-nano',
     connStatus: { status: 'ok', msg: '接続成功' },
     apiKey: 'sk-test-key-1234',
     sessionCost: 2.45,
-    lastUsedModel: 'gpt-5-nano',
+    lastUsedModel: 'gpt-5.4-nano',
     freeRemaining: null,
   },
 };
@@ -52,11 +52,11 @@ export const ProMode: Story = {
 /** Free mode 残り少ない */
 export const WithRateLimit: Story = {
   args: {
-    modelId: 'auto',
+    modelId: 'gpt-5.4-nano',
     connStatus: { status: 'idle', msg: '' },
     apiKey: '',
     sessionCost: 0,
-    lastUsedModel: 'gpt-5-nano',
+    lastUsedModel: 'gpt-5.4-nano',
     freeRemaining: { remaining: 3, limit: 50, resetAt: Date.now() + 3600_000 },
   },
 };
diff --git a/src/components/modals/SettingsModal.tsx b/src/components/modals/SettingsModal.tsx
index 3278170..249d35b 100644
--- a/src/components/modals/SettingsModal.tsx
+++ b/src/components/modals/SettingsModal.tsx
@@ -15,7 +15,7 @@ import {
 } from 'lucide-react';
 import {
   MODELS,
-  AUTO_MODEL_ID,
+  DEFAULT_MODEL_ID,
   isProMode,
   isLocalProvider,
   PROVIDER_DEFAULTS,
@@ -193,12 +193,16 @@ export const SettingsModal: React.FC<SettingsModalProps> = ({
                 </button>
               </div>
             </div>
-            {isProMode(apiKey) && (
+            {isProMode(apiKey) ? (
               <p className="mt-1 text-xs text-emerald-600 dark:text-emerald-400">
                 ✓ プロモード有効 — フル機能・高深度分析が使えます
               </p>
-            )}
-            {!isProMode(apiKey) && (
+            ) : apiKey.trim().length > 0 ? (
+              <p className="mt-1 text-xs text-amber-600 dark:text-amber-400">
+                ⚠ APIキーの形式が正しくありません（sk-...
+                で始まる20文字以上のキーを入力してください）
+              </p>
+            ) : (
               <p className={`mt-1 text-xs ${T.t3}`}>
                 入力するとプロモードに切り替わり、全機能が使えます
               </p>
@@ -271,24 +275,21 @@ export const SettingsModal: React.FC<SettingsModalProps> = ({
             <p className={`text-xs font-medium ${T.t2} mb-1.5`}>モデル</p>
             <div className="flex gap-1.5 flex-wrap">
               {MODELS.map((m) => {
-                const isAuto = m.id === AUTO_MODEL_ID;
+                const isProOnly = m.id !== DEFAULT_MODEL_ID;
+                const locked = isProOnly && !proMode;
                 const isSelected = modelId === m.id;
-                let cls: string;
-                if (isSelected && isAuto) {
-                  cls =
-                    'bg-emerald-600 dark:bg-emerald-700 border-emerald-500 dark:border-emerald-500 text-white font-medium';
-                } else if (isSelected) {
-                  cls =
-                    'bg-slate-800 dark:bg-slate-700 border-slate-600 dark:border-slate-500 text-slate-100 font-medium';
-                } else if (isAuto) {
-                  cls = `${T.btnGhost} border-emerald-200 dark:border-emerald-700/60`;
-                } else {
-                  cls = `${T.btnGhost} border-slate-200 dark:border-slate-700/60`;
-                }
+                const cls = locked
+                  ? 'opacity-50 cursor-not-allowed border-slate-200 dark:border-slate-700/60 bg-slate-100 dark:bg-slate-800/40 text-slate-400 dark:text-slate-500'
+                  : isSelected
+                    ? 'bg-slate-800 dark:bg-slate-700 border-slate-600 dark:border-slate-500 text-slate-100 font-medium'
+                    : `${T.btnGhost} border-slate-200 dark:border-slate-700/60`;
                 return (
                   <button
                     key={m.id}
+                    disabled={locked}
+                    title={locked ? 'APIキーを入力するとProモードで利用できます' : undefined}
                     onClick={() => {
+                      if (locked) return;
                       setModelId(m.id);
                       setConnStatus({ status: 'idle', msg: '' });
                     }}
@@ -298,16 +299,15 @@ export const SettingsModal: React.FC<SettingsModalProps> = ({
                     <span className={isSelected ? 'opacity-70 font-normal' : `${T.t3} font-normal`}>
                       {m.cost} · {m.t}
                     </span>
+                    {locked && <span className="ml-1 text-[9px] opacity-60">Pro</span>}
                   </button>
                 );
               })}
             </div>
-            <p
-              className={`mt-1.5 text-[10px] ${modelId === AUTO_MODEL_ID ? 'text-emerald-600 dark:text-emerald-400' : T.t3}`}
-            >
-              {modelId === AUTO_MODEL_ID
-                ? 'タスクに応じて最適なモデルを自動選択します（おすすめ）'
-                : '迷ったら「Auto」がおすすめです'}
+            <p className={`mt-1.5 text-[10px] ${T.t3}`}>
+              {proMode
+                ? '選択したモデルは自動で保存されます'
+                : 'APIキーを入力すると上位モデルが選択可能になります'}
             </p>
             {(sessionCost > 0 || lastUsedModel) && (
               <div className={`mt-2 flex items-center gap-3 text-[10px] ${T.t3}`}>
diff --git a/src/components/results/RichText.tsx b/src/components/results/RichText.tsx
index c5c1d94..0b3b537 100644
--- a/src/components/results/RichText.tsx
+++ b/src/components/results/RichText.tsx
@@ -79,7 +79,7 @@ export const RichText: React.FC<RichTextProps> = React.memo(({ text }) => {
     const hdr = tableRows[0],
       body = tableRows.slice(2);
     elements.push(
-      <div key={`tbl-${i}`} className="overflow-x-auto my-2">
+      <div key={`tbl-${i}`} className="overflow-x-auto my-4">
         <table className="w-full text-xs border-collapse">
           <thead>
             <tr>
@@ -89,7 +89,7 @@ export const RichText: React.FC<RichTextProps> = React.memo(({ text }) => {
                 .map((c, ci) => (
                   <th
                     key={ci}
-                    className="px-2 py-1.5 text-left bg-slate-100 dark:bg-slate-800/60 border border-slate-200 dark:border-slate-700/60 text-slate-700 dark:text-slate-300 font-semibold"
+                    className="px-3 py-2 text-left bg-slate-100 dark:bg-slate-800/60 border border-slate-200 dark:border-slate-700/60 text-slate-700 dark:text-slate-300 font-semibold"
                   >
                     {c.trim()}
                   </th>
@@ -105,7 +105,7 @@ export const RichText: React.FC<RichTextProps> = React.memo(({ text }) => {
                   .map((c, ci) => (
                     <td
                       key={ci}
-                      className="px-2 py-1 border border-slate-200 dark:border-slate-700/60 text-slate-600 dark:text-slate-400"
+                      className="px-3 py-1.5 border border-slate-200 dark:border-slate-700/60 text-slate-600 dark:text-slate-400"
                     >
                       {ri(c.trim())}
                     </td>
@@ -134,7 +134,7 @@ export const RichText: React.FC<RichTextProps> = React.memo(({ text }) => {
     }
     if (ln.startsWith('### '))
       elements.push(
-        <h4 key={i} className="text-sm font-semibold text-slate-800 dark:text-slate-200 mt-3 mb-1">
+        <h4 key={i} className="text-sm font-semibold text-slate-800 dark:text-slate-200 mt-5 mb-2">
           {ri(ln.slice(4))}
         </h4>,
       );
@@ -142,24 +142,24 @@ export const RichText: React.FC<RichTextProps> = React.memo(({ text }) => {
       elements.push(
         <h3
           key={i}
-          className="text-base font-bold text-slate-900 dark:text-slate-100 mt-4 mb-1.5 border-b border-slate-200 dark:border-slate-700/60 pb-1"
+          className="text-base font-bold text-slate-900 dark:text-slate-100 mt-6 mb-2.5 border-b border-slate-200 dark:border-slate-700/60 pb-1.5"
         >
           {ri(ln.slice(3))}
         </h3>,
       );
     else if (ln.startsWith('# '))
       elements.push(
-        <h2 key={i} className="text-lg font-bold text-slate-900 dark:text-slate-100 mt-4 mb-2">
+        <h2 key={i} className="text-lg font-bold text-slate-900 dark:text-slate-100 mt-6 mb-3">
           {ri(ln.slice(2))}
         </h2>,
       );
     else if (ln.startsWith('---'))
-      elements.push(<hr key={i} className="border-slate-200 dark:border-slate-700/60 my-3" />);
+      elements.push(<hr key={i} className="border-slate-200 dark:border-slate-700/60 my-4" />);
     else if (ln.match(/^[-*]\s/))
       elements.push(
         <div
           key={i}
-          className="flex gap-2 text-sm text-slate-600 dark:text-slate-400 leading-relaxed ml-2"
+          className="flex gap-2 text-sm text-slate-600 dark:text-slate-400 leading-relaxed ml-3"
         >
           <span className="text-slate-400 dark:text-slate-500 shrink-0 mt-0.5">•</span>
           <span>{ri(ln.replace(/^[-*]\s/, ''))}</span>
@@ -169,7 +169,7 @@ export const RichText: React.FC<RichTextProps> = React.memo(({ text }) => {
       elements.push(
         <div
           key={i}
-          className="flex gap-2 text-sm text-slate-600 dark:text-slate-400 leading-relaxed ml-2"
+          className="flex gap-2 text-sm text-slate-600 dark:text-slate-400 leading-relaxed ml-3"
         >
           <span className="text-slate-400 dark:text-slate-500 shrink-0 font-medium">
             {ln.match(/^(\d+)\./)?.[1] || ''}.
@@ -177,7 +177,7 @@ export const RichText: React.FC<RichTextProps> = React.memo(({ text }) => {
           <span>{ri(ln.replace(/^\d+\.\s/, ''))}</span>
         </div>,
       );
-    else if (ln.trim() === '') elements.push(<div key={i} className="h-1.5" />);
+    else if (ln.trim() === '') elements.push(<div key={i} className="h-3" />);
     else
       elements.push(
         <p key={i} className="text-sm text-slate-600 dark:text-slate-400 leading-relaxed">
@@ -187,5 +187,5 @@ export const RichText: React.FC<RichTextProps> = React.memo(({ text }) => {
     i++;
   }
   if (inTable) flushTable();
-  return <div className="space-y-0.5">{elements}</div>;
+  return <div className="space-y-1.5">{elements}</div>;
 });
diff --git a/src/components/support/SupportAIChat.tsx b/src/components/support/SupportAIChat.tsx
index 46552e8..9458e66 100644
--- a/src/components/support/SupportAIChat.tsx
+++ b/src/components/support/SupportAIChat.tsx
@@ -4,7 +4,7 @@ import { T } from '../../constants/theme';
 import { callAI, callAIWithKey, isProMode } from '../../constants/models';
 import { SUPPORT_SYSTEM_PROMPT } from './supportData';
 
-const SUPPORT_MODEL = 'gpt-4.1-nano';
+const SUPPORT_MODEL = 'gpt-5.4-nano';
 
 interface Message {
   role: 'user' | 'assistant';
diff --git a/src/components/support/supportData.ts b/src/components/support/supportData.ts
index 755df28..0cf4b8d 100644
--- a/src/components/support/supportData.ts
+++ b/src/components/support/supportData.ts
@@ -63,7 +63,7 @@ export const FAQ_CATEGORIES: FAQCategory[] = [
       },
       {
         q: 'APIキーの利用料金は？',
-        a: 'gpt-4.1-nanoなら1回あたり約0.5〜0.8円、gpt-5-nanoでも数円程度です。通常の利用であれば月数百円程度に収まります。',
+        a: 'gpt-5.4-nanoなら1回あたり約0.5〜1円程度です。通常の利用であれば月数百円程度に収まります。',
       },
     ],
   },
@@ -162,8 +162,8 @@ HR SaaS、DX推進、新規事業、マーケ戦略、オペ改革など多業
 
 【設定】
 ・APIキー: ヘッダー右端の歯車アイコンから設定。sk-...形式のOpenAI APIキー
-・モデル選択: gpt-5-nano(最速), gpt-5-mini(バランス), gpt-4.1-mini(コスパ), gpt-4.1-nano(最安)
-・API料金目安: gpt-4.1-nanoで1回0.5〜0.8円、gpt-5-nanoで数円程度
+・モデル選択: gpt-5.4-nano(最速・低コスト), gpt-5.4-mini(高精度)
+・API料金目安: gpt-5.4-nanoで1回約0.5〜1円程度
 
 【UI機能】
 ・レイアウト: デスクトップでは左右パネルをドラッグまたはプリセット(30:70/40:60/50:50)で調整
diff --git a/src/constants/models.ts b/src/constants/models.ts
index 4f2c59d..841fe4e 100644
--- a/src/constants/models.ts
+++ b/src/constants/models.ts
@@ -1,7 +1,10 @@
 import { ModelInfo, ChatMessage, LLMProvider } from '../types';
 
-/** APIキーがPro mode（ユーザー所有）か判定 */
-export const isProMode = (apiKey: string): boolean => apiKey.trim().startsWith('sk-');
+/** APIキーがPro mode（ユーザー所有）か判定。OpenAI キーは sk- で始まり 20 文字以上 */
+export const isProMode = (apiKey: string): boolean => {
+  const k = apiKey.trim();
+  return k.startsWith('sk-') && k.length >= 20;
+};
 
 const friendlyError = (status: number, body: string): string => {
   if (status === 429)
@@ -30,7 +33,8 @@ const friendlyError = (status: number, body: string): string => {
 };
 
 export const AUTO_MODEL_ID = 'auto';
-export const DEFAULT_MODEL_ID = AUTO_MODEL_ID;
+export const DEFAULT_MODEL_ID = 'gpt-5.4-nano';
+export const MODEL_STORAGE_KEY = 'ai-brainstorm-model';
 
 const API_ENDPOINT = '/api/openai';
 
@@ -52,19 +56,14 @@ export const PROVIDER_DEFAULTS: Record<
 export const isLocalProvider = (p: LLMProvider): boolean => p !== 'openai';
 
 export const MODELS: ModelInfo[] = [
-  { id: AUTO_MODEL_ID, label: 'Auto', t: '自動選択', cost: '$~$$' },
-  { id: 'gpt-5-nano', label: '5 Nano', t: '最速', cost: '$' },
-  { id: 'gpt-5-mini', label: '5 Mini', t: 'バランス', cost: '$$' },
-  { id: 'gpt-4.1-mini', label: '4.1 Mini', t: 'コスパ', cost: '$' },
-  { id: 'gpt-4.1-nano', label: '4.1 Nano', t: '最安', cost: '$' },
+  { id: 'gpt-5.4-nano', label: '5.4 Nano', t: '最速・低コスト', cost: '$' },
+  { id: 'gpt-5.4-mini', label: '5.4 Mini', t: '高精度', cost: '$$' },
 ];
 
 /** モデル別コスト単価 (JPY / 1M tokens) */
 export const MODEL_COSTS: Record<string, { inputPerM: number; outputPerM: number }> = {
-  'gpt-5-nano': { inputPerM: 10, outputPerM: 40 },
-  'gpt-5-mini': { inputPerM: 50, outputPerM: 200 },
-  'gpt-4.1-mini': { inputPerM: 40, outputPerM: 160 },
-  'gpt-4.1-nano': { inputPerM: 10, outputPerM: 45 },
+  'gpt-5.4-nano': { inputPerM: 10, outputPerM: 40 },
+  'gpt-5.4-mini': { inputPerM: 50, outputPerM: 200 },
 };
 
 /** API呼び出し結果（usage トークン数 + rate limit 情報を含む） */
@@ -75,7 +74,7 @@ export interface APICallResult {
 }
 
 export const testConn = async (modelId: string, apiKey = ''): Promise<string> => {
-  const resolvedId = modelId === AUTO_MODEL_ID ? 'gpt-5-nano' : modelId;
+  const resolvedId = modelId === AUTO_MODEL_ID ? DEFAULT_MODEL_ID : modelId;
   const usesCompletionTokens = resolvedId.startsWith('gpt-5') || resolvedId.startsWith('o');
   const tokenParam = usesCompletionTokens ? { max_completion_tokens: 100 } : { max_tokens: 100 };
   const headers: Record<string, string> = { 'Content-Type': 'application/json' };
diff --git a/src/hooks/useAI.ts b/src/hooks/useAI.ts
index 62b3fa6..554e50c 100644
--- a/src/hooks/useAI.ts
+++ b/src/hooks/useAI.ts
@@ -9,6 +9,8 @@ import {
   testConn,
   testConnLocal,
   DEFAULT_MODEL_ID,
+  MODEL_STORAGE_KEY,
+  MODELS,
   isProMode,
   isLocalProvider,
 } from '../constants/models';
@@ -52,7 +54,23 @@ function buildCompetitiveIntelContext(form: BrainstormForm): string {
 }
 
 export const useAI = () => {
-  const [modelId, setModelId] = useState(DEFAULT_MODEL_ID);
+  const [modelId, setModelIdState] = useState(() => {
+    try {
+      const saved = localStorage.getItem(MODEL_STORAGE_KEY);
+      if (saved && MODELS.some((m) => m.id === saved)) return saved;
+    } catch {
+      /* ignore */
+    }
+    return DEFAULT_MODEL_ID;
+  });
+  const setModelId = useCallback((id: string) => {
+    setModelIdState(id);
+    try {
+      localStorage.setItem(MODEL_STORAGE_KEY, id);
+    } catch {
+      /* ignore */
+    }
+  }, []);
   const [connStatus, setConnStatus] = useState<ConnStatus>({ status: 'idle', msg: '' });
 
   const [loading, setLoading] = useState(false);
@@ -130,7 +148,9 @@ export const useAI = () => {
   ): Promise<{ content: string; resolvedModel: string }> => {
     const isLocal = isLocalProvider(provider);
     const pro = isLocal || isProMode(apiKey);
-    const currentModel = isLocal ? localModel || modelId : modelId;
+    // Free モードでは nano に強制（mini は Pro 専用）
+    const rawModel = isLocal ? localModel || modelId : modelId;
+    const currentModel = !pro && rawModel !== DEFAULT_MODEL_ID ? DEFAULT_MODEL_ID : rawModel;
     const decision = selectModel(currentModel, routerInput, pro, isLocal);
     const resolvedId = decision.modelId;
 
diff --git a/src/utils/modelRouter.ts b/src/utils/modelRouter.ts
index 2b94bc4..4a2461b 100644
--- a/src/utils/modelRouter.ts
+++ b/src/utils/modelRouter.ts
@@ -20,8 +20,8 @@ export function estimateTokens(messages: ChatMessage[]): number {
   return messages.reduce((sum, m) => sum + Math.ceil(m.content.length / 2), 0);
 }
 
-const NANO = 'gpt-5-nano';
-const MINI = 'gpt-5-mini';
+const NANO = 'gpt-5.4-nano';
+const MINI = 'gpt-5.4-mini';
 
 /**
  * Auto モード時のモデル自動選択
diff --git a/vite.config.ts b/vite.config.ts
index bbc3e95..1d77510 100644
--- a/vite.config.ts
+++ b/vite.config.ts
@@ -19,8 +19,14 @@ export default defineConfig(({ mode }) => {
           rewrite: (path) => path.replace(/^\/api\/openai/, '/v1/chat/completions'),
           configure: (proxy) => {
             proxy.on('proxyReq', (proxyReq) => {
-              if (env.OPENAI_API_KEY)
+              // Pro モード: ユーザーの API キーを優先
+              const userKey = proxyReq.getHeader('x-api-key') as string | undefined;
+              if (userKey) {
+                proxyReq.setHeader('Authorization', `Bearer ${userKey}`);
+                proxyReq.removeHeader('x-api-key');
+              } else if (env.OPENAI_API_KEY) {
                 proxyReq.setHeader('Authorization', `Bearer ${env.OPENAI_API_KEY}`);
+              }
             });
           },
         },