diff --git a/frontend/src/pages/ChatView.tsx b/frontend/src/pages/ChatView.tsx
index 3d191e42..45a0a05c 100644
--- a/frontend/src/pages/ChatView.tsx
+++ b/frontend/src/pages/ChatView.tsx
@@ -223,9 +223,11 @@ export function ChatView() {
           onChange={(e) => setModel(e.target.value)}
           disabled={messages.running}
         >
+          <option value="claude-fable-5">Fable 5</option>
           <option value="claude-opus-4-7">Opus 4.7</option>
           <option value="claude-opus-4-7:max">Opus 4.7 Max</option>
           <option value="claude-opus-4-6">Opus 4.6</option>
+          <option value="claude-sonnet-5">Sonnet 5</option>
           <option value="claude-sonnet-4-6">Sonnet 4.6</option>
           <option value="claude-haiku-4-5">Haiku 4.5</option>
         </select>
diff --git a/frontend/src/pages/DesktopChatView.tsx b/frontend/src/pages/DesktopChatView.tsx
index 1e85533b..a201ef1e 100644
--- a/frontend/src/pages/DesktopChatView.tsx
+++ b/frontend/src/pages/DesktopChatView.tsx
@@ -198,9 +198,11 @@ export function DesktopChatView() {
               onChange={(e) => setModel(e.target.value)}
               disabled={messages.running}
             >
+              <option value="claude-fable-5">Fable 5</option>
               <option value="claude-opus-4-7">Opus 4.7</option>
               <option value="claude-opus-4-7:max">Opus 4.7 Max</option>
               <option value="claude-opus-4-6">Opus 4.6</option>
+              <option value="claude-sonnet-5">Sonnet 5</option>
               <option value="claude-sonnet-4-6">Sonnet 4.6</option>
               <option value="claude-haiku-4-5">Haiku 4.5</option>
             </select>
diff --git a/packages/harness/src/providers/anthropic-vertex.ts b/packages/harness/src/providers/anthropic-vertex.ts
index 17066abe..ef517961 100644
--- a/packages/harness/src/providers/anthropic-vertex.ts
+++ b/packages/harness/src/providers/anthropic-vertex.ts
@@ -16,6 +16,7 @@ const log = createLogger('provider:anthropic');
 /** Vertex AI model name mapping (Vertex uses different naming). */
 const VERTEX_MODEL_MAP: Record<string, string> = {
   'claude-opus-4-6': 'claude-opus-4-6@20250514',
+  'claude-sonnet-5': 'claude-sonnet-5',
   'claude-sonnet-4-6': 'claude-sonnet-4-6@20250514',
   'claude-haiku-4-5': 'claude-3-5-haiku@20241022',
 };
diff --git a/server/__tests__/model-spec.test.ts b/server/__tests__/model-spec.test.ts
index 095ec4e5..7473cd0b 100644
--- a/server/__tests__/model-spec.test.ts
+++ b/server/__tests__/model-spec.test.ts
@@ -62,4 +62,15 @@ describe('resolveThinking', () => {
   it('returns undefined for haiku', () => {
     expect(resolveThinking('claude-haiku-4-5')).toBeUndefined();
   });
+
+  it('returns undefined for fable (always-on thinking, must not pass disabled)', () => {
+    expect(resolveThinking('claude-fable-5')).toBeUndefined();
+  });
+
+  it('returns 10k budget for sonnet 5', () => {
+    expect(resolveThinking('claude-sonnet-5')).toEqual({
+      type: 'enabled',
+      budgetTokens: 10_000,
+    });
+  });
 });
diff --git a/server/chat.ts b/server/chat.ts
index 50579b77..3d668ae6 100644
--- a/server/chat.ts
+++ b/server/chat.ts
@@ -365,10 +365,14 @@ export function getRepoConfig() {
   return _cachedConfig;
 }
 
+export const FALLBACK_MODEL = 'claude-opus-4-6';
+
 export const AVAILABLE_MODELS = [
+  { id: 'claude-fable-5', label: 'Fable 5', desc: 'Most capable (direct API only)' },
   { id: 'claude-opus-4-7', label: 'Opus 4.7', desc: 'Adaptive thinking' },
   { id: 'claude-opus-4-7:max', label: 'Opus 4.7 Max', desc: 'Max thinking (128k)' },
   { id: 'claude-opus-4-6', label: 'Opus 4.6', desc: 'Previous Opus' },
+  { id: 'claude-sonnet-5', label: 'Sonnet 5', desc: 'Latest Sonnet' },
   { id: 'claude-sonnet-4-6', label: 'Sonnet 4.6', desc: 'Balanced' },
   { id: 'claude-haiku-4-5', label: 'Haiku 4.5', desc: 'Fastest' },
 ];
@@ -437,6 +441,8 @@ export function resolveThinking(
   spec?: string,
 ): { type: 'adaptive' } | { type: 'enabled'; budgetTokens: number } | undefined {
   const { model, effort } = parseModelSpec(spec);
+  // Fable: thinking is always on, omit the param to let it default (explicit 'disabled' returns 400)
+  if (model.includes('fable')) return undefined;
   if (model.includes('opus') && effort === 'max') return { type: 'enabled', budgetTokens: 128_000 };
   if (!model || model.includes('opus')) return { type: 'adaptive' };
   if (model.includes('sonnet')) return { type: 'enabled', budgetTokens: 10_000 };
@@ -746,7 +752,7 @@ export async function startChat(
     telosTaskId?: string;
     agentName?: string;
   },
-) {
+): Promise<void> {
   return withSpanAsync(
     'chat.start',
     {
@@ -1082,12 +1088,37 @@ async function _startChatInner(
     );
   } catch (err: unknown) {
     const message = err instanceof Error ? err.message : 'Unknown error';
+    const isModelError =
+      message.includes('not found') ||
+      message.includes('404') ||
+      message.includes('not have access') ||
+      message.includes('disallowed');
+    const requestedModel = parseModelSpec(options.model).model;
+    const canFallback = isModelError && requestedModel && requestedModel !== FALLBACK_MODEL;
+
     if (message.includes('No conversation found') && options.resume) {
       log.warn('SDK rejected resume, session expired', { sessionId: options.resume, cwd });
       send(transport, {
         type: 'error',
         error: 'Session expired. Send your message again to start fresh.',
       });
+    } else if (canFallback) {
+      log.warn('Model unavailable, falling back', {
+        requested: requestedModel,
+        fallback: FALLBACK_MODEL,
+        error: message,
+      });
+      send(transport, {
+        type: 'system',
+        text: `Model "${requestedModel}" unavailable, falling back to ${FALLBACK_MODEL}.`,
+      });
+      const failedSession = registry.get(clientId);
+      if (failedSession) cleanupSessionWorktrees(failedSession);
+      registry.abort(clientId);
+      return startChat(transport, clientId, prompt, {
+        ...options,
+        model: FALLBACK_MODEL,
+      });
     } else {
       log.error('startChat failed after register, cleaning up', { clientId, error: message });
       send(transport, { type: 'error', error: message });