diff --git a/package.json b/package.json
index e7aa14bf..1f0dd1ef 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@kaitranntt/ccs",
-  "version": "7.39.0",
+  "version": "7.39.0-dev.5",
   "description": "Claude Code Switch - Instant profile switching between Claude Sonnet 4.5 and GLM 4.6",
   "keywords": [
     "cli",
diff --git a/src/cliproxy/auth/provider-refreshers/index.ts b/src/cliproxy/auth/provider-refreshers/index.ts
index 89df2ab2..3c642340 100644
--- a/src/cliproxy/auth/provider-refreshers/index.ts
+++ b/src/cliproxy/auth/provider-refreshers/index.ts
@@ -2,7 +2,12 @@
  * Provider Token Refreshers
  *
  * Exports refresh functions for each OAuth provider.
- * Currently only Gemini is implemented; others return placeholder errors.
+ *
+ * Refresh responsibility:
+ * - CCS-managed: gemini (CCS refreshes tokens directly via Google OAuth)
+ * - CLIProxy-delegated: codex, agy, kiro, ghcp, qwen, iflow
+ *   (CLIProxyAPIPlus handles refresh automatically in background)
+ * - Not implemented: claude
  */
 
 import { CLIProxyProvider } from '../../types';
@@ -13,6 +18,29 @@ export interface ProviderRefreshResult {
   success: boolean;
   error?: string;
   expiresAt?: number;
+  /** True if refresh is delegated to CLIProxy (not handled by CCS) */
+  delegated?: boolean;
+}
+
+/**
+ * Providers where CLIProxyAPIPlus owns token refresh.
+ * CLIProxyAPIPlus runs background refresh automatically (e.g. kiro: every 1 min).
+ * CCS should not attempt to refresh these — just trust CLIProxy.
+ */
+const CLIPROXY_DELEGATED_REFRESH: CLIProxyProvider[] = [
+  'codex',
+  'agy',
+  'kiro',
+  'ghcp',
+  'qwen',
+  'iflow',
+];
+
+/**
+ * Check if a provider's token refresh is delegated to CLIProxy
+ */
+export function isRefreshDelegated(provider: CLIProxyProvider): boolean {
+  return CLIPROXY_DELEGATED_REFRESH.includes(provider);
 }
 
 /**
@@ -35,6 +63,10 @@ export async function refreshToken(
     case 'iflow':
     case 'kiro':
     case 'ghcp':
+      // CLIProxyAPIPlus handles refresh for these providers automatically.
+      // No action needed from CCS — report success with delegated flag.
+      return { success: true, delegated: true };
+
     case 'claude':
       return {
         success: false,
diff --git a/src/cliproxy/auth/token-expiry-checker.ts b/src/cliproxy/auth/token-expiry-checker.ts
index 066f1d9b..a8f13177 100644
--- a/src/cliproxy/auth/token-expiry-checker.ts
+++ b/src/cliproxy/auth/token-expiry-checker.ts
@@ -10,6 +10,7 @@ import * as path from 'path';
 import { CLIProxyProvider } from '../types';
 import { CLIPROXY_PROFILES } from '../../auth/profile-detector';
 import { getProviderAccounts, getAccountTokenPath } from '../account-manager';
+import { isRefreshDelegated } from './provider-refreshers';
 
 /** Preemptive refresh time: refresh tokens 45 minutes before expiry */
 export const PREEMPTIVE_REFRESH_MINUTES = 45;
@@ -74,9 +75,13 @@ export function getTokenExpiryInfo(
     const content = fs.readFileSync(tokenPath, 'utf-8');
     const data: TokenData = JSON.parse(content);
 
-    // Validate refresh_token exists (required for refresh)
-    if (!data.refresh_token || typeof data.refresh_token !== 'string') {
-      return null;
+    // Validate refresh_token exists (required for CCS-managed refresh).
+    // CLIProxy-delegated providers may not expose refresh_token in token files —
+    // CLIProxyAPIPlus manages refresh internally, so skip this check for them.
+    if (!isRefreshDelegated(provider)) {
+      if (!data.refresh_token || typeof data.refresh_token !== 'string') {
+        return null;
+      }
     }
 
     // Calculate expiry time with validation
diff --git a/src/cliproxy/auth/token-manager.ts b/src/cliproxy/auth/token-manager.ts
index 41ef2092..bec57b39 100644
--- a/src/cliproxy/auth/token-manager.ts
+++ b/src/cliproxy/auth/token-manager.ts
@@ -317,6 +317,13 @@ export function displayAuthStatus(): void {
  * Ensure OAuth token is valid for provider, refreshing if expired or expiring soon.
  * This prevents UND_ERR_SOCKET errors caused by expired tokens during API calls.
  *
+ * Refresh responsibility:
+ * - gemini: CCS refreshes directly via Google OAuth
+ * - codex, agy, kiro, ghcp, qwen, iflow: CLIProxyAPIPlus handles refresh
+ *   automatically in background (e.g. kiro refreshes every 1 min).
+ *   CCS only checks if token file exists (authentication state).
+ * - claude: not yet implemented
+ *
  * @param provider The CLIProxy provider
  * @param verbose Log progress if true
  * @returns Object with valid status and whether refresh occurred
@@ -325,14 +332,12 @@ export async function ensureTokenValid(
   provider: CLIProxyProvider,
   verbose = false
 ): Promise<{ valid: boolean; refreshed: boolean; error?: string }> {
-  // Currently only Gemini uses oauth_creds.json with expiry_date
-  // Other providers (agy, codex) use CLIProxyAPI's internal auth management
   if (provider === 'gemini') {
     const { ensureGeminiTokenValid } = await import('./gemini-token-refresh');
     return ensureGeminiTokenValid(verbose);
   }
 
-  // For other providers, assume token is valid if authenticated
-  // CLIProxyAPI handles token refresh internally for antigravity/codex
+  // For CLIProxy-delegated providers, token refresh is handled by CLIProxyAPIPlus.
+  // CCS only verifies the token file exists (authentication state).
   return { valid: isAuthenticated(provider), refreshed: false };
 }
diff --git a/src/cliproxy/auth/token-refresh-worker.ts b/src/cliproxy/auth/token-refresh-worker.ts
index 071e63d9..4e1c5304 100644
--- a/src/cliproxy/auth/token-refresh-worker.ts
+++ b/src/cliproxy/auth/token-refresh-worker.ts
@@ -7,7 +7,7 @@
 
 import { CLIProxyProvider } from '../types';
 import { getAllTokenExpiryInfo, TokenExpiryInfo } from './token-expiry-checker';
-import { refreshToken } from './provider-refreshers';
+import { refreshToken, isRefreshDelegated } from './provider-refreshers';
 
 /** Worker configuration */
 export interface TokenRefreshConfig {
@@ -182,7 +182,10 @@ export class TokenRefreshWorker {
 
     try {
       const tokens = getAllTokenExpiryInfo();
-      const tokensNeedingRefresh = tokens.filter((t) => t.needsRefresh);
+      // Skip CLIProxy-delegated providers — they handle their own refresh
+      const tokensNeedingRefresh = tokens.filter(
+        (t) => t.needsRefresh && !isRefreshDelegated(t.provider)
+      );
 
       if (tokensNeedingRefresh.length === 0) {
         this.log('[OK] All tokens valid, no refresh needed');
diff --git a/src/cliproxy/config/extended-context-config.ts b/src/cliproxy/config/extended-context-config.ts
index 3021419c..e69a5592 100644
--- a/src/cliproxy/config/extended-context-config.ts
+++ b/src/cliproxy/config/extended-context-config.ts
@@ -85,7 +85,25 @@ export function applyExtendedContextConfig(
   const baseModel = envVars.ANTHROPIC_MODEL || '';
   const cleanModelId = stripModelSuffixes(baseModel);
 
+  // Tier model env vars to apply/strip extended context suffix
+  const tierModels = [
+    'ANTHROPIC_DEFAULT_OPUS_MODEL',
+    'ANTHROPIC_DEFAULT_SONNET_MODEL',
+    'ANTHROPIC_DEFAULT_HAIKU_MODEL',
+  ] as const;
+
   if (!shouldApplyExtendedContext(provider, cleanModelId, extendedContextOverride)) {
+    // Strip [1m] suffix from models that no longer support extended context
+    // (e.g., user had it enabled before backend dropped support)
+    if (envVars.ANTHROPIC_MODEL?.toLowerCase().endsWith('[1m]')) {
+      envVars.ANTHROPIC_MODEL = envVars.ANTHROPIC_MODEL.replace(/\[1m\]$/i, '');
+    }
+    for (const tierVar of tierModels) {
+      const model = envVars[tierVar];
+      if (model?.toLowerCase().endsWith('[1m]')) {
+        envVars[tierVar] = model.replace(/\[1m\]$/i, '');
+      }
+    }
     return;
   }
 
@@ -95,11 +113,6 @@ export function applyExtendedContextConfig(
   }
 
   // Apply to tier models if they support extended context
-  const tierModels = [
-    'ANTHROPIC_DEFAULT_OPUS_MODEL',
-    'ANTHROPIC_DEFAULT_SONNET_MODEL',
-    'ANTHROPIC_DEFAULT_HAIKU_MODEL',
-  ] as const;
 
   for (const tierVar of tierModels) {
     const model = envVars[tierVar];
diff --git a/src/cliproxy/model-catalog.ts b/src/cliproxy/model-catalog.ts
index 82b1371e..3c0e02fe 100644
--- a/src/cliproxy/model-catalog.ts
+++ b/src/cliproxy/model-catalog.ts
@@ -78,7 +78,7 @@ export const MODEL_CATALOG: Partial<Record<CLIProxyProvider, ProviderCatalog>> =
       {
         id: 'gemini-claude-opus-4-6-thinking',
         name: 'Claude Opus 4.6 Thinking',
-        description: 'Latest flagship, 1M context, extended thinking',
+        description: 'Latest flagship, extended thinking',
         thinking: {
           type: 'budget',
           min: 1024,
@@ -86,7 +86,9 @@ export const MODEL_CATALOG: Partial<Record<CLIProxyProvider, ProviderCatalog>> =
           zeroAllowed: true,
           dynamicAllowed: true,
         },
-        extendedContext: true,
+        // TODO: Re-enable when Antigravity backend supports 1M context (currently 256k)
+        // extendedContext: true,
+        extendedContext: false,
       },
       {
         id: 'gemini-claude-opus-4-5-thinking',
diff --git a/src/cliproxy/tool-sanitization-proxy.ts b/src/cliproxy/tool-sanitization-proxy.ts
index 78ddecdb..87c3b811 100644
--- a/src/cliproxy/tool-sanitization-proxy.ts
+++ b/src/cliproxy/tool-sanitization-proxy.ts
@@ -439,10 +439,61 @@ export class ToolSanitizationProxy {
         (upstreamRes) => {
           clientRes.writeHead(upstreamRes.statusCode || 200, upstreamRes.headers);
 
-          // If no changes were made, just pipe through
+          // Track upstream SSE lifecycle events (guards against empty proxy responses)
+          const lifecycle = {
+            hasContent: false,
+            hasData: false,
+            hasMessageStart: false,
+            hasMessageDelta: false,
+            hasMessageStop: false,
+            /** Scan text for SSE lifecycle events and update tracking flags */
+            update(text: string) {
+              if (text.includes('"content_block_start"')) this.hasContent = true;
+              if (text.includes('"message_start"')) this.hasMessageStart = true;
+              if (text.includes('"message_delta"')) this.hasMessageDelta = true;
+              if (text.includes('"message_stop"')) this.hasMessageStop = true;
+            },
+          };
+          const isSuccessResponse =
+            (upstreamRes.statusCode || 200) >= 200 && (upstreamRes.statusCode || 200) < 300;
+
+          // If no changes were made, intercept to detect empty responses.
+          // In the real failure case (issue #350), upstream sends message_start but
+          // NOT message_delta/message_stop, so the synthetic response completes the
+          // stream. If upstream DID send message_stop, Claude Code treats the second
+          // synthetic block as additional content in the same conversation turn.
           if (!mapper.hasChanges()) {
-            upstreamRes.pipe(clientRes);
-            upstreamRes.on('end', () => resolve());
+            upstreamRes.on('data', (chunk: Buffer) => {
+              lifecycle.hasData = true;
+              lifecycle.update(chunk.toString('utf8'));
+              // Respect backpressure: pause upstream if client can't keep up
+              const canContinue = clientRes.write(chunk);
+              if (!canContinue) {
+                upstreamRes.pause();
+                clientRes.once('drain', () => upstreamRes.resume());
+              }
+            });
+            upstreamRes.on('end', () => {
+              try {
+                if (!lifecycle.hasContent && isSuccessResponse && lifecycle.hasData) {
+                  this.writeLog(
+                    'warn',
+                    '[tool-sanitization-proxy] Empty response detected from upstream (no content blocks). Injecting synthetic response to prevent client crash.'
+                  );
+                  clientRes.write(
+                    this.buildSyntheticErrorResponse(
+                      lifecycle.hasMessageStart,
+                      lifecycle.hasMessageDelta,
+                      lifecycle.hasMessageStop
+                    )
+                  );
+                }
+                clientRes.end();
+              } catch {
+                // Client may have disconnected — safe to ignore
+              }
+              resolve();
+            });
             upstreamRes.on('error', reject);
             return;
           }
@@ -451,6 +502,7 @@ export class ToolSanitizationProxy {
           let buffer = '';
 
           upstreamRes.on('data', (chunk: Buffer) => {
+            lifecycle.hasData = true;
             buffer += chunk.toString('utf8');
 
             // Process complete SSE events
@@ -460,18 +512,41 @@ export class ToolSanitizationProxy {
             for (const event of events) {
               if (!event.trim()) continue;
 
+              lifecycle.update(event);
+
               const processedEvent = this.processSSEEvent(event, mapper);
               clientRes.write(processedEvent + '\n\n');
             }
           });
 
           upstreamRes.on('end', () => {
-            // Process any remaining buffer
-            if (buffer.trim()) {
-              const processedEvent = this.processSSEEvent(buffer, mapper);
-              clientRes.write(processedEvent + '\n\n');
+            try {
+              // Process any remaining buffer
+              if (buffer.trim()) {
+                lifecycle.update(buffer);
+                const processedEvent = this.processSSEEvent(buffer, mapper);
+                clientRes.write(processedEvent + '\n\n');
+              }
+
+              // Safety net: if upstream sent data but no content blocks, inject synthetic response
+              if (!lifecycle.hasContent && isSuccessResponse && lifecycle.hasData) {
+                this.writeLog(
+                  'warn',
+                  '[tool-sanitization-proxy] Empty response detected from upstream (no content blocks). Injecting synthetic response to prevent client crash.'
+                );
+                clientRes.write(
+                  this.buildSyntheticErrorResponse(
+                    lifecycle.hasMessageStart,
+                    lifecycle.hasMessageDelta,
+                    lifecycle.hasMessageStop
+                  )
+                );
+              }
+
+              clientRes.end();
+            } catch {
+              // Client may have disconnected — safe to ignore
             }
-            clientRes.end();
             resolve();
           });
 
@@ -542,4 +617,43 @@ export class ToolSanitizationProxy {
 
     return processedLines.join('\n');
   }
+
+  /**
+   * Build a synthetic minimal SSE response when upstream returns empty content.
+   * Prevents Claude Code from crashing with "No assistant message found".
+   * Omits lifecycle events that upstream already sent to avoid protocol violations.
+   */
+  private buildSyntheticErrorResponse(
+    upstreamSentMessageStart = false,
+    upstreamSentMessageDelta = false,
+    upstreamSentMessageStop = false
+  ): string {
+    const events: string[] = [];
+
+    // Only include message_start if upstream didn't already send one
+    if (!upstreamSentMessageStart) {
+      const msgId = `msg_synthetic_${Date.now()}`;
+      events.push(
+        `event: message_start\ndata: {"type":"message_start","message":{"id":"${msgId}","type":"message","role":"assistant","content":[],"model":"unknown","stop_reason":null,"stop_sequence":null,"usage":{"input_tokens":0,"output_tokens":0}}}`
+      );
+    }
+
+    events.push(
+      `event: content_block_start\ndata: {"type":"content_block_start","index":0,"content_block":{"type":"text","text":""}}`,
+      `event: content_block_delta\ndata: {"type":"content_block_delta","index":0,"delta":{"type":"text_delta","text":"[Proxy Error] The upstream API returned an empty response. This typically occurs when the proxy drops unsigned thinking blocks during sub-agent execution. Please retry the request."}}`,
+      `event: content_block_stop\ndata: {"type":"content_block_stop","index":0}`
+    );
+
+    if (!upstreamSentMessageDelta) {
+      events.push(
+        `event: message_delta\ndata: {"type":"message_delta","delta":{"stop_reason":"end_turn","stop_sequence":null},"usage":{"input_tokens":0,"output_tokens":0}}`
+      );
+    }
+
+    if (!upstreamSentMessageStop) {
+      events.push(`event: message_stop\ndata: {"type":"message_stop"}`);
+    }
+
+    return events.join('\n\n') + '\n\n';
+  }
 }
diff --git a/tests/unit/cliproxy/extended-context-config.test.ts b/tests/unit/cliproxy/extended-context-config.test.ts
index 76d11610..e7b2f722 100644
--- a/tests/unit/cliproxy/extended-context-config.test.ts
+++ b/tests/unit/cliproxy/extended-context-config.test.ts
@@ -151,4 +151,23 @@ describe('applyExtendedContextConfig', () => {
     applyExtendedContextConfig(env, 'gemini', undefined);
     expect(env.ANTHROPIC_MODEL).toBeUndefined();
   });
+
+  it('strips [1m] suffix from models that no longer support extended context', () => {
+    // Simulates user who had [1m] in saved settings before support was removed
+    const env: NodeJS.ProcessEnv = {
+      ANTHROPIC_MODEL: 'gemini-claude-opus-4-6-thinking[1m]',
+      ANTHROPIC_DEFAULT_OPUS_MODEL: 'gemini-claude-opus-4-6-thinking[1m]',
+    };
+    applyExtendedContextConfig(env, 'agy', undefined);
+    expect(env.ANTHROPIC_MODEL).toBe('gemini-claude-opus-4-6-thinking');
+    expect(env.ANTHROPIC_DEFAULT_OPUS_MODEL).toBe('gemini-claude-opus-4-6-thinking');
+  });
+
+  it('strips [1m] suffix when --no-1m is explicit even if model has it', () => {
+    const env: NodeJS.ProcessEnv = {
+      ANTHROPIC_MODEL: 'gemini-2.5-pro[1m]',
+    };
+    applyExtendedContextConfig(env, 'gemini', false);
+    expect(env.ANTHROPIC_MODEL).toBe('gemini-2.5-pro');
+  });
 });
diff --git a/tests/unit/cliproxy/tool-sanitization-proxy-integration.test.ts b/tests/unit/cliproxy/tool-sanitization-proxy-integration.test.ts
index 038fc13c..8bbfc5d0 100644
--- a/tests/unit/cliproxy/tool-sanitization-proxy-integration.test.ts
+++ b/tests/unit/cliproxy/tool-sanitization-proxy-integration.test.ts
@@ -490,4 +490,221 @@ describe('ToolSanitizationProxy Integration', () => {
       }
     });
   });
+
+  describe('Empty Response Safety Net (Issue #350)', () => {
+    it('injects synthetic response when streaming 200 has no content blocks', async () => {
+      const proxy = new ToolSanitizationProxy({
+        upstreamBaseUrl: `http://127.0.0.1:${mockUpstreamPort}`,
+      });
+      const port = await proxy.start();
+
+      // Upstream sends message_start + message_delta + message_stop but NO content blocks
+      mockResponse = {
+        status: 200,
+        stream: true,
+        body: [
+          'event: message_start\n',
+          'data: {"type":"message_start","message":{"id":"msg_test","type":"message","role":"assistant","content":[],"model":"test","stop_reason":null}}\n\n',
+          'event: message_delta\n',
+          'data: {"type":"message_delta","delta":{"stop_reason":"end_turn"},"usage":{"input_tokens":10,"output_tokens":0}}\n\n',
+          'event: message_stop\n',
+          'data: {"type":"message_stop"}\n\n',
+        ],
+      };
+
+      try {
+        const response = await fetch(`http://127.0.0.1:${port}/v1/messages`, {
+          method: 'POST',
+          headers: { 'Content-Type': 'application/json' },
+          body: JSON.stringify({
+            stream: true,
+            tools: [{ name: 'valid_tool' }],
+          }),
+        });
+
+        const text = await response.text();
+        // Should contain the synthetic error message
+        expect(text).toContain('[Proxy Error]');
+        expect(text).toContain('content_block_start');
+        expect(text).toContain('content_block_delta');
+        expect(text).toContain('content_block_stop');
+        // Should NOT have duplicate lifecycle events (upstream already sent them)
+        const messageStartCount = (text.match(/"type":"message_start"/g) || []).length;
+        expect(messageStartCount).toBe(1);
+        const messageDeltaCount = (text.match(/"type":"message_delta"/g) || []).length;
+        expect(messageDeltaCount).toBe(1);
+        const messageStopCount = (text.match(/"type":"message_stop"/g) || []).length;
+        expect(messageStopCount).toBe(1);
+      } finally {
+        proxy.stop();
+      }
+    });
+
+    it('does NOT inject synthetic response when content blocks exist', async () => {
+      const proxy = new ToolSanitizationProxy({
+        upstreamBaseUrl: `http://127.0.0.1:${mockUpstreamPort}`,
+      });
+      const port = await proxy.start();
+
+      mockResponse = {
+        status: 200,
+        stream: true,
+        body: [
+          'event: message_start\n',
+          'data: {"type":"message_start","message":{"id":"msg_ok","type":"message","role":"assistant","content":[],"model":"test"}}\n\n',
+          'event: content_block_start\n',
+          'data: {"type":"content_block_start","index":0,"content_block":{"type":"text","text":""}}\n\n',
+          'event: content_block_delta\n',
+          'data: {"type":"content_block_delta","index":0,"delta":{"type":"text_delta","text":"Hello"}}\n\n',
+          'event: content_block_stop\n',
+          'data: {"type":"content_block_stop","index":0}\n\n',
+          'event: message_delta\n',
+          'data: {"type":"message_delta","delta":{"stop_reason":"end_turn"}}\n\n',
+          'event: message_stop\n',
+          'data: {"type":"message_stop"}\n\n',
+        ],
+      };
+
+      try {
+        const response = await fetch(`http://127.0.0.1:${port}/v1/messages`, {
+          method: 'POST',
+          headers: { 'Content-Type': 'application/json' },
+          body: JSON.stringify({
+            stream: true,
+            tools: [{ name: 'valid_tool' }],
+          }),
+        });
+
+        const text = await response.text();
+        expect(text).toContain('Hello');
+        expect(text).not.toContain('[Proxy Error]');
+      } finally {
+        proxy.stop();
+      }
+    });
+
+    it('does NOT inject synthetic response for 4xx/5xx errors', async () => {
+      const proxy = new ToolSanitizationProxy({
+        upstreamBaseUrl: `http://127.0.0.1:${mockUpstreamPort}`,
+      });
+      const port = await proxy.start();
+
+      // 429 rate limit with SSE body but no content blocks
+      mockResponse = {
+        status: 429,
+        stream: true,
+        body: [
+          'event: error\n',
+          'data: {"type":"error","error":{"type":"rate_limit_error","message":"Too many requests"}}\n\n',
+        ],
+      };
+
+      try {
+        const response = await fetch(`http://127.0.0.1:${port}/v1/messages`, {
+          method: 'POST',
+          headers: { 'Content-Type': 'application/json' },
+          body: JSON.stringify({
+            stream: true,
+            tools: [{ name: 'valid_tool' }],
+          }),
+        });
+
+        expect(response.status).toBe(429);
+        const text = await response.text();
+        expect(text).not.toContain('[Proxy Error]');
+        expect(text).toContain('rate_limit_error');
+      } finally {
+        proxy.stop();
+      }
+    });
+
+    it('injects synthetic response via SSE processing path (with sanitized tool names)', async () => {
+      const proxy = new ToolSanitizationProxy({
+        upstreamBaseUrl: `http://127.0.0.1:${mockUpstreamPort}`,
+      });
+      const port = await proxy.start();
+
+      // Upstream returns empty content — no content_block_start events
+      // Using sanitized tool name ('foo__bar__bar' → 'foo__bar') triggers the SSE processing path
+      mockResponse = {
+        status: 200,
+        stream: true,
+        body: [
+          'event: message_start\n',
+          'data: {"type":"message_start","message":{"id":"msg_sse","type":"message","role":"assistant","content":[],"model":"test","stop_reason":null}}\n\n',
+          'event: message_delta\n',
+          'data: {"type":"message_delta","delta":{"stop_reason":"end_turn"},"usage":{"input_tokens":10,"output_tokens":0}}\n\n',
+          'event: message_stop\n',
+          'data: {"type":"message_stop"}\n\n',
+        ],
+      };
+
+      try {
+        const response = await fetch(`http://127.0.0.1:${port}/v1/messages`, {
+          method: 'POST',
+          headers: { 'Content-Type': 'application/json' },
+          body: JSON.stringify({
+            stream: true,
+            tools: [{ name: 'foo__bar__bar' }], // Triggers sanitization → SSE processing path
+          }),
+        });
+
+        const text = await response.text();
+        expect(text).toContain('[Proxy Error]');
+        expect(text).toContain('content_block_start');
+        // No duplicate lifecycle events
+        const messageStartCount = (text.match(/"type":"message_start"/g) || []).length;
+        expect(messageStartCount).toBe(1);
+        const messageDeltaCount = (text.match(/"type":"message_delta"/g) || []).length;
+        expect(messageDeltaCount).toBe(1);
+        const messageStopCount = (text.match(/"type":"message_stop"/g) || []).length;
+        expect(messageStopCount).toBe(1);
+      } finally {
+        proxy.stop();
+      }
+    });
+
+    it('injects synthetic response when upstream sends only message_start (real failure mode)', async () => {
+      const proxy = new ToolSanitizationProxy({
+        upstreamBaseUrl: `http://127.0.0.1:${mockUpstreamPort}`,
+      });
+      const port = await proxy.start();
+
+      // Real failure: upstream sends message_start then abruptly ends — no message_delta or message_stop
+      mockResponse = {
+        status: 200,
+        stream: true,
+        body: [
+          'event: message_start\n',
+          'data: {"type":"message_start","message":{"id":"msg_abrupt","type":"message","role":"assistant","content":[],"model":"test","stop_reason":null}}\n\n',
+        ],
+      };
+
+      try {
+        const response = await fetch(`http://127.0.0.1:${port}/v1/messages`, {
+          method: 'POST',
+          headers: { 'Content-Type': 'application/json' },
+          body: JSON.stringify({
+            stream: true,
+            tools: [{ name: 'valid_tool' }],
+          }),
+        });
+
+        const text = await response.text();
+        expect(text).toContain('[Proxy Error]');
+        expect(text).toContain('content_block_start');
+        expect(text).toContain('message_delta');
+        expect(text).toContain('message_stop');
+        // Only 1 of each — upstream sent message_start, synthetic provides the rest
+        const messageStartCount = (text.match(/"type":"message_start"/g) || []).length;
+        expect(messageStartCount).toBe(1);
+        const messageDeltaCount = (text.match(/"type":"message_delta"/g) || []).length;
+        expect(messageDeltaCount).toBe(1);
+        const messageStopCount = (text.match(/"type":"message_stop"/g) || []).length;
+        expect(messageStopCount).toBe(1);
+      } finally {
+        proxy.stop();
+      }
+    });
+  });
 });
diff --git a/ui/src/components/ui/badge.tsx b/ui/src/components/ui/badge.tsx
index 45811bfd..ab895f8e 100644
--- a/ui/src/components/ui/badge.tsx
+++ b/ui/src/components/ui/badge.tsx
@@ -23,8 +23,7 @@ const badgeVariants = cva(
 );
 
 interface BadgeProps
-  extends React.HTMLAttributes<HTMLDivElement>,
-    VariantProps<typeof badgeVariants> {}
+  extends React.HTMLAttributes<HTMLDivElement>, VariantProps<typeof badgeVariants> {}
 
 function Badge({ className, variant, ...props }: BadgeProps) {
   return <div className={cn(badgeVariants({ variant }), className)} {...props} />;
diff --git a/ui/src/lib/model-catalogs.ts b/ui/src/lib/model-catalogs.ts
index bbd70c1d..b9244d66 100644
--- a/ui/src/lib/model-catalogs.ts
+++ b/ui/src/lib/model-catalogs.ts
@@ -15,8 +15,10 @@ export const MODEL_CATALOGS: Record<string, ProviderCatalog> = {
       {
         id: 'gemini-claude-opus-4-6-thinking',
         name: 'Claude Opus 4.6 Thinking',
-        description: 'Latest flagship, 1M context, extended thinking',
-        extendedContext: true,
+        description: 'Latest flagship, extended thinking',
+        // TODO: Re-enable when Antigravity backend supports 1M context (currently 256k)
+        // extendedContext: true,
+        extendedContext: false,
         presetMapping: {
           default: 'gemini-claude-opus-4-6-thinking',
           opus: 'gemini-claude-opus-4-6-thinking',