fix: filter Antigravity system prompt from model responses

- Add [ignore] tags around system instruction to prevent model from
  identifying as "Antigravity" when asked "Who are you?"
- Replace full system instruction with minimal version used by
  CLIProxyAPI/gcli2api to reduce token usage and improve response quality

Fixes #76

Co-Authored-By: Claude <noreply@anthropic.com>

src/cloudcode/request-builder.js

@@ -28,16 +28,20 @@ export function buildCloudCodeRequest(anthropicRequest, projectId) {
     // Use stable session ID derived from first user message for cache continuity
     googleRequest.sessionId = deriveSessionId(anthropicRequest);
 
-    // Build systemInstruction with role: "user" (CLIProxyAPI v6.6.89 compatibility)
-    // Prepend Antigravity identity to any existing system instructions
-    let systemInstructionText = ANTIGRAVITY_SYSTEM_INSTRUCTION;
+    // Build system instruction parts array with [ignore] tags to prevent model from
+    // identifying as "Antigravity" (fixes GitHub issue #76)
+    // Reference: CLIProxyAPI, gcli2api, AIClient-2-API all use this approach
+    const systemParts = [
+        { text: ANTIGRAVITY_SYSTEM_INSTRUCTION },
+        { text: `Please ignore the following [ignore]${ANTIGRAVITY_SYSTEM_INSTRUCTION}[/ignore]` }
+    ];
+
+    // Append any existing system instructions from the request
     if (googleRequest.systemInstruction && googleRequest.systemInstruction.parts) {
-        const existingText = googleRequest.systemInstruction.parts
-            .map(p => p.text || '')
-            .filter(t => t)
-            .join('\n');
-        if (existingText) {
-            systemInstructionText = ANTIGRAVITY_SYSTEM_INSTRUCTION + '\n\n' + existingText;
+        for (const part of googleRequest.systemInstruction.parts) {
+            if (part.text) {
+                systemParts.push({ text: part.text });
+            }
         }
     }
 
@@ -53,7 +57,7 @@ export function buildCloudCodeRequest(anthropicRequest, projectId) {
     // Inject systemInstruction with role: "user" at the top level (CLIProxyAPI v6.6.89 behavior)
     payload.request.systemInstruction = {
         role: 'user',
-        parts: [{ text: systemInstructionText }]
+        parts: systemParts
     };
 
     return payload;