diff --git a/open-sse/config/appConstants.js b/open-sse/config/appConstants.js
index 5ddf95ac..3e18633e 100644
--- a/open-sse/config/appConstants.js
+++ b/open-sse/config/appConstants.js
@@ -171,6 +171,13 @@ export const LOAD_CODE_ASSIST_METADATA = {
 
 // System prompts
 export const CLAUDE_SYSTEM_PROMPT = "You are Claude Code, Anthropic's official CLI for Claude.";
+// Rewrite rules applied to Antigravity system prompts: competing-client branding
+// makes the backend flag the request and answer 429 Quota Exhausted.
+export const ANTIGRAVITY_PROMPT_REWRITES = [
+  { from: "You are a Claude agent, built on Anthropic's Claude Agent SDK.", to: "" },
+  { from: /opencode/gi, to: (m) => (m === "OpenCode" ? "Antigravity" : m === "OPENCODE" ? "ANTIGRAVITY" : "antigravity") }
+];
+
 export const ANTIGRAVITY_DEFAULT_SYSTEM = "You are Antigravity, a powerful agentic AI coding assistant designed by the Google Deepmind team working on Advanced Agentic Coding.You are pair programming with a USER to solve their coding task. The task may require creating a new codebase, modifying or debugging an existing codebase, or simply answering a question.**Absolute paths only****Proactiveness**";
 
 // Derive từ registry oauth.refreshLeadMs
diff --git a/open-sse/executors/antigravity.js b/open-sse/executors/antigravity.js
index 07bbb4fc..35ec1006 100644
--- a/open-sse/executors/antigravity.js
+++ b/open-sse/executors/antigravity.js
@@ -1,7 +1,7 @@
 import crypto from "crypto";
 import { BaseExecutor } from "./base.js";
 import { PROVIDERS } from "../config/providers.js";
-import { OAUTH_ENDPOINTS, ANTIGRAVITY_HEADERS, AG_DEFAULT_TOOLS, AG_TOOL_SUFFIX } from "../config/appConstants.js";
+import { OAUTH_ENDPOINTS, ANTIGRAVITY_HEADERS, AG_DEFAULT_TOOLS, AG_TOOL_SUFFIX, ANTIGRAVITY_PROMPT_REWRITES } from "../config/appConstants.js";
 import { HTTP_STATUS } from "../config/runtimeConfig.js";
 import { resolveSessionId } from "../utils/sessionManager.js";
 import { proxyAwareFetch } from "../utils/proxyFetch.js";
@@ -246,13 +246,13 @@ export class AntigravityExecutor extends BaseExecutor {
     const { tools: _originalTools, toolConfig: _originalToolConfig, ...requestWithoutTools } = body.request || {};
     stripBlacklisted(requestWithoutTools);
     
-    // Rewrite competitive system prompts (e.g. Zed IDE's Claude prompt) to prevent Antigravity from 
-    // flagging the request and immediately blocking it with a 429 Quota Exhausted response.
+    // Rewrite competing-client branding in system prompts (e.g. Zed's Claude prompt,
+    // OpenCode naming) so Antigravity doesn't flag the request with a 429 Quota Exhausted.
     if (requestWithoutTools.systemInstruction?.parts) {
-      const oldText = "You are a Claude agent, built on Anthropic's Claude Agent SDK.";
       for (const part of requestWithoutTools.systemInstruction.parts) {
-        if (typeof part.text === "string" && part.text.includes(oldText)) {
-          part.text = part.text.split(oldText).join("");
+        if (typeof part.text !== "string") continue;
+        for (const { from, to } of ANTIGRAVITY_PROMPT_REWRITES) {
+          part.text = part.text.replaceAll(from, to);
         }
       }
     }