fix(antigravity): strip any competitor system prompt, not one literal

Google answers a systemInstruction advertising a rival agent with 429 RESOURCE_EXHAUSTED - a content refusal shaped like a quota error, which is why it survived the endpoint and backoff fixes. Upstream PR #3223 stripped a single Zed literal, so Claude Code / Hermes wording still tripped it. Match the sentence shape instead. Check fails on the old one-literal code and passes here.

open-sse/executors/antigravity-competitor-strip.check.mjs

@@ -0,0 +1,57 @@
+/**
+ * #3223-generalised: Antigravity answers a system prompt that advertises a rival
+ * agent with 429 RESOURCE_EXHAUSTED. Upstream stripped one Zed literal; every
+ * other client wording still tripped it. This pins the shapes we must strip.
+ *
+ * Run: node open-sse/executors/antigravity-competitor-strip.check.mjs
+ */
+import assert from 'node:assert/strict';
+import { readFileSync } from 'node:fs';
+import { dirname, resolve } from 'node:path';
+import { fileURLToPath } from 'node:url';
+import vm from 'node:vm';
+
+const here = dirname(fileURLToPath(import.meta.url));
+const src = readFileSync(
+  process.env.AG_SOURCE || resolve(here, 'antigravity.js'),
+  'utf8'
+);
+
+// Lift the strip block out of buildRequest and run it against a fake request.
+const start = src.indexOf('if (requestWithoutTools.systemInstruction?.parts)');
+assert.ok(start >= 0, 'competitor-strip block must exist');
+const end = src.indexOf('\n    }\n', src.indexOf('for (const part', start)) + 6;
+const block = src.slice(start, end);
+
+const strip = (text) => {
+  const requestWithoutTools = { systemInstruction: { parts: [{ text }] } };
+  vm.runInNewContext(block, { requestWithoutTools });
+  return requestWithoutTools.systemInstruction.parts[0].text;
+};
+
+// The literal upstream already handled — must keep working.
+assert.ok(
+  !strip("You are a Claude agent, built on Anthropic's Claude Agent SDK. Be terse.")
+    .includes('Claude'),
+  'Zed wording must still be stripped'
+);
+
+// The wordings upstream missed.
+for (const prompt of [
+  "You are Claude Code, Anthropic's official CLI for Claude. Help the user.",
+  'You are Claude, made by Anthropic!',
+  'you are a claude agent. lowercase should not save it.',
+]) {
+  const out = strip(prompt);
+  assert.ok(!/claude/i.test(out), `must strip competitor branding from: ${prompt}`);
+}
+
+// Must not eat an innocent prompt.
+const benign = 'You are a helpful assistant. Answer in French.';
+assert.equal(strip(benign), benign, 'benign prompts must pass through untouched');
+
+// Non-string parts must not throw.
+const req = { systemInstruction: { parts: [{ inlineData: {} }] } };
+vm.runInNewContext(block, { requestWithoutTools: req });
+
+console.log('antigravity-competitor-strip: all passed');

open-sse/executors/antigravity.js

@@ -246,17 +246,27 @@ 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 competitive system prompts before they reach Antigravity. Google
+    // inspects systemInstruction and answers a prompt that advertises a rival
+    // agent with 429 RESOURCE_EXHAUSTED — a content refusal wearing a quota
+    // error's clothes, which is why it survives every backoff and endpoint fix.
+    // Upstream PR #3223 stripped ONE literal (Zed's "You are a Claude agent,
+    // built on Anthropic's Claude Agent SDK."), so every other client — Claude
+    // Code, Hermes, Cline — still trips it. Match the shape instead.
+    // ponytail: regex over a prompt-classifier; widen the alternation if a new
+    // client wording slips through.
     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;
+        part.text = part.text
+          // "You are Claude Code, Anthropic's official CLI for Claude." /
+          // "You are a Claude agent, built on Anthropic's Claude Agent SDK."
+          .replace(/You are (?:a |an )?Claude\b[^.!?]*[.!?]\s*/gi, "")
+          // Residual vendor branding in the same sentence position.
+          .replace(/\b(?:Anthropic's|Anthropic)\s+(?:official\s+)?(?:CLI|Claude Agent SDK)\b[^.!?]*[.!?]\s*/gi, "")
+          .trimStart();
       }
     }
-    }
-
     const generationConfig = { ...(requestWithoutTools.generationConfig || {}) };
     if (generationConfig.maxOutputTokens > MAX_ANTIGRAVITY_OUTPUT_TOKENS) {
       generationConfig.maxOutputTokens = MAX_ANTIGRAVITY_OUTPUT_TOKENS;