diff --git a/open-sse/config/appConstants.js b/open-sse/config/appConstants.js
index 106cba50..62ddfb6b 100644
--- a/open-sse/config/appConstants.js
+++ b/open-sse/config/appConstants.js
@@ -179,6 +179,10 @@ export const CLAUDE_SYSTEM_PROMPT = "You are Claude Code, Anthropic's official C
 export const ANTIGRAVITY_PROMPT_REWRITES = [
   { from: "You are a Claude agent, built on Anthropic's Claude Agent SDK.", to: "" },
   { from: /You are Hermes Agent,\s*(an intelligent AI assistant)(?: created by Nous Research)?\./gi, to: "You are Hermes Agent. You are $1." },
+  // Claude Code prepends this line to its system prompt. The Claude-format translator strips it,
+  // but OpenAI-format clients (e.g. proxies that convert Claude Code to /v1/chat/completions)
+  // pass it through, and any system text containing it gets a fake 429 RESOURCE_EXHAUSTED.
+  { from: /^x-anthropic-billing-header:[^\n]*(?:\r?\n)*/gim, to: "" },
   { from: /opencode/gi, to: (m) => (m === "OpenCode" ? "Antigravity" : m === "OPENCODE" ? "ANTIGRAVITY" : "antigravity") }
 ];
 
diff --git a/tests/unit/antigravity-billing-header-rewrite.test.js b/tests/unit/antigravity-billing-header-rewrite.test.js
new file mode 100644
index 00000000..e324294f
--- /dev/null
+++ b/tests/unit/antigravity-billing-header-rewrite.test.js
@@ -0,0 +1,38 @@
+import { describe, expect, it } from "vitest";
+
+import { AntigravityExecutor } from "../../open-sse/executors/antigravity.js";
+import { openaiToAntigravityRequest } from "../../open-sse/translator/request/openai-to-gemini.js";
+
+const HEADER = "x-anthropic-billing-header: cc_version=2.1.275.f15; cc_entrypoint=cli;";
+
+function systemTextSentToAntigravity(systemContent) {
+  // OpenAI-format client (e.g. a proxy converting Claude Code to /v1/chat/completions).
+  const body = openaiToAntigravityRequest("gemini-3.8-flash-tiered", {
+    messages: [
+      { role: "system", content: systemContent },
+      { role: "user", content: "hi" },
+    ],
+  }, true);
+  const finalBody = new AntigravityExecutor().transformRequest("gemini-3.8-flash-tiered", body, true, {});
+  return finalBody.request.systemInstruction.parts.map((p) => p.text).join("\n");
+}
+
+describe("Antigravity strips the Claude Code billing header from system prompts", () => {
+  it("removes the header line prepended by Claude Code", () => {
+    const text = systemTextSentToAntigravity(`${HEADER}\n\nYou are Claude Code, Anthropic's official CLI for Claude.`);
+    expect(text).not.toContain("x-anthropic-billing-header");
+    expect(text).toContain("You are Claude Code, Anthropic's official CLI for Claude.");
+  });
+
+  it("removes the header when it is not the first line", () => {
+    const text = systemTextSentToAntigravity(`Some preamble\n${HEADER}\nRest of prompt`);
+    expect(text).not.toContain("x-anthropic-billing-header");
+    expect(text).toContain("Some preamble");
+    expect(text).toContain("Rest of prompt");
+  });
+
+  it("leaves prompts without the header untouched", () => {
+    const text = systemTextSentToAntigravity("You are a helpful assistant.");
+    expect(text).toContain("You are a helpful assistant.");
+  });
+});