diff --git a/src/adapters/google.ts b/src/adapters/google.ts
index 8829dc07846..685cd2177cd 100644
--- a/src/adapters/google.ts
+++ b/src/adapters/google.ts
@@ -57,7 +57,6 @@ const GOOGLE_BREVITY_INSTRUCTION = [
 
 const ANTIGRAVITY_REJECTED_CLAUDE_SDK_PARAGRAPH =
   "You are a Claude agent, built on Anthropic's Claude Agent SDK.";
-
 /**
  * CCA Flash generations that reject the Claude-Agent identity paragraph.
  *
@@ -102,6 +101,20 @@ function stripAntigravityRejectedClaudeSdkParagraph(systemText: string): string
     .join("\n\n");
 }
 
+/**
+ * Strips Claude Code CLI's internal billing header (`x-anthropic-billing-header: ...`)
+ * at the start of the system prompt, because Cloud Code Assist / Google Antigravity inspects
+ * `systemInstruction` and rejects requests containing Anthropic billing metadata with
+ * HTTP 429 RESOURCE_EXHAUSTED.
+ *
+ * Matching is restricted to the prompt start (`^` without the `/m` multiline flag) so that
+ * user prompts discussing billing headers in intermediate lines are never modified, and
+ * prompts without a billing header preserve their leading whitespace untouched.
+ */
+function stripAntigravityBillingHeader(systemText: string): string {
+  return systemText.replace(/^x-anthropic-billing-header:[^\n]*\n*/, "");
+}
+
 /**
  * Documented output ceiling for a Google-surface model, or `undefined` when the id is not
  * recognized.
@@ -276,6 +289,7 @@ function messagesToGeminiFormat(
   parsed: OcxParsedRequest,
   identityModelId: string,
   stripRejectedClaudeSdkParagraph = false,
+  isCloudCodeAssist = false,
 ): { systemInstruction?: unknown; contents: unknown[]; replayedCallIds: string[] } {
   // Neutralize Codex's GPT-5 identity line (Gemini/Antigravity share this path) so a routed model
   // never misreports as GPT-5/OpenAI, and never leaks the proxy identity upstream.
@@ -285,9 +299,12 @@ function messagesToGeminiFormat(
     ...(toolCatalogNudge ? [toolCatalogNudge] : []),
     GOOGLE_BREVITY_INSTRUCTION,
   ].join("\n\n"), identityModelId);
-  const systemText = stripRejectedClaudeSdkParagraph
-    ? stripAntigravityRejectedClaudeSdkParagraph(identifiedSystemText)
+  let systemText = isCloudCodeAssist
+    ? stripAntigravityBillingHeader(identifiedSystemText)
     : identifiedSystemText;
+  if (stripRejectedClaudeSdkParagraph) {
+    systemText = stripAntigravityRejectedClaudeSdkParagraph(systemText);
+  }
   const systemInstruction = { parts: [{ text: systemText }] };
 
   const contents: unknown[] = [];
@@ -833,12 +850,14 @@ export function createGoogleAdapter(provider: OcxProviderConfig): ProviderAdapte
         && /^gemini-/.test(routedModelId) && !isImageCapableModel(parsed.modelId);
       // AI Studio's `-tiered` spelling is wire-only; CCA aliases may migrate to another generation.
       const identityModelId = provider.googleMode === "cloud-code-assist" ? routedModelId : parsed.modelId;
-      const stripRejectedClaudeSdkParagraph = provider.googleMode === "cloud-code-assist"
+      const isCloudCodeAssist = provider.googleMode === "cloud-code-assist";
+      const stripRejectedClaudeSdkParagraph = isCloudCodeAssist
         && rejectsClaudeSdkParagraph(parsed.modelId, routedModelId);
       const { systemInstruction, contents, replayedCallIds } = messagesToGeminiFormat(
         parsed,
         identityModelId,
         stripRejectedClaudeSdkParagraph,
+        isCloudCodeAssist,
       );
       lastInjectedCallIds = [...replayedCallIds];
       lastReasoningReplayScope = parsed._reasoningReplayScope;
diff --git a/tests/adapters/google/google-adapter.test.ts b/tests/adapters/google/google-adapter.test.ts
index 1dd5a7c97a4..01d8e2602b8 100644
--- a/tests/adapters/google/google-adapter.test.ts
+++ b/tests/adapters/google/google-adapter.test.ts
@@ -350,6 +350,64 @@ describe("google adapter — Antigravity system prompt compatibility", () => {
 
     expect(systemInstructionText(body)).toContain(REJECTED_CLAUDE_SDK_PARAGRAPH);
   });
+
+  test("removes x-anthropic-billing-header for Cloud Code Assist models", async () => {
+    const parsed: OcxParsedRequest = {
+      ...systemPromptParsed("gemini-3.8-flash"),
+      context: {
+        systemPrompt: [
+          "x-anthropic-billing-header: cc_version=2.1.236.b88; cc_entrypoint=sdk-cli;",
+          "You are Claude Code.",
+        ],
+        messages: [{ role: "user", content: [{ type: "text", text: "hi" }] }],
+        tools: [],
+      },
+    };
+    const envelope = JSON.parse((await createGoogleAdapter(ccaProvider).buildRequest(parsed)).body) as {
+      request: Record<string, unknown>;
+    };
+
+    expect(systemInstructionText(envelope.request)).not.toContain("x-anthropic-billing-header");
+    expect(systemInstructionText(envelope.request)).toContain("You are Claude Code.");
+  });
+
+  test("preserves x-anthropic-billing-header outside Cloud Code Assist", async () => {
+    const parsed: OcxParsedRequest = {
+      ...systemPromptParsed("gemini-3.8-flash"),
+      context: {
+        systemPrompt: [
+          "x-anthropic-billing-header: cc_version=2.1.236.b88; cc_entrypoint=sdk-cli;",
+          "You are Claude Code.",
+        ],
+        messages: [{ role: "user", content: [{ type: "text", text: "hi" }] }],
+        tools: [],
+      },
+    };
+    const body = await geminiBody(parsed);
+
+    expect(systemInstructionText(body)).toContain("x-anthropic-billing-header");
+    expect(systemInstructionText(body)).toContain("You are Claude Code.");
+  });
+
+  test("preserves non-leading x-anthropic-billing-header and leading whitespace in Cloud Code Assist", async () => {
+    const parsed: OcxParsedRequest = {
+      ...systemPromptParsed("gemini-3.8-flash"),
+      context: {
+        systemPrompt: [
+          "  leading indentation",
+          "x-anthropic-billing-header: cc_version=2.1.236.b88; cc_entrypoint=sdk-cli;",
+        ],
+        messages: [{ role: "user", content: [{ type: "text", text: "hi" }] }],
+        tools: [],
+      },
+    };
+    const envelope = JSON.parse((await createGoogleAdapter(ccaProvider).buildRequest(parsed)).body) as {
+      request: Record<string, unknown>;
+    };
+
+    expect(systemInstructionText(envelope.request)).toContain("  leading indentation");
+    expect(systemInstructionText(envelope.request)).toContain("x-anthropic-billing-header");
+  });
 });
 
 describe("google adapter — tool_choice on the wire", () => {