diff --git a/src/adapters/google.ts b/src/adapters/google.ts
index d2f40a96e7..01189a5938 100644
--- a/src/adapters/google.ts
+++ b/src/adapters/google.ts
@@ -54,6 +54,16 @@ const GOOGLE_BREVITY_INSTRUCTION = [
   "- This applies only to intermediate progress text. Your final answer after the work is done is exempt: write it in full and at whatever length the task requires.",
 ].join("\n");
 
+const ANTIGRAVITY_REJECTED_CLAUDE_SDK_PARAGRAPH =
+  "You are a Claude agent, built on Anthropic's Claude Agent SDK.";
+
+function stripAntigravityRejectedClaudeSdkParagraph(systemText: string): string {
+  return systemText
+    .split("\n\n")
+    .filter(paragraph => paragraph !== ANTIGRAVITY_REJECTED_CLAUDE_SDK_PARAGRAPH)
+    .join("\n\n");
+}
+
 /**
  * Documented output ceiling for a Google-surface model, or `undefined` when the id is not
  * recognized.
@@ -227,15 +237,19 @@ function geminiOrphanToolResultParts(msg: OcxToolResultMessage): unknown[] {
 function messagesToGeminiFormat(
   parsed: OcxParsedRequest,
   identityModelId: string,
+  stripRejectedClaudeSdkParagraph = 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.
   const toolCatalogNudge = buildNonOpenAIToolCatalogNudgeForTools(parsed.context.tools, parsed.options.toolChoice);
-  const systemText = identifyRoutedModel([
+  const identifiedSystemText = identifyRoutedModel([
     ...(parsed.context.systemPrompt ?? []),
     ...(toolCatalogNudge ? [toolCatalogNudge] : []),
     GOOGLE_BREVITY_INSTRUCTION,
   ].join("\n\n"), identityModelId);
+  const systemText = stripRejectedClaudeSdkParagraph
+    ? stripAntigravityRejectedClaudeSdkParagraph(identifiedSystemText)
+    : identifiedSystemText;
   const systemInstruction = { parts: [{ text: systemText }] };
 
   const contents: unknown[] = [];
@@ -733,7 +747,13 @@ export function createGoogleAdapter(provider: OcxProviderConfig): ProviderAdapte
           : resolveDirectGeminiWireModelId(parsed.modelId, provider.directGeminiWireRenames !== false);
       // 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 { systemInstruction, contents, replayedCallIds } = messagesToGeminiFormat(parsed, identityModelId);
+      const stripRejectedClaudeSdkParagraph = provider.googleMode === "cloud-code-assist"
+        && parsed.modelId === "gemini-3.7-flash";
+      const { systemInstruction, contents, replayedCallIds } = messagesToGeminiFormat(
+        parsed,
+        identityModelId,
+        stripRejectedClaudeSdkParagraph,
+      );
       lastInjectedCallIds = [...replayedCallIds];
       lastReasoningReplayScope = parsed._reasoningReplayScope;
       const tools = toolsToGeminiFormat(parsed);
diff --git a/tests/google-adapter.test.ts b/tests/google-adapter.test.ts
index d067af9589..e7311eae62 100644
--- a/tests/google-adapter.test.ts
+++ b/tests/google-adapter.test.ts
@@ -21,6 +21,14 @@ async function geminiBody(parsed: OcxParsedRequest): Promise<Record<string, unkn
   return JSON.parse(body);
 }
 
+function systemInstructionText(body: Record<string, unknown>): string {
+  const instruction = body.systemInstruction as { parts?: Array<{ text?: string }> } | undefined;
+  return instruction?.parts?.[0]?.text ?? "";
+}
+
+const REJECTED_CLAUDE_SDK_PARAGRAPH =
+  "You are a Claude agent, built on Anthropic's Claude Agent SDK.";
+
 describe("google adapter — tool result images", () => {
   test("tool-result screenshots ride along as inline_data beside the functionResponse", async () => {
     const contents = await geminiContents(parsedWith([
@@ -239,6 +247,57 @@ describe("google adapter — tool-call ids on the wire", () => {
   });
 });
 
+describe("google adapter — Antigravity system prompt compatibility", () => {
+  const ccaProvider = {
+    ...provider,
+    googleMode: "cloud-code-assist",
+    baseUrl: "https://daily-cloudcode-pa.googleapis.com",
+    project: "proj-123",
+  } as const;
+
+  function systemPromptParsed(modelId: string): OcxParsedRequest {
+    return {
+      modelId,
+      stream: false,
+      options: {},
+      context: {
+        systemPrompt: [`prefix\n\n${REJECTED_CLAUDE_SDK_PARAGRAPH}\n\nsuffix`],
+        messages: [{ role: "user", content: "hi" }],
+        tools: [],
+      },
+    } as unknown as OcxParsedRequest;
+  }
+
+  test("removes only the rejected standalone paragraph for CCA Gemini 3.7 Flash", async () => {
+    const parsed = systemPromptParsed("gemini-3.7-flash");
+    const ccaEnvelope = JSON.parse((await createGoogleAdapter(ccaProvider).buildRequest(parsed)).body) as {
+      request: Record<string, unknown>;
+    };
+    const directBody = await geminiBody(parsed);
+    const directText = systemInstructionText(directBody);
+
+    expect(systemInstructionText(ccaEnvelope.request)).toBe(
+      directText.replace(`${REJECTED_CLAUDE_SDK_PARAGRAPH}\n\n`, ""),
+    );
+    expect(systemInstructionText(ccaEnvelope.request)).not.toContain(REJECTED_CLAUDE_SDK_PARAGRAPH);
+  });
+
+  test("preserves the paragraph for another Cloud Code Assist model", async () => {
+    const parsed = systemPromptParsed("gemini-3.6-flash");
+    const envelope = JSON.parse((await createGoogleAdapter(ccaProvider).buildRequest(parsed)).body) as {
+      request: Record<string, unknown>;
+    };
+
+    expect(systemInstructionText(envelope.request)).toContain(REJECTED_CLAUDE_SDK_PARAGRAPH);
+  });
+
+  test("preserves the paragraph outside Cloud Code Assist", async () => {
+    const body = await geminiBody(systemPromptParsed("gemini-3.7-flash"));
+
+    expect(systemInstructionText(body)).toContain(REJECTED_CLAUDE_SDK_PARAGRAPH);
+  });
+});
+
 describe("google adapter — tool_choice on the wire", () => {
   const TOOLS = [
     { name: "get_weather", parameters: { type: "object", properties: {} } },