fix(google): append continue nudge for Claude-on-Antigravity prefill

src/adapters/google.ts

@@ -498,6 +498,17 @@ export function createGoogleAdapter(provider: OcxProviderConfig): ProviderAdapte
           } else {
             sanitizeAntigravityClaudeSignatures(contents);
           }
+          // Claude-on-Antigravity rejects assistant-tail (model-tail in Gemini terms) histories
+          // as prefill: "This model does not support assistant message prefill. The conversation
+          // must end with a user message." Context compaction, previous_response_id expansion,
+          // and interrupted-turn replay can all produce a model-tail history. Append a user
+          // "(continue)" nudge, mirroring the anthropic adapter's tail guard (src/adapters/anthropic.ts).
+          if (/claude/i.test(wireModelId)) {
+            const last = contents.length > 0 ? contents[contents.length - 1] as { role?: string } : undefined;
+            if (!last || last.role === "model") {
+              contents.push({ role: "user", parts: [{ text: "(continue)" }] });
+            }
+          }
         }
         const envelope = {
           model: wireModelId,

tests/google-claude-prefill-guard.test.ts

@@ -0,0 +1,91 @@
+import { describe, expect, test } from "bun:test";
+import { createGoogleAdapter as createGoogleAdapterProduction } from "../src/adapters/google";
+import type { OcxMessage, OcxParsedRequest, OcxProviderConfig } from "../src/types";
+import { withTestTranslatorBudget } from "./helpers/translator-budget";
+
+const createGoogleAdapter = (...args: Parameters<typeof createGoogleAdapterProduction>) =>
+  withTestTranslatorBudget(createGoogleAdapterProduction(...args));
+
+const provider = {
+  adapter: "google",
+  baseUrl: "https://daily-cloudcode-pa.googleapis.com",
+  googleMode: "cloud-code-assist",
+  project: "proj-123",
+  apiKey: "ya29.token",
+} as OcxProviderConfig;
+
+function parsed(messages: OcxMessage[], modelId = "claude-opus-4-6-thinking"): OcxParsedRequest {
+  return {
+    modelId,
+    stream: false,
+    options: {},
+    context: { messages, systemPrompt: [], tools: [] },
+  } as unknown as OcxParsedRequest;
+}
+
+async function envelopeContents(p: OcxParsedRequest): Promise<{ role: string; parts: unknown[] }[]> {
+  const { body } = await createGoogleAdapter(provider).buildRequest(p);
+  const envelope = JSON.parse(body);
+  return envelope.request.contents;
+}
+
+describe("google claude prefill guard", () => {
+  test("appends a user continue nudge when CCA Claude context ends with model turn", async () => {
+    const contents = await envelopeContents(parsed([
+      { role: "user", content: "start", timestamp: 0 },
+      { role: "assistant", content: [{ type: "text", text: "partial answer" }], model: "claude", timestamp: 0 },
+    ]));
+
+    expect(contents.at(-1)).toEqual({ role: "user", parts: [{ text: "(continue)" }] });
+  });
+
+  test("leaves context ending with user unchanged", async () => {
+    const contents = await envelopeContents(parsed([
+      { role: "assistant", content: [{ type: "text", text: "answer" }], model: "claude", timestamp: 0 },
+      { role: "user", content: "follow up", timestamp: 0 },
+    ]));
+
+    expect(contents.at(-1)!.role).toBe("user");
+    expect(JSON.stringify(contents.at(-1))).not.toContain("(continue)");
+  });
+
+  test("appends nudge when CCA Claude context is empty", async () => {
+    const contents = await envelopeContents(parsed([]));
+
+    expect(contents.at(-1)).toEqual({ role: "user", parts: [{ text: "(continue)" }] });
+  });
+
+  test("does not append nudge after tool result (tool result maps to user role)", async () => {
+    const contents = await envelopeContents(parsed([
+      {
+        role: "assistant",
+        content: [{ type: "toolCall", id: "call_1", name: "read_file", arguments: { path: "README.md" } }],
+        model: "claude",
+        timestamp: 0,
+      },
+      {
+        role: "toolResult",
+        toolCallId: "call_1",
+        toolName: "read_file",
+        content: "contents",
+        isError: false,
+        timestamp: 0,
+      },
+    ] as OcxMessage[]));
+
+    // toolResult maps to role:"user" in Gemini format, so no nudge needed
+    expect(contents.at(-1)!.role).toBe("user");
+    expect(JSON.stringify(contents.at(-1))).not.toContain("(continue)");
+  });
+
+  test("does not append nudge for non-Claude models on Antigravity", async () => {
+    const contents = await envelopeContents(parsed([
+      { role: "user", content: "start", timestamp: 0 },
+      { role: "assistant", content: [{ type: "text", text: "answer" }], model: "gemini", timestamp: 0 },
+    ], "gemini-3.7-flash"));
+
+    // Gemini natively accepts model-tail; no nudge
+    expect(contents.at(-1)!.role).toBe("model");
+    expect(JSON.stringify(contents)).not.toContain("(continue)");
+  });
+});