fix: enforce user-ending Antigravity turns

Author: ualtinok

ARCHITECTURE.md

@@ -36,7 +36,7 @@
 **Payload Transformation:**
 - Purpose: Resolves client-supplied model names into logical Antigravity identifiers, injects tool description hardening rules, strips Claude thinking blocks, and sanitizes payload fields during model family swaps.
 - Location: `packages/core/src/transform/` and `packages/core/src/model-registry.ts`
-- Contains: `resolveModelWithTier`, `applyClaudeTransforms`, `applyGeminiTransforms`, `sanitizeCrossModelPayload`.
+- Contains: `resolveModelWithTier`, `applyClaudeTransforms`, `applyGeminiTransforms`, `sanitizeCrossModelPayload`, `getGemini36FlashAntigravityModel`.
 - Depends on: `packages/core/src/constants.ts`
 - Used by: Host request interceptors and streaming parsers.
 
@@ -89,9 +89,9 @@
 - Pattern: Raw socket read-write buffer stream with custom chunked-transfer decoding.
 
 **`ModelResolver` (`resolveModelWithTier`):**
-- Purpose: Maps external AI model tags (e.g. `claude-3-7-sonnet`) into internal Google Antigravity identifiers, specifying thinking budgets and custom header styles.
+- Purpose: Maps external AI model tags (e.g. `claude-3-7-sonnet`, `gemini-3.6-flash`) into internal Google Antigravity identifiers, specifying thinking budgets and custom header styles.
 - Location: `packages/core/src/transform/model-resolver.ts`
-- Pattern: Regular expression and alias lookup maps.
+- Pattern: Regular expression and alias lookup maps backed by tiered route definitions in `packages/core/src/model-registry.ts`.
 
 **`ensureProjectContext`:**
 - Purpose: Automatically initializes, caches, and maintains valid Google Cloud project mappings for standard or enterprise accounts.

STRUCTURE.md

@@ -17,7 +17,7 @@ antigravity-auth/
 **`packages/core`:**
 - Purpose: Provides harness-agnostic utilities for Google Antigravity integrations.
 - Contains: Direct TCP/TLS socket transport, Google OAuth PKCE auth flow, Google Cloud project bootstrapping, Claude and Gemini request/response transforms, device fingerprint generators, and centralized model registries.
-- Key files: `packages/core/src/agy-transport.ts` (TCP/TLS socket), `packages/core/src/project.ts` (project resolution), `packages/core/src/transform/model-resolver.ts` (model mapping).
+- Key files: `packages/core/src/agy-transport.ts` (TCP/TLS socket), `packages/core/src/project.ts` (project resolution), `packages/core/src/transform/model-resolver.ts` (model mapping), `packages/core/src/model-registry.ts` (model definitions & route maps).
 
 **`packages/opencode`:**
 - Purpose: Integrates the Antigravity authentication and request transformation logic into the OpenCode host environment.
@@ -42,6 +42,7 @@ antigravity-auth/
 **Core Logic:**
 - `packages/core/src/agy-transport.ts`: Custom TCP/TLS transport socket implementation.
 - `packages/core/src/project.ts`: Project resolution, context loading, and GCP project provisioning.
+- `packages/core/src/model-registry.ts`: Model definitions, quota groups, and Gemini Flash tier routing maps.
 - `packages/core/src/transform/cross-model-sanitizer.ts`: Payload cleanup when switching model families.
 - `packages/opencode/src/plugin/accounts.ts`: Multi-account selection, rotation, metrics, and health scores.
 

packages/opencode/src/plugin/request.test.ts

@@ -1313,6 +1313,32 @@ it("removes x-api-key header", () => {
       expect(wrapped.request.labels.model_enum).toBe(modelEnum);
     });
 
+    it("appends a user continuation when a Gemini AGY request ends with a model turn", () => {
+      const result = prepareAntigravityRequest(
+        "https://generativelanguage.googleapis.com/v1beta/models/antigravity-gemini-3.6-flash:generateContent",
+        {
+          method: "POST",
+          body: JSON.stringify({
+            contents: [
+              { role: "user", parts: [{ text: "Summarize this context" }] },
+              { role: "model", parts: [{ text: "Summary complete" }] },
+            ],
+          }),
+        },
+        mockAccessToken,
+        mockProjectId,
+        undefined,
+        "antigravity",
+      );
+
+      const wrapped = JSON.parse(result.init.body as string);
+      expect(wrapped.request.contents).toEqual([
+        { role: "user", parts: [{ text: "Summarize this context" }] },
+        { role: "model", parts: [{ text: "Summary complete" }] },
+        { role: "user", parts: [{ text: "[Continue]" }] },
+      ]);
+    });
+
     it("preserves uppercase Gemini schemas for properties named thinking", () => {
       const result = prepareAntigravityRequest(
         "https://generativelanguage.googleapis.com/v1beta/models/antigravity-gemini-3.6-flash:generateContent",

packages/opencode/src/plugin/request.ts

@@ -1243,10 +1243,11 @@ export function prepareAntigravityRequest(
           }
         }
 
-        // Guard against assistant prefill: Claude rejects conversations ending
-        // with model/assistant messages. After context compaction, the conversation
-        // can end with a model message — append synthetic user message to fix.
-        if (isClaude) {
+        // AGY rejects every request that ends with a model turn. Enforce this at
+        // the final wire boundary because host races, recovery, or sanitization can
+        // otherwise leave a model/assistant entry last. Claude fallback transports
+        // have the same assistant-prefill restriction.
+        if (headerStyle === "antigravity" || isClaude) {
           for (const req of requestObjects) {
             if (Array.isArray((req as any).contents)) {
               const contents = (req as any).contents;
@@ -1819,10 +1820,11 @@ export function prepareAntigravityRequest(
           }
         }
 
-        // Guard against assistant prefill: Claude rejects conversations ending
-        // with model/assistant messages. After context compaction, the conversation
-        // can end with a model message — append synthetic user message to fix.
-        if (isClaude) {
+        // AGY rejects every request that ends with a model turn. Enforce this at
+        // the final wire boundary because host races, recovery, or sanitization can
+        // otherwise leave a model/assistant entry last. Claude fallback transports
+        // have the same assistant-prefill restriction.
+        if (headerStyle === "antigravity" || isClaude) {
           if (Array.isArray(requestPayload.contents)) {
             const lastContent = requestPayload.contents[requestPayload.contents.length - 1] as any;
             if (lastContent?.role === "model" || lastContent?.role === "assistant") {