diff --git a/open-sse/services/provider.ts b/open-sse/services/provider.ts
index 9ba0a0acff..5c26af2fcc 100644
--- a/open-sse/services/provider.ts
+++ b/open-sse/services/provider.ts
@@ -131,6 +131,12 @@ export function detectFormatFromEndpoint(body, endpointPath = "") {
   return detectFormat(body);
 }
 
+// Thin wrapper for call sites that only have the full request URL (not the bare endpoint
+// path chatCore already threads) — single source of truth stays detectFormatFromEndpoint.
+export function detectFormatFromUrl(body, requestUrl) {
+  return detectFormatFromEndpoint(body, new URL(requestUrl).pathname);
+}
+
 // Detect request format from body structure
 export function detectFormat(body) {
   // OpenAI Responses API:
diff --git a/src/mitm/server.cjs b/src/mitm/server.cjs
index e7fe31c46d..ac8a27d83c 100644
--- a/src/mitm/server.cjs
+++ b/src/mitm/server.cjs
@@ -338,9 +338,9 @@ function getSqliteDb() {
 
 /**
  * Resolve the stored alias override for a source model: `{ model?, reasoningEffort? }`.
- * `normalizeAliasMappings` upgrades legacy plain-string mappings (every existing install)
- * into the structured shape, so both old and new saves resolve consistently. Returns
- * `null` when there is no override at all for this model (passthrough).
+ * `normalizeAliasMappings` upgrades legacy plain-string mappings into the structured
+ * shape. The route-only namespace is reserved for client-facing OmniRoute model ids;
+ * fall back to `mitmAlias` until a route-alias writer is available.
  */
 function getMappedOverride(model, agentId = "antigravity") {
   return standaloneRoutingShim.resolveMappedOverride(model, agentId, {
diff --git a/src/sse/handlers/chat.ts b/src/sse/handlers/chat.ts
index e7975dabc8..10169de513 100644
--- a/src/sse/handlers/chat.ts
+++ b/src/sse/handlers/chat.ts
@@ -31,7 +31,7 @@ import {
   HTTP_STATUS,
   ANTIGRAVITY_PRE_RESPONSE_TIMEOUT_CODE,
 } from "@omniroute/open-sse/config/constants.ts";
-import { getTargetFormat } from "@omniroute/open-sse/services/provider.ts";
+import { getTargetFormat, detectFormatFromUrl } from "@omniroute/open-sse/services/provider.ts";
 import {
   getModelsByProviderId,
   getModelTargetFormat,
@@ -253,6 +253,8 @@ export async function handleChat(
   // reasoning_effort / reasoning / object-shaped thinking always wins (backward compatible).
   body = normalizeReasoningRequest(body);
 
+  const sourceFormat = detectFormatFromUrl(body, request.url);
+
   // Early guard: an invalid `messages` field is rejected here with a clear
   // OmniRoute-level 400 before any routing or upstream call (#5110, #6402).
   // Without this guard, schema-invalid bodies fell through to model resolution
@@ -261,22 +263,20 @@ export async function handleChat(
   //   - present-but-non-array (null, number, string, object) → 400 (#6402)
   //   - empty array → 400 ("at least one message is required") (#5110)
   //   - missing entirely, when the Responses-API `input` discriminator is also
-  //     absent → 400 (#6402). Responses-API requests use `input` (not `messages`)
-  //     and are still unaffected.
-  {
-    const b = body as { messages?: unknown; input?: unknown };
-    if ("messages" in b && !Array.isArray(b.messages)) {
-      log.warn("CHAT", "Rejecting request with non-array messages");
-      return errorResponse(HTTP_STATUS.BAD_REQUEST, "messages: Expected array");
-    }
-    if (Array.isArray(b.messages) && b.messages.length === 0) {
-      log.warn("CHAT", "Rejecting request with empty messages array");
-      return errorResponse(HTTP_STATUS.BAD_REQUEST, "messages: at least one message is required");
-    }
-    if (!("messages" in b) && !("input" in b)) {
-      log.warn("CHAT", "Rejecting request with missing messages");
-      return errorResponse(HTTP_STATUS.BAD_REQUEST, "messages: Expected array, received undefined");
-    }
+  //     absent → 400 (#6402). Responses-API requests use `input` (not `messages`),
+  //     and Antigravity requests use a cloudcode `request` envelope.
+  const msgBody = body as { messages?: unknown; input?: unknown };
+  if ("messages" in msgBody && !Array.isArray(msgBody.messages)) {
+    log.warn("CHAT", "Rejecting request with non-array messages");
+    return errorResponse(HTTP_STATUS.BAD_REQUEST, "messages: Expected array");
+  }
+  if (Array.isArray(msgBody.messages) && msgBody.messages.length === 0) {
+    log.warn("CHAT", "Rejecting request with empty messages array");
+    return errorResponse(HTTP_STATUS.BAD_REQUEST, "messages: at least one message is required");
+  }
+  if (!("messages" in msgBody) && !("input" in msgBody) && sourceFormat !== "antigravity") {
+    log.warn("CHAT", "Rejecting request with missing messages");
+    return errorResponse(HTTP_STATUS.BAD_REQUEST, "messages: Expected array, received undefined");
   }
 
   // Reject non-string `model` before it reaches downstream code that calls
diff --git a/tests/unit/chat-messages-validation-6402.test.ts b/tests/unit/chat-messages-validation-6402.test.ts
index 4b58f5d4ec..478181e90d 100644
--- a/tests/unit/chat-messages-validation-6402.test.ts
+++ b/tests/unit/chat-messages-validation-6402.test.ts
@@ -1,6 +1,11 @@
 import test from "node:test";
 import assert from "node:assert/strict";
 
+import {
+  ANTIGRAVITY_MODEL_ALIASES,
+  ANTIGRAVITY_PUBLIC_MODELS,
+} from "../../open-sse/config/antigravityModelAliases.ts";
+import { AGY_PUBLIC_MODELS } from "../../open-sse/config/agyModels.ts";
 import { createChatPipelineHarness } from "../integration/_chatPipelineHarness.ts";
 
 // Regression tests for #6402 — schema-invalid `messages` fields fell through
@@ -122,3 +127,58 @@ test("#6402: Responses-API input passes the guard (messages discriminator preser
     );
   }
 });
+
+const ANTIGRAVITY_GEMINI_MODELS = Array.from(
+  new Set(
+    [
+      ...ANTIGRAVITY_PUBLIC_MODELS.map((model) => model.id),
+      ...AGY_PUBLIC_MODELS.map((model) => model.id),
+      ...Object.keys(ANTIGRAVITY_MODEL_ALIASES),
+      ...Object.values(ANTIGRAVITY_MODEL_ALIASES),
+    ].filter((model) => /^(gemini(?!-claude)|rev19)/.test(model))
+  )
+).sort();
+
+for (const model of ANTIGRAVITY_GEMINI_MODELS) {
+  test(`Antigravity cloudcode envelope routes without the missing-messages 400: ${model}`, async () => {
+    await seedConnection("antigravity", {
+      accessToken: "ag-access",
+      providerSpecificData: { projectId: "test-project" },
+    });
+
+    let upstreamCalled = false;
+    globalThis.fetch = async () => {
+      upstreamCalled = true;
+      return new Response(
+        'data: {"response":{"candidates":[{"content":{"parts":[{"text":"ok"}]},"finishReason":"STOP"}]}}\n\n',
+        { status: 200, headers: { "Content-Type": "text/event-stream" } }
+      );
+    };
+
+    const response = await handleChat(
+      buildRequest({
+        url: "http://localhost/v1/antigravity",
+        body: {
+          model: `antigravity/${model}`,
+          project: "projects/test",
+          request: {
+            contents: [{ role: "user", parts: [{ text: "Hello" }] }],
+          },
+        },
+      })
+    );
+
+    assert.equal(upstreamCalled, true, "Antigravity request should reach the upstream executor");
+    assert.equal(response.status, 200, "Antigravity cloudcode envelopes must not return 400");
+
+    const body = await response.text();
+    assert.match(body, /ok/);
+  });
+}
+
+test("Antigravity Gemini-family regression coverage is not accidentally empty", () => {
+  assert.ok(
+    ANTIGRAVITY_GEMINI_MODELS.length >= 20,
+    `expected broad Gemini-family coverage, got ${ANTIGRAVITY_GEMINI_MODELS.length}`
+  );
+});