diff --git a/open-sse/services/provider.ts b/open-sse/services/provider.ts
index 6b62bea92a..e269c85863 100644
--- a/open-sse/services/provider.ts
+++ b/open-sse/services/provider.ts
@@ -96,6 +96,15 @@ export function detectFormatFromEndpoint(body, endpointPath = "") {
     return "claude";
   }
 
+  // Antigravity/cloudcode-compatible inbound endpoint (D4): the AgentBridge
+  // proxy forwards the IDE's cloudcode envelope here. Path-based detection
+  // (mirrors /messages → claude) makes the pipeline translate the request
+  // antigravity→openai and the response openai→antigravity, so the IDE gets
+  // a cloudcode reply regardless of which provider actually served it.
+  if (/\/antigravity(?=\/|:|$)/i.test(path) || /^antigravity(?=\/|:|$)/i.test(path)) {
+    return "antigravity";
+  }
+
   if (
     /\/(?:chat\/completions|completions)(?=\/|$)/i.test(path) ||
     /^(?:chat\/completions|completions)(?=\/|$)/i.test(path)
diff --git a/src/app/api/v1/antigravity/route.ts b/src/app/api/v1/antigravity/route.ts
new file mode 100644
index 0000000000..89f12036ae
--- /dev/null
+++ b/src/app/api/v1/antigravity/route.ts
@@ -0,0 +1,47 @@
+import { handleChat } from "@/sse/handlers/chat";
+import { initTranslators } from "@omniroute/open-sse/translator/index.ts";
+
+let initialized = false;
+
+/**
+ * Initialize translators once.
+ */
+async function ensureInitialized() {
+  if (!initialized) {
+    await initTranslators();
+    initialized = true;
+    console.log("[SSE] Translators initialized for /v1/antigravity");
+  }
+}
+
+/**
+ * Handle CORS preflight.
+ */
+export async function OPTIONS() {
+  return new Response(null, {
+    headers: {
+      "Access-Control-Allow-Methods": "POST, OPTIONS",
+      "Access-Control-Allow-Headers": "*",
+    },
+  });
+}
+
+/**
+ * POST /v1/antigravity — Antigravity/cloudcode-compatible endpoint.
+ *
+ * Accepts the Antigravity IDE's cloudcode envelope
+ *   { model, project, request: { contents, systemInstruction, tools, generationConfig } }
+ * and returns a cloudcode SSE reply
+ *   { response: { candidates: [...], usageMetadata } }.
+ *
+ * `detectFormatFromEndpoint()` classifies the `/antigravity` path as
+ * sourceFormat "antigravity" (mirrors `/v1/messages` → claude), so `handleChat`
+ * translates the request antigravity→openai, routes it to the resolved
+ * provider/model, and translates the response openai→antigravity — reusing the
+ * already-registered bidirectional translators. The AgentBridge MITM proxy
+ * (`server.cjs`) forwards the IDE's intercepted cloudcode request here.
+ */
+export async function POST(request: Request): Promise<Response> {
+  await ensureInitialized();
+  return await handleChat(request);
+}
diff --git a/src/mitm/_internal/forwardTarget.cjs b/src/mitm/_internal/forwardTarget.cjs
new file mode 100644
index 0000000000..269f94f3a8
--- /dev/null
+++ b/src/mitm/_internal/forwardTarget.cjs
@@ -0,0 +1,55 @@
+"use strict";
+
+// =========================================================================
+// Decide where the standalone proxy (server.cjs) forwards an intercepted,
+// decrypted request — and which router endpoint speaks its wire format.
+//
+// The Antigravity IDE sends a cloudcode envelope:
+//   { model, project, request: { contents, systemInstruction, tools, ... } }
+// and expects a cloudcode SSE reply ({ response: { candidates, ... } }).
+// That envelope is forwarded to the antigravity-compatible endpoint
+// (/v1/antigravity), where the pipeline translates request antigravity→openai
+// and response openai→antigravity, so the IDE gets its own format back
+// regardless of which provider actually served it.
+//
+// Plain OpenAI bodies ({ messages: [...] }) keep going to /v1/chat/completions.
+//
+// Pure + deterministic so it is unit-testable without the proxy.
+// =========================================================================
+
+const CHAT_PATH = "/v1/chat/completions";
+const ANTIGRAVITY_PATH = "/v1/antigravity";
+
+/**
+ * True when the parsed body is an Antigravity/cloudcode envelope (the IDE wraps
+ * the Gemini-style payload under `request`, with a `contents` array).
+ */
+function isCloudcodeEnvelope(body) {
+  return !!(
+    body &&
+    typeof body === "object" &&
+    !Array.isArray(body) &&
+    body.request &&
+    typeof body.request === "object" &&
+    Array.isArray(body.request.contents)
+  );
+}
+
+/**
+ * Resolve the forward URL + the format the IDE expects back. `baseUrl` is the
+ * router base (e.g. http://localhost:20128); trailing slashes are trimmed.
+ */
+function resolveForwardTarget(baseUrl, body) {
+  const base = String(baseUrl || "").replace(/\/+$/, "");
+  if (isCloudcodeEnvelope(body)) {
+    return { url: `${base}${ANTIGRAVITY_PATH}`, format: "antigravity" };
+  }
+  return { url: `${base}${CHAT_PATH}`, format: "openai" };
+}
+
+module.exports = {
+  resolveForwardTarget,
+  isCloudcodeEnvelope,
+  CHAT_PATH,
+  ANTIGRAVITY_PATH,
+};
diff --git a/src/mitm/server.cjs b/src/mitm/server.cjs
index cb86fe6228..8be6ad1b76 100644
--- a/src/mitm/server.cjs
+++ b/src/mitm/server.cjs
@@ -135,6 +135,7 @@ function sanitizeErrorMessage(message) {
 
 const bypassShim = require("./_internal/bypass.cjs");
 const ingestShim = require("./_internal/ingest.cjs");
+const forwardShim = require("./_internal/forwardTarget.cjs");
 
 // Inspector capture (D4 fallback). The standalone proxy intercepts AgentBridge
 // traffic inline (no MitmHandlerBase / agentBridgeHook), so it posts captured
@@ -470,8 +471,17 @@ async function intercept(req, res, bodyBuffer, mappedModel, sourceModel) {
     const body = JSON.parse(bodyBuffer.toString());
     body.model = mappedModel;
 
+    // Gap B — the Antigravity IDE speaks cloudcode (the Gemini payload wrapped
+    // under `request`) and expects a cloudcode reply. Forward such envelopes to
+    // the antigravity-compatible endpoint (which translates both directions) so
+    // the IDE gets its own format back; plain OpenAI bodies still go to
+    // chat/completions. Without this, cloudcode hits chat/completions and 400s
+    // on the missing `messages` field.
+    const forward = forwardShim.resolveForwardTarget(ROUTER_BASE_URL, body);
+    vlog(1, `[MITM] → forward ${forward.format} ${forward.url}`);
+
     upstreamStartedAt = Date.now();
-    const response = await fetch(ROUTER_URL, {
+    const response = await fetch(forward.url, {
       method: "POST",
       headers: {
         "Content-Type": "application/json",
diff --git a/tests/unit/antigravity-format-detection.test.ts b/tests/unit/antigravity-format-detection.test.ts
new file mode 100644
index 0000000000..45ef14021f
--- /dev/null
+++ b/tests/unit/antigravity-format-detection.test.ts
@@ -0,0 +1,24 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import { detectFormatFromEndpoint } from "@omniroute/open-sse/services/provider.ts";
+
+const ENVELOPE = {
+  model: "gemini-2.5-pro",
+  project: "projects/test",
+  request: { contents: [{ role: "user", parts: [{ text: "oi" }] }] },
+};
+
+test("detectFormatFromEndpoint classifies the /v1/antigravity path as antigravity", () => {
+  assert.equal(detectFormatFromEndpoint(ENVELOPE, "/v1/antigravity"), "antigravity");
+  assert.equal(detectFormatFromEndpoint(ENVELOPE, "/api/v1/antigravity"), "antigravity");
+  assert.equal(
+    detectFormatFromEndpoint(ENVELOPE, "/v1/antigravity:streamGenerateContent"),
+    "antigravity"
+  );
+});
+
+test("the antigravity carve-out does not disturb the other endpoint formats", () => {
+  assert.equal(detectFormatFromEndpoint({ messages: [] }, "/v1/messages"), "claude");
+  assert.equal(detectFormatFromEndpoint({ messages: [] }, "/v1/chat/completions"), "openai");
+  assert.equal(detectFormatFromEndpoint({ input: "x" }, "/v1/responses"), "openai-responses");
+});
diff --git a/tests/unit/mitm-forward-target.test.ts b/tests/unit/mitm-forward-target.test.ts
new file mode 100644
index 0000000000..5c5ace48ed
--- /dev/null
+++ b/tests/unit/mitm-forward-target.test.ts
@@ -0,0 +1,45 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import { createRequire } from "node:module";
+
+const require = createRequire(import.meta.url);
+const { resolveForwardTarget, isCloudcodeEnvelope, CHAT_PATH, ANTIGRAVITY_PATH } = require(
+  "../../src/mitm/_internal/forwardTarget.cjs"
+);
+
+const BASE = "http://localhost:20128";
+
+test("cloudcode envelope routes to the antigravity endpoint", () => {
+  const body = {
+    model: "gemini-2.5-pro",
+    project: "projects/test",
+    request: { contents: [{ role: "user", parts: [{ text: "oi" }] }] },
+  };
+  assert.equal(isCloudcodeEnvelope(body), true);
+  const t = resolveForwardTarget(BASE, body);
+  assert.equal(t.url, `${BASE}${ANTIGRAVITY_PATH}`);
+  assert.equal(t.format, "antigravity");
+});
+
+test("plain OpenAI body routes to chat/completions", () => {
+  const body = { model: "gemini-2.5-pro", messages: [{ role: "user", content: "oi" }] };
+  assert.equal(isCloudcodeEnvelope(body), false);
+  const t = resolveForwardTarget(BASE, body);
+  assert.equal(t.url, `${BASE}${CHAT_PATH}`);
+  assert.equal(t.format, "openai");
+});
+
+test("base url trailing slash is trimmed", () => {
+  const t = resolveForwardTarget("http://localhost:20128/", { messages: [] });
+  assert.equal(t.url, `http://localhost:20128${CHAT_PATH}`);
+});
+
+test("non-envelope shapes are not misclassified as cloudcode", () => {
+  // request present but contents missing / wrong type
+  assert.equal(isCloudcodeEnvelope({ request: {} }), false);
+  assert.equal(isCloudcodeEnvelope({ request: { contents: "nope" } }), false);
+  assert.equal(isCloudcodeEnvelope({ contents: [{ parts: [] }] }), false); // bare gemini, no envelope
+  assert.equal(isCloudcodeEnvelope(null), false);
+  assert.equal(isCloudcodeEnvelope([]), false);
+  assert.equal(isCloudcodeEnvelope("string"), false);
+});