diff --git a/mcp-server/index.ts b/mcp-server/index.ts
index 5576f79..c144b43 100644
--- a/mcp-server/index.ts
+++ b/mcp-server/index.ts
@@ -23,6 +23,13 @@ import { roleBand, SCENES, SCENES_VERSION, FACE_RENDER_DIM, type RoleBand } from
 import { ICONS, ICONS_VERSION, DEFAULT_NEGATIVE, certToSlug, type IconRecipe } from "./icon_recipes.js";
 
 const BASE = process.env.LAKEHOUSE_URL || "http://localhost:3100";
+// G5 cutover prep (2026-05-01): when GO_LAKEHOUSE_URL is set, the
+// `/_go/*` pass-through routes to the Go gateway. Off-by-default
+// (empty = disabled, returns 503 on /_go/*). Reversible via unset.
+// Doesn't modify any existing tool — additive parallel path so
+// operators can validate Go-side handlers under real Bun-frontend
+// shape without touching the production Rust path.
+const GO_BASE = process.env.GO_LAKEHOUSE_URL || "";
 const PORT = parseInt(process.env.MCP_PORT || "3700");
 
 // ─── Staffer roster — used by the per-staffer hot-swap index (G). ────
@@ -711,6 +718,26 @@ async function main() {
           return new Response(await r.text(), { status: r.status, headers: { "Content-Type": "application/json" } });
         }
 
+        // G5 cutover slice: pass-through to GO_BASE (Go gateway). Same
+        // shape as /api/* but points at the Go side. Returns 503 when
+        // GO_LAKEHOUSE_URL isn't set so callers know the cutover slice
+        // is off rather than silently routing to Rust. No body
+        // transformation — caller responsible for sending Go-shaped
+        // requests (e.g. /v1/embed not /ai/embed).
+        if (url.pathname.startsWith("/_go/")) {
+          if (!GO_BASE) {
+            return err("GO_LAKEHOUSE_URL not set; /_go/* cutover slice is disabled", 503);
+          }
+          const path = url.pathname.replace("/_go", "");
+          const body = req.method !== "GET" ? await req.text() : undefined;
+          try {
+            const r = await fetch(`${GO_BASE}${path}`, { method: req.method, headers: { "Content-Type": "application/json" }, body });
+            return new Response(await r.text(), { status: r.status, headers: { "Content-Type": "application/json" } });
+          } catch (e) {
+            return err(`Go gateway unreachable at ${GO_BASE}: ${e}`, 502);
+          }
+        }
+
         // Proof — narrative HTML served from mcp-server/proof.html.
         // Live tests consumed client-side via /proof.json.
         if (url.pathname === "/proof") {