diff --git a/__tests__/lib/j0di3/j0di3-proxy.test.ts b/__tests__/lib/j0di3/j0di3-proxy.test.ts
index d08812724..5c92bfbf5 100644
--- a/__tests__/lib/j0di3/j0di3-proxy.test.ts
+++ b/__tests__/lib/j0di3/j0di3-proxy.test.ts
@@ -15,16 +15,19 @@ vi.mock("axios", () => ({
     },
 }));
 
-import j0di3 from "@/lib/j0di3-client";
 import axios from "axios";
+import j0di3 from "@/lib/j0di3-client";
 import { j0di3Proxy } from "@/lib/j0di3-proxy";
 
 const mockJ0di3 = j0di3 as unknown as Mock;
 const mockIsAxiosError = axios.isAxiosError as Mock;
 
-function createMockReqRes(
-    overrides: Partial<NextApiRequest> = {}
-): { req: NextApiRequest; res: NextApiResponse } {
+const VALID_TROOP_ID = "00000000-0000-4000-8000-000000000001";
+
+function createMockReqRes(overrides: Partial<NextApiRequest> = {}): {
+    req: NextApiRequest;
+    res: NextApiResponse;
+} {
     const req = {
         method: "POST",
         body: { question: "What is React?" },
@@ -34,7 +37,7 @@ function createMockReqRes(
             name: "Test",
             email: "test@test.com",
             role: "STUDENT",
-            troopId: "troop-uuid-123",
+            troopId: VALID_TROOP_ID,
         },
         ...overrides,
     } as unknown as NextApiRequest;
@@ -59,7 +62,7 @@ describe("j0di3Proxy", () => {
         expect(mockJ0di3).toHaveBeenCalledWith({
             method: "POST",
             url: "/api/v1/learning/explain",
-            data: { question: "What is React?", troop_id: "troop-uuid-123" },
+            data: { question: "What is React?", troop_id: VALID_TROOP_ID },
             params: undefined,
         });
         expect(res.json).toHaveBeenCalledWith({ response: "React is a library" });
@@ -77,7 +80,7 @@ describe("j0di3Proxy", () => {
             method: "GET",
             url: "/api/v1/challenges/recommended",
             data: undefined,
-            params: expect.objectContaining({ troop_id: "troop-uuid-123" }),
+            params: expect.objectContaining({ troop_id: VALID_TROOP_ID }),
         });
     });
 
@@ -110,6 +113,23 @@ describe("j0di3Proxy", () => {
         );
     });
 
+    it("returns 400 when troopId is not a valid UUID", async () => {
+        const consoleSpy = vi.spyOn(console, "warn").mockImplementation(() => {});
+        const handler = j0di3Proxy("POST", "/api/v1/learning/explain");
+        const { req, res } = createMockReqRes();
+        (req as any).user.troopId = "not-a-uuid";
+
+        await handler(req, res);
+
+        expect(res.status).toHaveBeenCalledWith(400);
+        expect(res.json).toHaveBeenCalledWith(
+            expect.objectContaining({ error: expect.stringContaining("troop profile is invalid") })
+        );
+        expect(mockJ0di3).not.toHaveBeenCalled();
+        expect(consoleSpy).toHaveBeenCalled();
+        consoleSpy.mockRestore();
+    });
+
     it("forwards J0dI3 error status and message on axios error", async () => {
         const axiosError = {
             response: {
diff --git a/src/lib/j0di3-proxy.ts b/src/lib/j0di3-proxy.ts
index 5e55b7035..6c2c6900b 100644
--- a/src/lib/j0di3-proxy.ts
+++ b/src/lib/j0di3-proxy.ts
@@ -1,14 +1,22 @@
 import axios from "axios";
 import type { NextApiRequest, NextApiResponse } from "next";
-import { requireAuth, type AuthenticatedRequest } from "@/lib/rbac";
 import j0di3 from "@/lib/j0di3-client";
+import { type AuthenticatedRequest, requireAuth } from "@/lib/rbac";
 
 type Method = "GET" | "POST" | "PATCH" | "DELETE";
 
-export function j0di3Proxy(
-    method: Method,
-    path: string | ((req: NextApiRequest) => string)
-) {
+// J0dI3 issues UUIDs for troop IDs (see ensure-troop.ts). Reject anything
+// that isn't shaped like one before making the external call, so a stale
+// session or bad migration surfaces as a clear user-actionable error
+// instead of an opaque 4xx/5xx from the backend.
+const UUID_RE = /^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/i;
+
+function redactTroopId(value: string): string {
+    if (value.length <= 8) return `${value.slice(0, 2)}…`;
+    return `${value.slice(0, 4)}…${value.slice(-4)}`;
+}
+
+export function j0di3Proxy(method: Method, path: string | ((req: NextApiRequest) => string)) {
     return requireAuth(async (req: AuthenticatedRequest, res: NextApiResponse) => {
         const url = typeof path === "function" ? path(req) : path;
         const troopId = req.user!.troopId;
@@ -19,9 +27,19 @@ export function j0di3Proxy(
                 .json({ error: "No J0dI3 troop profile linked. Please sign out and back in." });
         }
 
+        if (!UUID_RE.test(troopId)) {
+            console.warn(
+                "[j0di3-proxy] Rejected malformed troopId for user",
+                req.user!.id,
+                redactTroopId(troopId)
+            );
+            return res.status(400).json({
+                error: "Your troop profile is invalid. Please sign out and sign back in to refresh it.",
+            });
+        }
+
         try {
-            const body =
-                method !== "GET" ? { ...req.body, troop_id: troopId } : undefined;
+            const body = method !== "GET" ? { ...req.body, troop_id: troopId } : undefined;
 
             const { data } = await j0di3({
                 method,
@@ -30,12 +48,14 @@ export function j0di3Proxy(
                 params: method === "GET" ? { ...req.query, troop_id: troopId } : undefined,
             });
 
-
             res.json(data);
         } catch (error: unknown) {
             if (axios.isAxiosError(error) && error.response) {
                 const status = error.response.status;
-                const message = error.response.data?.detail || error.response.data?.error || "J0dI3 request failed";
+                const message =
+                    error.response.data?.detail ||
+                    error.response.data?.error ||
+                    "J0dI3 request failed";
                 return res.status(status).json({ error: message });
             }
             console.error("[j0di3-proxy] Unexpected error:", error);