cat1: mixed-server (2.7) + whoami per-user execution proof (2.4) PASS via tunnel-registered ref server

categories/cat1-functional/tests/probes.md

@@ -56,3 +56,27 @@ removed since first probe: ['Brightdata_ScrapeAsMarkdown']
 metadata). Corollary: the edit did **not** propagate while unsaved (draft); it appeared only after
 **Save** — correct/expected, not a defect. Propagation was effectively immediate (next poll).
 
+## Mixed prebuilt + custom + whoami execution proof (2.7, 2.4) — full self-hosted chain
+Registered the reference server (`arcade-mcp`, toolkit `ArcadeEvalRef`) as a **self-hosted Arcade
+server** via a Cloudflare tunnel (dashboard Add Server → **Arcade** type; URI = trycloudflare URL,
+Secret = `ARCADE_WORKER_SECRET`), then added Echo/Add/Whoami to `zeb-gateway-test`. (`arcade deploy`
+hosted is cloud-only — see LIVE-POC finding — so the register path is used.)
+
+Gateway lists **10 tools in one flat list — prebuilt + custom coexist**:
+```
+prebuilt (7): GoogleDocs x4, Slack x2, Youtube x1     (Arcade-cloud `main`)
+custom   (3): ArcadeEvalRef_Add, _Echo, _Whoami       (our self-hosted server, via tunnel)
+```
+Invocation (full chain client -> gateway -> Engine -> Cloudflare tunnel -> local server):
+```
+ArcadeEvalRef_Echo(text="hello-from-A") as A  ->  "hello-from-A"   (isError: False)
+```
+**Per-user EXECUTION proof (whoami):**
+```
+whoami as A (user-a@servicetitan.com)  ->  "user-a@servicetitan.com"
+whoami as B (user-b@servicetitan.com)  ->  "user-b@servicetitan.com"
+```
+Each caller's `Arcade-User-ID` is injected into `context.user_id` and returned — the tool provably
+executes as the calling user (distinct identity per caller, no shared/service identity). Also
+validates **cat-9 Stage-2** (full tunnel-registration chain) end-to-end.
+