MeshGemma

An offline disaster-mesh app that runs Gemma 4 on the phone.

When the internet, cell towers, and routers are all down, MeshGemma lets a group of iPhones form a peer-to-peer mesh over Bluetooth and WiFi-Direct, share signed incident reports and chat, navigate on a fully offline map, and run Gemma 4 E2B on-device to analyze photos, summarize the situation, and compress field reports for low-bandwidth radio uplink — all with zero connectivity.

---

Features

• Off-grid mesh networking — phones discover each other and sync over two independent message transports, no router or internet required.
• Signed, tamper-evident messages — every incident, chat, and location update is an Ed25519-signed envelope with a Lamport clock; peers verify before they trust.
• Multi-hop forwarding + catch-up — envelopes relay through intermediate phones, and a phone that reconnects automatically receives the messages it missed.
• On-device Gemma 4 E2B — text and vision inference runs directly on the iPhone via llama.rn (llama.cpp) with Metal GPU offload. No server needed.
• On-device photo analysis — attach a JPEG to an incident and Gemma describes the scene, hazards, people, and urgency, locally on the device.
• Offline tactical map — MapLibre with pre-cached OpenStreetMap tiles; pan, locate, and see colored incident pins with no network.
• Radio-uplink compression — Gemma compresses up to 50 mesh records into a ≤200-byte JSON payload sized for a low-bandwidth government radio link.
• Optional Mac-host mode — for speed, the AI features can also point at a llama-server on a Mac instead of running on-device.

App tour — five tabs

Tab	What it does
Mesh Scanner	Live view of discovered peers (mDNS, MultipeerConnectivity, BLE proximity).
Tactical Map	Offline MapLibre map with preload-area caching and incident pins.
Local Feed	GPS-tagged, signed incident log with optional photo attachments.
Analysis LPU	Gemma chat + photo analysis (LOCAL on-device or REMOTE Mac host) + the radio-uplink panel.
Chat	Peer-to-peer signed text chat across the mesh.

Architecture overview

Two message transports feed one signed-envelope handler with per-transport dedup; a third BLE layer is presence-only:

1. WiFi / TCP via mDNS — phones on the same network auto-discover and exchange envelopes over a TCP server on port 4000. Highest throughput, multi-hop forwarding, catch-up on discovery.
2. MultipeerConnectivity — Apple's Bluetooth + WiFi-Direct framework. Truly off-grid: airplane mode on, Bluetooth on, no router. ~30 m range.
3. BLE proximity beacon — advertises the node's public key + short ID so phones light up on each other's radar. Presence-only (no data bus).

Signed wire format — CryptoService produces a SignedEnvelope: { v, pubKey, lamport, ts, payload, sig }. Payloads are canonicalized with a recursive sorted-key serializer, signed with Ed25519 (tweetnacl), and stamped with a Lamport clock that jumps a safety reserve ahead on every boot to prevent counter reuse after a crash.

On-device Gemma 4 — LocalLlamaService downloads the Gemma 4 E2B model (gemma-4-E2B-it-UD-IQ2_M.gguf, ~2.29 GB) plus its vision projector (mmproj-F16.gguf, ~986 MB), ~3.28 GB total. It runs inference through llama.rn (a React Native binding of llama.cpp), offloading all layers to the iOS Metal GPU. initMultimodal() loads the projector so the same context handles both text and image input.

Offline map — MapTileService drives MapLibre's native OfflineManager, pre-downloading OpenStreetMap raster tiles into the SDK's SQLite store so the map renders with no connectivity.

Radio compression — RadioCompressService asks Gemma (on-device or Mac-host) to crunch the incident feed into a strict ≤200-byte JSON payload, with a one-shot retry if the model returns malformed or oversized JSON.

For a deeper code tour, see ARCHITECTURE.md.

Tech stack

• Expo SDK 54, React Native 0.81, TypeScript — iOS app.
• llama.rn 0.12 (React Native binding of llama.cpp) — on-device Gemma 4 E2B, iOS Metal.
• @maplibre/maplibre-react-native — offline vector/raster maps.
• tweetnacl — Ed25519 signing.
• react-native-tcp-socket + react-native-zeroconf — TCP mesh + mDNS discovery.
• react-native-ble-plx — BLE scanning.
• Two custom Swift Expo modules under modules/ — mesh-multipeer (MultipeerConnectivity) and mesh-ble-peripheral (BLE peripheral beacon).

Setup & running

This is a custom Expo dev-client app — it does not run in Expo Go (the native mesh modules require a prebuild). Full step-by-step build and two-iPhone demo instructions are in DEMO_RUNBOOK.md.

Quick start:

npm install
npx expo prebuild --clean
cd ios && pod install && cd ..
npx expo run:ios --device

Honest deferrals

We label what is real and what is simulated, on-camera and in the code:

• Radio uplink TX is a simulated animation. There is no LoRa hardware in this build. The radio panel genuinely uses Gemma to compress records to a ≤200-byte payload, but the "TRANSMIT" step is an honestly-labeled animation (ENCODING → TX @ 868 MHz → ACK) — a persistent banner says so. No real RF is transmitted.
• The BLE transport is presence-only. The BLE beacon makes phones visible to each other on the scanner radar; it is not used as a data bus. Mesh data flows over TCP/mDNS and MultipeerConnectivity.

Hackathon tracks

• Main track
• Impact: Global Resilience
• Special Technology: llama.cpp

Team

Jasper Groen and Guus Adema.

License

CC-BY 4.0.