diff --git a/profiles/deck/configuration.nix b/profiles/deck/configuration.nix
index 89b4d79..ae96153 100644
--- a/profiles/deck/configuration.nix
+++ b/profiles/deck/configuration.nix
@@ -19,6 +19,7 @@
     ../../services/eternal-terminal
     ../../services/matrix-conduit
     ../../services/nginx-matrix
+    ../../services/llm-steamdeck
     ./switch-emu.nix
   ];
 
diff --git a/services/default.nix b/services/default.nix
index 226478a..33d6b65 100644
--- a/services/default.nix
+++ b/services/default.nix
@@ -67,6 +67,7 @@
     ./reactive-resume
     ./anki-sync-server
     ./winnie
+    ./llm-steamdeck
     # ./kanidm
     # ./syncserver
     # ./newt # attendre maj flakes
diff --git a/services/llm-steamdeck/README.md b/services/llm-steamdeck/README.md
new file mode 100644
index 0000000..3769a0d
--- /dev/null
+++ b/services/llm-steamdeck/README.md
@@ -0,0 +1,186 @@
+# Steam Deck LLM Service - Usage Guide
+
+## Enable in your configuration
+
+The service is already imported in `profiles/deck/configuration.nix`. Enable it by adding:
+
+```nix
+{ config, lib, pkgs, ... }: {
+  services.llm-steamdeck = {
+    enable = true;
+
+    # Optional: customize model (defaults to Qwen3.5-9B Q4_K_M)
+    # modelRepo = "bartowski/Qwen_Qwen3.5-9B-GGUF";
+    # modelFile = "Qwen3.5-9B-Q4_K_M.gguf";
+    # contextSize = 8192;
+
+    # Optional: server settings
+    # port = 8080;
+    # host = "0.0.0.0";  # Use "127.0.0.1" for local-only
+
+    # Optional: performance tuning
+    # gpuLayers = 99;        # All layers to GPU (Vulkan)
+    # threads = 8;           # Steam Deck has 8 cores
+    # batchSize = 512;
+    # ubatchSize = 512;
+    # flashAttn = true;
+
+    # Optional: sampling defaults
+    # temperature = 0.7;
+    # topP = 0.95;
+    # topK = 20;
+    # repeatPenalty = 1.1;
+
+    # Optional: extra llama-server args
+    # extraArgs = [ "--reasoning" "on" ];  # Enable Qwen3.5 thinking mode
+  };
+}
+```
+
+## Model options
+
+| Model | Repo | File | Size | Notes |
+|-------|------|------|------|-------|
+| **Qwen3.5-9B Q4_K_M** (default) | `bartowski/Qwen_Qwen3.5-9B-GGUF` | `Qwen3.5-9B-Q4_K_M.gguf` | ~6.17 GB | Best quality/speed balance |
+| Qwen3.5-9B Q4_K_M (TheStageAI) | `TheStageAI/Qwen3.5-9B-GGUF` | `Qwen3.5-9B-M-TS-Q4_K_M.gguf` | ~5.06 GB | Smaller, slightly lower quality |
+| Qwen3.5-9B Q5_K_M | `bartowski/Qwen_Qwen3.5-9B-GGUF` | `Qwen3.5-9B-Q5_K_M.gguf` | ~7.11 GB | Higher quality, more VRAM |
+| Qwen3.5-9B Q3_K_M | `bartowski/Qwen_Qwen3.5-9B-GGUF` | `Qwen3.5-9B-Q3_K_M.gguf` | ~5.18 GB | Lower quality, faster |
+
+## Service management
+
+```bash
+# Check status
+systemctl status llm-steamdeck
+
+# View logs
+journalctl -u llm-steamdeck -f
+
+# Restart
+systemctl restart llm-steamdeck
+
+# Stop
+systemctl stop llm-steamdeck
+
+# Force model re-download
+rm -rf /var/lib/llm-steamdeck && systemctl restart llm-steamdeck-download llm-steamdeck
+```
+
+## API usage
+
+The server exposes an OpenAI-compatible API at `http://deck:8080/v1` (or `localhost:8080` from the Deck).
+
+### Chat completions
+
+```bash
+curl -X POST http://deck:8080/v1/chat/completions \
+  -H "Content-Type: application/json" \
+  -d '{
+    "model": "qwen3.5-9b",
+    "messages": [
+      {"role": "user", "content": "Explain quantum computing in simple terms"}
+    ],
+    "temperature": 0.7,
+    "max_tokens": 512
+  }'
+```
+
+### With thinking/reasoning mode (Qwen3.5)
+
+```bash
+curl -X POST http://deck:8080/v1/chat/completions \
+  -H "Content-Type: application/json" \
+  -d '{
+    "model": "qwen3.5-9b",
+    "messages": [
+      {"role": "user", "content": "Solve this step by step: 25 * 37"}
+    ],
+    "extra_body": {
+      "chat_template_kwargs": {"enable_thinking": true}
+    }
+  }'
+```
+
+### Streaming
+
+```bash
+curl -X POST http://deck:8080/v1/chat/completions \
+  -H "Content-Type: application/json" \
+  -d '{
+    "model": "qwen3.5-9b",
+    "messages": [{"role": "user", "content": "Write a haiku about Steam Deck"}],
+    "stream": true
+  }'
+```
+
+### Web UI
+
+Open `http://deck:8080` in a browser for the built-in llama.cpp web interface.
+
+## Performance notes for Steam Deck
+
+- **Expected speed**: ~10-13 tokens/s (Qwen3.5-9B Q4_K_M, all layers on GPU via Vulkan/RADV)
+- **Prefill latency**: ~30-60s for 4000-token prompts (weak GPU compute)
+- **RAM usage**: ~8-9 GB total (model + KV cache + OS)
+- **Headless mode**: For max context (16K), run without gamescope/desktop:
+  ```bash
+  systemctl stop plasma-plasma-kwin_wayland@0.service sddm
+  # Then start llm-steamdeck
+  ```
+- **Vulkan**: Uses RADV (Mesa) - ensure `hardware.opengl.enable = true` and `hardware.opengl.driSupport = true`
+
+## Troubleshooting
+
+### OOM / crash on start
+- Reduce `gpuLayers` to 80 or lower
+- Reduce `contextSize` to 4096
+- Use smaller quantization (Q3_K_M)
+
+### Slow prefill
+- This is normal for Steam Deck GPU (8 CU RDNA 2, ~1.6 TFLOPS)
+- Keep prompts under 2000 tokens for responsive chat
+- Use smaller `batchSize` / `ubatchSize` (256)
+
+### Vulkan not working
+```bash
+# Verify Vulkan
+vulkaninfo | grep -E "(GPU|deviceName|driver)"
+
+# Check RADV
+RADV_PERFTEST=aco,nggc,mesh_shaders AMD_DEBUG=w32ge,w32cs \
+  llama-server --model /var/lib/llm-steamdeck/... -ngl 99 --port 8081
+```
+
+### Model download fails
+- Check network connectivity
+- For gated models, set `hfToken` via sops secret
+- Manual download:
+  ```bash
+  huggingface-cli download bartowski/Qwen_Qwen3.5-9B-GGUF \
+    --include "Qwen3.5-9B-Q4_K_M.gguf" \
+    --local-dir /var/lib/llm-steamdeck/bartowski_Qwen_Qwen3.5-9B-GGUF
+  ```
+
+## Integration with Hermes / other tools
+
+Add to your Hermes/OpenAI client config:
+```yaml
+providers:
+  - name: "deck-qwen"
+    type: "openai"
+    base_url: "http://deck.local:8080/v1"
+    api_key: "dummy"  # llama-server doesn't require auth
+    models: ["qwen3.5-9b"]
+```
+
+## Updating the model
+
+```bash
+# Stop service
+systemctl stop llm-steamdeck
+
+# Remove old model
+rm -rf /var/lib/llm-steamdeck/bartowski_Qwen_Qwen3.5-9B-GGUF
+
+# Restart (triggers re-download)
+systemctl start llm-steamdeck-download llm-steamdeck
+```
\ No newline at end of file
diff --git a/services/llm-steamdeck/default.nix b/services/llm-steamdeck/default.nix
new file mode 100644
index 0000000..1e61b42
--- /dev/null
+++ b/services/llm-steamdeck/default.nix
@@ -0,0 +1,293 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  cfg = config.services.llm-steamdeck;
+
+  # Default model configuration - Qwen3.5-9B Q4_K_M (~6.17 GB)
+  defaultModel = {
+    repo = "bartowski/Qwen_Qwen3.5-9B-GGUF";
+    file = "Qwen3.5-9B-Q4_K_M.gguf";
+    contextSize = 8192;
+  };
+
+  # Alternative: TheStageAI Qwen3.5-9B Q4_K_M (~5.06 GB, slightly smaller)
+  altModel = {
+    repo = "TheStageAI/Qwen3.5-9B-GGUF";
+    file = "Qwen3.5-9B-M-TS-Q4_K_M.gguf";
+    contextSize = 8192;
+  };
+
+  # Model to use (can be overridden)
+  model = if cfg.modelRepo != null then {
+    repo = cfg.modelRepo;
+    file = cfg.modelFile or "Qwen3.5-9B-Q4_K_M.gguf";
+    contextSize = cfg.contextSize or 8192;
+  } else defaultModel;
+
+  # Download model on first run using llama.cpp's built-in HF support
+  modelPath = "/var/lib/llm-steamdeck/${model.repo}/${model.file}";
+
+in {
+  options.services.llm-steamdeck = {
+    enable = mkEnableOption "llama.cpp OpenAI-compatible server for Steam Deck (Vulkan)";
+
+    # Model configuration
+    modelRepo = mkOption {
+      type = types.nullOr types.str;
+      default = null;
+      description = "Hugging Face repository for the GGUF model (e.g. 'bartowski/Qwen_Qwen3.5-9B-GGUF'). Null uses default Qwen3.5-9B Q4_K_M.";
+    };
+    modelFile = mkOption {
+      type = types.str;
+      default = "Qwen3.5-9B-Q4_K_M.gguf";
+      description = "GGUF filename within the repository.";
+    };
+    contextSize = mkOption {
+      type = types.int;
+      default = 8192;
+      description = "Context window size. 8192 recommended for 16GB RAM; 16384 possible in headless mode.";
+    };
+
+    # Server configuration
+    port = mkOption {
+      type = types.portNumber;
+      default = 8080;
+      description = "Port for the OpenAI-compatible API server.";
+    };
+    host = mkOption {
+      type = types.str;
+      default = "0.0.0.0";
+      description = "Host to bind to. Use '127.0.0.1' for local-only access.";
+    };
+
+    # GPU/performance options
+    gpuLayers = mkOption {
+      type = types.int;
+      default = 99;
+      description = "Number of layers to offload to GPU (Vulkan). 99 = all layers. Reduce if OOM.";
+    };
+    threads = mkOption {
+      type = types.int;
+      default = 8;
+      description = "CPU threads for llama.cpp (Steam Deck has 8 cores).";
+    };
+    batchSize = mkOption {
+      type = types.int;
+      default = 512;
+      description = "Batch size for prompt processing. Lower if prefill OOM.";
+    };
+    ubatchSize = mkOption {
+      type = types.int;
+      default = 512;
+      description = "Micro-batch size for prompt processing.";
+    };
+    flashAttn = mkOption {
+      type = types.bool;
+      default = true;
+      description = "Enable flash attention (requires llama.cpp with Qwen3.5 support).";
+    };
+
+    # Sampling defaults (can be overridden per-request)
+    temperature = mkOption {
+      type = types.float;
+      default = 0.7;
+      description = "Default sampling temperature.";
+    };
+    topP = mkOption {
+      type = types.float;
+      default = 0.95;
+      description = "Default top-p sampling.";
+    };
+    topK = mkOption {
+      type = types.int;
+      default = 20;
+      description = "Default top-k sampling.";
+    };
+    repeatPenalty = mkOption {
+      type = types.float;
+      default = 1.1;
+      description = "Default repeat penalty.";
+    };
+
+    # Service management
+    user = mkOption {
+      type = types.str;
+      default = "deck";
+      description = "User to run the service as.";
+    };
+    group = mkOption {
+      type = types.str;
+      default = "deck";
+      description = "Group for the service.";
+    };
+    extraArgs = mkOption {
+      type = types.listOf types.str;
+      default = [ ];
+      description = "Extra arguments passed to llama-server.";
+    };
+
+    # Model download
+    autoDownload = mkOption {
+      type = types.bool;
+      default = true;
+      description = "Automatically download model on first start via llama.cpp HF support.";
+    };
+    hfToken = mkOption {
+      type = types.nullOr types.str;
+      default = null;
+      description = "Hugging Face token for private/gated models (set via sops secret).";
+    };
+
+    # Web UI
+    enableWebUI = mkOption {
+      type = types.bool;
+      default = true;
+      description = "Enable built-in web UI at /";
+    };
+  };
+
+  config = mkIf cfg.enable {
+    # Ensure Vulkan support for RADV (Steam Deck AMD GPU)
+    hardware.opengl.enable = true;
+    hardware.opengl.driSupport = true;
+    hardware.opengl.driSupport32Bit = true;
+
+    # Vulkan ICD loader for llama.cpp
+    environment.systemPackages = with pkgs; [
+      vulkan-tools
+      vulkan-loader
+      mesa
+      mesa.drivers
+      mesa_glu
+    ];
+
+    # Create model directory with proper permissions
+    systemd.tmpfiles.rules = [
+      "d /var/lib/llm-steamdeck 0755 ${cfg.user} ${cfg.group} - -"
+      "d /var/lib/llm-steamdeck/${model.repo} 0755 ${cfg.user} ${cfg.group} - -"
+    ];
+
+    # Pre-download model if autoDownload enabled (using llama-cli --hf-repo)
+    systemd.services.llm-steamdeck-download = {
+      description = "Download LLM model from Hugging Face";
+      documentation = [ "https://github.com/ggml-org/llama.cpp" ];
+      wantedBy = [ "multi-user.target" ];
+      before = [ "llm-steamdeck.service" ];
+      conditionPathExists = "!${modelPath}";
+      serviceConfig = {
+        Type = "oneshot";
+        User = cfg.user;
+        Group = cfg.group;
+        ExecStart = ''
+          ${pkgs.llama-cpp-vulkan}/bin/llama-cli \
+            --hf-repo ${model.repo} \
+            --hf-file ${model.file} \
+            --model ${modelPath} \
+            -p " " \
+            -n 1 \
+            ${lib.optionalString (cfg.hfToken != null) "--hf-token ${cfg.hfToken}"}
+        '';
+        Environment = [
+          "HOME=/home/${cfg.user}"
+          "XDG_CACHE_HOME=/home/${cfg.user}/.cache"
+        ];
+        StateDirectory = "llm-steamdeck";
+        TimeoutStartSec = 1800; # 30 min for download
+      };
+    };
+
+    # Main llama-server service
+    systemd.services.llm-steamdeck = {
+      description = "llama.cpp OpenAI-compatible server (Vulkan) for Steam Deck";
+      documentation = [
+        "https://github.com/ggml-org/llama.cpp"
+        "https://huggingface.co/${model.repo}"
+      ];
+      after = [ "network-online.target" "llm-steamdeck-download.service" ];
+      wants = [ "network-online.target" ];
+      serviceConfig = {
+        Type = "simple";
+        User = cfg.user;
+        Group = cfg.group;
+        WorkingDirectory = "/var/lib/llm-steamdeck";
+        ExecStart = ''
+          ${pkgs.llama-cpp-vulkan}/bin/llama-server \
+            --model ${modelPath} \
+            --host ${cfg.host} \
+            --port ${toString cfg.port} \
+            --ctx-size ${toString model.contextSize} \
+            --n-gpu-layers ${toString cfg.gpuLayers} \
+            --threads ${toString cfg.threads} \
+            --batch-size ${toString cfg.batchSize} \
+            --ubatch-size ${toString cfg.ubatchSize} \
+            --temp ${toString cfg.temperature} \
+            --top-p ${toString cfg.topP} \
+            --top-k ${toString cfg.topK} \
+            --repeat-penalty ${toString cfg.repeatPenalty} \
+            ${lib.optionalString cfg.flashAttn "--flash-attn"} \
+            ${lib.optionalString cfg.enableWebUI "--web-ui"} \
+            ${lib.optionalString (cfg.hfToken != null) "--hf-token ${cfg.hfToken}"} \
+            ${concatStringsSep " " cfg.extraArgs}
+        '';
+        Restart = "on-failure";
+        RestartSec = 10;
+        TimeoutStartSec = 120;
+        TimeoutStopSec = 30;
+        # Memory limits for Steam Deck 16GB unified RAM
+        MemoryMax = "12G";
+        MemorySwapMax = "2G";
+        # GPU access
+        SupplementaryGroups = [ "video" "render" ];
+        Environment = [
+          "VK_ICD_FILENAMES=/etc/vulkan/icd.d/radeon_icd.x86_64.json:/etc/vulkan/icd.d/radeon_icd.i686.json"
+          "RADV_PERFTEST=aco,nggc,mesh_shaders"
+          "AMD_DEBUG=w32ge,w32cs"
+          "HOME=/home/${cfg.user}"
+          "XDG_CACHE_HOME=/home/${cfg.user}/.cache"
+          "XDG_RUNTIME_DIR=/run/user/1000"
+        ];
+        # Logging
+        StandardOutput = "journal";
+        StandardError = "journal";
+        SyslogIdentifier = "llm-steamdeck";
+      };
+    };
+
+    # Enable and start services
+    systemd.services.llm-steamdeck-download.enable = cfg.autoDownload;
+    systemd.services.llm-steamdeck.enable = true;
+
+    # Firewall opening for local network access
+    networking.firewall.allowedTCPPorts = [ cfg.port ];
+
+    # User setup (ensure deck user has video/render groups)
+    users.users.${cfg.user} = {
+      extraGroups = [ "video" "render" ];
+    };
+
+    # Convenience: systemd user service alias for easy management
+    systemd.user.services.llm-steamdeck = {
+      description = "User-level alias for llm-steamdeck";
+      serviceConfig = {
+        Type = "oneshot";
+        ExecStart = "${pkgs.systemd}/bin/systemctl --user start llm-steamdeck.service";
+        RemainAfterExit = true;
+      };
+    };
+
+    # Logrotate for llama-server logs (uses systemd journal, logrotate not needed for journald)
+    # services.logrotate = {
+    #   enable = true;
+    #   settings."llm-steamdeck" = {
+    #     path = [ "/var/log/journal/*/system.journal" ];
+    #     rotate = 7;
+    #     daily = true;
+    #     compress = true;
+    #     missingok = true;
+    #     notifempty = true;
+    #   };
+    # };
+  };
+}
\ No newline at end of file