diff --git a/internal/agent/engines/llamacpp.go b/internal/agent/engines/llamacpp.go
index fb7bf22..ed5cc29 100644
--- a/internal/agent/engines/llamacpp.go
+++ b/internal/agent/engines/llamacpp.go
@@ -277,6 +277,9 @@ func (l *LlamaCpp) bin() string {
 	return "llama-server"
 }
 
+// Binary is the llama-server this engine runs.
+func (l *LlamaCpp) Binary() string { return l.bin() }
+
 // FallsBackToCPU says that llama.cpp keeps running when its GPU backend does
 // not load: --n-gpu-layers asks for layers on a GPU it then has none of, and
 // the server comes up and answers on the CPU, at a tenth of the speed, with no
diff --git a/internal/cli/enginecheck.go b/internal/cli/enginecheck.go
index bed0d0b..ebc506b 100644
--- a/internal/cli/enginecheck.go
+++ b/internal/cli/enginecheck.go
@@ -4,10 +4,15 @@ import (
 	"context"
 	"fmt"
 	"io"
+	"net"
+	"net/http"
+	"os"
 	"os/exec"
 	"path/filepath"
 	"sort"
+	"strconv"
 	"strings"
+	"sync"
 	"time"
 
 	"github.com/spf13/cobra"
@@ -22,6 +27,7 @@ func newEngineCheckCmd() *cobra.Command {
 		nodes   int
 		tp      int
 		perNode int
+		model   string
 	)
 
 	cmd := &cobra.Command{
@@ -31,6 +37,10 @@ func newEngineCheckCmd() *cobra.Command {
 			"engines rename and remove flags between versions. When that happens the\n" +
 			"failure arrives minutes into a model load, in an engine's own error\n" +
 			"message, on a node nobody was watching.\n\n" +
+			"For llama.cpp on a machine with NVIDIA GPUs it then loads a small GGUF\n" +
+			"(--model, or the smallest in the weights cache) with every layer on the GPU\n" +
+			"and asks for one token: a build whose CUDA backend does not load computes on\n" +
+			"the CPU, and that is where it is caught.\n\n" +
 			"This asks the engine itself. It runs the engine's --help and checks every\n" +
 			"flag llmgw would pass against it, which needs no GPU — a CLI prints its\n" +
 			"help on any machine. Run it after upgrading an engine.\n\n" +
@@ -49,6 +59,11 @@ func newEngineCheckCmd() *cobra.Command {
 			// rarely on PATH: a check that cannot find the engine reports
 			// nothing rather than reporting a problem.
 			cfg := engines.Config{}
+			// The build the agent installed, when none is named: the newest
+			// under /opt/llmgw/engines, which is where every node keeps it.
+			if bin == "" && name == "llamacpp" {
+				bin = installedLlama()
+			}
 			if bin != "" {
 				switch name {
 				case "llamacpp":
@@ -135,6 +150,11 @@ func newEngineCheckCmd() *cobra.Command {
 			var deviceErr error
 			if fb, ok := eng.(engines.CPUFallback); ok && help == "" {
 				deviceErr = checkDevices(cmd.Context(), name, fb, out)
+				// Seeing a GPU is not computing on it: a small model loaded
+				// on it and asked for a token is.
+				if b, ok := eng.(interface{ Binary() string }); ok && deviceErr == nil && model != "none" {
+					deviceErr = computeOnGPU(cmd.Context(), b.Binary(), model, out)
+				}
 			}
 
 			fmt.Fprintf(out, "%s: %d flags checked", name, checked)
@@ -160,7 +180,9 @@ func newEngineCheckCmd() *cobra.Command {
 
 	f := cmd.Flags()
 	f.StringVar(&help, "help-output", "", "a saved --help to check against, instead of running the engine")
-	f.StringVar(&bin, "bin", "", "where the engine lives, when it is not on PATH")
+	f.StringVar(&bin, "bin", "", "where the engine lives (default: the build the agent installed, else PATH)")
+	f.StringVar(&model, "model", "", "a GGUF file to load on the GPU and ask for a token (default: the smallest "+
+		"in this node's weights cache; none skips it)")
 	f.IntVar(&nodes, "nodes", 2, "shape the command for a replica of this many nodes")
 	f.IntVar(&tp, "tp", 8, "tensor parallel degree")
 	f.IntVar(&perNode, "gpus-per-node", 4, "GPUs each node contributes")
@@ -286,3 +308,128 @@ func checkDevices(ctx context.Context, name string, fb engines.CPUFallback, out
 	fmt.Fprintf(out, "computes on: %s\n", strings.Join(devs, "; "))
 	return nil
 }
+
+// installedLlama is the newest llama-server the agent installed here.
+func installedLlama() string {
+	m, _ := filepath.Glob("/opt/llmgw/engines/llamacpp/*/llama-server")
+	if len(m) == 0 {
+		return ""
+	}
+	sort.Strings(m)
+	return m[len(m)-1]
+}
+
+// weightDirs are where a node keeps models it has fetched.
+var weightDirs = []string{"/var/lib/llmgw-weights", "/var/lib/llmgw-models", "/srv/models"}
+
+// smallestGGUF is the smallest GGUF file under dirs, for a check that only
+// needs something to load.
+func smallestGGUF(dirs []string) string {
+	best, size := "", int64(0)
+	for _, d := range dirs {
+		_ = filepath.WalkDir(d, func(p string, e os.DirEntry, err error) error {
+			if err != nil || e.IsDir() || !strings.HasSuffix(strings.ToLower(p), ".gguf") ||
+				strings.Contains(strings.ToLower(filepath.Base(p)), "mmproj") {
+				return nil
+			}
+			if info, err := e.Info(); err == nil && (best == "" || info.Size() < size) {
+				best, size = p, info.Size()
+			}
+			return nil
+		})
+	}
+	return best
+}
+
+// computeOnGPU loads a model with every layer on the GPU, asks for one
+// token, and reads from llama.cpp's own output where the layers went.
+func computeOnGPU(ctx context.Context, bin, model string, out io.Writer) error {
+	if _, err := exec.LookPath("nvidia-smi"); err != nil {
+		return nil
+	}
+	if model == "" {
+		if model = smallestGGUF(weightDirs); model == "" {
+			fmt.Fprintln(out, "no GGUF in this node's weights cache to compute with; --model PATH runs one")
+			return nil
+		}
+	}
+	ln, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		return err
+	}
+	port := ln.Addr().(*net.TCPAddr).Port
+	_ = ln.Close()
+	ctx, cancel := context.WithTimeout(ctx, 3*time.Minute)
+	defer cancel()
+	var logs syncBuffer
+	c := exec.CommandContext(ctx, bin, "-m", model, "--host", "127.0.0.1", "--port", strconv.Itoa(port), //nolint:gosec // the engine's own binary
+		"--n-gpu-layers", "999", "--ctx-size", "512", "--parallel", "1")
+	c.Stdout, c.Stderr = &logs, &logs
+	if err := c.Start(); err != nil {
+		return fmt.Errorf("starting %s: %w", bin, err)
+	}
+	defer func() { _ = c.Process.Kill(); _ = c.Wait() }()
+	base := "http://127.0.0.1:" + strconv.Itoa(port)
+	start := time.Now()
+	for {
+		resp, err := http.Get(base + "/health") //nolint:gosec,noctx // loopback, bounded by the loop
+		if err == nil {
+			_ = resp.Body.Close()
+			if resp.StatusCode == http.StatusOK {
+				break
+			}
+		}
+		if ctx.Err() != nil {
+			return fmt.Errorf("%s did not come up with %s: %s", bin, filepath.Base(model), tailLines(logs.String(), 5))
+		}
+		time.Sleep(500 * time.Millisecond)
+	}
+	req, _ := http.NewRequestWithContext(ctx, http.MethodPost, base+"/completion",
+		strings.NewReader(`{"prompt":"Hello","n_predict":1}`))
+	req.Header.Set("Content-Type", "application/json")
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return fmt.Errorf("asking it for a token: %w", err)
+	}
+	_ = resp.Body.Close()
+	took := time.Since(start)
+	comp, said := engines.ReadLlamaCompute(strings.Split(logs.String(), "\n"))
+	if !said {
+		fmt.Fprintf(out, "computed a token with %s, but llama.cpp did not say where\n", filepath.Base(model))
+		return nil
+	}
+	if comp.OnTheCPU() {
+		return fmt.Errorf("%s computed on the CPU though every layer was asked for on the GPU (%d of %d offloaded): "+
+			"its CUDA backend did not load; ldd on its libggml-cuda.so names the missing library",
+			filepath.Base(model), comp.Offloaded, comp.Total)
+	}
+	fmt.Fprintf(out, "computed a token on %s: %s, %d of %d layers offloaded, in %s\n", comp.Backend,
+		strings.Join(comp.Devices, "; "), comp.Offloaded, comp.Total, took.Round(100*time.Millisecond))
+	return nil
+}
+
+func tailLines(s string, n int) string {
+	l := strings.Split(strings.TrimSpace(s), "\n")
+	if len(l) > n {
+		l = l[len(l)-n:]
+	}
+	return strings.Join(l, " | ")
+}
+
+// syncBuffer is a buffer two writers share.
+type syncBuffer struct {
+	mu sync.Mutex
+	b  strings.Builder
+}
+
+func (s *syncBuffer) Write(p []byte) (int, error) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	return s.b.Write(p)
+}
+
+func (s *syncBuffer) String() string {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	return s.b.String()
+}
diff --git a/internal/cli/enginecheck_gpu_test.go b/internal/cli/enginecheck_gpu_test.go
new file mode 100644
index 0000000..61fec52
--- /dev/null
+++ b/internal/cli/enginecheck_gpu_test.go
@@ -0,0 +1,66 @@
+package cli
+
+import (
+	"bytes"
+	"fmt"
+	"net/http"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+// TestMain lets the test binary stand in for llama-server: run with
+// LLMGW_FAKE_LLAMA set, it prints what llama.cpp prints and answers.
+func TestMain(m *testing.M) {
+	if mode := os.Getenv("LLMGW_FAKE_LLAMA"); mode != "" {
+		fakeLlama(mode)
+		return
+	}
+	os.Exit(m.Run())
+}
+
+func fakeLlama(mode string) {
+	port := ""
+	for i, a := range os.Args {
+		if a == "--port" && i+1 < len(os.Args) {
+			port = os.Args[i+1]
+		}
+	}
+	if mode == "gpu" {
+		fmt.Println("llama_model_load_from_file_impl: using device CUDA0 (Quadro RTX 6000) - 23800 MiB free")
+		fmt.Println("load_tensors: offloaded 25/25 layers to GPU")
+	} else {
+		fmt.Println("warning: no usable GPU found, --gpu-layers option will be ignored")
+		fmt.Println("load_tensors: offloaded 0/25 layers to GPU")
+	}
+	http.HandleFunc("/health", func(w http.ResponseWriter, _ *http.Request) { w.WriteHeader(http.StatusOK) })
+	http.HandleFunc("/completion", func(w http.ResponseWriter, _ *http.Request) { _, _ = w.Write([]byte(`{"content":"!"}`)) })
+	_ = http.ListenAndServe("127.0.0.1:"+port, nil) //nolint:gosec // a test's stand-in
+}
+
+func TestEngineCheckComputesATokenOnTheGPU(t *testing.T) {
+	// nvidia-smi present, so the check runs.
+	dir := t.TempDir()
+	smi := filepath.Join(dir, "nvidia-smi")
+	if err := os.WriteFile(smi, []byte("#!/bin/sh\nexit 0\n"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv("PATH", dir+string(os.PathListSeparator)+os.Getenv("PATH"))
+	model := filepath.Join(dir, "tiny.gguf")
+	_ = os.WriteFile(model, []byte("GGUF"), 0o644)
+	self, _ := os.Executable()
+
+	t.Setenv("LLMGW_FAKE_LLAMA", "gpu")
+	var out bytes.Buffer
+	if err := computeOnGPU(t.Context(), self, model, &out); err != nil || !strings.Contains(out.String(), "on CUDA") ||
+		!strings.Contains(out.String(), "25 of 25") {
+		t.Fatalf("on the GPU: %v %s", err, out.String())
+	}
+
+	t.Setenv("LLMGW_FAKE_LLAMA", "cpu")
+	out.Reset()
+	if err := computeOnGPU(t.Context(), self, model, &out); err == nil || !strings.Contains(err.Error(), "computed on the CPU") {
+		t.Fatalf("a build that falls back to the CPU: %v %s", err, out.String())
+	}
+}