diff --git a/internal/api/api_test.go b/internal/api/api_test.go
new file mode 100644
index 0000000..4701f53
--- /dev/null
+++ b/internal/api/api_test.go
@@ -0,0 +1,430 @@
+package api_test
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"io"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"strings"
+	"testing"
+
+	"github.com/uopi/uopi/internal/api"
+	"github.com/uopi/uopi/internal/broker"
+	"github.com/uopi/uopi/internal/datasource/stub"
+	"github.com/uopi/uopi/internal/storage"
+)
+
+// ── Test helpers ─────────────────────────────────────────────────────────────
+
+func setup(t *testing.T) (*httptest.Server, func()) {
+	t.Helper()
+
+	ctx, cancel := context.WithCancel(context.Background())
+
+	log := slog.New(slog.NewTextHandler(io.Discard, nil))
+	brk := broker.New(ctx, log)
+	ds := stub.New()
+	if err := ds.Connect(ctx); err != nil {
+		t.Fatal("stub connect:", err)
+	}
+	brk.Register(ds)
+
+	dir := t.TempDir()
+	store, err := storage.New(dir)
+	if err != nil {
+		t.Fatal("storage.New:", err)
+	}
+
+	mux := http.NewServeMux()
+	api.New(brk, nil, store, "", log).Register(mux, "/api/v1")
+
+	srv := httptest.NewServer(mux)
+	return srv, func() {
+		srv.Close()
+		cancel()
+	}
+}
+
+func get(t *testing.T, srv *httptest.Server, path string) *http.Response {
+	t.Helper()
+	resp, err := http.Get(srv.URL + path)
+	if err != nil {
+		t.Fatal("GET", path, err)
+	}
+	return resp
+}
+
+func postJSON(t *testing.T, srv *httptest.Server, path string, body any) *http.Response {
+	t.Helper()
+	b, err := json.Marshal(body)
+	if err != nil {
+		t.Fatal("json.Marshal:", err)
+	}
+	resp, err := http.Post(srv.URL+path, "application/json", bytes.NewReader(b))
+	if err != nil {
+		t.Fatal("POST", path, err)
+	}
+	return resp
+}
+
+func postRaw(t *testing.T, srv *httptest.Server, path, ct string, body []byte) *http.Response {
+	t.Helper()
+	resp, err := http.Post(srv.URL+path, ct, bytes.NewReader(body))
+	if err != nil {
+		t.Fatal("POST", path, err)
+	}
+	return resp
+}
+
+func putRaw(t *testing.T, srv *httptest.Server, path, ct string, body []byte) *http.Response {
+	t.Helper()
+	req, err := http.NewRequest(http.MethodPut, srv.URL+path, bytes.NewReader(body))
+	if err != nil {
+		t.Fatal("NewRequest PUT:", err)
+	}
+	req.Header.Set("Content-Type", ct)
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatal("PUT", path, err)
+	}
+	return resp
+}
+
+func deleteReq(t *testing.T, srv *httptest.Server, path string) *http.Response {
+	t.Helper()
+	req, err := http.NewRequest(http.MethodDelete, srv.URL+path, nil)
+	if err != nil {
+		t.Fatal("NewRequest DELETE:", err)
+	}
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatal("DELETE", path, err)
+	}
+	return resp
+}
+
+func readJSON(t *testing.T, resp *http.Response, v any) {
+	t.Helper()
+	defer resp.Body.Close()
+	if err := json.NewDecoder(resp.Body).Decode(v); err != nil {
+		t.Fatal("decode response:", err)
+	}
+}
+
+func assertStatus(t *testing.T, resp *http.Response, want int) {
+	t.Helper()
+	if resp.StatusCode != want {
+		body, _ := io.ReadAll(resp.Body)
+		resp.Body.Close()
+		t.Fatalf("expected HTTP %d, got %d: %s", want, resp.StatusCode, bytes.TrimSpace(body))
+	}
+}
+
+// ── /api/v1/datasources ───────────────────────────────────────────────────────
+
+func TestListDataSources(t *testing.T) {
+	srv, teardown := setup(t)
+	defer teardown()
+
+	resp := get(t, srv, "/api/v1/datasources")
+	assertStatus(t, resp, http.StatusOK)
+
+	var sources []struct {
+		Name   string `json:"name"`
+		Status string `json:"status"`
+	}
+	readJSON(t, resp, &sources)
+
+	if len(sources) == 0 {
+		t.Fatal("expected at least one data source")
+	}
+	found := false
+	for _, s := range sources {
+		if s.Name == "stub" {
+			found = true
+			if s.Status != "connected" {
+				t.Errorf("stub status = %q, want connected", s.Status)
+			}
+		}
+	}
+	if !found {
+		t.Error("stub data source not in list")
+	}
+}
+
+// ── /api/v1/signals ───────────────────────────────────────────────────────────
+
+func TestListSignalsMissingDs(t *testing.T) {
+	srv, teardown := setup(t)
+	defer teardown()
+
+	resp := get(t, srv, "/api/v1/signals")
+	assertStatus(t, resp, http.StatusBadRequest)
+}
+
+func TestListSignalsUnknownDs(t *testing.T) {
+	srv, teardown := setup(t)
+	defer teardown()
+
+	resp := get(t, srv, "/api/v1/signals?ds=nonexistent")
+	assertStatus(t, resp, http.StatusNotFound)
+}
+
+func TestListSignalsStub(t *testing.T) {
+	srv, teardown := setup(t)
+	defer teardown()
+
+	resp := get(t, srv, "/api/v1/signals?ds=stub")
+	assertStatus(t, resp, http.StatusOK)
+
+	var signals []struct {
+		Name string `json:"name"`
+		Type string `json:"type"`
+	}
+	readJSON(t, resp, &signals)
+
+	if len(signals) == 0 {
+		t.Fatal("expected signals from stub data source")
+	}
+}
+
+// ── /api/v1/signals/search ────────────────────────────────────────────────────
+
+func TestSearchSignals(t *testing.T) {
+	srv, teardown := setup(t)
+	defer teardown()
+
+	resp := get(t, srv, "/api/v1/signals/search?q=sine")
+	assertStatus(t, resp, http.StatusOK)
+
+	var signals []struct{ Name string `json:"name"` }
+	readJSON(t, resp, &signals)
+
+	for _, s := range signals {
+		if !strings.Contains(s.Name, "sine") {
+			t.Errorf("signal %q does not match query 'sine'", s.Name)
+		}
+	}
+}
+
+func TestSearchSignalsEmpty(t *testing.T) {
+	srv, teardown := setup(t)
+	defer teardown()
+
+	// Empty query should return all signals.
+	resp := get(t, srv, "/api/v1/signals/search")
+	assertStatus(t, resp, http.StatusOK)
+
+	var signals []any
+	readJSON(t, resp, &signals)
+	if len(signals) == 0 {
+		t.Error("expected signals for empty query")
+	}
+}
+
+// ── /api/v1/interfaces ────────────────────────────────────────────────────────
+
+const sampleXML = `<interface id="" name="Test Panel" version="1" w="800" h="600"></interface>`
+
+func TestInterfaceCRUD(t *testing.T) {
+	srv, teardown := setup(t)
+	defer teardown()
+
+	// List — initially empty
+	resp := get(t, srv, "/api/v1/interfaces")
+	assertStatus(t, resp, http.StatusOK)
+	var list []any
+	readJSON(t, resp, &list)
+	if len(list) != 0 {
+		t.Fatalf("expected empty interface list, got %d", len(list))
+	}
+
+	// Create
+	resp = postRaw(t, srv, "/api/v1/interfaces", "application/xml", []byte(sampleXML))
+	assertStatus(t, resp, http.StatusCreated)
+	var created struct{ ID string `json:"id"` }
+	readJSON(t, resp, &created)
+	if created.ID == "" {
+		t.Fatal("expected non-empty ID from create")
+	}
+
+	// Get
+	resp = get(t, srv, "/api/v1/interfaces/"+created.ID)
+	assertStatus(t, resp, http.StatusOK)
+	body, _ := io.ReadAll(resp.Body)
+	resp.Body.Close()
+	if !bytes.Contains(body, []byte("Test Panel")) {
+		t.Errorf("GET response missing interface name: %s", body)
+	}
+
+	// Update
+	updated := `<interface id="" name="Updated Panel" version="2" w="1024" h="768"></interface>`
+	resp = putRaw(t, srv, "/api/v1/interfaces/"+created.ID, "application/xml", []byte(updated))
+	assertStatus(t, resp, http.StatusNoContent)
+
+	// Verify update
+	resp = get(t, srv, "/api/v1/interfaces/"+created.ID)
+	assertStatus(t, resp, http.StatusOK)
+	body, _ = io.ReadAll(resp.Body)
+	resp.Body.Close()
+	if !bytes.Contains(body, []byte("Updated Panel")) {
+		t.Errorf("GET after update missing new name: %s", body)
+	}
+
+	// List — now one entry
+	resp = get(t, srv, "/api/v1/interfaces")
+	assertStatus(t, resp, http.StatusOK)
+	readJSON(t, resp, &list)
+	if len(list) != 1 {
+		t.Fatalf("expected 1 interface in list, got %d", len(list))
+	}
+
+	// Clone
+	req, _ := http.NewRequest(http.MethodPost, srv.URL+"/api/v1/interfaces/"+created.ID+"/clone", nil)
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatal("clone:", err)
+	}
+	assertStatus(t, resp, http.StatusCreated)
+	var cloned struct{ ID string `json:"id"` }
+	readJSON(t, resp, &cloned)
+	if cloned.ID == created.ID {
+		t.Error("clone produced same ID as original")
+	}
+
+	// Delete original
+	resp = deleteReq(t, srv, "/api/v1/interfaces/"+created.ID)
+	assertStatus(t, resp, http.StatusNoContent)
+
+	// Get after delete → 404
+	resp = get(t, srv, "/api/v1/interfaces/"+created.ID)
+	assertStatus(t, resp, http.StatusNotFound)
+
+	// Delete non-existent → 404
+	resp = deleteReq(t, srv, "/api/v1/interfaces/"+created.ID)
+	assertStatus(t, resp, http.StatusNotFound)
+}
+
+func TestInterfaceInvalidXML(t *testing.T) {
+	srv, teardown := setup(t)
+	defer teardown()
+
+	resp := postRaw(t, srv, "/api/v1/interfaces", "application/xml", []byte("not xml at all"))
+	assertStatus(t, resp, http.StatusBadRequest)
+}
+
+func TestInterfacePathTraversal(t *testing.T) {
+	srv, teardown := setup(t)
+	defer teardown()
+
+	// Attempt to read a file outside the storage dir via path traversal in the ID.
+	resp := get(t, srv, "/api/v1/interfaces/../etc/passwd")
+	// Should be 404 (invalid ID), not 200 or 500.
+	if resp.StatusCode == http.StatusOK {
+		t.Error("path traversal succeeded — expected 404")
+	}
+}
+
+// ── Synthetic (disabled — synth is nil) ──────────────────────────────────────
+
+func TestSyntheticDisabled(t *testing.T) {
+	srv, teardown := setup(t)
+	defer teardown()
+
+	resp := get(t, srv, "/api/v1/synthetic")
+	assertStatus(t, resp, http.StatusOK)
+	var out []any
+	readJSON(t, resp, &out)
+	if len(out) != 0 {
+		t.Errorf("expected empty list when synthetic is nil, got %v", out)
+	}
+}
+
+func TestSyntheticCreateDisabled(t *testing.T) {
+	srv, teardown := setup(t)
+	defer teardown()
+
+	resp := postJSON(t, srv, "/api/v1/synthetic", map[string]any{"name": "test"})
+	assertStatus(t, resp, http.StatusServiceUnavailable)
+}
+
+// ── Channel Finder (not configured) ──────────────────────────────────────────
+
+func TestChannelFinderNotConfigured(t *testing.T) {
+	srv, teardown := setup(t)
+	defer teardown()
+
+	resp := get(t, srv, "/api/v1/channel-finder?q=test")
+	assertStatus(t, resp, http.StatusNotImplemented)
+}
+
+// ── /healthz ──────────────────────────────────────────────────────────────────
+
+func TestHealthz(t *testing.T) {
+	// healthz is registered in server.go, not api.go, so test it directly.
+	mux := http.NewServeMux()
+	mux.HandleFunc("/healthz", func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write([]byte("ok"))
+	})
+	srv := httptest.NewServer(mux)
+	defer srv.Close()
+
+	resp, err := http.Get(srv.URL + "/healthz")
+	if err != nil {
+		t.Fatal(err)
+	}
+	assertStatus(t, resp, http.StatusOK)
+	body, _ := io.ReadAll(resp.Body)
+	resp.Body.Close()
+	if string(body) != "ok" {
+		t.Errorf("healthz body = %q, want ok", body)
+	}
+}
+
+// ── Storage ID validation ─────────────────────────────────────────────────────
+
+func TestStorageValidateID(t *testing.T) {
+	dir := t.TempDir()
+	store, err := storage.New(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Create a real interface first to get a valid ID.
+	id, err := store.Create([]byte(sampleXML))
+	if err != nil {
+		t.Fatal("create:", err)
+	}
+
+	// Valid ID works.
+	data, err := store.Get(id)
+	if err != nil {
+		t.Fatalf("Get(%q): %v", id, err)
+	}
+	if len(data) == 0 {
+		t.Error("expected data, got empty")
+	}
+
+	// Malicious IDs must be rejected.
+	malicious := []string{"../etc/passwd", "../../secret", "a/b", "a\x00b", ""}
+	for _, bad := range malicious {
+		if _, err := store.Get(bad); err == nil {
+			t.Errorf("Get(%q) should have failed, got nil error", bad)
+		}
+		if err := store.Update(bad, []byte(sampleXML)); err == nil {
+			t.Errorf("Update(%q) should have failed", bad)
+		}
+		if err := store.Delete(bad); err == nil {
+			t.Errorf("Delete(%q) should have failed", bad)
+		}
+	}
+}
+
+// ── Ensure os is used (blank import guard) ─────────────────────────────────────
+
+var _ = os.DevNull
diff --git a/internal/broker/broker_bench_test.go b/internal/broker/broker_bench_test.go
new file mode 100644
index 0000000..6652262
--- /dev/null
+++ b/internal/broker/broker_bench_test.go
@@ -0,0 +1,101 @@
+package broker_test
+
+import (
+	"testing"
+	"time"
+
+	"github.com/uopi/uopi/internal/broker"
+)
+
+// BenchmarkFanOut measures the end-to-end latency and throughput of the broker
+// fan-out with varying numbers of downstream clients.
+
+func BenchmarkFanOut1Client(b *testing.B)   { benchFanOut(b, 1) }
+func BenchmarkFanOut10Clients(b *testing.B)  { benchFanOut(b, 10) }
+func BenchmarkFanOut20Clients(b *testing.B)  { benchFanOut(b, 20) }
+func BenchmarkFanOut100Clients(b *testing.B) { benchFanOut(b, 100) }
+
+func benchFanOut(b *testing.B, nClients int) {
+	b.Helper()
+
+	brk, cancel := newBroker(b)
+	defer cancel()
+
+	ref := broker.SignalRef{DS: "stub", Name: "counter_fast"}
+
+	// Subscribe N downstream clients.
+	type sub struct {
+		ch     chan broker.Update
+		cancel func()
+	}
+	subs := make([]sub, nClients)
+	for i := range subs {
+		ch := make(chan broker.Update, 512)
+		cancelFn, err := brk.Subscribe(ref, ch)
+		if err != nil {
+			b.Skip("signal not available:", err)
+		}
+		subs[i] = sub{ch, cancelFn}
+	}
+	defer func() {
+		for _, s := range subs {
+			s.cancel()
+		}
+	}()
+
+	// Warm-up: wait for the first update to arrive on every client.
+	timeout := time.After(3 * time.Second)
+	for _, s := range subs {
+		select {
+		case <-s.ch:
+		case <-timeout:
+			b.Skip("timed out waiting for first update — signal may not exist in stub")
+		}
+	}
+
+	b.ResetTimer()
+	b.ReportAllocs()
+
+	for range b.N {
+		// Drain one update from every client channel.
+		for _, s := range subs {
+			select {
+			case <-s.ch:
+			case <-time.After(2 * time.Second):
+				b.Fatal("timed out waiting for update")
+			}
+		}
+	}
+}
+
+// newBroker is defined in broker_test.go (same package broker_test) and shared
+// across test files.  It returns a broker pre-populated with the stub DS.
+
+// BenchmarkSubscribeUnsubscribe measures the overhead of adding and removing
+// a subscription (the fast path — signal is already active).
+func BenchmarkSubscribeUnsubscribe(b *testing.B) {
+	brk, cancel := newBroker(b)
+	defer cancel()
+
+	ref := broker.SignalRef{DS: "stub", Name: "sine_1hz"}
+
+	// Prime the signal so the fanOut goroutine exists.
+	ch0 := make(chan broker.Update, 256)
+	unsub0, err := brk.Subscribe(ref, ch0)
+	if err != nil {
+		b.Fatal(err)
+	}
+	defer unsub0()
+
+	b.ResetTimer()
+	b.ReportAllocs()
+
+	for range b.N {
+		ch := make(chan broker.Update, 16)
+		unsub, err := brk.Subscribe(ref, ch)
+		if err != nil {
+			b.Fatal(err)
+		}
+		unsub()
+	}
+}
diff --git a/internal/metrics/metrics.go b/internal/metrics/metrics.go
new file mode 100644
index 0000000..91b3a15
--- /dev/null
+++ b/internal/metrics/metrics.go
@@ -0,0 +1,82 @@
+// Package metrics provides in-process Prometheus-format instrumentation.
+// All counters and gauges use atomic operations and are safe for concurrent use.
+package metrics
+
+import (
+	"fmt"
+	"net/http"
+	"sync/atomic"
+	"time"
+)
+
+var startTime = time.Now()
+
+// Counters and gauges — updated by callers in ws.go and api.go.
+var (
+	wsConns       atomic.Int64 // current open WebSocket connections (gauge)
+	msgIn         atomic.Int64 // total WS messages received (counter)
+	msgOut        atomic.Int64 // total WS messages sent (counter)
+	writeOps      atomic.Int64 // total signal write operations (counter)
+	historyReqs   atomic.Int64 // total history requests served (counter)
+)
+
+// IncWsConns increments the active WebSocket connection gauge.
+func IncWsConns() { wsConns.Add(1) }
+
+// DecWsConns decrements the active WebSocket connection gauge.
+func DecWsConns() { wsConns.Add(-1) }
+
+// IncMsgIn increments the total inbound WS message counter.
+func IncMsgIn() { msgIn.Add(1) }
+
+// IncMsgOut increments the total outbound WS message counter.
+func IncMsgOut() { msgOut.Add(1) }
+
+// IncWrites increments the signal write operation counter.
+func IncWrites() { writeOps.Add(1) }
+
+// IncHistoryReqs increments the history request counter.
+func IncHistoryReqs() { historyReqs.Add(1) }
+
+// Handler returns an http.HandlerFunc that renders Prometheus-format metrics.
+// activeSubs is called each request to read the current number of unique signal
+// subscriptions from the broker; pass nil to omit the metric.
+func Handler(activeSubs func() int) http.HandlerFunc {
+	return func(w http.ResponseWriter, _ *http.Request) {
+		w.Header().Set("Content-Type", "text/plain; version=0.0.4; charset=utf-8")
+
+		uptime := time.Since(startTime).Seconds()
+		subs := 0
+		if activeSubs != nil {
+			subs = activeSubs()
+		}
+
+		fmt.Fprintf(w, "# HELP uopi_uptime_seconds Seconds elapsed since the server started\n")
+		fmt.Fprintf(w, "# TYPE uopi_uptime_seconds counter\n")
+		fmt.Fprintf(w, "uopi_uptime_seconds %.3f\n\n", uptime)
+
+		fmt.Fprintf(w, "# HELP uopi_ws_connections Current number of open WebSocket connections\n")
+		fmt.Fprintf(w, "# TYPE uopi_ws_connections gauge\n")
+		fmt.Fprintf(w, "uopi_ws_connections %d\n\n", wsConns.Load())
+
+		fmt.Fprintf(w, "# HELP uopi_signal_subscriptions Current number of unique upstream signal subscriptions\n")
+		fmt.Fprintf(w, "# TYPE uopi_signal_subscriptions gauge\n")
+		fmt.Fprintf(w, "uopi_signal_subscriptions %d\n\n", subs)
+
+		fmt.Fprintf(w, "# HELP uopi_ws_messages_in_total Total WebSocket messages received from clients\n")
+		fmt.Fprintf(w, "# TYPE uopi_ws_messages_in_total counter\n")
+		fmt.Fprintf(w, "uopi_ws_messages_in_total %d\n\n", msgIn.Load())
+
+		fmt.Fprintf(w, "# HELP uopi_ws_messages_out_total Total WebSocket messages sent to clients\n")
+		fmt.Fprintf(w, "# TYPE uopi_ws_messages_out_total counter\n")
+		fmt.Fprintf(w, "uopi_ws_messages_out_total %d\n\n", msgOut.Load())
+
+		fmt.Fprintf(w, "# HELP uopi_write_ops_total Total signal write operations\n")
+		fmt.Fprintf(w, "# TYPE uopi_write_ops_total counter\n")
+		fmt.Fprintf(w, "uopi_write_ops_total %d\n\n", writeOps.Load())
+
+		fmt.Fprintf(w, "# HELP uopi_history_requests_total Total historical data requests served\n")
+		fmt.Fprintf(w, "# TYPE uopi_history_requests_total counter\n")
+		fmt.Fprintf(w, "uopi_history_requests_total %d\n\n", historyReqs.Load())
+	}
+}
diff --git a/web/src/SyntheticWizard.tsx b/web/src/SyntheticWizard.tsx
new file mode 100644
index 0000000..da976a7
--- /dev/null
+++ b/web/src/SyntheticWizard.tsx
@@ -0,0 +1,182 @@
+import { h } from 'preact';
+import { useState } from 'preact/hooks';
+
+interface Props {
+  onClose: () => void;
+  onCreated: () => void;
+}
+
+interface NodeParam {
+  label: string;
+  key: string;
+  type: 'number' | 'text';
+  default: string;
+}
+
+const NODE_TYPES: Array<{ type: string; label: string; params: NodeParam[] }> = [
+  { type: 'gain',         label: 'Gain',            params: [{ label: 'Factor',     key: 'factor',  type: 'number', default: '1' }] },
+  { type: 'offset',       label: 'Offset',          params: [{ label: 'Offset',     key: 'offset',  type: 'number', default: '0' }] },
+  { type: 'moving_avg',   label: 'Moving Average',  params: [{ label: 'Window (n)', key: 'n',       type: 'number', default: '10' }] },
+  { type: 'rms',          label: 'RMS',             params: [{ label: 'Window (n)', key: 'n',       type: 'number', default: '10' }] },
+  { type: 'lowpass',      label: 'Lowpass Filter',  params: [{ label: 'Cutoff Hz',  key: 'cutoff',  type: 'number', default: '1' }, { label: 'Sample Hz', key: 'sample_rate', type: 'number', default: '10' }] },
+  { type: 'highpass',     label: 'Highpass Filter', params: [{ label: 'Cutoff Hz',  key: 'cutoff',  type: 'number', default: '1' }, { label: 'Sample Hz', key: 'sample_rate', type: 'number', default: '10' }] },
+  { type: 'derivative',   label: 'Derivative',      params: [] },
+  { type: 'integral',     label: 'Integral',        params: [] },
+  { type: 'clamp',        label: 'Clamp',           params: [{ label: 'Min', key: 'min', type: 'number', default: '0' }, { label: 'Max', key: 'max', type: 'number', default: '100' }] },
+  { type: 'formula',      label: 'Formula',         params: [{ label: 'Expression (use "x")', key: 'expr', type: 'text', default: 'x * 1.0' }] },
+];
+
+export default function SyntheticWizard({ onClose, onCreated }: Props) {
+  const [name, setName] = useState('');
+  const [inputDs, setInputDs] = useState('stub');
+  const [inputSig, setInputSig] = useState('sine_1hz');
+  const [nodeType, setNodeType] = useState('gain');
+  const [params, setParams] = useState<Record<string, string>>({});
+  const [unit, setUnit] = useState('');
+  const [desc, setDesc] = useState('');
+  const [dispLow, setDispLow] = useState('0');
+  const [dispHigh, setDispHigh] = useState('100');
+  const [saving, setSaving] = useState(false);
+  const [error, setError] = useState('');
+
+  const nodeDef = NODE_TYPES.find(n => n.type === nodeType)!;
+
+  function getParam(key: string, def: string): string {
+    return params[key] ?? def;
+  }
+
+  function setParam(key: string, val: string) {
+    setParams(p => ({ ...p, [key]: val }));
+  }
+
+  async function handleCreate() {
+    if (!name.trim()) { setError('Name is required'); return; }
+    if (!inputSig.trim()) { setError('Input signal is required'); return; }
+
+    const nodeParams: Record<string, any> = {};
+    for (const p of nodeDef.params) {
+      const raw = getParam(p.key, p.default);
+      nodeParams[p.key] = p.type === 'number' ? parseFloat(raw) : raw;
+    }
+
+    const def = {
+      name: name.trim(),
+      ds: inputDs,
+      signal: inputSig.trim(),
+      pipeline: [{ type: nodeType, params: nodeParams }],
+      meta: {
+        unit: unit || undefined,
+        description: desc || undefined,
+        displayLow: parseFloat(dispLow) || 0,
+        displayHigh: parseFloat(dispHigh) || 100,
+      },
+    };
+
+    setSaving(true);
+    setError('');
+    try {
+      const res = await fetch('/api/v1/synthetic', {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify(def),
+      });
+      if (!res.ok) {
+        const j = await res.json().catch(() => ({ error: res.statusText }));
+        throw new Error(j.error ?? res.statusText);
+      }
+      onCreated();
+      onClose();
+    } catch (err) {
+      setError(err instanceof Error ? err.message : String(err));
+    } finally {
+      setSaving(false);
+    }
+  }
+
+  return (
+    <div class="wizard-backdrop" onClick={onClose}>
+      <div class="wizard" onClick={(e) => e.stopPropagation()}>
+        <div class="wizard-header">
+          <span>New Synthetic Signal</span>
+          <button class="icon-btn" onClick={onClose}>✕</button>
+        </div>
+
+        <div class="wizard-body">
+          {error && <p class="wizard-error">{error}</p>}
+
+          <div class="wizard-section-title">Signal identity</div>
+          <div class="wizard-field">
+            <label>Name</label>
+            <input class="prop-input" value={name}
+              placeholder="e.g. smoothed_pressure"
+              onInput={(e) => setName((e.target as HTMLInputElement).value)} />
+          </div>
+
+          <div class="wizard-section-title">Input signal</div>
+          <div class="wizard-field wizard-field-row">
+            <div class="wizard-field">
+              <label>Data source</label>
+              <input class="prop-input" value={inputDs}
+                placeholder="stub"
+                onInput={(e) => setInputDs((e.target as HTMLInputElement).value)} />
+            </div>
+            <div class="wizard-field" style="flex:2;">
+              <label>Signal name</label>
+              <input class="prop-input" value={inputSig}
+                placeholder="sine_1hz"
+                onInput={(e) => setInputSig((e.target as HTMLInputElement).value)} />
+            </div>
+          </div>
+
+          <div class="wizard-section-title">Processing</div>
+          <div class="wizard-field">
+            <label>Node type</label>
+            <select class="prop-select" value={nodeType}
+              onChange={(e) => { setNodeType((e.target as HTMLSelectElement).value); setParams({}); }}>
+              {NODE_TYPES.map(n => <option key={n.type} value={n.type}>{n.label}</option>)}
+            </select>
+          </div>
+          {nodeDef.params.map(p => (
+            <div key={p.key} class="wizard-field">
+              <label>{p.label}</label>
+              <input class="prop-input" type={p.type === 'number' ? 'number' : 'text'}
+                value={getParam(p.key, p.default)}
+                onInput={(e) => setParam(p.key, (e.target as HTMLInputElement).value)} />
+            </div>
+          ))}
+
+          <div class="wizard-section-title">Metadata (optional)</div>
+          <div class="wizard-field wizard-field-row">
+            <div class="wizard-field">
+              <label>Unit</label>
+              <input class="prop-input" value={unit} placeholder="m/s"
+                onInput={(e) => setUnit((e.target as HTMLInputElement).value)} />
+            </div>
+            <div class="wizard-field">
+              <label>Display low</label>
+              <input class="prop-input" type="number" value={dispLow}
+                onInput={(e) => setDispLow((e.target as HTMLInputElement).value)} />
+            </div>
+            <div class="wizard-field">
+              <label>Display high</label>
+              <input class="prop-input" type="number" value={dispHigh}
+                onInput={(e) => setDispHigh((e.target as HTMLInputElement).value)} />
+            </div>
+          </div>
+          <div class="wizard-field">
+            <label>Description</label>
+            <input class="prop-input" value={desc} placeholder="Optional description"
+              onInput={(e) => setDesc((e.target as HTMLInputElement).value)} />
+          </div>
+        </div>
+
+        <div class="wizard-footer">
+          <button class="toolbar-btn" onClick={onClose}>Cancel</button>
+          <button class="toolbar-btn toolbar-btn-primary" onClick={handleCreate} disabled={saving}>
+            {saving ? 'Creating…' : 'Create Signal'}
+          </button>
+        </div>
+      </div>
+    </div>
+  );
+}