diff --git a/poller/internal/bus/discovery_responder_test.go b/poller/internal/bus/discovery_responder_test.go
new file mode 100644
index 0000000..dc240d0
--- /dev/null
+++ b/poller/internal/bus/discovery_responder_test.go
@@ -0,0 +1,219 @@
+package bus
+
+import (
+	"encoding/json"
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestDiscoveryResponder_Subscribe(t *testing.T) {
+	nc, cleanup := startTestNATS(t)
+	defer cleanup()
+
+	dr := NewDiscoveryResponder(nc)
+	if err := dr.Start(); err != nil {
+		t.Fatalf("Start() returned error: %v", err)
+	}
+	defer dr.Stop()
+
+	if dr.sub == nil {
+		t.Fatal("expected subscription to be set after Start()")
+	}
+
+	// Verify subscription subject and queue group
+	if dr.sub.Subject != "device.discover.snmp" {
+		t.Errorf("expected subject 'device.discover.snmp', got %q", dr.sub.Subject)
+	}
+	if dr.sub.Queue != "discover-workers" {
+		t.Errorf("expected queue 'discover-workers', got %q", dr.sub.Queue)
+	}
+}
+
+func TestDiscoveryResponder_InvalidJSON(t *testing.T) {
+	nc, cleanup := startTestNATS(t)
+	defer cleanup()
+
+	dr := NewDiscoveryResponder(nc)
+	if err := dr.Start(); err != nil {
+		t.Fatalf("Start: %v", err)
+	}
+	defer dr.Stop()
+
+	reply, err := nc.Request("device.discover.snmp", []byte("{invalid json"), 5*time.Second)
+	if err != nil {
+		t.Fatalf("NATS request failed: %v", err)
+	}
+
+	var resp DiscoveryResponse
+	if err := json.Unmarshal(reply.Data, &resp); err != nil {
+		t.Fatalf("unmarshal response: %v", err)
+	}
+
+	if resp.Error == "" {
+		t.Error("expected non-empty error for invalid JSON")
+	}
+	if !strings.Contains(resp.Error, "invalid request") {
+		t.Errorf("expected error to contain 'invalid request', got %q", resp.Error)
+	}
+}
+
+func TestDiscoveryResponder_MissingIPAddress(t *testing.T) {
+	nc, cleanup := startTestNATS(t)
+	defer cleanup()
+
+	dr := NewDiscoveryResponder(nc)
+	if err := dr.Start(); err != nil {
+		t.Fatalf("Start: %v", err)
+	}
+	defer dr.Stop()
+
+	req := DiscoveryRequest{
+		SNMPVersion: "v2c",
+		Community:   "public",
+	}
+	reqData, _ := json.Marshal(req)
+
+	reply, err := nc.Request("device.discover.snmp", reqData, 5*time.Second)
+	if err != nil {
+		t.Fatalf("NATS request failed: %v", err)
+	}
+
+	var resp DiscoveryResponse
+	if err := json.Unmarshal(reply.Data, &resp); err != nil {
+		t.Fatalf("unmarshal response: %v", err)
+	}
+
+	if resp.Error == "" {
+		t.Error("expected non-empty error for missing ip_address")
+	}
+	if !strings.Contains(resp.Error, "ip_address") {
+		t.Errorf("expected error to mention 'ip_address', got %q", resp.Error)
+	}
+}
+
+func TestDiscoveryResponder_ResponseFields(t *testing.T) {
+	// Verify the DiscoveryResponse JSON field names match the spec.
+	resp := DiscoveryResponse{
+		SysObjectID: "1.3.6.1.4.1.14988.1",
+		SysDescr:    "RouterOS RB750Gr3",
+		SysName:     "router1.local",
+	}
+
+	data, err := json.Marshal(resp)
+	if err != nil {
+		t.Fatalf("marshal: %v", err)
+	}
+
+	var raw map[string]interface{}
+	if err := json.Unmarshal(data, &raw); err != nil {
+		t.Fatalf("unmarshal to map: %v", err)
+	}
+
+	for _, field := range []string{"sys_object_id", "sys_descr", "sys_name"} {
+		if _, ok := raw[field]; !ok {
+			t.Errorf("expected field %q in JSON output", field)
+		}
+	}
+
+	// Verify error field with omitempty
+	errResp := DiscoveryResponse{Error: "test error"}
+	errData, _ := json.Marshal(errResp)
+	var errRaw map[string]interface{}
+	_ = json.Unmarshal(errData, &errRaw)
+	if _, ok := errRaw["error"]; !ok {
+		t.Error("expected 'error' field in error response JSON")
+	}
+}
+
+func TestDiscoveryResponder_Stop_Unsubscribes(t *testing.T) {
+	nc, cleanup := startTestNATS(t)
+	defer cleanup()
+
+	dr := NewDiscoveryResponder(nc)
+	if err := dr.Start(); err != nil {
+		t.Fatalf("Start: %v", err)
+	}
+
+	if !dr.sub.IsValid() {
+		t.Fatal("expected subscription to be valid before Stop()")
+	}
+
+	dr.Stop()
+
+	if dr.sub.IsValid() {
+		t.Error("expected subscription to be invalid after Stop()")
+	}
+}
+
+func TestDiscoveryResponder_InvalidSNMPVersion(t *testing.T) {
+	nc, cleanup := startTestNATS(t)
+	defer cleanup()
+
+	dr := NewDiscoveryResponder(nc)
+	if err := dr.Start(); err != nil {
+		t.Fatalf("Start: %v", err)
+	}
+	defer dr.Stop()
+
+	req := DiscoveryRequest{
+		IPAddress:   "10.0.0.1",
+		SNMPVersion: "v4",
+	}
+	reqData, _ := json.Marshal(req)
+
+	reply, err := nc.Request("device.discover.snmp", reqData, 5*time.Second)
+	if err != nil {
+		t.Fatalf("NATS request failed: %v", err)
+	}
+
+	var resp DiscoveryResponse
+	if err := json.Unmarshal(reply.Data, &resp); err != nil {
+		t.Fatalf("unmarshal response: %v", err)
+	}
+
+	if resp.Error == "" {
+		t.Error("expected non-empty error for invalid snmp_version")
+	}
+}
+
+func TestDiscoveryResponder_DefaultPort(t *testing.T) {
+	// Test that requests with zero port default to 161.
+	// We verify this indirectly -- the request should not fail validation
+	// (it will fail on SNMP connect, which is expected since there's no device).
+	nc, cleanup := startTestNATS(t)
+	defer cleanup()
+
+	dr := NewDiscoveryResponder(nc)
+	if err := dr.Start(); err != nil {
+		t.Fatalf("Start: %v", err)
+	}
+	defer dr.Stop()
+
+	req := DiscoveryRequest{
+		IPAddress:   "192.0.2.1", // TEST-NET, unreachable
+		SNMPVersion: "v2c",
+		Community:   "public",
+		// SNMPPort intentionally 0 -- should default to 161
+	}
+	reqData, _ := json.Marshal(req)
+
+	reply, err := nc.Request("device.discover.snmp", reqData, 10*time.Second)
+	if err != nil {
+		t.Fatalf("NATS request failed: %v", err)
+	}
+
+	var resp DiscoveryResponse
+	if err := json.Unmarshal(reply.Data, &resp); err != nil {
+		t.Fatalf("unmarshal response: %v", err)
+	}
+
+	// The request passed validation (no "ip_address" or "snmp_version" error).
+	// It should fail on SNMP probe (unreachable device), not on validation.
+	if resp.Error != "" && strings.Contains(resp.Error, "ip_address") {
+		t.Error("request with zero port should not fail ip_address validation")
+	}
+	if resp.Error != "" && strings.Contains(resp.Error, "snmp_version") {
+		t.Error("request with zero port should not fail snmp_version validation")
+	}
+}