feat: include active alert count in MCP server instructions

Add InstructionsFunc callback to ServerConfig, called during each
initialize handshake to generate dynamic instructions. The lab-monitoring
server uses this to query Alertmanager and include a count of active
non-silenced alerts, so the LLM can proactively inform the user.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/lab-monitoring/main.go

@@ -168,10 +168,15 @@ func runServe(c *cli.Context) error {
 
 	logger := log.New(os.Stderr, "[mcp] ", log.LstdFlags)
 	config := mcp.DefaultMonitoringConfig()
-	server := mcp.NewGenericServer(logger, config)
 
 	prom := monitoring.NewPrometheusClient(c.String("prometheus-url"))
 	am := monitoring.NewAlertmanagerClient(c.String("alertmanager-url"))
+
+	config.InstructionsFunc = func() string {
+		return monitoring.AlertSummary(am)
+	}
+
+	server := mcp.NewGenericServer(logger, config)
 	monitoring.RegisterHandlers(server, prom, am)
 
 	transport := c.String("transport")

internal/mcp/server.go

@@ -30,6 +30,9 @@ type ServerConfig struct {
 	Version string
 	// Instructions are the server instructions sent to clients.
 	Instructions string
+	// InstructionsFunc, if set, is called during initialization to generate
+	// dynamic instructions. Its return value is appended to Instructions.
+	InstructionsFunc func() string
 	// DefaultChannel is the default channel to use when no revision is specified.
 	DefaultChannel string
 	// SourceName is the name of the source repository (e.g., "nixpkgs", "home-manager").
@@ -244,6 +247,13 @@ func (s *Server) handleInitialize(req *Request) *Response {
 	s.logger.Printf("Client: %s %s, protocol: %s",
 		params.ClientInfo.Name, params.ClientInfo.Version, params.ProtocolVersion)
 
+	instructions := s.config.Instructions
+	if s.config.InstructionsFunc != nil {
+		if extra := s.config.InstructionsFunc(); extra != "" {
+			instructions += "\n\n" + extra
+		}
+	}
+
 	result := InitializeResult{
 		ProtocolVersion: ProtocolVersion,
 		Capabilities: Capabilities{
@@ -255,7 +265,7 @@ func (s *Server) handleInitialize(req *Request) *Response {
 			Name:    s.config.Name,
 			Version: s.config.Version,
 		},
-		Instructions: s.config.Instructions,
+		Instructions: instructions,
 	}
 
 	return &Response{

internal/monitoring/handlers.go

@@ -10,6 +10,50 @@ import (
 	"git.t-juice.club/torjus/labmcp/internal/mcp"
 )
 
+// AlertSummary queries Alertmanager for active (non-silenced) alerts and returns
+// a short summary string. Returns empty string if there are no alerts or if
+// Alertmanager is unreachable.
+func AlertSummary(am *AlertmanagerClient) string {
+	active := true
+	silenced := false
+	inhibited := false
+	alerts, err := am.ListAlerts(context.Background(), AlertFilters{
+		Active:   &active,
+		Silenced: &silenced,
+		Inhibited: &inhibited,
+	})
+	if err != nil || len(alerts) == 0 {
+		return ""
+	}
+
+	// Count by severity
+	severities := make(map[string]int)
+	for _, a := range alerts {
+		sev := a.Labels["severity"]
+		if sev == "" {
+			sev = "unknown"
+		}
+		severities[sev]++
+	}
+
+	var parts []string
+	// Show critical first if present
+	if n, ok := severities["critical"]; ok {
+		parts = append(parts, fmt.Sprintf("%d critical", n))
+		delete(severities, "critical")
+	}
+	if n, ok := severities["warning"]; ok {
+		parts = append(parts, fmt.Sprintf("%d warning", n))
+		delete(severities, "warning")
+	}
+	for sev, n := range severities {
+		parts = append(parts, fmt.Sprintf("%d %s", n, sev))
+	}
+
+	return fmt.Sprintf("ALERT STATUS: %d active alert(s) firing (%s). Use list_alerts for details. Let the user know about this.",
+		len(alerts), strings.Join(parts, ", "))
+}
+
 // RegisterHandlers registers all monitoring tool handlers on the MCP server.
 func RegisterHandlers(server *mcp.Server, prom *PrometheusClient, am *AlertmanagerClient) {
 	server.RegisterTool(listAlertsTool(), makeListAlertsHandler(am))