diff --git a/server/internal/grpc/server.go b/server/internal/grpc/server.go
index f60c461..8e7f9e7 100644
--- a/server/internal/grpc/server.go
+++ b/server/internal/grpc/server.go
@@ -112,7 +112,7 @@ func (s *vantageServer) SyncMonitors(ctx context.Context, req *pb.SyncMonitorsRe
 	if err != nil {
 		return nil, status.Errorf(codes.Unauthenticated, "invalid agent token")
 	}
-	monitors, err := services.ListMonitorsForRunner(srv.ServerID)
+	monitors, err := services.ListMonitorsForRunner(srv.OrgID, srv.ServerID)
 	if err != nil {
 		return nil, status.Errorf(codes.Internal, "list monitors")
 	}
@@ -137,7 +137,8 @@ func (s *vantageServer) SyncMonitors(ctx context.Context, req *pb.SyncMonitorsRe
 }
 
 func (s *vantageServer) ReportChecks(ctx context.Context, req *pb.ReportChecksRequest) (*pb.ReportChecksResponse, error) {
-	if _, err := services.ValidateAgentToken(req.ServerId, req.AgentToken); err != nil {
+	srv, err := services.ValidateAgentToken(req.ServerId, req.AgentToken)
+	if err != nil {
 		return nil, status.Errorf(codes.Unauthenticated, "invalid agent token")
 	}
 	for _, r := range req.Results {
@@ -146,7 +147,9 @@ func (s *vantageServer) ReportChecks(ctx context.Context, req *pb.ReportChecksRe
 			t := time.Unix(r.CertExpiryUnix, 0)
 			res.CertExpiry = &t
 		}
-		if err := services.IngestResult(r.MonitorId, res); err != nil {
+		// A rejected monitor (wrong org, or not run by this agent) is skipped,
+		// not fatal — the rest of the batch is still legitimate.
+		if err := services.IngestResult(srv.OrgID, srv.ServerID, r.MonitorId, res); err != nil {
 			log.Printf("ingest check %s: %v", r.MonitorId, err)
 		}
 	}
diff --git a/server/internal/monitorsched/scheduler.go b/server/internal/monitorsched/scheduler.go
index f3b33a8..214f4c9 100644
--- a/server/internal/monitorsched/scheduler.go
+++ b/server/internal/monitorsched/scheduler.go
@@ -35,7 +35,7 @@ func loop(ctx context.Context) {
 	var mu sync.Mutex
 
 	sync := func() {
-		monitors, err := services.ListMonitorsForRunner(models.RunnerServer)
+		monitors, err := services.ListMonitorsForRunner("", models.RunnerServer)
 		if err != nil {
 			log.Printf("monitorsched: list monitors: %v", err)
 			return
@@ -88,7 +88,7 @@ func runMonitor(ctx context.Context, m models.Monitor) {
 
 	run := func() {
 		res := checker.Run(ctx, spec)
-		if err := services.IngestResult(m.MonitorID, res); err != nil {
+		if err := services.IngestResult("", models.RunnerServer, m.MonitorID, res); err != nil {
 			log.Printf("monitorsched: ingest %s: %v", m.MonitorID, err)
 		}
 	}
diff --git a/server/internal/services/monitors.go b/server/internal/services/monitors.go
index 4de9ec6..38427bc 100644
--- a/server/internal/services/monitors.go
+++ b/server/internal/services/monitors.go
@@ -3,6 +3,7 @@ package services
 import (
 	"context"
 	"errors"
+	"fmt"
 	"log"
 	"time"
 
@@ -51,12 +52,19 @@ func ListMonitors(orgID string) ([]models.Monitor, error) {
 }
 
 // ListMonitorsForRunner returns enabled monitors whose Runner matches runner.
-// Agent/scheduler path — a cross-org sweep (mirrors MarkOfflineServers), so it
-// intentionally has no org filter.
-func ListMonitorsForRunner(runner string) ([]models.Monitor, error) {
+// Runner is client-supplied at write time, so an agent fetching its own work
+// must scope by the org of its authenticated server record — otherwise another
+// org could point a monitor at that server_id and have it run their checks.
+// An empty orgID means the cross-org server-scheduler sweep (mirrors
+// MarkOfflineServers) and is only ever passed with runner == RunnerServer.
+func ListMonitorsForRunner(orgID, runner string) ([]models.Monitor, error) {
 	ctx, cancel := monCtx()
 	defer cancel()
-	cur, err := db.Col("monitors").Find(ctx, bson.M{"runner": runner, "enabled": true})
+	filter := bson.M{"runner": runner, "enabled": true}
+	if orgID != "" {
+		filter["org_id"] = orgID
+	}
+	cur, err := db.Col("monitors").Find(ctx, filter)
 	if err != nil {
 		return nil, err
 	}
@@ -98,12 +106,29 @@ func getMonitorByID(monitorID string) (*models.Monitor, error) {
 	return &m, nil
 }
 
+// validateRunner rejects a runner that is neither the reserved server-scheduler
+// value nor a server in the org. The value is client-supplied and is later
+// consumed by an agent's own monitor fetch, so ownership has to be proven at
+// the write boundary.
+func validateRunner(orgID, runner string) error {
+	if runner == "" || runner == models.RunnerServer {
+		return nil
+	}
+	if _, err := GetServer(orgID, runner); err != nil {
+		return fmt.Errorf("runner server %s not found", runner)
+	}
+	return nil
+}
+
 func CreateMonitor(orgID string, m *models.Monitor) (*models.Monitor, error) {
 	ctx, cancel := monCtx()
 	defer cancel()
 	if err := validateChannelIDs(orgID, m.ChannelIDs); err != nil {
 		return nil, err
 	}
+	if err := validateRunner(orgID, m.Runner); err != nil {
+		return nil, err
+	}
 	m.OrgID = orgID
 	m.MonitorID = uuid.NewString()
 	m.CreatedAt = time.Now()
@@ -131,6 +156,11 @@ func UpdateMonitor(orgID, monitorID string, upd bson.M) error {
 			return err
 		}
 	}
+	if runner, ok := upd["runner"].(string); ok {
+		if err := validateRunner(orgID, runner); err != nil {
+			return err
+		}
+	}
 	_, err := db.Col("monitors").UpdateOne(ctx, bson.M{"monitor_id": monitorID, "org_id": orgID}, bson.M{"$set": upd})
 	return err
 }
@@ -191,7 +221,13 @@ func UptimeRollups(monitorID string, since time.Time) ([]models.Rollup, error) {
 // incidents on up<->down transitions, rolls up the hourly bucket, and fires
 // notifications on transition. Both the server scheduler and agent-reported
 // results funnel through here.
-func IngestResult(monitorID string, res checker.Result) error {
+//
+// monitorID is client-supplied on the agent path, so the caller passes the org
+// and runner it is authenticated as: orgID is the reporting agent's server org
+// and runner is its server_id. A result is only applied to a monitor owned by
+// that org and assigned to that runner. An empty orgID is the in-process server
+// scheduler, which passes runner == RunnerServer.
+func IngestResult(orgID, runner, monitorID string, res checker.Result) error {
 	ctx, cancel := monCtx()
 	defer cancel()
 
@@ -199,6 +235,12 @@ func IngestResult(monitorID string, res checker.Result) error {
 	if err != nil || m == nil {
 		return err
 	}
+	if orgID != "" && m.OrgID != orgID {
+		return fmt.Errorf("monitor %s belongs to another org", monitorID)
+	}
+	if m.Runner != runner {
+		return fmt.Errorf("monitor %s is not run by %s", monitorID, runner)
+	}
 
 	now := time.Now()
 	prev := m.State.Status