diff --git a/checker/interactive.go b/checker/interactive.go
index bb7158e..7e8b190 100644
--- a/checker/interactive.go
+++ b/checker/interactive.go
@@ -16,10 +16,12 @@ package checker
 
 import (
 	"bytes"
+	"context"
 	"encoding/json"
 	"fmt"
 	"html/template"
 	"log"
+	"maps"
 	"net/http"
 	"time"
 )
@@ -104,14 +106,19 @@ func (s *Server) handleCheckSubmit(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	related := s.collectRelatedObservations(r.Context(), opts, data)
+
 	if s.definition != nil {
-		obs := &mapObservationGetter{data: map[ObservationKey]json.RawMessage{
-			s.provider.Key(): raw,
-		}}
+		obs := &mapObservationGetter{
+			data: map[ObservationKey]json.RawMessage{
+				s.provider.Key(): raw,
+			},
+			related: related,
+		}
 		result.States = s.evaluateRules(r.Context(), obs, opts, nil)
 	}
 
-	ctx := NewReportContext(raw, nil)
+	ctx := NewReportContext(raw, related)
 
 	if reporter, ok := s.provider.(CheckerHTMLReporter); ok {
 		html, rerr := reporter.GetHTMLReport(ctx)
@@ -134,6 +141,86 @@ func (s *Server) handleCheckSubmit(w http.ResponseWriter, r *http.Request) {
 	s.renderCheckResult(w, result)
 }
 
+// collectRelatedObservations runs sibling providers declared via
+// InteractiveRelatedProviders and returns their results keyed by the
+// sibling's observation key. Sibling errors are logged and skipped.
+func (s *Server) collectRelatedObservations(ctx context.Context, opts CheckerOptions, data any) map[ObservationKey][]RelatedObservation {
+	irp, ok := s.provider.(InteractiveRelatedProviders)
+	if !ok {
+		return nil
+	}
+	siblings := irp.RelatedProviders()
+	if len(siblings) == 0 {
+		return nil
+	}
+
+	var entries []DiscoveryEntry
+	if dp, ok := s.provider.(DiscoveryPublisher); ok {
+		e, err := dp.DiscoverEntries(data)
+		if err != nil {
+			log.Printf("interactive: DiscoverEntries failed: %v", err)
+		} else {
+			entries = e
+		}
+	}
+
+	related := make(map[ObservationKey][]RelatedObservation, len(siblings))
+	for _, sp := range siblings {
+		sOpts := cloneOptions(opts)
+		siblingID := ""
+		if dp, ok := sp.(CheckerDefinitionProvider); ok {
+			if def := dp.Definition(); def != nil {
+				siblingID = def.ID
+				if len(entries) > 0 {
+					fillDiscoveryEntryOption(sOpts, def, entries)
+				}
+			}
+		}
+		sData, err := sp.Collect(ctx, sOpts)
+		if err != nil {
+			log.Printf("interactive: sibling %q Collect failed: %v", sp.Key(), err)
+			continue
+		}
+		raw, err := json.Marshal(sData)
+		if err != nil {
+			log.Printf("interactive: sibling %q marshal failed: %v", sp.Key(), err)
+			continue
+		}
+		related[sp.Key()] = append(related[sp.Key()], RelatedObservation{
+			CheckerID:   siblingID,
+			Key:         sp.Key(),
+			Data:        raw,
+			CollectedAt: time.Now(),
+		})
+	}
+	return related
+}
+
+func cloneOptions(opts CheckerOptions) CheckerOptions {
+	out := make(CheckerOptions, len(opts))
+	maps.Copy(out, opts)
+	return out
+}
+
+// fillDiscoveryEntryOption mirrors the host's AutoFill wiring: it writes
+// entries into every option in def tagged AutoFill == AutoFillDiscoveryEntries.
+func fillDiscoveryEntryOption(opts CheckerOptions, def *CheckerDefinition, entries []DiscoveryEntry) {
+	scopes := [][]CheckerOptionDocumentation{
+		def.Options.AdminOpts,
+		def.Options.UserOpts,
+		def.Options.DomainOpts,
+		def.Options.ServiceOpts,
+		def.Options.RunOpts,
+	}
+	for _, scope := range scopes {
+		for _, f := range scope {
+			if f.AutoFill == AutoFillDiscoveryEntries {
+				opts[f.Id] = entries
+			}
+		}
+	}
+}
+
 func (s *Server) checkPageTitle() string {
 	if s.definition != nil && s.definition.Name != "" {
 		return s.definition.Name
diff --git a/checker/interactive_test.go b/checker/interactive_test.go
index d85948b..dc93b93 100644
--- a/checker/interactive_test.go
+++ b/checker/interactive_test.go
@@ -243,3 +243,136 @@ func (b *bareInteractiveProvider) RenderForm() []CheckerOptionField {
 func (b *bareInteractiveProvider) ParseForm(r *http.Request) (CheckerOptions, error) {
 	return CheckerOptions{"domain": r.FormValue("domain")}, nil
 }
+
+type siblingProvider struct {
+	key        ObservationKey
+	id         string
+	entriesOpt string
+	gotOpts    CheckerOptions
+	payload    any
+}
+
+func (s *siblingProvider) Key() ObservationKey { return s.key }
+func (s *siblingProvider) Collect(ctx context.Context, opts CheckerOptions) (any, error) {
+	s.gotOpts = opts
+	return s.payload, nil
+}
+func (s *siblingProvider) Definition() *CheckerDefinition {
+	return &CheckerDefinition{
+		ID: s.id,
+		Options: CheckerOptionsDocumentation{
+			RunOpts: []CheckerOptionDocumentation{
+				{Id: s.entriesOpt, Type: "array", AutoFill: AutoFillDiscoveryEntries},
+			},
+		},
+	}
+}
+
+type primaryWithSibling struct {
+	key     ObservationKey
+	def     *CheckerDefinition
+	entries []DiscoveryEntry
+	sibling ObservationProvider
+}
+
+func (p *primaryWithSibling) Key() ObservationKey { return p.key }
+func (p *primaryWithSibling) Collect(ctx context.Context, opts CheckerOptions) (any, error) {
+	return map[string]string{"primary": "ok"}, nil
+}
+func (p *primaryWithSibling) Definition() *CheckerDefinition { return p.def }
+func (p *primaryWithSibling) RenderForm() []CheckerOptionField {
+	return []CheckerOptionField{{Id: "domain", Type: "string"}}
+}
+func (p *primaryWithSibling) ParseForm(r *http.Request) (CheckerOptions, error) {
+	return CheckerOptions{"domain": r.FormValue("domain")}, nil
+}
+func (p *primaryWithSibling) DiscoverEntries(data any) ([]DiscoveryEntry, error) {
+	return p.entries, nil
+}
+func (p *primaryWithSibling) RelatedProviders() []ObservationProvider {
+	return []ObservationProvider{p.sibling}
+}
+
+type relatedAssertRule struct {
+	key ObservationKey
+}
+
+func (r *relatedAssertRule) Name() string        { return "related_assert" }
+func (r *relatedAssertRule) Description() string { return "" }
+func (r *relatedAssertRule) Evaluate(ctx context.Context, obs ObservationGetter, opts CheckerOptions) []CheckState {
+	related, err := obs.GetRelated(ctx, r.key)
+	if err != nil {
+		return []CheckState{{Status: StatusError, Message: err.Error()}}
+	}
+	if len(related) == 0 {
+		return []CheckState{{Status: StatusCrit, Message: "no related observation"}}
+	}
+	return []CheckState{{Status: StatusOK, Message: "saw related observation"}}
+}
+
+func TestCheck_Submit_RunsSiblingAndExposesRelated(t *testing.T) {
+	sibling := &siblingProvider{
+		key:        "sibling_key",
+		id:         "sibling",
+		entriesOpt: "endpoints",
+		payload:    map[string]string{"sibling": "ok"},
+	}
+	entry := DiscoveryEntry{Type: "fake.v1", Ref: "r1"}
+	primary := &primaryWithSibling{
+		key: "primary_key",
+		def: &CheckerDefinition{
+			ID:    "primary",
+			Rules: []CheckRule{&relatedAssertRule{key: sibling.key}},
+		},
+		entries: []DiscoveryEntry{entry},
+		sibling: sibling,
+	}
+
+	srv := NewServer(primary)
+	defer srv.Close()
+
+	rec := postForm(srv.Handler(), "/check", url.Values{"domain": {"example.com"}})
+	if rec.Code != http.StatusOK {
+		t.Fatalf("POST /check = %d, want 200", rec.Code)
+	}
+	body := rec.Body.String()
+	if !strings.Contains(body, "saw related observation") {
+		t.Errorf("rule did not see related observation; body:\n%s", body)
+	}
+
+	got, ok := sibling.gotOpts[sibling.entriesOpt].([]DiscoveryEntry)
+	if !ok {
+		t.Fatalf("sibling opts missing %q or wrong type: %#v", sibling.entriesOpt, sibling.gotOpts[sibling.entriesOpt])
+	}
+	if len(got) != 1 || got[0].Ref != entry.Ref {
+		t.Errorf("sibling saw entries %v, want [%v]", got, entry)
+	}
+
+	if v, _ := sibling.gotOpts["domain"].(string); v != "example.com" {
+		t.Errorf("sibling did not receive primary domain opt, got %q", v)
+	}
+}
+
+func TestCheck_Submit_NoSibling_LeavesRelatedEmpty(t *testing.T) {
+	p := &interactiveProvider{
+		testProvider: &testProvider{
+			key: "test",
+			definition: &CheckerDefinition{
+				ID:    "test",
+				Rules: []CheckRule{&relatedAssertRule{key: "other"}},
+			},
+		},
+		fields: []CheckerOptionField{{Id: "domain", Type: "string"}},
+	}
+	srv := NewServer(p)
+	defer srv.Close()
+
+	rec := postForm(srv.Handler(), "/check", url.Values{"domain": {"example.com"}})
+	if rec.Code != http.StatusOK {
+		t.Fatalf("POST /check = %d, want 200", rec.Code)
+	}
+	body := rec.Body.String()
+	if !strings.Contains(body, "no related observation") {
+		t.Errorf("rule should have seen no related observation; body:\n%s", body)
+	}
+}
diff --git a/checker/server.go b/checker/server.go
index bfec550..4478baf 100644
--- a/checker/server.go
+++ b/checker/server.go
@@ -380,9 +380,13 @@ func (s *Server) handleReport(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, http.StatusOK, metrics)
 }
 
-// mapObservationGetter implements ObservationGetter backed by a static map.
+// mapObservationGetter implements ObservationGetter backed by static maps.
+// Both fields are optional: Get reads from data, GetRelated reads from
+// related. Leaving related nil preserves the pre-existing "no lineage"
+// behavior used by the remote /evaluate path.
 type mapObservationGetter struct {
-	data map[ObservationKey]json.RawMessage
+	data    map[ObservationKey]json.RawMessage
+	related map[ObservationKey][]RelatedObservation
 }
 
 func (g *mapObservationGetter) Get(ctx context.Context, key ObservationKey, dest any) error {
@@ -393,13 +397,13 @@ func (g *mapObservationGetter) Get(ctx context.Context, key ObservationKey, dest
 	return json.Unmarshal(raw, dest)
 }
 
-// GetRelated always returns nil in the remote /evaluate path: the host that
-// invokes /evaluate does not (currently) carry cross-checker related data in
-// ExternalEvaluateRequest. Consumers that need related observations must run
-// evaluation locally with a host-side ObservationContext that resolves
-// lineage.
+// GetRelated returns the pre-resolved related observations for key, or nil
+// when none were seeded. The remote /evaluate path leaves related nil
+// because ExternalEvaluateRequest does not currently carry cross-checker
+// lineage; the interactive /check path can seed it from sibling providers
+// declared via InteractiveRelatedProviders.
 func (g *mapObservationGetter) GetRelated(ctx context.Context, key ObservationKey) ([]RelatedObservation, error) {
-	return nil, nil
+	return g.related[key], nil
 }
 
 func writeJSON(w http.ResponseWriter, status int, v any) {
diff --git a/checker/types.go b/checker/types.go
index c218dbb..1e1ad0e 100644
--- a/checker/types.go
+++ b/checker/types.go
@@ -372,6 +372,21 @@ type CheckerDefinitionProvider interface {
 	Definition() *CheckerDefinition
 }
 
+// InteractiveRelatedProviders is an optional interface an interactive
+// ObservationProvider can co-implement to declare sibling providers whose
+// Collect the SDK runs in-process during /check. Their results are
+// exposed as RelatedObservations on ObservationGetter and ReportContext,
+// mirroring the cross-checker lineage a happyDomain host resolves.
+//
+// For each sibling the SDK seeds options from the primary and, when the
+// primary implements DiscoveryPublisher, writes its entries into any
+// sibling option tagged AutoFill == AutoFillDiscoveryEntries. Sibling
+// errors are logged and skipped so the primary result still reaches the
+// user.
+type InteractiveRelatedProviders interface {
+	RelatedProviders() []ObservationProvider
+}
+
 // CheckerDefinition is the complete definition of a checker, registered via init().
 type CheckerDefinition struct {
 	ID              string                      `json:"id"`