diff --git a/cmd/server/chunked_load.go b/cmd/server/chunked_load.go
index e04b03f7..143044e4 100644
--- a/cmd/server/chunked_load.go
+++ b/cmd/server/chunked_load.go
@@ -106,37 +106,59 @@ func (s *PacketStore) fireChunkCallbacks(rowsThisChunk, totalRows int) {
 }
 
 // RunStartupLoad orchestrates the startup load sequence:
-//  1. start LoadChunked (async)
-//  2. caller waits for FirstChunkReady to bind the HTTP listener
-//  3. spawn the background fill loader AFTER LoadChunked completes,
-//     so s.oldestLoaded is set before the bg loader reads it (#1809)
+//  1. run LoadChunked synchronously (FirstChunkReady is signaled
+//     internally so callers waiting on it in parallel can bind HTTP)
+//  2. on success, run loadBackgroundChunks synchronously so
+//     s.oldestLoaded is guaranteed set before the bg loader reads
+//     it (#1809).
 //
-// chunkSize=0 uses the LoadChunked default. Blocks until LoadChunked
-// AND any background loader have finished. Callers that want to bind
-// the HTTP listener at FirstChunkReady should run this in a goroutine
-// and wait on FirstChunkReady() themselves.
+// chunkSize=0 uses the LoadChunked default. The function blocks until
+// LoadChunked AND any background loader have finished. Callers that
+// want to bind the HTTP listener at FirstChunkReady should run this
+// in a goroutine and wait on FirstChunkReady() themselves.
+//
+// Steady-state contracts post-return:
+//   - LoadChunked error: backgroundLoadFailed=true, backgroundLoadDone
+//     stays false, backgroundLoadErr non-empty. Returns the error.
+//   - hotStartupHours == 0: backgroundLoadDone=true, failed=false
+//     (no background work was needed).
+//   - hotStartupHours > 0 success: terminal state is whatever
+//     loadBackgroundChunks set (done=true on full coverage,
+//     failed=true on partial / chunk errors — see #1690).
 //
 // Issue #1809 root cause: previously main.go spawned loadBackgroundChunks
 // at FirstChunkReady while LoadChunked was still merging the remainder
 // of the hot window. s.oldestLoaded is only assigned at the end of
-// LoadChunked (chunked_load.go:330), so the bg loader read "" and
-// bailed → coverage gate trips → backgroundLoadFailed=true. Gating the
-// bg loader on LoadChunked completion preserves the FirstChunkReady
-// HTTP-bind parallelism while ensuring oldestLoaded has a valid floor
-// when the bg loader starts.
+// LoadChunked, so the bg loader read "" and bailed → coverage gate
+// trips → backgroundLoadFailed=true. Running the bg loader after
+// LoadChunked returns preserves the FirstChunkReady HTTP-bind
+// parallelism while ensuring oldestLoaded has a valid floor when the
+// bg loader starts.
 func (s *PacketStore) RunStartupLoad(chunkSize int) error {
-	loadErrCh := make(chan error, 1)
-	go func() {
-		loadErrCh <- s.LoadChunked(chunkSize)
-	}()
-	// Block until LoadChunked returns. Callers that want to bind their
-	// HTTP listener earlier can wait on FirstChunkReady() in parallel.
-	if err := <-loadErrCh; err != nil {
+	if err := s.LoadChunked(chunkSize); err != nil {
+		// Pick a terminal state on the error path so /api/healthz +
+		// backgroundLoadComplete don't stay undefined (done=false,
+		// failed=false) forever.
+		s.bgErrMu.Lock()
+		s.backgroundLoadErr = fmt.Sprintf("LoadChunked failed: %v", err)
+		s.bgErrMu.Unlock()
+		s.backgroundLoadFailed.Store(true)
 		return err
 	}
-	if s.hotStartupHours > 0 {
-		s.loadBackgroundChunks()
+	if s.hotStartupHours <= 0 {
+		// No bg work required → terminal steady state is done=true,
+		// failed=false. Without this the healthz probe would see
+		// backgroundLoadComplete=false indefinitely.
+		s.backgroundLoadDone.Store(true)
+		s.backgroundLoadProgress.Store(100)
+		return nil
 	}
+	// INFO signal between LoadChunked completion and the bg loader
+	// kick-off. The post-mortem of #1809 needed exactly this line to
+	// confirm the bg loader actually started after oldestLoaded was set.
+	log.Printf("[store] LoadChunked complete (oldestLoaded=%q) — starting background fill loader (retentionHours=%gh, hotStartupHours=%gh)",
+		s.oldestLoaded, s.retentionHours, s.hotStartupHours)
+	s.loadBackgroundChunks()
 	return nil
 }
 
diff --git a/cmd/server/main.go b/cmd/server/main.go
index 522ae660..3e0219e5 100644
--- a/cmd/server/main.go
+++ b/cmd/server/main.go
@@ -242,8 +242,10 @@ func main() {
 		log.Printf("[store] RunStartupLoad completed before first-chunk signal (empty DB?)")
 	}
 	if store.hotStartupHours > 0 {
-		log.Printf("[store] background load will start after LoadChunked completes: filling retentionHours=%gh from hotStartupHours=%gh",
-			store.retentionHours, store.hotStartupHours)
+		log.Printf("[store] hot-startup window configured: hotStartupHours=%gh, retentionHours=%gh (background fill loader runs after LoadChunked succeeds — see RunStartupLoad)",
+			store.hotStartupHours, store.retentionHours)
+	} else {
+		log.Printf("[store] hot-startup disabled (hotStartupHours=0) — no background fill loader will run")
 	}
 	go func() {
 		if err := <-loadErrCh; err != nil {
diff --git a/cmd/server/runstartup_load_test.go b/cmd/server/runstartup_load_test.go
new file mode 100644
index 00000000..9b686668
--- /dev/null
+++ b/cmd/server/runstartup_load_test.go
@@ -0,0 +1,248 @@
+package main
+
+// Tests for RunStartupLoad branch behavior and #1809 invariants
+// (PR #1811 round-1 followups B2/B3/B4/B5).
+//
+// The pre-#1811 RunStartupLoad left several steady states undefined:
+//   * hotStartupHours == 0 → backgroundLoadDone stayed false forever
+//   * LoadChunked error    → both done & failed stayed false
+//   * empty DB + no bg work needed → backgroundLoadDone stayed false
+//
+// These tests codify the post-#1811 contract:
+//   * LoadChunked error → backgroundLoadFailed=true, done=false
+//   * hotStartupHours == 0 → backgroundLoadDone=true, failed=false,
+//     bg loader NOT called
+//   * empty DB + hot window → backgroundLoadDone reflects coverage
+//     (1.0 on empty DB → done=true, failed=false)
+//   * call ordering inside RunStartupLoad: LoadChunked completes
+//     before loadBackgroundChunks executes (so oldestLoaded is set)
+
+import (
+	"database/sql"
+	"fmt"
+	"path/filepath"
+	"testing"
+	"time"
+)
+
+// TestRunStartupLoad_HotStartupHoursZero_SetsDoneImmediately covers B3:
+// when hotStartupHours == 0 the bg loader has no work to do; healthz
+// must NOT be stuck on backgroundLoadComplete=false.
+func TestRunStartupLoad_HotStartupHoursZero_SetsDoneImmediately(t *testing.T) {
+	dir := t.TempDir()
+	dbPath := filepath.Join(dir, "test.db")
+	nowSec := time.Now().UTC().Unix()
+	createTestDBWithLastSeen(t, dbPath, 10, 1, nowSec,
+		30*time.Minute, 30*time.Minute)
+
+	db, err := OpenDB(dbPath)
+	if err != nil {
+		t.Fatalf("OpenDB: %v", err)
+	}
+	defer db.conn.Close()
+
+	store := NewPacketStore(db, &PacketStoreConfig{
+		RetentionHours:  168,
+		HotStartupHours: 0, // disable hot window → bg loader must not run
+	})
+
+	if err := store.RunStartupLoad(500); err != nil {
+		t.Fatalf("RunStartupLoad: %v", err)
+	}
+	if !store.backgroundLoadDone.Load() {
+		t.Fatalf("backgroundLoadDone must be true when hotStartupHours=0 (no bg work needed)")
+	}
+	if store.backgroundLoadFailed.Load() {
+		t.Fatalf("backgroundLoadFailed must be false on the no-bg-work path; got error=%q",
+			store.BackgroundLoadError())
+	}
+}
+
+// TestRunStartupLoad_LoadChunkedError_SetsFailedTerminal covers B2:
+// when LoadChunked errors, the steady state must be terminal
+// (failed=true) — not the pre-fix (done=false, failed=false).
+func TestRunStartupLoad_LoadChunkedError_SetsFailedTerminal(t *testing.T) {
+	dir := t.TempDir()
+	dbPath := filepath.Join(dir, "test.db")
+	nowSec := time.Now().UTC().Unix()
+	createTestDBWithLastSeen(t, dbPath, 5, 1, nowSec,
+		30*time.Minute, 30*time.Minute)
+
+	db, err := OpenDB(dbPath)
+	if err != nil {
+		t.Fatalf("OpenDB: %v", err)
+	}
+	// Close the underlying connection to force LoadChunked to fail on
+	// its very first query. We're explicitly verifying the failure path
+	// terminal state, not the success path.
+	_ = db.conn.Close()
+
+	store := NewPacketStore(db, &PacketStoreConfig{
+		RetentionHours:  168,
+		HotStartupHours: 1,
+	})
+
+	loadErr := store.RunStartupLoad(500)
+	if loadErr == nil {
+		t.Fatalf("RunStartupLoad must return an error when LoadChunked fails")
+	}
+	if !store.backgroundLoadFailed.Load() {
+		t.Fatalf("backgroundLoadFailed must be true after LoadChunked error (terminal state)")
+	}
+	if store.backgroundLoadDone.Load() {
+		t.Fatalf("backgroundLoadDone must remain false on LoadChunked error")
+	}
+	if store.BackgroundLoadError() == "" {
+		t.Fatalf("BackgroundLoadError must be non-empty after LoadChunked failure")
+	}
+}
+
+// TestRunStartupLoad_EmptyDB_SetsDoneTerminal covers B4: empty DB with
+// hot window > 0 — oldestLoaded stays "" because there are no packets.
+// loadBackgroundChunks must reach its coverage block (totalInDB==0 →
+// ratio=1.0) and set done=true rather than leaving the store stuck.
+func TestRunStartupLoad_EmptyDB_SetsDoneTerminal(t *testing.T) {
+	dir := t.TempDir()
+	dbPath := filepath.Join(dir, "test.db")
+	createTestDBWithLastSeen(t, dbPath, 0, 0, time.Now().UTC().Unix(),
+		30*time.Minute, 30*time.Minute)
+
+	db, err := OpenDB(dbPath)
+	if err != nil {
+		t.Fatalf("OpenDB: %v", err)
+	}
+	defer db.conn.Close()
+
+	store := NewPacketStore(db, &PacketStoreConfig{
+		RetentionHours:  168,
+		HotStartupHours: 1,
+	})
+
+	if err := store.RunStartupLoad(500); err != nil {
+		t.Fatalf("RunStartupLoad on empty DB: %v", err)
+	}
+	if !store.backgroundLoadDone.Load() {
+		t.Fatalf("backgroundLoadDone must be true after empty-DB load (nothing to load == complete)")
+	}
+	if store.backgroundLoadFailed.Load() {
+		t.Fatalf("backgroundLoadFailed must be false on empty DB; got %q",
+			store.BackgroundLoadError())
+	}
+}
+
+// TestRunStartupLoad_BgLoaderRunsAfterLoadChunkedSets_OldestLoaded
+// covers B5/B6: assert the in-process call ordering inside
+// RunStartupLoad. The OnChunkLoaded hook fires from LoadChunked; the
+// loadBackgroundChunks panic guard fires only if oldestLoaded=="" at
+// entry. So observing the chunk callback strictly before the bg loader
+// (which is exercised via the loop continuing without panic) is the
+// minimum guarantee. If a future refactor re-introduces the parallel
+// spawn pattern, the runtime assertion in loadBackgroundChunks will
+// trip and this test will fail.
+func TestRunStartupLoad_BgLoaderRunsAfterLoadChunkedSets_OldestLoaded(t *testing.T) {
+	dir := t.TempDir()
+	dbPath := filepath.Join(dir, "test.db")
+	nowSec := time.Now().UTC().Unix()
+	createTestDBWithLastSeen(t, dbPath, 50, 1, nowSec,
+		30*time.Minute, 30*time.Minute)
+
+	db, err := OpenDB(dbPath)
+	if err != nil {
+		t.Fatalf("OpenDB: %v", err)
+	}
+	defer db.conn.Close()
+
+	store := NewPacketStore(db, &PacketStoreConfig{
+		RetentionHours:  168,
+		HotStartupHours: 1,
+	})
+
+	// Hook: LoadChunked fires OnChunkLoaded after each chunk merge.
+	// We record whether it fired before RunStartupLoad returned. The
+	// runtime assertion in loadBackgroundChunks ensures the bg loader
+	// observes a non-empty oldestLoaded; if a future refactor parallels
+	// the bg-loader spawn with LoadChunked, that assertion panics.
+	chunkSeen := false
+	store.OnChunkLoaded(func(rowsThisChunk, totalRows int) {
+		chunkSeen = true
+	})
+
+	if err := store.RunStartupLoad(500); err != nil {
+		t.Fatalf("RunStartupLoad: %v", err)
+	}
+	if !chunkSeen {
+		t.Fatalf("LoadChunked OnChunkLoaded did not fire — chunk loop did not execute before bg loader")
+	}
+	if store.oldestLoaded == "" {
+		t.Fatalf("oldestLoaded is empty after RunStartupLoad — bg loader would have read \"\" and bailed")
+	}
+}
+
+// TestLoadBackgroundChunks_PanicsOnOldestLoadedEmpty_Invariant covers the
+// runtime assertion (A7). Manually populate s.packets without setting
+// oldestLoaded and call loadBackgroundChunks directly — the panic guard
+// must fire so future refactors cannot silently re-introduce the
+// #1809 race.
+func TestLoadBackgroundChunks_PanicsOnOldestLoadedEmpty_Invariant(t *testing.T) {
+	dir := t.TempDir()
+	dbPath := filepath.Join(dir, "test.db")
+	conn, err := sql.Open("sqlite", dbPath+"?_journal_mode=WAL")
+	if err != nil {
+		t.Fatal(err)
+	}
+	// Create the bare minimum schema so OpenDB succeeds; we don't care
+	// about row count — only the guard at the top of the function.
+	if _, err := conn.Exec(`CREATE TABLE transmissions (
+		id INTEGER PRIMARY KEY, raw_hex TEXT, hash TEXT, first_seen TEXT,
+		route_type INTEGER, payload_type INTEGER, payload_version INTEGER,
+		decoded_json TEXT, last_seen INTEGER NOT NULL DEFAULT 0)`); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := conn.Exec(`CREATE TABLE observations (
+		id INTEGER PRIMARY KEY, transmission_id INTEGER, observer_id TEXT,
+		observer_name TEXT, direction TEXT, snr REAL, rssi REAL,
+		score INTEGER, path_json TEXT, timestamp TEXT, raw_hex TEXT)`); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := conn.Exec(`CREATE TABLE observers (rowid INTEGER PRIMARY KEY, id TEXT, name TEXT, iata TEXT)`); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := conn.Exec(`CREATE TABLE nodes (pubkey TEXT PRIMARY KEY, name TEXT, role TEXT, lat REAL, lon REAL, last_seen TEXT, first_seen TEXT, frequency REAL)`); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := conn.Exec(`CREATE TABLE schema_version (version INTEGER)`); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := conn.Exec(`INSERT INTO schema_version (version) VALUES (1)`); err != nil {
+		t.Fatal(err)
+	}
+	conn.Close()
+
+	db, err := OpenDB(dbPath)
+	if err != nil {
+		t.Fatalf("OpenDB: %v", err)
+	}
+	defer db.conn.Close()
+
+	store := NewPacketStore(db, &PacketStoreConfig{
+		RetentionHours:  168,
+		HotStartupHours: 1,
+	})
+	// Simulate the #1809 race: packets present, oldestLoaded never set.
+	store.mu.Lock()
+	store.packets = append(store.packets, &StoreTx{ID: 1, Hash: "deadbeef", FirstSeen: "2025-01-01T00:00:00Z"})
+	store.oldestLoaded = ""
+	store.mu.Unlock()
+
+	defer func() {
+		r := recover()
+		if r == nil {
+			t.Fatalf("loadBackgroundChunks must panic when oldestLoaded=\"\" with packets in store (#1809 invariant)")
+		}
+		msg := fmt.Sprintf("%v", r)
+		if msg == "" {
+			t.Fatalf("panic message must be non-empty")
+		}
+	}()
+	store.loadBackgroundChunks()
+}
diff --git a/cmd/server/store.go b/cmd/server/store.go
index 4b4322e9..3a9967f6 100644
--- a/cmd/server/store.go
+++ b/cmd/server/store.go
@@ -421,6 +421,13 @@ type PacketStore struct {
 	//     Read order MUST be: load backgroundLoadDone first; only if true
 	//     is backgroundLoadFailed meaningful.
 	// 0 = disabled (current behavior). Background loader fills the rest.
+	//
+	// IMMUTABILITY: hotStartupHours is set ONCE in NewPacketStore (with
+	// optional clamp against retentionHours) and is NEVER mutated
+	// afterward. Readers (LoadChunked, RunStartupLoad,
+	// loadBackgroundChunks, GetPerfStoreStats) intentionally read it
+	// without s.mu. Do not add a write path without also adding the
+	// lock to every reader — see #1809 / #1811.
 	hotStartupHours        float64
 	backgroundLoadDone     atomic.Bool
 	backgroundLoadFailed   atomic.Bool
@@ -1418,6 +1425,22 @@ func (s *PacketStore) loadChunk(from, to time.Time) error {
 // chunks are merged it rebuilds analytics indexes once. Chunk errors are
 // handled by advancing past the failed window so the loop always terminates.
 func (s *PacketStore) loadBackgroundChunks() {
+	// #1809 invariant: oldestLoaded MUST be set before the bg loader
+	// runs whenever the in-memory store has packets. The original bug
+	// was a parallel spawn that read oldestLoaded="" and silently
+	// bailed → coverage gate trips → backgroundLoadFailed=true. Encode
+	// the precondition here so a future refactor that re-introduces
+	// the race fails loudly instead of silently shipping the same
+	// regression. Empty store + empty oldestLoaded is the legitimate
+	// "empty DB" path and is allowed.
+	s.mu.RLock()
+	oldestAtEntry := s.oldestLoaded
+	packetCountAtEntry := len(s.packets)
+	s.mu.RUnlock()
+	if oldestAtEntry == "" && packetCountAtEntry > 0 {
+		panic(fmt.Sprintf("loadBackgroundChunks: oldestLoaded=\"\" with %d packets in store — LoadChunked must run to completion first (#1809)", packetCountAtEntry))
+	}
+
 	if s.retentionHours <= 0 {
 		s.backgroundLoadDone.Store(true)
 		return