diff --git a/CHANGELOG.md b/CHANGELOG.md
index c339e9c..fb4dfc5 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -45,6 +45,10 @@ APIs change between minors without a deprecation period.
   an endpoint it does not measure as a queue, so an external model (OpenAI, another llmgw cluster) was held to what a
   saturation of 0.85 allows of "in flight / 4"; the rest waited and timed out with 504. An outside provider queues for
   itself and says 429 when full, which its breaker already hears: it no longer counts towards saturation.
+- **A valid key told to slow down under load, every thirty seconds.** When a busy key's cached entry expired, every
+  request in flight missed the cache together and each took one of the address's allowance for unknown keys: past a
+  hundred or so streams at once, a valid key got 429 "too many unrecognised api keys". Requests missing the same key
+  at once now share one read and one allowance.
 
 ## v0.16.1 — 2026-10-30
 
diff --git a/internal/auth/auth.go b/internal/auth/auth.go
index c425554..2eb8c98 100644
--- a/internal/auth/auth.go
+++ b/internal/auth/auth.go
@@ -15,6 +15,7 @@ import (
 	"time"
 
 	"gitea.mynodes.xyz/ilker/llmgw/internal/store"
+	"golang.org/x/sync/singleflight"
 )
 
 // Principal is who a request is, once authenticated.
@@ -118,6 +119,12 @@ type Authenticator struct {
 	// lookups rations the database reads of keys not in the cache, per
 	// client address.
 	lookups *buckets
+
+	// resolving shares one database read between the requests that miss the
+	// cache for the same key at once: when a busy key's entry expired, every
+	// request in flight missed together, each took from the client's ration,
+	// and a valid key was told to slow down.
+	resolving singleflight.Group
 }
 
 type entry struct {
@@ -166,26 +173,37 @@ func (a *Authenticator) AuthenticateFrom(ctx context.Context, presented, client
 		return e.principal, e.err
 	}
 
-	if client != "" && !a.lookups.allow(client, a.now()) {
-		return Principal{}, ErrThrottled
-	}
-	p, err := a.resolve(ctx, hash)
-	if err != nil && !isRejection(err) {
-		// A database problem is not an authentication answer, so it must not be
-		// cached as one. Failing the request is right — the alternative is
-		// serving traffic whose authorization is unknown.
+	// One read for every request missing this key at once, rationed once.
+	v, err, _ := a.resolving.Do(id, func() (any, error) {
+		if e, ok := a.lookup(id); ok { // filled while this waited its turn
+			return e, nil
+		}
+		if client != "" && !a.lookups.allow(client, a.now()) {
+			return nil, ErrThrottled
+		}
+		p, err := a.resolve(ctx, hash)
+		if err != nil && !isRejection(err) {
+			// A database problem is not an authentication answer, so it must
+			// not be cached as one. Failing the request is right — the
+			// alternative is serving traffic whose authorization is unknown.
+			return nil, err
+		}
+		ttl := a.cfg.TTL
+		if err != nil {
+			ttl = a.cfg.NegativeTTL
+		}
+		e := entry{principal: p, err: err, expires: a.now().Add(ttl)}
+		a.store_(id, e)
+		return e, nil
+	})
+	if err != nil {
 		return Principal{}, err
 	}
-
-	ttl := a.cfg.TTL
-	if err != nil {
-		ttl = a.cfg.NegativeTTL
+	e := v.(entry)
+	if e.err == nil {
+		a.markUsed(e.principal.KeyID)
 	}
-	a.store_(id, entry{principal: p, err: err, expires: a.now().Add(ttl)})
-	if err == nil {
-		a.markUsed(p.KeyID)
-	}
-	return p, err
+	return e.principal, e.err
 }
 
 func (a *Authenticator) resolve(ctx context.Context, hash []byte) (Principal, error) {
diff --git a/internal/auth/auth_test.go b/internal/auth/auth_test.go
index f335158..14ea2ab 100644
--- a/internal/auth/auth_test.go
+++ b/internal/auth/auth_test.go
@@ -478,3 +478,41 @@ func TestTheCacheDoesNotGrowWithoutBound(t *testing.T) {
 		t.Errorf("%d entries", n)
 	}
 }
+
+// A busy key whose entry has expired: every request in flight misses at once,
+// and they share one read and one of the address's allowance. Each taking
+// its own told a valid key to slow down, every thirty seconds, under load.
+func TestABusyKeyMissingTheCacheTogetherIsReadOnceAndNotThrottled(t *testing.T) {
+	s, secret, _ := fixture(t)
+	c := &slowStore{countingStore: countingStore{Store: s}, delay: 50 * time.Millisecond}
+	a := New(c, DefaultConfig())
+	var wg sync.WaitGroup
+	var throttled atomic.Int64
+	for range 200 {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			if _, err := a.AuthenticateFrom(context.Background(), secret, "203.0.113.9"); errors.Is(err, ErrThrottled) {
+				throttled.Add(1)
+			} else if err != nil {
+				t.Error(err)
+			}
+		}()
+	}
+	wg.Wait()
+	if throttled.Load() != 0 || c.lookups.Load() != 1 {
+		t.Errorf("%d throttled, %d database reads", throttled.Load(), c.lookups.Load())
+	}
+}
+
+// slowStore is a countingStore whose reads take a while, so the requests
+// missing together are still waiting when the first one asks.
+type slowStore struct {
+	countingStore
+	delay time.Duration
+}
+
+func (s *slowStore) KeyByHash(ctx context.Context, hash []byte) (store.Key, error) {
+	time.Sleep(s.delay)
+	return s.countingStore.KeyByHash(ctx, hash)
+}