diff --git a/core/apiauth/cache.go b/core/apiauth/cache.go
new file mode 100644
index 000000000..136b5c336
--- /dev/null
+++ b/core/apiauth/cache.go
@@ -0,0 +1,87 @@
+package apiauth
+
+import (
+	"maps"
+	"sync"
+	"time"
+)
+
+const maxLivenessEntries = 1024
+
+type livenessEntry struct {
+	userID     string
+	epoch      int
+	lastUsedAt time.Time
+	expires    time.Time
+}
+
+// livenessCache bounds how long a node trusts "this grant exists" without asking the DB.
+type livenessCache struct {
+	mu      sync.Mutex
+	ttl     time.Duration
+	gen     uint64
+	entries map[string]livenessEntry
+	evicted map[string]uint64 // grant id -> generation of its last eviction
+	floor   uint64            // fills started before the last trim of evicted are dropped
+}
+
+func newLivenessCache(ttl time.Duration) *livenessCache {
+	return &livenessCache{ttl: ttl, entries: map[string]livenessEntry{}, evicted: map[string]uint64{}}
+}
+
+func (c *livenessCache) begin() uint64 {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	return c.gen
+}
+
+func (c *livenessCache) get(id string, now time.Time) (livenessEntry, bool) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	e, ok := c.entries[id]
+	if !ok || !now.Before(e.expires) {
+		return livenessEntry{}, false
+	}
+	return e, true
+}
+
+// put ignores a fill whose DB read started before the grant was last evicted.
+func (c *livenessCache) put(id string, e livenessEntry, now time.Time, started uint64) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	if started < c.floor || c.evicted[id] > started {
+		return
+	}
+	if len(c.entries) >= maxLivenessEntries {
+		maps.DeleteFunc(c.entries, func(_ string, v livenessEntry) bool { return !now.Before(v.expires) })
+	}
+	if len(c.evicted) >= maxLivenessEntries {
+		clear(c.evicted)
+		c.floor = c.gen
+	}
+	e.expires = now.Add(c.ttl)
+	c.entries[id] = e
+}
+
+func (c *livenessCache) evict(id string) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.gen++
+	c.evicted[id] = c.gen
+	delete(c.entries, id)
+}
+
+func (c *livenessCache) markUsed(id string, at time.Time) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	if e, ok := c.entries[id]; ok {
+		e.lastUsedAt = at
+		c.entries[id] = e
+	}
+}
+
+func (c *livenessCache) len() int {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	return len(c.entries)
+}
diff --git a/core/apiauth/cache_test.go b/core/apiauth/cache_test.go
new file mode 100644
index 000000000..999f3e24a
--- /dev/null
+++ b/core/apiauth/cache_test.go
@@ -0,0 +1,84 @@
+package apiauth
+
+import (
+	"fmt"
+	"time"
+
+	. "github.com/onsi/ginkgo/v2"
+	. "github.com/onsi/gomega"
+)
+
+var _ = Describe("livenessCache", func() {
+	var c *livenessCache
+	var t0 time.Time
+
+	BeforeEach(func() {
+		c = newLivenessCache(30 * time.Second)
+		t0 = time.Date(2026, 9, 25, 12, 0, 0, 0, time.UTC)
+	})
+
+	It("returns an entry until its TTL passes", func() {
+		c.put("g1", livenessEntry{userID: "u1", epoch: 2}, t0, c.begin())
+		e, ok := c.get("g1", t0.Add(29*time.Second))
+		Expect(ok).To(BeTrue())
+		Expect(e.userID).To(Equal("u1"))
+		Expect(e.epoch).To(Equal(2))
+		_, ok = c.get("g1", t0.Add(30*time.Second))
+		Expect(ok).To(BeFalse())
+	})
+
+	It("forgets evicted entries", func() {
+		c.put("g1", livenessEntry{userID: "u1"}, t0, c.begin())
+		c.evict("g1")
+		_, ok := c.get("g1", t0)
+		Expect(ok).To(BeFalse())
+	})
+
+	It("drops a fill that started before an eviction of the same grant", func() {
+		started := c.begin()                                  // a request reads the grant from the DB...
+		c.evict("g1")                                         // ...a logout deletes and evicts it...
+		c.put("g1", livenessEntry{userID: "u1"}, t0, started) // ...then the slow request tries to cache it
+		_, ok := c.get("g1", t0)
+		Expect(ok).To(BeFalse())
+	})
+
+	It("still accepts fills of other grants and later fills of the same grant", func() {
+		started := c.begin()
+		c.evict("g1")
+		c.put("g2", livenessEntry{userID: "u2"}, t0, started)
+		_, ok := c.get("g2", t0)
+		Expect(ok).To(BeTrue())
+		c.put("g1", livenessEntry{userID: "u1"}, t0, c.begin())
+		_, ok = c.get("g1", t0)
+		Expect(ok).To(BeTrue())
+	})
+
+	It("still drops a racing fill after the eviction log is trimmed", func() {
+		started := c.begin()
+		c.evict("g1")
+		for i := range maxLivenessEntries {
+			c.evict(fmt.Sprint("other", i))
+		}
+		c.put("fresh", livenessEntry{}, t0, c.begin())
+		c.put("g1", livenessEntry{userID: "u1"}, t0, started)
+		_, ok := c.get("g1", t0)
+		Expect(ok).To(BeFalse())
+	})
+
+	It("records the last use without extending the TTL", func() {
+		c.put("g1", livenessEntry{userID: "u1"}, t0, c.begin())
+		c.markUsed("g1", t0.Add(10*time.Second))
+		e, _ := c.get("g1", t0.Add(11*time.Second))
+		Expect(e.lastUsedAt).To(Equal(t0.Add(10 * time.Second)))
+		_, ok := c.get("g1", t0.Add(30*time.Second))
+		Expect(ok).To(BeFalse())
+	})
+
+	It("drops expired entries when it grows", func() {
+		for i := range maxLivenessEntries {
+			c.put(fmt.Sprint(i), livenessEntry{}, t0, c.begin())
+		}
+		c.put("fresh", livenessEntry{}, t0.Add(time.Minute), c.begin())
+		Expect(c.len()).To(Equal(1))
+	})
+})