diff --git a/anthropic-java-core/src/main/kotlin/com/anthropic/core/auth/CachingAccessTokenProvider.kt b/anthropic-java-core/src/main/kotlin/com/anthropic/core/auth/CachingAccessTokenProvider.kt
index f8164e2a1..a36cb4c82 100644
--- a/anthropic-java-core/src/main/kotlin/com/anthropic/core/auth/CachingAccessTokenProvider.kt
+++ b/anthropic-java-core/src/main/kotlin/com/anthropic/core/auth/CachingAccessTokenProvider.kt
@@ -3,6 +3,7 @@ package com.anthropic.core.auth
 import java.time.Duration
 import java.time.Instant
 import java.util.concurrent.CompletableFuture
+import java.util.concurrent.ForkJoinPool
 import java.util.concurrent.locks.ReentrantLock
 import kotlin.concurrent.withLock
 
@@ -191,11 +192,10 @@ internal class CachingAccessTokenProvider(
         // arriving during `prior` will join us.
         asyncInFlight = future
         asyncInFlightForce = true
-        prior.whenComplete { _, _ ->
-            provider.getAsync(baseUrl, true).whenComplete { token, err ->
-                completeAsync(future, baseUrl, token, err, advisory = false)
-            }
-        }
+        prior.whenCompleteAsync(
+            { _, _ -> launchAsyncRefresh(baseUrl, future, force = true, advisory = false) },
+            ForkJoinPool.commonPool(),
+        )
         return future
     }
 
@@ -205,9 +205,16 @@ internal class CachingAccessTokenProvider(
         force: Boolean,
         advisory: Boolean,
     ) {
-        provider.getAsync(baseUrl, force).whenComplete { token, err ->
-            completeAsync(future, baseUrl, token, err, advisory)
-        }
+        val refresh =
+            try {
+                provider.getAsync(baseUrl, force)
+            } catch (e: Exception) {
+                // A provider can fail before returning its future. Release the same slot
+                // and settle the same waiters as an exceptionally completed future.
+                completeAsync(future, baseUrl, null, e, advisory)
+                return
+            }
+        refresh.whenComplete { token, err -> completeAsync(future, baseUrl, token, err, advisory) }
     }
 
     private fun completeAsync(
diff --git a/anthropic-java-core/src/test/kotlin/com/anthropic/auth/CachingAccessTokenProviderSynchronousFailureTest.kt b/anthropic-java-core/src/test/kotlin/com/anthropic/auth/CachingAccessTokenProviderSynchronousFailureTest.kt
new file mode 100644
index 000000000..b497bcacd
--- /dev/null
+++ b/anthropic-java-core/src/test/kotlin/com/anthropic/auth/CachingAccessTokenProviderSynchronousFailureTest.kt
@@ -0,0 +1,136 @@
+package com.anthropic.auth
+
+import com.anthropic.core.auth.AccessToken
+import com.anthropic.core.auth.AccessTokenProvider
+import com.anthropic.core.auth.CachingAccessTokenProvider
+import java.time.Instant
+import java.util.concurrent.CompletableFuture
+import java.util.concurrent.ExecutionException
+import java.util.concurrent.TimeUnit
+import org.assertj.core.api.Assertions.assertThat
+import org.assertj.core.api.Assertions.assertThatThrownBy
+import org.junit.jupiter.api.Test
+
+internal class CachingAccessTokenProviderSynchronousFailureTest {
+    private val baseUrl = "https://api.anthropic.com"
+    private val failure = IllegalStateException("provider failed before returning a future")
+    private val fresh = AccessToken("fresh", expiresAt = null)
+
+    private fun provider(call: (Boolean) -> CompletableFuture<AccessToken>): AccessTokenProvider =
+        object : AccessTokenProvider {
+            override fun get(baseUrl: String, forceRefresh: Boolean): AccessToken =
+                throw UnsupportedOperationException("sync path not used")
+
+            override fun getAsync(baseUrl: String, forceRefresh: Boolean) = call(forceRefresh)
+        }
+
+    private fun assertFailed(future: CompletableFuture<AccessToken>) {
+        assertThatThrownBy { future.get(5, TimeUnit.SECONDS) }
+            .isInstanceOf(ExecutionException::class.java)
+            .hasCause(failure)
+    }
+
+    @Test
+    fun foregroundFailureCompletesTheFutureAndAllowsAnotherFetch() {
+        var calls = 0
+        val cache =
+            CachingAccessTokenProvider(
+                provider {
+                    if (++calls == 1) throw failure
+                    CompletableFuture.completedFuture(fresh)
+                }
+            )
+        assertFailed(cache.getAsync(baseUrl, false))
+        assertThat(cache.getAsync(baseUrl, false).get(1, TimeUnit.SECONDS)).isSameAs(fresh)
+        assertThat(calls).isEqualTo(2)
+    }
+
+    @Test
+    fun mandatoryFailureDoesNotLeaveLaterCallsWaiting() {
+        var calls = 0
+        val cache =
+            CachingAccessTokenProvider(
+                provider {
+                    when (++calls) {
+                        1 ->
+                            CompletableFuture.completedFuture(AccessToken("expired", Instant.EPOCH))
+                        2 -> throw failure
+                        else -> CompletableFuture.completedFuture(fresh)
+                    }
+                }
+            )
+        cache.getAsync(baseUrl, false).get(1, TimeUnit.SECONDS)
+        assertFailed(cache.getAsync(baseUrl, false))
+        assertThat(cache.getAsync(baseUrl, false).get(1, TimeUnit.SECONDS)).isSameAs(fresh)
+        assertThat(calls).isEqualTo(3)
+    }
+
+    @Test
+    fun advisoryFailureStillServesTheTokenAndUsesBackoff() {
+        var now = Instant.parse("2026-01-01T00:00:00Z")
+        val stale = AccessToken("stale", now.plusSeconds(60))
+        var calls = 0
+        val cache =
+            CachingAccessTokenProvider(
+                provider {
+                    when (++calls) {
+                        1 -> CompletableFuture.completedFuture(stale)
+                        2 -> throw failure
+                        else -> CompletableFuture.completedFuture(fresh)
+                    }
+                },
+                clock = { now },
+            )
+        assertThat(cache.getAsync(baseUrl, false).get(1, TimeUnit.SECONDS)).isSameAs(stale)
+        repeat(3) {
+            assertThat(cache.getAsync(baseUrl, false).get(1, TimeUnit.SECONDS)).isSameAs(stale)
+        }
+        assertThat(calls).isEqualTo(2)
+        now = now.plusSeconds(6)
+        assertThat(cache.getAsync(baseUrl, false).get(1, TimeUnit.SECONDS)).isSameAs(stale)
+        assertThat(cache.getAsync(baseUrl, false).get(1, TimeUnit.SECONDS)).isSameAs(fresh)
+        assertThat(calls).isEqualTo(3)
+    }
+
+    @Test
+    fun chainedForceFailureCompletesAllWaitersAndReleasesTheSlot() {
+        val pending = CompletableFuture<AccessToken>()
+        val forces = mutableListOf<Boolean>()
+        val cache =
+            CachingAccessTokenProvider(
+                provider { force ->
+                    forces.add(force)
+                    when (forces.size) {
+                        1 -> pending
+                        2 -> throw failure
+                        else -> CompletableFuture.completedFuture(fresh)
+                    }
+                }
+            )
+        val first = cache.getAsync(baseUrl, false)
+        val forced = cache.getAsync(baseUrl, true)
+        val joined = cache.getAsync(baseUrl, true)
+        assertThat(joined).isSameAs(forced)
+        pending.completeExceptionally(IllegalArgumentException("prior attempt failed"))
+        assertThat(first.isCompletedExceptionally).isTrue()
+        assertFailed(forced)
+        assertFailed(joined)
+        assertThat(cache.getAsync(baseUrl, false).get(1, TimeUnit.SECONDS)).isSameAs(fresh)
+        assertThat(forces).containsExactly(false, true, false)
+    }
+
+    @Test
+    fun alreadyExceptionalFutureRetainsExistingBehavior() {
+        var calls = 0
+        val cache =
+            CachingAccessTokenProvider(
+                provider {
+                    if (++calls == 1)
+                        CompletableFuture<AccessToken>().also { it.completeExceptionally(failure) }
+                    else CompletableFuture.completedFuture(fresh)
+                }
+            )
+        assertFailed(cache.getAsync(baseUrl, false))
+        assertThat(cache.getAsync(baseUrl, false).get(1, TimeUnit.SECONDS)).isSameAs(fresh)
+    }
+}