diff --git a/CHANGELOG.md b/CHANGELOG.md
index a390056d46..e3bb6e423d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -78,6 +78,13 @@ to docs, or any other relevant information.
   specific to the Temporal CLI dev server implementation and may no longer be valid if that implementation changes.
 
 ### Fixed
+- Fixed workflow task completion pagination, which threw `UnsupportedOperationException` before
+  sending the first page. A completion that needed pagination timed out on every attempt instead
+  of being sent. The same fix lets `RpcRetryOptions.newBuilder(options).addDoNotRetry(...)` extend
+  existing options, which also threw `UnsupportedOperationException`.
+- `RpcRetryOptions` no longer changes when the builder that created it is reused. Previously,
+  `addDoNotRetry` calls made after `build()` or `validateBuildWithDefaults()` also appeared in
+  the options already built.
 - `ActivityClient` now sends the `temporal-namespace` gRPC header like `WorkflowClient` and `ScheduleClient` do,
   so standalone activity calls are no longer rejected with `PERMISSION_DENIED` by namespace-scoped auth (e.g.
   Temporal Cloud API keys).
diff --git a/temporal-serviceclient/src/main/java/io/temporal/serviceclient/RpcRetryOptions.java b/temporal-serviceclient/src/main/java/io/temporal/serviceclient/RpcRetryOptions.java
index bddcca5a14..61c874b82c 100644
--- a/temporal-serviceclient/src/main/java/io/temporal/serviceclient/RpcRetryOptions.java
+++ b/temporal-serviceclient/src/main/java/io/temporal/serviceclient/RpcRetryOptions.java
@@ -95,7 +95,7 @@ private Builder(RpcRetryOptions options) {
       this.congestionInitialInterval = options.getCongestionInitialInterval();
       this.maximumInterval = options.getMaximumInterval();
       this.maximumJitterCoefficient = options.getMaximumJitterCoefficient();
-      this.doNotRetry = options.getDoNotRetry();
+      this.doNotRetry = new ArrayList<>(options.getDoNotRetry());
     }
 
     /**
@@ -413,7 +413,7 @@ private RpcRetryOptions(
     this.maximumAttempts = maximumAttempts;
     this.maximumInterval = maximumInterval;
     this.maximumJitterCoefficient = maximumJitterCoefficient;
-    this.doNotRetry = Collections.unmodifiableList(doNotRetry);
+    this.doNotRetry = Collections.unmodifiableList(new ArrayList<>(doNotRetry));
   }
 
   public Duration getInitialInterval() {
diff --git a/temporal-serviceclient/src/test/java/io/temporal/serviceclient/RpcRetryOptionsTest.java b/temporal-serviceclient/src/test/java/io/temporal/serviceclient/RpcRetryOptionsTest.java
index 8b66a2ded3..56c8e89aab 100644
--- a/temporal-serviceclient/src/test/java/io/temporal/serviceclient/RpcRetryOptionsTest.java
+++ b/temporal-serviceclient/src/test/java/io/temporal/serviceclient/RpcRetryOptionsTest.java
@@ -2,7 +2,12 @@
 
 import static org.junit.Assert.assertEquals;
 
+import io.grpc.Status;
 import java.time.Duration;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
 import org.junit.Test;
 
 public class RpcRetryOptionsTest {
@@ -27,4 +32,42 @@ public void setRetryOptionsMergesCongestionInitialInterval() {
     assertEquals(Duration.ofMillis(200), merged.getInitialInterval());
     assertEquals(Duration.ofSeconds(7), merged.getCongestionInitialInterval());
   }
+
+  @Test
+  public void aBuilderCopiedFromOptionsCanAddDoNotRetryItems() {
+    RpcRetryOptions source =
+        RpcRetryOptions.newBuilder()
+            .addDoNotRetry(Status.Code.NOT_FOUND, null)
+            .validateBuildWithDefaults();
+
+    RpcRetryOptions copy =
+        RpcRetryOptions.newBuilder(source)
+            .addDoNotRetry(Status.Code.ABORTED, null)
+            .validateBuildWithDefaults();
+
+    assertEquals(Arrays.asList(Status.Code.NOT_FOUND, Status.Code.ABORTED), codes(copy));
+    assertEquals(Collections.singletonList(Status.Code.NOT_FOUND), codes(source));
+  }
+
+  @Test
+  public void reusingABuilderDoesNotChangeOptionsItAlreadyBuilt() {
+    RpcRetryOptions.Builder builder =
+        RpcRetryOptions.newBuilder().addDoNotRetry(Status.Code.NOT_FOUND, null);
+    RpcRetryOptions validated = builder.validateBuildWithDefaults();
+    RpcRetryOptions built = builder.build();
+
+    builder.addDoNotRetry(Status.Code.ABORTED, null);
+
+    assertEquals(Collections.singletonList(Status.Code.NOT_FOUND), codes(validated));
+    assertEquals(Collections.singletonList(Status.Code.NOT_FOUND), codes(built));
+    assertEquals(Arrays.asList(Status.Code.NOT_FOUND, Status.Code.ABORTED), codes(builder.build()));
+  }
+
+  private static List<Status.Code> codes(RpcRetryOptions options) {
+    List<Status.Code> codes = new ArrayList<>();
+    for (RpcRetryOptions.DoNotRetryItem item : options.getDoNotRetry()) {
+      codes.add(item.getCode());
+    }
+    return codes;
+  }
 }
diff --git a/temporal-testing/src/main/java/io/temporal/testing/internal/devserver/SdkJavaTestServerProfile.java b/temporal-testing/src/main/java/io/temporal/testing/internal/devserver/SdkJavaTestServerProfile.java
index 37073eb4bf..decae29151 100644
--- a/temporal-testing/src/main/java/io/temporal/testing/internal/devserver/SdkJavaTestServerProfile.java
+++ b/temporal-testing/src/main/java/io/temporal/testing/internal/devserver/SdkJavaTestServerProfile.java
@@ -150,7 +150,13 @@ private static List<String> repositoryServerArguments() {
         "--dynamic-config-value",
         "frontend.workerCommandsEnabled=true",
         "--dynamic-config-value",
-        "system.enableCancelActivityWorkerCommand=true");
+        "system.enableCancelActivityWorkerCommand=true",
+        "--dynamic-config-value",
+        "history.enableWorkflowTaskCompletionPagination=true",
+        // Pagination clears the gRPC request limit, but the recombined completion still persists as
+        // one transaction, so the persistence limit must be above it.
+        "--dynamic-config-value",
+        "system.transactionSizeLimit=33554432");
   }
 
   private static File workingDirectory() {