diff --git a/java/com/google/gerrit/acceptance/testsuite/change/PerCommentOperationsImpl.java b/java/com/google/gerrit/acceptance/testsuite/change/PerCommentOperationsImpl.java
index 5f046ca..0218731 100644
--- a/java/com/google/gerrit/acceptance/testsuite/change/PerCommentOperationsImpl.java
+++ b/java/com/google/gerrit/acceptance/testsuite/change/PerCommentOperationsImpl.java
@@ -56,6 +56,11 @@
   }
 
   static TestHumanComment toTestComment(HumanComment comment) {
-    return TestHumanComment.builder().uuid(comment.key.uuid).parentUuid(comment.parentUuid).build();
+    return TestHumanComment.builder()
+        .uuid(comment.key.uuid)
+        .parentUuid(comment.parentUuid)
+        .tag(comment.tag)
+        .unresolved(comment.unresolved)
+        .build();
   }
 }
diff --git a/java/com/google/gerrit/acceptance/testsuite/change/TestHumanComment.java b/java/com/google/gerrit/acceptance/testsuite/change/TestHumanComment.java
index 9bb026f..3a7f2ae 100644
--- a/java/com/google/gerrit/acceptance/testsuite/change/TestHumanComment.java
+++ b/java/com/google/gerrit/acceptance/testsuite/change/TestHumanComment.java
@@ -28,6 +28,12 @@
   /** UUID of another comment to which this comment is a reply. */
   public abstract Optional<String> parentUuid();
 
+  /** Tag of a comment. */
+  public abstract Optional<String> tag();
+
+  /** Unresolved state of a comment. */
+  public abstract boolean unresolved();
+
   static Builder builder() {
     return new AutoValue_TestHumanComment.Builder();
   }
@@ -38,6 +44,10 @@
 
     abstract Builder parentUuid(@Nullable String parentUuid);
 
+    abstract Builder tag(@Nullable String tag);
+
+    abstract Builder unresolved(boolean unresolved);
+
     abstract TestHumanComment build();
   }
 }
diff --git a/java/com/google/gerrit/server/CommentsUtil.java b/java/com/google/gerrit/server/CommentsUtil.java
index 30913f7..b83eae8 100644
--- a/java/com/google/gerrit/server/CommentsUtil.java
+++ b/java/com/google/gerrit/server/CommentsUtil.java
@@ -189,6 +189,12 @@
         .findFirst();
   }
 
+  public Optional<HumanComment> getPublishedHumanComment(ChangeNotes notes, String uuid) {
+    return publishedHumanCommentsByChange(notes).stream()
+        .filter(c -> c.key.uuid.equals(uuid))
+        .findFirst();
+  }
+
   public Optional<HumanComment> getDraft(ChangeNotes notes, IdentifiedUser user, Comment.Key key) {
     return draftByChangeAuthor(notes, user.getAccountId()).stream()
         .filter(c -> key.equals(c.key))
@@ -205,6 +211,10 @@
     return sort(Lists.newArrayList(notes.getRobotComments().values()));
   }
 
+  public Optional<RobotComment> getRobotComment(ChangeNotes notes, String uuid) {
+    return robotCommentsByChange(notes).stream().filter(c -> c.key.uuid.equals(uuid)).findFirst();
+  }
+
   public List<HumanComment> draftByChange(ChangeNotes notes) {
     List<HumanComment> comments = new ArrayList<>();
     for (Ref ref : getDraftRefs(notes.getChangeId())) {
diff --git a/java/com/google/gerrit/server/change/DraftCommentResource.java b/java/com/google/gerrit/server/change/DraftCommentResource.java
index e0648cf..19a495d 100644
--- a/java/com/google/gerrit/server/change/DraftCommentResource.java
+++ b/java/com/google/gerrit/server/change/DraftCommentResource.java
@@ -20,6 +20,7 @@
 import com.google.gerrit.extensions.restapi.RestResource;
 import com.google.gerrit.extensions.restapi.RestView;
 import com.google.gerrit.server.CurrentUser;
+import com.google.gerrit.server.notedb.ChangeNotes;
 import com.google.inject.TypeLiteral;
 
 public class DraftCommentResource implements RestResource {
@@ -50,6 +51,10 @@
     return comment;
   }
 
+  public ChangeNotes getNotes() {
+    return rev.getNotes();
+  }
+
   public String getId() {
     return comment.key.uuid;
   }
diff --git a/java/com/google/gerrit/server/restapi/change/CreateDraftComment.java b/java/com/google/gerrit/server/restapi/change/CreateDraftComment.java
index b749e6a..399da8f 100644
--- a/java/com/google/gerrit/server/restapi/change/CreateDraftComment.java
+++ b/java/com/google/gerrit/server/restapi/change/CreateDraftComment.java
@@ -81,6 +81,11 @@
       throw new BadRequestException("line must be >= 0");
     } else if (in.line != null && in.range != null && in.line != in.range.endLine) {
       throw new BadRequestException("range endLine must be on the same line as the comment");
+    } else if (in.inReplyTo != null
+        && !commentsUtil.getPublishedHumanComment(rsrc.getNotes(), in.inReplyTo).isPresent()
+        && !commentsUtil.getRobotComment(rsrc.getNotes(), in.inReplyTo).isPresent()) {
+      throw new BadRequestException(
+          String.format("Invalid inReplyTo, comment %s not found", in.inReplyTo));
     }
 
     try (BatchUpdate bu =
diff --git a/java/com/google/gerrit/server/restapi/change/PostReview.java b/java/com/google/gerrit/server/restapi/change/PostReview.java
index 9064dc8..32c1656 100644
--- a/java/com/google/gerrit/server/restapi/change/PostReview.java
+++ b/java/com/google/gerrit/server/restapi/change/PostReview.java
@@ -616,6 +616,7 @@
         ensureCommentNotOnMagicFilesOfAutoMerge(path, comment);
         ensureRangeIsValid(path, comment.range);
         ensureValidPatchsetLevelComment(path, comment);
+        ensureValidInReplyTo(revision.getNotes(), comment.inReplyTo);
       }
     }
   }
@@ -662,6 +663,16 @@
     }
   }
 
+  private void ensureValidInReplyTo(ChangeNotes changeNotes, String inReplyTo)
+      throws BadRequestException {
+    if (inReplyTo != null
+        && !commentsUtil.getPublishedHumanComment(changeNotes, inReplyTo).isPresent()
+        && !commentsUtil.getRobotComment(changeNotes, inReplyTo).isPresent()) {
+      throw new BadRequestException(
+          String.format("Invalid inReplyTo, comment %s not found", inReplyTo));
+    }
+  }
+
   private void checkRobotComments(
       RevisionResource revision, Map<String, List<RobotCommentInput>> in)
       throws BadRequestException, PatchListNotAvailableException {
diff --git a/java/com/google/gerrit/server/restapi/change/PutDraftComment.java b/java/com/google/gerrit/server/restapi/change/PutDraftComment.java
index f327f16..17a8ff4 100644
--- a/java/com/google/gerrit/server/restapi/change/PutDraftComment.java
+++ b/java/com/google/gerrit/server/restapi/change/PutDraftComment.java
@@ -31,6 +31,7 @@
 import com.google.gerrit.server.CommentsUtil;
 import com.google.gerrit.server.PatchSetUtil;
 import com.google.gerrit.server.change.DraftCommentResource;
+import com.google.gerrit.server.notedb.ChangeNotes;
 import com.google.gerrit.server.notedb.ChangeUpdate;
 import com.google.gerrit.server.patch.PatchListCache;
 import com.google.gerrit.server.patch.PatchListNotAvailableException;
@@ -55,6 +56,7 @@
   private final PatchSetUtil psUtil;
   private final Provider<CommentJson> commentJson;
   private final PatchListCache patchListCache;
+  private final ChangeNotes.Factory changeNotesFactory;
 
   @Inject
   PutDraftComment(
@@ -63,13 +65,15 @@
       CommentsUtil commentsUtil,
       PatchSetUtil psUtil,
       Provider<CommentJson> commentJson,
-      PatchListCache patchListCache) {
+      PatchListCache patchListCache,
+      ChangeNotes.Factory changeNotesFactory) {
     this.updateFactory = updateFactory;
     this.delete = delete;
     this.commentsUtil = commentsUtil;
     this.psUtil = psUtil;
     this.commentJson = commentJson;
     this.patchListCache = patchListCache;
+    this.changeNotesFactory = changeNotesFactory;
   }
 
   @Override
@@ -86,8 +90,12 @@
       throw new BadRequestException("patchset-level comments can't have side, range, or line");
     } else if (in.line != null && in.range != null && in.line != in.range.endLine) {
       throw new BadRequestException("range endLine must be on the same line as the comment");
+    } else if (in.inReplyTo != null
+        && !commentsUtil.getPublishedHumanComment(rsrc.getNotes(), in.inReplyTo).isPresent()
+        && !commentsUtil.getRobotComment(rsrc.getNotes(), in.inReplyTo).isPresent()) {
+      throw new BadRequestException(
+          String.format("Invalid inReplyTo, comment %s not found", in.inReplyTo));
     }
-
     try (BatchUpdate bu =
         updateFactory.create(rsrc.getChange().getProject(), rsrc.getUser(), TimeUtil.nowTs())) {
       Op op = new Op(rsrc.getComment().key, in);
diff --git a/java/com/google/gerrit/testing/TestCommentHelper.java b/java/com/google/gerrit/testing/TestCommentHelper.java
index 889bc18..76a5521 100644
--- a/java/com/google/gerrit/testing/TestCommentHelper.java
+++ b/java/com/google/gerrit/testing/TestCommentHelper.java
@@ -17,6 +17,7 @@
 import static java.util.stream.Collectors.toList;
 
 import com.google.common.collect.ImmutableList;
+import com.google.gerrit.entities.Change;
 import com.google.gerrit.extensions.api.GerritApi;
 import com.google.gerrit.extensions.api.changes.DraftInput;
 import com.google.gerrit.extensions.api.changes.ReviewInput;
@@ -148,13 +149,30 @@
     addRobotComment(targetChangeId, robotCommentInput, "robot comment test");
   }
 
+  public void addRobotComment(Change.Id targetChangeId, RobotCommentInput robotCommentInput)
+      throws Exception {
+    addRobotComment(targetChangeId, robotCommentInput, "robot comment test");
+  }
+
   public void addRobotComment(
       String targetChangeId, RobotCommentInput robotCommentInput, String message) throws Exception {
+    ReviewInput reviewInput = createReviewInput(robotCommentInput, message);
+    gApi.changes().id(targetChangeId).current().review(reviewInput);
+  }
+
+  public void addRobotComment(
+      Change.Id targetChangeId, RobotCommentInput robotCommentInput, String message)
+      throws Exception {
+    ReviewInput reviewInput = createReviewInput(robotCommentInput, message);
+    gApi.changes().id(targetChangeId.get()).current().review(reviewInput);
+  }
+
+  private ReviewInput createReviewInput(RobotCommentInput robotCommentInput, String message) {
     ReviewInput reviewInput = new ReviewInput();
     reviewInput.robotComments =
         Collections.singletonMap(robotCommentInput.path, ImmutableList.of(robotCommentInput));
     reviewInput.message = message;
     reviewInput.tag = ChangeMessagesUtil.AUTOGENERATED_TAG_PREFIX;
-    gApi.changes().id(targetChangeId).current().review(reviewInput);
+    return reviewInput;
   }
 }
diff --git a/javatests/com/google/gerrit/acceptance/api/revision/RobotCommentsIT.java b/javatests/com/google/gerrit/acceptance/api/revision/RobotCommentsIT.java
index 27b866b..1ad952c 100644
--- a/javatests/com/google/gerrit/acceptance/api/revision/RobotCommentsIT.java
+++ b/javatests/com/google/gerrit/acceptance/api/revision/RobotCommentsIT.java
@@ -14,9 +14,11 @@
 
 package com.google.gerrit.acceptance.api.revision;
 
+import static com.google.common.collect.ImmutableSet.toImmutableSet;
 import static com.google.common.collect.MoreCollectors.onlyElement;
 import static com.google.common.truth.Truth.assertThat;
 import static com.google.gerrit.acceptance.PushOneCommit.SUBJECT;
+import static com.google.gerrit.entities.Patch.COMMIT_MSG;
 import static com.google.gerrit.entities.Patch.PATCHSET_LEVEL;
 import static com.google.gerrit.extensions.client.ListChangesOption.MESSAGES;
 import static com.google.gerrit.extensions.common.testing.DiffInfoSubject.assertThat;
@@ -32,8 +34,11 @@
 import com.google.gerrit.acceptance.PushOneCommit;
 import com.google.gerrit.acceptance.UseClockStep;
 import com.google.gerrit.acceptance.config.GerritConfig;
+import com.google.gerrit.acceptance.testsuite.change.ChangeOperations;
+import com.google.gerrit.entities.Change;
 import com.google.gerrit.entities.Patch;
 import com.google.gerrit.extensions.api.changes.PublishChangeEditInput;
+import com.google.gerrit.extensions.api.changes.ReviewInput;
 import com.google.gerrit.extensions.api.changes.ReviewInput.RobotCommentInput;
 import com.google.gerrit.extensions.client.Comment;
 import com.google.gerrit.extensions.client.Side;
@@ -67,6 +72,7 @@
 
 public class RobotCommentsIT extends AbstractDaemonTest {
   @Inject private TestCommentHelper testCommentHelper;
+  @Inject private ChangeOperations changeOperations;
 
   private static final String PLAIN_TEXT_CONTENT_TYPE = "text/plain";
   private static final String GERRIT_COMMIT_MESSAGE_TYPE = "text/x-gerrit-commit-message";
@@ -320,6 +326,58 @@
   }
 
   @Test
+  public void robotCommentInvalidInReplyTo() throws Exception {
+    Change.Id changeId = changeOperations.newChange().create();
+    RobotCommentInput input = TestCommentHelper.createRobotCommentInput(PATCHSET_LEVEL);
+    input.inReplyTo = "invalid";
+    BadRequestException ex =
+        assertThrows(
+            BadRequestException.class, () -> testCommentHelper.addRobotComment(changeId, input));
+    assertThat(ex.getMessage()).contains("inReplyTo");
+  }
+
+  @Test
+  public void canCreateRobotCommentWithRobotCommentAsParent() throws Exception {
+    Change.Id changeId = changeOperations.newChange().create();
+    String parentRobotCommentUuid =
+        changeOperations.change(changeId).currentPatchset().newRobotComment().create();
+
+    ReviewInput.RobotCommentInput robotCommentInput =
+        TestCommentHelper.createRobotCommentInputWithMandatoryFields(COMMIT_MSG);
+    robotCommentInput.message = "comment reply";
+    robotCommentInput.inReplyTo = parentRobotCommentUuid;
+    testCommentHelper.addRobotComment(changeId, robotCommentInput);
+
+    RobotCommentInfo resultComment =
+        Iterables.getOnlyElement(
+            gApi.changes().id(changeId.get()).current().robotCommentsAsList().stream()
+                .filter(c -> c.message.equals("comment reply"))
+                .collect(toImmutableSet()));
+    assertThat(resultComment.inReplyTo).isEqualTo(parentRobotCommentUuid);
+  }
+
+  @Test
+  public void canCreateRobotCommentWithHumanCommentAsParent() throws Exception {
+    Change.Id changeId = changeOperations.newChange().create();
+    String changeIdString = changeOperations.change(changeId).get().changeId();
+    String parentCommentUuid =
+        changeOperations.change(changeId).currentPatchset().newComment().create();
+
+    ReviewInput.RobotCommentInput robotCommentInput =
+        TestCommentHelper.createRobotCommentInputWithMandatoryFields(COMMIT_MSG);
+    robotCommentInput.message = "comment reply";
+    robotCommentInput.inReplyTo = parentCommentUuid;
+    testCommentHelper.addRobotComment(changeIdString, robotCommentInput);
+
+    RobotCommentInfo resultComment =
+        Iterables.getOnlyElement(
+            gApi.changes().id(changeIdString).current().robotCommentsAsList().stream()
+                .filter(c -> c.message.equals("comment reply"))
+                .collect(toImmutableSet()));
+    assertThat(resultComment.inReplyTo).isEqualTo(parentCommentUuid);
+  }
+
+  @Test
   public void hugeRobotCommentIsRejected() {
     int defaultSizeLimit = 1 << 20;
     fixReplacementInfo.replacement = getStringFor(defaultSizeLimit + 1);
diff --git a/javatests/com/google/gerrit/acceptance/server/change/CommentsIT.java b/javatests/com/google/gerrit/acceptance/server/change/CommentsIT.java
index 88705e1..0cdff4e 100644
--- a/javatests/com/google/gerrit/acceptance/server/change/CommentsIT.java
+++ b/javatests/com/google/gerrit/acceptance/server/change/CommentsIT.java
@@ -14,10 +14,12 @@
 
 package com.google.gerrit.acceptance.server.change;
 
+import static com.google.common.collect.ImmutableSet.toImmutableSet;
 import static com.google.common.truth.Truth.assertThat;
 import static com.google.common.truth.Truth8.assertThat;
 import static com.google.gerrit.acceptance.PushOneCommit.FILE_NAME;
 import static com.google.gerrit.acceptance.PushOneCommit.SUBJECT;
+import static com.google.gerrit.entities.Patch.COMMIT_MSG;
 import static com.google.gerrit.entities.Patch.PATCHSET_LEVEL;
 import static com.google.gerrit.testing.GerritJUnit.assertThrows;
 import static com.google.gerrit.truth.MapSubject.assertThatMap;
@@ -51,7 +53,6 @@
 import com.google.gerrit.extensions.client.Side;
 import com.google.gerrit.extensions.common.ChangeInfo;
 import com.google.gerrit.extensions.common.CommentInfo;
-import com.google.gerrit.extensions.common.RobotCommentInfo;
 import com.google.gerrit.extensions.restapi.AuthException;
 import com.google.gerrit.extensions.restapi.BadRequestException;
 import com.google.gerrit.extensions.restapi.IdString;
@@ -658,6 +659,34 @@
   }
 
   @Test
+  public void putDraft_humanInReplyTo() throws Exception {
+    Change.Id changeId = changeOperations.newChange().create();
+    String parentCommentUuid =
+        changeOperations.change(changeId).currentPatchset().newComment().create();
+
+    DraftInput draft = newDraft(COMMIT_MSG, Side.REVISION, 0, "foo");
+    draft.inReplyTo = parentCommentUuid;
+    String createdDraftUuid = addDraft(changeId, draft).id;
+    TestHumanComment actual =
+        changeOperations.change(changeId).draftComment(createdDraftUuid).get();
+    assertThat(actual.parentUuid()).hasValue(parentCommentUuid);
+  }
+
+  @Test
+  public void putDraft_robotInReplyTo() throws Exception {
+    Change.Id changeId = changeOperations.newChange().create();
+    String parentRobotCommentUuid =
+        changeOperations.change(changeId).currentPatchset().newRobotComment().create();
+
+    DraftInput draft = newDraft(COMMIT_MSG, Side.REVISION, 0, "foo");
+    draft.inReplyTo = parentRobotCommentUuid;
+    String createdDraftUuid = addDraft(changeId, draft).id;
+    TestHumanComment actual =
+        changeOperations.change(changeId).draftComment(createdDraftUuid).get();
+    assertThat(actual.parentUuid()).hasValue(parentRobotCommentUuid);
+  }
+
+  @Test
   public void putDraft_idMismatch() throws Exception {
     String file = "file";
     PushOneCommit.Result r = createChange();
@@ -702,6 +731,16 @@
   }
 
   @Test
+  public void putDraft_invalidInReplyTo() throws Exception {
+    Change.Id changeId = changeOperations.newChange().create();
+    DraftInput draft = newDraft(COMMIT_MSG, Side.REVISION, 0, "foo");
+    draft.inReplyTo = "invalid";
+    BadRequestException exception =
+        assertThrows(BadRequestException.class, () -> addDraft(changeId, draft));
+    assertThat(exception.getMessage()).contains(String.format("%s not found", draft.inReplyTo));
+  }
+
+  @Test
   public void putDraft_updatePath() throws Exception {
     PushOneCommit.Result r = createChange();
     String changeId = r.getChangeId();
@@ -715,22 +754,62 @@
   }
 
   @Test
-  public void putDraft_updateInReplyToAndTag() throws Exception {
-    PushOneCommit.Result r = createChange();
-    String changeId = r.getChangeId();
-    String revId = r.getCommit().getName();
-    DraftInput draftInput1 = newDraft(FILE_NAME, Side.REVISION, 0, "foo");
-    CommentInfo commentInfo = addDraft(changeId, revId, draftInput1);
-    DraftInput draftInput2 = newDraft(FILE_NAME, Side.REVISION, 0, "bar");
-    String inReplyTo = "in_reply_to";
+  public void putDraft_updateInvalidInReplyTo() throws Exception {
+    Change.Id changeId = changeOperations.newChange().create();
+    DraftInput originalDraftInput = newDraft(FILE_NAME, Side.REVISION, 0, "foo");
+    CommentInfo originalDraft = addDraft(changeId, originalDraftInput);
+
+    DraftInput updatedDraftInput = newDraft(FILE_NAME, Side.REVISION, 0, "bar");
+    updatedDraftInput.inReplyTo = "invalid";
+    BadRequestException exception =
+        assertThrows(
+            BadRequestException.class,
+            () -> updateDraft(changeId, updatedDraftInput, originalDraft.id));
+    assertThat(exception.getMessage()).contains(String.format("Invalid inReplyTo"));
+  }
+
+  @Test
+  public void putDraft_updateHumanInReplyTo() throws Exception {
+    Change.Id changeId = changeOperations.newChange().create();
+    String parentCommentUuid =
+        changeOperations.change(changeId).currentPatchset().newComment().create();
+    DraftInput originalDraftInput = newDraft(FILE_NAME, Side.REVISION, 0, "foo");
+    CommentInfo originalDraft = addDraft(changeId, originalDraftInput);
+
+    DraftInput updateDraftInput = newDraft(FILE_NAME, Side.REVISION, 0, "bar");
+    updateDraftInput.inReplyTo = parentCommentUuid;
+    updateDraft(changeId, updateDraftInput, originalDraft.id);
+    assertThat(changeOperations.change(changeId).draftComment(originalDraft.id).get().parentUuid())
+        .hasValue(parentCommentUuid);
+  }
+
+  @Test
+  public void putDraft_updateRobotInReplyTo() throws Exception {
+    Change.Id changeId = changeOperations.newChange().create();
+    String parentRobotCommentUuid =
+        changeOperations.change(changeId).currentPatchset().newRobotComment().create();
+    DraftInput originalDraftInput = newDraft(FILE_NAME, Side.REVISION, 0, "foo");
+    CommentInfo originalDraft = addDraft(changeId, originalDraftInput);
+
+    DraftInput updateDraftInput = newDraft(FILE_NAME, Side.REVISION, 0, "bar");
+    updateDraftInput.inReplyTo = parentRobotCommentUuid;
+    updateDraft(changeId, updateDraftInput, originalDraft.id);
+    assertThat(changeOperations.change(changeId).draftComment(originalDraft.id).get().parentUuid())
+        .hasValue(parentRobotCommentUuid);
+  }
+
+  @Test
+  public void putDraft_updateTag() throws Exception {
+    Change.Id changeId = changeOperations.newChange().create();
+    DraftInput originalDraftInput = newDraft(FILE_NAME, Side.REVISION, 0, "foo");
+    CommentInfo originalDraft = addDraft(changeId, originalDraftInput);
+
+    DraftInput updateDraftInput = newDraft(FILE_NAME, Side.REVISION, 0, "bar");
     String tag = "täg";
-    draftInput2.inReplyTo = inReplyTo;
-    draftInput2.tag = tag;
-    updateDraft(changeId, revId, draftInput2, commentInfo.id);
-    com.google.gerrit.entities.Comment comment =
-        Iterables.getOnlyElement(commentsUtil.draftByChange(r.getChange().notes()));
-    assertThat(comment.parentUuid).isEqualTo(inReplyTo);
-    assertThat(comment.tag).isEqualTo(tag);
+    updateDraftInput.tag = tag;
+    updateDraft(changeId, updateDraftInput, originalDraft.id);
+    assertThat(changeOperations.change(changeId).draftComment(originalDraft.id).get().tag())
+        .hasValue(tag);
   }
 
   @Test
@@ -1474,26 +1553,73 @@
 
   @Test
   public void canCreateHumanCommentWithRobotCommentAsParentAndUnsetUnresolved() throws Exception {
-    PushOneCommit.Result result = createChange();
-    String changeId = result.getChangeId();
-    String ps1 = result.getCommit().name();
+    Change.Id changeId = changeOperations.newChange().create();
+    String parentRobotCommentUuid =
+        changeOperations.change(changeId).currentPatchset().newRobotComment().create();
 
-    testCommentHelper.addRobotComment(
-        result.getChangeId(),
-        TestCommentHelper.createRobotCommentInputWithMandatoryFields(FILE_NAME));
-    RobotCommentInfo robotCommentInfo =
-        Iterables.getOnlyElement(gApi.changes().id(changeId).current().robotCommentsAsList());
+    CommentInput createdCommentInput = newComment(COMMIT_MSG, "comment reply");
+    createdCommentInput.inReplyTo = parentRobotCommentUuid;
+    createdCommentInput.unresolved = null;
+    addComments(changeId, createdCommentInput);
 
-    CommentInput comment = newComment(FILE_NAME, "comment 1 reply");
-    comment.inReplyTo = robotCommentInfo.id;
-    comment.unresolved = null;
-    addComments(changeId, ps1, comment);
+    CommentInfo resultNewComment =
+        Iterables.getOnlyElement(
+            getPublishedCommentsAsList(changeId).stream()
+                .filter(c -> c.message.equals("comment reply"))
+                .collect(toImmutableSet()));
 
-    CommentInfo resultComment = Iterables.getOnlyElement(getPublishedCommentsAsList(changeId));
-    assertThat(resultComment.inReplyTo).isEqualTo(robotCommentInfo.id);
+    assertThat(resultNewComment.inReplyTo).isEqualTo(parentRobotCommentUuid);
 
     // Default unresolved is false.
-    assertThat(resultComment.unresolved).isFalse();
+    assertThat(resultNewComment.unresolved).isFalse();
+  }
+
+  @Test
+  public void canCreateHumanCommentWithHumanCommentAsParent() throws Exception {
+    Change.Id changeId = changeOperations.newChange().create();
+    String parentCommentUuid =
+        changeOperations.change(changeId).currentPatchset().newComment().create();
+
+    CommentInput createdCommentInput = newComment(COMMIT_MSG, "comment reply");
+    createdCommentInput.inReplyTo = parentCommentUuid;
+    addComments(changeId, createdCommentInput);
+
+    CommentInfo resultNewComment =
+        Iterables.getOnlyElement(
+            getPublishedCommentsAsList(changeId).stream()
+                .filter(c -> c.message.equals("comment reply"))
+                .collect(toImmutableSet()));
+    assertThat(resultNewComment.inReplyTo).isEqualTo(parentCommentUuid);
+  }
+
+  @Test
+  public void canCreateHumanCommentWithRobotCommentAsParent() throws Exception {
+    Change.Id changeId = changeOperations.newChange().create();
+    String parentRobotCommentUuid =
+        changeOperations.change(changeId).currentPatchset().newRobotComment().create();
+
+    CommentInput createdCommentInput = newComment(COMMIT_MSG, "comment reply");
+    createdCommentInput.inReplyTo = parentRobotCommentUuid;
+    addComments(changeId, createdCommentInput);
+
+    CommentInfo resultNewComment =
+        Iterables.getOnlyElement(
+            getPublishedCommentsAsList(changeId).stream()
+                .filter(c -> c.message.equals("comment reply"))
+                .collect(toImmutableSet()));
+    assertThat(resultNewComment.inReplyTo).isEqualTo(parentRobotCommentUuid);
+  }
+
+  @Test
+  public void cannotCreateCommentWithInvalidInReplyTo() throws Exception {
+    Change.Id changeId = changeOperations.newChange().create();
+
+    CommentInput comment = newComment(COMMIT_MSG, "comment 1 reply");
+    comment.inReplyTo = "invalid";
+
+    BadRequestException exception =
+        assertThrows(BadRequestException.class, () -> addComments(changeId, comment));
+    assertThat(exception.getMessage()).contains(String.format("%s not found", comment.inReplyTo));
   }
 
   private List<CommentInfo> getRevisionComments(String changeId, String revId) throws Exception {
@@ -1510,6 +1636,12 @@
     return comment;
   }
 
+  private void addComments(Change.Id changeId, CommentInput... commentInputs) throws Exception {
+    ReviewInput input = new ReviewInput();
+    input.comments = Arrays.stream(commentInputs).collect(groupingBy(c -> c.path));
+    gApi.changes().id(changeId.get()).current().review(input);
+  }
+
   private void addComments(String changeId, String revision, CommentInput... commentInputs)
       throws Exception {
     ReviewInput input = new ReviewInput();
@@ -1517,6 +1649,12 @@
     gApi.changes().id(changeId).revision(revision).review(input);
   }
 
+  private void addComments(String changeId, CommentInput... commentInputs) throws Exception {
+    ReviewInput input = new ReviewInput();
+    input.comments = Arrays.stream(commentInputs).collect(groupingBy(c -> c.path));
+    gApi.changes().id(changeId).current().review(input);
+  }
+
   /**
    * All the commits, which contain the target comment before, should still contain the comment with
    * the updated message. All the other metas of the commits should be exactly the same.
@@ -1610,11 +1748,31 @@
     return gApi.changes().id(changeId).revision(revId).createDraft(in).get();
   }
 
+  private CommentInfo addDraft(Change.Id changeId, String revId, DraftInput in) throws Exception {
+    return gApi.changes().id(changeId.get()).revision(revId).createDraft(in).get();
+  }
+
+  private CommentInfo addDraft(String changeId, DraftInput in) throws Exception {
+    return gApi.changes().id(changeId).current().createDraft(in).get();
+  }
+
+  private CommentInfo addDraft(Change.Id changeId, DraftInput in) throws Exception {
+    return gApi.changes().id(changeId.get()).current().createDraft(in).get();
+  }
+
   private void updateDraft(String changeId, String revId, DraftInput in, String uuid)
       throws Exception {
     gApi.changes().id(changeId).revision(revId).draft(uuid).update(in);
   }
 
+  private void updateDraft(String changeId, DraftInput in, String uuid) throws Exception {
+    gApi.changes().id(changeId).current().draft(uuid).update(in);
+  }
+
+  private void updateDraft(Change.Id changeId, DraftInput in, String uuid) throws Exception {
+    gApi.changes().id(changeId.get()).current().draft(uuid).update(in);
+  }
+
   private void deleteDraft(String changeId, String revId, String uuid) throws Exception {
     gApi.changes().id(changeId).revision(revId).draft(uuid).delete();
   }
@@ -1633,6 +1791,10 @@
     return gApi.changes().id(changeId).commentsAsList();
   }
 
+  private List<CommentInfo> getPublishedCommentsAsList(Change.Id changeId) throws Exception {
+    return gApi.changes().id(changeId.get()).commentsAsList();
+  }
+
   private Map<String, List<CommentInfo>> getDraftComments(String changeId, String revId)
       throws Exception {
     return gApi.changes().id(changeId).revision(revId).drafts();
diff --git a/javatests/com/google/gerrit/acceptance/testsuite/change/ChangeOperationsImplTest.java b/javatests/com/google/gerrit/acceptance/testsuite/change/ChangeOperationsImplTest.java
index a6e31eb..0bd6554 100644
--- a/javatests/com/google/gerrit/acceptance/testsuite/change/ChangeOperationsImplTest.java
+++ b/javatests/com/google/gerrit/acceptance/testsuite/change/ChangeOperationsImplTest.java
@@ -1175,6 +1175,39 @@
   }
 
   @Test
+  public void tagOfPublishedCommentCanBeRetrieved() {
+    Change.Id changeId = changeOperations.newChange().create();
+    String childCommentUuid =
+        changeOperations.change(changeId).currentPatchset().newComment().tag("tag").create();
+
+    TestHumanComment comment = changeOperations.change(changeId).comment(childCommentUuid).get();
+
+    assertThat(comment.tag()).value().isEqualTo("tag");
+  }
+
+  @Test
+  public void unresolvedOfUnresolvedPublishedCommentCanBeRetrieved() {
+    Change.Id changeId = changeOperations.newChange().create();
+    String childCommentUuid =
+        changeOperations.change(changeId).currentPatchset().newComment().unresolved().create();
+
+    TestHumanComment comment = changeOperations.change(changeId).comment(childCommentUuid).get();
+
+    assertThat(comment.unresolved()).isTrue();
+  }
+
+  @Test
+  public void unresolvedOfResolvedPublishedCommentCanBeRetrieved() {
+    Change.Id changeId = changeOperations.newChange().create();
+    String childCommentUuid =
+        changeOperations.change(changeId).currentPatchset().newComment().resolved().create();
+
+    TestHumanComment comment = changeOperations.change(changeId).comment(childCommentUuid).get();
+
+    assertThat(comment.unresolved()).isFalse();
+  }
+
+  @Test
   public void draftCommentCanBeRetrieved() {
     Change.Id changeId = changeOperations.newChange().create();
     String commentUuid = changeOperations.change(changeId).currentPatchset().newComment().create();