diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/git/MergeConflictException.java b/gerrit-extension-api/src/main/java/com/google/gerrit/extensions/restapi/MergeConflictException.java
similarity index 74%
rename from gerrit-server/src/main/java/com/google/gerrit/server/git/MergeConflictException.java
rename to gerrit-extension-api/src/main/java/com/google/gerrit/extensions/restapi/MergeConflictException.java
index 02bc8dc..a67db0f 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/git/MergeConflictException.java
+++ b/gerrit-extension-api/src/main/java/com/google/gerrit/extensions/restapi/MergeConflictException.java
@@ -12,11 +12,16 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-package com.google.gerrit.server.git;
+package com.google.gerrit.extensions.restapi;
 
-/** Indicates that the commit cannot be merged without conflicts. */
-public class MergeConflictException extends Exception {
+/**
+ * Indicates that a commit cannot be merged without conflicts.
+ * <p>
+ * Messages should be viewable by end users.
+ */
+public class MergeConflictException extends ResourceConflictException {
   private static final long serialVersionUID = 1L;
+
   public MergeConflictException(String msg) {
     super(msg, null);
   }
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/change/CherryPickChange.java b/gerrit-server/src/main/java/com/google/gerrit/server/change/CherryPickChange.java
index 1045cff..18527bd 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/change/CherryPickChange.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/change/CherryPickChange.java
@@ -17,6 +17,7 @@
 import com.google.common.base.Strings;
 import com.google.gerrit.common.FooterConstants;
 import com.google.gerrit.common.TimeUtil;
+import com.google.gerrit.extensions.restapi.MergeConflictException;
 import com.google.gerrit.extensions.restapi.RestApiException;
 import com.google.gerrit.reviewdb.client.Branch;
 import com.google.gerrit.reviewdb.client.Change;
@@ -33,7 +34,6 @@
 import com.google.gerrit.server.git.CodeReviewCommit;
 import com.google.gerrit.server.git.CodeReviewCommit.CodeReviewRevWalk;
 import com.google.gerrit.server.git.GitRepositoryManager;
-import com.google.gerrit.server.git.MergeConflictException;
 import com.google.gerrit.server.git.MergeException;
 import com.google.gerrit.server.git.MergeIdenticalTreeException;
 import com.google.gerrit.server.git.MergeUtil;
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/change/Rebase.java b/gerrit-server/src/main/java/com/google/gerrit/server/change/Rebase.java
index ebcbb19..60f285f 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/change/Rebase.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/change/Rebase.java
@@ -33,7 +33,6 @@
 import com.google.gerrit.reviewdb.server.ReviewDb;
 import com.google.gerrit.server.git.BatchUpdate;
 import com.google.gerrit.server.git.GitRepositoryManager;
-import com.google.gerrit.server.git.MergeConflictException;
 import com.google.gerrit.server.git.UpdateException;
 import com.google.gerrit.server.git.validators.CommitValidators;
 import com.google.gerrit.server.project.ChangeControl;
@@ -108,10 +107,6 @@
           .setRunHooks(true)
           .setValidatePolicy(CommitValidators.Policy.GERRIT));
       bu.execute();
-    } catch (UpdateException e) {
-      if (e.getCause() instanceof MergeConflictException) {
-        throw new ResourceConflictException(e.getCause().getMessage());
-      }
     }
     return json.create(OPTIONS).format(change.getId());
   }
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/change/RebaseChangeOp.java b/gerrit-server/src/main/java/com/google/gerrit/server/change/RebaseChangeOp.java
index 6769a5c..4130273 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/change/RebaseChangeOp.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/change/RebaseChangeOp.java
@@ -17,6 +17,7 @@
 import static com.google.common.base.Preconditions.checkState;
 
 import com.google.gerrit.common.Nullable;
+import com.google.gerrit.extensions.restapi.MergeConflictException;
 import com.google.gerrit.extensions.restapi.ResourceConflictException;
 import com.google.gerrit.extensions.restapi.RestApiException;
 import com.google.gerrit.reviewdb.client.PatchSet;
@@ -25,7 +26,6 @@
 import com.google.gerrit.server.git.BatchUpdate;
 import com.google.gerrit.server.git.BatchUpdate.ChangeContext;
 import com.google.gerrit.server.git.BatchUpdate.RepoContext;
-import com.google.gerrit.server.git.MergeConflictException;
 import com.google.gerrit.server.git.MergeUtil;
 import com.google.gerrit.server.git.validators.CommitValidators;
 import com.google.gerrit.server.project.ChangeControl;
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/git/MergeUtil.java b/gerrit-server/src/main/java/com/google/gerrit/server/git/MergeUtil.java
index 5fdefbe..2bd96bc 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/git/MergeUtil.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/git/MergeUtil.java
@@ -23,6 +23,7 @@
 import com.google.gerrit.common.FooterConstants;
 import com.google.gerrit.common.Nullable;
 import com.google.gerrit.common.data.LabelType;
+import com.google.gerrit.extensions.restapi.MergeConflictException;
 import com.google.gerrit.reviewdb.client.Account;
 import com.google.gerrit.reviewdb.client.Branch;
 import com.google.gerrit.reviewdb.client.Change;
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/git/strategy/CherryPick.java b/gerrit-server/src/main/java/com/google/gerrit/server/git/strategy/CherryPick.java
index 566a730..fc76020 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/git/strategy/CherryPick.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/git/strategy/CherryPick.java
@@ -16,6 +16,7 @@
 
 import com.google.common.collect.Lists;
 import com.google.gerrit.common.TimeUtil;
+import com.google.gerrit.extensions.restapi.MergeConflictException;
 import com.google.gerrit.extensions.restapi.RestApiException;
 import com.google.gerrit.reviewdb.client.Change;
 import com.google.gerrit.reviewdb.client.PatchSet;
@@ -29,7 +30,6 @@
 import com.google.gerrit.server.git.CodeReviewCommit;
 import com.google.gerrit.server.git.CommitMergeStatus;
 import com.google.gerrit.server.git.GroupCollector;
-import com.google.gerrit.server.git.MergeConflictException;
 import com.google.gerrit.server.git.MergeException;
 import com.google.gerrit.server.git.MergeIdenticalTreeException;
 import com.google.gerrit.server.git.MergeTip;
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/git/strategy/RebaseIfNecessary.java b/gerrit-server/src/main/java/com/google/gerrit/server/git/strategy/RebaseIfNecessary.java
index 5ef33a6..5070b84 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/git/strategy/RebaseIfNecessary.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/git/strategy/RebaseIfNecessary.java
@@ -16,6 +16,7 @@
 
 import com.google.common.collect.Lists;
 import com.google.gerrit.common.TimeUtil;
+import com.google.gerrit.extensions.restapi.MergeConflictException;
 import com.google.gerrit.extensions.restapi.RestApiException;
 import com.google.gerrit.reviewdb.client.Change;
 import com.google.gerrit.reviewdb.client.PatchSet;
@@ -24,7 +25,6 @@
 import com.google.gerrit.server.git.BatchUpdate;
 import com.google.gerrit.server.git.CodeReviewCommit;
 import com.google.gerrit.server.git.CommitMergeStatus;
-import com.google.gerrit.server.git.MergeConflictException;
 import com.google.gerrit.server.git.MergeException;
 import com.google.gerrit.server.git.MergeTip;
 import com.google.gerrit.server.git.RebaseSorter;
@@ -111,15 +111,12 @@
             newCommits.put(newPatchSet.getId().getParentKey(),
                 mergeTip.getCurrentTip());
             setRefLogIdent();
-          } catch (UpdateException e) {
-            if (e.getCause() instanceof MergeConflictException) {
-              n.setStatusCode(CommitMergeStatus.REBASE_MERGE_CONFLICT);
-            }
-            throw new MergeException("Cannot rebase " + n.name(), e);
+          } catch (MergeConflictException e) {
+            n.setStatusCode(CommitMergeStatus.REBASE_MERGE_CONFLICT);
+            throw new MergeException(
+                "Cannot rebase " + n.name() + ": " + e.getMessage(), e);
           } catch (NoSuchChangeException | OrmException | IOException
-              | RestApiException e) {
-            // TODO(dborowitz): Allow Submit to unwrap ResourceConflictException
-            // so it can turn into a 409.
+              | RestApiException | UpdateException e) {
             throw new MergeException("Cannot rebase " + n.name(), e);
           }
         }