diff --git a/cassandra-analytics-cdc-sidecar/src/main/java/org/apache/cassandra/cdc/sidecar/SidecarCdcCommitLogSegment.java b/cassandra-analytics-cdc-sidecar/src/main/java/org/apache/cassandra/cdc/sidecar/SidecarCdcCommitLogSegment.java
index 30662e29f..ebfb78230 100644
--- a/cassandra-analytics-cdc-sidecar/src/main/java/org/apache/cassandra/cdc/sidecar/SidecarCdcCommitLogSegment.java
+++ b/cassandra-analytics-cdc-sidecar/src/main/java/org/apache/cassandra/cdc/sidecar/SidecarCdcCommitLogSegment.java
@@ -81,7 +81,7 @@ public FileType fileType()
 
             public long size()
             {
-                return segment.idx;
+                return segment.size;
             }
 
             @Nullable
@@ -106,7 +106,7 @@ public String path()
 
     public long maxOffset()
     {
-        return segment.idx;
+        return segment.size;
     }
 
     @Override
diff --git a/cassandra-analytics-cdc-sidecar/src/test/java/org/apache/cassandra/cdc/sidecar/SidecarCdcCommitLogSegmentTest.java b/cassandra-analytics-cdc-sidecar/src/test/java/org/apache/cassandra/cdc/sidecar/SidecarCdcCommitLogSegmentTest.java
new file mode 100644
index 000000000..c14dc9ff3
--- /dev/null
+++ b/cassandra-analytics-cdc-sidecar/src/test/java/org/apache/cassandra/cdc/sidecar/SidecarCdcCommitLogSegmentTest.java
@@ -0,0 +1,82 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.cassandra.cdc.sidecar;
+
+import org.junit.jupiter.api.Test;
+
+import o.a.c.sidecar.client.shaded.common.response.data.CdcSegmentInfo;
+import org.apache.cassandra.cdc.stats.ICdcStats;
+import org.apache.cassandra.spark.data.partitioner.CassandraInstance;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.Mockito.mock;
+
+/**
+ * Unit tests for SidecarCdcCommitLogSegment class
+ */
+public class SidecarCdcCommitLogSegmentTest
+{
+    /**
+     * Test that maxOffset() and source().size() return segment.size instead of segment.idx.
+     * This prevents AssertionError in CdcRandomAccessReader when buffer calculations expect
+     * the wrong file size, causing the assertion "buffer.remaining() == 0" to fail.
+     */
+    @Test
+    public void testMaxOffsetAndSizeReturnFileSizeNotSegmentId()
+    {
+        // Use real values from the bug: file size 83544 bytes, segment ID 1770159718743
+        long fileSize = 83544L;
+        long segmentId = 1770159718743L;
+
+        CdcSegmentInfo segmentInfo = new CdcSegmentInfo(
+            "CommitLog-7-1770159718743.log",
+            fileSize,
+            segmentId,
+            false,
+            System.currentTimeMillis()
+        );
+
+        // Create test stub with stats field
+        TestSidecarCdcClient testSidecarClient = new TestSidecarCdcClient();
+        SidecarCdcClient.ClientConfig clientConfig = SidecarCdcClient.ClientConfig.create();
+
+        SidecarCdcCommitLogSegment segment = new SidecarCdcCommitLogSegment(
+            testSidecarClient,
+            mock(CassandraInstance.class),
+            segmentInfo,
+            clientConfig
+        );
+
+        // maxOffset() and source().size() must return file size, not segment ID
+        assertThat(segment.maxOffset()).isEqualTo(fileSize);
+        assertThat(segment.source().size()).isEqualTo(fileSize);
+    }
+
+    /**
+     * Test stub for SidecarCdcClient
+     */
+    private static class TestSidecarCdcClient extends SidecarCdcClient
+    {
+        TestSidecarCdcClient()
+        {
+            super(null, null, ICdcStats.STUB);
+        }
+    }
+}