diff --git a/docs/validation/Database-Merged-Chat-2026-09-06.md b/docs/validation/Database-Merged-Chat-2026-09-06.md
new file mode 100644
index 0000000..6d97d6a
--- /dev/null
+++ b/docs/validation/Database-Merged-Chat-2026-09-06.md
@@ -0,0 +1,41 @@
+# 数据库合并聊天记录展开 — 2026-09-06
+
+## 调用方式
+
+```text
+WxAgent.Host db merged --account <account-fingerprint> --chat filehelper --local-id 149
+WxAgent.Host db merged --account <account-fingerprint> --chat filehelper --local-id 149 --include-content
+```
+
+可选 `--database <relative-path>` 消除不同消息分库 local ID 重复时的歧义；支持 `--key-file` 与 `--timeout`。只使用既有账户密钥缓存，不调用内存扫描器，也不打开聊天窗口。
+
+默认返回父记录身份、条目路径、类型、脱敏发送人标识、原消息 ID、时间戳和正文长度；正文、显示名称、合并标题和描述为 null。`--include-content` 显式启用正文/名称。源 `hashusername` 不是可确认的真实 wxid，因此不把它伪装为 wxid。
+
+## 实现与安全
+
+- 将数据库 `local_type` 模型和转换从 Int32 修正为 **Int64**；实测合并类型为 `81604378673`。原先直接读取此记录会发生 Int32 溢出。
+- 精确按聊天表及 `local_id` 参数查询，逐个消息分库检查；多处命中必须显式选择分库，不返回第一个碰巧匹配的消息。
+- 解析 `appmsg/type=19 → recorditem → recordinfo/datalist/dataitem`，保留顺序、重复正文、原始字符串 ID、秒级 Unix 时间、显示时间和发送人名称。嵌套记录拥有独立路径，不把重复 dataid 当作去重依据。
+- XML 禁止 DTD/外部实体，限制文档字符数、深度、嵌套记录层数及总条目数。声明数量与实际不符、文本缺失、非法时间等明确报错，不用空结果掩盖损坏。
+- Hex/Zstd 解码有尺寸上限，避免压缩消息无限膨胀。
+- 每次 SQLCipher 打开前，缓存密钥重新通过**当前目标数据库 page-1 HMAC**校验；连接仍为 `Mode=ReadOnly` 与 `query_only=ON`。
+- 移除原来的 `CREATE TABLE` 写拒绝探针，改为读取 `sqlite3_db_readonly`，不尝试任何数据库写入。打开失败时也释放连接。
+- 仅输出允许的附件元数据，不导出 CDN 地址/AES 密钥，不下载附件或绕过协议。
+
+## 真机验收
+
+- 微信 `4.1.13.63`，Windows `10.1.1.101`。
+- 最终发布：`C:\Users\Rogee\wx-agent\releases\db-merged-20260906T120104`。
+- EXE SHA-256：`5DB6A6422F6DAD9AE76C6C4D5A522576C7B7EB0534396434A536190D4E0DEEF8`。
+- Linux **129/129 Core 测试通过**，完整 Release build 无警告/错误。
+- `Test-DatabaseMerged.ps1` 在 **SSH Session 0** 运行，5/5 通过：默认脱敏输出、显式正文输出、数据库只读状态、非法 ID、缺失 ID。数据库读操作无需微信所在交互桌面；没有在 Session 0 执行 UI 自动化。
+- 既有交互会话另行执行 doctor、inspect-ui、`smoke --read-only`，3/3 通过。
+- 文件传输助手中父记录 local ID **149** 展开 **16 条文本消息**。
+- 用独立 Python XML 解析器逐条对照数据库原始嵌入 XML：正文、发送人、原始消息 ID、时间戳 **各 16/16 完全一致**。
+- 全程发送/转发次数 **0**。完整正文仅保留于忽略的私有验证产物，不进入 Git。
+
+脱敏证据：[`evidence/Database-Merged-20260906/`](evidence/Database-Merged-20260906/)。第一次验证脚本误读 `metadata.WritesRejected` 的嵌套层次，修正脚本后同一二进制通过；并非数据库接受了写入。
+
+## 验收范围
+
+真实样本为 16 条文本；嵌套记录、附件元数据、恶意/截断 XML、重复条目和解压上限有离线测试。尚无混合图片/视频/文件、嵌套合并的真机样本；附件正文/原文件下载不属于本次展开结果。字段缺失保留 null，不补造身份或时间。
diff --git a/docs/validation/evidence/Database-Merged-20260906/baseline-summary.json b/docs/validation/evidence/Database-Merged-20260906/baseline-summary.json
new file mode 100644
index 0000000..85bacd3
--- /dev/null
+++ b/docs/validation/evidence/Database-Merged-20260906/baseline-summary.json
@@ -0,0 +1,18 @@
+{
+  "Checks": [
+    {
+      "Stage": "doctor",
+      "ExitCode": 0
+    },
+    {
+      "Stage": "inspect-ui",
+      "ExitCode": 0
+    },
+    {
+      "Stage": "smoke-readonly",
+      "ExitCode": 0
+    }
+  ],
+  "SendsAttempted": 0,
+  "Success": true
+}
diff --git a/docs/validation/evidence/Database-Merged-20260906/invalid-id.json b/docs/validation/evidence/Database-Merged-20260906/invalid-id.json
new file mode 100644
index 0000000..b03f722
--- /dev/null
+++ b/docs/validation/evidence/Database-Merged-20260906/invalid-id.json
@@ -0,0 +1,4 @@
+{
+  "error": "InvalidArgument",
+  "Message": "--local-id must be a positive 64-bit integer."
+}
diff --git a/docs/validation/evidence/Database-Merged-20260906/merged-default.json b/docs/validation/evidence/Database-Merged-20260906/merged-default.json
new file mode 100644
index 0000000..2f8c720
--- /dev/null
+++ b/docs/validation/evidence/Database-Merged-20260906/merged-default.json
@@ -0,0 +1,307 @@
+{
+  "account": "a2e8a1eaab7fd5fbd3806525c0d9cce750c28f4efb8e7cfcabc9e3e8660e0277",
+  "database": "message/message_0.db",
+  "chatId": "sha256:7F7D698D0AE00B0D",
+  "recordId": "a2e8a1eaab7fd5fbd3806525c0d9cce750c28f4efb8e7cfcabc9e3e8660e0277:message/message_0.db:149",
+  "parent": {
+    "LocalId": 149,
+    "ServerId": 7498992953238207040,
+    "Type": 81604378673,
+    "Timestamp": "2026-09-06T09:18:20+00:00"
+  },
+  "record": {
+    "title": null,
+    "description": null,
+    "count": 16,
+    "messages": [
+      {
+        "path": "0",
+        "dataId": "d7a3cf5d308ef865ededc7cda76cdf06",
+        "dataType": 1,
+        "senderId": "sha256:6221F48E5D88469F",
+        "senderName": null,
+        "sourceLocalId": null,
+        "sourceServerId": "5158611296847210182",
+        "timestamp": "2026-08-28T08:13:35+00:00",
+        "displayTime": "2026-08-28 16:13",
+        "text": null,
+        "length": 14,
+        "contentAvailable": true,
+        "title": null,
+        "format": null,
+        "sizeBytes": null,
+        "nestedRecord": null
+      },
+      {
+        "path": "1",
+        "dataId": "a55bf6920207cc484914173ca7ec98ff",
+        "dataType": 1,
+        "senderId": "sha256:2F0AF7C1B8E3D684",
+        "senderName": null,
+        "sourceLocalId": null,
+        "sourceServerId": "2505042190386030576",
+        "timestamp": "2026-08-28T08:14:27+00:00",
+        "displayTime": "2026-08-28 16:14",
+        "text": null,
+        "length": 1,
+        "contentAvailable": true,
+        "title": null,
+        "format": null,
+        "sizeBytes": null,
+        "nestedRecord": null
+      },
+      {
+        "path": "2",
+        "dataId": "30831307ce3e4909af357ca3a0d36492",
+        "dataType": 1,
+        "senderId": "sha256:2F0AF7C1B8E3D684",
+        "senderName": null,
+        "sourceLocalId": null,
+        "sourceServerId": "4314853433147875167",
+        "timestamp": "2026-08-28T08:14:31+00:00",
+        "displayTime": "2026-08-28 16:14",
+        "text": null,
+        "length": 3,
+        "contentAvailable": true,
+        "title": null,
+        "format": null,
+        "sizeBytes": null,
+        "nestedRecord": null
+      },
+      {
+        "path": "3",
+        "dataId": "835abcae449f3e156d2e4961c42138ff",
+        "dataType": 1,
+        "senderId": "sha256:6221F48E5D88469F",
+        "senderName": null,
+        "sourceLocalId": null,
+        "sourceServerId": "6850076042613883819",
+        "timestamp": "2026-08-28T08:15:35+00:00",
+        "displayTime": "2026-08-28 16:15",
+        "text": null,
+        "length": 41,
+        "contentAvailable": true,
+        "title": null,
+        "format": null,
+        "sizeBytes": null,
+        "nestedRecord": null
+      },
+      {
+        "path": "4",
+        "dataId": "fd7fcef26f3408b3e24fcf526f8c101b",
+        "dataType": 1,
+        "senderId": "sha256:2F0AF7C1B8E3D684",
+        "senderName": null,
+        "sourceLocalId": null,
+        "sourceServerId": "6621524893657137135",
+        "timestamp": "2026-08-28T08:16:12+00:00",
+        "displayTime": "2026-08-28 16:16",
+        "text": null,
+        "length": 3,
+        "contentAvailable": true,
+        "title": null,
+        "format": null,
+        "sizeBytes": null,
+        "nestedRecord": null
+      },
+      {
+        "path": "5",
+        "dataId": "6fe59e85d75f439e66c9871458e322c1",
+        "dataType": 1,
+        "senderId": "sha256:6221F48E5D88469F",
+        "senderName": null,
+        "sourceLocalId": null,
+        "sourceServerId": "6108206286854864988",
+        "timestamp": "2026-08-28T08:16:59+00:00",
+        "displayTime": "2026-08-28 16:16",
+        "text": null,
+        "length": 58,
+        "contentAvailable": true,
+        "title": null,
+        "format": null,
+        "sizeBytes": null,
+        "nestedRecord": null
+      },
+      {
+        "path": "6",
+        "dataId": "b0195dfb34d6552e9697717c85e3244d",
+        "dataType": 1,
+        "senderId": "sha256:2F0AF7C1B8E3D684",
+        "senderName": null,
+        "sourceLocalId": null,
+        "sourceServerId": "6291818813835930172",
+        "timestamp": "2026-08-28T08:17:32+00:00",
+        "displayTime": "2026-08-28 16:17",
+        "text": null,
+        "length": 9,
+        "contentAvailable": true,
+        "title": null,
+        "format": null,
+        "sizeBytes": null,
+        "nestedRecord": null
+      },
+      {
+        "path": "7",
+        "dataId": "7865e3c4ef4610909cbda0034897852b",
+        "dataType": 1,
+        "senderId": "sha256:6221F48E5D88469F",
+        "senderName": null,
+        "sourceLocalId": null,
+        "sourceServerId": "8805680531905882697",
+        "timestamp": "2026-08-28T08:17:57+00:00",
+        "displayTime": "2026-08-28 16:17",
+        "text": null,
+        "length": 19,
+        "contentAvailable": true,
+        "title": null,
+        "format": null,
+        "sizeBytes": null,
+        "nestedRecord": null
+      },
+      {
+        "path": "8",
+        "dataId": "ec62b6595e5f3391381b82b847d47de3",
+        "dataType": 1,
+        "senderId": "sha256:2F0AF7C1B8E3D684",
+        "senderName": null,
+        "sourceLocalId": null,
+        "sourceServerId": "7935299570123740393",
+        "timestamp": "2026-08-28T08:18:07+00:00",
+        "displayTime": "2026-08-28 16:18",
+        "text": null,
+        "length": 3,
+        "contentAvailable": true,
+        "title": null,
+        "format": null,
+        "sizeBytes": null,
+        "nestedRecord": null
+      },
+      {
+        "path": "9",
+        "dataId": "8871d9bfe9c261850b155f737b9e0fc4",
+        "dataType": 1,
+        "senderId": "sha256:6221F48E5D88469F",
+        "senderName": null,
+        "sourceLocalId": null,
+        "sourceServerId": "5334061395816435285",
+        "timestamp": "2026-08-28T08:18:11+00:00",
+        "displayTime": "2026-08-28 16:18",
+        "text": null,
+        "length": 8,
+        "contentAvailable": true,
+        "title": null,
+        "format": null,
+        "sizeBytes": null,
+        "nestedRecord": null
+      },
+      {
+        "path": "10",
+        "dataId": "06987bf3be56f93fe26f516ee9fce712",
+        "dataType": 1,
+        "senderId": "sha256:6221F48E5D88469F",
+        "senderName": null,
+        "sourceLocalId": null,
+        "sourceServerId": "3775467287450770804",
+        "timestamp": "2026-08-28T08:18:21+00:00",
+        "displayTime": "2026-08-28 16:18",
+        "text": null,
+        "length": 5,
+        "contentAvailable": true,
+        "title": null,
+        "format": null,
+        "sizeBytes": null,
+        "nestedRecord": null
+      },
+      {
+        "path": "11",
+        "dataId": "811bbaf41661bc93de162cb79051d30c",
+        "dataType": 1,
+        "senderId": "sha256:6221F48E5D88469F",
+        "senderName": null,
+        "sourceLocalId": null,
+        "sourceServerId": "5606979100728674670",
+        "timestamp": "2026-08-28T08:18:33+00:00",
+        "displayTime": "2026-08-28 16:18",
+        "text": null,
+        "length": 4,
+        "contentAvailable": true,
+        "title": null,
+        "format": null,
+        "sizeBytes": null,
+        "nestedRecord": null
+      },
+      {
+        "path": "12",
+        "dataId": "1b54a0acfa9af3f87612f1ad1d899615",
+        "dataType": 1,
+        "senderId": "sha256:2F0AF7C1B8E3D684",
+        "senderName": null,
+        "sourceLocalId": null,
+        "sourceServerId": "8768702616063460855",
+        "timestamp": "2026-08-28T08:18:34+00:00",
+        "displayTime": "2026-08-28 16:18",
+        "text": null,
+        "length": 1,
+        "contentAvailable": true,
+        "title": null,
+        "format": null,
+        "sizeBytes": null,
+        "nestedRecord": null
+      },
+      {
+        "path": "13",
+        "dataId": "8f995c7506723cb9bbac6428de6e48da",
+        "dataType": 1,
+        "senderId": "sha256:2F0AF7C1B8E3D684",
+        "senderName": null,
+        "sourceLocalId": null,
+        "sourceServerId": "540359944811197130",
+        "timestamp": "2026-08-28T08:18:39+00:00",
+        "displayTime": "2026-08-28 16:18",
+        "text": null,
+        "length": 2,
+        "contentAvailable": true,
+        "title": null,
+        "format": null,
+        "sizeBytes": null,
+        "nestedRecord": null
+      },
+      {
+        "path": "14",
+        "dataId": "6444ca65742edd89728ec3872ad10bef",
+        "dataType": 1,
+        "senderId": "sha256:6221F48E5D88469F",
+        "senderName": null,
+        "sourceLocalId": null,
+        "sourceServerId": "3039346887632908543",
+        "timestamp": "2026-08-28T08:19:05+00:00",
+        "displayTime": "2026-08-28 16:19",
+        "text": null,
+        "length": 9,
+        "contentAvailable": true,
+        "title": null,
+        "format": null,
+        "sizeBytes": null,
+        "nestedRecord": null
+      },
+      {
+        "path": "15",
+        "dataId": "2489f0db398df842a99962035db3f82e",
+        "dataType": 1,
+        "senderId": "sha256:6221F48E5D88469F",
+        "senderName": null,
+        "sourceLocalId": null,
+        "sourceServerId": "679561649162720019",
+        "timestamp": "2026-08-28T08:19:16+00:00",
+        "displayTime": "2026-08-28 16:19",
+        "text": null,
+        "length": 8,
+        "contentAvailable": true,
+        "title": null,
+        "format": null,
+        "sizeBytes": null,
+        "nestedRecord": null
+      }
+    ]
+  }
+}
diff --git a/docs/validation/evidence/Database-Merged-20260906/missing-id.json b/docs/validation/evidence/Database-Merged-20260906/missing-id.json
new file mode 100644
index 0000000..bce40db
--- /dev/null
+++ b/docs/validation/evidence/Database-Merged-20260906/missing-id.json
@@ -0,0 +1,4 @@
+{
+  "error": "ControlNotFound",
+  "Message": "The selected database message was not found."
+}
diff --git a/docs/validation/evidence/Database-Merged-20260906/readonly.json b/docs/validation/evidence/Database-Merged-20260906/readonly.json
new file mode 100644
index 0000000..fe6f2e6
--- /dev/null
+++ b/docs/validation/evidence/Database-Merged-20260906/readonly.json
@@ -0,0 +1,3 @@
+{
+  "WritesRejected": true
+}
diff --git a/docs/validation/evidence/Database-Merged-20260906/summary.json b/docs/validation/evidence/Database-Merged-20260906/summary.json
new file mode 100644
index 0000000..97c9bfe
--- /dev/null
+++ b/docs/validation/evidence/Database-Merged-20260906/summary.json
@@ -0,0 +1,34 @@
+{
+  "BinarySha256": "5DB6A6422F6DAD9AE76C6C4D5A522576C7B7EB0534396434A536190D4E0DEEF8",
+  "SendsAttempted": 0,
+  "Checks": [
+    {
+      "ExpectedExit": 0,
+      "ExitCode": 0,
+      "Stage": "merged-default"
+    },
+    {
+      "ExpectedExit": 0,
+      "ExitCode": 0,
+      "Stage": "merged-content.private"
+    },
+    {
+      "ExpectedExit": 0,
+      "ExitCode": 0,
+      "Stage": "database-readonly"
+    },
+    {
+      "ExpectedExit": 1,
+      "ExitCode": 1,
+      "Stage": "invalid-id"
+    },
+    {
+      "ExpectedExit": 1,
+      "ExitCode": 1,
+      "Stage": "missing-id"
+    }
+  ],
+  "Success": true,
+  "ProcessSessionId": 0,
+  "RecordCount": 16
+}
diff --git a/docs/validation/evidence/Database-Merged-20260906/xml-cross-check.json b/docs/validation/evidence/Database-Merged-20260906/xml-cross-check.json
new file mode 100644
index 0000000..94c59b9
--- /dev/null
+++ b/docs/validation/evidence/Database-Merged-20260906/xml-cross-check.json
@@ -0,0 +1,9 @@
+{
+  "MessageCount": 16,
+  "ExactTextMatches": 16,
+  "ExactSenderMatches": 16,
+  "ExactSourceIdMatches": 16,
+  "ExactTimestampMatches": 16,
+  "IndependentParser": "Python xml.etree.ElementTree",
+  "ContentIncluded": false
+}
diff --git a/scripts/windows/Test-DatabaseMerged.ps1 b/scripts/windows/Test-DatabaseMerged.ps1
new file mode 100644
index 0000000..050696d
--- /dev/null
+++ b/scripts/windows/Test-DatabaseMerged.ps1
@@ -0,0 +1,43 @@
+param(
+    [Parameter(Mandatory=$true)][string]$Executable,
+    [Parameter(Mandatory=$true)][string]$OutputDirectory,
+    [Parameter(Mandatory=$true)][string]$Account,
+    [string]$ChatId='filehelper',
+    [Parameter(Mandatory=$true)][long]$LocalId,
+    [Parameter(Mandatory=$true)][int]$ExpectedCount
+)
+$ErrorActionPreference='Stop'
+$exe=(Resolve-Path $Executable).Path
+if(Test-Path $OutputDirectory){throw 'Use a fresh evidence directory.'}
+$out=(New-Item -ItemType Directory $OutputDirectory).FullName
+$checks=New-Object System.Collections.Generic.List[object]
+function Check([string]$Name,[string[]]$Arguments,[int]$ExpectedExit=0){
+    $raw=(& $exe @Arguments 2> "$out\$Name.stderr.txt" | Out-String)
+    $code=$LASTEXITCODE
+    $raw | Set-Content "$out\$Name.json" -Encoding UTF8
+    $checks.Add(@{Stage=$Name;ExitCode=$code;ExpectedExit=$ExpectedExit})
+    if($code -ne $ExpectedExit){throw "$Name returned unexpected exit code."}
+    return ($raw | ConvertFrom-Json)
+}
+try {
+    $default=Check 'merged-default' @('db','merged','--account',$Account,'--chat',$ChatId,'--local-id',"$LocalId",'--timeout','60')
+    if($default.record.count -ne $ExpectedCount){throw 'Unexpected record count.'}
+    if($null -ne $default.record.title -or $null -ne $default.record.description -or @($default.record.messages | Where-Object {$null -ne $_.text -or $null -ne $_.senderName}).Count){throw 'Default output leaked record content.'}
+    $full=Check 'merged-content.private' @('db','merged','--account',$Account,'--chat',$ChatId,'--local-id',"$LocalId",'--include-content','--timeout','60')
+    if($full.record.count -ne $ExpectedCount -or $full.parent.LocalId -ne $LocalId){throw 'Selected record mismatch.'}
+    if(@($full.record.messages | Where-Object {$_.dataType -ne 1 -or $null -eq $_.text -or !$_.senderName -or !$_.timestamp -or !$_.sourceServerId}).Count){throw 'Expected complete text-message metadata in this fixture.'}
+    if(@($full.record.messages.path | Select-Object -Unique).Count -ne $ExpectedCount){throw 'Message paths are not unique.'}
+    $metadata=Check 'database-readonly' @('db','query','--account',$Account,'--database',$full.database,'--timeout','60')
+    if(!$metadata.metadata.WritesRejected){throw 'Database is not reported read-only.'}
+    $invalid=Check 'invalid-id' @('db','merged','--account',$Account,'--chat',$ChatId,'--local-id','0','--timeout','30') 1
+    if($invalid.error -ne 'InvalidArgument'){throw 'Wrong invalid-ID error.'}
+    $missing=Check 'missing-id' @('db','merged','--account',$Account,'--chat',$ChatId,'--local-id','9223372036854775807','--timeout','60') 1
+    if($missing.error -ne 'ControlNotFound'){throw 'Wrong missing-ID error.'}
+} catch {
+    @{Message=$_.Exception.Message;Line=$_.InvocationInfo.ScriptLineNumber} | ConvertTo-Json | Set-Content "$out\error.json" -Encoding UTF8
+} finally {
+    $success=($checks.Count -eq 5 -and @($checks | Where-Object {$_.ExitCode -ne $_.ExpectedExit}).Count -eq 0 -and !(Test-Path "$out\error.json"))
+    @{Success=$success;Checks=@($checks.ToArray());ProcessSessionId=[Diagnostics.Process]::GetCurrentProcess().SessionId;RecordCount=$ExpectedCount;SendsAttempted=0;BinarySha256=(Get-FileHash $exe -Algorithm SHA256).Hash} |
+        ConvertTo-Json -Depth 6 | Set-Content "$out\summary.json" -Encoding UTF8
+}
+if(!$success){exit 1}
diff --git a/src/WxAgent.Core/WechatDbMessage.cs b/src/WxAgent.Core/WechatDbMessage.cs
index 33bdf61..56f66cf 100644
--- a/src/WxAgent.Core/WechatDbMessage.cs
+++ b/src/WxAgent.Core/WechatDbMessage.cs
@@ -12,7 +12,7 @@ public sealed record DbMessage(
     string? SenderWxId,
     string? SenderName,
     string? SenderAvatarUrl,
-    int Type,
+    long Type,
     string Content,
     DateTimeOffset Timestamp,
     bool? IsSelf);
@@ -26,6 +26,8 @@ public static class WechatDbMessage
     public static string DecodeContent(string hexContent, bool compressed)
     {
         if (string.IsNullOrEmpty(hexContent)) return string.Empty;
+        if (hexContent.Length > WechatMergedChatParser.MaxXmlCharacters * 2)
+            throw new InvalidDataException("Database message content exceeds the decoding limit.");
         byte[] bytes;
         try
         {
@@ -43,9 +45,18 @@ public static class WechatDbMessage
                 using var source = new MemoryStream(bytes);
                 using var decompressor = new DecompressionStream(source);
                 using var reader = new StreamReader(decompressor, Encoding.UTF8);
-                return reader.ReadToEnd();
+                var decoded = new StringBuilder();
+                var buffer = new char[4096];
+                int read;
+                while ((read = reader.Read(buffer, 0, buffer.Length)) != 0)
+                {
+                    if (decoded.Length + read > WechatMergedChatParser.MaxXmlCharacters)
+                        throw new InvalidDataException("Decompressed message exceeds the decoding limit.");
+                    decoded.Append(buffer, 0, read);
+                }
+                return decoded.ToString();
             }
-            catch (Exception exception) when (exception is not OutOfMemoryException)
+            catch (Exception exception) when (exception is not (OutOfMemoryException or InvalidDataException))
             {
                 return string.Empty;
             }
diff --git a/src/WxAgent.Core/WechatMergedChat.cs b/src/WxAgent.Core/WechatMergedChat.cs
new file mode 100644
index 0000000..0a5e8f8
--- /dev/null
+++ b/src/WxAgent.Core/WechatMergedChat.cs
@@ -0,0 +1,117 @@
+using System.Globalization;
+using System.Xml;
+using System.Xml.Linq;
+
+namespace WxAgent.Core;
+
+public sealed record WechatMergedMessage(
+    string Path, string? DataId, int DataType, string? SenderName, string? SenderHash,
+    string? SourceLocalId, string? SourceServerId, DateTimeOffset? Timestamp, string? DisplayTime,
+    string? Text, string? Title, string? Format, long? SizeBytes, WechatMergedChat? NestedRecord);
+
+public sealed record WechatMergedChat(string? Title, string? Description, IReadOnlyList<WechatMergedMessage> Messages);
+
+public sealed record DbMergedChat(string DatabaseRelativePath, DbMessage Parent, WechatMergedChat Record);
+
+public static class WechatMergedChatParser
+{
+    public const int MaxXmlCharacters = 4 * 1024 * 1024;
+    public const int MaxMessages = 5000;
+    public const int MaxNesting = 8;
+
+    public static WechatMergedChat Parse(string xml)
+    {
+        var budget = MaxXmlCharacters * 2;
+        var remaining = MaxMessages;
+        try
+        {
+            var root = Load(xml, ref budget);
+            if (root.Name == "msg")
+            {
+                var app = root.Element("appmsg") ?? throw Invalid("Missing appmsg.");
+                if (app.Element("type")?.Value != "19") throw Invalid("Not a merged-chat app message.");
+                root = ReadRecord(app.Element("recorditem") ?? throw Invalid("Missing recorditem."), ref budget);
+            }
+            return ParseRecord(root, "", 0, ref remaining, ref budget);
+        }
+        catch (XmlException ex) { throw new InvalidDataException("Invalid merged-chat XML.", ex); }
+        catch (ArgumentOutOfRangeException ex) { throw new InvalidDataException("Invalid merged-chat timestamp.", ex); }
+    }
+
+    private static WechatMergedChat ParseRecord(XElement record, string parentPath, int depth, ref int remaining, ref int budget)
+    {
+        if (depth >= MaxNesting) throw Invalid("Nested record limit exceeded.");
+        if (record.Name != "recordinfo") throw Invalid("Expected recordinfo.");
+        var list = record.Element("datalist") ?? throw Invalid("Missing datalist.");
+        var items = list.Elements().ToArray();
+        if (items.Any(item => item.Name != "dataitem")) throw Invalid("Unknown datalist element.");
+        if (items.Length > remaining) throw Invalid("Merged-message count limit exceeded.");
+        remaining -= items.Length;
+        var count = (string?)list.Attribute("count");
+        if (count is not null && (!int.TryParse(count, NumberStyles.None, CultureInfo.InvariantCulture, out var expected) || expected != items.Length))
+            throw Invalid("Declared merged-message count does not match the embedded record.");
+        var messages = new List<WechatMergedMessage>(items.Length);
+        for (var index = 0; index < items.Length; index++)
+        {
+            var item = items[index];
+            if (!int.TryParse((string?)item.Attribute("datatype"), NumberStyles.None, CultureInfo.InvariantCulture, out var type))
+                throw Invalid("Missing or invalid merged-message datatype.");
+            var path = parentPath.Length == 0 ? index.ToString(CultureInfo.InvariantCulture) : $"{parentPath}/{index}";
+            var text = Value(item, "datadesc");
+            if (type == 1 && text is null) throw Invalid("Text record has no datadesc.");
+            var seconds = Integer(item, "srcMsgCreateTime");
+            var source = item.Element("dataitemsource");
+            var nestedElement = item.Element("recordinfo") ?? item.Element("recorditem");
+            var nested = nestedElement is null ? null : ParseRecord(ReadRecord(nestedElement, ref budget), path, depth + 1, ref remaining, ref budget);
+            messages.Add(new WechatMergedMessage(path, (string?)item.Attribute("dataid"), type,
+                Value(item, "sourcename"), source is null ? null : Value(source, "hashusername"),
+                Value(item, "srcMsgLocalId"), Value(item, "fromnewmsgid"),
+                seconds is null ? null : DateTimeOffset.FromUnixTimeSeconds(seconds.Value), Value(item, "sourcetime"),
+                text, Value(item, "datatitle"), Value(item, "datafmt"), Integer(item, "datasize"), nested));
+        }
+        return new WechatMergedChat(Value(record, "title"), Value(record, "desc"), messages);
+    }
+
+    private static XElement ReadRecord(XElement element, ref int budget)
+    {
+        if (element.Name == "recordinfo" && element.HasElements) return element;
+        if (element.HasElements)
+        {
+            var children = element.Elements().ToArray();
+            if (children.Length != 1 || children[0].Name != "recordinfo") throw Invalid("Ambiguous embedded record.");
+            return children[0];
+        }
+        return Load(element.Value, ref budget);
+    }
+
+    private static XElement Load(string xml, ref int budget)
+    {
+        if (string.IsNullOrWhiteSpace(xml) || xml.Length > MaxXmlCharacters || xml.Length > budget)
+            throw Invalid("Merged-chat XML is empty or exceeds the size limit.");
+        budget -= xml.Length;
+        var settings = new XmlReaderSettings
+        {
+            DtdProcessing = DtdProcessing.Prohibit, XmlResolver = null,
+            MaxCharactersInDocument = MaxXmlCharacters, MaxCharactersFromEntities = 0
+        };
+        // Preflight depth before building a DOM; untrusted forwarded XML must stay bounded.
+        using (var check = XmlReader.Create(new StringReader(xml), settings))
+            while (check.Read())
+                if (check.Depth > 64) throw Invalid("XML depth limit exceeded.");
+        using var reader = XmlReader.Create(new StringReader(xml), settings);
+        return XElement.Load(reader, LoadOptions.PreserveWhitespace);
+    }
+
+    private static string? Value(XElement element, string name) => element.Element(name)?.Value;
+
+    private static long? Integer(XElement element, string name)
+    {
+        var text = Value(element, name);
+        if (string.IsNullOrEmpty(text)) return null;
+        if (!long.TryParse(text, NumberStyles.None, CultureInfo.InvariantCulture, out var number))
+            throw Invalid($"Invalid {name}.");
+        return number;
+    }
+
+    private static InvalidDataException Invalid(string message) => new(message);
+}
diff --git a/src/WxAgent.Host/Program.cs b/src/WxAgent.Host/Program.cs
index acedaaa..3ba4465 100644
--- a/src/WxAgent.Host/Program.cs
+++ b/src/WxAgent.Host/Program.cs
@@ -545,6 +545,30 @@ try
             });
             return 0;
         }
+        case "db" when args[1] == "merged":
+        {
+            var accountId = GetRequiredOption(args, "--account");
+            var chatId = GetRequiredOption(args, "--chat");
+            if (!long.TryParse(GetRequiredOption(args, "--local-id"), System.Globalization.NumberStyles.None,
+                    System.Globalization.CultureInfo.InvariantCulture, out var localId) || localId <= 0)
+                throw new WxAgentException(WxAgentErrorCode.InvalidArgument, "--local-id must be a positive 64-bit integer.");
+            var accounts = await DatabaseKeyStore.LoadAsync(GetOption(args, "--key-file"), cancellationToken);
+            var account = accounts.SingleOrDefault(item => string.Equals(item.AccountRootFingerprint, accountId, StringComparison.OrdinalIgnoreCase))
+                ?? throw new WxAgentException(WxAgentErrorCode.DatabaseKeyNotFound, "Account fingerprint was not found in the key store.");
+            var result = await WechatMessageDbReader.ReadMergedAsync(account.AccountRootPath, account.Databases, chatId, localId,
+                cancellationToken, GetOption(args, "--database"));
+            var includeContent = HasOption(args, "--include-content");
+            WriteJson(new
+            {
+                account = account.AccountRootFingerprint,
+                database = result.DatabaseRelativePath,
+                chatId = includeContent ? chatId : MaskIdentifier(chatId),
+                recordId = $"{account.AccountRootFingerprint}:{result.DatabaseRelativePath}:{localId}",
+                parent = new { result.Parent.LocalId, result.Parent.ServerId, result.Parent.Type, result.Parent.Timestamp },
+                record = ToMergedOutput(result.Record, includeContent)
+            });
+            return 0;
+        }
         case "db" when args[1] == "messages":
         {
             var accountId = GetRequiredOption(args, "--account");
@@ -793,6 +817,9 @@ static int ValidateCommandLine(string[] values)
         case "query":
             ValidateOptions(values, 2, ["--account", "--database", "--key-file", "--timeout"], []);
             return 30;
+        case "merged":
+            ValidateOptions(values, 2, ["--account", "--chat", "--local-id", "--database", "--key-file", "--timeout"], ["--include-content"]);
+            return 60;
         case "messages":
             ValidateOptions(values, 2, ["--account", "--chat", "--key-file", "--limit", "--timeout"], ["--include-content"]);
             return 60;
@@ -846,6 +873,32 @@ static string? GetOption(string[] values, string name)
 static string GetRequiredOption(string[] values, string name) =>
     GetOption(values, name) ?? throw new WxAgentException(WxAgentErrorCode.InvalidArgument, $"Missing required option {name}.");
 
+static object ToMergedOutput(WechatMergedChat record, bool includeContent) => new
+{
+    title = includeContent ? record.Title : null,
+    description = includeContent ? record.Description : null,
+    count = record.Messages.Count,
+    messages = record.Messages.Select(message => new
+    {
+        path = message.Path,
+        dataId = message.DataId,
+        dataType = message.DataType,
+        senderId = MaskIdentifier(message.SenderHash ?? message.SenderName),
+        senderName = includeContent ? message.SenderName : null,
+        sourceLocalId = message.SourceLocalId,
+        sourceServerId = message.SourceServerId,
+        timestamp = message.Timestamp,
+        displayTime = message.DisplayTime,
+        text = includeContent ? message.Text : null,
+        length = message.Text?.Length,
+        contentAvailable = message.Text is not null,
+        title = includeContent ? message.Title : null,
+        format = message.Format,
+        sizeBytes = message.SizeBytes,
+        nestedRecord = message.NestedRecord is null ? null : ToMergedOutput(message.NestedRecord, includeContent)
+    })
+};
+
 static string? MaskIdentifier(string? identifier) =>
     string.IsNullOrEmpty(identifier)
         ? identifier
@@ -950,6 +1003,7 @@ WxAgent.Host commands:
   db status [--key-file <path>] [--timeout 30]
   db query --account <fingerprint> --database <relative-path> [--key-file <path>] [--timeout 30]
   db messages --account <fingerprint> --chat <wxid> [--limit 50] [--include-content] [--key-file <path>] [--timeout 60]
+  db merged --account <fingerprint> --chat <wxid> --local-id <id> [--database <relative-path>] [--include-content] [--key-file <path>] [--timeout 60]
   db contacts --account <fingerprint> [--contains <text>] [--include-content] [--key-file <path>] [--timeout 60]
 
 Chat commands default to File Transfer Assistant; send/monitor and listener-smoke accept --session.
diff --git a/src/WxAgent.Windows/SqlCipherDatabaseReader.cs b/src/WxAgent.Windows/SqlCipherDatabaseReader.cs
index d166dd2..3077596 100644
--- a/src/WxAgent.Windows/SqlCipherDatabaseReader.cs
+++ b/src/WxAgent.Windows/SqlCipherDatabaseReader.cs
@@ -37,24 +37,12 @@ public static class SqlCipherDatabaseReader
                 }
             }
 
-            var writesRejected = false;
-            try
-            {
-                var probeName = "__wxagent_read_only_probe_" + Guid.NewGuid().ToString("N");
-                await using var command = connection.CreateCommand();
-                // nosemgrep:csharp-sqli - The name is a random GUID produced above; it cannot be attacker-controlled.
-                command.CommandText = $"CREATE TABLE {probeName}(value INTEGER);";
-                command.CommandTimeout = 10;
-                await command.ExecuteNonQueryAsync(cancellationToken);
-            }
-            catch (SqliteException exception) when (exception.SqliteErrorCode == 8)
-            {
-                writesRejected = true;
-            }
+            // Inspect SQLite's connection state; never attempt writes against the user's database.
+            var writesRejected = SQLitePCL.raw.sqlite3_db_readonly(connection.Handle, "main") == 1;
 
             if (!writesRejected)
             {
-                throw new WxAgentException(WxAgentErrorCode.DatabaseOpenFailed, "Read-only SQLCipher connection unexpectedly accepted a write.");
+                throw new WxAgentException(WxAgentErrorCode.DatabaseOpenFailed, "SQLCipher connection did not report read-only mode.");
             }
 
             return new DatabaseMetadata(cipherVersion, sqliteVersion, count, names, writesRejected);
@@ -118,6 +106,12 @@ public static class SqlCipherDatabaseReader
 
     private static async Task<SqliteConnection> OpenReadOnlyAsync(string databasePath, string hexKey, CancellationToken cancellationToken)
     {
+        var page = new byte[SqlCipherPageVerifier.PageSize];
+        await using (var file = new FileStream(databasePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite | FileShare.Delete,
+                         SqlCipherPageVerifier.PageSize, FileOptions.Asynchronous))
+            await file.ReadExactlyAsync(page, cancellationToken).ConfigureAwait(false);
+        if (!SqlCipherPageVerifier.VerifyHexKey(page, hexKey))
+            throw new WxAgentException(WxAgentErrorCode.DatabaseOpenFailed, "The cached key failed current database page-1 HMAC verification; refresh the selected account key.");
         var connectionString = new SqliteConnectionStringBuilder
         {
             DataSource = Path.GetFullPath(databasePath),
@@ -128,9 +122,9 @@ public static class SqlCipherDatabaseReader
         }.ToString();
 
         var connection = new SqliteConnection(connectionString);
-        await connection.OpenAsync(cancellationToken);
         try
         {
+            await connection.OpenAsync(cancellationToken).ConfigureAwait(false);
             await using (var command = connection.CreateCommand())
             {
                 // nosemgrep:csharp-sqli - hexKey is validated to exactly 64 hex chars before this call; no injection is possible.
diff --git a/src/WxAgent.Windows/WechatMessageDbReader.cs b/src/WxAgent.Windows/WechatMessageDbReader.cs
index fb50857..75316b4 100644
--- a/src/WxAgent.Windows/WechatMessageDbReader.cs
+++ b/src/WxAgent.Windows/WechatMessageDbReader.cs
@@ -7,7 +7,7 @@ namespace WxAgent.Windows;
 public static class WechatMessageDbReader
 {
     public static async Task<IReadOnlyList<DbMessage>> ReadAsync(
-        string accountRootPath, IReadOnlyList<DatabaseKeyEvidence> databases, string chatId, int limit, CancellationToken cancellationToken)
+        string accountRootPath, IReadOnlyList<DatabaseKeyEvidence> databases, string chatId, int limit, CancellationToken cancellationToken, long? localId = null)
     {
         ArgumentException.ThrowIfNullOrWhiteSpace(accountRootPath);
         ArgumentException.ThrowIfNullOrWhiteSpace(chatId);
@@ -30,9 +30,10 @@ public static class WechatMessageDbReader
         var sql = "SELECT m.local_id, m.server_id, m.local_type, m.create_time, " +
             "hex(m.message_content) AS hex_content, m.WCDB_CT_message_content AS is_compressed, n.user_name AS sender_wxid " +
             $"FROM \"{tableName}\" m LEFT JOIN Name2Id n ON m.real_sender_id = n.rowid " +
-            "ORDER BY m.create_time DESC LIMIT $limit;";
+            "WHERE ($localId IS NULL OR m.local_id = $localId) " +
+            "ORDER BY m.create_time DESC, m.local_id DESC LIMIT $limit;";
         var rows = await SqlCipherDatabaseReader.QueryRowsAsync(messageDatabase.Value.Path, messageDatabase.Value.EncKey, sql,
-            [new KeyValuePair<string, object?>("$limit", (long)limit)], cancellationToken).ConfigureAwait(false);
+            [new KeyValuePair<string, object?>("$limit", (long)limit), new KeyValuePair<string, object?>("$localId", localId)], cancellationToken).ConfigureAwait(false);
 
         var senders = rows.Select(row => row.GetValueOrDefault("sender_wxid") as string)
             .Where(sender => !string.IsNullOrEmpty(sender)).Cast<string>().Distinct(StringComparer.Ordinal).ToArray();
@@ -53,7 +54,7 @@ public static class WechatMessageDbReader
                 senderWxId,
                 contact?.Name,
                 contact?.AvatarUrl,
-                Convert.ToInt32(row.GetValueOrDefault("local_type") ?? 0, CultureInfo.InvariantCulture),
+                Convert.ToInt64(row.GetValueOrDefault("local_type") ?? 0L, CultureInfo.InvariantCulture),
                 content,
                 DateTimeOffset.FromUnixTimeSeconds(Convert.ToInt64(row.GetValueOrDefault("create_time") ?? 0L, CultureInfo.InvariantCulture)),
                 WechatDbMessage.IsSelf(senderWxId, accountRootDirectoryName)));
@@ -62,6 +63,29 @@ public static class WechatMessageDbReader
         return messages;
     }
 
+    public static async Task<DbMergedChat> ReadMergedAsync(string accountRootPath, IReadOnlyList<DatabaseKeyEvidence> databases,
+        string chatId, long localId, CancellationToken cancellationToken, string? databaseRelativePath = null)
+    {
+        if (localId <= 0) throw new WxAgentException(WxAgentErrorCode.InvalidArgument, "A positive local message ID is required.");
+        var candidates = databases.Where(database => database.RelativePath.StartsWith("message/", StringComparison.OrdinalIgnoreCase)
+            && (databaseRelativePath is null || string.Equals(database.RelativePath, databaseRelativePath.Replace('\\', '/'), StringComparison.OrdinalIgnoreCase))).ToArray();
+        if (candidates.Length == 0) throw new WxAgentException(WxAgentErrorCode.DatabaseKeyNotFound, "No matching message database has a cached key.");
+        (string Path, DbMessage Message)? found = null;
+        foreach (var database in candidates)
+        {
+            cancellationToken.ThrowIfCancellationRequested();
+            var selected = databases.Where(entry => !entry.RelativePath.StartsWith("message/", StringComparison.OrdinalIgnoreCase) || entry == database).ToArray();
+            var rows = await ReadAsync(accountRootPath, selected, chatId, 2, cancellationToken, localId).ConfigureAwait(false);
+            if (rows.Count == 0) continue;
+            if (rows.Count != 1 || found is not null)
+                throw new WxAgentException(WxAgentErrorCode.InvalidOperationState, "The local ID is ambiguous across message shards; specify --database.");
+            found = (database.RelativePath, rows[0]);
+        }
+        if (found is null) throw new WxAgentException(WxAgentErrorCode.ControlNotFound, "The selected database message was not found.");
+        try { return new DbMergedChat(found.Value.Path, found.Value.Message, WechatMergedChatParser.Parse(found.Value.Message.Content)); }
+        catch (InvalidDataException ex) { throw new WxAgentException(WxAgentErrorCode.DatabaseOpenFailed, ex.Message, ex); }
+    }
+
     private static async Task<IReadOnlyDictionary<string, WechatContactName>> ReadContactsAsync(
         string accountRootPath, IReadOnlyList<DatabaseKeyEvidence> databases, IReadOnlyList<string> senders, CancellationToken cancellationToken)
     {
@@ -100,7 +124,11 @@ public static class WechatMessageDbReader
         foreach (var database in candidates)
         {
             cancellationToken.ThrowIfCancellationRequested();
-            var path = Path.GetFullPath(Path.Combine(accountRootPath, database.RelativePath.Replace('/', Path.DirectorySeparatorChar)));
+            var root = Path.GetFullPath(accountRootPath).TrimEnd(Path.DirectorySeparatorChar, Path.AltDirectorySeparatorChar) + Path.DirectorySeparatorChar;
+            var relative = database.RelativePath.Replace('/', Path.DirectorySeparatorChar);
+            var path = Path.GetFullPath(Path.Combine(root, relative));
+            if (Path.IsPathRooted(relative) || !path.StartsWith(root, StringComparison.OrdinalIgnoreCase))
+                throw new WxAgentException(WxAgentErrorCode.InvalidArgument, "The database path must remain inside the selected account root.");
             var sql = "SELECT name FROM sqlite_master WHERE type = 'table' AND name = $name;";
             var rows = await SqlCipherDatabaseReader.QueryRowsAsync(path, database.EncKey, sql,
                 [new KeyValuePair<string, object?>("$name", tableName)], cancellationToken).ConfigureAwait(false);
diff --git a/tests/WxAgent.Core.Tests/WechatMergedChatTests.cs b/tests/WxAgent.Core.Tests/WechatMergedChatTests.cs
new file mode 100644
index 0000000..e6de4d5
--- /dev/null
+++ b/tests/WxAgent.Core.Tests/WechatMergedChatTests.cs
@@ -0,0 +1,84 @@
+using System.Text;
+using System.Text.Json;
+using System.Xml.Linq;
+using WxAgent.Core;
+using Xunit;
+using ZstdSharp;
+
+namespace WxAgent.Core.Tests;
+
+public sealed class WechatMergedChatTests
+{
+    [Fact]
+    public void ParsesEmbeddedTextMetadataWithoutDeduplicatingRepeatedMessages()
+    {
+        var result = WechatMergedChatParser.Parse(Wrap(Record(Item("same\n\ntext"), Item("same\n\ntext"))));
+        Assert.Equal(2, result.Messages.Count);
+        Assert.Equal("0", result.Messages[0].Path);
+        Assert.Equal("1", result.Messages[1].Path);
+        Assert.Equal("same\n\ntext", result.Messages[0].Text);
+        Assert.Equal("Synthetic sender", result.Messages[0].SenderName);
+        Assert.Equal("opaque-hash", result.Messages[0].SenderHash);
+        Assert.Equal("18446744073709551615", result.Messages[0].SourceServerId);
+        Assert.Equal(DateTimeOffset.FromUnixTimeSeconds(1700000000), result.Messages[0].Timestamp);
+    }
+
+    [Fact]
+    public void SupportsNestedRecordsAndExplicitAttachmentMetadata()
+    {
+        var attachment = new XElement("dataitem", new XAttribute("datatype", 8),
+            new XElement("datatitle", "example.txt"), new XElement("datafmt", "txt"), new XElement("datasize", 42),
+            new XElement("cdn_dataurl", "must-not-be-exported"), new XElement("aeskey", "must-not-be-exported"));
+        var nested = new XElement("dataitem", new XAttribute("datatype", 17), Record(Item("nested")));
+        var result = WechatMergedChatParser.Parse(Record(attachment, nested).ToString());
+        Assert.Null(result.Messages[0].Text);
+        Assert.Equal(42, result.Messages[0].SizeBytes);
+        Assert.Equal("1/0", result.Messages[1].NestedRecord!.Messages[0].Path);
+        Assert.DoesNotContain("must-not-be-exported", JsonSerializer.Serialize(result));
+    }
+
+    [Fact]
+    public void RejectsTruncationMissingPayloadAndInvalidTimestamp()
+    {
+        var record = Record(Item("hello"));
+        record.Element("datalist")!.SetAttributeValue("count", 2);
+        Assert.Throws<InvalidDataException>(() => WechatMergedChatParser.Parse(record.ToString()));
+        Assert.Throws<InvalidDataException>(() => WechatMergedChatParser.Parse(Record(new XElement("dataitem", new XAttribute("datatype", 1))).ToString()));
+        var item = Item("hello");
+        item.Element("srcMsgCreateTime")!.Value = long.MaxValue.ToString();
+        Assert.Throws<InvalidDataException>(() => WechatMergedChatParser.Parse(Record(item).ToString()));
+    }
+
+    [Fact]
+    public void RejectsDtdAndBoundViolations()
+    {
+        Assert.Throws<InvalidDataException>(() => WechatMergedChatParser.Parse("<!DOCTYPE recordinfo [<!ENTITY x SYSTEM 'file:///not-read'>]><recordinfo>&x;</recordinfo>"));
+        Assert.Throws<InvalidDataException>(() => WechatMergedChatParser.Parse(new string('x', WechatMergedChatParser.MaxXmlCharacters + 1)));
+        var nested = Record(Item("innermost"));
+        for (var i = 0; i < WechatMergedChatParser.MaxNesting; i++)
+            nested = Record(new XElement("dataitem", new XAttribute("datatype", 17), nested));
+        Assert.Throws<InvalidDataException>(() => WechatMergedChatParser.Parse(nested.ToString()));
+        Assert.Throws<InvalidDataException>(() => WechatMergedChatParser.Parse(Record(Enumerable.Range(0, WechatMergedChatParser.MaxMessages + 1).Select(_ => Item("x")).ToArray()).ToString()));
+    }
+
+    [Fact]
+    public void PreservesComposite64BitMessageTypeAndBoundsZstdDecoding()
+    {
+        var message = new DbMessage(149, 123, "filehelper", null, null, null, 81604378673L, "xml", DateTimeOffset.UnixEpoch, null);
+        using var json = JsonDocument.Parse(JsonSerializer.Serialize(message));
+        Assert.Equal(81604378673L, json.RootElement.GetProperty("Type").GetInt64());
+        using var output = new MemoryStream();
+        using (var compressed = new CompressionStream(output))
+            compressed.Write(Encoding.UTF8.GetBytes(new string('x', WechatMergedChatParser.MaxXmlCharacters + 1)));
+        Assert.Throws<InvalidDataException>(() => WechatDbMessage.DecodeContent(Convert.ToHexString(output.ToArray()), true));
+    }
+
+    private static XElement Item(string text) => new("dataitem", new XAttribute("datatype", 1), new XAttribute("dataid", "repeated-id"),
+        new XElement("datadesc", text), new XElement("sourcename", "Synthetic sender"), new XElement("srcMsgLocalId", "7"),
+        new XElement("srcMsgCreateTime", "1700000000"), new XElement("fromnewmsgid", "18446744073709551615"),
+        new XElement("dataitemsource", new XElement("hashusername", "opaque-hash")));
+    private static XElement Record(params XElement[] items) => new("recordinfo", new XElement("title", "Synthetic record"),
+        new XElement("datalist", new XAttribute("count", items.Length), items));
+    private static string Wrap(XElement record) => new XElement("msg", new XElement("appmsg", new XElement("type", 19),
+        new XElement("recorditem", new XCData(record.ToString(SaveOptions.DisableFormatting))))).ToString();
+}