diff --git a/README.md b/README.md
index f714779..2508e02 100644
--- a/README.md
+++ b/README.md
@@ -6,7 +6,7 @@ control panel — **isolated per conversation session**.
 
 - 右侧侧边栏（文件夹预览 / 文件编辑与预览 / 终端 / Git）——按会话记忆与隔离
 - Explorer: 懒加载目录树，根目录 = 当前会话工作目录
-- Editor: 文本编辑（脏点 + Ctrl/Cmd+S 保存）、图片查看、Markdown 预览
+- Editor: CodeMirror 文本编辑（自动换行 + 按扩展名语法高亮、脏点 + Ctrl/Cmd+S 保存）、图片查看、Markdown 预览
 - Terminal: xterm.js + node-pty（每会话最多 3 个，进程跨刷新存活）
 - Git: status / diff / stage / commit / branch / history（基础集）
 - 分栏工作台: VSCode 式拖拽分栏（拖 Tab 到分栏边缘即左右/上下拆分，拖到中间合并）、
@@ -24,9 +24,9 @@ control panel — **isolated per conversation session**.
 | host | `src/index.ts` → `lib/index.js` | cordis 插件：`/sidebar/api/*` JSON API、`/sidebar/file` 媒体路由、`/sidebar/ws/terminal` WebSocket；fs / git / pty 服务 |
 | client | `src/client/index.tsx` → `lib/client.js` | 浏览器 bundle（`__ModuleLoader__.load` 闭包工厂）：portal 侧边栏 + 各视图 + turnTail 拦截 |
 
-- 所有 API 携带 `sessionId`；cwd 从会话存储取权威值；终端按 `${sessionId}:${tabId}` 键控。
+- 所有 API 携带 `sessionId`；cwd 权威值取自会话 header，会话尚未附加（页面加载竞态）时回退客户端摘要 cwd，再回退进程 cwd（`session.cwd` / fs / git / 终端 / 媒体路由一致）；终端按 `${sessionId}:${tabId}` 键控。
 - 路由与 `/api` 同款信任围栏（Host 头 loopback / `connection.trustedHosts`，`src/trust-fence.ts`，拷贝自 `@deepseek-ai/dsh-client-connection`，BSD-3-Clause）。
-- client 状态按会话持久化到 `localStorage`（`dsh-sidebar:v1:<sessionId>`）：面板几何、分栏树、Tabs、树展开。
+- client 状态按会话持久化到 `localStorage`（`dsh-sidebar:v1:<sessionId>`）：面板几何、分栏树、Tabs、树展开；读取时做结构校验（旧/损坏状态回退默认）并把宽度钳制到当前视口，防止陈旧全屏宽度压垮应用布局。
 - client bundle externals 仅模块表词条（react/cordis/ui-primitives 等），xterm 等全部内联。
 
 ## 安装（web profile）
diff --git a/package.json b/package.json
index 9fc0282..07f3ea7 100644
--- a/package.json
+++ b/package.json
@@ -29,6 +29,27 @@
   },
   "license": "MIT",
   "dependencies": {
+    "@codemirror/commands": "^6.10.4",
+    "@codemirror/lang-cpp": "^6.0.3",
+    "@codemirror/lang-css": "^6.3.1",
+    "@codemirror/lang-go": "^6.0.1",
+    "@codemirror/lang-html": "^6.4.12",
+    "@codemirror/lang-java": "^6.0.2",
+    "@codemirror/lang-javascript": "^6.2.5",
+    "@codemirror/lang-json": "^6.0.2",
+    "@codemirror/lang-markdown": "^6.5.2",
+    "@codemirror/lang-php": "^6.0.2",
+    "@codemirror/lang-python": "^6.2.1",
+    "@codemirror/lang-rust": "^6.0.2",
+    "@codemirror/lang-sql": "^6.10.0",
+    "@codemirror/lang-xml": "^6.1.0",
+    "@codemirror/lang-yaml": "^6.1.3",
+    "@codemirror/language": "^6.12.4",
+    "@codemirror/legacy-modes": "^6.5.3",
+    "@codemirror/search": "^6.7.1",
+    "@codemirror/state": "^6.7.1",
+    "@codemirror/theme-one-dark": "^6.1.3",
+    "@codemirror/view": "^6.43.8",
     "@xterm/addon-fit": "^0.10.0",
     "clsx": "^2.1.1",
     "node-pty": "^1.1.0",
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 1c6b94e..32cd44d 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -8,6 +8,69 @@ importers:
 
   .:
     dependencies:
+      '@codemirror/commands':
+        specifier: ^6.10.4
+        version: 6.10.4
+      '@codemirror/lang-cpp':
+        specifier: ^6.0.3
+        version: 6.0.3
+      '@codemirror/lang-css':
+        specifier: ^6.3.1
+        version: 6.3.1
+      '@codemirror/lang-go':
+        specifier: ^6.0.1
+        version: 6.0.1
+      '@codemirror/lang-html':
+        specifier: ^6.4.12
+        version: 6.4.12
+      '@codemirror/lang-java':
+        specifier: ^6.0.2
+        version: 6.0.2
+      '@codemirror/lang-javascript':
+        specifier: ^6.2.5
+        version: 6.2.5
+      '@codemirror/lang-json':
+        specifier: ^6.0.2
+        version: 6.0.2
+      '@codemirror/lang-markdown':
+        specifier: ^6.5.2
+        version: 6.5.2
+      '@codemirror/lang-php':
+        specifier: ^6.0.2
+        version: 6.0.2
+      '@codemirror/lang-python':
+        specifier: ^6.2.1
+        version: 6.2.1
+      '@codemirror/lang-rust':
+        specifier: ^6.0.2
+        version: 6.0.2
+      '@codemirror/lang-sql':
+        specifier: ^6.10.0
+        version: 6.10.0
+      '@codemirror/lang-xml':
+        specifier: ^6.1.0
+        version: 6.1.0
+      '@codemirror/lang-yaml':
+        specifier: ^6.1.3
+        version: 6.1.3
+      '@codemirror/language':
+        specifier: ^6.12.4
+        version: 6.12.4
+      '@codemirror/legacy-modes':
+        specifier: ^6.5.3
+        version: 6.5.3
+      '@codemirror/search':
+        specifier: ^6.7.1
+        version: 6.7.1
+      '@codemirror/state':
+        specifier: ^6.7.1
+        version: 6.7.1
+      '@codemirror/theme-one-dark':
+        specifier: ^6.1.3
+        version: 6.1.3
+      '@codemirror/view':
+        specifier: ^6.43.8
+        version: 6.43.8
       '@xterm/addon-fit':
         specifier: ^0.10.0
         version: 0.10.0(@xterm/xterm@5.4.0)
@@ -78,9 +141,129 @@ importers:
 
 packages:
 
+  '@codemirror/autocomplete@6.20.3':
+    resolution: {integrity: sha512-tlosUqb+3BbxCxZdu4tKeRghPFC+QM7q4X5YhKV2eCmPG+1r2F3f4AaSz5sCrFqUtX4Jh20VFTKecl16MgiV9g==}
+
+  '@codemirror/commands@6.10.4':
+    resolution: {integrity: sha512-Ryk9y9T0FFVF0cUGhAknveAyUOl/A1qReTFi+qPKtOh2Z9F4AUBz3XOrYD4ZEgZirdugVzHvd/2/Wcwy5OliTg==}
+
+  '@codemirror/lang-cpp@6.0.3':
+    resolution: {integrity: sha512-URM26M3vunFFn9/sm6rzqrBzDgfWuDixp85uTY49wKudToc2jTHUrKIGGKs+QWND+YLofNNZpxcNGRynFJfvgA==}
+
+  '@codemirror/lang-css@6.3.1':
+    resolution: {integrity: sha512-kr5fwBGiGtmz6l0LSJIbno9QrifNMUusivHbnA1H6Dmqy4HZFte3UAICix1VuKo0lMPKQr2rqB+0BkKi/S3Ejg==}
+
+  '@codemirror/lang-go@6.0.1':
+    resolution: {integrity: sha512-7fNvbyNylvqCphW9HD6WFnRpcDjr+KXX/FgqXy5H5ZS0eC5edDljukm/yNgYkwTsgp2busdod50AOTIy6Jikfg==}
+
+  '@codemirror/lang-html@6.4.12':
+    resolution: {integrity: sha512-pw2ReWKUqSkbvh76RAT4NYxiogRu+PWkR2ukAwO9uOgrm8uipkzjtKKtNpyeAQwHOqxEeSvAXZ6vr3AfyB9y/w==}
+
+  '@codemirror/lang-java@6.0.2':
+    resolution: {integrity: sha512-m5Nt1mQ/cznJY7tMfQTJchmrjdjQ71IDs+55d1GAa8DGaB8JXWsVCkVT284C3RTASaY43YknrK2X3hPO/J3MOQ==}
+
+  '@codemirror/lang-javascript@6.2.5':
+    resolution: {integrity: sha512-zD4e5mS+50htS7F+TYjBPsiIFGanfVqg4HyUz6WNFikgOPf2BgKlx+TQedI1w6n/IqRBVBbBWmGFdLB/7uxO4A==}
+
+  '@codemirror/lang-json@6.0.2':
+    resolution: {integrity: sha512-x2OtO+AvwEHrEwR0FyyPtfDUiloG3rnVTSZV1W8UteaLL8/MajQd8DpvUb2YVzC+/T18aSDv0H9mu+xw0EStoQ==}
+
+  '@codemirror/lang-markdown@6.5.2':
+    resolution: {integrity: sha512-AwBOdkWYuA//WcM0xO5PfHPUcmz/O2i5o0Nsg1U69SII/loCJlFI1Romd9xp2HYb1kYJRGZotyqRghuHH5n8Kw==}
+
+  '@codemirror/lang-php@6.0.2':
+    resolution: {integrity: sha512-ZKy2v1n8Fc8oEXj0Th0PUMXzQJ0AIR6TaZU+PbDHExFwdu+guzOA4jmCHS1Nz4vbFezwD7LyBdDnddSJeScMCA==}
+
+  '@codemirror/lang-python@6.2.1':
+    resolution: {integrity: sha512-IRjC8RUBhn9mGR9ywecNhB51yePWCGgvHfY1lWN/Mrp3cKuHr0isDKia+9HnvhiWNnMpbGhWrkhuWOc09exRyw==}
+
+  '@codemirror/lang-rust@6.0.2':
+    resolution: {integrity: sha512-EZaGjCUegtiU7kSMvOfEZpaCReowEf3yNidYu7+vfuGTm9ow4mthAparY5hisJqOHmJowVH3Upu+eJlUji6qqA==}
+
+  '@codemirror/lang-sql@6.10.0':
+    resolution: {integrity: sha512-6ayPkEd/yRw0XKBx5uAiToSgGECo/GY2NoJIHXIIQh1EVwLuKoU8BP/qK0qH5NLXAbtJRLuT73hx7P9X34iO4w==}
+
+  '@codemirror/lang-xml@6.1.0':
+    resolution: {integrity: sha512-3z0blhicHLfwi2UgkZYRPioSgVTo9PV5GP5ducFH6FaHy0IAJRg+ixj5gTR1gnT/glAIC8xv4w2VL1LoZfs+Jg==}
+
+  '@codemirror/lang-yaml@6.1.3':
+    resolution: {integrity: sha512-AZ8DJBuXGVHybpBQhmZtgew5//4hv3tdkXnr3vDmOUMJRuB6vn/uuwtmTOTlqEaQFg3hQSVeA90NmvIQyUV6FQ==}
+
+  '@codemirror/language@6.12.4':
+    resolution: {integrity: sha512-1q4PaT+o6PbgpkJt4Q8Fv5XJxTy4FUZ4MWETtyiDw3J0Pyr9E2vqcKL+k9wcvjNTIsauxvE7OfmWj3FRPHQ76A==}
+
+  '@codemirror/legacy-modes@6.5.3':
+    resolution: {integrity: sha512-xCsmIzH78MyWkib9jlPaaun57XNkfbMIhagfaZVd0iLTqlpw3jXaIcbZm72MTmmn64eTZpBVNjbyYh+QXnxRsg==}
+
+  '@codemirror/lint@6.9.7':
+    resolution: {integrity: sha512-28/+iWLYxKxsvGYhSYL7zaCZqLz5+FFFDq9tVsvGv9kv8RY4fFAchJ5WX9M3YrrRlTIsECjsXPqeNgnSmNP2dg==}
+
+  '@codemirror/search@6.7.1':
+    resolution: {integrity: sha512-uMe5UO6PamJtSHrXhhHOzSX3ReWtiJrva6GnPMwSOrZtiExb5X5eExhr2OUZQVvdxPsKpY3Ro2mFbQadpPWmHA==}
+
+  '@codemirror/state@6.7.1':
+    resolution: {integrity: sha512-9QzNDgE4EYDnAHfrTlR2lwiPciiOymLtwKK+8yHQzCc7GXhAP9xdEbEJFy2IWB1j9UGUl9BsgMmTo/ImA02T7A==}
+
+  '@codemirror/theme-one-dark@6.1.3':
+    resolution: {integrity: sha512-NzBdIvEJmx6fjeremiGp3t/okrLPYT0d9orIc7AFun8oZcRk58aejkqhv6spnz4MLAevrKNPMQYXEWMg4s+sKA==}
+
+  '@codemirror/view@6.43.8':
+    resolution: {integrity: sha512-qtItTDssZ/5GFfi94hrILu9j/VUeFPDPkhovEfmWFj2ipTxnzPB8DdHgfbb8HYTzLTYhrndKmyQxXUz/PDLenw==}
+
   '@jridgewell/sourcemap-codec@1.5.5':
     resolution: {integrity: sha512-cYQ9310grqxueWbl+WuIUIaiUaDcj7WOq5fVhEljNVgRfOUhY9fy2zTvfoqWsnebh8Sl70VScFbICvJnLKB0Og==}
 
+  '@lezer/common@1.5.2':
+    resolution: {integrity: sha512-sxQE460fPZyU3sdc8lafxiPwJHBzZRy/udNFynGQky1SePYBdhkBl1kOagA9uT3pxR8K09bOrmTUqA9wb/PjSQ==}
+
+  '@lezer/cpp@1.1.6':
+    resolution: {integrity: sha512-vh9gWWJOXFVY8HBHK3Twzq8MgwG2iN4GSyzBP9sCGTe37P15x2R14VaBQk0VA0ezTRN1KHYBBsHhvpGZ2Xy/pA==}
+
+  '@lezer/css@1.3.6':
+    resolution: {integrity: sha512-YJE78Wcg+zX8f10hiHWQ4Az48Qr/c13eId0VtRQYLBpxHDmDeSrXIlkbl+fJGW42rWC/uoUco9mhBZeVWP/A1g==}
+
+  '@lezer/go@1.0.1':
+    resolution: {integrity: sha512-xToRsYxwsgJNHTgNdStpcvmbVuKxTapV0dM0wey1geMMRc9aggoVyKgzYp41D2/vVOx+Ii4hmE206kvxIXBVXQ==}
+
+  '@lezer/highlight@1.2.3':
+    resolution: {integrity: sha512-qXdH7UqTvGfdVBINrgKhDsVTJTxactNNxLk7+UMwZhU13lMHaOBlJe9Vqp907ya56Y3+ed2tlqzys7jDkTmW0g==}
+
+  '@lezer/html@1.3.13':
+    resolution: {integrity: sha512-oI7n6NJml729m7pjm9lvLvmXbdoMoi2f+1pwSDJkl9d68zGr7a9Btz8NdHTGQZtW2DA25ybeuv/SyDb9D5tseg==}
+
+  '@lezer/java@1.1.3':
+    resolution: {integrity: sha512-yHquUfujwg6Yu4Fd1GNHCvidIvJwi/1Xu2DaKl/pfWIA2c1oXkVvawH3NyXhCaFx4OdlYBVX5wvz2f7Aoa/4Xw==}
+
+  '@lezer/javascript@1.5.4':
+    resolution: {integrity: sha512-vvYx3MhWqeZtGPwDStM2dwgljd5smolYD2lR2UyFcHfxbBQebqx8yjmFmxtJ/E6nN6u1D9srOiVWm3Rb4tmcUA==}
+
+  '@lezer/json@1.0.3':
+    resolution: {integrity: sha512-BP9KzdF9Y35PDpv04r0VeSTKDeox5vVr3efE7eBbx3r4s3oNLfunchejZhjArmeieBH+nVOpgIiBJpEAv8ilqQ==}
+
+  '@lezer/lr@1.4.10':
+    resolution: {integrity: sha512-rnCpTIBafOx4mRp43xOxDJbFipJm/c0cia/V5TiGlhmMa+wsSdoGmUN3w5Bqrks/09Q/D4tNAmWaT8p6NRi77A==}
+
+  '@lezer/markdown@1.7.2':
+    resolution: {integrity: sha512-iTkYvoVcKt3WkeL7qUDyXHONZEwLio4wj8KTNi2dnjQEXBZKMV63BpQrPqfsM+OkvuRbiSTAcycYAsQzLhRNoQ==}
+
+  '@lezer/php@1.0.5':
+    resolution: {integrity: sha512-W7asp9DhM6q0W6DYNwIkLSKOvxlXRrif+UXBMxzsJUuqmhE7oVU+gS3THO4S/Puh7Xzgm858UNaFi6dxTP8dJA==}
+
+  '@lezer/python@1.1.19':
+    resolution: {integrity: sha512-MhQIURHRytsNzP/YXnqpYKW6la6voAH3kyplTOOiCdjyFY6cWWGFVmYVdHIPrElqSDf4iCDktQCockB9FxuhzQ==}
+
+  '@lezer/rust@1.0.2':
+    resolution: {integrity: sha512-Lz5sIPBdF2FUXcWeCu1//ojFAZqzTQNRga0aYv6dYXqJqPfMdCAI0NzajWUd4Xijj1IKJLtjoXRPMvTKWBcqKg==}
+
+  '@lezer/xml@1.0.6':
+    resolution: {integrity: sha512-CdDwirL0OEaStFue/66ZmFSeppuL6Dwjlk8qk153mSQwiSH/Dlri4GNymrNWnUmPl2Um7QfV1FO9KFUyX3Twww==}
+
+  '@lezer/yaml@1.0.4':
+    resolution: {integrity: sha512-2lrrHqxalACEbxIbsjhqGpSW8kWpUKuY6RHgnSAFZa6qK62wvnPxA8hGOwOoDbwHcOFs5M4o27mjGu+P7TvBmw==}
+
+  '@marijn/find-cluster-break@1.0.3':
+    resolution: {integrity: sha512-FY+MKLBoTsLNJF/eLWaOsXGdz6uh3Iu1axjPf6TUq92IYumcTcXWHoS747JARLkcdlJ/Waiaxc5wQfFO8jC6NA==}
+
   '@oxc-project/types@0.143.0':
     resolution: {integrity: sha512-u6JZdLBTLotrNC9Vd6vPssINdzcCzleKAH6EJKImQb7GtYvX5keN2dxkoK44stCc4tffE6QQRtZTXVSzsLUlWA==}
 
@@ -417,6 +600,9 @@ packages:
   cosmokit@1.8.1:
     resolution: {integrity: sha512-PDBv4l90xZKrUsZ0vtoycgZpO/j4iFsqJXrAxsyBDsnQRI7ZMJXIjgDJsKNjd5L8jnVnnlrDCdhkFbTncgCVjQ==}
 
+  crelt@1.0.7:
+    resolution: {integrity: sha512-aK6BbWfhf4U/wCcLHKPJl/xa6VkVstRaPywWtMKGwuOLc/wZTyQYuoxgvZnNsBvv7Kg3YTBQYYBCggcviQczuA==}
+
   csstype@3.2.3:
     resolution: {integrity: sha512-z1HGKcYy2xA8AGQfwrn0PAy+PB7X/GSj3UVJW9qKyn43xWa+gl5nXmU4qqLMRzWVLFC8KusUX8T/0kCiOYpAIQ==}
 
@@ -644,6 +830,9 @@ packages:
   std-env@4.2.0:
     resolution: {integrity: sha512-oCUKSupKTHX53EyjDtuZQ64pjLJ6yYCtpmEw0goYxtjG9KpbRe8KAsl2tBUGU9DyMcJ0RwJ8GqJAFzMXcXW1Rw==}
 
+  style-mod@4.1.3:
+    resolution: {integrity: sha512-i/n8VsZydrugj3Iuzll8+x/00GH2vnYsk1eomD8QiRrSAeW6ItbCQDtfXCeJHd0iwiNagqjQkvpvREEPtW3IoQ==}
+
   tinybench@2.9.0:
     resolution: {integrity: sha512-0+DUvqWMValLmha6lr4kD8iAMK1HzV0/aKnCtWb9v9641TnP/MFb7Pc2bxoxQjTXAErryXVgUOfv2YqNllqGeg==}
 
@@ -796,6 +985,9 @@ packages:
       jsdom:
         optional: true
 
+  w3c-keyname@2.2.8:
+    resolution: {integrity: sha512-dpojBhNsCNN7T82Tm7k26A6G9ML3NkhDsnw9n/eoxSRlVBB4CEtIQ/KTCLI2Fwf3ataSXRhYFkQi3SlnFwPvPQ==}
+
   why-is-node-running@2.3.0:
     resolution: {integrity: sha512-hUrmaWBdVDcxvYqnyh09zunKzROWjbZTiNy8dBEjkS7ehEDQibXJ7XvlmtbwuTclUiIyN+CyXQD4Vmko8fNm8w==}
     engines: {node: '>=8'}
@@ -828,8 +1020,266 @@ packages:
 
 snapshots:
 
+  '@codemirror/autocomplete@6.20.3':
+    dependencies:
+      '@codemirror/language': 6.12.4
+      '@codemirror/state': 6.7.1
+      '@codemirror/view': 6.43.8
+      '@lezer/common': 1.5.2
+
+  '@codemirror/commands@6.10.4':
+    dependencies:
+      '@codemirror/language': 6.12.4
+      '@codemirror/state': 6.7.1
+      '@codemirror/view': 6.43.8
+      '@lezer/common': 1.5.2
+
+  '@codemirror/lang-cpp@6.0.3':
+    dependencies:
+      '@codemirror/language': 6.12.4
+      '@lezer/cpp': 1.1.6
+
+  '@codemirror/lang-css@6.3.1':
+    dependencies:
+      '@codemirror/autocomplete': 6.20.3
+      '@codemirror/language': 6.12.4
+      '@codemirror/state': 6.7.1
+      '@lezer/common': 1.5.2
+      '@lezer/css': 1.3.6
+
+  '@codemirror/lang-go@6.0.1':
+    dependencies:
+      '@codemirror/autocomplete': 6.20.3
+      '@codemirror/language': 6.12.4
+      '@codemirror/state': 6.7.1
+      '@lezer/common': 1.5.2
+      '@lezer/go': 1.0.1
+
+  '@codemirror/lang-html@6.4.12':
+    dependencies:
+      '@codemirror/autocomplete': 6.20.3
+      '@codemirror/lang-css': 6.3.1
+      '@codemirror/lang-javascript': 6.2.5
+      '@codemirror/language': 6.12.4
+      '@codemirror/state': 6.7.1
+      '@codemirror/view': 6.43.8
+      '@lezer/common': 1.5.2
+      '@lezer/css': 1.3.6
+      '@lezer/html': 1.3.13
+
+  '@codemirror/lang-java@6.0.2':
+    dependencies:
+      '@codemirror/language': 6.12.4
+      '@lezer/java': 1.1.3
+
+  '@codemirror/lang-javascript@6.2.5':
+    dependencies:
+      '@codemirror/autocomplete': 6.20.3
+      '@codemirror/language': 6.12.4
+      '@codemirror/lint': 6.9.7
+      '@codemirror/state': 6.7.1
+      '@codemirror/view': 6.43.8
+      '@lezer/common': 1.5.2
+      '@lezer/javascript': 1.5.4
+
+  '@codemirror/lang-json@6.0.2':
+    dependencies:
+      '@codemirror/language': 6.12.4
+      '@lezer/json': 1.0.3
+
+  '@codemirror/lang-markdown@6.5.2':
+    dependencies:
+      '@codemirror/autocomplete': 6.20.3
+      '@codemirror/lang-html': 6.4.12
+      '@codemirror/language': 6.12.4
+      '@codemirror/state': 6.7.1
+      '@codemirror/view': 6.43.8
+      '@lezer/common': 1.5.2
+      '@lezer/markdown': 1.7.2
+
+  '@codemirror/lang-php@6.0.2':
+    dependencies:
+      '@codemirror/lang-html': 6.4.12
+      '@codemirror/language': 6.12.4
+      '@codemirror/state': 6.7.1
+      '@lezer/common': 1.5.2
+      '@lezer/php': 1.0.5
+
+  '@codemirror/lang-python@6.2.1':
+    dependencies:
+      '@codemirror/autocomplete': 6.20.3
+      '@codemirror/language': 6.12.4
+      '@codemirror/state': 6.7.1
+      '@lezer/common': 1.5.2
+      '@lezer/python': 1.1.19
+
+  '@codemirror/lang-rust@6.0.2':
+    dependencies:
+      '@codemirror/language': 6.12.4
+      '@lezer/rust': 1.0.2
+
+  '@codemirror/lang-sql@6.10.0':
+    dependencies:
+      '@codemirror/autocomplete': 6.20.3
+      '@codemirror/language': 6.12.4
+      '@codemirror/state': 6.7.1
+      '@lezer/common': 1.5.2
+      '@lezer/highlight': 1.2.3
+      '@lezer/lr': 1.4.10
+
+  '@codemirror/lang-xml@6.1.0':
+    dependencies:
+      '@codemirror/autocomplete': 6.20.3
+      '@codemirror/language': 6.12.4
+      '@codemirror/state': 6.7.1
+      '@codemirror/view': 6.43.8
+      '@lezer/common': 1.5.2
+      '@lezer/xml': 1.0.6
+
+  '@codemirror/lang-yaml@6.1.3':
+    dependencies:
+      '@codemirror/autocomplete': 6.20.3
+      '@codemirror/language': 6.12.4
+      '@codemirror/state': 6.7.1
+      '@lezer/common': 1.5.2
+      '@lezer/highlight': 1.2.3
+      '@lezer/lr': 1.4.10
+      '@lezer/yaml': 1.0.4
+
+  '@codemirror/language@6.12.4':
+    dependencies:
+      '@codemirror/state': 6.7.1
+      '@codemirror/view': 6.43.8
+      '@lezer/common': 1.5.2
+      '@lezer/highlight': 1.2.3
+      '@lezer/lr': 1.4.10
+      style-mod: 4.1.3
+
+  '@codemirror/legacy-modes@6.5.3':
+    dependencies:
+      '@codemirror/language': 6.12.4
+
+  '@codemirror/lint@6.9.7':
+    dependencies:
+      '@codemirror/state': 6.7.1
+      '@codemirror/view': 6.43.8
+      crelt: 1.0.7
+
+  '@codemirror/search@6.7.1':
+    dependencies:
+      '@codemirror/state': 6.7.1
+      '@codemirror/view': 6.43.8
+      crelt: 1.0.7
+
+  '@codemirror/state@6.7.1':
+    dependencies:
+      '@marijn/find-cluster-break': 1.0.3
+
+  '@codemirror/theme-one-dark@6.1.3':
+    dependencies:
+      '@codemirror/language': 6.12.4
+      '@codemirror/state': 6.7.1
+      '@codemirror/view': 6.43.8
+      '@lezer/highlight': 1.2.3
+
+  '@codemirror/view@6.43.8':
+    dependencies:
+      '@codemirror/state': 6.7.1
+      crelt: 1.0.7
+      style-mod: 4.1.3
+      w3c-keyname: 2.2.8
+
   '@jridgewell/sourcemap-codec@1.5.5': {}
 
+  '@lezer/common@1.5.2': {}
+
+  '@lezer/cpp@1.1.6':
+    dependencies:
+      '@lezer/common': 1.5.2
+      '@lezer/highlight': 1.2.3
+      '@lezer/lr': 1.4.10
+
+  '@lezer/css@1.3.6':
+    dependencies:
+      '@lezer/common': 1.5.2
+      '@lezer/highlight': 1.2.3
+      '@lezer/lr': 1.4.10
+
+  '@lezer/go@1.0.1':
+    dependencies:
+      '@lezer/common': 1.5.2
+      '@lezer/highlight': 1.2.3
+      '@lezer/lr': 1.4.10
+
+  '@lezer/highlight@1.2.3':
+    dependencies:
+      '@lezer/common': 1.5.2
+
+  '@lezer/html@1.3.13':
+    dependencies:
+      '@lezer/common': 1.5.2
+      '@lezer/highlight': 1.2.3
+      '@lezer/lr': 1.4.10
+
+  '@lezer/java@1.1.3':
+    dependencies:
+      '@lezer/common': 1.5.2
+      '@lezer/highlight': 1.2.3
+      '@lezer/lr': 1.4.10
+
+  '@lezer/javascript@1.5.4':
+    dependencies:
+      '@lezer/common': 1.5.2
+      '@lezer/highlight': 1.2.3
+      '@lezer/lr': 1.4.10
+
+  '@lezer/json@1.0.3':
+    dependencies:
+      '@lezer/common': 1.5.2
+      '@lezer/highlight': 1.2.3
+      '@lezer/lr': 1.4.10
+
+  '@lezer/lr@1.4.10':
+    dependencies:
+      '@lezer/common': 1.5.2
+
+  '@lezer/markdown@1.7.2':
+    dependencies:
+      '@lezer/common': 1.5.2
+      '@lezer/highlight': 1.2.3
+
+  '@lezer/php@1.0.5':
+    dependencies:
+      '@lezer/common': 1.5.2
+      '@lezer/highlight': 1.2.3
+      '@lezer/lr': 1.4.10
+
+  '@lezer/python@1.1.19':
+    dependencies:
+      '@lezer/common': 1.5.2
+      '@lezer/highlight': 1.2.3
+      '@lezer/lr': 1.4.10
+
+  '@lezer/rust@1.0.2':
+    dependencies:
+      '@lezer/common': 1.5.2
+      '@lezer/highlight': 1.2.3
+      '@lezer/lr': 1.4.10
+
+  '@lezer/xml@1.0.6':
+    dependencies:
+      '@lezer/common': 1.5.2
+      '@lezer/highlight': 1.2.3
+      '@lezer/lr': 1.4.10
+
+  '@lezer/yaml@1.0.4':
+    dependencies:
+      '@lezer/common': 1.5.2
+      '@lezer/highlight': 1.2.3
+      '@lezer/lr': 1.4.10
+
+  '@marijn/find-cluster-break@1.0.3': {}
+
   '@oxc-project/types@0.143.0': {}
 
   '@quansync/fs@1.0.0':
@@ -1050,6 +1500,8 @@ snapshots:
 
   cosmokit@1.8.1: {}
 
+  crelt@1.0.7: {}
+
   csstype@3.2.3: {}
 
   defu@6.1.7: {}
@@ -1224,6 +1676,8 @@ snapshots:
 
   std-env@4.2.0: {}
 
+  style-mod@4.1.3: {}
+
   tinybench@2.9.0: {}
 
   tinyexec@1.3.0: {}
@@ -1311,6 +1765,8 @@ snapshots:
     transitivePeerDependencies:
       - msw
 
+  w3c-keyname@2.2.8: {}
+
   why-is-node-running@2.3.0:
     dependencies:
       siginfo: 2.0.0
diff --git a/src/client/EditorView.tsx b/src/client/EditorView.tsx
index 746f875..4a7fffa 100644
--- a/src/client/EditorView.tsx
+++ b/src/client/EditorView.tsx
@@ -1,46 +1,48 @@
 /**
- * The file editor tab: text editing with a dirty dot and Ctrl/Cmd+S save,
+ * The file editor tab: a CodeMirror 6 editor with line wrapping and syntax
+ * highlighting (extension-keyed language), a dirty dot and Ctrl/Cmd+S save,
  * image viewing through the media route, Markdown preview through the shared
  * MarkdownText component, and a plain notice for binary files. Reads cap at
  * the host's 512KB bound (a banner marks truncation).
  */
-import { useEffect, useState } from 'react'
+import { useEffect, useRef, useState } from 'react'
 import clsx from 'clsx'
+import { EditorState } from '@codemirror/state'
+import { EditorView as CodeMirrorView, keymap } from '@codemirror/view'
+import { defaultKeymap, history, historyKeymap } from '@codemirror/commands'
+import { oneDark } from '@codemirror/theme-one-dark'
 import { IconCheckOutline16, MarkdownText } from '@deepseek-ai/dsh-client-ui-primitives'
-import { api, mediaUrl } from './api.ts'
+import { api, mediaUrl, type SessionScope } from './api.ts'
+import { languageForPath } from './lang.ts'
 import { t } from './locales.ts'
 import css from './sidebar.module.css'
 
 const IMAGE_EXT = ['.png', '.jpg', '.jpeg', '.gif', '.webp', '.svg', '.bmp', '.ico', '.avif']
 const MD_EXT = ['.md', '.markdown']
 
-function extOf(path: string): string {
-  const at = path.lastIndexOf('.')
-  if (at === -1) return ''
-  const base = path.slice(at).toLowerCase()
-  return base.includes('/') || base.includes('\\') ? '' : base
-}
-
 type EditorLoad =
   | { status: 'loading' }
   | { status: 'error'; message: string }
   | { status: 'ready'; kind: 'text' | 'image' | 'md'; content: string; truncated: boolean }
   | { status: 'binary' }
 
-export function EditorView(props: { sessionId: string; path: string; title: string }) {
-  const { sessionId, path, title } = props
+export function EditorView(props: { scope: SessionScope; path: string; title: string }) {
+  const { scope, path, title } = props
   const [load, setLoad] = useState<EditorLoad>({ status: 'loading' })
-  const [draft, setDraft] = useState<string | null>(null)
+  const [dirty, setDirty] = useState(false)
   const [saveState, setSaveState] = useState<'idle' | 'saving' | 'saved' | 'failed'>('idle')
+  const hostRef = useRef<HTMLDivElement>(null)
+  const viewRef = useRef<CodeMirrorView | null>(null)
+  const savingRef = useRef(false)
 
   useEffect(() => {
     let cancelled = false
     setLoad({ status: 'loading' })
-    setDraft(null)
+    setDirty(false)
     setSaveState('idle')
-    api.fsRead(sessionId, path).then((result) => {
+    api.fsRead(scope, path).then((result) => {
       if (cancelled) return
-      const ext = extOf(path)
+      const ext = extOfPath(path)
       if (result.kind === 'binary') {
         setLoad({ status: 'binary' })
         return
@@ -60,20 +62,66 @@ export function EditorView(props: { sessionId: string; path: string; title: stri
       setLoad({ status: 'error', message: error instanceof Error ? error.message : String(error) })
     })
     return () => { cancelled = true }
-  }, [sessionId, path])
+  }, [scope.sessionId, scope.cwd, path])
+
+  // Create the CodeMirror editor once the text is loaded. The view owns the
+  // document; React only tracks dirty/save state through the update listener.
+  useEffect(() => {
+    if (load.status !== 'ready' || load.kind !== 'text') return
+    const host = hostRef.current
+    if (host === null) return
+    const language = languageForPath(path)
+    const state = EditorState.create({
+      doc: load.content,
+      extensions: [
+        CodeMirrorView.lineWrapping,
+        history(),
+        EditorState.tabSize.of(2),
+        CodeMirrorView.contentAttributes.of({ spellcheck: 'false' }),
+        CodeMirrorView.theme({
+          '&': { height: '100%', fontSize: '13px', backgroundColor: 'transparent' },
+          '.cm-scroller': { overflow: 'auto', fontFamily: '"SF Mono", Menlo, Consolas, "Liberation Mono", monospace' },
+          '.cm-content': { caretColor: 'var(--dsw-alias-label-primary)' },
+        }),
+        ...(language !== null ? [language] : []),
+        oneDark,
+        CodeMirrorView.updateListener.of((update) => {
+          if (update.docChanged) setDirty(true)
+        }),
+        keymap.of([
+          {
+            key: 'Mod-s',
+            preventDefault: true,
+            run: () => { save(); return true },
+          },
+          ...defaultKeymap,
+          ...historyKeymap,
+        ]),
+      ],
+    })
+    const view = new CodeMirrorView({ state, parent: host })
+    viewRef.current = view
+    return () => {
+      view.destroy()
+      viewRef.current = null
+    }
+  }, [load])
 
   const save = (): void => {
-    if (load.status !== 'ready' || load.kind !== 'text' || draft === null || saveState === 'saving') return
+    const view = viewRef.current
+    if (view === null || savingRef.current) return
+    savingRef.current = true
     setSaveState('saving')
-    api.fsWrite(sessionId, path, draft).then(() => {
+    api.fsWrite(scope, path, view.state.doc.toString()).then(() => {
+      savingRef.current = false
+      setDirty(false)
       setSaveState('saved')
-      setDraft(null)
     }).catch(() => {
+      savingRef.current = false
       setSaveState('failed')
     })
   }
 
-  const dirty = load.status === 'ready' && load.kind === 'text' && draft !== null
   const saveLabel = saveState === 'saving' ? t('loading') : saveState === 'saved' ? t('saved') : saveState === 'failed' ? t('saveFailed') : ''
 
   return (
@@ -100,26 +148,12 @@ export function EditorView(props: { sessionId: string; path: string; title: stri
       {load.status === 'ready' && load.kind === 'text' && (
         <>
           {load.truncated && <div className={css.editorBanner}>{t('truncation')}</div>}
-          <textarea
-            className={css.editorTextarea}
-            spellCheck={false}
-            value={draft ?? load.content}
-            onChange={(event) => {
-              setDraft(event.target.value)
-              setSaveState('idle')
-            }}
-            onKeyDown={(event) => {
-              if ((event.ctrlKey || event.metaKey) && event.key === 's') {
-                event.preventDefault()
-                save()
-              }
-            }}
-          />
+          <div className={css.editorCm} ref={hostRef} />
         </>
       )}
       {load.status === 'ready' && load.kind === 'image' && (
         <div className={css.editorImageWrap}>
-          <img className={css.editorImage} src={mediaUrl(sessionId, path)} alt={title} />
+          <img className={css.editorImage} src={mediaUrl(scope, path)} alt={title} />
         </div>
       )}
       {load.status === 'ready' && load.kind === 'md' && (
@@ -130,3 +164,11 @@ export function EditorView(props: { sessionId: string; path: string; title: stri
     </div>
   )
 }
+
+/** The lowercased file extension of a path ('' when none). */
+function extOfPath(path: string): string {
+  const at = path.lastIndexOf('.')
+  if (at === -1) return ''
+  const base = path.slice(at).toLowerCase()
+  return base.includes('/') || base.includes('\\') ? '' : base
+}
diff --git a/src/client/ExplorerView.tsx b/src/client/ExplorerView.tsx
index 0e4ed73..403cf12 100644
--- a/src/client/ExplorerView.tsx
+++ b/src/client/ExplorerView.tsx
@@ -45,12 +45,12 @@ export function ExplorerView(props: {
   const loadDir = useCallback((dir: string) => {
     if (dataRef.current[dir] !== undefined) return
     storeLevel(dir, {})
-    api.fsTree(sessionId, dir).then((listing) => {
+    api.fsTree({ sessionId, cwd }, dir).then((listing) => {
       storeLevel(dir, { entries: listing.entries })
     }).catch((error: unknown) => {
       storeLevel(dir, { error: error instanceof Error ? error.message : String(error) })
     })
-  }, [sessionId, storeLevel])
+  }, [sessionId, cwd, storeLevel])
 
   useEffect(() => {
     // Load the visible set; already-loaded levels (kept in the cache) are
diff --git a/src/client/GitView.tsx b/src/client/GitView.tsx
index bb2d559..8940d87 100644
--- a/src/client/GitView.tsx
+++ b/src/client/GitView.tsx
@@ -11,7 +11,7 @@ import {
   DiffBlock, IconBranchOutline16, IconRefreshOutline16, IconTrashOutline16,
   Input,
 } from '@deepseek-ai/dsh-client-ui-primitives'
-import type { GitLogEntry, GitStatusEntry, GitStatusResult } from './api.ts'
+import type { GitLogEntry, GitStatusEntry, GitStatusResult, SessionScope } from './api.ts'
 import { api } from './api.ts'
 import { t } from './locales.ts'
 import css from './sidebar.module.css'
@@ -25,8 +25,8 @@ function badgeOf(entry: GitStatusEntry): string {
   return '?'
 }
 
-export function GitView(props: { sessionId: string }) {
-  const { sessionId } = props
+export function GitView(props: { scope: SessionScope }) {
+  const { scope } = props
   const [status, setStatus] = useState<GitStatusResult | null>(null)
   const [loading, setLoading] = useState(true)
   const [error, setError] = useState<string | null>(null)
@@ -47,9 +47,9 @@ export function GitView(props: { sessionId: string }) {
     setError(null)
     try {
       const [statusResult, branchResult, logResult] = await Promise.all([
-        api.gitStatus(sessionId),
-        api.gitBranch(sessionId).catch(() => ({ current: '', names: [] as string[] })),
-        api.gitLog(sessionId).catch(() => [] as GitLogEntry[]),
+        api.gitStatus(scope),
+        api.gitBranch(scope).catch(() => ({ current: '', names: [] as string[] })),
+        api.gitLog(scope).catch(() => [] as GitLogEntry[]),
       ])
       setStatus(statusResult)
       setBranchNames(branchResult.names)
@@ -59,7 +59,7 @@ export function GitView(props: { sessionId: string }) {
     } finally {
       setLoading(false)
     }
-  }, [sessionId])
+  }, [scope.sessionId, scope.cwd])
 
   useEffect(() => { void refresh() }, [refresh])
 
@@ -69,12 +69,12 @@ export function GitView(props: { sessionId: string }) {
     setDiffStaged(staged)
     setDiffOld(null)
     setDiffNew('')
-    const oldPromise = api.gitShow(sessionId, 'HEAD', entry.path)
+    const oldPromise = api.gitShow(scope, 'HEAD', entry.path)
       .then(result => result.content)
       .catch(() => null)
     const newPromise = staged
-      ? api.gitShow(sessionId, ':', entry.path).then(result => result.content ?? '').catch(() => '')
-      : api.fsRead(sessionId, entry.path)
+      ? api.gitShow(scope, ':', entry.path).then(result => result.content ?? '').catch(() => '')
+      : api.fsRead(scope, entry.path)
         .then(result => (result.kind === 'text' ? result.content : ''))
         .catch(() => '')
     const [oldText, newText] = await Promise.all([oldPromise, newPromise])
@@ -85,8 +85,8 @@ export function GitView(props: { sessionId: string }) {
   const stageEntry = async (entry: GitStatusEntry, staged: boolean): Promise<void> => {
     setBusy(true)
     try {
-      if (staged) await api.gitUnstage(sessionId, entry.path)
-      else await api.gitStage(sessionId, entry.path)
+      if (staged) await api.gitUnstage(scope, entry.path)
+      else await api.gitStage(scope, entry.path)
       setDiffPath(null)
       await refresh()
     } finally {
@@ -97,8 +97,8 @@ export function GitView(props: { sessionId: string }) {
   const stageAll = async (staged: boolean): Promise<void> => {
     setBusy(true)
     try {
-      if (staged) await api.gitUnstage(sessionId)
-      else await api.gitStage(sessionId)
+      if (staged) await api.gitUnstage(scope)
+      else await api.gitStage(scope)
       setDiffPath(null)
       await refresh()
     } finally {
@@ -112,7 +112,7 @@ export function GitView(props: { sessionId: string }) {
     setBusy(true)
     setCommitError(null)
     try {
-      await api.gitCommit(sessionId, message)
+      await api.gitCommit(scope, message)
       setCommitMsg('')
       setDiffPath(null)
       await refresh()
@@ -128,7 +128,7 @@ export function GitView(props: { sessionId: string }) {
     setBusy(true)
     setCommitError(null)
     try {
-      await api.gitCheckout(sessionId, branch)
+      await api.gitCheckout(scope, branch)
       setDiffPath(null)
       await refresh()
     } catch (reason) {
diff --git a/src/client/Sidebar.tsx b/src/client/Sidebar.tsx
index fd0d31c..c12c70d 100644
--- a/src/client/Sidebar.tsx
+++ b/src/client/Sidebar.tsx
@@ -27,6 +27,7 @@ import { EditorView } from './EditorView.tsx'
 import { TerminalView } from './TerminalView.tsx'
 import { GitView } from './GitView.tsx'
 import { t } from './locales.ts'
+import { api } from './api.ts'
 import { openSidebarFile } from './intercept.tsx'
 import css from './sidebar.module.css'
 
@@ -40,6 +41,7 @@ function TabContent(props: {
   ctx: Context
 }) {
   const { tab, sessionId, cwd, expanded, onToggleDir, ctx } = props
+  const scope = { sessionId, cwd }
   switch (tab.type) {
     case 'explorer':
       return (
@@ -52,11 +54,11 @@ function TabContent(props: {
         />
       )
     case 'git':
-      return <GitView sessionId={sessionId} />
+      return <GitView scope={scope} />
     case 'terminal':
-      return <TerminalView sessionId={sessionId} tabId={tab.id} />
+      return <TerminalView scope={scope} tabId={tab.id} />
     case 'editor':
-      return <EditorView sessionId={sessionId} path={tab.path ?? ''} title={tab.title} />
+      return <EditorView scope={scope} path={tab.path ?? ''} title={tab.title} />
   }
 }
 
@@ -95,7 +97,23 @@ export function Sidebar(props: { ctx: Context }) {
 
   const state = snapshot.state
   const sessionId = snapshot.sessionId
-  const cwd = sessionId === undefined ? undefined : sessionList.byId[sessionId]?.cwd
+  const summaryCwd = sessionId === undefined ? undefined : sessionList.byId[sessionId]?.cwd
+
+  // While the session's header is still hydrating (or the session is blank),
+  // the list summary may carry no cwd; ask the host once (it falls back to
+  // the process cwd) so the explorer root and terminal cwd are real from
+  // first paint instead of showing "no session".
+  const [fetchedCwd, setFetchedCwd] = useState<string | undefined>(undefined)
+  useEffect(() => {
+    setFetchedCwd(undefined)
+    if (sessionId === undefined || summaryCwd !== undefined) return
+    let cancelled = false
+    api.sessionCwd({ sessionId })
+      .then(result => { if (!cancelled) setFetchedCwd(result.cwd) })
+      .catch(() => { /* the explorer/git rows surface their own errors */ })
+    return () => { cancelled = true }
+  }, [sessionId, summaryCwd])
+  const cwd = summaryCwd ?? fetchedCwd
 
   // Panel width drag (left edge strip).
   const widthDrag = useRef({ startX: 0, startWidth: 0 })
@@ -103,9 +121,13 @@ export function Sidebar(props: { ctx: Context }) {
 
   // Layout push: the app shell gives up the panel's width while it is open
   // (0 while collapsed), so the conversation and input bar are squeezed
-  // instead of covered. Dragging disables the layout transition.
+  // instead of covered. The margin is capped at the viewport so a stale
+  // persisted width (e.g. fullscreen on a bigger window) can never crush
+  // the app shell to zero. Dragging disables the layout transition.
   useEffect(() => {
-    const width = snapshot.state?.panelOpen === true ? snapshot.state.width : 0
+    const width = snapshot.state?.panelOpen === true
+      ? Math.min(snapshot.state.width, window.innerWidth)
+      : 0
     document.documentElement.style.setProperty('--dsh-sidebar-width', `${width}px`)
   }, [snapshot.state?.panelOpen, snapshot.state?.width])
   useEffect(() => {
@@ -215,7 +237,7 @@ export function Sidebar(props: { ctx: Context }) {
       */}
       <div
         className={clsx(css.panel, !state.panelOpen && css.panelHidden)}
-        style={{ width: state.width }}
+        style={{ width: Math.min(state.width, window.innerWidth) }}
         data-dragging={draggingWidth || undefined}
       >
           <div
diff --git a/src/client/TerminalView.tsx b/src/client/TerminalView.tsx
index ac14191..9dae45c 100644
--- a/src/client/TerminalView.tsx
+++ b/src/client/TerminalView.tsx
@@ -5,17 +5,22 @@
  * type:"resize". Transient disconnects (page refresh, host restart) reconnect
  * automatically; a server-side refusal (close code 1011 with a reason, e.g.
  * a failed pty spawn) stops the loop and shows the reason with a manual
- * retry, so the banner never spins forever.
+ * retry, and repeated unreasoned failures surface the close code after three
+ * attempts, so the banner never spins forever.
  */
 import { useEffect, useRef, useState } from 'react'
 import { Terminal } from 'xterm'
 import { FitAddon } from '@xterm/addon-fit'
 import 'xterm/css/xterm.css'
 import { t } from './locales.ts'
+import type { SessionScope } from './api.ts'
 import css from './sidebar.module.css'
 
-export function TerminalView(props: { sessionId: string; tabId: string }) {
-  const { sessionId, tabId } = props
+/** How many consecutive unreasoned failures before showing the error banner. */
+const FAILURE_LIMIT = 3
+
+export function TerminalView(props: { scope: SessionScope; tabId: string }) {
+  const { scope, tabId } = props
   const hostRef = useRef<HTMLDivElement>(null)
   const [connected, setConnected] = useState(false)
   const [fatal, setFatal] = useState<string | null>(null)
@@ -40,10 +45,12 @@ export function TerminalView(props: { sessionId: string; tabId: string }) {
     let socket: WebSocket | null = null
     let closed = false
     let retry: number | undefined
+    let failures = 0
 
     const wsUrl = (): string => {
       const proto = location.protocol === 'https:' ? 'wss' : 'ws'
-      const params = new URLSearchParams({ sessionId, tab: tabId })
+      const params = new URLSearchParams({ sessionId: scope.sessionId, tab: tabId })
+      if (scope.cwd !== undefined && scope.cwd !== '') params.set('cwd', scope.cwd)
       return `${proto}//${location.host}/sidebar/ws/terminal?${params.toString()}`
     }
 
@@ -57,6 +64,7 @@ export function TerminalView(props: { sessionId: string; tabId: string }) {
       if (closed) return
       socket = new WebSocket(wsUrl())
       socket.onopen = () => {
+        failures = 0
         setConnected(true)
         setFatal(null)
         sendResize()
@@ -72,6 +80,15 @@ export function TerminalView(props: { sessionId: string; tabId: string }) {
           setFatal(event.reason)
           return
         }
+        // Unreasoned drops (upgrade rejected, host down, mid-handshake
+        // refusal) normally recover on the next attempt; after a few
+        // consecutive failures stop spinning and show the close code.
+        failures += 1
+        if (failures >= FAILURE_LIMIT) {
+          const detail = event.reason !== '' ? ` (${event.code}: ${event.reason})` : ` (${event.code})`
+          setFatal(`${t('terminalConnectFailed')}${detail}`)
+          return
+        }
         if (!closed) retry = window.setTimeout(connect, 2000)
       }
       socket.onerror = () => {
@@ -109,7 +126,7 @@ export function TerminalView(props: { sessionId: string; tabId: string }) {
       term.dispose()
       connectRef.current = null
     }
-  }, [sessionId, tabId])
+  }, [scope.sessionId, scope.cwd, tabId])
 
   return (
     <div className={css.terminalWrap}>
diff --git a/src/client/api.ts b/src/client/api.ts
index c1308b3..36057f8 100644
--- a/src/client/api.ts
+++ b/src/client/api.ts
@@ -1,7 +1,10 @@
 /**
  * Typed fetch wrapper over the /sidebar JSON API. Every call posts to
- * `/sidebar/api/<method>` with the sessionId; failures surface as
- * {@link SidebarApiError} with the wire error code.
+ * `/sidebar/api/<method>` with the sessionId and — when known — the session's
+ * cwd from the client's own list summary. The host prefers its attached
+ * session header and uses the summary cwd only while the session is still
+ * hydrating at page load (a detached session would otherwise fail the
+ * request). Failures surface as {@link SidebarApiError} with the wire code.
  */
 
 /** One wire failure. */
@@ -71,37 +74,51 @@ async function call<T>(method: string, payload: Record<string, unknown>, signal?
   return parsed.value as T
 }
 
-/** The sidebar API surface (sessionId is threaded through every call). */
+/** One request's session scope: the conversation id plus its cwd when known. */
+export interface SessionScope {
+  sessionId: string
+  /** The session's working directory from the client list summary (optional). */
+  cwd?: string
+}
+
+/** Fold a scope into a JSON payload ({cwd} only when present). */
+function scopePayload(scope: SessionScope, extra: Record<string, unknown>): Record<string, unknown> {
+  return { sessionId: scope.sessionId, ...(scope.cwd !== undefined && scope.cwd !== '' ? { cwd: scope.cwd } : {}), ...extra }
+}
+
+/** The sidebar API surface (session scope threaded through every call). */
 export const api = {
-  sessionCwd: (sessionId: string, signal?: AbortSignal) =>
-    call<{ sessionId: string; cwd: string; root: string; parent: string | null }>('session.cwd', { sessionId }, signal),
-  fsTree: (sessionId: string, path: string, signal?: AbortSignal) =>
-    call<{ path: string; entries: FsEntry[]; truncated: boolean }>('fs.tree', { sessionId, path }, signal),
-  fsRead: (sessionId: string, path: string, signal?: AbortSignal) =>
-    call<FsTextResult | FsBinaryResult>('fs.read', { sessionId, path }, signal),
-  fsWrite: (sessionId: string, path: string, content: string) =>
-    call<{ ok: true }>('fs.write', { sessionId, path, content }),
-  gitStatus: (sessionId: string, signal?: AbortSignal) =>
-    call<GitStatusResult>('git.status', { sessionId }, signal),
-  gitDiff: (sessionId: string, path: string | undefined, staged: boolean, signal?: AbortSignal) =>
-    call<{ diff: string }>('git.diff', { sessionId, ...(path !== undefined ? { path } : {}), staged }, signal),
-  gitShow: (sessionId: string, rev: string, path: string, signal?: AbortSignal) =>
-    call<{ content: string | null }>('git.show', { sessionId, rev, path }, signal),
-  gitStage: (sessionId: string, path?: string) =>
-    call<{ ok: true }>('git.stage', { sessionId, ...(path !== undefined ? { path } : {}) }),
-  gitUnstage: (sessionId: string, path?: string) =>
-    call<{ ok: true }>('git.unstage', { sessionId, ...(path !== undefined ? { path } : {}) }),
-  gitCommit: (sessionId: string, message: string) =>
-    call<{ ok: true }>('git.commit', { sessionId, message }),
-  gitBranch: (sessionId: string, signal?: AbortSignal) =>
-    call<{ current: string; names: string[] }>('git.branch', { sessionId }, signal),
-  gitCheckout: (sessionId: string, branch: string) =>
-    call<{ ok: true }>('git.checkout', { sessionId, branch }),
-  gitLog: (sessionId: string, signal?: AbortSignal) =>
-    call<GitLogEntry[]>('git.log', { sessionId }, signal),
+  sessionCwd: (scope: SessionScope, signal?: AbortSignal) =>
+    call<{ sessionId: string; cwd: string; root: string; parent: string | null }>('session.cwd', scopePayload(scope, {}), signal),
+  fsTree: (scope: SessionScope, path: string, signal?: AbortSignal) =>
+    call<{ path: string; entries: FsEntry[]; truncated: boolean }>('fs.tree', scopePayload(scope, { path }), signal),
+  fsRead: (scope: SessionScope, path: string, signal?: AbortSignal) =>
+    call<FsTextResult | FsBinaryResult>('fs.read', scopePayload(scope, { path }), signal),
+  fsWrite: (scope: SessionScope, path: string, content: string) =>
+    call<{ ok: true }>('fs.write', scopePayload(scope, { path, content })),
+  gitStatus: (scope: SessionScope, signal?: AbortSignal) =>
+    call<GitStatusResult>('git.status', scopePayload(scope, {}), signal),
+  gitDiff: (scope: SessionScope, path: string | undefined, staged: boolean, signal?: AbortSignal) =>
+    call<{ diff: string }>('git.diff', scopePayload(scope, { ...(path !== undefined ? { path } : {}), staged }), signal),
+  gitShow: (scope: SessionScope, rev: string, path: string, signal?: AbortSignal) =>
+    call<{ content: string | null }>('git.show', scopePayload(scope, { rev, path }), signal),
+  gitStage: (scope: SessionScope, path?: string) =>
+    call<{ ok: true }>('git.stage', scopePayload(scope, { ...(path !== undefined ? { path } : {}) })),
+  gitUnstage: (scope: SessionScope, path?: string) =>
+    call<{ ok: true }>('git.unstage', scopePayload(scope, { ...(path !== undefined ? { path } : {}) })),
+  gitCommit: (scope: SessionScope, message: string) =>
+    call<{ ok: true }>('git.commit', scopePayload(scope, { message })),
+  gitBranch: (scope: SessionScope, signal?: AbortSignal) =>
+    call<{ current: string; names: string[] }>('git.branch', scopePayload(scope, {}), signal),
+  gitCheckout: (scope: SessionScope, branch: string) =>
+    call<{ ok: true }>('git.checkout', scopePayload(scope, { branch })),
+  gitLog: (scope: SessionScope, signal?: AbortSignal) =>
+    call<GitLogEntry[]>('git.log', scopePayload(scope, {}), signal),
 }
 
 /** Absolute URL of the media route for one path (images only). */
-export function mediaUrl(sessionId: string, path: string): string {
-  return `/sidebar/file?sessionId=${encodeURIComponent(sessionId)}&path=${encodeURIComponent(path)}`
+export function mediaUrl(scope: SessionScope, path: string): string {
+  const params = new URLSearchParams({ sessionId: scope.sessionId, path })
+  if (scope.cwd !== undefined && scope.cwd !== '') params.set('cwd', scope.cwd)
+  return `/sidebar/file?${params.toString()}`
 }
diff --git a/src/client/index.tsx b/src/client/index.tsx
index 54b52c4..2726a44 100644
--- a/src/client/index.tsx
+++ b/src/client/index.tsx
@@ -57,20 +57,52 @@ class SidebarBoundary extends Component<{ children: ReactNode }, { error: string
  * @param ctx - the client cordis context (slots, sessions).
  */
 export function apply(ctx: Context): void {
-  ctx.effect(() => {
-    const host = document.createElement('div')
-    host.setAttribute('data-dsh-better-sidebar', '')
-    document.body.appendChild(host)
-    const root = createRoot(host)
-    root.render(createElement(SidebarBoundary, null, createElement(Sidebar, { ctx })))
-    return () => {
-      root.unmount()
-      host.remove()
+  // A failure anywhere in the client lifecycle must never take the app down
+  // silently: log with the plugin prefix and pin a visible diagnostic strip
+  // to the page so a blank panel is never the only symptom.
+  const fail = (phase: string, error: unknown): void => {
+    console.error(`[dsh-better-sidebar] ${phase} error:`, error)
+    try {
+      const bar = document.createElement('div')
+      bar.style.cssText = 'position:fixed;left:8px;bottom:8px;z-index:2147483000;max-width:70vw;padding:8px 12px;'
+        + 'font:12px/1.5 ui-monospace,SFMono-Regular,Menlo,monospace;color:#f2a1a1;background:#1b1b22;'
+        + 'border:1px solid #f2a1a1;border-radius:8px;white-space:pre-wrap'
+      bar.textContent = `[dsh-better-sidebar] ${phase} error: ${error instanceof Error ? error.message : String(error)}`
+      document.body.appendChild(bar)
+    } catch {
+      // Nothing left to report with.
     }
-  }, 'dsh-better-sidebar: sidebar mount')
+  }
+  try {
+    ctx.effect(() => {
+      try {
+        const host = document.createElement('div')
+        host.setAttribute('data-dsh-better-sidebar', '')
+        document.body.appendChild(host)
+        const root = createRoot(host)
+        root.render(createElement(SidebarBoundary, null, createElement(Sidebar, { ctx })))
+        return () => {
+          root.unmount()
+          host.remove()
+        }
+      } catch (error) {
+        fail('mount', error)
+        return undefined
+      }
+    }, 'dsh-better-sidebar: sidebar mount')
 
-  ctx.effect(
-    () => registerTurnTailInterception(ctx),
-    'dsh-better-sidebar: turn-tail interception',
-  )
+    ctx.effect(
+      () => {
+        try {
+          return registerTurnTailInterception(ctx)
+        } catch (error) {
+          fail('interception', error)
+          return undefined
+        }
+      },
+      'dsh-better-sidebar: turn-tail interception',
+    )
+  } catch (error) {
+    fail('load', error)
+  }
 }
diff --git a/src/client/lang.ts b/src/client/lang.ts
new file mode 100644
index 0000000..f8f811d
--- /dev/null
+++ b/src/client/lang.ts
@@ -0,0 +1,92 @@
+/**
+ * Syntax highlighting for the file editor: extension → CodeMirror language
+ * mapping. The key derivation is pure and unit-tested; the factories pull in
+ * the CodeMirror language packages (bundled into the client).
+ */
+import { Language, LanguageSupport, StreamLanguage } from '@codemirror/language'
+import { javascript } from '@codemirror/lang-javascript'
+import { json } from '@codemirror/lang-json'
+import { python } from '@codemirror/lang-python'
+import { html } from '@codemirror/lang-html'
+import { css } from '@codemirror/lang-css'
+import { xml } from '@codemirror/lang-xml'
+import { yaml } from '@codemirror/lang-yaml'
+import { sql } from '@codemirror/lang-sql'
+import { java } from '@codemirror/lang-java'
+import { cpp } from '@codemirror/lang-cpp'
+import { rust } from '@codemirror/lang-rust'
+import { go } from '@codemirror/lang-go'
+import { php } from '@codemirror/lang-php'
+import { shell } from '@codemirror/legacy-modes/mode/shell'
+import { toml } from '@codemirror/legacy-modes/mode/toml'
+import { nginx } from '@codemirror/legacy-modes/mode/nginx'
+import { dockerFile } from '@codemirror/legacy-modes/mode/dockerfile'
+import { properties } from '@codemirror/legacy-modes/mode/properties'
+
+/** The lowercased file extension of a path ('' when none). */
+export function extOf(path: string): string {
+  const at = path.lastIndexOf('.')
+  if (at === -1) return ''
+  const base = path.slice(at + 1).toLowerCase()
+  return base.includes('/') || base.includes('\\') ? '' : base
+}
+
+/** Language key for an extension, or null for plain text. Pure (tested). */
+export function languageKeyForExt(ext: string): string | null {
+  switch (ext) {
+    case 'js': case 'mjs': case 'cjs': return 'js'
+    case 'jsx': return 'jsx'
+    case 'ts': case 'mts': case 'cts': return 'ts'
+    case 'tsx': return 'tsx'
+    case 'json': case 'jsonc': return 'json'
+    case 'py': case 'pyw': return 'python'
+    case 'html': case 'htm': return 'html'
+    case 'css': return 'css'
+    case 'xml': case 'xsl': return 'xml'
+    case 'yaml': case 'yml': return 'yaml'
+    case 'sql': return 'sql'
+    case 'java': return 'java'
+    case 'c': case 'h': return 'c'
+    case 'cc': case 'cpp': case 'cxx': case 'hpp': case 'hh': case 'hxx': return 'cpp'
+    case 'rs': return 'rust'
+    case 'go': return 'go'
+    case 'php': return 'php'
+    case 'sh': case 'bash': case 'zsh': return 'shell'
+    case 'toml': return 'toml'
+    case 'nginx': case 'conf': return 'nginx'
+    case 'dockerfile': case 'docker': return 'dockerfile'
+    case 'properties': case 'env': return 'properties'
+    default: return null
+  }
+}
+
+const FACTORIES: Record<string, () => Language | LanguageSupport> = {
+  js: () => javascript({ jsx: true }),
+  jsx: () => javascript({ jsx: true }),
+  ts: () => javascript({ typescript: true }),
+  tsx: () => javascript({ typescript: true, jsx: true }),
+  json: () => json(),
+  python: () => python(),
+  html: () => html(),
+  css: () => css(),
+  xml: () => xml(),
+  yaml: () => yaml(),
+  sql: () => sql(),
+  java: () => java(),
+  c: () => cpp(),
+  cpp: () => cpp(),
+  rust: () => rust(),
+  go: () => go(),
+  php: () => php(),
+  shell: () => StreamLanguage.define(shell),
+  toml: () => StreamLanguage.define(toml),
+  nginx: () => StreamLanguage.define(nginx),
+  dockerfile: () => StreamLanguage.define(dockerFile),
+  properties: () => StreamLanguage.define(properties),
+}
+
+/** The CodeMirror language support for a path, or null for plain text. */
+export function languageForPath(path: string): Language | LanguageSupport | null {
+  const key = languageKeyForExt(extOf(path))
+  return key === null ? null : FACTORIES[key]!()
+}
diff --git a/src/client/locales.ts b/src/client/locales.ts
index 2ab4923..80c13c3 100644
--- a/src/client/locales.ts
+++ b/src/client/locales.ts
@@ -21,6 +21,7 @@ const zh = {
   expandFullscreen: '展开为全屏',
   restoreFullscreen: '还原宽度',
   terminalError: '终端连接失败',
+  terminalConnectFailed: '终端多次连接失败',
   terminalRetry: '重试',
   refresh: '刷新',
   save: '保存',
@@ -82,6 +83,7 @@ const en: Record<keyof typeof zh, string> = {
   expandFullscreen: 'Expand to full width',
   restoreFullscreen: 'Restore width',
   terminalError: 'Terminal connection failed',
+  terminalConnectFailed: 'Terminal failed to connect repeatedly',
   terminalRetry: 'Retry',
   refresh: 'Refresh',
   save: 'Save',
diff --git a/src/client/sidebar.module.css b/src/client/sidebar.module.css
index c66275a..5213baa 100644
--- a/src/client/sidebar.module.css
+++ b/src/client/sidebar.module.css
@@ -538,19 +538,21 @@
   background: var(--dsw-alias-state-warn-tertiary);
 }
 
-.editorTextarea {
+.editorCm {
   flex: 1;
   min-height: 0;
-  width: 100%;
-  padding: 10px 12px;
-  border: none;
-  outline: none;
-  resize: none;
+  overflow: hidden;
   background: transparent;
-  color: var(--dsw-alias-label-primary);
-  font: var(--dsw-font-markdown-code-block-small);
-  tab-size: 2;
-  white-space: pre;
+}
+
+/* CodeMirror fills the pane; the theme supplies its own token palette
+   (a designed dark theme, same policy as the xterm palette). */
+.editorCm :global(.cm-editor) {
+  height: 100%;
+}
+
+.editorCm :global(.cm-editor.cm-focused) {
+  outline: none;
 }
 
 .editorImageWrap {
diff --git a/src/client/state.ts b/src/client/state.ts
index 1ad90e3..78b23a4 100644
--- a/src/client/state.ts
+++ b/src/client/state.ts
@@ -366,10 +366,8 @@ function loadState(sessionId: string): SidebarState {
   try {
     const raw = localStorage.getItem(`${STORAGE_PREFIX}:${sessionId}`)
     if (raw !== null) {
-      const parsed = JSON.parse(raw) as SidebarState
-      if (parsed !== null && typeof parsed === 'object' && 'splits' in parsed && 'width' in parsed) {
-        return parsed
-      }
+      const sanitized = sanitizeState(JSON.parse(raw) as unknown)
+      if (sanitized !== undefined) return sanitized
     }
   } catch {
     // Corrupt or unavailable storage: fall through to the default.
@@ -383,6 +381,86 @@ function loadState(sessionId: string): SidebarState {
   return makeDefaultState(contentHalf)
 }
 
+/**
+ * Structural validation of one persisted state. A malformed or stale shape
+ * (older layouts, hand-edited storage) must fall back to the default instead
+ * of crashing the panel on every reload; the restored width is also clamped
+ * to the current viewport so a stale fullscreen width can never crush the
+ * app shell (margin-right larger than the window) or cover the whole screen.
+ * @returns a clean state, or undefined to fall back to the default.
+ */
+export function sanitizeState(parsed: unknown): SidebarState | undefined {
+  if (parsed === null || typeof parsed !== 'object') return undefined
+  const record = parsed as Record<string, unknown>
+  if (typeof record.panelOpen !== 'boolean') return undefined
+  if (typeof record.width !== 'number' || !Number.isFinite(record.width)) return undefined
+  if (typeof record.nextTerminal !== 'number' || !Number.isInteger(record.nextTerminal) || record.nextTerminal < 1) {
+    return undefined
+  }
+  if (typeof record.activePane !== 'string' && record.activePane !== null) return undefined
+  if (!Array.isArray(record.expanded) || record.expanded.some(item => typeof item !== 'string')) return undefined
+  const splits = sanitizeNode(record.splits)
+  if (splits === undefined) return undefined
+  const maxWidth = typeof window !== 'undefined' ? window.innerWidth : Infinity
+  return {
+    panelOpen: record.panelOpen,
+    width: Math.max(PANEL_MIN, Math.min(record.width, maxWidth)),
+    activePane: typeof record.activePane === 'string' ? record.activePane : null,
+    nextTerminal: record.nextTerminal,
+    expanded: record.expanded as string[],
+    splits,
+  }
+}
+
+/** Validate one split-tree node (leaf or split) and rebuild it cleanly. */
+function sanitizeNode(node: unknown): SplitNode | undefined {
+  if (node === null || typeof node !== 'object') return undefined
+  const record = node as Record<string, unknown>
+  if (record.kind === 'leaf') {
+    if (typeof record.id !== 'string' || !Array.isArray(record.tabs)) return undefined
+    const tabs: SidebarTab[] = []
+    for (const tab of record.tabs) {
+      if (tab === null || typeof tab !== 'object') return undefined
+      const candidate = tab as Record<string, unknown>
+      if (typeof candidate.id !== 'string' || typeof candidate.title !== 'string') return undefined
+      if (
+        candidate.type !== 'explorer' && candidate.type !== 'git'
+        && candidate.type !== 'editor' && candidate.type !== 'terminal'
+      ) {
+        return undefined
+      }
+      tabs.push({
+        id: candidate.id,
+        type: candidate.type,
+        title: candidate.title,
+        ...(typeof candidate.path === 'string' ? { path: candidate.path } : {}),
+      })
+    }
+    const active = typeof record.active === 'string' ? record.active : null
+    if (active !== null && !tabs.some(tab => tab.id === active)) return undefined
+    return { kind: 'leaf', id: record.id, tabs, active }
+  }
+  if (record.kind === 'split') {
+    if (typeof record.id !== 'string' || (record.dir !== 'row' && record.dir !== 'col')) return undefined
+    if (!Array.isArray(record.children) || !Array.isArray(record.sizes)) return undefined
+    const children: SplitNode[] = []
+    for (const child of record.children) {
+      const clean = sanitizeNode(child)
+      if (clean === undefined) return undefined
+      children.push(clean)
+    }
+    if (children.length < 2) return undefined
+    if (
+      record.sizes.length !== children.length
+      || record.sizes.some(size => typeof size !== 'number' || !Number.isFinite(size) || size <= 0)
+    ) {
+      return undefined
+    }
+    return { kind: 'split', id: record.id, dir: record.dir, sizes: record.sizes as number[], children }
+  }
+  return undefined
+}
+
 /** The session-scoped store: one state per conversation, localStorage-backed. */
 export class SidebarStore {
   private readonly bySession = new Map<string, SidebarState>()
diff --git a/src/index.ts b/src/index.ts
index 8c0ec3b..9ef7464 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -63,14 +63,28 @@ function trustedHostsOf(ctx: Context): string[] {
   return []
 }
 
-/** Resolve a session's authoritative working directory or throw not-found. */
-function sessionCwdOf(ctx: Context, sessionId: string): string {
+/**
+ * Resolve a session's authoritative working directory. The attached session
+ * header wins; while the session is still hydrating from persistence (the
+ * web client attaches the current conversation a moment after page load, so
+ * the very first sidebar requests can arrive detached) the caller's own
+ * list-summary cwd is used; the process cwd is the last resort (blank
+ * sessions have no cwd anywhere yet). Never throws for a missing cwd, so
+ * explorer/git/terminal work from first paint instead of surfacing
+ * "session ... has no working directory".
+ */
+function sessionCwdOf(ctx: Context, sessionId: string, clientCwd?: string): string {
   const session = ctx.sessions.get(sessionId)
-  const cwd = session?.header.cwd
-  if (cwd === undefined) {
-    throw new SidebarError('not-found', `session "${sessionId}" has no working directory`, 404)
+  const headerCwd = session?.header.cwd
+  if (headerCwd !== undefined && headerCwd !== '') return headerCwd
+  if (clientCwd !== undefined && clientCwd !== '') {
+    try {
+      return requireAbsolute(clientCwd)
+    } catch {
+      throw new SidebarError('bad-request', `invalid working directory "${clientCwd}"`)
+    }
   }
-  return cwd
+  return process.cwd()
 }
 
 /** Text read of a file with the size cap; binary detection via NUL probe. */
@@ -104,7 +118,9 @@ type ApiMethod = (payload: unknown) => Promise<unknown> | unknown
 function buildApi(ctx: Context, ptyManager: PtyManager): Record<string, ApiMethod> {
   const cwdOf = (payload: unknown): { sessionId: string; cwd: string } => {
     const sessionId = requireString(payload, 'sessionId')
-    return { sessionId, cwd: sessionCwdOf(ctx, sessionId) }
+    const record = payload as { cwd?: unknown } | null
+    const clientCwd = typeof record?.cwd === 'string' && record.cwd !== '' ? record.cwd : undefined
+    return { sessionId, cwd: sessionCwdOf(ctx, sessionId, clientCwd) }
   }
   return {
     'session.cwd': (payload) => {
@@ -256,7 +272,7 @@ export function apply(ctx: Context): void {
         const sessionId = url.searchParams.get('sessionId')
         const raw = url.searchParams.get('path')
         if (sessionId === null || raw === null) throw new SidebarError('bad-request', 'sessionId and path are required')
-        const cwd = sessionCwdOf(ctx, sessionId)
+        const cwd = sessionCwdOf(ctx, sessionId, url.searchParams.get('cwd') ?? undefined)
         const path = requireAbsolute(raw)
         if (!path.startsWith(cwd)) {
           // Only files under the session cwd are served as media (the editor
@@ -311,7 +327,7 @@ async function attachTerminal(
       ws.close(1008, 'sessionId and tab are required')
       return
     }
-    const cwd = sessionCwdOf(ctx, sessionId)
+    const cwd = sessionCwdOf(ctx, sessionId, url.searchParams.get('cwd') ?? undefined)
     const handle = ptyManager.open(sessionId, tabId, cwd, 80, 24)
     // Replay the transcript, then follow live output.
     if (handle.transcript !== '') ws.send(handle.transcript)
diff --git a/tests/smoke.spec.ts b/tests/smoke.spec.ts
index 789b5cd..10e78b9 100644
--- a/tests/smoke.spec.ts
+++ b/tests/smoke.spec.ts
@@ -16,7 +16,7 @@ interface FakeContext {
     register: (route: SidebarWebRoute) => () => void
     registerUpgrade: (route: SidebarWebUpgradeRoute) => () => void
   }
-  sessions: { get: (id: string) => undefined }
+  sessions: { get: (id: string) => { header: { cwd?: string } } | undefined }
   effect: (fn: () => void | (() => void), label?: string) => void
 }
 
@@ -126,3 +126,74 @@ describe('host plugin smoke', () => {
     expect(listing.truncated).toBe(false)
   })
 })
+
+describe('session cwd resolution over the API route', () => {
+  interface CtxOverrides {
+    sessions?: { get: (id: string) => { header: { cwd?: string } } | undefined }
+  }
+
+  const mount = (overrides: CtxOverrides = {}): SidebarWebRoute => {
+    const routes: SidebarWebRoute[] = []
+    const ctx = {
+      loader: { entries: () => [] },
+      httpServer: {
+        register: (route: SidebarWebRoute) => { routes.push(route); return () => {} },
+        registerUpgrade: (route: SidebarWebUpgradeRoute) => { void route; return () => {} },
+      },
+      sessions: overrides.sessions ?? { get: () => undefined },
+      // The vendored cordis runs registration effects immediately.
+      effect: (fn: () => void | (() => void)) => { fn() },
+    }
+    apply(ctx as never)
+    return routes.find(route => route.path === '/sidebar/api')!
+  }
+
+  const invoke = async (route: SidebarWebRoute, payload: unknown): Promise<{ ok: boolean; value?: { cwd: string }; error?: { message: string } }> => {
+    const body = Buffer.from(JSON.stringify(payload))
+    const req = {
+      method: 'POST',
+      url: '/sidebar/api/session.cwd',
+      headers: { host: '127.0.0.1:3080' },
+      [Symbol.asyncIterator]: async function* () { yield body },
+    } as never
+    const out: { status: number; body: string } = { status: 200, body: '' }
+    const res = {
+      writeHead: (status: number) => { out.status = status },
+      end: (chunk: unknown) => { out.body += String(chunk ?? '') },
+    } as never
+    await route.handler(req, res)
+    return JSON.parse(out.body) as { ok: boolean; value?: { cwd: string }; error?: { message: string } }
+  }
+
+  it('uses the client summary cwd while the session is detached', async () => {
+    const route = mount()
+    const result = await invoke(route, { sessionId: 's-detached', cwd: '/tmp/summary-cwd' })
+    expect(result.ok).toBe(true)
+    expect(result.value?.cwd).toBe('/tmp/summary-cwd')
+  })
+
+  it('falls back to the process cwd with no summary cwd', async () => {
+    const route = mount()
+    const result = await invoke(route, { sessionId: 's-unknown' })
+    expect(result.ok).toBe(true)
+    expect(result.value?.cwd).toBe(process.cwd())
+  })
+
+  it('prefers the attached session header over the client summary', async () => {
+    const route = mount({
+      sessions: {
+        get: (id) => id === 's-attached' ? { header: { cwd: '/attached-cwd' } } : undefined,
+      },
+    })
+    const result = await invoke(route, { sessionId: 's-attached', cwd: '/tmp/summary-cwd' })
+    expect(result.ok).toBe(true)
+    expect(result.value?.cwd).toBe('/attached-cwd')
+  })
+
+  it('rejects a non-absolute client cwd', async () => {
+    const route = mount()
+    const result = await invoke(route, { sessionId: 's-detached', cwd: 'relative/path' })
+    expect(result.ok).toBe(false)
+    expect(result.error?.message).toMatch(/invalid working directory/)
+  })
+})
diff --git a/tests/unit.spec.ts b/tests/unit.spec.ts
index ee15b2e..2e9c216 100644
--- a/tests/unit.spec.ts
+++ b/tests/unit.spec.ts
@@ -3,8 +3,9 @@ import { compareEntries, parentOf, rootLabel, requireAbsolute } from '../src/fs-
 import { parseLogLines, parsePorcelainZ } from '../src/git.ts'
 import {
   activateTab, closeTab, makeDefaultState, moveTab, moveTabToEdge, openTab,
-  resizeSplit, splitPane, toggleExpanded, type SidebarState, type SplitNode,
+  resizeSplit, sanitizeState, splitPane, toggleExpanded, type SidebarState, type SplitNode,
 } from '../src/client/state.ts'
+import { extOf, languageKeyForExt } from '../src/client/lang.ts'
 import { producedForClosing, resolveSidebarPath, selectProducedFiles } from '../src/client/produced-files.ts'
 import { defaultShell, ensureSpawnHelper } from '../src/pty-manager.ts'
 
@@ -260,3 +261,57 @@ describe('produced-files derivation', () => {
     expect(resolveSidebarPath(undefined, 'a.ts')).toBe('a.ts')
   })
 })
+
+describe('persisted state sanitization', () => {
+  it('accepts a well-formed state unchanged (node environment: no width clamp)', () => {
+    const state = makeDefaultState(400)
+    const clean = sanitizeState(JSON.parse(JSON.stringify(state)))
+    expect(clean).toEqual(state)
+  })
+
+  it('clamps undersized widths to the panel minimum', () => {
+    const state = { ...makeDefaultState(400), width: 10 }
+    const clean = sanitizeState(JSON.parse(JSON.stringify(state)))
+    expect(clean?.width).toBe(280)
+  })
+
+  it('rejects malformed shapes instead of crashing the panel', () => {
+    expect(sanitizeState(null)).toBeUndefined()
+    expect(sanitizeState('nope')).toBeUndefined()
+    expect(sanitizeState({})).toBeUndefined()
+    expect(sanitizeState({ ...makeDefaultState(400), width: 'wide' })).toBeUndefined()
+    expect(sanitizeState({ ...makeDefaultState(400), panelOpen: 1 })).toBeUndefined()
+    // A split whose sizes do not match its children is rejected.
+    const withSplit = JSON.parse(JSON.stringify(makeDefaultState(400)))
+    withSplit.splits = { kind: 'split', id: 's1', dir: 'row', sizes: [0.5], children: [] }
+    expect(sanitizeState(withSplit)).toBeUndefined()
+    // Unknown tab types (older layouts) are rejected.
+    const withBadTab = JSON.parse(JSON.stringify(makeDefaultState(400)))
+    withBadTab.splits.tabs[0].type = 'watcher'
+    expect(sanitizeState(withBadTab)).toBeUndefined()
+    // An active id that no tab carries is rejected.
+    const withBadActive = JSON.parse(JSON.stringify(makeDefaultState(400)))
+    withBadActive.splits.active = 'ghost-tab'
+    expect(sanitizeState(withBadActive)).toBeUndefined()
+  })
+})
+
+describe('editor language mapping', () => {
+  it('derives extensions from paths', () => {
+    expect(extOf('/a/b/main.tsx')).toBe('tsx')
+    expect(extOf('README.MD')).toBe('md')
+    expect(extOf('/a/b/.gitignore')).toBe('gitignore')
+    expect(extOf('noext')).toBe('')
+  })
+
+  it('maps common extensions to languages and falls back to plain text', () => {
+    expect(languageKeyForExt('tsx')).toBe('tsx')
+    expect(languageKeyForExt('js')).toBe('js')
+    expect(languageKeyForExt('py')).toBe('python')
+    expect(languageKeyForExt('yaml')).toBe('yaml')
+    expect(languageKeyForExt('sh')).toBe('shell')
+    expect(languageKeyForExt('txt')).toBeNull()
+    expect(languageKeyForExt('log')).toBeNull()
+    expect(languageKeyForExt('')).toBeNull()
+  })
+})