Merge pull request #213 from Ark0N/feat/file-viewer-edit-mode

File Viewer: edit mode for text files (edit + save in the viewer)

src/config/file-editing.ts

@@ -0,0 +1,152 @@
+/**
+ * @fileoverview File Viewer edit-mode policy (issue #212).
+ *
+ * Pure, IO-free policy for which workspace files the in-viewer editor may read
+ * for editing and write back. Consumed by the `edit=1` branch of
+ * `GET /api/sessions/:id/file-content` and by `PUT /api/sessions/:id/file-content`
+ * in `src/web/routes/file-routes.ts`.
+ *
+ * Design (docs/file-viewer-edit-plan.md):
+ * - ALLOWLIST of text extensions/basenames, not a blocklist — matching the
+ *   attachment-guard precedent. `svg` and `env` are deliberately absent: svg is
+ *   treated as untrusted on the read side, and `.env` is sensitive-path blocked
+ *   anyway; excluding them here keeps a single obvious refusal.
+ * - The `.git/` subtree is denied outright: `.git/hooks/*` is code execution and
+ *   a corrupted index looks unrecoverable to a user who wanted to fix a typo.
+ * - EOL helpers exist because a browser <textarea> normalizes to LF; the server
+ *   re-applies the file's original ending so a two-line edit of a CRLF file does
+ *   not become a whole-file diff. Mixed-EOL files normalize to the dominant
+ *   style (documented lossy edge).
+ */
+
+/** Hard cap for edit-mode reads AND writes (bytes of file content). */
+export const MAX_EDITABLE_BYTES = 512 * 1024;
+
+/** Lowercase extensions (no dot) the editor will open and save. */
+export const EDITABLE_EXTENSIONS: ReadonlySet<string> = new Set([
+  // JS/TS ecosystem
+  'ts',
+  'tsx',
+  'js',
+  'jsx',
+  'mjs',
+  'cjs',
+  'json',
+  'jsonc',
+  // Docs / plain text
+  'md',
+  'mdx',
+  'txt',
+  'rst',
+  'adoc',
+  // Web
+  'css',
+  'scss',
+  'less',
+  'html',
+  'htm',
+  'xml',
+  // Config
+  'yml',
+  'yaml',
+  'toml',
+  'ini',
+  'cfg',
+  'conf',
+  'properties',
+  // Shell
+  'sh',
+  'bash',
+  'zsh',
+  'fish',
+  // Languages
+  'py',
+  'rb',
+  'go',
+  'rs',
+  'java',
+  'kt',
+  'swift',
+  'c',
+  'h',
+  'cpp',
+  'hpp',
+  'cc',
+  'cs',
+  'php',
+  'sql',
+  'graphql',
+  'proto',
+  'lua',
+  'pl',
+  'r',
+  'jl',
+  'tf',
+  'gradle',
+  // Data / misc text
+  'csv',
+  'tsv',
+  'log',
+  'diff',
+  'patch',
+]);
+
+/** Extensionless (or dot-led) file names that are still editable text. */
+export const EDITABLE_BASENAMES: ReadonlySet<string> = new Set([
+  'dockerfile',
+  'makefile',
+  'license',
+  'readme',
+  'changelog',
+  'authors',
+  'codeowners',
+  'procfile',
+  '.gitignore',
+  '.gitattributes',
+  '.dockerignore',
+  '.prettierignore',
+  '.prettierrc',
+  '.editorconfig',
+  '.nvmrc',
+  '.npmrc',
+  '.eslintignore',
+]);
+
+/** Whether a file name (basename only) is eligible for in-viewer editing. */
+export function isEditableFileName(fileName: string): boolean {
+  const lower = fileName.toLowerCase();
+  if (EDITABLE_BASENAMES.has(lower)) return true;
+  const dot = lower.lastIndexOf('.');
+  // No extension (or a bare dotfile like `.bashrc`): only the basename list applies.
+  if (dot <= 0) return false;
+  return EDITABLE_EXTENSIONS.has(lower.slice(dot + 1));
+}
+
+/**
+ * Whether a workspace-relative path is denied for editing regardless of its
+ * extension. Currently: anything inside a `.git` directory at any depth.
+ */
+export function isDeniedEditRelativePath(relativePath: string): boolean {
+  return relativePath.split('/').some((segment) => segment === '.git');
+}
+
+export type FileEol = 'lf' | 'crlf';
+
+/** Dominant line-ending style of a text buffer (LF when tied or single-line). */
+export function detectEol(text: string): FileEol {
+  let crlf = 0;
+  let lf = 0;
+  for (let i = 0; i < text.length; i++) {
+    if (text.charCodeAt(i) === 10) {
+      if (i > 0 && text.charCodeAt(i - 1) === 13) crlf++;
+      else lf++;
+    }
+  }
+  return crlf > lf ? 'crlf' : 'lf';
+}
+
+/** Normalize every line ending in `text` to the requested style. */
+export function applyEol(text: string, eol: FileEol): string {
+  const normalized = text.replace(/\r\n/g, '\n');
+  return eol === 'crlf' ? normalized.replace(/\n/g, '\r\n') : normalized;
+}

src/types/common.ts

@@ -97,6 +97,22 @@ export interface FilesystemBrowseData {
   truncated: boolean;
 }
 
+/** Response payload for `PUT /api/sessions/:id/file-content` (File Viewer edit mode). */
+export interface FileWriteData {
+  /** Workspace-relative path as submitted */
+  path: string;
+  /** Size of the written content in bytes */
+  size: number;
+  /** mtime of the file after the write */
+  mtimeMs: number;
+  /** sha256 hex of the written bytes — the client's next baseHash */
+  hash: string;
+  /** Line count of the written content */
+  totalLines: number;
+  /** Line-ending style that was applied */
+  eol: 'lf' | 'crlf';
+}
+
 export type CleanupResourceType = 'timer' | 'interval' | 'watcher' | 'listener' | 'stream';
 
 /**

src/web/public/i18n.js

@@ -601,6 +601,9 @@
     'Select a run to view its agents': '选择一次运行以查看其智能体',
     'Source type filter': '来源类型筛选',
     'Copy content': '复制内容',
+    'Edit file': '编辑文件',
+    'Unsaved changes': '未保存的更改',
+    Saved: '已保存',
     'Export as JSON': '导出为 JSON',
     'Export as Markdown': '导出为 Markdown',
     'Mark all read': '全部标为已读',

src/web/public/index.html

@@ -422,11 +422,18 @@
         <div class="file-preview-header">
           <span class="file-preview-title" id="filePreviewTitle">file.ts</span>
           <div class="file-preview-actions">
+            <button class="btn-icon-sm file-preview-edit-btn" id="filePreviewEditBtn" onclick="app.enterFilePreviewEdit()" title="Edit file" aria-label="Edit file" hidden><svg width="13" height="13" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="M17 3a2.85 2.83 0 1 1 4 4L7.5 20.5 2 22l1.5-5.5z"/></svg></button>
             <button class="btn-icon-sm" onclick="app.copyFilePreviewContent()" title="Copy content">&#x2398;</button>
             <button class="btn-icon-sm" onclick="app.closeFilePreview()" title="Close">&times;</button>
           </div>
         </div>
         <div class="file-preview-body" id="filePreviewBody"></div>
+        <div class="file-preview-editbar" id="filePreviewEditBar" hidden>
+          <span class="file-preview-dirty" id="filePreviewDirty" hidden>Unsaved changes</span>
+          <span class="file-preview-editbar-spacer"></span>
+          <button class="file-preview-editbar-btn" onclick="app.cancelFilePreviewEdit()">Cancel</button>
+          <button class="file-preview-editbar-btn file-preview-editbar-btn--save" id="filePreviewSaveBtn" onclick="app.saveFilePreviewEdit()" disabled>Save</button>
+        </div>
         <div class="file-preview-footer" id="filePreviewFooter"></div>
       </div>
     </div>

src/web/public/mobile.css

@@ -1871,6 +1871,33 @@ html.mobile-init .file-browser-panel {
     bottom: calc(44px + 2rem + var(--safe-area-bottom));
   }
 
+  /* File preview window: full screen on phones. --app-height tracks the visual
+     viewport (KeyboardHandler), so the edit textarea + Save bar stay above the
+     OS keyboard instead of hiding behind it. Footer/edit bar pad for the home
+     indicator. */
+  .file-preview-window {
+    width: 100vw;
+    max-width: 100vw;
+    height: var(--app-height, 100vh);
+    max-height: var(--app-height, 100vh);
+    border-radius: 0;
+    border-left: none;
+    border-right: none;
+  }
+
+  .file-preview-editbar {
+    padding-bottom: calc(0.4rem + var(--safe-area-bottom));
+  }
+
+  .file-preview-overlay .file-preview-footer {
+    padding-bottom: calc(0.35rem + var(--safe-area-bottom));
+  }
+
+  /* >=16px or iOS Safari auto-zooms the page on focus */
+  .file-preview-body textarea.file-preview-editor {
+    font-size: 16px;
+  }
+
   /* Notification drawer - full width on mobile, includes safe area padding */
   .notification-drawer {
     width: 100%;

src/web/public/panels-ui.js

@@ -3200,6 +3200,9 @@ Object.assign(CodemanApp.prototype, {
 
     if (!overlay || !bodyEl) return;
 
+    // Edit mode: reset any prior editor state whenever a preview (re)loads.
+    this._resetFilePreviewEdit();
+
     // Show overlay with loading state
     overlay.classList.add('visible');
     titleEl.textContent = filePath;
@@ -3298,6 +3301,13 @@ Object.assign(CodemanApp.prototype, {
         bodyEl.innerHTML = `<pre><code>${escapeHtml(data.content)}</code></pre>`;
         const truncNote = data.truncated ? ` (showing 500/${data.totalLines} lines)` : '';
         footerEl.textContent = `${data.totalLines} lines \u2022 ${this.formatFileSize(data.size)}${truncNote}`;
+        // Edit affordance only when the server says an edit=1 re-fetch would
+        // succeed (workspace text file inside the allowlist and size cap).
+        if (data.editable) {
+          this.filePreviewEditTarget = { sessionId, filePath };
+          const editBtn = this.$('filePreviewEditBtn');
+          if (editBtn) editBtn.hidden = false;
+        }
       }
     } catch (err) {
       console.error('Failed to preview file:', err);
@@ -3306,6 +3316,8 @@ Object.assign(CodemanApp.prototype, {
   },
 
   closeFilePreview() {
+    if (this.filePreviewEdit?.dirty && !confirm('Discard unsaved changes?')) return;
+    this._resetFilePreviewEdit();
     const overlay = this.$('filePreviewOverlay');
     if (overlay) {
       overlay.classList.remove('visible');
@@ -3313,6 +3325,172 @@ Object.assign(CodemanApp.prototype, {
     this.filePreviewContent = '';
   },
 
+  // ═══════════════════════════════════════════════════════════════
+  // File Viewer edit mode (issue #212 — docs/file-viewer-edit-plan.md)
+  // ═══════════════════════════════════════════════════════════════
+
+  _resetFilePreviewEdit() {
+    this.filePreviewEdit = null;
+    this.filePreviewEditTarget = null;
+    const editBtn = this.$('filePreviewEditBtn');
+    if (editBtn) editBtn.hidden = true;
+    const editBar = this.$('filePreviewEditBar');
+    if (editBar) editBar.hidden = true;
+    const dirtyEl = this.$('filePreviewDirty');
+    if (dirtyEl) dirtyEl.hidden = true;
+    const saveBtn = this.$('filePreviewSaveBtn');
+    if (saveBtn) {
+      saveBtn.disabled = true;
+      saveBtn.textContent = 'Save';
+    }
+  },
+
+  async enterFilePreviewEdit() {
+    const target = this.filePreviewEditTarget;
+    if (!target || this.filePreviewEdit) return;
+    const bodyEl = this.$('filePreviewBody');
+    const footerEl = this.$('filePreviewFooter');
+    if (!bodyEl) return;
+
+    // Always re-fetch with edit=1: the preview buffer may be line-truncated and
+    // a truncated buffer must never become an edit buffer. Parse the envelope
+    // even on non-ok responses so the specific refusal ("too large to edit
+    // here") reaches the toast instead of a generic failure.
+    let data;
+    try {
+      const res = await fetch(
+        `/api/sessions/${target.sessionId}/file-content?path=${encodeURIComponent(target.filePath)}&edit=1`
+      );
+      const result = await res.json().catch(() => null);
+      if (!result || result.success !== true) {
+        throw new Error(result?.error || `Failed to load file for editing (HTTP ${res.status})`);
+      }
+      data = result.data;
+    } catch (err) {
+      this.showToast(err.message, 'error');
+      return;
+    }
+
+    this.filePreviewEdit = {
+      sessionId: target.sessionId,
+      filePath: target.filePath,
+      baseHash: data.hash,
+      eol: data.eol,
+      original: data.content,
+      dirty: false,
+      saving: false,
+    };
+
+    const textarea = document.createElement('textarea');
+    textarea.className = 'file-preview-editor';
+    textarea.spellcheck = false;
+    textarea.setAttribute('autocapitalize', 'off');
+    textarea.setAttribute('autocorrect', 'off');
+    textarea.setAttribute('autocomplete', 'off');
+    textarea.wrap = 'off';
+    textarea.value = data.content;
+    textarea.addEventListener('input', () => this._onFilePreviewEditInput());
+    bodyEl.innerHTML = '';
+    bodyEl.appendChild(textarea);
+    // Deliberately no autofocus: on phones that would pop the OS keyboard
+    // before the user has scrolled to the line they want to change.
+
+    const editBtn = this.$('filePreviewEditBtn');
+    if (editBtn) editBtn.hidden = true;
+    const editBar = this.$('filePreviewEditBar');
+    if (editBar) editBar.hidden = false;
+    if (footerEl) {
+      const eolNote = data.eol === 'crlf' ? ' • CRLF' : '';
+      footerEl.textContent = `Editing • ${data.totalLines} lines • ${this.formatFileSize(data.size)}${eolNote}`;
+    }
+  },
+
+  _onFilePreviewEditInput() {
+    const edit = this.filePreviewEdit;
+    if (!edit) return;
+    const textarea = this.$('filePreviewBody')?.querySelector('textarea.file-preview-editor');
+    if (!textarea) return;
+    edit.dirty = textarea.value !== edit.original;
+    const dirtyEl = this.$('filePreviewDirty');
+    if (dirtyEl) dirtyEl.hidden = !edit.dirty;
+    const saveBtn = this.$('filePreviewSaveBtn');
+    if (saveBtn) saveBtn.disabled = !edit.dirty || edit.saving;
+  },
+
+  cancelFilePreviewEdit() {
+    const edit = this.filePreviewEdit;
+    if (!edit) return;
+    if (edit.dirty && !confirm('Discard unsaved changes?')) return;
+    const { sessionId, filePath } = edit;
+    this._resetFilePreviewEdit();
+    this.openFilePreview(filePath, sessionId);
+  },
+
+  async saveFilePreviewEdit(force = false) {
+    const edit = this.filePreviewEdit;
+    if (!edit || edit.saving) return;
+    const textarea = this.$('filePreviewBody')?.querySelector('textarea.file-preview-editor');
+    if (!textarea) return;
+
+    edit.saving = true;
+    const saveBtn = this.$('filePreviewSaveBtn');
+    if (saveBtn) {
+      saveBtn.disabled = true;
+      saveBtn.textContent = 'Saving…';
+    }
+    const restoreSaveState = () => {
+      edit.saving = false;
+      if (saveBtn) saveBtn.textContent = 'Save';
+      this._onFilePreviewEditInput();
+    };
+
+    let result = null;
+    let status = 0;
+    try {
+      const res = await fetch(`/api/sessions/${edit.sessionId}/file-content`, {
+        method: 'PUT',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({
+          path: edit.filePath,
+          content: textarea.value,
+          baseHash: edit.baseHash,
+          eol: edit.eol ?? undefined, // Zod .optional() rejects null
+          force: force || undefined,
+        }),
+      });
+      status = res.status;
+      result = await res.json().catch(() => null);
+    } catch (err) {
+      restoreSaveState();
+      this.showToast(`Save failed: ${err.message}`, 'error');
+      return;
+    }
+
+    if (status === 409 || result?.errorCode === 'CONFLICT') {
+      restoreSaveState();
+      if (
+        confirm(
+          'File changed on disk since you loaded it.\nOK overwrites it with your version; Cancel keeps your draft open.'
+        )
+      ) {
+        this.saveFilePreviewEdit(true);
+      }
+      return;
+    }
+    if (!result || result.success !== true) {
+      restoreSaveState();
+      this.showToast(`Save failed: ${result?.error || `HTTP ${status}`}`, 'error');
+      return;
+    }
+
+    const { sessionId, filePath } = edit;
+    this._resetFilePreviewEdit();
+    this.showToast('Saved', 'success');
+    // Re-open in read mode — re-fetching shows the truth on disk (including the
+    // server-side EOL normalization) rather than trusting the local buffer.
+    this.openFilePreview(filePath, sessionId);
+  },
+
   // ═══════════════════════════════════════════════════════════════
   // Attachment Cards (detected documents/images)
   // ═══════════════════════════════════════════════════════════════
@@ -3749,8 +3927,13 @@ Object.assign(CodemanApp.prototype, {
   },
 
   copyFilePreviewContent() {
-    if (this.filePreviewContent) {
-      navigator.clipboard.writeText(this.filePreviewContent).then(() => {
+    // While editing, copy the live editor buffer (not the stale preview text).
+    const editTextarea = this.filePreviewEdit
+      ? this.$('filePreviewBody')?.querySelector('textarea.file-preview-editor')
+      : null;
+    const content = editTextarea ? editTextarea.value : this.filePreviewContent;
+    if (content) {
+      navigator.clipboard.writeText(content).then(() => {
         this.showToast('Copied to clipboard', 'success');
       }).catch(() => {
         this.showToast('Failed to copy', 'error');

src/web/public/styles.css

@@ -9430,6 +9430,84 @@ kbd {
   flex-shrink: 0;
 }
 
+/* ---- File Viewer edit mode (issue #212) ---- */
+
+.file-preview-body textarea.file-preview-editor {
+  display: block;
+  width: 100%;
+  height: 100%;
+  margin: 0;
+  padding: 0.75rem;
+  border: none;
+  outline: none;
+  resize: none;
+  background: var(--bg-dark);
+  color: var(--text);
+  font-family: var(--font-mono);
+  font-size: 0.8rem;
+  line-height: 1.5;
+  white-space: pre;
+  overflow-wrap: normal;
+  overflow: auto;
+  tab-size: 4;
+}
+
+.file-preview-editbar {
+  display: flex;
+  align-items: center;
+  gap: 0.5rem;
+  padding: 0.4rem 0.75rem;
+  border-top: 1px solid var(--border);
+  background: var(--bg-input);
+  flex-shrink: 0;
+}
+
+.file-preview-editbar[hidden] {
+  display: none;
+}
+
+.file-preview-editbar-spacer {
+  flex: 1;
+}
+
+.file-preview-dirty {
+  font-size: 0.7rem;
+  color: var(--warning, #e5c07b);
+}
+
+.file-preview-dirty::before {
+  content: '\25CF  ';
+}
+
+.file-preview-editbar-btn {
+  padding: 0.3rem 0.9rem;
+  font-size: 0.75rem;
+  border-radius: 6px;
+  border: 1px solid var(--control-border);
+  background: var(--bg-input);
+  color: var(--text);
+  cursor: pointer;
+}
+
+.file-preview-editbar-btn:hover {
+  background: var(--bg-hover, rgba(255, 255, 255, 0.08));
+}
+
+.file-preview-editbar-btn--save {
+  background: var(--accent);
+  border-color: var(--accent);
+  color: #fff;
+}
+
+.file-preview-editbar-btn--save:hover {
+  background: var(--accent-hover);
+}
+
+.file-preview-editbar-btn--save:disabled {
+  opacity: 0.45;
+  cursor: default;
+}
+
 /* ========== Log Viewer Windows (Floating) ========== */
 
 .log-viewer-window {

src/web/routes/file-routes.ts

@@ -1,12 +1,16 @@
 /**
  * @fileoverview File browser and streaming routes.
- * Provides directory listing, file content preview, raw file serving, and tail streaming.
+ * Provides directory listing, file content preview, raw file serving, tail
+ * streaming, and the File Viewer edit-mode write path (edit=1 read +
+ * PUT /api/sessions/:id/file-content; policy in src/config/file-editing.ts,
+ * design in docs/file-viewer-edit-plan.md).
  */
 
 import { FastifyInstance, type FastifyReply } from 'fastify';
-import { basename as pathBasename, extname, isAbsolute, join, relative, resolve, sep } from 'node:path';
+import { basename as pathBasename, dirname, extname, isAbsolute, join, relative, resolve, sep } from 'node:path';
 import { createReadStream, realpathSync, type ReadStream } from 'node:fs';
 import fs from 'node:fs/promises';
+import { createHash, randomBytes } from 'node:crypto';
 import { homedir } from 'node:os';
 import type {
   ApiResponse,
@@ -14,6 +18,7 @@ import type {
   FilesystemBrowseEntry,
   FilesystemBrowseRoot,
   FilesystemPreviewKind,
+  FileWriteData,
 } from '../../types.js';
 import { ApiErrorCode, createErrorResponse, getErrorMessage } from '../../types.js';
 import { fileStreamManager } from '../../file-stream-manager.js';
@@ -44,7 +49,14 @@ import type { SessionAttachmentHistoryItem, SessionState } from '../../types/ses
 import { isSensitivePath } from '../sensitive-path.js';
 import { SseEvent } from '../sse-events.js';
 import type { ConfigPort, EventPort, SessionPort } from '../ports/index.js';
-import { FilesystemBrowseQuerySchema, FilesystemPreviewQuerySchema } from '../schemas.js';
+import { FilesystemBrowseQuerySchema, FilesystemPreviewQuerySchema, FileWriteSchema } from '../schemas.js';
+import {
+  MAX_EDITABLE_BYTES,
+  applyEol,
+  detectEol,
+  isDeniedEditRelativePath,
+  isEditableFileName,
+} from '../../config/file-editing.js';
 
 const MIME_TYPES: Record<string, string> = {
   png: 'image/png',
@@ -453,6 +465,71 @@ function appendDownloadFlag(url: string): string {
   return `${url}${url.includes('?') ? '&' : '?'}download=true`;
 }
 
+// ===== File Viewer edit mode (issue #212) =====
+// Policy lives in src/config/file-editing.ts; design in docs/file-viewer-edit-plan.md.
+
+function sha256Hex(buf: Buffer): string {
+  return createHash('sha256').update(buf).digest('hex');
+}
+
+/** NUL byte in the first 8KB — same binary signal the plain read path uses. */
+function sniffsBinary(buf: Buffer): boolean {
+  const sniffLength = Math.min(buf.length, 8192);
+  for (let i = 0; i < sniffLength; i++) {
+    if (buf[i] === 0) return true;
+  }
+  return false;
+}
+
+/**
+ * Structured-throw variant for the edit read/write paths. Identical mechanics to
+ * throwFilesystemPickerError (rendered by the central route error handler both
+ * in prod and in the app.inject() test harness); a separate name only so edit
+ * failures grep distinctly.
+ */
+function throwFileEditError(statusCode: number, code: ApiErrorCode, message: string): never {
+  throw Object.assign(new Error(message), {
+    statusCode,
+    body: createErrorResponse(code, message),
+  });
+}
+
+/**
+ * Gate a resolved workspace file for edit-mode read/write. Throws a structured
+ * error when the file may not be edited; returns void when it may. Order
+ * matters for the message a user sees: confinement (the caller's 404) →
+ * sensitive/blocked (403) → .git (403) → extension allowlist (400).
+ */
+function assertEditableTarget(resolvedPath: string, relativePath: string, blockedTrees: readonly string[]): void {
+  if (isSensitivePath(resolvedPath) || isBlockedAttachmentPath(resolvedPath, blockedTrees)) {
+    throwFileEditError(403, ApiErrorCode.FORBIDDEN, 'Editing this file is blocked');
+  }
+  if (isDeniedEditRelativePath(relativePath)) {
+    throwFileEditError(403, ApiErrorCode.FORBIDDEN, 'Files under .git cannot be edited');
+  }
+  if (!isEditableFileName(pathBasename(resolvedPath))) {
+    throwFileEditError(400, ApiErrorCode.INVALID_INPUT, 'This file type is not editable');
+  }
+}
+
+/**
+ * Decode a candidate edit buffer, refusing binary and non-UTF-8 content. The
+ * round-trip compare is what protects against silent corruption: decoding
+ * latin-1 (or any non-UTF-8) bytes yields U+FFFD replacements, and writing
+ * those back would destroy the original bytes. A UTF-8 BOM round-trips and is
+ * deliberately preserved.
+ */
+function decodeEditableText(buf: Buffer): string {
+  if (sniffsBinary(buf)) {
+    throwFileEditError(400, ApiErrorCode.INVALID_INPUT, 'Binary files cannot be edited');
+  }
+  const text = buf.toString('utf8');
+  if (!Buffer.from(text, 'utf8').equals(buf)) {
+    throwFileEditError(400, ApiErrorCode.INVALID_INPUT, 'Only UTF-8 text files can be edited');
+  }
+  return text;
+}
+
 function getSessionAttachmentHistory(
   ctx: SessionPort & ConfigPort,
   sessionId: string,
@@ -864,7 +941,12 @@ export function registerFileRoutes(app: FastifyInstance, ctx: SessionPort & Even
   // Get file content for preview (File Browser)
   app.get('/api/sessions/:id/file-content', async (req) => {
     const { id } = req.params as { id: string };
-    const { path: filePath, lines, raw } = req.query as { path?: string; lines?: string; raw?: string };
+    const {
+      path: filePath,
+      lines,
+      raw,
+      edit,
+    } = req.query as { path?: string; lines?: string; raw?: string; edit?: string };
     const session = findSessionOrFail(ctx, id, req);
 
     if (!filePath) {
@@ -876,7 +958,52 @@ export function registerFileRoutes(app: FastifyInstance, ctx: SessionPort & Even
     if (!validated) {
       return createErrorResponse(ApiErrorCode.NOT_FOUND, 'File not found');
     }
-    const { resolvedPath } = validated;
+    const { resolvedPath, relativePath } = validated;
+
+    // Read-for-edit: never truncated (a truncated buffer must never become an
+    // edit buffer), tighter size cap, full editability gate, and the hash/eol
+    // the client must echo back on PUT. Outside the shared try/catch below so
+    // its structured errors keep their status codes instead of collapsing into
+    // OPERATION_FAILED.
+    if (edit === '1' || edit === 'true') {
+      const guard = await loadAttachmentGuardConfig();
+      assertEditableTarget(resolvedPath, relativePath, guard.blockedTrees);
+
+      let editStat;
+      try {
+        editStat = await fs.stat(resolvedPath);
+      } catch {
+        throwFileEditError(404, ApiErrorCode.NOT_FOUND, 'File not found');
+      }
+      if (!editStat.isFile()) {
+        throwFileEditError(400, ApiErrorCode.INVALID_INPUT, 'Only regular files can be edited');
+      }
+      if (editStat.size > MAX_EDITABLE_BYTES) {
+        throwFileEditError(
+          413,
+          ApiErrorCode.INVALID_INPUT,
+          `File too large to edit here (${Math.ceil(editStat.size / 1024)}KB > ${MAX_EDITABLE_BYTES / 1024}KB limit)`
+        );
+      }
+
+      const editBuf = await fs.readFile(resolvedPath);
+      const editText = decodeEditableText(editBuf);
+      return {
+        success: true,
+        data: {
+          path: filePath,
+          content: editText,
+          size: editBuf.length,
+          mtimeMs: editStat.mtimeMs,
+          totalLines: editText.split('\n').length,
+          truncated: false,
+          extension: filePath.split('.').pop()?.toLowerCase() || '',
+          editable: true,
+          hash: sha256Hex(editBuf),
+          eol: detectEol(editText),
+        },
+      };
+    }
 
     try {
       const stat = await fs.stat(resolvedPath);
@@ -998,6 +1125,19 @@ export function registerFileRoutes(app: FastifyInstance, ctx: SessionPort & Even
       const truncatedContent = allLines.length > maxLines;
       const displayContent = truncatedContent ? allLines.slice(0, maxLines).join('\n') : content;
 
+      // Additive edit-mode advertisement: whether an edit=1 re-fetch would
+      // succeed. The UTF-8 round-trip compare is a cheap memcmp and mirrors
+      // decodeEditableText; no hash here — the Edit action re-fetches with
+      // edit=1, which is where the baseHash comes from.
+      const guard = await loadAttachmentGuardConfig();
+      const editable =
+        isEditableFileName(pathBasename(resolvedPath)) &&
+        !isDeniedEditRelativePath(relativePath) &&
+        !isSensitivePath(resolvedPath) &&
+        !isBlockedAttachmentPath(resolvedPath, guard.blockedTrees) &&
+        stat.size <= MAX_EDITABLE_BYTES &&
+        Buffer.from(content, 'utf8').equals(buf);
+
       return {
         success: true,
         data: {
@@ -1007,6 +1147,7 @@ export function registerFileRoutes(app: FastifyInstance, ctx: SessionPort & Even
           totalLines: allLines.length,
           truncated: truncatedContent,
           extension: ext,
+          editable,
         },
       };
     } catch (err) {
@@ -1014,6 +1155,121 @@ export function registerFileRoutes(app: FastifyInstance, ctx: SessionPort & Even
     }
   });
 
+  // File Viewer edit mode: save a text file back into the session workspace.
+  // Edit-in-place ONLY — there is deliberately no O_CREAT path in this handler,
+  // so it can never create, and it never deletes. Confinement is identical to
+  // the read path (realpath + workspace boundary + ownership via
+  // findSessionOrFail), plus the sensitive-path/attachment-guard blocklists and
+  // the extension allowlist. Concurrency is optimistic: the client echoes the
+  // sha256 it loaded (baseHash) and a mismatch is a 409 unless force is set.
+  // bodyLimit: JSON escaping can expand content up to ~6x (each control char
+  // becomes \uXXXX), so the 512KB content cap needs headroom over Fastify's
+  // 1MB default.
+  app.put(
+    '/api/sessions/:id/file-content',
+    { bodyLimit: 4 * 1024 * 1024 },
+    async (req): Promise<ApiResponse<FileWriteData>> => {
+      const { id } = req.params as { id: string };
+      const session = findSessionOrFail(ctx, id, req);
+      const body = parseBody(FileWriteSchema, req.body);
+
+      // Exact byte cap — the schema's .max() counts UTF-16 code units and is
+      // only a coarse pre-filter.
+      if (Buffer.byteLength(body.content, 'utf8') > MAX_EDITABLE_BYTES) {
+        throwFileEditError(413, ApiErrorCode.INVALID_INPUT, `Content too large (${MAX_EDITABLE_BYTES / 1024}KB limit)`);
+      }
+
+      const validated = validateSessionFilePath(session.workingDir, body.path);
+      if (!validated) {
+        // Covers missing files, traversal, and symlink escapes alike — a write
+        // target that fails confinement is reported identically to a missing
+        // one, matching the read route.
+        throwFileEditError(404, ApiErrorCode.NOT_FOUND, 'File not found');
+      }
+      const { resolvedPath, relativePath } = validated;
+
+      const guard = await loadAttachmentGuardConfig();
+      assertEditableTarget(resolvedPath, relativePath, guard.blockedTrees);
+
+      let stat;
+      try {
+        stat = await fs.stat(resolvedPath);
+      } catch {
+        throwFileEditError(404, ApiErrorCode.NOT_FOUND, 'File not found');
+      }
+      if (!stat.isFile()) {
+        throwFileEditError(400, ApiErrorCode.INVALID_INPUT, 'Only regular files can be edited');
+      }
+      if (stat.size > MAX_EDITABLE_BYTES) {
+        throwFileEditError(
+          413,
+          ApiErrorCode.INVALID_INPUT,
+          `File too large to edit here (${MAX_EDITABLE_BYTES / 1024}KB limit)`
+        );
+      }
+
+      const currentBuf = await fs.readFile(resolvedPath);
+      const currentText = decodeEditableText(currentBuf);
+      const currentHash = sha256Hex(currentBuf);
+      if (currentHash !== body.baseHash && !body.force) {
+        throwFileEditError(
+          409,
+          ApiErrorCode.CONFLICT,
+          'File changed on disk since it was loaded — reload it or overwrite'
+        );
+      }
+
+      // Re-apply the file's original line endings (a <textarea> normalizes to
+      // LF; without this a two-line edit of a CRLF file rewrites every line).
+      const eol = body.eol ?? detectEol(currentText);
+      const outText = applyEol(body.content, eol);
+      const outBuf = Buffer.from(outText, 'utf8');
+      if (outBuf.length > MAX_EDITABLE_BYTES) {
+        throwFileEditError(413, ApiErrorCode.INVALID_INPUT, `Content too large (${MAX_EDITABLE_BYTES / 1024}KB limit)`);
+      }
+
+      // Atomic replace: O_EXCL temp in the same directory, then rename.
+      // 'wx' cannot follow a pre-existing symlink and rename() replaces (not
+      // follows) a symlink in the final component, which closes the
+      // validate-then-write TOCTOU window. fchmod because open()'s mode is
+      // masked by the process umask; fsync so the rename never publishes a
+      // partially-durable file. Trade-off (same as vim's default): the inode
+      // changes, so hardlinks keep the old content.
+      const fileMode = stat.mode & 0o777;
+      const tmpPath = join(
+        dirname(resolvedPath),
+        `.${pathBasename(resolvedPath)}.codeman-tmp-${randomBytes(6).toString('hex')}`
+      );
+      let handle;
+      try {
+        handle = await fs.open(tmpPath, 'wx', fileMode);
+        await handle.chmod(fileMode);
+        await handle.writeFile(outBuf);
+        await handle.sync();
+        await handle.close();
+        handle = undefined;
+        await fs.rename(tmpPath, resolvedPath);
+      } catch (err) {
+        if (handle) await handle.close().catch(() => {});
+        await fs.unlink(tmpPath).catch(() => {});
+        throwFileEditError(500, ApiErrorCode.OPERATION_FAILED, `Failed to save file: ${getErrorMessage(err)}`);
+      }
+
+      const newStat = await fs.stat(resolvedPath).catch(() => undefined);
+      return {
+        success: true,
+        data: {
+          path: body.path,
+          size: outBuf.length,
+          mtimeMs: newStat?.mtimeMs ?? Date.now(),
+          hash: sha256Hex(outBuf),
+          totalLines: outText.split('\n').length,
+          eol,
+        },
+      };
+    }
+  );
+
   // Serve raw file content (for images/binary files)
   app.get('/api/sessions/:id/file-raw', async (req, reply) => {
     const { id } = req.params as { id: string };

src/web/schemas.ts

@@ -16,6 +16,7 @@ import {
   MIN_TERMINAL_BUFFER_BYTES,
   MIN_TERMINAL_SCROLLBACK_LINES,
 } from '../config/terminal-history.js';
+import { MAX_EDITABLE_BYTES } from '../config/file-editing.js';
 
 // ========== Path Validation ==========
 
@@ -83,6 +84,30 @@ export const FilesystemPreviewQuerySchema = z.object({
     .optional(),
 });
 
+/**
+ * Body validation for `PUT /api/sessions/:id/file-content` (File Viewer edit
+ * mode). `content.max()` counts UTF-16 code units, which for UTF-8 output is
+ * always <= the byte length, so it is a coarse pre-filter that never rejects
+ * valid content; the handler enforces the exact MAX_EDITABLE_BYTES byte cap.
+ * Workspace containment and symlink resolution are enforced by the route via
+ * validateSessionFilePath after parsing.
+ */
+export const FileWriteSchema = z
+  .object({
+    path: z
+      .string()
+      .min(1)
+      .max(4096)
+      .refine((p) => !p.includes('\0') && !p.includes('\n') && !p.includes('\r'), {
+        message: 'Invalid path',
+      }),
+    content: z.string().max(MAX_EDITABLE_BYTES),
+    baseHash: z.string().regex(/^[a-f0-9]{64}$/, 'baseHash must be a sha256 hex digest'),
+    eol: z.enum(['lf', 'crlf']).optional(),
+    force: z.boolean().optional(),
+  })
+  .strict();
+
 // ========== Env Var Allowlist ==========
 
 /** Allowlisted env var key prefixes */