Edit text files outside the workspace in the file panel

[steve-cooks]

Problem

When I open a text file from outside the thread's workspace in the right panel, it's read-only. For example, I might open ~/.codex/AGENTS.md or ~/.claude/CLAUDE.md from a chat link. Files inside the workspace can already be edited in place and save automatically. So to fix one line in my global agent instructions, I have to switch to VS Code or another editor.

Codex desktop lets you edit these files directly from its side panel.

What exists today

• Web and desktop edit workspace text files in place with the existing editor. A file outside the workspace, a chat attachment, or a file truncated at 1 MB opens in the read-only source preview instead (FilePreviewPanel.tsx).
• The server reads host paths (since fix: ~/ file links open in the file panel for projects outside the home folder #20950), but WorkspaceFileSystem.writeFile only writes inside known workspaces.
• [Feature][Mobile] Edit workspace Markdown files directly, including AGENTS.md #14438's triage describes this split. Allow line comments on files outside the thread's workspace #21326 asks for line comments on these same external files.

Proposed scope

• Use the existing editor for complete text files outside the workspace that T3 Code can already read and display.
• Require the existing filesystem:write scope. Read-only connections, truncated files, attachments and media stay read-only.
• Make the write conditional: the save checks the file is unchanged since it was read. If it changed, the save is refused and nothing is overwritten. This avoids the overwrite-a-newer-save problem raised on fix(web): retry failed final file saves after editor close #20738.
• Web and desktop only. Mobile editing stays with [Mobile] Edit workspace text files directly from the Files viewer #16500.

Out of scope: creating, renaming or deleting files outside the workspace, and any instructions or skills manager.

Open questions

1. Should any readable absolute path be writable, or only certain locations? For example, the home directory, or only files the panel already opened.
2. Does feat: install skills from a source, view their files, and let agents change Tools #17691's "T3 never writes an agent's config files" stance apply to a user editing these files by hand?
3. Should the conditional write also apply to workspace saves, or stay limited to external files for now?

I'm happy to implement a focused PR if maintainers approve this direction and scope.

Created with Claude Opus 5.5 High in Claude Code, running in T3 Code.

[steve-cooks]

@juliusmarminge, could you take a look when you have a moment? This touches the write-safety concern you raised on #20738. I would like a yes or no on direction and scope before writing any code. The three open questions:

1. Should any readable absolute path be writable from the file panel, or only certain locations (for example the home directory, or only files the panel already opened)?
2. Does the "T3 never writes an agent's config files" stance from feat: install skills from a source, view their files, and let agents change Tools #17691 also apply to a user editing files like ~/.codex/AGENTS.md by hand?
3. Should saves that check the file is unchanged since it was read also cover workspace autosaves, or stay limited to external files for now?