[BUG]

### Preflight Checklist

- [x] I have searched [existing issues](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20state%3Aopen%20label%3Abug) and this hasn't been reported yet
- [x] This is a single bug report (please file separate reports for different bugs)
- [x] I am using the latest version of Claude Code

### What's Wrong?

Creating a Routine (via desktop app: Customize → More → Routines → New local routine) fails 100% of the time with the toast "Failed to save routine. You can try again." The same failure reproduces via the scheduled-tasks MCP tool's create_scheduled_task call inside a Claude Code session, with the underlying error "path moved between validation and open".

This is not caused by antivirus, file permissions, OneDrive, or org policy: writing a file directly to the exact same path the app fails on (C:\Users\\.claude\scheduled-tasks\\SKILL.md) succeeds instantly with no interference. The bug appears to be a race condition in the app's own save logic between a path-validation step and the subsequent open-for-write step.

### What Should Happen?

The routine/scheduled task should save successfully, writing its full frontmatter and prompt content to SKILL.md, and then appear in the Routines list / list_scheduled_tasks output.

### Error Messages/Logs

```shell
Desktop UI toast: Failed to save routine. You can try again.

CLI tool error (identical underlying cause):

Failed to create scheduled task: path moved between validation and open
```

### Steps to Reproduce

1. Open Claude desktop app → Customize → More → Routines → New local routine.
2. Fill in Name, Description, Instructions (any values), leave trigger as Manual/Daily, set a working directory (e.g. `C:\Temp\test`).
3. Click Create.
4. Observe error toast: "Failed to save routine. You can try again."
5. Repeat with a different name — same result every time.

Equivalent reproduction via Claude Code session (no desktop UI needed):

1. Call the scheduled-tasks MCP tool's create_scheduled_task with any taskId, prompt, description, and a cronExpression.
2. Observe: Failed to create scheduled task: path moved between validation and open.
3. Inspect C:\Users\\.claude\scheduled-tasks\\: a directory and an empty (0-byte) SKILL.md were created, but the content write never completed.

### Claude Model

None

### Is this a regression?

I don't know

### Last Working Version

_No response_

### Claude Code Version

2.1.275 (Claude Code) — desktop app

### Platform

Anthropic API

### Operating System

Windows

### Terminal/Shell

Other

### Additional Information

- Ruled out CrowdStrike Falcon/antivirus interference: manually writing to the exact failing path succeeded with no issues.

- Ruled out file permission issues: ACLs on .claude\scheduled-tasks are standard (SYSTEM/Administrators/user, full control).

- Ruled out OneDrive sync: .claude is a plain local directory, not OneDrive-redirected or symlinked.

- Ruled out org/workspace policy: the org-level "Routines" feature toggle is enabled (Default: on), and no local managed-settings.json policy exists on this machine.

- Every failed attempt leaves an orphaned empty directory + 0-byte SKILL.md under .claude\scheduled-tasks\\, which list_scheduled_tasks correctly ignores as invalid, but which are never cleaned up automatically.

Reproduce the failure through the desktop Routines flow and the scheduled-tasks MCP tool's create_scheduled_task call on Windows. Inspect the save path under .claude\scheduled-tasks and the validation-to-open sequence, including the orphaned empty SKILL.md. Done means a routine saves its complete content without the race error and appears in the Routines list or list_scheduled_tasks output.

Written by the indexing model from the issue text.