[BUG] similar to: anthropics/claude-code #15301

Preflight Checklist

• [x] I have searched existing issues 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?

[BUG] Custom slash commands autocomplete but return "Unknown command" on execution (Claude Code Desktop)
Environment: Claude Code Desktop (macOS), model Opus 4.8. Custom project commands in .claude/commands/*.md.
Summary: Custom slash commands appear in autocomplete/Recents but every invocation returns Unknown command: /name. Did you mean…, including as the first message of a brand-new session.

What Should Happen?

Expected: command executes.
Actual: "Unknown command." Filename shows in autocomplete, so it's discovered but not registered as runnable. Same behavior for other untracked commands (e.g. /day12) added in the same batch. Commands present at an earlier launch (/day1–/day9) still run.
Workaround: "Read .claude/commands/<name>.md and execute it" works fine.
Possible regression: Did not occur on the previous model/app version; began after updating to Opus 4.8.

Error Messages/Logs

Steps to Reproduce

Steps to reproduce:

Add a new file .claude/commands/seal-day11.md (valid frontmatter, plain ASCII) to a project.
Fully quit and relaunch the app.
Open a new session; type /seal-day11 as the first message.

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

claude-opus-4-7[1m]

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

Using claude app claude code and claude co-work. Not in shell or terminal.