Remote Control: built-in slash commands fail because a "Message sent at" system-reminder is prepended ahead of the leading slash

Summary

When a built-in slash command (e.g. /clear, /compact) is sent from a Remote Control session, it is delivered to the model as plain text and silently no-ops instead of being intercepted locally. This appears to be the root cause behind the symptom reported in #29156 (closed as duplicate) and #28764 (closed as not-planned/stale) — both describe slash commands being sent as prompts in Remote Control, but neither identifies the mechanism.

Root cause (observed)

Messages arriving over Remote Control have a system-reminder prepended by the harness. The message the session actually receives looks like:

<system-reminder>Message sent at Sat 2026-06-13 14:45:07 UTC.</system-reminder>
/clear

The local slash-command parser only intercepts a command when the leading / is at position 0 of the message. Because the injected Message sent at … reminder now precedes it, the command is no longer first, the parser skips it, and the raw /clear text is forwarded to the model — which has no way to execute a client-side command.

Impact

All client-side slash commands (/clear, /compact, /context, /usage, etc.) are effectively unusable over Remote Control. Tool-invoked / custom skill commands are unaffected, since they don't depend on the leading-slash parser.

Expected behavior

The command parser should strip or ignore the injected Message sent at … system-reminder before checking for a leading slash (or detect the command after the reminder), so that slash commands sent over Remote Control are intercepted locally as they are in a local terminal session.

Steps to reproduce

1. Start a session and attach Remote Control via /rc (/remote-control).
2. From the phone/web client, send /clear.
3. Observe: the conversation is not cleared; the message is treated as a prompt.

Environment

• Claude Code Remote Control (research preview)
• Local host: Windows 11

Related

• #29156 (closed as duplicate) — same symptom
• #28764 (closed as not-planned/stale) — related Remote Control slash-command behavior