Options passed to a custom skill invocation are dropped before reaching the model

Describe the bug

When invoking a custom skill using /skill-name --option value, Copilot CLI records the complete input but omits the option arguments from the prompt dispatched to the skill.

Environment:

• GitHub Copilot CLI 1.0.81-6
• Windows
• Session: 2da2d958-a8cd-4eeb-9760-241fac5b53ed
• Occurred around 2026-08-21 04:10:27 UTC

The session event log contains the complete command and arguments. However, the model-visible invocation contains only that the skill was explicitly invoked plus its SKILL.md instructions. Because the arguments are absent, the skill follows its no-arguments behavior.

Affected version

1.0.81-6

Steps to reproduce the behavior

1. Define a repository skill that supports command-line-style arguments and has different behavior when no arguments are supplied.
2. Invoke it with arguments, for example:
   /evaluate-meeting-prep --slides week-over-week,speed,tokens --current "Model=C:\path\to\cohort"
3. Observe that the skill loads successfully.
4. Observe that the skill behaves as if no arguments were supplied and displays its interactive slide-selection menu.
5. Inspect the session event log: the original user.message contains the complete command, including --slides and --current, but those arguments are absent from the prompt delivered to the model with the skill context.

Expected behavior

When a custom skill is invoked as /skill-name <arguments>, the complete argument text should be included in the model-visible skill invocation. In this example, the skill should receive --slides week-over-week,speed,tokens and the supplied cohort paths, then proceed without asking the user to select slides again.

Additional context

• Copilot CLI: 1.0.81-6
• Operating system: Windows x64
• Shell: PowerShell
• Session ID: 2da2d958-a8cd-4eeb-9760-241fac5b53ed
• Timestamp: 2026-08-21 04:10:27 UTC

Diagnostic detail: the session's raw user.message event contains the complete 521-character invocation, including --slides, --current, --prior, and --outdir. The subsequent model-visible skill invocation contains the loaded skill instructions but none of those options. This indicates the options are lost during skill dispatch rather than input parsing.

Windows PowerShell 上の GitHub Copilot CLI でカスタム skill の呼び出しを再現し、その後、raw の user.message イベントとモデルから見える skill の呼び出しを比較します。skill のディスパッチ経路を追跡して、オプションテキストがどこで破棄されるのかを特定します。--slides、--current、--prior、--outdir を含む完全な引数がモデルに届き、skill が引数なしの場合の動作をスキップすれば完了です。