/compact cannot recover a session after the CAPI Responses payload reaches the 5 MB limit`

Describe the bug

Description

When a Copilot CLI session reaches the 5 MB CAPI Responses request limit, normal prompts fail as expected—but /compact also fails. This leaves no way to reduce the active context while preserving and continuing the session. Basically this kills my session and I cannot continue in it which is a huge kill in productivity and flow.

Error

The request is too large to send through CAPI Responses. Try shortening the conversation or prompt. (5.0 MB request; 5.0 MB limit)

Environment

• GitHub Copilot CLI: 1.0.78-0
• OS: macOS 26.5.2
• Architecture: arm64

Affected version

1.0.78-0

Steps to reproduce the behavior

Steps to reproduce

1. Continue an interactive Copilot CLI session until its request context reaches 5 MB.
2. Submit another prompt.
3. Observe the request-size error.
4. Run /compact.
5. Observe that compaction cannot complete either.

Expected behavior

Expected behavior

/compact should remain usable after the normal request limit is reached, perhaps by truncating or chunking context locally before requesting a summary. The complete transcript should remain stored locally.

Ideally, Copilot CLI should also compact automatically before reaching an unrecoverable size.

Actual behavior

Both regular prompts and /compact are blocked. The session history still exists, but the session cannot be continued; the only practical workaround is starting a new session and manually transferring context.

Additional context

Additional detail

This occurs when the UI reports only approximately 69–70% context usage (about 1.1M context shown), rather than near 100%.

The context meter appears to track token capacity, while CAPI applies a separate 5 MB serialized-request limit. The CLI neither warns about this lower effective limit nor compacts before reaching it.

I have attached a screenshot showing the session at 70% context when the failure occurred.

まず、CAPI Responses リクエストが 5 MB の上限に達したときに、インタラクティブな Copilot CLI セッションで失敗を再現し、その後 /compact を実行します。コンパクションとリクエストサイズ処理の経路を追跡します。/compact によってコンテキストを削減して既存のセッションを継続でき、完全なトランスクリプトがローカルに保存されたままになれば完了です。