Bug: Cannot add new messages to conversation - 'Request too large (max 20MB)' error blocks further input

Bug Description

I'm unable to add any new messages to an existing conversation. Every time I try to send a new message, I get the following error:

Request too large (max 20MB). Try with a smaller file.

The issue is that I am not attaching any file — I'm just trying to send a regular text message. This completely blocks me from continuing the conversation, since even a short message triggers the error.

Expected Behavior

• I should be able to continue sending messages in a conversation.
• If the conversation context has grown too large, Claude Code should auto-compact the conversation (as documented) rather than hard-blocking all input.
• The error message is misleading because it suggests the problem is a "file" when no file is being attached.

Actual Behavior

• Any new message (even a one-word reply) is rejected with Request too large (max 20MB). Try with a smaller file.
• The conversation becomes completely unusable — there's no way to recover it or continue.
• No file is attached to the request.

Impact

• Loss of conversation context / in-progress work.
• No clear recovery path: the user cannot reduce the request size because they're not the one making it large.
• The error message points the user toward a file-size fix that doesn't apply.

Suggested Fixes

1. Trigger auto-compaction before hitting the 20MB request limit, not after.
2. If the limit is hit, surface a clearer error message explaining that the conversation context (not a file) is too large, and offer a recovery action (e.g. /compact, start a new session while preserving memory, export the transcript).
3. Ideally, allow the user to send a final /compact command even when the context is over the limit, so the conversation can be salvaged.

Environment

• Platform: darwin (macOS)
• OS: Darwin 25.2.0
• Claude Code: VSCode native extension