Handle "Prompt is too long" error gracefully

[fenix011]

Summary

When resuming a session whose context exceeds the model's limit, the JSON-RPC layer returns a raw error:

{"jsonrpc":"2.0","id":4,"error":{"code":-32603,"message":"Internal error: Prompt is too long"}}

agent-shell displays this as a generic error dialog:

╭─
  ⚠ Error (-32603) ⚠
  Internal error: Prompt is too long
  [Details]
╰─

This is confusing — the user doesn't know what happened or how to recover.

By contrast, the Claude Code CLI handles the same situation with a friendly message ("Context is too long...") and offers the /compact command to summarize and continue.

Proposed improvement

Detect this specific error (code -32603, message matching "Prompt is too long") and provide an actionable recovery path, for example:

• Show a clearer message: "Context too long — compact the conversation to continue"
• Add a clickable button to trigger compaction (if the agent supports a compact/summarize operation via ACP)
• Or automatically compact and retry

Steps to reproduce

1. Open a session in agent-shell and have a long conversation
2. Close/kill the buffer
3. Resume the same session — the accumulated context triggers the error

[xenodium]

Thanks for filing.

Add a clickable button to trigger compaction (if the agent supports a compact/summarize operation via ACP)

Is there a way to tell if the agent supports the feature via ACP? You'll have to inspect traffic to find out https://github.com/xenodium/agent-shell?tab=readme-ov-file#how-do-i-viewget-agent-client-protocol-traffic