API returns 500 when user input contains nested message-shaped JSON

Description

Pasting JSONL conversation transcripts (which contain role/content fields) into the chat input causes HTTP 500 Internal Server Error. The API returns 500 instead of 400, suggesting the input reaches parsing before validation.

Steps to Reproduce

1. Copy a JSONL line from a Claude Code transcript (contains "role": "user", "content": "..." fields)
2. Paste it into the Claude Code chat input
3. Observe HTTP 500 Internal Server Error

This is also reproducible indirectly: tool results that render role/content fields in their output can trigger it.

Expected Behavior

The API should either:

• Handle nested message-shaped JSON in user input without error, or
• Return a 400 Bad Request with a clear error message if such input is not supported

Actual Behavior

HTTP 500 Internal Server Error — an unhandled server-side failure.

Environment

• Claude Code v2.1.39