Bug: Claude UserPromptSubmit hook may corrupt sessions with empty cached text blocks

[Taimylife]

Claude Code regression: UserPromptSubmit hook appears to cause empty cached text blocks

Environment:

• Linux
• Claude Code 2.1.245
• IJFW enabled as the Claude plugin
• Claude sessions using the IJFW UserPromptSubmit hook

After enabling IJFW's full hook set globally, fresh Claude sessions repeatedly failed with:

API Error: 400 messages.N.content.1.text: cache_control cannot be set for empty text blocks

The failure made normal messages and /compact fail. Inspecting the affected JSONL transcripts found multiple assistant records containing only { "type": "text", "text": "" }. Removing those empty records from a backup-restored transcript allowed recovery.

The IJFW pre-prompt.sh log also showed internal router/module failures during this period (identifier redeclaration and a bad module path). Disabling only IJFW's UserPromptSubmit hook stopped further global injection; the remaining session-start/compaction/session-end hooks and MCP server were not the target.

Could maintainers confirm:

1. Whether pre-prompt.sh can ever emit an empty hookSpecificOutput.additionalContext or otherwise lead Claude Code to serialize an empty text block;
2. Whether this hook is safe on Claude Code 2.1.245; and
3. The recommended fix/workaround that preserves IJFW memory without corrupting Claude sessions?

I have local backups and can provide redacted hook output if useful.