fix: make FOLLOW_UP_REMINDER opt-in via env var

[leo-dower]

Summary

Makes the FOLLOW_UP_REMINDER appended to ask_question answers opt-in via a new environment variable NOTEBOOKLM_FOLLOW_UP_REMINDER (default false). Addresses #28.

Why

The hardcoded reminder currently appended to every ask_question answer reads:

"EXTREMELY IMPORTANT: Is that ALL you need to know? [...] before you reply to the user, review their original request and this answer. If anything is still unclear or missing, ask me another question first."

This pattern — caps EXTREMELY IMPORTANT, imperative language directed at the assistant ("before you reply to the user"), instructions to take follow-up tool actions — is functionally indistinguishable from adversarial prompt injection. Well-aligned assistants (e.g., Claude Code) are instructed by their system prompts to flag suspected prompt injection in tool output before continuing. In practice this means every ask_question call generates a user-facing security warning, which is noisy and confusing.

See my comment on #28 for more context: #28 (comment)

Changes

• src/config.ts — adds followUpReminderEnabled: boolean to the Config interface, defaults to false in DEFAULTS, and adds env var override via NOTEBOOKLM_FOLLOW_UP_REMINDER in applyEnvOverrides.
• src/tools/handlers.ts — wraps the FOLLOW_UP_REMINDER concatenation in a ternary that checks CONFIG.followUpReminderEnabled. The constant itself is preserved unchanged so users who opt in get the exact same behavior as before.
• docs/configuration.md — documents the new env var in the Response formatting section.

Default choice

I set the default to false (opt-in) for three reasons:

1. The current default actively conflicts with the safety behavior of well-aligned assistants.
2. For less robust models, unprompted follow-up reminders can create token waste loops — the concern already raised in Feature request: Add option to disable FOLLOW_UP_REMINDER and multi-pass strategy #28.
3. Users who value the engagement behavior can opt in with a single env var; users who don't shouldn't have to discover the flag exists.

Happy to flip the default to true (preserving backward compatibility) if you prefer — just let me know and I'll update the PR. The flag itself is the important part.

Test plan

• npm run build passes without errors
• Compiled dist/tools/handlers.js shows the conditional append
• Compiled dist/config.js includes the new default and env var override
• Manual test with NOTEBOOKLM_FOLLOW_UP_REMINDER=true — reminder should appear
• Manual test with env var unset — reminder should NOT appear (default behavior changes)

I can provide manual test logs if useful.

Backward compatibility

Behavioral change: existing users who rely on the reminder will need to set NOTEBOOKLM_FOLLOW_UP_REMINDER=true in their environment to preserve the old behavior. This is documented in docs/configuration.md. If this is a concern, I can invert the default to true (opt-out) and still solve the security flagging issue for users who set it to false.

🤖 Generated with Claude Code

[PleasePrompto]

Thanks @leo-dower — your security framing was exactly right and v2.0.0 ships the same default: FOLLOW_UP_REMINDER is off by default, opt back in via NOTEBOOKLM_FOLLOW_UP_REMINDER=true. Closes #28. Crediting you in the release notes 🙏