# Task 04 — Deflect Prompt: Plain-Text-Only Line (Prevention)

**Phase:** `71_scaffolding_guardrails` · **Story:** n/a (owner request from chat, 2026-09-03)

## Objective
Close the door at the prompt: the deflected (LOW) turn offers no tools, so any tool
markup there is always wrong — add one plain-text-only instruction line to the
`DEFLECT_MODE` body. This is the owner-permitted change to the otherwise-locked LOW
prompt; the `DEFLECT_MODE` marker and everything else in the prompt stay put (the
E2E mock keys on the marker's presence, not the wording).

## Work
1. `app/rag/prompts.py` — `build_deflect_prompt`: append one line to the
   `DEFLECT_MODE` body (after "…propose 2-3 alternative questions."):
   "Reply in plain text only — you have no tools in this mode." The weak-hit
   title list follows exactly as today; `build_high_prompt`, `PERSONA`,
   `TOOLS_SECTION`, and the section order are untouched. Update the module
   docstring: the phase-71 note (owner-permitted 2026-09-03 LOW-prompt line;
   the marker-keying contract unchanged).
2. `tests/unit/test_prompts.py` (and `test_chat_gate.py` where the deflection body
   is pinned) — pins: the new line present in the LOW prompt; the `DEFLECT_MODE`
   marker still present (mock keying); the HIGH prompt unchanged (byte-identical
   pins stay green — the line must not leak into HIGH); the byte-identical-when-
   empty steering/overview behavior unchanged.

## Testing & Quality
- Unit: the prompt pins above.
- Coverage: **>90%** on this task's modified code (`app/rag/prompts.py`).

## Completion Criteria
- [ ] `uv run pytest tests/unit/test_prompts.py tests/unit/test_chat_gate.py -v
      --no-cov` green.
- [ ] LOW prompt = pre-phase text + exactly the one new line (a diff pin, or a
      reconstruction pin in the test).
- [ ] `uv run ruff check . && uv run pyright` clean.