Build-order step 5. Wrap untrusted content in a quarantine fence a downstream
trusted prompt can rely on: everything between the delimiters is data, never
instructions.
Two load-bearing properties:
- Per-call cryptographic nonce (secrets.token_hex, 128-bit) in the delimiter, so
an attacker embedded in the payload cannot forge the matching closing marker to
break out — the nonce is unpredictable and fresh every call.
- Marker-strip FIRST: fabricated fence markers already in the payload are removed
(any/no nonce, case-insensitive, ReDoS-safe negated-class regex) and flagged as
fence:marker-injection (HIGH, LLM01) before wrapping — defense in depth against
a lucky guess of the static skeleton, and it surfaces the attempt.
Pure text -> (fenced_text, report, nonce); only mutation is the marker-strip.
Nonce exposed so the caller can reference the fence in the trusted prompt.
10 tests: wrap/preserve, per-call randomness, nonce length, breakout containment,
marker-strip (+ case-insensitive), prose-word FP guard, source, empty input.
[skip-docs]: README positioning + honest-limitations is a deliberate build-order
step-11 deliverable (steps 1-4 likewise left README frozen). README status line
("pre-implementation") is stale and flagged for the step-11 refresh.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01K8GmKRCdsPjWYAKWsNgeQS
