[lorenzozanee]

Summary

• The llmlingua engine shields XML tags and negation/absolute words from the uncased default backend, restores source casing and edge whitespace on re-stitch, and fail-opens on empty replies.

Related Issues

• Closes fix(backend): compression: llmlingua default TinyBERT model lower-cases output, drops negations and breaks tags/whitespace #13455
• Related to fix(backend): ultra heuristic prunes polarity/negation words and collapses newlines, inverting instructions #13454, fix(backend): lossy compression engines rewrite instruction blocks in user-role messages #13453

Validation

• Change type: other (compression engine)
• Focused tests and category gates from the golden path
• npm run lint
• Reconciled with the current active release base; focused checks rerun afterward
• Production-code changes include a new or updated automated test in this PR

Tests Added Or Updated

• tests/unit/compression/llmlingua-fidelity.test.ts (new, 6 tests)
• Related existing suites re-run: llmlingua-failopen, llmlingua-ultra-entry, llmlingua-worker, llmlingua-model-store, llmlingua-worker-resolution — all green locally.

Coverage Notes

• Change touches open-sse/; covered by the new fidelity tests plus the related llmlingua suites above.

Reviewer Notes

• The negation wordlist is heuristic; reviewers may ask for it to become an engine option.
• Local validation only; CI runs on this PR.