ADR-075: Neutral cross-encoder reranker protocol 

Context 

nr_ai_search measured (NRFE-3960, its ADR-029) that a cross-encoder reranker over the bi-encoder candidate pool lifts top-1 retrieval accuracy where the bi-encoder alone and a naive LLM reranker do not — on the real BMDV corpus from 6/9 to 8/9. It shipped the capability as a consumer-private client (RerankerClientInterface over its own RetrievedDocument DTO) plus an HTTP sidecar in its Build/reranker serving the one heavy dependency (sentence-transformers + torch) outside PHP.

Its ADR-029 kept that placement deliberately consumer-side because no second consumer existed. The revisit trigger has fired: cross-encoder scoring is wanted for nr_llm-side ranking too (upgrading embed-and-rank from bi-encoder to cross-encoder quality, e.g. link-target ranking), which makes it a shared capability — exactly the category ADR-050 assigns to nr_llm, as long as it stays stateless.

Decision 

1. Neutral protocol in nr_llm. Netresearch\NrLlm\Service\Rerank\RerankerInterface:

rerank(string $query, array $candidates): array
// $candidates: list<array{id: string, text: string}>
// return:      list<array{id: string, score: float}>

No consumer DTO crosses the boundary. Implementations:

• HttpReranker — speaks the sidecar contract (POST {endpoint}/rerank {"query", "documents": [{"id", "text"}]} → {"scores": [{"id", "score"}]}, input order). Pools above the sidecar's batch cap (RERANKER_MAX_DOCUMENTS, default 128) are split into sequential requests; per-pair scoring makes the split score-neutral. A transport failure, non-200 status or off-protocol body throws the typed RerankerException (Service\Rerank\Exception) — nr_llm never decides degradation.
• NullReranker — one entry per candidate in input order with a uniform score of 0.0. The uniform value carries no ranking signal (a stable sort preserves the caller's ordering) and keeps the result shape-identical to HttpReranker so consumer merge code needs no null branch.

2. Selection rule. RerankerFactory builds the RerankerInterface container service from the extension configuration: an empty rerankerEndpoint selects NullReranker, a configured endpoint selects HttpReranker with the configurable rerankerTimeout (default 30 s — a CPU cross-encoder can be slow for a wide pool). Unreadable configuration fails open to NullReranker. This mirrors the selection nr_ai_search's RerankerClientFactory made consumer-side.

3. Sidecar moves alongside the client. Build/reranker (app.py, Dockerfile, requirements.txt, README.md) now lives in nr_llm so client and server version together. The HTTP contract is unchanged.

4. What stays consumer-side. DTO mapping (id/text extraction, score attachment), the ordering merge (including how an unscored candidate ranks), the degradation policy on RerankerException (e.g. fall back to the pre-rerank cosine ordering), and any score-threshold gate (the score scale is model-specific).

Boundary with ADR-050. This supersedes the nr_llm-side reading of ADR-050 / nr_ai_search ADR-029 that cross-encoder reranking lives only in nr_ai_search. ADR-050's guardrail is untouched: the reranker is a stateless capability (candidates in, scores out) — no persistent index, no chunking, no reindex pipeline. nr_ai_search amends its ADR-028/ADR-029 in its own adoption change.

Public-service count: 29 → 30 (after ADR-076's 28 → 29). The RerankerInterface factory-built entry joins Category A (documented downstream contract). This ADR is the new count authority, superseding ADR-076's count. The breakdown is now 17 + 5 + 1 + 5 + 2 = 30 (PublicServicesPolicyTest).

Consequences 

• A consumer depends on RerankerInterface and drops its own client, factory and sidecar copy; only its DTO mapping, merge, degradation and gate code remain.
• Two new extension-configuration keys: rerankerEndpoint, rerankerTimeout. Both default to off/30 s — nothing changes until an operator opts in.
• The sidecar container is an optional runtime dependency of nr_llm deployments that enable reranking; nr_llm itself never requires it.
• RerankerException implements NrLlmExceptionInterface (ADR-053), so blanket consumer catch blocks keep working.