[Router] 2381 semantic_similarity classifier (M5)

[SlawomirNowaczyk]

Implements the semantic_similarity classifier — the first model-backed classifier in the generic routing engine — against the existing Classifier interface. It scores an input by max cosine similarity against a set of reference_phrases, reported as a single score under the empty-string key and consumed via a classifier-band condition. Closes #2381.

Changes

• SemanticSimilarityClassifier (routing_policy.cpp): embeds the input via ClassifierServices::embed, computes cosine against each reference exemplar, and returns max as Score::labels[""] (read by conditions through Score::primary()). Declares no labels.
• Reference-phrase caching: exemplars are embedded once and cached on the instance. Since classifiers are shared across concurrent router requests (AGENTS.md invariant Lock transformers version for cpu #8), the lazy cache is guarded by a mutex; the input embedding is recomputed per request.
• Registry wiring (make_classifier): semantic_similarity now constructs the real classifier instead of throwing "not implemented". Requires model + non-empty reference_phrases; rejects labels/default_label.
• Failure handling: any embed failure (missing service, throw, empty/mismatched vectors) yields Score::ok = false so the owning band applies its on_error policy rather than letting an exception escape.
• Tests: extended FakeClassifierServices with per-(model, text) embeddings and embed call counters; added test_routing_policy_semantic.cpp covering max-cosine selection, reference-phrase caching (embed-once), inclusive band boundaries incl. the default min_score: 0.5, and on_error; updated the registry test for the now-accepted type.

Notes

• The cosine score is clamped into [0,1] to satisfy the band contract (guards against floating-point overshoot past 1.0 for near-identical vectors).
• Out of scope per the issue: the real embed() ↔ Router wiring ([Router] ClassifierServices to Router wiring (M-svc) #2384) and the other classifier types (classifier/llm). No schema change — the existing route_policy.schema.json already describes reference_phrases.

Testing

ctest -R RoutingPolicy — all four suites (Contract, Evaluator, Registry, Semantic) pass.

[eddierichter-amd]

This looks great! Only feedback is can you add the RoutingPolicySemanticTest to the CI?

[SlawomirNowaczyk]

This looks great! Only feedback is can you add the RoutingPolicySemanticTest to the CI?

Thanks, I forgot about it. Should be done now :)

[ramkrishna2910]

Thanks for this — it's clean and was approved, but we're consolidating #2381 on the unified version in #2483, which builds on this work and additionally collapses semantic_similarity onto the generic classifier label→score contract (no magic empty key) and supports multi-concept scoring. Since the schema is still unreleased, taking the breaking cleanup now is the cheap moment to do it.

No action needed here. I'll close this once #2483 merges (it carries your initial commit forward).

[SlawomirNowaczyk]

Made obsolete by #2483