HybridQuery.yieldCombinedScoreAs() emits invalid FT.HYBRID (COMBINE arg-count off-by-2) → silent fallback to FT.AGGREGATE

Start at the HybridQuery implementation for yieldCombinedScoreAs() and the FT.HYBRID command construction used by index.query(). Add a regression test covering RRF and LINEAR with yieldCombinedScoreAs, verifying that the generated COMBINE count includes the two yield tokens and the native FT.HYBRID command is accepted.

Summary

Calling .yieldCombinedScoreAs(...) on a HybridQuery produces an FT.HYBRID command whose COMBINE RRF <n> argument count does not include the appended YIELD_SCORE_AS <name> tokens. Those two tokens therefore fall outside the COMBINE clause, Redis rejects the command with YIELD_SCORE_AS: Unknown argument, and the client silently falls back to AggregateHybridQuery (FT.AGGREGATE) — which combines with a hardcoded LINEAR weighting instead of the requested RRF, changing both the result set and the ranking, with only a WARN surfaced to the caller.

Environment

• com.redis:redisvl 0.13.1
• Redis 8.6.2 (built-in query engine; FT.HYBRID available)
• Jedis 7.3.0
• Java 17+ (reproduced on JDK 25)

Reproduction

HybridQuery query = HybridQuery.builder()
    .text("creature crossed the road at night")
    .textFieldName("observed")
    .vector(queryVector)               // float[1536]
    .vectorFieldName("embedding")
    .textScorer("BM25STD")
    .combinationMethod(HybridQuery.CombinationMethod.RRF)
    .rrfWindow(20)
    .rrfConstant(60)
    .numResults(5)
    .returnFields(List.of("id", "state", "county", "classification", "observed"))
    .yieldCombinedScoreAs("score")     // <-- triggers the bug
    .build();
index.query(query);

Log:

WARN com.redis.vl.index.SearchIndex - FT.HYBRID failed, falling back to
AggregateHybridQuery (FT.AGGREGATE): YIELD_SCORE_AS: Unknown argument

Captured command (redis-cli MONITOR)

The command redisvl-java actually sends (vector blobs elided as <vec>):

FT.HYBRID bigfoot SEARCH <vec> SCORER BM25STD VSIM @embedding $vector KNN 2 K 5
  COMBINE RRF 4 WINDOW 20 CONSTANT 60.0 YIELD_SCORE_AS score
  LOAD 5 @id @state @county @classification @observed LIMIT 0 5 PARAMS 2 vector <vec>

COMBINE RRF 4 is followed by WINDOW 20 CONSTANT 60.0 (4 tokens) and then YIELD_SCORE_AS score. The count 4 omits the two YIELD_SCORE_AS score tokens, so they are parsed outside the COMBINE clause → YIELD_SCORE_AS: Unknown argument.

For comparison, the equivalent redisvl (Python 0.20.1) command, which Redis accepts:

FT.HYBRID bigfoot SEARCH <vec> SCORER BM25STD VSIM @embedding $vector
  COMBINE RRF 6 WINDOW 20 CONSTANT 60 YIELD_SCORE_AS score
  LOAD 5 ... LIMIT 0 5 PARAMS 2 vector <vec>

Here COMBINE RRF 6 correctly counts all six following tokens (WINDOW 20 CONSTANT 60 YIELD_SCORE_AS score).

Root cause

The COMBINE RRF <n> argument count is derived from the window/constant args but is not incremented when YIELD_SCORE_AS <name> is appended.

Impact

• The combined score cannot be retrieved on the native FT.HYBRID path (without the yield, scores come back as 1.0).
• The silent fallback to FT.AGGREGATE uses a hardcoded LINEAR blend (0.3*text_score + 0.7*vector_similarity), not the requested RRF, so the result set and ranking differ from what was configured — and the caller only sees a WARN.

Suggested fix

Include the YIELD_SCORE_AS <name> tokens (2) in the COMBINE RRF <n> (and COMBINE LINEAR <n>) argument count when yieldCombinedScoreAs is set — i.e. emit COMBINE RRF 6 … rather than COMBINE RRF 4 … in the example above.

Separately, consider surfacing the FT.HYBRID → FT.AGGREGATE fallback more prominently (it silently changes the combination algorithm).