FloatVectorSimilarityQuery and ByteVectorSimilarityQuery with traversalSimilarity == resultSimilarity (the 3- and 4-argument constructors: the plain ones in 9.10.0-10.4.0, the deprecated Explicit classes in 10.5.2 and main) silently drop documents whose similarity is exactly resultSimilarity, because the collector's minCompetitiveSimilarity() rises to the inclusive threshold and every searcher treats an equal score as not competitive

Finding LFV-467Area: search / vector similarity-threshold queries (ExplicitVectorSimilarityCollector with the exhaustive scan and the HNSW graph search)Priority: lowImpact: silent wrong resultsNovelty: new

Finding page for LFV-467

AI-assisted finding, confirmed by automated checks, not yet triaged by a human Lucene committer. This site files nothing upstream.

What it shows

You index vectors with the default codec and run FloatVectorSimilarityQuery.Explicit(field, target, 1.0f) on main or 10.5.2 (the plain FloatVectorSimilarityQuery(field, target, 1.0f) on 9.10.0-10.4.0), which sets traversalSimilarity = resultSimilarity, to find every exact copy of target under EUCLIDEAN, and you expect every document with similarity >= 1.0, as collect() ('similarity >= resultSimilarity'), explain() ('Score above threshold') and the query's own exact fallback all count them. Instead only the first copy comes back: a 65-vector segment with copies at doc 0 and doc 64 returns [0] and count() 1, the same query with a MatchAllDocsQuery filter returns [0, 64], a 4,000-vector HNSW segment returns 1 of its 12 copies, and a traversalSimilarity one ulp lower returns all of them (byte vectors at an exact 0.5 behave the same). Once a hit at the threshold is collected, ExplicitVectorSimilarityCollector.minCompetitiveSimilarity() = min(traversalSimilarity, maxSimilarity) equals the inclusive result threshold, and both the exhaustive scan (a 64-vector batch is collected only if its best score > minCompetitiveSimilarity()) and the graph search (score >= nextUp(minCompetitiveSimilarity())) treat an equal score as not competitive; the one-line fix returns min(nextDown(traversalSimilarity), maxSimilarity).

FloatVectorSimilarityQuery.Explicit with traversalSimilarity == resultSimilarity drops a document whose similarity equals the threshold once an earlier hit at the threshold was collected (65 vectors, exact copies of the target at doc 0 and doc 64, EUCLIDEAN threshold 1.0).

Expected vs actual

Expected hits [0, 64] (both have similarity 1.0 >= resultSimilarity); main returns [0].

Files

• src/org/apache/lucene/search/TestExplicitSimilarityThresholdTie.java (JUnit test)

Run it

• Tested on Lucene main e1faeba08bde (2026-11-02): still reproduces.
• Tested on Lucene main 645d4fe14a20 (2026-10-28): reproduced.
• Recorded outcome of run 1 on Lucene main 645d4fe14a20: 1 test(s) run, 1 failed.
• How to read the outcome: a failing test is the finding reproducing (the failure message states expected against actual).
• Seeds: a rerun with the same seed replays the same random choices. Assertions off (-da -dsa -Dtests.asserts=false) is the production-like setting.

# Lucene checkout at the tested commit, built with: ./gradlew --no-daemon assemble   (JDK 25)
LUCENE=/path/to/lucene; V=12.0.0-SNAPSHOT
CP=$LUCENE/lucene/core/build/libs/lucene-core-$V.jar
CP=$CP:$LUCENE/lucene/codecs/build/libs/lucene-codecs-$V.jar
CP=$CP:$LUCENE/lucene/test-framework/build/libs/lucene-test-framework-$V.jar
CP=$CP:randomizedtesting-runner-2.9.2.jar:junit-4.13.2.jar:hamcrest-3.0.jar   # test-framework dependencies (Maven Central)
CP=$CP:junit-platform-console-standalone-6.1.3.jar   # JUnit Platform console launcher (Maven Central)

javac -d classes -cp "$CP" $(find src -name '*.java')

# run 1: assertions on, seed DEADBEEF
java -ea -esa --add-modules jdk.incubator.vector --enable-native-access=ALL-UNNAMED -Dtests.seed=DEADBEEF \
     -cp "classes:$CP" org.junit.platform.console.ConsoleLauncher \
     execute --select-class org.apache.lucene.search.TestExplicitSimilarityThresholdTie

Gradle alternative: copy the test into the matching module's src/test/ tree of the checkout and run ./gradlew -p lucene/<module> test --tests <class> -Ptests.seed=<seed> (add -Ptests.asserts=false for a run without assertions).

Feedback

Open an issue in lucene-ai-bug-hunt. Please do not open issues or pull requests in apache/lucene about this finding unless a Lucene committer agrees it should be filed.

Snapshot of 2026-11-04. AI-assisted; not yet triaged by a human Lucene committer. Feedback: open an issue in lucene-ai-bug-hunt.