KNearestFuzzyClassifier ignores its documented 'field^boost' syntax: an entry such as 'title^10' in textFieldNames is looked up as a literal field name, so that field contributes nothing, and with a single boosted field assignClass returns null

Finding LFV-801Area: classification / KNearestFuzzyClassifier (textFieldNames 'field^boost' syntax; NearestFuzzyQuery field lookup)Priority: lowImpact: silent wrong resultsNovelty: new

Finding page for LFV-801

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

What it shows

You build a KNearestFuzzyClassifier with a text field written as 'text^2' or 'title^10', as its constructor javadoc says you may, and expect the same classes as with the plain field name (with that field weighted more). Instead the entry is used verbatim as the name of a field called 'text^2', which does not exist, so that field adds no terms to the query: with one text field assignClass returns null and getClasses an empty list, and with several fields the boosted one is silently ignored. The sibling KNearestNeighborClassifier, whose javadoc has the same sentence, parses the boost and classifies normally.

The test shows that KNearestFuzzyClassifier reads a documented boosted field name such as text^2 as a literal field name, so the field contributes nothing.

Expected vs actual

Expected assignClass("apple") with "text^2" to give the same class as with "text" (fruit); actual: null (-ea and -da).

Files

• src/org/apache/lucene/classification/TestKNearestFuzzyClassifierBoostedField.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=$CP:$LUCENE/lucene/analysis/common/build/libs/lucene-analysis-common-$V.jar
CP=$CP:$LUCENE/lucene/classification/build/libs/lucene-classification-$V.jar
CP=$CP:$LUCENE/lucene/grouping/build/libs/lucene-grouping-$V.jar
CP=$CP:$LUCENE/lucene/queries/build/libs/lucene-queries-$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.classification.TestKNearestFuzzyClassifierBoostedField

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.