ExpressionRescorer.explain evaluates the expression with _score = 0 (the value of SortRescorer's placeholder explanation) instead of the first-pass score, so the explanation shows the wrong _score and expression values; the rescored ranking is correct
Finding LFV-174Area: expressions / ExpressionRescorer explain (Expression.getRescorer, ExpressionValueSource.explain over SortRescorer)Priority: lowImpact: minorNovelty: new
AI-assisted finding, confirmed by automated checks, not yet triaged by a human Lucene committer. This site files nothing upstream.
What it shows
You rescore first-pass hits with an expression that reads _score, for example '_score * 100 + popularity' through Expression.getRescorer, and ask the rescorer to explain a hit. You expect what the ExpressionRescorer javadoc promises, an explanation 'showing the value of each variable': _score equal to the first-pass score the rescore used, and the expression value computed from it (107.0 for a first-pass score of 1.0 and popularity 7). Instead, since 7.0.0, ExpressionRescorer hands SortRescorer's explanation, whose value is always 0, to the expression as the score explanation, so the explanation reads '7.0 = _score * 100 + popularity' with a _score detail of 0.0, while the rescore itself (order, sort values, scores) is correct.
The test shows that ExpressionRescorer.explain for '_score * 100 + popularity' evaluates the expression with SortRescorer's 0.0 placeholder as _score instead of the first-pass score (one doc, MatchAllDocsQuery).
Expected vs actual
Expected: the _score detail is the first-pass score 1.0. Actual: 0.0 (root 7.0 while the rescore sorts by 107.0).
Files
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/expressions/build/libs/lucene-expressions-$V.jar
CP=$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.expressions.TestExpressionRescorerExplainScoreThe module(s) expressions also need their third-party runtime jars; ./gradlew -p lucene/<module> dependencies lists them.
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.