FreeTextSuggester.lookup throws AssertionError (-ea) or NullPointerException (-da) when a word the analyzer removed (a stop word) comes right before the key's last word, because it reads the count of the filler-only context '_', which the n-gram model never stores (lookup('the w') with the default constructor and EnglishAnalyzer)

Finding LFV-662Area: suggest / FreeTextSuggester (n-gram model built with ShingleFilter; lookup context counts after analysis holes)Priority: mediumImpact: crashNovelty: known upstream

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 FreeTextSuggester with the default constructor and the stock EnglishAnalyzer over 'the wizard of oz' and call lookup('the w'), expecting the suggestion 'wizard': lookup documents only IllegalArgumentException and IllegalStateException, and the upstream tests testEndingHole and testTwoEndingHoles assert results for a stop-word analyzer. Instead lookup throws AssertionError at FreeTextSuggester.java:576 with -ea, or NullPointerException in decodeWeight (:724) with -da: the removed 'the' leaves a hole, so the bigram-level query token is '_ w', which matches the stored shingle '_ wizard', and lookup then reads the count of its context '_', which ShingleFilter never emits and the model therefore never stores. The same happens on the upstream testEndingHole model for lookup('wizard of o'), one keystroke after the query that test asserts.

Expected vs actual

Expected: The test passes: it asserts the documented behaviour.
Actual: Lookup throws AssertionError at FreeTextSuggester.java:576 with -ea, or NullPointerException in decodeWeight (:724) with -da: the removed 'the' leaves a hole, so the bigram-level query token is '_ w', which matches the stored shingle '_ wizard', and lookup then reads the count of its context '_', which ShingleFilter never emits and the model therefore never stores.

Files

src/org/apache/lucene/search/suggest/analyzing/TestFreeTextSuggesterStopWordContext.java (JUnit test)

Run it

Tested on Lucene main e1faeba08bde (2026-11-02): still reproduces.
Tested on Lucene main a9779ef3c193 (2026-10-26): reproduced.
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:$LUCENE/lucene/suggest/build/libs/lucene-suggest-$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 off (production-like), seed DEADBEEF
java -da -dsa -Dtests.asserts=false --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.suggest.analyzing.TestFreeTextSuggesterStopWordContext

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 2026-11-03. AI-assisted; not yet triaged by a human Lucene committer. Feedback: open an issue in lucene-ai-bug-hunt.