Fuse recent and lexical skills in shadow selection (#38204)

## What changed

- Add the `lru_plus_lexical_v1` selector, combining the 50 most recent skills
  with weighted lexical matches through reciprocal rank fusion.
- Include the combined selector in the shadow-selection experiment and propagate
  query and candidate truncation metadata from both inputs.
- Generalize the existing reciprocal-rank fusion helper to accept a rank
  constant while preserving the existing lexical/character selector behavior.

## Testing

- Cover lexical-only selection, merging recent and matching skills, promotion
  from agreement between both rankings, and weak overlaps.
- Extend the skills extension test to verify invocation metrics for the new
  selector alongside `lru_v1`.

GitOrigin-RevId: 1b7b82bc97ea919c719cd05a009351b701019306

codex-rs/ext/skills/src/dynamic_skill_selector.rs

@@ -2,6 +2,7 @@ mod character_ngram;
 mod character_routing_card;
 mod fielded_bm25;
 mod lru;
+mod lru_plus_lexical;
 mod multi_query_lexical;
 mod routing_card_lexical;
 mod rrf_lexical_char;
@@ -11,6 +12,7 @@ pub(crate) use character_routing_card::CharacterRoutingCardSkillSelector;
 use codex_skills::SkillDependencies;
 pub(crate) use fielded_bm25::FieldedBm25SkillSelector;
 pub(crate) use lru::LruSkillSelector;
+pub(crate) use lru_plus_lexical::LruPlusLexicalSkillSelector;
 pub(crate) use multi_query_lexical::MultiQueryLexicalSkillSelector;
 pub(crate) use routing_card_lexical::RoutingCardLexicalSkillSelector;
 pub(crate) use rrf_lexical_char::RrfLexicalCharSkillSelector;

codex-rs/ext/skills/src/dynamic_skill_selector/lru_plus_lexical.rs

@@ -0,0 +1,60 @@
+use super::CheapSkillSelection;
+use super::CheapSkillSelector;
+use super::LruSkillSelector;
+use super::SkillSelectionDocument;
+use super::WeightedLexicalSkillSelector;
+use super::rrf_lexical_char::fuse_rankings_with_constant;
+
+const MAX_RESULTS: usize = 50;
+const RRF_K: usize = 10;
+
+#[derive(Clone, Debug, Default)]
+pub(crate) struct LruPlusLexicalSkillSelector {
+    lru_selector: LruSkillSelector,
+}
+
+impl LruPlusLexicalSkillSelector {
+    pub(crate) fn new(lru_selector: LruSkillSelector) -> Self {
+        Self { lru_selector }
+    }
+}
+
+impl CheapSkillSelector for LruPlusLexicalSkillSelector {
+    fn method(&self) -> &'static str {
+        "lru_plus_lexical_v1"
+    }
+
+    fn select(
+        &self,
+        query: &str,
+        documents: &[SkillSelectionDocument<'_>],
+        limit: usize,
+    ) -> CheapSkillSelection {
+        if limit == 0 {
+            return CheapSkillSelection::default();
+        }
+
+        let recent = self
+            .lru_selector
+            .select(query, documents, /*limit*/ MAX_RESULTS);
+        let lexical =
+            WeightedLexicalSkillSelector.select(query, documents, /*limit*/ MAX_RESULTS);
+        let candidate_ids = fuse_rankings_with_constant(
+            [&recent.candidate_ids, &lexical.candidate_ids],
+            limit.min(MAX_RESULTS),
+            /*rank_constant*/ RRF_K,
+        );
+
+        CheapSkillSelection {
+            candidate_ids,
+            query_term_count: recent.query_term_count.max(lexical.query_term_count),
+            query_truncated: recent.query_truncated || lexical.query_truncated,
+            candidate_set_truncated: recent.candidate_set_truncated
+                || lexical.candidate_set_truncated,
+        }
+    }
+}
+
+#[cfg(test)]
+#[path = "lru_plus_lexical_tests.rs"]
+mod tests;

codex-rs/ext/skills/src/dynamic_skill_selector/lru_plus_lexical_tests.rs

@@ -0,0 +1,119 @@
+use pretty_assertions::assert_eq;
+
+use super::*;
+
+#[test]
+fn combined_selector_uses_lexical_results_before_invocation_history_exists() {
+    let documents = [
+        SkillSelectionDocument {
+            id: 10,
+            name: "ci",
+            short_description: None,
+            description: "Investigate failing checks.",
+            dependencies: None,
+        },
+        SkillSelectionDocument {
+            id: 20,
+            name: "python-tools",
+            short_description: None,
+            description: "Manage Python environments.",
+            dependencies: None,
+        },
+    ];
+
+    let selection = LruPlusLexicalSkillSelector::new(LruSkillSelector::default()).select(
+        "manage python environments",
+        &documents,
+        /*limit*/ 50,
+    );
+
+    assert_eq!(vec![20], selection.candidate_ids);
+}
+
+#[test]
+fn combined_selector_merges_recent_and_newly_matching_skills() {
+    let documents = [
+        SkillSelectionDocument {
+            id: 10,
+            name: "ci",
+            short_description: None,
+            description: "Investigate failing checks.",
+            dependencies: None,
+        },
+        SkillSelectionDocument {
+            id: 20,
+            name: "python-tools",
+            short_description: None,
+            description: "Manage Python environments.",
+            dependencies: None,
+        },
+        SkillSelectionDocument {
+            id: 30,
+            name: "monorepo",
+            short_description: None,
+            description: "Follow repository conventions.",
+            dependencies: None,
+        },
+    ];
+
+    let selection = LruPlusLexicalSkillSelector::new(LruSkillSelector::new(vec![30, 10])).select(
+        "manage python environments",
+        &documents,
+        /*limit*/ 50,
+    );
+
+    assert_eq!(vec![20, 30, 10], selection.candidate_ids);
+}
+
+#[test]
+fn combined_selector_promotes_skills_supported_by_both_signals() {
+    let documents = [
+        SkillSelectionDocument {
+            id: 10,
+            name: "ci",
+            short_description: None,
+            description: "Investigate failing checks.",
+            dependencies: None,
+        },
+        SkillSelectionDocument {
+            id: 30,
+            name: "monorepo",
+            short_description: None,
+            description: "Follow repository conventions.",
+            dependencies: None,
+        },
+    ];
+
+    let selection = LruPlusLexicalSkillSelector::new(LruSkillSelector::new(vec![30, 10])).select(
+        "investigate failing ci checks",
+        &documents,
+        /*limit*/ 50,
+    );
+
+    assert_eq!(vec![10, 30], selection.candidate_ids);
+}
+
+#[test]
+fn combined_fusion_keeps_a_top_ranked_skill_ahead_of_a_weak_overlap() {
+    let recent = std::iter::once(1)
+        .chain(100..148)
+        .chain(std::iter::once(2))
+        .collect::<Vec<_>>();
+    let lexical = (200..249).chain(std::iter::once(2)).collect::<Vec<_>>();
+
+    let fused = fuse_rankings_with_constant(
+        [&recent, &lexical],
+        /*limit*/ MAX_RESULTS,
+        /*rank_constant*/ RRF_K,
+    );
+    let top_ranked = fused
+        .iter()
+        .position(|candidate| *candidate == 1)
+        .expect("top-ranked skill should be selected");
+    let weak_overlap = fused
+        .iter()
+        .position(|candidate| *candidate == 2)
+        .expect("weak overlapping skill should be selected");
+
+    assert!(top_ranked < weak_overlap);
+}

codex-rs/ext/skills/src/dynamic_skill_selector/rrf_lexical_char.rs

@@ -53,12 +53,20 @@ impl CheapSkillSelector for RrfLexicalCharSkillSelector {
 }
 
 fn fuse_rankings<const N: usize>(rankings: [&[usize]; N], limit: usize) -> Vec<usize> {
+    fuse_rankings_with_constant(rankings, limit, RRF_K)
+}
+
+pub(super) fn fuse_rankings_with_constant<const N: usize>(
+    rankings: [&[usize]; N],
+    limit: usize,
+    rank_constant: usize,
+) -> Vec<usize> {
     let mut scores = HashMap::<usize, f64>::new();
     let mut best_ranks = HashMap::<usize, usize>::new();
     for ranking in rankings {
         for (index, id) in ranking.iter().copied().enumerate() {
             let rank = index + 1;
-            *scores.entry(id).or_default() += 1.0 / (RRF_K + rank) as f64;
+            *scores.entry(id).or_default() += 1.0 / (rank_constant + rank) as f64;
             best_ranks
                 .entry(id)
                 .and_modify(|best_rank| *best_rank = (*best_rank).min(rank))

codex-rs/ext/skills/src/shadow_selection_experiment.rs

@@ -21,6 +21,7 @@ use crate::dynamic_skill_selector::CharacterRoutingCardSkillSelector;
 use crate::dynamic_skill_selector::CheapSkillSelection;
 use crate::dynamic_skill_selector::CheapSkillSelector;
 use crate::dynamic_skill_selector::FieldedBm25SkillSelector;
+use crate::dynamic_skill_selector::LruPlusLexicalSkillSelector;
 use crate::dynamic_skill_selector::LruSkillSelector;
 use crate::dynamic_skill_selector::MultiQueryLexicalSkillSelector;
 use crate::dynamic_skill_selector::RoutingCardLexicalSkillSelector;
@@ -120,7 +121,8 @@ impl ShadowSelectionExperiment {
             .collect();
         let routing_selector = CharacterRoutingCardSkillSelector::new(catalog, host_snapshot);
         let lru_selector = LruSkillSelector::new(recent_skill_ids);
-        let mut ranked_selections = Vec::with_capacity(self.selectors.len() + 2);
+        let lru_plus_lexical_selector = LruPlusLexicalSkillSelector::new(lru_selector.clone());
+        let mut ranked_selections = Vec::with_capacity(self.selectors.len() + 3);
 
         for selector in self
             .selectors
@@ -129,6 +131,7 @@ impl ShadowSelectionExperiment {
             .chain([
                 &routing_selector as &dyn CheapSkillSelector,
                 &lru_selector as &dyn CheapSkillSelector,
+                &lru_plus_lexical_selector as &dyn CheapSkillSelector,
             ])
         {
             let start = Instant::now();

codex-rs/ext/skills/tests/skills_extension.rs

@@ -932,7 +932,7 @@ async fn shadow_lru_selector_recovers_a_skill_invoked_on_an_earlier_turn() -> Te
         .flat_map(opentelemetry_sdk::metrics::data::ScopeMetrics::metrics)
         .find(|metric| metric.name() == "codex.skills.shadow_selection.invocation")
         .ok_or("shadow invocation metric should be recorded")?;
-    let mut lru_hits = match metric.data() {
+    let mut selector_hits = match metric.data() {
         AggregatedMetrics::U64(MetricData::Sum(sum)) => sum
             .data_points()
             .filter_map(|point| {
@@ -941,7 +941,7 @@ async fn shadow_lru_selector_recovers_a_skill_invoked_on_an_earlier_turn() -> Te
                     .find(|attribute| attribute.key.as_str() == "method")?
                     .value
                     .as_str();
-                if method != "lru_v1" {
+                if method != "lru_v1" && method != "lru_plus_lexical_v1" {
                     return None;
                 }
                 let hit = point
@@ -950,16 +950,20 @@ async fn shadow_lru_selector_recovers_a_skill_invoked_on_an_earlier_turn() -> Te
                     .value
                     .as_str()
                     .to_string();
-                Some((hit, point.value()))
+                Some((method.to_string(), hit, point.value()))
             })
             .collect::<Vec<_>>(),
         data => panic!("unexpected shadow invocation metric data: {data:?}"),
     };
-    lru_hits.sort();
+    selector_hits.sort();
 
     assert_eq!(
-        vec![("false".to_string(), 1), ("true".to_string(), 1)],
-        lru_hits
+        vec![
+            ("lru_plus_lexical_v1".to_string(), "true".to_string(), 2),
+            ("lru_v1".to_string(), "false".to_string(), 1),
+            ("lru_v1".to_string(), "true".to_string(), 1),
+        ],
+        selector_hits
     );
     Ok(())
 }