feat(libsy): let stage algorithms fall_open tier be set at runtime

crates/libsy/src/algorithms/stage.rs

@@ -10,23 +10,23 @@
 //!
 //! Signals do not decide every turn. An under-threshold turn abstains and falls
 //! through to the optional [`LlmTaskClassifier`] — the capability route's judge,
-//! joined in unchanged — and then to the picker's default tier. The judge is
-//! asked per turn and its verdict is never pinned to the session.
+//! joined in unchanged — and then to the picker's default tier, or to an
+//! override a decider ahead of stage set in its place.
 //!
 use std::sync::Arc;
 
 use async_trait::async_trait;
 
-use super::fall_through::{DefaultTarget, FallThrough};
+use super::fall_through::FallThrough;
 use super::llm_class::{LlmClassifierConfig, LlmTaskClassifier, TaskClassifierConfig};
 use super::util::prompts::{SystemPromptProcessor, TargetPrompts};
 use super::util::stage::{
-    DecisionSource, HandoffNoteConfig, PickerMode, StageClassifier, StageTargets,
-    record_decision_source, record_routing_decision,
+    DecisionSource, HandoffNoteConfig, PickerMode, StageClassifier, StageTargets, Tier,
+    fall_open_tier, record_decision_source, record_routing_decision,
 };
 use super::util::tool_signals::{DEFAULT_RECENT_WINDOW, ToolSignalProcessor};
 use crate::core::algorithm::{Algorithm, Driver};
-use crate::core::classifier::{Classification, Classifier};
+use crate::core::classifier::{Classification, Classifier, Score};
 use crate::core::state::State;
 use crate::{LibsyError, Result};
 use switchyard_protocol::{ModelId, Request, Response};
@@ -65,6 +65,32 @@ impl Classifier<State> for SourceStamp {
     }
 }
 
+/// Closes the cascade at zero confidence: a fallback, not a judgement.
+struct FallOpen {
+    targets: StageTargets,
+    default_tier: Tier,
+}
+
+#[async_trait]
+impl Classifier<State> for FallOpen {
+    async fn score(
+        &self,
+        state: &mut State,
+        _request: &mut Request,
+        _driver: Option<&Driver>,
+    ) -> Result<(Classification, Option<Response>)> {
+        let tier = fall_open_tier(state).unwrap_or(self.default_tier);
+        let target = self.targets.name(tier).clone();
+        Ok((
+            Classification::Scores(vec![Score {
+                target,
+                confidence: 0.0,
+            }]),
+            None,
+        ))
+    }
+}
+
 /// The capability judge a stage router falls through to.
 pub struct LlmFallback {
     /// Target the judge model is called through. It is not a routing
@@ -165,9 +191,11 @@ fn build_route(
     // The tiers are a fixed pair; their targets are whatever the deployment calls
     // them, and the classifier scores onto those names.
     let targets = StageTargets::new(capable.clone(), efficient.clone());
-    // The picker's mode fixes the fallback tier up front, so the terminal
-    // classifier is a constant rather than a per-turn lookup.
-    let fall_open = targets.name(config.mode.default_tier()).to_string();
+    let default_tier = config.mode.default_tier();
+    let fall_open = FallOpen {
+        targets: targets.clone(),
+        default_tier,
+    };
 
     let mut classifier = StageClassifier::new(targets, config.mode, config.confidence_threshold);
     if let Some(notes) = config.handoff_notes {
@@ -194,10 +222,9 @@ fn build_route(
             source: DecisionSource::LlmClassifier,
         }));
     }
-    // Nothing behind this, so the turn lands on the picker's default tier —
-    // including when the judge could not tell.
+    // Nothing behind this, so no turn is left unrouted.
     router = router.with_classifier(Arc::new(SourceStamp {
-        inner: Arc::new(DefaultTarget::new(fall_open)),
+        inner: Arc::new(fall_open),
         source: DecisionSource::FallOpen,
     }));
     // Runs on the post-decision hook, so it applies to the target the cascade
@@ -219,8 +246,8 @@ mod tests {
     };
 
     use super::*;
-    use crate::algorithms::util::stage::DECISION_SOURCE_KEY;
-    use crate::core::classifier::Score;
+    use crate::algorithms::util::stage::{DECISION_SOURCE_KEY, clear_fall_open, set_fall_open};
+    use crate::core::processor::{Event, Processor};
     use crate::core::state::StateValue;
     use crate::core::testing::{Serve, reply, test_drive};
     use switchyard_protocol::{Metadata, Response};
@@ -465,6 +492,64 @@ mod tests {
         }
     }
 
+    /// Stands in for a decider ahead of stage: sets the override once, clears it
+    /// once, and leaves the turns between alone.
+    #[derive(Default)]
+    struct TierDecider {
+        requests: Mutex<u32>,
+    }
+
+    #[async_trait]
+    impl Processor<State> for TierDecider {
+        async fn process(&self, state: &mut State, event: Event<'_>) -> Result<()> {
+            if matches!(event, Event::Request(_)) {
+                let mut requests = self.requests.lock();
+                *requests += 1;
+                match *requests {
+                    1 => set_fall_open(state, Tier::Efficient),
+                    4 => clear_fall_open(state),
+                    _ => {}
+                }
+            }
+            Ok(())
+        }
+    }
+
+    #[tokio::test]
+    async fn an_override_replaces_the_picker_default_and_leaves_the_signals_alone() -> Result<()> {
+        let recorder = Arc::new(Recorder::default());
+        // The picker would fall open to "strong"; the override says "weak".
+        let config = StageRouterConfig::new(PickerMode::CapableFirst, 0.5);
+        let route: Arc<dyn Algorithm> = Arc::new(
+            build_route(ModelId::from("strong"), ModelId::from("weak"), config)?
+                .with_processor(Arc::new(TierDecider::default())),
+        );
+
+        test_drive(route.clone(), turn_request(false), recorder.serve()).await?;
+        test_drive(route.clone(), turn_request(false), recorder.serve()).await?;
+        test_drive(route.clone(), turn_request(true), recorder.serve()).await?;
+        test_drive(route.clone(), turn_request(false), recorder.serve()).await?;
+
+        let routed = recorder.routed();
+        assert_eq!(
+            routed[0].target, "weak",
+            "an undecided turn takes the override"
+        );
+        assert_eq!(
+            routed[1].target, "weak",
+            "which outlives the turn that set it"
+        );
+        assert_eq!(
+            routed[2].target, "strong",
+            "a critical failure still reaches the signals"
+        );
+        assert_eq!(
+            routed[3].target, "strong",
+            "clearing restores the picker default"
+        );
+        Ok(())
+    }
+
     #[tokio::test]
     async fn a_signal_driven_escalation_hands_the_note_to_the_model() -> Result<()> {
         let recorder = Arc::new(Recorder::default());

crates/libsy/src/algorithms/util/stage.rs

@@ -81,13 +81,22 @@ impl Tier {
     /// Stable label for stats and the [`routing_tier`](Classifier::routing_tier)
     /// hook, independent of what the tiers' targets are called. These are the
     /// strings the capability route reports too, so a deployment running both
-    /// sees one tier vocabulary.
+    /// sees one tier vocabulary. Also the encoding a default-tier override is
+    /// stored under, so changing these strings invalidates one.
     fn label(self) -> &'static str {
         match self {
             Self::Capable => "strong",
             Self::Efficient => "weak",
         }
     }
+
+    fn from_label(label: &str) -> Option<Self> {
+        match label {
+            "strong" => Some(Self::Capable),
+            "weak" => Some(Self::Efficient),
+            _ => None,
+        }
+    }
 }
 
 /// The targets a stage router's two tiers route to.
@@ -153,6 +162,32 @@ impl PickerMode {
 /// `State.extra` key under which the turn's [`DecisionSource`] is recorded.
 pub const DECISION_SOURCE_KEY: &str = "decision_source";
 
+/// `State.extra` key under which a default-tier override is recorded.
+const FALL_OPEN_KEY: &str = "fall_open";
+
+/// Overrides the default tier undecided turns fall open to, until
+/// [`clear_fall_open`] restores the picker's. Held in session state, so it
+/// survives later requests only when they carry a session id.
+pub fn set_fall_open(state: &mut State, tier: Tier) {
+    state.extra.insert(
+        FALL_OPEN_KEY.to_string(),
+        StateValue::String(tier.label().to_string()),
+    );
+}
+
+/// Restores the picker's default tier.
+pub fn clear_fall_open(state: &mut State) {
+    state.extra.remove(FALL_OPEN_KEY);
+}
+
+/// The default-tier override for this session, if any.
+pub(crate) fn fall_open_tier(state: &State) -> Option<Tier> {
+    match state.extra.get(FALL_OPEN_KEY) {
+        Some(StateValue::String(label)) => Tier::from_label(label),
+        _ => None,
+    }
+}
+
 /// Record which component decided the turn.
 pub(crate) fn record_decision_source(state: &mut State, source: DecisionSource) {
     state.extra.insert(

crates/libsy/src/lib.rs

@@ -37,8 +37,8 @@ pub use algorithms::util::tool_signals::{DEFAULT_RECENT_WINDOW, ToolSignals};
 // core (scorer, picker, and the `StageClassifier`).
 pub use algorithms::util::stage::{
     CodingAgentDimensions, DECISION_SOURCE_KEY, DecisionSource, HandoffNoteConfig, PickOutcome,
-    PickerMode, ScoreResult, StageClassifier, StageTargets, Tier, dimensions_from_signal,
-    pick_tier, score_signal,
+    PickerMode, ScoreResult, StageClassifier, StageTargets, Tier, clear_fall_open,
+    dimensions_from_signal, pick_tier, score_signal, set_fall_open,
 };
 
 mod observability;