switchyard_libsy/algorithms/util/
subagent.rs

1// SPDX-FileCopyrightText: Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
2// SPDX-License-Identifier: Apache-2.0
3
4//! Sub-agent override as a single SDK component.
5//!
6//! [`SubagentOverride`] scores one fixed worker target for requests carrying delegated
7//! sub-agent work ([`Metadata::is_subagent_work`]) and abstains for everything else, so a
8//! cascade falls through to its later classifiers on ordinary traffic.
9//!
10//! It is stateless and holds only the worker's *name*: the fall-through cascade resolves it
11//! against its
12//! target set. Keeping the policy independent of any memory of past decisions is what lets
13//! it compose with a stateful classifier such as
14//! [`AffinityRouter`](crate::algorithms::AffinityRouter) — the override decides *which*
15//! target delegated work belongs on, affinity decides *how long* a decision lives, and
16//! neither needs to know about the other.
17
18use async_trait::async_trait;
19
20use crate::Result;
21use crate::core::algorithm::Driver;
22use crate::core::classifier::{Classification, Classifier, Score};
23use switchyard_protocol::{Metadata, Request, Response};
24
25/// Scores a fixed worker target for delegated sub-agent work; abstains otherwise.
26pub struct SubagentOverride {
27    /// Name of the worker target, resolved by the cascade against its target set.
28    worker: String,
29}
30
31impl SubagentOverride {
32    /// Creates an override scoring `worker` for delegated sub-agent work.
33    ///
34    /// `worker` must name a target in the cascade's set, or routing a sub-agent request
35    /// fails with [`LibsyError::TargetNotFound`](crate::LibsyError::TargetNotFound).
36    pub fn new(worker: impl Into<String>) -> Self {
37        Self {
38            worker: worker.into(),
39        }
40    }
41}
42
43#[async_trait]
44impl<S> Classifier<S> for SubagentOverride
45where
46    S: Send + 'static,
47{
48    async fn score(
49        &self,
50        _state: &mut S,
51        request: &mut Request,
52        _driver: Option<&Driver>,
53    ) -> Result<(Classification, Option<Response>)> {
54        // Delegated *work* only. A harness maintenance turn (e.g. Codex `compact`) carries
55        // sub-agent lineage but is not delegated work, so it abstains and routes normally.
56        let is_delegated_work = request
57            .metadata
58            .as_ref()
59            .is_some_and(Metadata::is_subagent_work);
60        Ok((
61            Classification::Scores(if is_delegated_work {
62                vec![Score {
63                    confidence: 1.0,
64                    target: self.worker.clone(),
65                }]
66            } else {
67                Vec::new()
68            }),
69            None,
70        ))
71    }
72}
73
74#[cfg(test)]
75mod tests {
76    use super::*;
77    use std::collections::BTreeMap;
78    use switchyard_protocol::text_request;
79
80    fn request(headers: &[(&str, &str)]) -> Request {
81        let metadata = (!headers.is_empty()).then(|| {
82            Metadata::from_headers(
83                &headers
84                    .iter()
85                    .map(|(name, value)| ((*name).to_string(), (*value).to_string()))
86                    .collect::<BTreeMap<_, _>>(),
87            )
88        });
89        Request {
90            llm_request: text_request(Some("auto".to_string()), "hi"),
91            raw_request: None,
92            metadata,
93        }
94    }
95
96    /// Scores `headers` through the override, returning the winning target if it scored.
97    async fn selected(headers: &[(&str, &str)]) -> Result<Option<String>> {
98        let mut state = ();
99        let classification = SubagentOverride::new("worker")
100            .score(&mut state, &mut request(headers), None)
101            .await?;
102        Ok(classification.0.argmax(false)?.map(|score| score.target))
103    }
104
105    #[tokio::test]
106    async fn requests_without_metadata_abstain() -> Result<()> {
107        assert_eq!(selected(&[]).await?, None);
108        Ok(())
109    }
110
111    #[tokio::test]
112    async fn subagent_work_scores_the_worker() -> Result<()> {
113        // Claude Code child-agent lineage.
114        let claude = &[
115            ("x-claude-code-session-id", "root"),
116            ("x-claude-code-agent-id", "child-1"),
117        ];
118        assert_eq!(selected(claude).await?, Some("worker".to_string()));
119
120        // Codex delegated-work kinds.
121        assert_eq!(
122            selected(&[("x-openai-subagent", "review")]).await?,
123            Some("worker".to_string())
124        );
125        assert_eq!(
126            selected(&[("x-openai-subagent", "collab_spawn")]).await?,
127            Some("worker".to_string())
128        );
129        Ok(())
130    }
131
132    #[tokio::test]
133    async fn harness_maintenance_turns_abstain() -> Result<()> {
134        assert_eq!(selected(&[("x-openai-subagent", "compact")]).await?, None);
135        assert_eq!(
136            selected(&[("x-switchyard-is-subagent", "false")]).await?,
137            None
138        );
139        Ok(())
140    }
141
142    #[tokio::test]
143    async fn delegated_work_is_scored_definitively() -> Result<()> {
144        // Confidence 1.0 under `Scores` (never `Ambiguous`), so the cascade stops here
145        // rather than consulting later classifiers.
146        let mut state = ();
147        let classification = SubagentOverride::new("worker")
148            .score(
149                &mut state,
150                &mut request(&[("x-openai-subagent", "review")]),
151                None,
152            )
153            .await?;
154        match classification.0 {
155            Classification::Scores(scores) => {
156                assert_eq!(scores.len(), 1);
157                assert_eq!(scores[0].confidence, 1.0);
158            }
159            Classification::Ambiguous(_) => panic!("override must score definitively"),
160        }
161        Ok(())
162    }
163}
switchyard_libsy/algorithms/util/subagent.rs

switchyard_libsy/algorithms/util/
subagent.rs
