diff --git a/rust/fleet-data/src/lib.rs b/rust/fleet-data/src/lib.rs
index 4f59e67..63569b7 100644
--- a/rust/fleet-data/src/lib.rs
+++ b/rust/fleet-data/src/lib.rs
@@ -8,3 +8,4 @@ pub mod panes;
 pub mod plan;
 pub mod scores;
 pub mod tmux;
+pub mod toposort;
diff --git a/rust/fleet-data/src/toposort.rs b/rust/fleet-data/src/toposort.rs
new file mode 100644
index 0000000..8b23323
--- /dev/null
+++ b/rust/fleet-data/src/toposort.rs
@@ -0,0 +1,144 @@
+//! Kahn-style topological-wave assignment for plan subtasks.
+//!
+//! Extracted from the byte-identical copies that previously lived in
+//! `fleet-plan-tree::waves()` and `fleet-waves::waves()`. Behaviour is
+//! preserved verbatim: missing predecessors collapse to level 0, indices
+//! inside each wave are sorted ascending, and cycles in `depends_on` are
+//! NOT handled (the recursive resolver will overflow the stack on a cycle).
+//! Callers must ensure their plan.json is acyclic — the producer side
+//! (Colony plan publisher) already guarantees this.
+
+use crate::plan::Subtask;
+use std::collections::HashMap;
+
+/// Assign each subtask to a wave such that every `depends_on` predecessor
+/// sits in a strictly lower wave. Returns `out[level] = Vec<subtask_index>`
+/// with indices sorted ascending inside each wave.
+pub fn waves(subtasks: &[Subtask]) -> Vec<Vec<u32>> {
+    let mut level: HashMap<u32, u32> = HashMap::new();
+    let by_idx: HashMap<u32, &Subtask> =
+        subtasks.iter().map(|s| (s.subtask_index, s)).collect();
+    for s in subtasks {
+        resolve(s.subtask_index, &by_idx, &mut level);
+    }
+    let max = level.values().copied().max().unwrap_or(0);
+    let mut out: Vec<Vec<u32>> = (0..=max).map(|_| Vec::new()).collect();
+    let mut idxs: Vec<u32> = level.keys().copied().collect();
+    idxs.sort();
+    for i in idxs {
+        out[level[&i] as usize].push(i);
+    }
+    out
+}
+
+fn resolve(idx: u32, by: &HashMap<u32, &Subtask>, memo: &mut HashMap<u32, u32>) -> u32 {
+    if let Some(&v) = memo.get(&idx) {
+        return v;
+    }
+    let s = match by.get(&idx) {
+        Some(s) => s,
+        None => {
+            memo.insert(idx, 0);
+            return 0;
+        }
+    };
+    let lvl = if s.depends_on.is_empty() {
+        0
+    } else {
+        s.depends_on
+            .iter()
+            .map(|d| resolve(*d, by, memo))
+            .max()
+            .unwrap_or(0)
+            + 1
+    };
+    memo.insert(idx, lvl);
+    lvl
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::plan::Subtask;
+
+    fn st(idx: u32, deps: Vec<u32>) -> Subtask {
+        Subtask {
+            subtask_index: idx,
+            title: format!("t{idx}"),
+            description: String::new(),
+            file_scope: Vec::new(),
+            depends_on: deps,
+            capability_hint: None,
+            spec_row_id: None,
+            status: "available".into(),
+            claimed_by_session_id: None,
+            claimed_by_agent: None,
+            completed_summary: None,
+        }
+    }
+
+    #[test]
+    fn empty_graph_yields_single_empty_wave() {
+        // Preserved historical behaviour: with no nodes, `max = 0` so the
+        // output is a single empty wave (not an empty Vec).
+        let out = waves(&[]);
+        assert_eq!(out, vec![Vec::<u32>::new()]);
+    }
+
+    #[test]
+    fn single_node_lands_in_wave_zero() {
+        let out = waves(&[st(0, vec![])]);
+        assert_eq!(out, vec![vec![0]]);
+    }
+
+    #[test]
+    fn linear_chain_one_node_per_wave() {
+        let plan = vec![st(0, vec![]), st(1, vec![0]), st(2, vec![1]), st(3, vec![2])];
+        let out = waves(&plan);
+        assert_eq!(out, vec![vec![0], vec![1], vec![2], vec![3]]);
+    }
+
+    #[test]
+    fn two_parallel_chains_share_waves() {
+        // 0 -> 1 -> 2
+        // 10 -> 11 -> 12
+        let plan = vec![
+            st(0, vec![]),
+            st(1, vec![0]),
+            st(2, vec![1]),
+            st(10, vec![]),
+            st(11, vec![10]),
+            st(12, vec![11]),
+        ];
+        let out = waves(&plan);
+        assert_eq!(out, vec![vec![0, 10], vec![1, 11], vec![2, 12]]);
+    }
+
+    #[test]
+    fn node_with_multiple_deps_takes_max_plus_one() {
+        // 0, 1 are roots; 2 depends on a long chain via 1; 3 depends on both.
+        let plan = vec![
+            st(0, vec![]),
+            st(1, vec![]),
+            st(2, vec![1]),
+            st(3, vec![0, 2]),
+        ];
+        let out = waves(&plan);
+        assert_eq!(out, vec![vec![0, 1], vec![2], vec![3]]);
+    }
+
+    #[test]
+    fn missing_dependency_treated_as_level_zero() {
+        // Subtask 1 depends on a non-existent 99 — preserved behaviour
+        // memoises the unknown index at level 0, so it is emitted in wave
+        // 0 alongside any real roots and 1 lands at level 1.
+        let plan = vec![st(1, vec![99])];
+        let out = waves(&plan);
+        assert_eq!(out, vec![vec![99], vec![1]]);
+    }
+
+    // NOTE: cycles in `depends_on` are NOT handled — the recursive resolver
+    // overflows the stack. Adding an executable cycle test would crash the
+    // test binary, so the invariant is documented at the module level
+    // instead. Producers (Colony plan publisher) guarantee acyclic input.
+}
diff --git a/rust/fleet-plan-tree/src/main.rs b/rust/fleet-plan-tree/src/main.rs
index 5a1658b..1452586 100644
--- a/rust/fleet-plan-tree/src/main.rs
+++ b/rust/fleet-plan-tree/src/main.rs
@@ -23,6 +23,7 @@ use std::process::Command;
 use std::time::{Duration, Instant};
 
 use fleet_data::plan::{self, Plan, Subtask};
+use fleet_data::toposort::waves;
 use fleet_ui::{
     card::card,
     chip::{status_chip, ChipKind},
@@ -934,51 +935,6 @@ fn resolve_plan_path() -> Option<PathBuf> {
     plan::newest_plan(&PathBuf::from(root)).ok().flatten()
 }
 
-fn waves(subtasks: &[Subtask]) -> Vec<Vec<u32>> {
-    let mut level: std::collections::HashMap<u32, u32> = std::collections::HashMap::new();
-    let by_idx: std::collections::HashMap<u32, &Subtask> =
-        subtasks.iter().map(|s| (s.subtask_index, s)).collect();
-    fn resolve(
-        idx: u32,
-        by: &std::collections::HashMap<u32, &Subtask>,
-        memo: &mut std::collections::HashMap<u32, u32>,
-    ) -> u32 {
-        if let Some(&v) = memo.get(&idx) {
-            return v;
-        }
-        let s = match by.get(&idx) {
-            Some(s) => s,
-            None => {
-                memo.insert(idx, 0);
-                return 0;
-            }
-        };
-        let lvl = if s.depends_on.is_empty() {
-            0
-        } else {
-            s.depends_on
-                .iter()
-                .map(|d| resolve(*d, by, memo))
-                .max()
-                .unwrap_or(0)
-                + 1
-        };
-        memo.insert(idx, lvl);
-        lvl
-    }
-    for s in subtasks {
-        resolve(s.subtask_index, &by_idx, &mut level);
-    }
-    let max = level.values().copied().max().unwrap_or(0);
-    let mut out: Vec<Vec<u32>> = (0..=max).map(|_| Vec::new()).collect();
-    let mut idxs: Vec<u32> = level.keys().copied().collect();
-    idxs.sort();
-    for i in idxs {
-        out[level[&i] as usize].push(i);
-    }
-    out
-}
-
 fn rollup(plan: &Plan) -> (usize, usize, usize, usize) {
     let mut available = 0;
     let mut claimed = 0;
diff --git a/rust/fleet-waves/src/main.rs b/rust/fleet-waves/src/main.rs
index 6006a69..e1b1d41 100644
--- a/rust/fleet-waves/src/main.rs
+++ b/rust/fleet-waves/src/main.rs
@@ -11,9 +11,8 @@
 //                title, agent-initial badges on the right.
 //
 // Data: fleet-data::plan (newest plan.json under openspec/plans/*). Waves
-// come from a Kahn topological sort of `Subtask.depends_on`; copied inline
-// from fleet-plan-tree::waves() so this binary doesn't bump fleet-ui (locked
-// by another agent during warp-borrow validation).
+// come from a Kahn topological sort of `Subtask.depends_on`, provided by
+// `fleet_data::toposort::waves`.
 //
 // Visual notes:
 //   - Idle bars use IOS_CARD_BG (#2c2c30) — slightly above the background
@@ -36,7 +35,10 @@ use std::{
     time::{Duration, Instant, SystemTime, UNIX_EPOCH},
 };
 
-use fleet_data::plan::{self, Plan, Subtask};
+use fleet_data::plan::{self, Plan};
+#[cfg(test)]
+use fleet_data::plan::Subtask;
+use fleet_data::toposort::waves;
 use fleet_ui::{
     chip::{status_chip, ChipKind, CHIP_WIDTH},
     palette::*,
@@ -153,55 +155,6 @@ impl AppComponent<Msg, NoUserEvent> for App {
     }
 }
 
-// Kahn topological levels — assign each subtask to a wave such that all
-// `depends_on` predecessors are in lower waves. Copied verbatim from
-// fleet-plan-tree::waves(); kept inline to avoid promoting it into a shared
-// crate while fleet-ui is locked by another agent.
-fn waves(subtasks: &[Subtask]) -> Vec<Vec<u32>> {
-    let mut level: std::collections::HashMap<u32, u32> = std::collections::HashMap::new();
-    let by_idx: std::collections::HashMap<u32, &Subtask> =
-        subtasks.iter().map(|s| (s.subtask_index, s)).collect();
-    fn resolve(
-        idx: u32,
-        by: &std::collections::HashMap<u32, &Subtask>,
-        memo: &mut std::collections::HashMap<u32, u32>,
-    ) -> u32 {
-        if let Some(&v) = memo.get(&idx) {
-            return v;
-        }
-        let s = match by.get(&idx) {
-            Some(s) => s,
-            None => {
-                memo.insert(idx, 0);
-                return 0;
-            }
-        };
-        let lvl = if s.depends_on.is_empty() {
-            0
-        } else {
-            s.depends_on
-                .iter()
-                .map(|d| resolve(*d, by, memo))
-                .max()
-                .unwrap_or(0)
-                + 1
-        };
-        memo.insert(idx, lvl);
-        lvl
-    }
-    for s in subtasks {
-        resolve(s.subtask_index, &by_idx, &mut level);
-    }
-    let max = level.values().copied().max().unwrap_or(0);
-    let mut out: Vec<Vec<u32>> = (0..=max).map(|_| Vec::new()).collect();
-    let mut idxs: Vec<u32> = level.keys().copied().collect();
-    idxs.sort();
-    for i in idxs {
-        out[level[&i] as usize].push(i);
-    }
-    out
-}
-
 #[derive(Clone, Copy, PartialEq, Eq)]
 enum WaveStatus {
     Done,