diff --git a/src/Core/TemporalCoordinationDetection.fs b/src/Core/TemporalCoordinationDetection.fs
index fb250f57..e9e27e29 100644
--- a/src/Core/TemporalCoordinationDetection.fs
+++ b/src/Core/TemporalCoordinationDetection.fs
@@ -94,3 +94,57 @@ module TemporalCoordinationDetection =
         else
             [| for tau in -maxLag .. maxLag ->
                    tau, crossCorrelation xs ys tau |]
+
+    /// **Phase-locking value (PLV)** — the magnitude of the mean
+    /// complex phase-difference vector between two phase series.
+    /// Returns a value in `[0.0, 1.0]`:
+    ///
+    /// * `1.0` — perfect phase locking (the phase difference
+    ///   `φ_a[k] - φ_b[k]` is constant across the series; two
+    ///   actors whose events always arrive at the same phase
+    ///   offset look like this, which is the classical firefly-
+    ///   synchronization signature)
+    /// * `0.0` — phase differences uniformly spread around the
+    ///   unit circle (the null hypothesis of independent timing)
+    /// * in between — partial coordination
+    ///
+    /// Returns `None` when the input sequences are empty or of
+    /// unequal length; PLV is undefined for mismatched pairs
+    /// and silently truncating would invite a subtle detection
+    /// bug downstream.
+    ///
+    /// Input phases `phasesA` and `phasesB` are expected in
+    /// radians; the computation uses the Euler identity
+    /// `e^{iθ} = cos θ + i sin θ` and only depends on the
+    /// phase *difference*, so any consistent wrapping convention
+    /// (`[-π, π]` or `[0, 2π]`) works — callers do not need to
+    /// pre-unwrap.
+    ///
+    /// Complementary to `crossCorrelation`: cross-correlation
+    /// answers "do amplitudes move together?"; PLV answers "do
+    /// the events fire at matching phases?". Cartels that flatten
+    /// amplitude cross-correlation by adding noise may still
+    /// reveal themselves through preserved phase structure, and
+    /// vice versa. Detectors compose both.
+    ///
+    /// Provenance: primitive from Aaron's differentiable firefly
+    /// network design, formalized in Amara's 11th courier ferry
+    /// (`docs/aurora/2026-04-24-amara-temporal-coordination-
+    /// detection-cartel-graph-influence-surface-11th-ferry.md`,
+    /// §1 Signal model). Third graduation under the Otto-105
+    /// cadence.
+    let phaseLockingValue (phasesA: double seq) (phasesB: double seq) : double option =
+        let aArr = Seq.toArray phasesA
+        let bArr = Seq.toArray phasesB
+        if aArr.Length = 0 || aArr.Length <> bArr.Length then None
+        else
+            let mutable sumCos = 0.0
+            let mutable sumSin = 0.0
+            for i in 0 .. aArr.Length - 1 do
+                let d = aArr.[i] - bArr.[i]
+                sumCos <- sumCos + cos d
+                sumSin <- sumSin + sin d
+            let n = double aArr.Length
+            let meanCos = sumCos / n
+            let meanSin = sumSin / n
+            Some (sqrt (meanCos * meanCos + meanSin * meanSin))
diff --git a/tests/Tests.FSharp/Algebra/TemporalCoordinationDetection.Tests.fs b/tests/Tests.FSharp/Algebra/TemporalCoordinationDetection.Tests.fs
index f8acce01..265fa28c 100644
--- a/tests/Tests.FSharp/Algebra/TemporalCoordinationDetection.Tests.fs
+++ b/tests/Tests.FSharp/Algebra/TemporalCoordinationDetection.Tests.fs
@@ -99,3 +99,85 @@ let ``crossCorrelationProfile with negative maxLag returns empty array`` () =
     let xs = [ 1.0; 2.0; 3.0 ]
     let ys = [ 1.0; 2.0; 3.0 ]
     TemporalCoordinationDetection.crossCorrelationProfile xs ys -1 |> should equal ([||]: (int * double option) array)
+
+
+// ─── phaseLockingValue ─────────
+
+[<Fact>]
+let ``phaseLockingValue of identical phase series is 1`` () =
+    let phases = [ 0.0; 0.5; 1.0; 1.5; 2.0 ]
+    TemporalCoordinationDetection.phaseLockingValue phases phases
+    |> Option.map (fun v -> Math.Round(v, 9))
+    |> should equal (Some 1.0)
+
+[<Fact>]
+let ``phaseLockingValue with constant phase offset is 1 (perfect locking)`` () =
+    // Constant offset of pi/4 — the complex phase-difference
+    // vector is the same unit vector every step, so magnitude = 1.
+    let a = [ 0.0; 0.3; 0.6; 0.9; 1.2 ]
+    let offset = Math.PI / 4.0
+    let b = a |> List.map (fun x -> x + offset)
+    TemporalCoordinationDetection.phaseLockingValue a b
+    |> Option.map (fun v -> Math.Round(v, 9))
+    |> should equal (Some 1.0)
+
+[<Fact>]
+let ``phaseLockingValue of empty series is None`` () =
+    TemporalCoordinationDetection.phaseLockingValue [] [] |> should equal (None: double option)
+
+[<Fact>]
+let ``phaseLockingValue on mismatched-length series is None`` () =
+    // PLV is undefined for mismatched pairs. Silently truncating
+    // would mask a caller bug; None surfaces it.
+    let a = [ 0.0; 0.5; 1.0 ]
+    let b = [ 0.0; 0.5 ]
+    TemporalCoordinationDetection.phaseLockingValue a b |> should equal (None: double option)
+
+[<Fact>]
+let ``phaseLockingValue of anti-phase series is 1 (locking at pi offset)`` () =
+    // Two phase series that differ by exactly pi every step are
+    // perfectly anti-phase-locked; PLV measures the magnitude of
+    // the mean complex vector, which is 1 when the offset is
+    // constant (regardless of offset value).
+    let a = [ 0.0; 0.5; 1.0; 1.5 ]
+    let b = a |> List.map (fun x -> x + Math.PI)
+    TemporalCoordinationDetection.phaseLockingValue a b
+    |> Option.map (fun v -> Math.Round(v, 9))
+    |> should equal (Some 1.0)
+
+[<Fact>]
+let ``phaseLockingValue of uniformly-distributed differences is near 0`` () =
+    // Evenly-spaced phase differences spanning [0, 2*pi); the
+    // complex vectors sum to approximately zero by symmetry.
+    // Large N for the cancellation to be numerically clean.
+    let n = 360
+    let a = [ for _ in 0 .. n - 1 -> 0.0 ]
+    let b = [ for i in 0 .. n - 1 -> 2.0 * Math.PI * double i / double n ]
+    let plv =
+        TemporalCoordinationDetection.phaseLockingValue a b
+        |> Option.defaultValue -1.0
+    plv |> should (be lessThan) 1e-9
+
+[<Fact>]
+let ``phaseLockingValue is commutative`` () =
+    // Swapping arguments flips the sign of every phase difference,
+    // which negates sin but leaves cos unchanged; the magnitude of
+    // the mean complex vector is invariant.
+    let a = [ 0.0; 0.4; 0.8; 1.2; 1.6 ]
+    let b = [ 0.1; 0.3; 0.7; 1.4; 1.5 ]
+    let ab = TemporalCoordinationDetection.phaseLockingValue a b
+    let ba = TemporalCoordinationDetection.phaseLockingValue b a
+    ab |> Option.map (fun v -> Math.Round(v, 12))
+    |> should equal (ba |> Option.map (fun v -> Math.Round(v, 12)))
+
+[<Fact>]
+let ``phaseLockingValue handles single-element series`` () =
+    // N=1 is a degenerate case: the single complex vector has
+    // magnitude 1 regardless of phase. Not useful as a detector
+    // at that size (no statistical power), but the function
+    // must not crash and must return a defined value.
+    let a = [ 0.0 ]
+    let b = [ 0.0 ]
+    TemporalCoordinationDetection.phaseLockingValue a b
+    |> Option.map (fun v -> Math.Round(v, 9))
+    |> should equal (Some 1.0)