diff --git a/src/Core/TemporalCoordinationDetection.fs b/src/Core/TemporalCoordinationDetection.fs
index 9cf6d3aa..14909663 100644
--- a/src/Core/TemporalCoordinationDetection.fs
+++ b/src/Core/TemporalCoordinationDetection.fs
@@ -127,13 +127,32 @@ module TemporalCoordinationDetection =
     /// 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 =
+    /// Provenance: primitive from the human maintainer's
+    /// differentiable firefly-network design, formalized in an
+    /// external AI collaborator's 11th courier ferry (§1 Signal
+    /// model; ferry content tracked in the Otto-105 operationalize
+    /// queue, see `memory/MEMORY.md` "Amara's 11th ferry"). Third
+    /// graduation under the Otto-105 cadence.
+    /// Shared epsilon floor for phase-difference mean-vector
+    /// magnitude. Used ONLY by `meanPhaseOffset` +
+    /// `phaseLockingWithOffset` to decide when the offset
+    /// (angle of the mean vector) is mathematically
+    /// undefined. `phaseLockingValue` does not apply any
+    /// floor — it returns magnitude arbitrarily close to 0
+    /// as normal output. Value `1e-12` chosen well below
+    /// any physically-meaningful PLV magnitude.
+    let private phasePairEpsilon : double = 1e-12
+
+    /// Shared single-pass accumulation of sin/cos of phase
+    /// differences. Returns `None` on empty / mismatched
+    /// input, otherwise `Some (meanCos, meanSin, n)`. All
+    /// three phase-pair primitives (`phaseLockingValue`,
+    /// `meanPhaseOffset`, `phaseLockingWithOffset`) route
+    /// through this to avoid accumulation-logic drift.
+    let private meanPhaseDiffVector
+            (phasesA: double seq)
+            (phasesB: double seq)
+            : struct (double * double * int) option =
         let aArr = Seq.toArray phasesA
         let bArr = Seq.toArray phasesB
         if aArr.Length = 0 || aArr.Length <> bArr.Length then None
@@ -145,10 +164,102 @@ module TemporalCoordinationDetection =
                 sumCos <- sumCos + cos d
                 sumSin <- sumSin + sin d
             let n = double aArr.Length
-            let meanCos = sumCos / n
-            let meanSin = sumSin / n
+            Some (struct (sumCos / n, sumSin / n, aArr.Length))
+
+    let phaseLockingValue (phasesA: double seq) (phasesB: double seq) : double option =
+        match meanPhaseDiffVector phasesA phasesB with
+        | None -> None
+        | Some (struct (meanCos, meanSin, _)) ->
             Some (sqrt (meanCos * meanCos + meanSin * meanSin))
 
+    /// **Mean phase offset between two phase series** — the
+    /// argument (angle) of the same mean complex phase-
+    /// difference vector whose magnitude is the PLV. Returns
+    /// a value in `[-pi, pi]` (the full `System.Math.Atan2`
+    /// range, which includes both endpoints under IEEE-754
+    /// signed-zero semantics) when defined, or `None` when
+    /// input sequences are empty, of unequal length, or
+    /// when the mean vector has effectively zero magnitude
+    /// (direction is undefined). The epsilon floor
+    /// (`phasePairEpsilon`, 1e-12) applies ONLY to this
+    /// offset decision — `phaseLockingValue` does not
+    /// apply any floor and will return magnitude values
+    /// arbitrarily close to zero as normal output.
+    ///
+    /// Why this complements `phaseLockingValue`: PLV
+    /// magnitude alone cannot distinguish "same-time
+    /// locking" (offset near 0) from "anti-phase locking"
+    /// (offset near +/- pi) or "lead-lag locking" (offset
+    /// between). Both extremes return magnitude 1.0. Per
+    /// Amara 18th-ferry correction #6: cartel-detection
+    /// that relies on "PLV = 1 means synchronized action"
+    /// misreads anti-phase coordinators as same-time
+    /// coordinators. Downstream detectors should consume
+    /// BOTH magnitude (coherence of locking) and offset
+    /// (nature of locking: in-phase, anti-phase, lead-lag).
+    ///
+    /// Computation routes through the shared
+    /// `meanPhaseDiffVector` helper (single-pass sin/cos
+    /// accumulation). The `phasePairEpsilon` floor guards
+    /// `atan2` against reading a spurious argument from a
+    /// mean vector that is mathematically zero-length
+    /// (happens when phase differences are uniformly
+    /// distributed around the unit circle).
+    ///
+    /// Provenance: external AI collaborator's 18th
+    /// courier ferry, Part 2 correction #6 (§"Ten
+    /// required corrections"). The ferry absorb doc
+    /// lives under `docs/aurora/` when landed; at primitive-
+    /// ship time the substance is captured in session
+    /// memory + the related 19th-ferry DST-audit absorb
+    /// at `docs/aurora/2026-04-24-amara-dst-audit-deep-
+    /// research-plus-5-5-corrections-19th-ferry.md`.
+    /// Complements the original PLV primitive from the
+    /// 11th ferry without changing its contract.
+    /// Downstream score vectors (see
+    /// `Graph.coordinationRiskScore*`) consuming PLV
+    /// should add a separate offset-based term rather than
+    /// collapsing both into one scalar.
+    let meanPhaseOffset (phasesA: double seq) (phasesB: double seq) : double option =
+        match meanPhaseDiffVector phasesA phasesB with
+        | None -> None
+        | Some (struct (meanCos, meanSin, _)) ->
+            let magnitude = sqrt (meanCos * meanCos + meanSin * meanSin)
+            if magnitude < phasePairEpsilon then None
+            else Some (atan2 meanSin meanCos)
+
+    /// **Phase locking + offset together** — returns both the
+    /// PLV magnitude and the mean phase offset as a single
+    /// option-tuple, sharing the cos/sin accumulation pass
+    /// for callers that want both. Returns `None` under the
+    /// same input conditions as `phaseLockingValue` (empty
+    /// or length-mismatched series).
+    ///
+    /// When the mean complex vector has effectively zero
+    /// magnitude the offset field is set to `nan` rather
+    /// than `None`; the magnitude will itself be `< 1e-12`
+    /// which is the caller's reliable "offset is undefined"
+    /// signal. This keeps the return type flat for
+    /// downstream composition.
+    ///
+    /// Prefer this over separate `phaseLockingValue` +
+    /// `meanPhaseOffset` calls when you need both, to avoid
+    /// traversing the sequences twice. Use the individual
+    /// primitives when you only need one quantity, to keep
+    /// call sites honest about what they consume.
+    let phaseLockingWithOffset
+            (phasesA: double seq)
+            (phasesB: double seq)
+            : struct (double * double) option =
+        match meanPhaseDiffVector phasesA phasesB with
+        | None -> None
+        | Some (struct (meanCos, meanSin, _)) ->
+            let magnitude = sqrt (meanCos * meanCos + meanSin * meanSin)
+            let offset =
+                if magnitude < phasePairEpsilon then nan
+                else atan2 meanSin meanCos
+            Some (struct (magnitude, offset))
+
     /// **Significant lags from a correlation profile.** Returns the
     /// subset of lags from a `crossCorrelationProfile` where the
     /// absolute correlation meets or exceeds `threshold`. `None`
diff --git a/tests/Tests.FSharp/Algebra/TemporalCoordinationDetection.Tests.fs b/tests/Tests.FSharp/Algebra/TemporalCoordinationDetection.Tests.fs
index 8fc99d6f..efa27e18 100644
--- a/tests/Tests.FSharp/Algebra/TemporalCoordinationDetection.Tests.fs
+++ b/tests/Tests.FSharp/Algebra/TemporalCoordinationDetection.Tests.fs
@@ -181,6 +181,134 @@ let ``phaseLockingValue handles single-element series`` () =
     TemporalCoordinationDetection.phaseLockingValue a b
     |> Option.map (fun v -> Math.Round(v, 9))
     |> should equal (Some 1.0)
+
+// ─── meanPhaseOffset ─────────
+
+[<Fact>]
+let ``meanPhaseOffset of identical phase series is 0`` () =
+    let phases = [ 0.0; 0.5; 1.0; 1.5; 2.0 ]
+    TemporalCoordinationDetection.meanPhaseOffset phases phases
+    |> Option.map (fun v -> Math.Round(v, 9))
+    |> should equal (Some 0.0)
+
+[<Fact>]
+let ``meanPhaseOffset with constant pi/4 offset returns -pi/4`` () =
+    // b = a + pi/4, so phase difference a - b = -pi/4.
+    // atan2 reads the mean vector angle; the sign is the
+    // *difference* direction, consistent with PLV's
+    // a-minus-b convention.
+    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.meanPhaseOffset a b
+    |> Option.map (fun v -> Math.Round(v, 9))
+    |> should equal (Some (Math.Round(-offset, 9)))
+
+[<Fact>]
+let ``meanPhaseOffset distinguishes anti-phase from in-phase`` () =
+    // This is the 18th-ferry correction #6 regression test.
+    // Two series with pi offset have PLV = 1 (perfectly
+    // locked), but the offset tells us they are ANTI-phase,
+    // not same-time. Downstream detectors that rely on
+    // PLV = 1 => synchronized would misclassify this.
+    let a = [ 0.0; 0.5; 1.0; 1.5 ]
+    let b = a |> List.map (fun x -> x + Math.PI)
+    let antiPhaseOffset =
+        TemporalCoordinationDetection.meanPhaseOffset a b
+        |> Option.defaultValue 0.0
+    // a - b = -pi for every element; atan2(0, -1) = pi
+    Math.Round(abs antiPhaseOffset, 6) |> should equal (Math.Round(Math.PI, 6))
+
+    // Same-time locking has offset near 0 — contrast case
+    let c = a |> List.map id
+    TemporalCoordinationDetection.meanPhaseOffset a c
+    |> Option.map (fun v -> Math.Round(v, 9))
+    |> should equal (Some 0.0)
+
+[<Fact>]
+let ``meanPhaseOffset is None when mean vector has zero magnitude`` () =
+    // Uniformly-distributed phase differences sum to the
+    // zero vector; direction is undefined, so None.
+    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 ]
+    TemporalCoordinationDetection.meanPhaseOffset a b
+    |> should equal (None: double option)
+
+[<Fact>]
+let ``meanPhaseOffset of empty series is None`` () =
+    TemporalCoordinationDetection.meanPhaseOffset [] []
+    |> should equal (None: double option)
+
+[<Fact>]
+let ``meanPhaseOffset on mismatched-length series is None`` () =
+    let a = [ 0.0; 0.5; 1.0 ]
+    let b = [ 0.0; 0.5 ]
+    TemporalCoordinationDetection.meanPhaseOffset a b
+    |> should equal (None: double option)
+
+// ─── phaseLockingWithOffset ─────────
+
+[<Fact>]
+let ``phaseLockingWithOffset returns magnitude and offset together`` () =
+    let a = [ 0.0; 0.3; 0.6; 0.9; 1.2 ]
+    let offset = Math.PI / 6.0
+    let b = a |> List.map (fun x -> x + offset)
+    match TemporalCoordinationDetection.phaseLockingWithOffset a b with
+    | Some (struct (magnitude, observedOffset)) ->
+        Math.Round(magnitude, 9) |> should equal 1.0
+        Math.Round(observedOffset, 9) |> should equal (Math.Round(-offset, 9))
+    | None ->
+        failwith "expected Some tuple"
+
+[<Fact>]
+let ``phaseLockingWithOffset magnitude matches phaseLockingValue`` () =
+    // Consistency property: the magnitude field must be
+    // identical (within FP rounding) to the standalone
+    // phaseLockingValue result. If the two primitives
+    // disagree, downstream score vectors will silently
+    // carry inconsistent values depending on which primitive
+    // the caller happened to invoke.
+    let a = [ 0.1; 0.4; 0.9; 1.3; 1.7 ]
+    let b = [ 0.2; 0.5; 0.95; 1.4; 1.8 ]
+    // Explicit pattern-match + fail on None rather than
+    // using sentinel -1.0 with Option.defaultValue. Prior
+    // sentinel form would silently pass the equality
+    // assertion if BOTH primitives returned None, masking
+    // a real regression. Per reviewer thread 59WGi9:
+    // report the actual regression, don't paper over it.
+    match TemporalCoordinationDetection.phaseLockingWithOffset a b,
+          TemporalCoordinationDetection.phaseLockingValue a b with
+    | Some (struct (magnitudeFromPair, _)), Some magnitudeFromPlv ->
+        Math.Round(magnitudeFromPair, 12)
+        |> should equal (Math.Round(magnitudeFromPlv, 12))
+    | None, _ ->
+        failwith "phaseLockingWithOffset returned None on valid input"
+    | _, None ->
+        failwith "phaseLockingValue returned None on valid input"
+
+[<Fact>]
+let ``phaseLockingWithOffset flags zero-magnitude with nan offset`` () =
+    // Zero-magnitude mean vector: magnitude near 0, offset = nan.
+    // Caller's reliable "offset is undefined" signal is the
+    // near-zero magnitude, not the nan per se.
+    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 ]
+    match TemporalCoordinationDetection.phaseLockingWithOffset a b with
+    | Some (struct (magnitude, offset)) ->
+        magnitude |> should (be lessThan) 1e-9
+        System.Double.IsNaN(offset) |> should equal true
+    | None ->
+        failwith "expected Some tuple with nan offset on zero-magnitude mean"
+
+[<Fact>]
+let ``phaseLockingWithOffset returns None on empty or mismatched inputs`` () =
+    TemporalCoordinationDetection.phaseLockingWithOffset [] []
+    |> should equal (None: struct (double * double) option)
+    TemporalCoordinationDetection.phaseLockingWithOffset [ 0.0 ] [ 0.0; 1.0 ]
+    |> should equal (None: struct (double * double) option)
+
 // ─── significantLags ─────────
 
 [<Fact>]