diff --git a/swift/Sources/CoreAIShared/Runtime/VectorMath.swift b/swift/Sources/CoreAIShared/Runtime/VectorMath.swift
index 18b40110..bffd8e1b 100644
--- a/swift/Sources/CoreAIShared/Runtime/VectorMath.swift
+++ b/swift/Sources/CoreAIShared/Runtime/VectorMath.swift
@@ -17,3 +17,23 @@ public func sigmoid(_ x: Float) -> Float {
     let e = Foundation.exp(x)
     return e / (1 + e)
 }
+
+/// Cosine similarity of two equal-length vectors, accumulated in `Double` and returned as `Float`.
+///
+/// Length-mismatched inputs score `0`. For zero-norm inputs the convention matches the parity
+/// harnesses: two zero-norm (or empty) vectors are identical and score `1`, while a single
+/// zero-norm vector has no direction to compare and scores `0`.
+public func cosineSimilarity(_ a: [Float], _ b: [Float]) -> Float {
+    guard a.count == b.count else { return 0 }
+    var dot = 0.0
+    var normA = 0.0
+    var normB = 0.0
+    for i in a.indices {
+        dot += Double(a[i]) * Double(b[i])
+        normA += Double(a[i]) * Double(a[i])
+        normB += Double(b[i]) * Double(b[i])
+    }
+    let denom = (normA * normB).squareRoot()
+    guard denom > 0 else { return normA == normB ? 1 : 0 }
+    return Float(dot / denom)
+}
diff --git a/swift/Sources/Tools/diffusion-runner/DiffusionRunnerMain+Parity.swift b/swift/Sources/Tools/diffusion-runner/DiffusionRunnerMain+Parity.swift
index fd12bcaf..87cf7fe7 100644
--- a/swift/Sources/Tools/diffusion-runner/DiffusionRunnerMain+Parity.swift
+++ b/swift/Sources/Tools/diffusion-runner/DiffusionRunnerMain+Parity.swift
@@ -102,20 +102,6 @@ extension DiffusionRunner {
 
     // MARK: - Helpers
 
-    private func cosineSimilarity(_ a: [Float], _ b: [Float]) -> Float {
-        guard a.count == b.count, !a.isEmpty else { return 0 }
-        var dot: Float = 0
-        var normA: Float = 0
-        var normB: Float = 0
-        for i in 0..<a.count {
-            dot += a[i] * b[i]
-            normA += a[i] * a[i]
-            normB += b[i] * b[i]
-        }
-        let denom = sqrt(normA) * sqrt(normB)
-        return denom > 0 ? dot / denom : 0
-    }
-
     /// Widen a `[Float]` buffer into an `NDArray` of the requested scalar type.
     ///
     /// Not diffusion- or model-specific. Float targets defer to
diff --git a/swift/Sources/Tools/image-segmenter/ImageSegmentationRunnerMain.swift b/swift/Sources/Tools/image-segmenter/ImageSegmentationRunnerMain.swift
index 9c4ba51e..7fdb0a19 100644
--- a/swift/Sources/Tools/image-segmenter/ImageSegmentationRunnerMain.swift
+++ b/swift/Sources/Tools/image-segmenter/ImageSegmentationRunnerMain.swift
@@ -603,20 +603,6 @@ struct ImageSegmenterCLI: AsyncParsableCommand {
         return Float(10.0 * log10((peak * peak) / mse))
     }
 
-    private func cosineSimilarity(_ a: [Float], _ b: [Float]) -> Float {
-        guard a.count == b.count, !a.isEmpty else { return 0 }
-        var dot: Double = 0
-        var normA: Double = 0
-        var normB: Double = 0
-        for i in 0..<a.count {
-            dot += Double(a[i]) * Double(b[i])
-            normA += Double(a[i]) * Double(a[i])
-            normB += Double(b[i]) * Double(b[i])
-        }
-        let denom = (normA * normB).squareRoot()
-        return denom > 0 ? Float(dot / denom) : 0
-    }
-
     /// Return the leading slice of `ids` up to and including the first EOT
     /// (token id 49407) after position 0. Trailing pad tokens are excluded.
     /// If no EOT is found, returns the full sequence.
diff --git a/swift/Sources/Tools/speech-recognizer/SpeechParity.swift b/swift/Sources/Tools/speech-recognizer/SpeechParity.swift
index 86b4ee6b..255cc960 100644
--- a/swift/Sources/Tools/speech-recognizer/SpeechParity.swift
+++ b/swift/Sources/Tools/speech-recognizer/SpeechParity.swift
@@ -467,7 +467,7 @@ private func metricRow(
             ok: false)
     }
     let p = psnr(actual, ref)
-    let c = cosineSimilarity(actual, ref)
+    let c = Double(cosineSimilarity(actual, ref))
     let maxAbs = zip(actual, ref).reduce(Float(0)) { max($0, abs($1.0 - $1.1)) }
     let ok = (p.isInfinite || p >= psnrFloor) && c >= cosineFloor
     let psnrStr = p.isInfinite ? "INF" : String(format: "%.2f", p)
@@ -491,16 +491,3 @@ private func psnr(_ a: [Float], _ b: [Float]) -> Double {
     if peak == 0 { peak = 1 }
     return 10 * log10(peak * peak / mse)
 }
-
-private func cosineSimilarity(_ a: [Float], _ b: [Float]) -> Double {
-    var dot = 0.0
-    var na = 0.0
-    var nb = 0.0
-    for i in a.indices {
-        dot += Double(a[i]) * Double(b[i])
-        na += Double(a[i]) * Double(a[i])
-        nb += Double(b[i]) * Double(b[i])
-    }
-    guard na > 0 && nb > 0 else { return na == nb ? 1 : 0 }
-    return dot / (na.squareRoot() * nb.squareRoot())
-}
diff --git a/swift/Tests/CoreAISharedTests/VectorMathTests.swift b/swift/Tests/CoreAISharedTests/VectorMathTests.swift
index eebcbfae..61dc917f 100644
--- a/swift/Tests/CoreAISharedTests/VectorMathTests.swift
+++ b/swift/Tests/CoreAISharedTests/VectorMathTests.swift
@@ -34,4 +34,24 @@ struct VectorMathTests {
         #expect(sigmoid(-100) < 1e-40)
         #expect(sigmoid(-100).isNaN == false)
     }
+
+    // MARK: - cosineSimilarity
+
+    @Test("cosineSimilarity: identical, orthogonal, and opposite vectors")
+    func cosineKnownValues() {
+        #expect(abs(cosineSimilarity([1, 2, 3], [1, 2, 3]) - 1.0) < 1e-6)
+        #expect(abs(cosineSimilarity([1, 0], [0, 1])) < 1e-6)
+        #expect(abs(cosineSimilarity([1, 2, 3], [-1, -2, -3]) + 1.0) < 1e-6)
+    }
+
+    @Test("cosineSimilarity: zero-norm convention matches speech parity")
+    func cosineDegenerate() {
+        // Two zero-norm (or empty) vectors are identical and score 1; a single zero-norm vector
+        // has no direction to compare and scores 0; a length mismatch scores 0. This matches the
+        // speech parity convention so every harness that shares this helper agrees.
+        #expect(cosineSimilarity([], []) == 1)
+        #expect(cosineSimilarity([0, 0], [0, 0]) == 1)
+        #expect(cosineSimilarity([0, 0], [1, 2]) == 0)
+        #expect(cosineSimilarity([1, 2], [1, 2, 3]) == 0)
+    }
 }