diff --git a/RELEASE_NOTES.md b/RELEASE_NOTES.md
index d6d525f..d96928c 100644
--- a/RELEASE_NOTES.md
+++ b/RELEASE_NOTES.md
@@ -1,6 +1,8 @@
 ### Unreleased
 
+* Test coverage: Added tests for previously-untested public API functions `AsyncSeq.tryFirst`, `AsyncSeq.firstOrDefault`, `AsyncSeq.zipWithParallel`, `AsyncSeq.combineLatestWithAsync`, and `AsyncSeq.toObservable`. No functional changes.
 * Fixed Fable CI build: `Microsoft.Bcl.AsyncInterfaces` was pinned to a specific version (`10.0.7`) that was older than the version resolved transitively via `System.Threading.Channels`, causing a `NU1605` package downgrade error that made Fable's project cracker fail during `dotnet fable`. The reference now uses `Version="*"` (matching `System.Threading.Channels`) so both resolve consistently. (#334)
+* Tests: Added comprehensive tests for `AsyncSeq.zapp`, `AsyncSeq.zappAsync`, and `AsyncSeq.compareWithAsync`, which previously had no dedicated test coverage.
 
 ### 4.17.0
 
diff --git a/tests/FSharp.Control.AsyncSeq.Tests/AsyncSeqTests.fs b/tests/FSharp.Control.AsyncSeq.Tests/AsyncSeqTests.fs
index 770f04c..bd1d56f 100644
--- a/tests/FSharp.Control.AsyncSeq.Tests/AsyncSeqTests.fs
+++ b/tests/FSharp.Control.AsyncSeq.Tests/AsyncSeqTests.fs
@@ -5087,3 +5087,192 @@ let ``AsyncSeq.forall2Async returns true with async predicate`` () =
       (AsyncSeq.ofSeq [1;2;3])
     |> Async.RunSynchronously
   Assert.IsTrue(result)
+
+// ===== tryFirst / firstOrDefault =====
+
+[<Test>]
+let ``AsyncSeq.tryFirst returns Some first element for non-empty sequence`` () =
+  let result = AsyncSeq.tryFirst (AsyncSeq.ofSeq [1;2;3]) |> Async.RunSynchronously
+  Assert.AreEqual(Some 1, result)
+
+[<Test>]
+let ``AsyncSeq.tryFirst returns None for empty sequence`` () =
+  let result = AsyncSeq.tryFirst (AsyncSeq.empty<int>) |> Async.RunSynchronously
+  Assert.AreEqual(None, result)
+
+[<Test>]
+let ``AsyncSeq.firstOrDefault returns first element when non-empty`` () =
+  let result = AsyncSeq.firstOrDefault -1 (AsyncSeq.ofSeq [5;6;7]) |> Async.RunSynchronously
+  Assert.AreEqual(5, result)
+
+[<Test>]
+let ``AsyncSeq.firstOrDefault returns default when empty`` () =
+  let result = AsyncSeq.firstOrDefault -1 (AsyncSeq.empty<int>) |> Async.RunSynchronously
+  Assert.AreEqual(-1, result)
+
+// ===== zipWithParallel =====
+
+[<Test>]
+let ``AsyncSeq.zipWithParallel combines values from both sequences`` () =
+  let result =
+    AsyncSeq.zipWithParallel (fun a b -> a + b) (AsyncSeq.ofSeq [1;2;3]) (AsyncSeq.ofSeq [10;20;30])
+    |> AsyncSeq.toArrayAsync
+    |> Async.RunSynchronously
+  Assert.AreEqual([| 11;22;33 |], result)
+
+[<Test>]
+let ``AsyncSeq.zipWithParallel stops at shorter sequence`` () =
+  let result =
+    AsyncSeq.zipWithParallel (fun a b -> a, b) (AsyncSeq.ofSeq [1;2]) (AsyncSeq.ofSeq ["a";"b";"c"])
+    |> AsyncSeq.toArrayAsync
+    |> Async.RunSynchronously
+  Assert.AreEqual([| (1,"a"); (2,"b") |], result)
+
+// ===== combineLatestWithAsync =====
+
+[<Test>]
+let ``AsyncSeq.combineLatestWithAsync combines initial values then emits on each new update`` () =
+  let result =
+    AsyncSeq.combineLatestWithAsync
+      (fun a b -> async { return a + b })
+      (AsyncSeq.ofSeq [1;2])
+      (AsyncSeq.ofSeq [10])
+    |> AsyncSeq.toArrayAsync
+    |> Async.RunSynchronously
+  // First combination of initial pair, then subsequent updates to source1 combined with latest source2 value.
+  Assert.AreEqual([| 11; 12 |], result)
+
+[<Test>]
+let ``AsyncSeq.combineLatestWithAsync produces empty sequence when either source is empty`` () =
+  let result =
+    AsyncSeq.combineLatestWithAsync
+      (fun a b -> async { return a + b })
+      (AsyncSeq.empty<int>)
+      (AsyncSeq.ofSeq [1;2;3])
+    |> AsyncSeq.toArrayAsync
+    |> Async.RunSynchronously
+  Assert.AreEqual([||], result)
+
+// ===== toObservable =====
+
+type private TestObserver<'T>(onNext, onCompleted) =
+  interface IObserver<'T> with
+    member _.OnNext(v) = onNext v
+    member _.OnCompleted() = onCompleted ()
+    member _.OnError(_e) = ()
+
+[<Test>]
+let ``AsyncSeq.toObservable emits all values then completes`` () =
+  let received = ResizeArray<int>()
+  let completedEvent = new System.Threading.ManualResetEventSlim(false)
+  let observer = TestObserver<int>(received.Add, completedEvent.Set)
+  use _sub = (AsyncSeq.toObservable (AsyncSeq.ofSeq [1;2;3])).Subscribe(observer)
+  Assert.IsTrue(completedEvent.Wait(2000))
+  Assert.AreEqual([| 1;2;3 |], received.ToArray())
+
+[<Test>]
+let ``AsyncSeq.toObservable on empty sequence emits nothing`` () =
+  let received = ResizeArray<int>()
+  let completedEvent = new System.Threading.ManualResetEventSlim(false)
+  let observer = TestObserver<int>(received.Add, completedEvent.Set)
+  use _sub = (AsyncSeq.toObservable (AsyncSeq.empty<int>)).Subscribe(observer)
+  Assert.IsTrue(completedEvent.Wait(2000))
+  Assert.AreEqual([||], received.ToArray())
+// ===== zapp / zappAsync =====
+
+[<Test>]
+let ``AsyncSeq.zapp applies functions to corresponding elements`` () =
+  let fs = asyncSeq { yield (fun x -> x + 10); yield (fun x -> x * 2); yield (fun x -> x - 1) }
+  let vs = asyncSeq { yield 1; yield 2; yield 3 }
+  let result = AsyncSeq.zapp fs vs |> AsyncSeq.toArrayAsync |> Async.RunSynchronously
+  Assert.AreEqual([| 11; 4; 2 |], result)
+
+[<Test>]
+let ``AsyncSeq.zapp stops when functions run out`` () =
+  let fs = asyncSeq { yield (fun x -> x + 1); yield (fun x -> x + 2) }
+  let vs = asyncSeq { yield 10; yield 20; yield 30 }
+  let result = AsyncSeq.zapp fs vs |> AsyncSeq.toArrayAsync |> Async.RunSynchronously
+  Assert.AreEqual([| 11; 22 |], result)
+
+[<Test>]
+let ``AsyncSeq.zapp stops when values run out`` () =
+  let fs = asyncSeq { yield (fun x -> x + 1); yield (fun x -> x + 2); yield (fun x -> x + 3) }
+  let vs = asyncSeq { yield 5 }
+  let result = AsyncSeq.zapp fs vs |> AsyncSeq.toArrayAsync |> Async.RunSynchronously
+  Assert.AreEqual([| 6 |], result)
+
+[<Test>]
+let ``AsyncSeq.zapp on empty functions returns empty`` () =
+  let fs = AsyncSeq.empty<int -> int>
+  let vs = asyncSeq { yield 1; yield 2; yield 3 }
+  let result = AsyncSeq.zapp fs vs |> AsyncSeq.toArrayAsync |> Async.RunSynchronously
+  Assert.AreEqual([||], result)
+
+[<Test>]
+let ``AsyncSeq.zappAsync applies async functions to corresponding elements`` () =
+  let fs = asyncSeq {
+    yield (fun x -> async { return x + 10 })
+    yield (fun x -> async { return x * 3 })
+  }
+  let vs = asyncSeq { yield 5; yield 4 }
+  let result = AsyncSeq.zappAsync fs vs |> AsyncSeq.toArrayAsync |> Async.RunSynchronously
+  Assert.AreEqual([| 15; 12 |], result)
+
+[<Test>]
+let ``AsyncSeq.zappAsync on empty source returns empty`` () =
+  let fs = asyncSeq { yield (fun x -> async { return x + 1 }) }
+  let vs = AsyncSeq.empty<int>
+  let result = AsyncSeq.zappAsync fs vs |> AsyncSeq.toArrayAsync |> Async.RunSynchronously
+  Assert.AreEqual([||], result)
+
+// ===== compareWithAsync =====
+
+[<Test>]
+let ``AsyncSeq.compareWithAsync equal sequences returns 0`` () =
+  let result =
+    AsyncSeq.compareWithAsync
+      (fun a b -> async { return compare a b })
+      (AsyncSeq.ofSeq [1;2;3])
+      (AsyncSeq.ofSeq [1;2;3])
+    |> Async.RunSynchronously
+  Assert.AreEqual(0, result)
+
+[<Test>]
+let ``AsyncSeq.compareWithAsync shorter is less than longer`` () =
+  let result =
+    AsyncSeq.compareWithAsync
+      (fun a b -> async { return compare a b })
+      (AsyncSeq.ofSeq [1;2])
+      (AsyncSeq.ofSeq [1;2;3])
+    |> Async.RunSynchronously
+  Assert.IsTrue(result < 0)
+
+[<Test>]
+let ``AsyncSeq.compareWithAsync longer is greater than shorter`` () =
+  let result =
+    AsyncSeq.compareWithAsync
+      (fun a b -> async { return compare a b })
+      (AsyncSeq.ofSeq [1;2;3])
+      (AsyncSeq.ofSeq [1;2])
+    |> Async.RunSynchronously
+  Assert.IsTrue(result > 0)
+
+[<Test>]
+let ``AsyncSeq.compareWithAsync lexicographic difference`` () =
+  let result =
+    AsyncSeq.compareWithAsync
+      (fun a b -> async { return compare a b })
+      (AsyncSeq.ofSeq [1;3])
+      (AsyncSeq.ofSeq [1;2])
+    |> Async.RunSynchronously
+  Assert.IsTrue(result > 0)
+
+[<Test>]
+let ``AsyncSeq.compareWithAsync empty sequences returns 0`` () =
+  let result =
+    AsyncSeq.compareWithAsync
+      (fun a b -> async { return compare a b })
+      AsyncSeq.empty<int>
+      AsyncSeq.empty<int>
+    |> Async.RunSynchronously
+  Assert.AreEqual(0, result)