diff --git a/.github/workflows/pull_request.yml b/.github/workflows/pull_request.yml
index 188204d6..bd4f2101 100644
--- a/.github/workflows/pull_request.yml
+++ b/.github/workflows/pull_request.yml
@@ -50,6 +50,25 @@ jobs:
       - name: Build
         run: cmake --build .cmake-build
 
+  wasm-build:
+    name: Wasm Build
+    runs-on: ubuntu-latest
+    container:
+      image: swift:6.1-noble
+    steps:
+      - name: Checkout repo
+        uses: actions/checkout@v4
+      - name: Install Swift SDKs for WebAssembly
+        run: |
+          # TODO: We can replace these Swift SDKs with the swift.org one once it supports Foundation.
+          swift sdk install https://github.com/swiftwasm/swift/releases/download/swift-wasm-6.1-RELEASE/swift-wasm-6.1-RELEASE-wasm32-unknown-wasi.artifactbundle.zip --checksum 7550b4c77a55f4b637c376f5d192f297fe185607003a6212ad608276928db992
+          swift sdk install https://github.com/swiftwasm/swift/releases/download/swift-wasm-6.1-RELEASE/swift-wasm-6.1-RELEASE-wasm32-unknown-wasip1-threads.artifactbundle.zip --checksum 0dd273be28741f8e1eb00682c39bdc956361ed24b5572e183dd8a4e9d1c5f6ec
+          swift sdk list
+      - name: Build
+        run: |
+          swift build --swift-sdk wasm32-unknown-wasi --target ArgumentParser
+          swift build --swift-sdk wasm32-unknown-wasip1-threads --target ArgumentParser
+
   soundness:
     name: Soundness
     uses: swiftlang/github-workflows/.github/workflows/soundness.yml@main
diff --git a/Sources/ArgumentParser/Parsable Properties/CompletionKind.swift b/Sources/ArgumentParser/Parsable Properties/CompletionKind.swift
index 82739618..97573656 100644
--- a/Sources/ArgumentParser/Parsable Properties/CompletionKind.swift	
+++ b/Sources/ArgumentParser/Parsable Properties/CompletionKind.swift	
@@ -41,6 +41,9 @@ public struct CompletionKind {
     case directory
     case shellCommand(String)
     case custom(@Sendable ([String], Int, String) -> [String])
+    #if !canImport(Dispatch)
+    @available(*, unavailable, message: "DispatchSemaphore is unavailable")
+    #endif
     case customAsync(@Sendable ([String], Int, String) async -> [String])
     case customDeprecated(@Sendable ([String]) -> [String])
   }
@@ -181,11 +184,18 @@ public struct CompletionKind {
   ///
   /// The same as `custom(@Sendable @escaping ([String], Int, String) -> [String])`,
   /// except that the closure is asynchronous.
+  #if !canImport(Dispatch)
+  @available(*, unavailable, message: "DispatchSemaphore is unavailable")
+  #endif
   @available(macOS 10.15, macCatalyst 13, iOS 13, tvOS 13, watchOS 6, *)
   public static func custom(
     _ completion: @Sendable @escaping ([String], Int, String) async -> [String]
   ) -> CompletionKind {
+    #if !canImport(Dispatch)
+    fatalError("DispatchSemaphore is unavailable")
+    #else
     CompletionKind(kind: .customAsync(completion))
+    #endif
   }
 
   /// Deprecated; only kept for backwards compatibility.
diff --git a/Sources/ArgumentParser/Parsing/CommandParser.swift b/Sources/ArgumentParser/Parsing/CommandParser.swift
index b8d9b972..3f2f08cc 100644
--- a/Sources/ArgumentParser/Parsing/CommandParser.swift
+++ b/Sources/ArgumentParser/Parsing/CommandParser.swift
@@ -10,11 +10,15 @@
 //===----------------------------------------------------------------------===//
 
 #if swift(>=6.0)
+#if canImport(Dispatch)
 @preconcurrency private import class Dispatch.DispatchSemaphore
+#endif
 internal import class Foundation.NSLock
 internal import class Foundation.ProcessInfo
 #else
+#if canImport(Dispatch)
 @preconcurrency import class Dispatch.DispatchSemaphore
+#endif
 import class Foundation.NSLock
 import class Foundation.ProcessInfo
 #endif
@@ -474,12 +478,16 @@ extension CommandParser {
         completingPrefix
       )
     case .customAsync(let complete):
+      #if canImport(Dispatch)
       if #available(macOS 10.15, macCatalyst 13, iOS 13, tvOS 13, watchOS 6, *)
       {
         completions = try asyncCustomCompletions(from: args, complete: complete)
       } else {
         throw ParserError.invalidState
       }
+      #else
+      throw ParserError.invalidState
+      #endif
     case .customDeprecated(let complete):
       completions = complete(args)
     default:
@@ -528,11 +536,17 @@ private func parseCustomCompletionArguments(
   return (Array(args), completingArgumentIndex, completingPrefix)
 }
 
+#if !canImport(Dispatch)
+@available(*, unavailable, message: "DispatchSemaphore is unavailable")
+#endif
 @available(macOS 10.15, macCatalyst 13, iOS 13, tvOS 13, watchOS 6, *)
 private func asyncCustomCompletions(
   from args: [String],
   complete: @escaping @Sendable ([String], Int, String) async -> [String]
 ) throws -> [String] {
+  #if !canImport(Dispatch)
+  throw ParserError.invalidState
+  #else
   let (args, completingArgumentIndex, completingPrefix) =
     try parseCustomCompletionArguments(from: args)
 
@@ -550,6 +564,7 @@ private func asyncCustomCompletions(
 
   semaphore.wait()
   return completionsBox.value
+  #endif
 }
 
 // Helper class to make values sendable across concurrency boundaries