diff --git a/Sources/SKSupport/AsyncUtils.swift b/Sources/SKSupport/AsyncUtils.swift
index 216463fb1..0cfb2a94c 100644
--- a/Sources/SKSupport/AsyncUtils.swift
+++ b/Sources/SKSupport/AsyncUtils.swift
@@ -98,7 +98,7 @@ public extension Task where Failure == Never {
 ///
 /// If the task executing `withCancellableCheckedThrowingContinuation` gets
 /// cancelled, `cancel` is invoked with the handle that `operation` provided.
-public func withCancellableCheckedThrowingContinuation<Handle, Result>(
+public func withCancellableCheckedThrowingContinuation<Handle: Sendable, Result>(
   _ operation: (_ continuation: CheckedContinuation<Result, any Error>) -> Handle,
   cancel: @Sendable (Handle) -> Void
 ) async throws -> Result {
diff --git a/Sources/SKSupport/ThreadSafeBox.swift b/Sources/SKSupport/ThreadSafeBox.swift
index 5f56b8c66..a4f825eaf 100644
--- a/Sources/SKSupport/ThreadSafeBox.swift
+++ b/Sources/SKSupport/ThreadSafeBox.swift
@@ -24,7 +24,7 @@ extension NSLock {
 /// A thread safe container that contains a value of type `T`.
 ///
 /// - Note: Unchecked sendable conformance because value is guarded by a lock.
-public class ThreadSafeBox<T>: @unchecked Sendable {
+public class ThreadSafeBox<T: Sendable>: @unchecked Sendable {
   /// Lock guarding `_value`.
   private let lock = NSLock()
 
@@ -41,6 +41,11 @@ public class ThreadSafeBox<T>: @unchecked Sendable {
         _value = newValue
       }
     }
+    _modify {
+      lock.lock()
+      defer { lock.unlock() }
+      yield &_value
+    }
   }
 
   public init(initialValue: T) {
diff --git a/Sources/SKTestSupport/TestSourceKitLSPClient.swift b/Sources/SKTestSupport/TestSourceKitLSPClient.swift
index 83915ad10..299d5ff37 100644
--- a/Sources/SKTestSupport/TestSourceKitLSPClient.swift
+++ b/Sources/SKTestSupport/TestSourceKitLSPClient.swift
@@ -34,7 +34,7 @@ extension SourceKitLSPServer.Options {
 /// that the server sends to the client.
 public final class TestSourceKitLSPClient: MessageHandler {
   /// A function that takes a request and returns the request's response.
-  public typealias RequestHandler<Request: RequestType> = (Request) -> Request.Response
+  public typealias RequestHandler<Request: RequestType> = @Sendable (Request) -> Request.Response
 
   /// The ID that should be assigned to the next request sent to the `server`.
   /// `nonisolated(unsafe)` is fine because `nextRequestID` is atomic.
@@ -72,7 +72,7 @@ public final class TestSourceKitLSPClient: MessageHandler {
   ///
   /// `isOneShort` if the request handler should only serve a single request and should be removed from
   /// `requestHandlers` after it has been called.
-  private nonisolated(unsafe) var requestHandlers: ThreadSafeBox<[(requestHandler: Any, isOneShot: Bool)]> =
+  private nonisolated(unsafe) var requestHandlers: ThreadSafeBox<[(requestHandler: Sendable, isOneShot: Bool)]> =
     ThreadSafeBox(initialValue: [])
 
   /// A closure that is called when the `TestSourceKitLSPClient` is destructed.
diff --git a/Sources/SourceKitD/SourceKitD.swift b/Sources/SourceKitD/SourceKitD.swift
index 8aa470541..a58b1b3a2 100644
--- a/Sources/SourceKitD/SourceKitD.swift
+++ b/Sources/SourceKitD/SourceKitD.swift
@@ -15,6 +15,12 @@ import Dispatch
 import Foundation
 import SKSupport
 
+#if compiler(>=6)
+extension sourcekitd_api_request_handle_t: @retroactive @unchecked Sendable {}
+#else
+extension sourcekitd_api_request_handle_t: @unchecked Sendable {}
+#endif
+
 /// Access to sourcekitd API, taking care of initialization, shutdown, and notification handler
 /// multiplexing.
 ///