diff --git a/Sources/OpenAPIRuntime/Conversion/URLExtensions.swift b/Sources/OpenAPIRuntime/Conversion/URLExtensions.swift
index 432d78ae..ef94ab1d 100644
--- a/Sources/OpenAPIRuntime/Conversion/URLExtensions.swift
+++ b/Sources/OpenAPIRuntime/Conversion/URLExtensions.swift
@@ -14,19 +14,28 @@
 import Foundation
 
 extension URL {
+  private static func defaultOpenAPIServer () -> Self {
+    guard let url = URL(string: "/") else { fatalError("Failed to create an URL with the string '/'.") }
+    return url
+  }
+  
+  #if swift(>=6.0)
     /// Returns the default server URL of "/".
     ///
     /// Specification: https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.0.3.md#fixed-fields
-    public static let defaultOpenAPIServerURL: Self = {
-        guard let url = URL(string: "/") else { fatalError("Failed to create an URL with the string '/'.") }
-        return url
-    }()
-
-    /// Returns a validated server URL, or throws an error.
-    /// - Parameter string: A URL string.
-    /// - Throws: If the provided string doesn't convert to URL.
-    public init(validatingOpenAPIServerURL string: String) throws {
-        guard let url = Self(string: string) else { throw RuntimeError.invalidServerURL(string) }
-        self = url
-    }
+    public nonisolated(unsafe) static let defaultOpenAPIServerURL: Self = defaultOpenAPIServer()
+  #else
+    /// Returns the default server URL of "/".
+    ///
+    /// Specification: https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.0.3.md#fixed-fields
+    public static let defaultOpenAPIServerURL: Self = defaultOpenAPIServer()
+  #endif
+  
+  /// Returns a validated server URL, or throws an error.
+  /// - Parameter string: A URL string.
+  /// - Throws: If the provided string doesn't convert to URL.
+  public init(validatingOpenAPIServerURL string: String) throws {
+    guard let url = Self(string: string) else { throw RuntimeError.invalidServerURL(string) }
+    self = url
+  }
 }
diff --git a/Sources/OpenAPIRuntime/Multipart/MultipartFramesToRawPartsSequence.swift b/Sources/OpenAPIRuntime/Multipart/MultipartFramesToRawPartsSequence.swift
index c5823ec2..312f9bdf 100644
--- a/Sources/OpenAPIRuntime/Multipart/MultipartFramesToRawPartsSequence.swift
+++ b/Sources/OpenAPIRuntime/Multipart/MultipartFramesToRawPartsSequence.swift
@@ -342,8 +342,17 @@ extension MultipartFramesToRawPartsSequence {
             switch stateMachine.nextFromPartSequence() {
             case .returnNil: return nil
             case .fetchFrame:
+              let frame : Upstream.AsyncIterator.Element?
                 var upstream = upstream
-                let frame = try await upstream.next()
+              if #available(macOS 15, iOS 18.0, tvOS 18.0, watchOS 11.0, macCatalyst 18.0, visionOS 2.0, *) {
+                #if compiler(>=6.0)
+                  frame = try await upstream.next(isolation: self)
+                #else
+                  frame = try await upstream.next()
+                #endif
+              } else {
+                frame = try await upstream.next()
+              }
                 self.upstream = upstream
                 switch stateMachine.partReceivedFrame(frame) {
                 case .returnNil: return nil
@@ -365,8 +374,17 @@ extension MultipartFramesToRawPartsSequence {
             switch stateMachine.nextFromBodySubsequence() {
             case .returnNil: return nil
             case .fetchFrame:
+              let frame : Upstream.AsyncIterator.Element?
                 var upstream = upstream
-                let frame = try await upstream.next()
+              if #available(macOS 15, iOS 18.0, tvOS 18.0, watchOS 11.0, macCatalyst 18.0, visionOS 2.0, *) {
+                #if compiler(>=6.0)
+                  frame = try await upstream.next(isolation: self)
+                #else
+                  frame = try await upstream.next()
+                #endif
+              } else {
+                frame = try await upstream.next()
+              }
                 self.upstream = upstream
                 switch stateMachine.bodyReceivedFrame(frame) {
                 case .returnNil: return nil
diff --git a/Sources/OpenAPIRuntime/URICoder/Serialization/URISerializer.swift b/Sources/OpenAPIRuntime/URICoder/Serialization/URISerializer.swift
index 45d3b0da..5f00bcc9 100644
--- a/Sources/OpenAPIRuntime/URICoder/Serialization/URISerializer.swift
+++ b/Sources/OpenAPIRuntime/URICoder/Serialization/URISerializer.swift
@@ -47,19 +47,34 @@ struct URISerializer {
 }
 
 extension CharacterSet {
-
+  #if swift(>=6.0)
+    /// A character set of unreserved symbols only from RFC 6570 (excludes
+    /// alphanumeric characters).
+    fileprivate nonisolated(unsafe) static let unreservedSymbols: CharacterSet = .init(charactersIn: "-._~")
+    
+    /// A character set of unreserved characters from RFC 6570.
+    fileprivate nonisolated(unsafe) static let unreserved: CharacterSet = .alphanumerics.union(unreservedSymbols)
+    
+    /// A character set with only the space character.
+    fileprivate nonisolated(unsafe) static let space: CharacterSet = .init(charactersIn: " ")
+    
+    /// A character set of unreserved characters and a space.
+    fileprivate nonisolated(unsafe) static let unreservedAndSpace: CharacterSet = .unreserved.union(space)
+  #else
+    
     /// A character set of unreserved symbols only from RFC 6570 (excludes
     /// alphanumeric characters).
     fileprivate static let unreservedSymbols: CharacterSet = .init(charactersIn: "-._~")
-
+    
     /// A character set of unreserved characters from RFC 6570.
     fileprivate static let unreserved: CharacterSet = .alphanumerics.union(unreservedSymbols)
-
+    
     /// A character set with only the space character.
     fileprivate static let space: CharacterSet = .init(charactersIn: " ")
-
+    
     /// A character set of unreserved characters and a space.
     fileprivate static let unreservedAndSpace: CharacterSet = .unreserved.union(space)
+  #endif
 }
 
 extension URISerializer {