diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index 486bcd46..6778c1cb 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -142,7 +142,7 @@ jobs:
         run: |
           toolchain_path="/opt/swiftwasm"
           ./build-exec mkdir -p "$toolchain_path"
-          curl -v -L ${{ matrix.development-toolchain-download }} | ./build-exec tar xz --strip-component 1 -C "$toolchain_path"
+          curl -L ${{ matrix.development-toolchain-download }} | ./build-exec tar xz --strip-component 1 -C "$toolchain_path"
           echo "toolchain-path=$toolchain_path" >> $GITHUB_OUTPUT
           ./build-exec "$toolchain_path/usr/bin/swift" sdk install "${{ matrix.wasi-swift-sdk-download }}" --checksum "${{ matrix.wasi-swift-sdk-checksum }}"
           wasi_sdk_path=$(./build-exec "$toolchain_path/usr/bin/swift" sdk configure --show-configuration "${{ matrix.wasi-swift-sdk-id }}" wasm32-unknown-wasi | grep sdkRootPath: | cut -d: -f2)
diff --git a/Package.swift b/Package.swift
index 9e37db0a..cc6f66cf 100644
--- a/Package.swift
+++ b/Package.swift
@@ -4,12 +4,7 @@ import PackageDescription
 
 import class Foundation.ProcessInfo
 
-let DarwinPlatforms: [Platform]
-#if swift(<5.9)
-    DarwinPlatforms = [.macOS, .iOS, .watchOS, .tvOS]
-#else
-    DarwinPlatforms = [.macOS, .iOS, .watchOS, .tvOS, .visionOS]
-#endif
+let DarwinPlatforms: [Platform] = [.macOS, .iOS, .watchOS, .tvOS, .visionOS]
 
 let package = Package(
     name: "WasmKit",
@@ -120,8 +115,7 @@ let package = Package(
 
         .target(name: "WITExtractor"),
         .testTarget(name: "WITExtractorTests", dependencies: ["WITExtractor", "WIT"]),
-    ],
-    swiftLanguageVersions: [.v5]
+    ]
 )
 
 if ProcessInfo.processInfo.environment["SWIFTCI_USE_LOCAL_DEPS"] == nil {
diff --git a/Sources/WAT/Location.swift b/Sources/WAT/Location.swift
index c6bc387d..91a51df7 100644
--- a/Sources/WAT/Location.swift
+++ b/Sources/WAT/Location.swift
@@ -1,5 +1,5 @@
 /// A location in a WAT source file.
-public struct Location: Equatable, CustomDebugStringConvertible {
+public struct Location: Equatable, CustomDebugStringConvertible, Sendable {
     let index: Lexer.Index
     let source: String.UnicodeScalarView
 
diff --git a/Sources/WAT/WAT.swift b/Sources/WAT/WAT.swift
index 9d7c1650..974f3d3f 100644
--- a/Sources/WAT/WAT.swift
+++ b/Sources/WAT/WAT.swift
@@ -1,7 +1,7 @@
 import WasmParser
 
 /// Options for encoding a WebAssembly module into a binary format.
-public struct EncodeOptions {
+public struct EncodeOptions: Sendable {
     /// Whether to include the name section.
     public var nameSection: Bool
 
diff --git a/Sources/WIT/AST.swift b/Sources/WIT/AST.swift
index 497e4c49..f1ee8685 100644
--- a/Sources/WIT/AST.swift
+++ b/Sources/WIT/AST.swift
@@ -33,13 +33,13 @@ struct Version: Equatable, Hashable, CustomStringConvertible {
     }
 }
 
-public enum ASTItemSyntax: Equatable, Hashable {
+public enum ASTItemSyntax: Equatable, Hashable, Sendable {
     case interface(SyntaxNode<InterfaceSyntax>)
     case world(SyntaxNode<WorldSyntax>)
     case use(SyntaxNode<TopLevelUseSyntax>)
 }
 
-public struct PackageNameSyntax: Equatable, Hashable, CustomStringConvertible {
+public struct PackageNameSyntax: Equatable, Hashable, CustomStringConvertible, Sendable {
     public var namespace: Identifier
     public var name: Identifier
     var version: Version?
@@ -81,7 +81,7 @@ public struct WorldSyntax: Equatable, Hashable, SyntaxNodeProtocol {
     public var items: [WorldItemSyntax]
 }
 
-public enum WorldItemSyntax: Equatable, Hashable {
+public enum WorldItemSyntax: Equatable, Hashable, Sendable {
     case `import`(ImportSyntax)
     case export(ExportSyntax)
     case use(SyntaxNode<UseSyntax>)
@@ -89,19 +89,19 @@ public enum WorldItemSyntax: Equatable, Hashable {
     case include(IncludeSyntax)
 }
 
-public struct ImportSyntax: Equatable, Hashable {
+public struct ImportSyntax: Equatable, Hashable, Sendable {
     public var documents: DocumentsSyntax
     public var attributes: [AttributeSyntax]
     public var kind: ExternKindSyntax
 }
 
-public struct ExportSyntax: Equatable, Hashable {
+public struct ExportSyntax: Equatable, Hashable, Sendable {
     public var documents: DocumentsSyntax
     public var attributes: [AttributeSyntax]
     public var kind: ExternKindSyntax
 }
 
-public enum ExternKindSyntax: Equatable, Hashable {
+public enum ExternKindSyntax: Equatable, Hashable, Sendable {
     case interface(Identifier, [InterfaceItemSyntax])
     case path(UsePathSyntax)
     case function(Identifier, FunctionSyntax)
@@ -131,7 +131,7 @@ public struct TypeDefSyntax: Equatable, Hashable, SyntaxNodeProtocol {
     public var body: TypeDefBodySyntax
 }
 
-public enum TypeDefBodySyntax: Equatable, Hashable {
+public enum TypeDefBodySyntax: Equatable, Hashable, Sendable {
     case flags(FlagsSyntax)
     case resource(ResourceSyntax)
     case record(RecordSyntax)
@@ -141,11 +141,11 @@ public enum TypeDefBodySyntax: Equatable, Hashable {
     case alias(TypeAliasSyntax)
 }
 
-public struct TypeAliasSyntax: Equatable, Hashable {
+public struct TypeAliasSyntax: Equatable, Hashable, Sendable {
     public let typeRepr: TypeReprSyntax
 }
 
-public indirect enum TypeReprSyntax: Equatable, Hashable {
+public indirect enum TypeReprSyntax: Equatable, Hashable, Sendable {
     case bool
     case u8
     case u16
@@ -169,7 +169,7 @@ public indirect enum TypeReprSyntax: Equatable, Hashable {
     case stream(StreamSyntax)
 }
 
-public enum HandleSyntax: Equatable, Hashable {
+public enum HandleSyntax: Equatable, Hashable, Sendable {
     case own(resource: Identifier)
     case borrow(resource: Identifier)
 
@@ -181,65 +181,65 @@ public enum HandleSyntax: Equatable, Hashable {
     }
 }
 
-public struct ResourceSyntax: Equatable, Hashable {
+public struct ResourceSyntax: Equatable, Hashable, Sendable {
     var functions: [ResourceFunctionSyntax]
 }
 
-public enum ResourceFunctionSyntax: Equatable, Hashable {
+public enum ResourceFunctionSyntax: Equatable, Hashable, Sendable {
     case method(SyntaxNode<NamedFunctionSyntax>)
     case `static`(SyntaxNode<NamedFunctionSyntax>)
     case constructor(SyntaxNode<NamedFunctionSyntax>)
 }
 
-public struct RecordSyntax: Equatable, Hashable {
+public struct RecordSyntax: Equatable, Hashable, Sendable {
     public var fields: [FieldSyntax]
 }
 
-public struct FieldSyntax: Equatable, Hashable {
+public struct FieldSyntax: Equatable, Hashable, Sendable {
     public var documents: DocumentsSyntax
     public var name: Identifier
     public var type: TypeReprSyntax
     var textRange: TextRange
 }
 
-public struct FlagsSyntax: Equatable, Hashable {
+public struct FlagsSyntax: Equatable, Hashable, Sendable {
     public var flags: [FlagSyntax]
 }
 
-public struct FlagSyntax: Equatable, Hashable {
+public struct FlagSyntax: Equatable, Hashable, Sendable {
     public var documents: DocumentsSyntax
     public var name: Identifier
 }
 
-public struct VariantSyntax: Equatable, Hashable {
+public struct VariantSyntax: Equatable, Hashable, Sendable {
     public var cases: [CaseSyntax]
     var textRange: TextRange
 }
 
-public struct CaseSyntax: Equatable, Hashable {
+public struct CaseSyntax: Equatable, Hashable, Sendable {
     public var documents: DocumentsSyntax
     public var name: Identifier
     public var type: TypeReprSyntax?
     var textRange: TextRange
 }
 
-public struct EnumSyntax: Equatable, Hashable {
+public struct EnumSyntax: Equatable, Hashable, Sendable {
     public var cases: [EnumCaseSyntax]
     var textRange: TextRange
 }
 
-public struct EnumCaseSyntax: Equatable, Hashable {
+public struct EnumCaseSyntax: Equatable, Hashable, Sendable {
     public var documents: DocumentsSyntax
     public var name: Identifier
     var textRange: TextRange
 }
 
-public struct ResultSyntax: Equatable, Hashable {
+public struct ResultSyntax: Equatable, Hashable, Sendable {
     public let ok: TypeReprSyntax?
     public let error: TypeReprSyntax?
 }
 
-public struct StreamSyntax: Equatable, Hashable {
+public struct StreamSyntax: Equatable, Hashable, Sendable {
     var element: TypeReprSyntax?
     var end: TypeReprSyntax?
 }
@@ -256,20 +256,20 @@ public struct UnionSyntax: Equatable, Hashable, SyntaxNodeProtocol {
     var textRange: TextRange
 }
 
-public struct UnionCaseSyntax: Equatable, Hashable {
+public struct UnionCaseSyntax: Equatable, Hashable, Sendable {
     public var documents: DocumentsSyntax
     public var type: TypeReprSyntax
     var textRange: TextRange
 }
 
-public struct ParameterSyntax: Equatable, Hashable {
+public struct ParameterSyntax: Equatable, Hashable, Sendable {
     public var name: Identifier
     public var type: TypeReprSyntax
     var textRange: TextRange
 }
 public typealias ParameterList = [ParameterSyntax]
 
-public enum ResultListSyntax: Equatable, Hashable {
+public enum ResultListSyntax: Equatable, Hashable, Sendable {
     case named(ParameterList)
     case anon(TypeReprSyntax)
 
@@ -281,7 +281,7 @@ public enum ResultListSyntax: Equatable, Hashable {
     }
 }
 
-public struct FunctionSyntax: Equatable, Hashable {
+public struct FunctionSyntax: Equatable, Hashable, Sendable {
     public var parameters: ParameterList
     public var results: ResultListSyntax
     var textRange: TextRange
@@ -293,7 +293,7 @@ public struct UseSyntax: Equatable, Hashable, SyntaxNodeProtocol {
     public var names: [UseNameSyntax]
 }
 
-public enum UsePathSyntax: Equatable, Hashable {
+public enum UsePathSyntax: Equatable, Hashable, Sendable {
     case id(Identifier)
     case package(id: PackageNameSyntax, name: Identifier)
 
@@ -305,23 +305,23 @@ public enum UsePathSyntax: Equatable, Hashable {
     }
 }
 
-public struct UseNameSyntax: Equatable, Hashable {
+public struct UseNameSyntax: Equatable, Hashable, Sendable {
     public var name: Identifier
     public var asName: Identifier?
 }
 
-public struct IncludeSyntax: Equatable, Hashable {
+public struct IncludeSyntax: Equatable, Hashable, Sendable {
     var attributes: [AttributeSyntax]
     var from: UsePathSyntax
     var names: [IncludeNameSyntax]
 }
 
-public struct IncludeNameSyntax: Equatable, Hashable {
+public struct IncludeNameSyntax: Equatable, Hashable, Sendable {
     var name: Identifier
     var asName: Identifier
 }
 
-public struct Identifier: Equatable, Hashable, CustomStringConvertible {
+public struct Identifier: Equatable, Hashable, CustomStringConvertible, Sendable {
     public var text: String
     var textRange: TextRange
 
@@ -330,28 +330,28 @@ public struct Identifier: Equatable, Hashable, CustomStringConvertible {
     }
 }
 
-public struct DocumentsSyntax: Equatable, Hashable {
+public struct DocumentsSyntax: Equatable, Hashable, Sendable {
     var comments: [String]
 }
 
-public enum AttributeSyntax: Equatable, Hashable {
+public enum AttributeSyntax: Equatable, Hashable, Sendable {
     case since(SinceAttributeSyntax)
     case unstable(UnstableAttributeSyntax)
     case deprecated(DeprecatedAttributeSyntax)
 }
 
-public struct SinceAttributeSyntax: Equatable, Hashable {
+public struct SinceAttributeSyntax: Equatable, Hashable, Sendable {
     let version: Version
     let feature: Identifier?
     let textRange: TextRange
 }
 
-public struct UnstableAttributeSyntax: Equatable, Hashable {
+public struct UnstableAttributeSyntax: Equatable, Hashable, Sendable {
     let textRange: TextRange
     let feature: Identifier
 }
 
-public struct DeprecatedAttributeSyntax: Equatable, Hashable {
+public struct DeprecatedAttributeSyntax: Equatable, Hashable, Sendable {
     let textRange: TextRange
     let version: Version
 }
diff --git a/Sources/WIT/Diagnostics.swift b/Sources/WIT/Diagnostics.swift
index 03aae5b5..e93af79e 100644
--- a/Sources/WIT/Diagnostics.swift
+++ b/Sources/WIT/Diagnostics.swift
@@ -6,7 +6,7 @@ struct DiagnosticError: Error {
     let diagnostic: Diagnostic
 }
 
-public struct Diagnostic {
+public struct Diagnostic: Sendable {
 
     public let message: String
     var textRange: TextRange?
diff --git a/Sources/WIT/PackageResolver.swift b/Sources/WIT/PackageResolver.swift
index f54f60cc..cfd1e29a 100644
--- a/Sources/WIT/PackageResolver.swift
+++ b/Sources/WIT/PackageResolver.swift
@@ -1,7 +1,7 @@
 import Foundation
 
 /// A unit of WIT package managing a collection of WIT source files
-public final class PackageUnit: Hashable, CustomStringConvertible {
+public final class PackageUnit: Hashable, CustomStringConvertible, Sendable {
     public let packageName: PackageNameSyntax
     public let sourceFiles: [SyntaxNode<SourceFileSyntax>]
 
diff --git a/Sources/WIT/Semantics/RequestEvaluator.swift b/Sources/WIT/Semantics/RequestEvaluator.swift
index ad27e9b6..5ddba2bc 100644
--- a/Sources/WIT/Semantics/RequestEvaluator.swift
+++ b/Sources/WIT/Semantics/RequestEvaluator.swift
@@ -35,7 +35,7 @@ internal class Evaluator {
 
         // Check cyclical request
         if activeRequestsSet.contains(requestAsHashable) {
-            throw CyclicalRequestError(activeRequests: activeRequests + [request])
+            throw CyclicalRequestError(activeRequestDescriptions: activeRequests.map { "\($0)" } + ["\(request)"])
         }
 
         // Push the given request as an active request
@@ -64,11 +64,11 @@ internal class Evaluator {
 
 extension Evaluator {
     struct CyclicalRequestError: Error, CustomStringConvertible {
-        let activeRequests: [any EvaluationRequest]
+        let activeRequestDescriptions: [String]
 
         var description: String {
             var description = "==== Cycle detected! ====\n"
-            for (index, request) in activeRequests.enumerated() {
+            for (index, request) in activeRequestDescriptions.enumerated() {
                 let indent = String(repeating: "  ", count: index)
                 description += "\(indent)\\- \(request)\n"
             }
diff --git a/Sources/WIT/SyntaxNode.swift b/Sources/WIT/SyntaxNode.swift
index f2593b11..01c8a923 100644
--- a/Sources/WIT/SyntaxNode.swift
+++ b/Sources/WIT/SyntaxNode.swift
@@ -1,10 +1,10 @@
 /// An AST node that can be wrapped by ``SyntaxNode`` as a reference
-public protocol SyntaxNodeProtocol {}
+public protocol SyntaxNodeProtocol: Sendable {}
 
 /// An AST node with reference semantics to provide identity of a node.
 @dynamicMemberLookup
-public struct SyntaxNode<Syntax: SyntaxNodeProtocol>: Equatable, Hashable {
-    class Ref {
+public struct SyntaxNode<Syntax: SyntaxNodeProtocol>: Equatable, Hashable, Sendable {
+    final class Ref: Sendable {
         fileprivate let syntax: Syntax
         init(syntax: Syntax) {
             self.syntax = syntax
diff --git a/Sources/WasmKit/Component/ComponentTypes.swift b/Sources/WasmKit/Component/ComponentTypes.swift
index 1f4965d8..90711076 100644
--- a/Sources/WasmKit/Component/ComponentTypes.swift
+++ b/Sources/WasmKit/Component/ComponentTypes.swift
@@ -1,5 +1,5 @@
 /// Error type raised from Component Model operations
-public struct ComponentError<Content>: Error {
+public struct ComponentError<Content: Sendable>: Error {
     /// The content of the error
     public let content: Content
 
diff --git a/Sources/WasmKit/Execution/Errors.swift b/Sources/WasmKit/Execution/Errors.swift
index 08bd4273..07c1880b 100644
--- a/Sources/WasmKit/Execution/Errors.swift
+++ b/Sources/WasmKit/Execution/Errors.swift
@@ -3,12 +3,9 @@ import WasmTypes
 import struct WasmParser.Import
 
 /// The backtrace of the trap.
-struct Backtrace: CustomStringConvertible {
+struct Backtrace: CustomStringConvertible, Sendable {
     /// A symbol in the backtrace.
     struct Symbol {
-        /// The function that the symbol represents.
-        let function: Function
-
         /// The name of the symbol.
         let name: String?
     }
diff --git a/Sources/WasmKit/Execution/Execution.swift b/Sources/WasmKit/Execution/Execution.swift
index 3702ba72..4f86cb75 100644
--- a/Sources/WasmKit/Execution/Execution.swift
+++ b/Sources/WasmKit/Execution/Execution.swift
@@ -70,7 +70,6 @@ struct Execution {
             let symbolName = store.nameRegistry.symbolicate(.wasm(function))
             symbols.append(
                 Backtrace.Symbol(
-                    function: Function(handle: .wasm(function), store: store),
                     name: symbolName
                 )
             )
diff --git a/Sources/WasmParser/Stream/Stream.swift b/Sources/WasmParser/Stream/Stream.swift
index bf457aa4..46225c12 100644
--- a/Sources/WasmParser/Stream/Stream.swift
+++ b/Sources/WasmParser/Stream/Stream.swift
@@ -1,5 +1,5 @@
 @usableFromInline
-enum StreamError<Element>: Swift.Error, Equatable where Element: Hashable {
+enum StreamError<Element>: Swift.Error, Equatable where Element: Hashable & Sendable {
     case unexpectedEnd(expected: Set<Element>?)
     case unexpected(Element, index: Int, expected: Set<Element>?)
 }
diff --git a/Sources/WasmParser/WasmParser.swift b/Sources/WasmParser/WasmParser.swift
index f1fe12b8..0bca6049 100644
--- a/Sources/WasmParser/WasmParser.swift
+++ b/Sources/WasmParser/WasmParser.swift
@@ -175,7 +175,7 @@ public struct ExpressionParser {
 let WASM_MAGIC: [UInt8] = [0x00, 0x61, 0x73, 0x6D]
 
 /// Flags for enabling/disabling WebAssembly features
-public struct WasmFeatureSet: OptionSet {
+public struct WasmFeatureSet: OptionSet, Sendable {
     /// The raw value of the feature set
     public let rawValue: Int
 
@@ -206,7 +206,7 @@ public struct WasmFeatureSet: OptionSet {
 /// An error that occurs during parsing of a WebAssembly binary
 public struct WasmParserError: Swift.Error {
     @usableFromInline
-    struct Message {
+    struct Message: Sendable {
         let text: String
 
         init(_ text: String) {
diff --git a/Sources/WasmParser/WasmTypes.swift b/Sources/WasmParser/WasmTypes.swift
index e58ae783..2acd5212 100644
--- a/Sources/WasmParser/WasmTypes.swift
+++ b/Sources/WasmParser/WasmTypes.swift
@@ -189,7 +189,7 @@ public struct Global: Equatable {
 /// <https://webassembly.github.io/spec/core/syntax/modules.html#element-segments>
 public struct ElementSegment: Equatable {
     @usableFromInline
-    struct Flag: OptionSet {
+    struct Flag: OptionSet, Sendable {
         @usableFromInline let rawValue: UInt32
 
         @inlinable
diff --git a/Sources/WasmTypes/WasmTypes.swift b/Sources/WasmTypes/WasmTypes.swift
index 054e901c..3f0c9c61 100644
--- a/Sources/WasmTypes/WasmTypes.swift
+++ b/Sources/WasmTypes/WasmTypes.swift
@@ -2,7 +2,7 @@
 ///
 /// > Note:
 /// <https://webassembly.github.io/spec/core/syntax/types.html#function-types>
-public struct FunctionType: Equatable, Hashable {
+public struct FunctionType: Equatable, Hashable, Sendable {
     public init(parameters: [ValueType], results: [ValueType] = []) {
         self.parameters = parameters
         self.results = results
@@ -15,14 +15,14 @@ public struct FunctionType: Equatable, Hashable {
 }
 
 /// Reference types
-public enum ReferenceType: UInt8, Equatable, Hashable {
+public enum ReferenceType: UInt8, Equatable, Hashable, Sendable {
     /// A nullable reference type to a function.
     case funcRef
     /// A nullable external reference type.
     case externRef
 }
 
-public enum ValueType: Equatable, Hashable {
+public enum ValueType: Equatable, Hashable, Sendable {
     /// 32-bit signed or unsigned integer.
     case i32
     /// 64-bit signed or unsigned integer.
diff --git a/Tests/WITTests/Semantics/RequestEvaluatorTests.swift b/Tests/WITTests/Semantics/RequestEvaluatorTests.swift
index fc0ae422..cc7eb398 100644
--- a/Tests/WITTests/Semantics/RequestEvaluatorTests.swift
+++ b/Tests/WITTests/Semantics/RequestEvaluatorTests.swift
@@ -61,7 +61,7 @@ class RequestEvaluatorTests: XCTestCase {
         )
 
         let cyclicalRequestError = try XCTUnwrap(anyError as? Evaluator.CyclicalRequestError)
-        XCTAssertEqual(cyclicalRequestError.activeRequests.count, 3)
+        XCTAssertEqual(cyclicalRequestError.activeRequestDescriptions.count, 3)
     }
 
     func testThrowingRequest() throws {
diff --git a/Tests/WasmKitTests/Spectest/Spectest.swift b/Tests/WasmKitTests/Spectest/Spectest.swift
index 7dc094a6..82302c78 100644
--- a/Tests/WasmKitTests/Spectest/Spectest.swift
+++ b/Tests/WasmKitTests/Spectest/Spectest.swift
@@ -48,11 +48,11 @@ struct NullSpectestProgressReporter: SpectestProgressReporter {
 
 struct StderrSpectestProgressReporter: SpectestProgressReporter {
     func log(_ message: String, verbose: Bool) {
-        fputs(message + "\n", stderr)
+        try! FileHandle.standardError.write(contentsOf: Data((message + "\n").utf8))
     }
     func log(_ message: String, path: String, location: Location, verbose: Bool) {
         let (line, _) = location.computeLineAndColumn()
-        fputs("\(path):\(line): " + message + "\n", stderr)
+        try! FileHandle.standardError.write(contentsOf: Data(("\(path):\(line): " + message + "\n").utf8))
     }
 }