diff --git a/Sources/SmithyHTTPAPI/HTTPResponse.swift b/Sources/SmithyHTTPAPI/HTTPResponse.swift
index 0840d9990..20e430c1e 100644
--- a/Sources/SmithyHTTPAPI/HTTPResponse.swift
+++ b/Sources/SmithyHTTPAPI/HTTPResponse.swift
@@ -8,43 +8,48 @@
 import protocol Smithy.ResponseMessage
 import protocol Smithy.Stream
 import enum Smithy.ByteStream
-import class Foundation.DispatchQueue
+import class Foundation.NSRecursiveLock
 
-public class HTTPResponse: HTTPURLResponse, ResponseMessage {
+public final class HTTPResponse: ResponseMessage, @unchecked Sendable {
+    private var lock = NSRecursiveLock()
 
-    public var headers: Headers
-    public var body: ByteStream
-    public var reason: String?
+    private var _headers: Headers
+    public var headers: Headers {
+        get { lock.lock(); defer { lock.unlock() }; return _headers }
+        set { lock.lock(); defer { lock.unlock() }; self._headers = newValue }
+    }
+
+    private var _body: ByteStream
+    public var body: ByteStream {
+        get { lock.lock(); defer { lock.unlock() }; return _body }
+        set { lock.lock(); defer { lock.unlock() }; self._body = newValue }
+    }
 
     private var _statusCode: HTTPStatusCode
-    private let statusCodeQueue = DispatchQueue(label: "statusCodeSerialQueue")
     public var statusCode: HTTPStatusCode {
-        get {
-            statusCodeQueue.sync {
-                return _statusCode
-            }
-        }
-        set {
-            statusCodeQueue.sync {
-                self._statusCode = newValue
-            }
-        }
+        get { lock.lock(); defer { lock.unlock() }; return _statusCode }
+        set { lock.lock(); defer { lock.unlock() }; self._statusCode = newValue }
     }
 
+    public let reason: String?
+
     public init(
         headers: Headers = .init(),
         statusCode: HTTPStatusCode = .processing,
         body: ByteStream = .noStream,
-        reason: String? = nil) {
-        self.headers = headers
+        reason: String? = nil
+    ) {
+        self._headers = headers
         self._statusCode = statusCode
-        self.body = body
+        self._body = body
+        self.reason = reason
     }
 
     public init(headers: Headers = .init(), body: ByteStream, statusCode: HTTPStatusCode, reason: String? = nil) {
-        self.body = body
+        self._body = body
         self._statusCode = statusCode
-        self.headers = headers
+        self._headers = headers
+        self.reason = reason
     }
 
     /**
diff --git a/Sources/SmithyHTTPAPI/HTTPURLResponse.swift b/Sources/SmithyHTTPAPI/HTTPURLResponse.swift
deleted file mode 100644
index 3498db019..000000000
--- a/Sources/SmithyHTTPAPI/HTTPURLResponse.swift
+++ /dev/null
@@ -1,15 +0,0 @@
-//
-// Copyright Amazon.com Inc. or its affiliates.
-// All Rights Reserved.
-//
-// SPDX-License-Identifier: Apache-2.0
-//
-
-import enum Smithy.ByteStream
-
-protocol HTTPURLResponse {
-    var headers: Headers { get set }
-    var body: ByteStream { get set }
-    var statusCode: HTTPStatusCode { get set }
-    var reason: String? { get set }
-}
diff --git a/Sources/SmithyHTTPAPI/Headers.swift b/Sources/SmithyHTTPAPI/Headers.swift
index 6358ecc6f..bf9100359 100644
--- a/Sources/SmithyHTTPAPI/Headers.swift
+++ b/Sources/SmithyHTTPAPI/Headers.swift
@@ -5,8 +5,28 @@
 // SPDX-License-Identifier: Apache-2.0
 //
 
-public struct Headers: Sendable {
-    public var headers: [Header] = []
+import class Foundation.NSRecursiveLock
+
+public struct Headers: @unchecked Sendable {
+    private let lock = NSRecursiveLock()
+
+    private var _headers: [Header] = []
+    public var headers: [Header] {
+        get { access { $0 } }
+        set { mutate { $0 = newValue } }
+    }
+
+    private mutating func mutate(_ block: (inout [Header]) -> Void) {
+        lock.lock()
+        defer { lock.unlock() }
+        block(&_headers)
+    }
+
+    private func access<T>(_ block: ([Header]) throws -> T) rethrows -> T {
+        lock.lock()
+        defer { lock.unlock() }
+        return try block(_headers)
+    }
 
     /// Creates an empty instance.
     public init() {}
@@ -53,11 +73,13 @@ public struct Headers: Sendable {
     /// - Parameters:
     ///   - header:  The `Header` to be added or updated.
     public mutating func add(_ header: Header) {
-        guard let index = headers.index(of: header.name) else {
-            headers.append(header)
-            return
+        mutate { headers in
+            guard let index = headers.index(of: header.name) else {
+                headers.append(header)
+                return
+            }
+            headers[index].value.append(contentsOf: header.value)
         }
-        headers[index].value.append(contentsOf: header.value)
     }
 
     /// Case-insensitively updates the value of a `Header` by replacing the values of it or appends a `Header`
@@ -66,11 +88,13 @@ public struct Headers: Sendable {
     /// - Parameters:
     ///   - header:  The `Header` to be added or updated.
     public mutating func update(_ header: Header) {
-        guard let index = headers.index(of: header.name) else {
-            headers.append(header)
-            return
+        mutate { headers in
+            guard let index = headers.index(of: header.name) else {
+                headers.append(header)
+                return
+            }
+            headers.replaceSubrange(index...index, with: [header])
         }
-        headers.replaceSubrange(index...index, with: [header])
     }
 
     /// Case-insensitively updates the value of a `Header` by replacing the values of it or appends a `Header`
@@ -97,17 +121,20 @@ public struct Headers: Sendable {
     ///
     /// - Parameters:
     ///   - headers:  The `Headers` object.
-    public mutating func addAll(headers: Headers) {
-        self.headers.append(contentsOf: headers.headers)
+    public mutating func addAll(headers otherHeaders: Headers) {
+        mutate { headers in
+            headers.append(contentsOf: otherHeaders.headers)
+        }
     }
 
     /// Case-insensitively removes a `Header`, if it exists, from the instance.
     ///
     /// - Parameter name: The name of the `HTTPHeader` to remove.
     public mutating func remove(name: String) {
-        guard let index = headers.index(of: name) else { return }
-
-        headers.remove(at: index)
+        mutate { headers in
+            guard let index = headers.index(of: name) else { return }
+            headers.remove(at: index)
+        }
     }
 
     /// Case-insensitively find a header's values by name.
@@ -116,13 +143,14 @@ public struct Headers: Sendable {
     ///
     /// - Returns: The values of the header, if they exist.
     public func values(for name: String) -> [String]? {
-        guard let indices = headers.indices(of: name), !indices.isEmpty else { return nil }
-        var values = [String]()
-        for index in indices {
-            values.append(contentsOf: headers[index].value)
+        access { headers in
+            guard let indices = headers.indices(of: name), !indices.isEmpty else { return nil }
+            var values = [String]()
+            for index in indices {
+                values.append(contentsOf: headers[index].value)
+            }
+            return values
         }
-
-        return values
     }
 
     /// Case-insensitively find a header's value by name.
@@ -131,29 +159,28 @@ public struct Headers: Sendable {
     ///
     /// - Returns: The value of header as a comma delimited string, if it exists.
     public func value(for name: String) -> String? {
-        guard let values = values(for: name) else {
-            return nil
-        }
+        guard let values = values(for: name) else { return nil }
         return values.joined(separator: ",")
     }
 
     public func exists(name: String) -> Bool {
-        headers.index(of: name) != nil
+        access { $0.index(of: name) != nil }
     }
 
     /// The dictionary representation of all headers.
     ///
     /// This representation does not preserve the current order of the instance.
     public var dictionary: [String: [String]] {
-        let namesAndValues = headers.map { ($0.name, $0.value) }
-
-        return Dictionary(namesAndValues) { (first, last) -> [String] in
-            return first + last
+        access { headers in
+            let namesAndValues = headers.map { ($0.name, $0.value) }
+            return Dictionary(namesAndValues) { (first, last) -> [String] in
+                first + last
+            }
         }
     }
 
     public var isEmpty: Bool {
-        return self.headers.isEmpty
+        access { $0.isEmpty }
     }
 }
 
@@ -164,14 +191,18 @@ extension Headers: Equatable {
     ///   - rhs: The second `Headers` to compare.
     /// - Returns: `true` if the two values are equal irrespective of order, otherwise `false`.
     public static func == (lhs: Headers, rhs: Headers) -> Bool {
-        return lhs.headers.sorted() == rhs.headers.sorted()
+        lhs.access { lhsHeaders in
+            rhs.access { rhsHeaders in
+                lhsHeaders.sorted() == rhsHeaders.sorted()
+            }
+        }
     }
 }
 
 extension Headers: Hashable {
 
     public func hash(into hasher: inout Hasher) {
-        hasher.combine(headers.sorted())
+        access { hasher.combine($0.sorted()) }
     }
 }
 
diff --git a/smithy-swift-codegen/src/main/kotlin/software/amazon/smithy/swift/codegen/StructureGenerator.kt b/smithy-swift-codegen/src/main/kotlin/software/amazon/smithy/swift/codegen/StructureGenerator.kt
index d11d00fd2..88dcd287b 100644
--- a/smithy-swift-codegen/src/main/kotlin/software/amazon/smithy/swift/codegen/StructureGenerator.kt
+++ b/smithy-swift-codegen/src/main/kotlin/software/amazon/smithy/swift/codegen/StructureGenerator.kt
@@ -215,10 +215,11 @@ class StructureGenerator(
 
         writer.writeAvailableAttribute(model, shape)
         writer.openBlock(
-            "public struct \$struct.name:L: \$N, \$error.protocol:N, \$N, \$N {",
+            "public struct \$struct.name:L: \$N, \$error.protocol:N, \$N, \$N, \$N {",
             ClientRuntimeTypes.Core.ModeledError,
             ClientRuntimeTypes.Http.HttpError,
-            SwiftTypes.Error
+            SwiftTypes.Error,
+            SwiftTypes.Protocols.Sendable,
         )
             .call { generateErrorStructMembers() }
             .write("")
@@ -234,7 +235,7 @@ class StructureGenerator(
     private fun generateErrorStructMembers() {
         if (membersSortedByName.isNotEmpty()) {
             writer.write("")
-            writer.openBlock("public struct Properties {", "}") {
+            writer.openBlock("public struct Properties: \$N {", "}", SwiftTypes.Protocols.Sendable) {
                 membersSortedByName.forEach {
                     val (memberName, memberSymbol) = memberShapeDataContainer.getOrElse(it) { return@forEach }
                     writer.writeMemberDocs(model, it)
diff --git a/smithy-swift-codegen/src/test/kotlin/software/amazon/smithy/swift/codegen/basicshapes/StructureGeneratorTests.kt b/smithy-swift-codegen/src/test/kotlin/software/amazon/smithy/swift/codegen/basicshapes/StructureGeneratorTests.kt
index 536949b6b..8a21ba2f7 100644
--- a/smithy-swift-codegen/src/test/kotlin/software/amazon/smithy/swift/codegen/basicshapes/StructureGeneratorTests.kt
+++ b/smithy-swift-codegen/src/test/kotlin/software/amazon/smithy/swift/codegen/basicshapes/StructureGeneratorTests.kt
@@ -262,9 +262,9 @@ public struct RecursiveShapesInputOutputLists: Swift.Sendable {
 
         contents.shouldContain(swiftSettings.copyrightNotice)
         val expectedGeneratedStructure = """
-public struct MyError: ClientRuntime.ModeledError, ClientRuntime.ServiceError, ClientRuntime.HTTPError, Swift.Error {
+public struct MyError: ClientRuntime.ModeledError, ClientRuntime.ServiceError, ClientRuntime.HTTPError, Swift.Error, Swift.Sendable {
 
-    public struct Properties {
+    public struct Properties: Swift.Sendable {
         /// This is documentation about the member.
         public internal(set) var baz: Swift.Int? = nil
         public internal(set) var message: Swift.String? = nil