diff --git a/Sources/OpenAPIRuntime/Conversion/Converter+Server.swift b/Sources/OpenAPIRuntime/Conversion/Converter+Server.swift
index 7394568..7d36cb2 100644
--- a/Sources/OpenAPIRuntime/Conversion/Converter+Server.swift
+++ b/Sources/OpenAPIRuntime/Conversion/Converter+Server.swift
@@ -12,7 +12,11 @@
 //
 //===----------------------------------------------------------------------===//
 
+#if canImport(FoundationEssentials)
+import FoundationEssentials
+#else
 import Foundation
+#endif
 public import HTTPTypes
 
 extension Converter {
diff --git a/Sources/OpenAPIRuntime/Conversion/CurrencyExtensions.swift b/Sources/OpenAPIRuntime/Conversion/CurrencyExtensions.swift
index 27663b3..1200448 100644
--- a/Sources/OpenAPIRuntime/Conversion/CurrencyExtensions.swift
+++ b/Sources/OpenAPIRuntime/Conversion/CurrencyExtensions.swift
@@ -12,7 +12,11 @@
 //
 //===----------------------------------------------------------------------===//
 
+#if canImport(FoundationEssentials)
+import FoundationEssentials
+#else
 import Foundation
+#endif
 import HTTPTypes
 
 extension ParameterStyle {
@@ -382,12 +386,12 @@ extension Converter {
 
         guard let queryStart else {
             // No existing query substring, add the question mark.
-            request.path = path.appending("?\(escapedUriSnippet)\(fragment)")
+            request.path = path + "?\(escapedUriSnippet)\(fragment)"
             return
         }
 
         let query = pathAndAll[pathAndAll.index(after: queryStart)..<fragmentStart]
-        request.path = path.appending("?\(query)&\(escapedUriSnippet)\(fragment)")
+        request.path = path + "?\(query)&\(escapedUriSnippet)\(fragment)"
     }
 
     /// Returns the decoded value for the provided name of a query parameter.
diff --git a/Sources/OpenAPIRuntime/Conversion/ErrorExtensions.swift b/Sources/OpenAPIRuntime/Conversion/ErrorExtensions.swift
index d4100ab..520f6a1 100644
--- a/Sources/OpenAPIRuntime/Conversion/ErrorExtensions.swift
+++ b/Sources/OpenAPIRuntime/Conversion/ErrorExtensions.swift
@@ -12,7 +12,11 @@
 //
 //===----------------------------------------------------------------------===//
 
+#if canImport(FoundationEssentials)
+import FoundationEssentials
+#else
 import Foundation
+#endif
 
 extension DecodingError {
 
diff --git a/Sources/OpenAPIRuntime/Errors/ClientError.swift b/Sources/OpenAPIRuntime/Errors/ClientError.swift
index ffd53b0..595d12b 100644
--- a/Sources/OpenAPIRuntime/Errors/ClientError.swift
+++ b/Sources/OpenAPIRuntime/Errors/ClientError.swift
@@ -12,7 +12,11 @@
 //
 //===----------------------------------------------------------------------===//
 
+#if canImport(FoundationEssentials)
+public import FoundationEssentials
+#else
 public import Foundation
+#endif
 public import HTTPTypes
 
 /// An error thrown by a client performing an OpenAPI operation.
diff --git a/Sources/OpenAPIRuntime/Errors/ServerError.swift b/Sources/OpenAPIRuntime/Errors/ServerError.swift
index 6686d5a..b9568d3 100644
--- a/Sources/OpenAPIRuntime/Errors/ServerError.swift
+++ b/Sources/OpenAPIRuntime/Errors/ServerError.swift
@@ -12,7 +12,11 @@
 //
 //===----------------------------------------------------------------------===//
 
+#if canImport(FoundationEssentials)
+public import FoundationEssentials
+#else
 public import Foundation
+#endif
 public import HTTPTypes
 
 /// An error thrown by a server handling an OpenAPI operation.
diff --git a/Sources/OpenAPIRuntime/EventStreams/ServerSentEventsEncoding.swift b/Sources/OpenAPIRuntime/EventStreams/ServerSentEventsEncoding.swift
index 825542c..99fe959 100644
--- a/Sources/OpenAPIRuntime/EventStreams/ServerSentEventsEncoding.swift
+++ b/Sources/OpenAPIRuntime/EventStreams/ServerSentEventsEncoding.swift
@@ -12,7 +12,11 @@
 //
 //===----------------------------------------------------------------------===//
 
+#if canImport(FoundationEssentials)
+public import FoundationEssentials
+#else
 public import Foundation
+#endif
 
 /// A sequence that serializes Server-sent Events.
 public struct ServerSentEventsSerializationSequence<Upstream: AsyncSequence & Sendable>: Sendable
diff --git a/Sources/OpenAPIRuntime/Interface/CurrencyTypes.swift b/Sources/OpenAPIRuntime/Interface/CurrencyTypes.swift
index 0d8dfb3..232f7f7 100644
--- a/Sources/OpenAPIRuntime/Interface/CurrencyTypes.swift
+++ b/Sources/OpenAPIRuntime/Interface/CurrencyTypes.swift
@@ -12,7 +12,11 @@
 //
 //===----------------------------------------------------------------------===//
 
+#if canImport(FoundationEssentials)
+import FoundationEssentials
+#else
 import Foundation
+#endif
 public import HTTPTypes
 
 /// A container for request metadata already parsed and validated
diff --git a/Sources/OpenAPIRuntime/Multipart/MultipartBoundaryGenerator.swift b/Sources/OpenAPIRuntime/Multipart/MultipartBoundaryGenerator.swift
index cdea9a3..4bd71d4 100644
--- a/Sources/OpenAPIRuntime/Multipart/MultipartBoundaryGenerator.swift
+++ b/Sources/OpenAPIRuntime/Multipart/MultipartBoundaryGenerator.swift
@@ -12,7 +12,11 @@
 //
 //===----------------------------------------------------------------------===//
 
+#if canImport(FoundationEssentials)
+import FoundationEssentials
+#else
 import Foundation
+#endif
 
 /// A generator of a new boundary string used by multipart messages to separate parts.
 public protocol MultipartBoundaryGenerator: Sendable {
@@ -72,6 +76,6 @@ public struct RandomMultipartBoundaryGenerator: MultipartBoundaryGenerator {
     public func makeBoundary() -> String {
         var randomSuffix = [UInt8](repeating: 0, count: randomNumberSuffixLength)
         for i in randomSuffix.startIndex..<randomSuffix.endIndex { randomSuffix[i] = values.randomElement()! }
-        return boundaryPrefix.appending(String(decoding: randomSuffix, as: UTF8.self))
+        return boundaryPrefix + String(decoding: randomSuffix, as: UTF8.self)
     }
 }