diff --git a/Sources/AiSTKit/AuthenticationMiddleware.swift b/Sources/AiSTKit/AuthenticationMiddleware.swift
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+//
+//  AuthenticationMiddleware.swift
+//  SyntaxKit
+//
+//  Created by Leo Dion.
+//  Copyright © 2026 BrightDigit.
+//
+//  Permission is hereby granted, free of charge, to any person
+//  obtaining a copy of this software and associated documentation
+//  files (the “Software”), to deal in the Software without
+//  restriction, including without limitation the rights to use,
+//  copy, modify, merge, publish, distribute, sublicense, and/or
+//  sell copies of the Software, and to permit persons to whom the
+//  Software is furnished to do so, subject to the following
+//  conditions:
+//
+//  The above copyright notice and this permission notice shall be
+//  included in all copies or substantial portions of the Software.
+//
+//  THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND,
+//  EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
+//  OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+//  NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+//  HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+//  WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+//  FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+//  OTHER DEALINGS IN THE SOFTWARE.
+//
+
+public import HTTPTypes
+public import OpenAPIRuntime
+
+#if os(Linux)
+  @preconcurrency public import struct Foundation.URL
+#else
+  public import struct Foundation.URL
+#endif
+
+/// OpenAPI client middleware that adds Anthropic API authentication headers.
+///
+/// Injects the `x-api-key` and `anthropic-version` headers into every
+/// outgoing request before forwarding it to the next handler in the chain.
+public struct AuthenticationMiddleware: ClientMiddleware {
+  /// The Anthropic API version sent with every request.
+  private static let anthropicVersion = "2023-06-01"
+
+  /// The Anthropic API key used to authenticate requests.
+  private let apiKey: String
+
+  /// Creates a middleware that authenticates requests with the given API key.
+  /// - Parameter apiKey: The Anthropic API key to send in the `x-api-key` header.
+  public init(apiKey: String) {
+    self.apiKey = apiKey
+  }
+
+  /// Adds the Anthropic authentication headers, then forwards the request.
+  /// - Parameters:
+  ///   - request: The outgoing HTTP request.
+  ///   - body: The outgoing HTTP request body, if any.
+  ///   - baseURL: The base URL of the server.
+  ///   - operationID: The OpenAPI operation identifier for the request.
+  ///   - next: The next handler in the middleware chain.
+  /// - Returns: The HTTP response and body from the next handler.
+  public func intercept(
+    _ request: HTTPRequest,
+    body: HTTPBody?,
+    baseURL: URL,
+    operationID: String,
+    next: @Sendable (HTTPRequest, HTTPBody?, URL) async throws -> (HTTPResponse, HTTPBody?)
+  ) async throws -> (HTTPResponse, HTTPBody?) {
+    var request = request
+    if let name = HTTPField.Name("x-api-key") {
+      request.headerFields[name] = apiKey
+    }
+    if let name = HTTPField.Name("anthropic-version") {
+      request.headerFields[name] = Self.anthropicVersion
+    }
+    return try await next(request, body, baseURL)
+  }
+}