diff --git a/Sources/AnyLanguageModel/Models/AnthropicLanguageModel.swift b/Sources/AnyLanguageModel/Models/AnthropicLanguageModel.swift
index d074433b..97795f1a 100644
--- a/Sources/AnyLanguageModel/Models/AnthropicLanguageModel.swift
+++ b/Sources/AnyLanguageModel/Models/AnthropicLanguageModel.swift
@@ -41,8 +41,8 @@ public struct AnthropicLanguageModel: LanguageModel {
     /// The base URL for the API endpoint.
     public let baseURL: URL
 
-    /// The API key for authentication.
-    public let apiKey: String
+    /// The closure providing the API key for authentication.
+    private let tokenProvider: @Sendable () -> String
 
     /// The API version to use for requests.
     public let apiVersion: String
@@ -54,20 +54,19 @@ public struct AnthropicLanguageModel: LanguageModel {
     public let model: String
 
     private let urlSession: URLSession
-    private let headers: [String: String]
 
     /// Creates an Anthropic language model.
     ///
     /// - Parameters:
     ///   - baseURL: The base URL for the API endpoint. Defaults to Anthropic's official API.
-    ///   - apiKey: Your Anthropic API key.
+    ///   - apiKey: Your Anthropic API key or a closure that returns it.
     ///   - apiVersion: The API version to use for requests. Defaults to `2023-06-01`.
     ///   - betas: Optional beta version(s) of the API to use.
     ///   - model: The model identifier (for example, "claude-3-5-sonnet-20241022").
     ///   - session: The URL session to use for network requests.
     public init(
         baseURL: URL = defaultBaseURL,
-        apiKey: String,
+        apiKey tokenProvider: @escaping @autoclosure @Sendable () -> String,
         apiVersion: String = defaultAPIVersion,
         betas: [String]? = nil,
         model: String,
@@ -79,22 +78,11 @@ public struct AnthropicLanguageModel: LanguageModel {
         }
 
         self.baseURL = baseURL
-        self.apiKey = apiKey
+        self.tokenProvider = tokenProvider
         self.apiVersion = apiVersion
         self.betas = betas
         self.model = model
         self.urlSession = session
-
-        var headers: [String: String] = [
-            "x-api-key": apiKey,
-            "anthropic-version": apiVersion,
-        ]
-
-        if let betas = betas, !betas.isEmpty {
-            headers["anthropic-beta"] = betas.joined(separator: ",")
-        }
-
-        self.headers = headers
     }
 
     public func respond<Content>(
@@ -109,6 +97,9 @@ public struct AnthropicLanguageModel: LanguageModel {
             fatalError("AnthropicLanguageModel only supports generating String content")
         }
 
+        let url = baseURL.appendingPathComponent("v1/messages")
+        let headers = buildHeaders()
+
         let messages = [
             AnthropicMessage(role: .user, content: [.text(.init(text: prompt.description))])
         ]
@@ -126,8 +117,8 @@ public struct AnthropicLanguageModel: LanguageModel {
             options: options
         )
 
-        let url = baseURL.appendingPathComponent("v1/messages")
         let body = try JSONEncoder().encode(params)
+
         let message: AnthropicMessageResponse = try await urlSession.fetch(
             .post,
             url: url,
@@ -189,6 +180,8 @@ public struct AnthropicLanguageModel: LanguageModel {
             continuation in
             let task = Task { @Sendable in
                 do {
+                    let headers = buildHeaders()
+
                     // Convert available tools to Anthropic format
                     let anthropicTools: [AnthropicTool] = try session.tools.map { tool in
                         try convertToolToAnthropicFormat(tool)
@@ -247,6 +240,19 @@ public struct AnthropicLanguageModel: LanguageModel {
 
         return LanguageModelSession.ResponseStream(stream: stream)
     }
+
+    private func buildHeaders() -> [String: String] {
+        var headers: [String: String] = [
+            "x-api-key": tokenProvider(),
+            "anthropic-version": apiVersion,
+        ]
+
+        if let betas = betas, !betas.isEmpty {
+            headers["anthropic-beta"] = betas.joined(separator: ",")
+        }
+
+        return headers
+    }
 }
 
 // MARK: - Conversions
diff --git a/Sources/AnyLanguageModel/Models/OpenAILanguageModel.swift b/Sources/AnyLanguageModel/Models/OpenAILanguageModel.swift
index 1c2dae1c..c6f2f486 100644
--- a/Sources/AnyLanguageModel/Models/OpenAILanguageModel.swift
+++ b/Sources/AnyLanguageModel/Models/OpenAILanguageModel.swift
@@ -37,8 +37,8 @@ public struct OpenAILanguageModel: LanguageModel {
     /// The base URL for the API endpoint.
     public let baseURL: URL
 
-    /// The API key for authentication.
-    public let apiKey: String
+    /// The closure providing the API key for authentication.
+    private let tokenProvider: @Sendable () -> String
 
     /// The model identifier to use for generation.
     public let model: String
@@ -52,13 +52,13 @@ public struct OpenAILanguageModel: LanguageModel {
     ///
     /// - Parameters:
     ///   - baseURL: The base URL for the API endpoint. Defaults to OpenAI's official API.
-    ///   - apiKey: Your OpenAI API key.
+    ///   - apiKey: Your OpenAI API key or a closure that returns it.
     ///   - model: The model identifier (for example, "gpt-4" or "gpt-3.5-turbo").
     ///   - apiVariant: The API variant to use. Defaults to `.chatCompletions`.
     ///   - session: The URL session to use for network requests.
     public init(
         baseURL: URL = defaultBaseURL,
-        apiKey: String,
+        apiKey tokenProvider: @escaping @autoclosure @Sendable () -> String,
         model: String,
         apiVariant: APIVariant = .chatCompletions,
         session: URLSession = URLSession(configuration: .default)
@@ -69,7 +69,7 @@ public struct OpenAILanguageModel: LanguageModel {
         }
 
         self.baseURL = baseURL
-        self.apiKey = apiKey
+        self.tokenProvider = tokenProvider
         self.model = model
         self.apiVariant = apiVariant
         self.urlSession = session
@@ -140,7 +140,7 @@ public struct OpenAILanguageModel: LanguageModel {
             .post,
             url: url,
             headers: [
-                "Authorization": "Bearer \(apiKey)"
+                "Authorization": "Bearer \(tokenProvider())"
             ],
             body: body
         )
@@ -193,7 +193,7 @@ public struct OpenAILanguageModel: LanguageModel {
             .post,
             url: url,
             headers: [
-                "Authorization": "Bearer \(apiKey)"
+                "Authorization": "Bearer \(tokenProvider())"
             ],
             body: body
         )
@@ -269,7 +269,7 @@ public struct OpenAILanguageModel: LanguageModel {
                                 .post,
                                 url: url,
                                 headers: [
-                                    "Authorization": "Bearer \(apiKey)"
+                                    "Authorization": "Bearer \(tokenProvider())"
                                 ],
                                 body: body
                             )
@@ -332,7 +332,7 @@ public struct OpenAILanguageModel: LanguageModel {
                                 .post,
                                 url: url,
                                 headers: [
-                                    "Authorization": "Bearer \(apiKey)"
+                                    "Authorization": "Bearer \(tokenProvider())"
                                 ],
                                 body: body
                             )
diff --git a/Tests/AnyLanguageModelTests/OpenAILanguageModelTests.swift b/Tests/AnyLanguageModelTests/OpenAILanguageModelTests.swift
index e2f23330..d76c47af 100644
--- a/Tests/AnyLanguageModelTests/OpenAILanguageModelTests.swift
+++ b/Tests/AnyLanguageModelTests/OpenAILanguageModelTests.swift
@@ -18,7 +18,6 @@ struct OpenAILanguageModelTests {
         for apiVariant in [OpenAILanguageModel.APIVariant.chatCompletions, .responses] {
             let model = OpenAILanguageModel(apiKey: "test-key", model: "test-model", apiVariant: apiVariant)
             #expect(model.apiVariant == apiVariant)
-            #expect(model.apiKey == "test-key")
             #expect(model.model == "test-model")
         }
     }