diff --git a/Sources/ContainerizationOCI/Client/RegistryClient+Token.swift b/Sources/ContainerizationOCI/Client/RegistryClient+Token.swift
index 57eeb6ff2..442be902d 100644
--- a/Sources/ContainerizationOCI/Client/RegistryClient+Token.swift
+++ b/Sources/ContainerizationOCI/Client/RegistryClient+Token.swift
@@ -25,8 +25,10 @@ struct TokenRequest {
 
     /// The realm against which the token should be requested.
     let realm: String
-    /// The name of the service which hosts the resource.
-    let service: String
+    /// The name of the service which hosts the resource. Optional: the parameter is a Docker
+    /// registry convention, not a requirement of RFC 6750, and some registries (notably Google
+    /// Artifact Registry) omit it from their challenge entirely.
+    let service: String?
     /// Whether to return a refresh token along with the bearer token.
     let offlineToken: Bool
     /// String identifying the client.
@@ -36,7 +38,7 @@ struct TokenRequest {
 
     init(
         realm: String,
-        service: String,
+        service: String?,
         clientId: String,
         scope: String?,
         offlineToken: Bool = false
@@ -139,9 +141,11 @@ extension RegistryClient {
         }
         try validateRealm(components)
         components.queryItems = [
-            URLQueryItem(name: "client_id", value: request.clientId),
-            URLQueryItem(name: "service", value: request.service),
+            URLQueryItem(name: "client_id", value: request.clientId)
         ]
+        if let service = request.service {
+            components.queryItems?.append(URLQueryItem(name: "service", value: service))
+        }
         var scope = ""
         if let reqScope = request.scope {
             scope = reqScope
@@ -228,11 +232,8 @@ extension RegistryClient {
         guard let realm = bearerChallenge.realm else {
             throw ContainerizationError(.invalidArgument, message: "cannot parse realm from \(TokenRequest.authenticateHeaderName) header")
         }
-        guard let service = bearerChallenge.service else {
-            throw ContainerizationError(.invalidArgument, message: "cannot parse service from \(TokenRequest.authenticateHeaderName) header")
-        }
         let scope = bearerChallenge.scope
-        let tokenRequest = TokenRequest(realm: realm, service: service, clientId: self.clientID, scope: scope)
+        let tokenRequest = TokenRequest(realm: realm, service: bearerChallenge.service, clientId: self.clientID, scope: scope)
         return tokenRequest
     }
 
diff --git a/Tests/ContainerizationOCITests/TokenRequestServiceTests.swift b/Tests/ContainerizationOCITests/TokenRequestServiceTests.swift
new file mode 100644
index 000000000..13ac8ddf0
--- /dev/null
+++ b/Tests/ContainerizationOCITests/TokenRequestServiceTests.swift
@@ -0,0 +1,60 @@
+//===----------------------------------------------------------------------===//
+// Copyright © 2025-2026 Apple Inc. and the Containerization project authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//===----------------------------------------------------------------------===//
+
+import Foundation
+import Testing
+
+@testable import ContainerizationOCI
+
+/// `service` is a Docker registry convention, not a requirement of RFC 6750 section 3, so a
+/// conforming registry may omit it from the Bearer challenge. `parseWWWAuthenticateHeaders` has
+/// always modelled it as optional — see `AuthChallengeTests` — but `createTokenRequest` used to
+/// reject a challenge without it, which locked out every registry that omits it.
+struct TokenRequestServiceTests {
+    /// Google Artifact Registry's actual challenge, verbatim: realm only, no `service`.
+    private static let artifactRegistryChallenge = #"Bearer realm="https://us-central1-docker.pkg.dev/v2/token""#
+
+    /// Docker Hub's challenge, which does carry `service`.
+    private static let dockerHubChallenge =
+        #"Bearer realm="https://auth.docker.io/token",service="registry.docker.io""#
+
+    @Test
+    func acceptsChallengeWithoutService() throws {
+        let client = RegistryClient(host: "us-central1-docker.pkg.dev", scheme: "https")
+        let request = try client.createTokenRequest(parsing: [Self.artifactRegistryChallenge])
+
+        #expect(request.realm == "https://us-central1-docker.pkg.dev/v2/token")
+        #expect(request.service == nil)
+    }
+
+    @Test
+    func preservesServiceWhenPresent() throws {
+        let client = RegistryClient(host: "registry-1.docker.io", scheme: "https")
+        let request = try client.createTokenRequest(parsing: [Self.dockerHubChallenge])
+
+        #expect(request.service == "registry.docker.io")
+    }
+
+    /// `realm` stays mandatory — without it there is nowhere to send the token request.
+    @Test
+    func stillRejectsChallengeWithoutRealm() throws {
+        let client = RegistryClient(host: "registry.example.com", scheme: "https")
+
+        #expect(throws: (any Error).self) {
+            try client.createTokenRequest(parsing: [#"Bearer service="registry.example.com""#])
+        }
+    }
+}