Decode the error object in OpenAI Responses results

Sources/AnyLanguageModel/Models/OpenAILanguageModel.swift

@@ -1291,7 +1291,7 @@ private enum Responses {
         let id: String
         let output: [JSONValue]?
         let usage: ResponsesUsage?
-        let error: [JSONValue]?
+        let error: ResponseError?
         let outputText: String?
         let finishReason: String?
 
@@ -1304,6 +1304,12 @@ private enum Responses {
             case error = "error"
         }
     }
+
+    /// The `error` object of a response, which is `null` unless the response failed.
+    struct ResponseError: Decodable, Sendable {
+        let code: String?
+        let message: String?
+    }
 }
 
 // MARK: - Supporting Types

Tests/AnyLanguageModelTests/ProviderUsageTests.swift

@@ -859,6 +859,16 @@ import Testing
             #expect(UsageURLProtocol.recordedBodies.count == 2)
         }
 
+        @Test(arguments: [Provider.responses, .openResponses])
+        func responsesDecodeErrorObjects(_ provider: Provider) async throws {
+            UsageURLProtocol.reset()
+            var body = provider.response(counts: provider.counts)
+            body["error"] = ["code": "server_error", "message": "The model failed."]
+            UsageURLProtocol.enqueue(json: try Self.json(body))
+            let response = try await provider.makeSession().respond(to: "Hi")
+            #expect(response.content == "Hello")
+        }
+
         @Test(arguments: [Provider.chat, .responses, .openResponses, .gemini])
         func toolRoundsAccumulateUsage(_ provider: Provider) async throws {
             UsageURLProtocol.reset()