Return specific error for unrecognized payload in GenerativeAIService

FirebaseAI/Sources/Errors.swift

@@ -142,3 +142,13 @@ enum InvalidCandidateError: Error {
   case emptyContent(underlyingError: Error)
   case malformedContent(underlyingError: Error)
 }
+
+struct UnrecognizedRPCError: Error {
+  let responseBody: String
+}
+
+extension UnrecognizedRPCError: LocalizedError {
+  var errorDescription: String? {
+    return "Unrecognized error payload: \(responseBody)"
+  }
+}

FirebaseAI/Sources/GenerativeAIService.swift

@@ -264,8 +264,8 @@ struct GenerativeAIService {
       logRPCError(rpcError)
       return rpcError
     } catch {
-      // TODO: Return an error about an unrecognized error payload with the response body
-      return error
+      let responseString = String(data: responseData, encoding: .utf8) ?? ""
+      return UnrecognizedRPCError(responseBody: responseString)
     }
   }
 

FirebaseAI/Tests/Unit/GenerativeAIServiceTests.swift

@@ -0,0 +1,87 @@
+// Copyright 2026 Google LLC
+//
+// 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
+//
+//      http://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 FirebaseAppCheckInterop
+import FirebaseAuthInterop
+import FirebaseCore
+import XCTest
+
+@testable import FirebaseAILogic
+
+#if !os(watchOS)
+  @available(iOS 15.0, macOS 12.0, macCatalyst 15.0, tvOS 15.0, watchOS 8.0, *)
+  final class GenerativeAIServiceTests: XCTestCase {
+    let testModelName = "test-model"
+    let testModelResourceName =
+      "projects/test-project-id/locations/test-location/publishers/google/models/test-model"
+    let apiConfig = FirebaseAI.defaultVertexAIAPIConfig
+
+    var urlSession: URLSession!
+    var model: GenerativeModel!
+
+    override func setUp() async throws {
+      let configuration = URLSessionConfiguration.default
+      configuration.protocolClasses = [MockURLProtocol.self]
+      urlSession = try XCTUnwrap(URLSession(configuration: configuration))
+      model = GenerativeModel(
+        modelName: testModelName,
+        modelResourceName: testModelResourceName,
+        firebaseInfo: GenerativeModelTestUtil.testFirebaseInfo(),
+        apiConfig: apiConfig,
+        tools: nil,
+        requestOptions: RequestOptions(),
+        urlSession: urlSession
+      )
+    }
+
+    override func tearDown() {
+      MockURLProtocol.requestHandler = nil
+    }
+
+    func testGenerateContent_failure_unrecognizedErrorPayload() async throws {
+      let expectedStatusCode = 500
+      let responseBody = "Internal Server Error"
+
+      // We need to construct the handler to return specific data
+      let tempURL = FileManager.default.temporaryDirectory.appendingPathComponent(UUID().uuidString)
+      addTeardownBlock {
+        try? FileManager.default.removeItem(at: tempURL)
+      }
+
+      MockURLProtocol.requestHandler = { request in
+        let response = HTTPURLResponse(
+          url: request.url!,
+          statusCode: expectedStatusCode,
+          httpVersion: nil,
+          headerFields: nil
+        )!
+
+        try responseBody.write(to: tempURL, atomically: true, encoding: .utf8)
+        let stream = URL(fileURLWithPath: tempURL.path).lines
+        return (response, stream)
+      }
+
+      do {
+        _ = try await model.generateContent("test")
+        XCTFail("An error should have been thrown, but no error was thrown.")
+      } catch let GenerateContentError
+        .internalError(underlying: unrecognizedError as UnrecognizedRPCError) {
+        // MockURLProtocol appends a newline to the response.
+        XCTAssertEqual(unrecognizedError.responseBody, responseBody + "\n")
+      } catch {
+        XCTFail("Caught unexpected error: \(error)")
+      }
+    }
+  }
+#endif // !os(watchOS)