[AI] Add an implicit caching unit test (#15727)

Author: paulb777

FirebaseAI/Tests/Unit/GenerativeModelVertexAICachingTests.swift

@@ -0,0 +1,82 @@
+// 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
+
+@available(iOS 15.0, macOS 12.0, macCatalyst 15.0, tvOS 15.0, watchOS 8.0, *)
+final class GenerativeModelImplicitCachingTests: XCTestCase {
+  let testPrompt = "What sorts of questions can I ask you?"
+  let testModelName = "test-model"
+  let testModelResourceName =
+    "projects/test-project-id/locations/test-location/publishers/google/models/test-model"
+  let apiConfig = FirebaseAI.defaultVertexAIAPIConfig
+
+  let vertexSubdirectory = "mock-responses/vertexai"
+
+  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_success_implicitCaching() async throws {
+    MockURLProtocol
+      .requestHandler = try GenerativeModelTestUtil.httpRequestHandler(
+        forResource: "unary-success-implicit-caching",
+        withExtension: "json",
+        subdirectory: vertexSubdirectory
+      )
+
+    let response = try await model.generateContent(testPrompt)
+
+    XCTAssertEqual(response.candidates.count, 1)
+    let candidate = try XCTUnwrap(response.candidates.first)
+    let finishReason = try XCTUnwrap(candidate.finishReason)
+    XCTAssertEqual(finishReason, .stop)
+
+    let usageMetadata = try XCTUnwrap(response.usageMetadata)
+    XCTAssertEqual(usageMetadata.promptTokenCount, 12013)
+    XCTAssertEqual(usageMetadata.candidatesTokenCount, 15)
+    XCTAssertEqual(usageMetadata.totalTokenCount, 12101)
+
+    // Validate implicit caching fields
+    XCTAssertEqual(usageMetadata.cachedContentTokenCount, 11243)
+    XCTAssertEqual(usageMetadata.promptTokensDetails.count, 1)
+    let detail = try XCTUnwrap(usageMetadata.promptTokensDetails.first)
+    XCTAssertEqual(detail.modality, .text)
+    XCTAssertEqual(detail.tokenCount, 12013)
+    XCTAssertEqual(usageMetadata.thoughtsTokenCount, 73)
+  }
+}