KG-159: Make parts field nullable because Gemini models sometimes don't return it (#652)

Author: Ololoshechkin

prompt/prompt-executor/prompt-executor-clients/prompt-executor-google-client/build.gradle.kts

@@ -50,6 +50,7 @@ kotlin {
         jvmTest {
             dependencies {
                 implementation(kotlin("test-junit5"))
+                implementation(libs.ktor.client.mock)
             }
         }
     }

prompt/prompt-executor/prompt-executor-clients/prompt-executor-google-client/src/commonMain/kotlin/ai/koog/prompt/executor/clients/google/DataModel.kt

@@ -44,7 +44,7 @@ internal class GoogleRequest(
  */
 @Serializable
 internal class GoogleContent(
-    val parts: List<GooglePart>,
+    val parts: List<GooglePart>? = null,
     val role: String? = null,
 )
 

prompt/prompt-executor/prompt-executor-clients/prompt-executor-google-client/src/commonMain/kotlin/ai/koog/prompt/executor/clients/google/GoogleLLMClient.kt

@@ -220,7 +220,7 @@ public open class GoogleLLMClient(
 
         val request = createGoogleRequest(prompt, model, tools)
 
-        return withContext(Dispatchers.SuitableForIO) {
+        val response = withContext(Dispatchers.SuitableForIO) {
             val response = httpClient.post("$DEFAULT_PATH/${model.id}:$DEFAULT_METHOD_GENERATE_CONTENT") {
                 setBody(request)
             }
@@ -233,6 +233,13 @@ public open class GoogleLLMClient(
                 error("Error from GoogleAI API: ${response.status}: $errorBody")
             }
         }
+
+        // https://discuss.ai.google.dev/t/gemini-2-5-pro-with-empty-response-text/81175/219
+        if (response.candidates.isNotEmpty() && response.candidates.all { it.content?.parts?.isEmpty() == true }) {
+            logger.warn { "Content `parts` field is missing in the response from GoogleAI API: $response" }
+        }
+
+        return response
     }
 
     /**

prompt/prompt-executor/prompt-executor-clients/prompt-executor-google-client/src/jvmTest/kotlin/ai/koog/prompt/executor/clients/google/GoogleModelsTest.kt

@@ -1,10 +1,49 @@
 package ai.koog.prompt.executor.clients.google
 
+import ai.koog.prompt.dsl.prompt
 import ai.koog.prompt.executor.clients.list
 import ai.koog.prompt.llm.LLMProvider
+import ai.koog.prompt.message.Message
+import io.ktor.client.HttpClient
+import io.ktor.client.engine.mock.MockEngine
+import io.ktor.client.engine.mock.respond
+import io.ktor.http.HttpHeaders
+import io.ktor.http.HttpStatusCode
+import io.ktor.http.headersOf
+import io.ktor.utils.io.ByteReadChannel
+import kotlinx.coroutines.test.runTest
 import kotlin.test.Test
+import kotlin.test.assertEquals
 import kotlin.test.assertSame
 
+// "Bad" request from Gemini with missing `parts` field
+private val badRequest: String = """
+    {
+      "candidates": [
+        {
+          "content": {
+            "role": "model"
+          },
+          "finishReason": "STOP",
+          "index": 0
+        }
+      ],
+      "usageMetadata": {
+        "promptTokenCount": 36,
+        "totalTokenCount": 146,
+        "promptTokensDetails": [
+          {
+            "modality": "TEXT",
+            "tokenCount": 36
+          }
+        ],
+        "thoughtsTokenCount": 110
+      },
+      "modelVersion": "gemini-2.5-pro",
+      "responseId": "B0esaJmqKv-0xN8P-dzlwQY"
+    }
+""".trimIndent()
+
 class GoogleModelsTest {
 
     @Test
@@ -19,4 +58,33 @@ class GoogleModelsTest {
             )
         }
     }
+
+    @Test
+    fun `Test when FLASH_2_5 returns no parts GoogleLLMClient does not fail`() = runTest {
+        val mockEngine = MockEngine { request ->
+            respond(
+                content = ByteReadChannel(badRequest),
+                status = HttpStatusCode.OK,
+                headers = headersOf(HttpHeaders.ContentType, "application/json")
+            )
+        }
+
+        val googleClient = GoogleLLMClient(
+            apiKey = "test-key",
+            baseClient = HttpClient(mockEngine) // Ktor client would always respond with the json from above
+        )
+
+        val responses = googleClient.execute(
+            prompt = prompt("test") { user("What is the capital of France?") },
+            model = GoogleModels.Gemini2_5Flash
+        )
+
+        assertEquals(1, responses.size)
+        // When no parts returned -- content should be interpreted as empty
+        assertEquals("", responses.single().content)
+        // Also let's check some other fields parsing
+        assertEquals(Message.Role.Assistant, responses.single().role)
+        assertEquals(36, responses.single().metaInfo.inputTokensCount)
+        assertEquals(146, responses.single().metaInfo.totalTokensCount)
+    }
 }