feat(langchain4j): make modelName optional when ChatModel or StreamingChatModel is provided

contrib/langchain4j/src/main/java/com/google/adk/models/langchain4j/LangChain4j.java

@@ -57,6 +57,7 @@
 import dev.langchain4j.model.chat.ChatModel;
 import dev.langchain4j.model.chat.StreamingChatModel;
 import dev.langchain4j.model.chat.request.ChatRequest;
+import dev.langchain4j.model.chat.request.ChatRequestParameters;
 import dev.langchain4j.model.chat.request.ToolChoice;
 import dev.langchain4j.model.chat.request.json.JsonArraySchema;
 import dev.langchain4j.model.chat.request.json.JsonBooleanSchema;
@@ -100,6 +101,7 @@ public abstract class LangChain4j extends BaseLlm {
 
   public abstract ObjectMapper objectMapper();
 
+  @Nullable
   public abstract String modelName();
 
   @Nullable
@@ -126,7 +128,33 @@ public abstract static class Builder {
 
     public abstract Builder modelName(String modelName);
 
-    public abstract LangChain4j build();
+    abstract @Nullable ChatModel chatModel();
+
+    abstract @Nullable StreamingChatModel streamingChatModel();
+
+    abstract @Nullable String modelName();
+
+    abstract LangChain4j autoBuild();
+
+    public LangChain4j build() {
+      if (modelName() == null) {
+        String name = chatModel() == null ? null : nameOf(chatModel().defaultRequestParameters());
+        if (name == null && streamingChatModel() != null) {
+          name = nameOf(streamingChatModel().defaultRequestParameters());
+        }
+        if (name == null) {
+          throw new IllegalStateException(
+              "Set modelName(), or use a ChatModel or StreamingChatModel whose"
+                  + " defaultRequestParameters() declare a model name");
+        }
+        modelName(name);
+      }
+      return autoBuild();
+    }
+
+    private static @Nullable String nameOf(@Nullable ChatRequestParameters parameters) {
+      return parameters == null ? null : parameters.modelName();
+    }
   }
 
   public LangChain4j(ChatModel chatModel) {

contrib/langchain4j/src/test/java/com/google/adk/models/langchain4j/LangChain4jTest.java

@@ -16,6 +16,7 @@
 package com.google.adk.models.langchain4j;
 
 import static org.assertj.core.api.Assertions.*;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.Mockito.*;
 
@@ -31,6 +32,7 @@
 import dev.langchain4j.model.chat.ChatModel;
 import dev.langchain4j.model.chat.StreamingChatModel;
 import dev.langchain4j.model.chat.request.ChatRequest;
+import dev.langchain4j.model.chat.request.ChatRequestParameters;
 import dev.langchain4j.model.chat.request.json.JsonObjectSchema;
 import dev.langchain4j.model.chat.request.json.JsonStringSchema;
 import dev.langchain4j.model.chat.response.ChatResponse;
@@ -1145,4 +1147,89 @@ void testGenerateContentWithMalformedCharsetFallback() {
 
     assertThat(textContent.text()).isEqualTo(textPayload);
   }
+
+  @Test
+  @DisplayName("Should auto-detect model name from ChatModel when not explicitly set")
+  void testBuilderAutoDetectsModelNameFromChatModel() {
+    // Given
+    final ChatRequestParameters params = mock(ChatRequestParameters.class);
+    when(params.modelName()).thenReturn("auto-detected-model");
+    when(chatModel.defaultRequestParameters()).thenReturn(params);
+
+    // When
+    final LangChain4j lc4j = LangChain4j.builder().chatModel(chatModel).build();
+
+    // Then
+    assertThat(lc4j.modelName()).isEqualTo("auto-detected-model");
+    assertThat(lc4j.model()).isEqualTo("auto-detected-model");
+  }
+
+  @Test
+  @DisplayName("Should auto-detect model name from StreamingChatModel when not explicitly set")
+  void testBuilderAutoDetectsModelNameFromStreamingChatModel() {
+    // Given
+    final ChatRequestParameters params = mock(ChatRequestParameters.class);
+    when(params.modelName()).thenReturn("auto-detected-streaming-model");
+    when(streamingChatModel.defaultRequestParameters()).thenReturn(params);
+
+    // When
+    final LangChain4j lc4j = LangChain4j.builder().streamingChatModel(streamingChatModel).build();
+
+    // Then
+    assertThat(lc4j.modelName()).isEqualTo("auto-detected-streaming-model");
+    assertThat(lc4j.model()).isEqualTo("auto-detected-streaming-model");
+  }
+
+  @Test
+  @DisplayName("Should prefer explicit model name over auto-detected one")
+  void testBuilderPrefersExplicitModelName() {
+    // Given
+    final ChatRequestParameters params = mock(ChatRequestParameters.class);
+    when(params.modelName()).thenReturn("auto-detected-model");
+    when(chatModel.defaultRequestParameters()).thenReturn(params);
+
+    // When
+    final LangChain4j lc4j =
+        LangChain4j.builder().chatModel(chatModel).modelName("explicit-model").build();
+
+    // Then
+    assertThat(lc4j.modelName()).isEqualTo("explicit-model");
+  }
+
+  @Test
+  @DisplayName("Should fall back to StreamingChatModel name when ChatModel has none")
+  void testBuilderFallsBackToStreamingModelNameWhenChatModelHasNone() {
+    // Given
+    final ChatRequestParameters emptyParams = mock(ChatRequestParameters.class);
+    when(chatModel.defaultRequestParameters()).thenReturn(emptyParams);
+    final ChatRequestParameters streamingParams = mock(ChatRequestParameters.class);
+    when(streamingParams.modelName()).thenReturn("streaming-model");
+    when(streamingChatModel.defaultRequestParameters()).thenReturn(streamingParams);
+
+    // When
+    final LangChain4j lc4j =
+        LangChain4j.builder().chatModel(chatModel).streamingChatModel(streamingChatModel).build();
+
+    // Then
+    assertThat(lc4j.modelName()).isEqualTo("streaming-model");
+  }
+
+  @Test
+  @DisplayName("Should fail when no source declares a model name")
+  void testBuilderFailsWhenNoModelNameAvailable() {
+    // Given
+    final ChatRequestParameters emptyParams = mock(ChatRequestParameters.class);
+    when(chatModel.defaultRequestParameters()).thenReturn(emptyParams);
+    when(streamingChatModel.defaultRequestParameters()).thenReturn(emptyParams);
+
+    // When / Then
+    assertThrows(
+        IllegalStateException.class,
+        () ->
+            LangChain4j.builder()
+                .chatModel(chatModel)
+                .streamingChatModel(streamingChatModel)
+                .build());
+    assertThrows(IllegalStateException.class, () -> LangChain4j.builder().build());
+  }
 }