From c3a19a1920fda937647d84099377f6b4c0f97196 Mon Sep 17 00:00:00 2001
From: Thanaphoom Babparn <tp.babparn@gmail.com>
Date: Mon, 8 Jun 2026 15:34:36 +0700
Subject: [PATCH] Make modelName optional if providing ChatModel or
 StreamingChatModel

---
 .../adk/models/langchain4j/LangChain4j.java   | 29 ++++++++++-
 .../models/langchain4j/LangChain4jTest.java   | 48 +++++++++++++++++++
 2 files changed, 76 insertions(+), 1 deletion(-)

diff --git a/contrib/langchain4j/src/main/java/com/google/adk/models/langchain4j/LangChain4j.java b/contrib/langchain4j/src/main/java/com/google/adk/models/langchain4j/LangChain4j.java
index b35c68f89..f2ce914e1 100644
--- a/contrib/langchain4j/src/main/java/com/google/adk/models/langchain4j/LangChain4j.java
+++ b/contrib/langchain4j/src/main/java/com/google/adk/models/langchain4j/LangChain4j.java
@@ -76,6 +76,7 @@
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Objects;
 import java.util.UUID;
 import org.jspecify.annotations.Nullable;
 import org.slf4j.Logger;
@@ -100,6 +101,7 @@ public abstract class LangChain4j extends BaseLlm {
 
   public abstract ObjectMapper objectMapper();
 
+  @Nullable
   public abstract String modelName();
 
   @Nullable
@@ -126,7 +128,32 @@ 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 (Objects.isNull(modelName())) {
+        // Try to extract modelName from chatModel or streamingChatModel
+        if (!Objects.isNull(chatModel())
+            && !Objects.isNull(chatModel().defaultRequestParameters())) {
+          modelName(chatModel().defaultRequestParameters().modelName());
+        } else if (!Objects.isNull(streamingChatModel())
+            && !Objects.isNull(streamingChatModel().defaultRequestParameters())) {
+          modelName(streamingChatModel().defaultRequestParameters().modelName());
+        }
+      }
+      // Up to this step, if modelName still null - Fail fast
+      if (modelName() == null) {
+        throw new IllegalStateException(
+            "modelName is required. Either set it explicitly via modelName() or provide a ChatModel/StreamingChatModel");
+      }
+      return autoBuild();
+    }
   }
 
   public LangChain4j(ChatModel chatModel) {
diff --git a/contrib/langchain4j/src/test/java/com/google/adk/models/langchain4j/LangChain4jTest.java b/contrib/langchain4j/src/test/java/com/google/adk/models/langchain4j/LangChain4jTest.java
index affacd7c3..60e3e59b8 100644
--- a/contrib/langchain4j/src/test/java/com/google/adk/models/langchain4j/LangChain4jTest.java
+++ b/contrib/langchain4j/src/test/java/com/google/adk/models/langchain4j/LangChain4jTest.java
@@ -31,6 +31,7 @@
 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 +1146,51 @@ void testGenerateContentWithMalformedCharsetFallback() {
 
     assertThat(textContent.text()).isEqualTo(textPayload);
   }
+  
+  @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");
+  }
 }