Remove mandatory model name in LangChain4j integration

[philippart-s]

🔴 Required Information

Please ensure all items in this section are completed to allow for efficient
triaging. Requests without complete information may be rejected / deprioritized.
If an item is not applicable to you - please mark it as N/A

Is your feature request related to a specific problem?

When I'm using the LangChain4J integration I need to set twice the model name: once in the LangCHain4j configuration and one when using the LangChain4j builder.

import com.google.adk.agents.LlmAgent;
import com.google.adk.models.langchain4j.LangChain4j;
import dev.langchain4j.model.anthropic.AnthropicChatModel;

// 1. Initialize LangChain4j model
AnthropicChatModel claudeModel = AnthropicChatModel.builder()
    .apiKey(System.getenv("ANTHROPIC_API_KEY"))
    .modelName("claude-sonnet-4-6")
    .build();

// 2. Wrap the LangChain4j model for ADK
LangChain4j adkModel = LangChain4j.builder()
    .chatModel(claudeModel)
    .modelName("claude-sonnet-4-6") // <--- 👀 here
    .build();

// 3. Create your agent
LlmAgent agent = LlmAgent.builder()
    .name("science-teacher")
    .description("A helpful science teacher")
    .model(adkModel)
    .instruction("You are a helpful science teacher that explains concepts clearly.")
    .build();

Describe the Solution You'd Like

As the model name is configured in the LangChain4j configuration, it should be more maintenable to not set it again with the LangChain4J builder.

import com.google.adk.agents.LlmAgent;
import com.google.adk.models.langchain4j.LangChain4j;
import dev.langchain4j.model.anthropic.AnthropicChatModel;

// 1. Initialize LangChain4j model
AnthropicChatModel claudeModel = AnthropicChatModel.builder()
    .apiKey(System.getenv("ANTHROPIC_API_KEY"))
    .modelName("claude-sonnet-4-6")
    .build();

// 2. Wrap the LangChain4j model for ADK
LangChain4j adkModel = LangChain4j.builder()
    .chatModel(claudeModel)
    .build();

// 3. Create your agent
LlmAgent agent = LlmAgent.builder()
    .name("science-teacher")
    .description("A helpful science teacher")
    .model(adkModel)
    .instruction("You are a helpful science teacher that explains concepts clearly.")
    .build();

Impact on your work

Minore impact, it's just more readable and maintainable.

Willingness to contribute

Are you interested in implementing this feature yourself or submitting a PR?
No

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[marttp]

I'd like to take a look at this.