LangChain4j.toParts() throws NPE when AiMessage.text() is null

[anandnair2005]

Description

LangChain4j.toParts() throws a NullPointerException when processing an AiMessage with a null text field. This occurs in multi-agent setups where the LLM returns tool-call-only responses (no text content).

The toParts() method passes aiMessage.text() directly to Part.builder().text() without a null check. The AutoValue-generated Part.Builder.text() rejects null values, causing the NPE.

AiMessage.text() returning null is a known LangChain4j behavior for tool-call-only responses — see langchain4j#2686 and langchain4j#986.

Stack Trace

java.lang.NullPointerException
    at com.google.adk.models.langchain4j.AutoValue_Part$Builder.text(AutoValue_Part.java)
    at com.google.adk.models.langchain4j.LangChain4j.toParts(LangChain4j.java)
    at com.google.adk.models.langchain4j.LangChain4j.lambda$generateContent$1(LangChain4j.java)

Steps to Reproduce

1. Set up a router agent with sub-agents using the LangChain4j bridge
2. Use an OpenAI-compatible provider (e.g., OpenRouter) as the LLM backend
3. Send a query that triggers sub-agent delegation
4. When the LLM response contains tool calls but no text, toParts() crashes

The issue is intermittent (~1 in 10-20 requests) because it depends on whether the LLM includes a text field alongside tool calls.

Expected Behavior

Null text should be handled gracefully — skip the text part when aiMessage.text() is null, rather than crashing.

Affected Versions

0.7.0, 0.8.0, 0.9.0

Environment

• Java 21+
• LangChain4j with OpenRouter (OpenAI-compatible API)
• Multi-agent router with specialist sub-agents
• Cloud Run deployment