Exception Handling in @Tool Methods for LangChain4j: How Errors Are Communicated to LLMs

When integrating tools with language models using LangChain4j, robust error handling is crucial for smooth interactions. The way exceptions thrown by @Tool annotated methods are managed can significantly affect how an LLM interprets failures and decides on next steps. 

Exception Handling Behavior

If a method annotated with @Tool throws an Exception, LangChain4j catches this exception internally. Instead of failing silently or crashing, the framework extracts the message from the exception (e.getMessage()). This exception message is sent directly to the LLM as the tool's execution result.

 

Why is this important?

·      The LLM receives explicit feedback on what went wrong during the tool’s execution.

·      This enables the LLM to understand the error context and effectively adjust its instructions or parameters to retry the tool call.

·      For example, if the tool failed due to invalid input or missing data, then LLM might correct its request and invoke the tool again with better input.

·      This feedback loop improves the system’s fault tolerance and intelligence by making it capable of recovering from errors autonomously.

 

Best Practices for Exception Messages

·      Ensure exception messages are clear, concise, and descriptive to help the LLM understand the exact issue.

·      Avoid exposing sensitive information or overly technical stack traces in exception messages.

·      Use custom exceptions where possible to provide meaningful domain-specific error descriptions.

·      Consider logging full exception details internally for debugging while sending user-friendly messages to the LLM.

 

In summary, Exception handling in @Tool methods is not just about catching errors, it’s about communicating failures back to the LLM in a way that enables corrective action. This design choice in LangChain4j makes your tools more interactive, reliable, and intelligent.

 

Example 

@Tool("Take degrees as input and return the equivalent value in radians")
public Double degreesToRadians(double degrees) throws IllegalArgumentException {
    System.out.println("***Converting Degrees to Radians***");

    // Validate input
    if (Double.isNaN(degrees)) {
        throw new IllegalArgumentException("Input is not a valid number (NaN). Please provide a numeric value.");
    }
    if (Double.isInfinite(degrees)) {
        throw new IllegalArgumentException("Input is infinite. Please provide a finite numeric value.");
    }
    // Optionally, you can restrict degrees to a valid range, for example -360 to 360:
    if (degrees < -360 || degrees > 360) {
        throw new IllegalArgumentException("Input degrees must be between -360 and 360.");
    }

    return Math.toRadians(degrees);
}

 

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