The tool returned an empty AssistantMessage. Converting to…

Error message

The tool returned an empty AssistantMessage. Converting to conventional response.

What it means

MessageToolCallResultConverter converts a tool's return value into a string for ToolResponseMessage. An AssistantMessage with empty text (and no media) carries no usable content, so the converter logs this warning and returns the conventional JSON '"Done"' response.

Solutions

Return a non-empty string or ToolResponseMessage from the tool instead of an empty AssistantMessage
Check text before constructing AssistantMessage and fall back to a meaningful placeholder
Handle media separately — media in AssistantMessage throws UnsupportedOperationException here
Log/inspect the tool pipeline to find why the wrapped generation returned empty text

Example fix

// before
return new AssistantMessage(""); // warns, becomes "Done"
// after
String text = generate();
return new AssistantMessage(text != null && !text.isBlank() ? text : "No content generated");

Defensive patterns

Strategy: validation

Validate before calling

if (toolResult instanceof AssistantMessage am && (am.getText() == null || am.getText().isBlank()) && (am.getMedia() == null || am.getMedia().isEmpty())) { log.warn("Tool produced empty AssistantMessage"); }

Type guard

static boolean hasUsableText(AssistantMessage m) { return m != null && m.getText() != null && !m.getText().isBlank(); }

Prevention

Return plain Strings or ToolResponseMessage from tools rather than raw AssistantMessage
Validate LLM output for emptiness inside tools before wrapping
Never put media in AssistantMessage returned to this converter (it throws UnsupportedOperationException)

When it happens

Trigger: A @Tool-annotated method (or ToolCallback) returns an AssistantMessage whose getText() is empty/blank and getMedia() is empty, e.g. new AssistantMessage("").

Common situations: Tool wraps an LLM call that returned empty content; tool builds AssistantMessage programmatically with null/blank text; upstream model outage yields empty message passed through as tool result.

Understand the failure class

Background: "must not be empty", "cannot be empty" — required-field validation errors across open-source libraries — this error's family across 41 libraries.

Related errors

Currently Spring AI ToolResponseMessage only supports text…

AI-assisted analysis of alibaba/spring-ai-alibaba@f82da0b50f (2026-09-09). Data as JSON: /api/errors/4eea30fba4e9bb1a. Report an issue: GitHub.

Appendix: source

Thrown at spring-ai-alibaba-agent-framework/src/main/java/com/alibaba/cloud/ai/graph/agent/MessageToolCallResultConverter.java:49

public class MessageToolCallResultConverter implements ToolCallResultConverter {

	private static final Logger logger = LoggerFactory.getLogger(MessageToolCallResultConverter.class);

	/**
	 * Currently Spring AI ToolResponseMessage only supports text type, that's why the return type of this method is String.
	 * More types like image/audio/video/file can be supported in the future.
	 */
	public String convert(@Nullable Object result, @Nullable Type returnType) {
		if (returnType == Void.TYPE) {
			logger.debug("The tool has no return type. Converting to conventional response.");
			return JsonParser.toJson("Done");
		} else if (result instanceof AssistantMessage assistantMessage) {
			if (StringUtils.hasLength(assistantMessage.getText())) {
				return assistantMessage.getText();
			} else if (CollectionUtils.isNotEmpty(assistantMessage.getMedia())) {
				throw new UnsupportedOperationException("Currently Spring AI ToolResponseMessage only supports text type, that's why the return type of this method is String. More types like image/audio/video/file can be supported in the future.");
			}
			logger.warn("The tool returned an empty AssistantMessage. Converting to conventional response.");
			return JsonParser.toJson("Done");
		} else {
			logger.debug("Converting tool result to JSON.");
			return JsonParser.toJson(result);
		}
	}
}

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