alibaba/spring-ai-alibaba · error · IllegalArgumentException

Cannot merge incompatible types: {} and {}

Error message

Cannot merge incompatible types: {} and {}

What it means

The default apply() merge strategy merges maps generically, but for other values it requires oldValue and newValue to be the same class; if classes differ it throws IllegalArgumentException listing both class names. This guards state channel reducers from silently combining incompatible value shapes during graph state updates.

Source

Thrown at spring-ai-alibaba-graph-core/src/main/java/com/alibaba/cloud/ai/graph/state/strategy/MergeStrategy.java:59

			newValue = newValueOptional.orElse(null);
		}

		if (newValue == null) {
			return oldValue;
		}

		if (oldValue == null) {
			return newValue;
		}

		if (oldValue instanceof Map && newValue instanceof Map) {
			Map<Object, Object> mergedMap = new HashMap<>((Map<?, ?>) oldValue);
			mergedMap.putAll((Map<?, ?>) newValue);
			return mergedMap;
		}

		if (oldValue.getClass() != newValue.getClass()) {
			throw new IllegalArgumentException(
					     "Cannot merge incompatible types: " +
					     oldValue.getClass().getName() + " and " + newValue.getClass().getName()
			);
		}

		if (shouldMergeObjectFields(oldValue.getClass())) {
			return mergeObjectFields(oldValue, newValue).orElse(newValue);
		}

		return newValue;
	}

	private Optional<Object> mergeObjectFields(Object oldValue, Object newValue) {
		try {
			Object mergedValue = createInstance(oldValue.getClass());
			for (Field field : mergeableFields(oldValue.getClass())) {
				field.setAccessible(true);
				Object oldFieldValue = field.get(oldValue);

View on GitHub (pinned to f82da0b50f)

Solutions

1. Make all writers to the channel return the same type
2. Add a custom MergeStrategy/reducer for that channel that handles both types
3. Coerce/convert values to a common type before returning them from nodes

Example fix

// before
state.put("result", "text-summary");   // node A
state.put("result", Map.of("k","v")); // node B -> throws
// after
// both nodes emit the same shape, e.g.
state.put("result", Map.of("summary", "text-summary"));

if (oldValue != null && newValue != null && oldValue.getClass() != newValue.getClass()) { /* coerce or route to custom reducer first */ }

try { merged = strategy.apply(channel, oldValue, newValue); } catch (IllegalArgumentException e) { log.error("Channel {} type conflict: {}", channel, e.getMessage()); }

When it happens

Trigger: Two graph nodes write to the same state channel in one superstep and return values of different types (e.g. a Map and a String, or Integer and Long); a custom reducer delegates to this strategy with heterogeneous values.

Common situations: Parallel/branching workflows where branches emit different result types into a shared key; refactoring a node's return type without updating other writers; numeric boxing differences (Integer vs Long) across nodes.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• No default output or error next node provided
• Sub-agents must be BaseAgent
• Loop flow requires a valid LoopStrategy. Got: null
• Routing sub-agents must be BaseAgent for merge support
• Human feedback metadata must be of type InterruptionMetadata

AI-assisted analysis of alibaba/spring-ai-alibaba@f82da0b50f (2026-09-09). Data as JSON: /api/errors/e8cce6d9c8fdefea. Report an issue: GitHub.