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packages/@n8n/nodes-langchain/utils/agent-execution/serializeIntermediateSteps.ts

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+/**
+ * Converts intermediateSteps messageLog entries from LangChain class instances
+ * to plain objects. This ensures the data structure is consistent between
+ * runtime (expression evaluation) and UI display (after JSON serialization).
+ *
+ * Without this, AIMessage instances have properties like `content` and
+ * `tool_calls` directly, but their `toJSON()` wraps them under `kwargs`,
+ * causing expressions built from UI inspection to fail at runtime.
+ */
+export function serializeIntermediateSteps(
+	steps: Array<{ action: { messageLog?: unknown[] }; [key: string]: unknown }>,
+): void {
+	for (const step of steps) {
+		if (step.action.messageLog) {
+			step.action.messageLog = step.action.messageLog.map((msg) => {
+				if (
+					msg &&
+					typeof msg === 'object' &&
+					typeof (msg as Record<string, unknown>).toJSON === 'function'
+				) {
+					return serializeMessage(msg);
+				}
+				return msg;
+			});
+		}
+	}
+}
+
+function serializeMessage(msg: unknown): Record<string, unknown> {
+	const m = msg as Record<string, unknown>;
+	const result: Record<string, unknown> = {};
+
+	for (const key of Object.keys(m)) {
+		if (key === 'toJSON' || key === '_getType') continue;
+		result[key] = m[key];
+	}
+
+	// Ensure type is included (may come from a getter on the prototype)
+	if (
+		!('type' in result) &&
+		typeof (m as Record<string, (...args: unknown[]) => unknown>)._getType === 'function'
+	) {
+		result.type = (m as Record<string, (...args: unknown[]) => unknown>)._getType();
+	}
+
+	return result;
+}