FlowiseAI/Flowise · error · Error

Returned message history must be an array

Error message

Returned message history must be an array

What it means

ChatPromptTemplate.init executes a user-supplied `messageHistoryCode` snippet in a sandbox, JSON.parses the response, and verifies it is an array. If the code returns a JSON object, primitive, or string-encoded non-array, the Array.isArray check fails and throws.

Source

Thrown at packages/components/nodes/prompts/ChatPromptTemplate/ChatPromptTemplate.ts:139

            const databaseEntities = options.databaseEntities as IDatabaseEntity
            const variables = await getVars(appDataSource, databaseEntities, nodeData, options)
            const flow = {
                chatflowId: options.chatflowid,
                sessionId: options.sessionId,
                chatId: options.chatId
            }

            const sandbox = createCodeExecutionSandbox('', variables, flow)

            try {
                const response = await executeJavaScriptCode(messageHistoryCode, sandbox, {
                    libraries: ['axios', '@langchain/core']
                })

                const parsedResponse = JSON.parse(response)

                if (!Array.isArray(parsedResponse)) {
                    throw new Error('Returned message history must be an array')
                }
                prompt = ChatPromptTemplate.fromMessages([
                    SystemMessagePromptTemplate.fromTemplate(systemMessagePrompt),
                    ...parsedResponse,
                    HumanMessagePromptTemplate.fromTemplate(humanMessagePrompt)
                ])
            } catch (e) {
                throw new Error(e)
            }
        }

        let promptValues: ICommonObject = {}
        if (promptValuesStr) {
            try {
                promptValues = typeof promptValuesStr === 'object' ? promptValuesStr : JSON.parse(promptValuesStr)
            } catch (exception) {
                throw new Error("Invalid JSON in the ChatPromptTemplate's promptValues: " + exception)
            }

View on GitHub (pinned to abe4a8601a)

Solutions

  1. Ensure messageHistoryCode returns a JSON array literal: `[{role:'user',content:'...'}, ...]`.
  2. If fetching from an API, map/extract the array before returning.
  3. JSON.stringify the value inside the code to confirm shape.
  4. Wrap the response: `return JSON.stringify(Array.isArray(x) ? x : [x])`.

Example fix

// before
const response = await executeJavaScriptCode(messageHistoryCode, sandbox, {...})
const parsedResponse = JSON.parse(response)
if (!Array.isArray(parsedResponse)) throw new Error('Returned message history must be an array')
// after
const response = await executeJavaScriptCode(messageHistoryCode, sandbox, {...})
const parsedResponse = JSON.parse(response)
const arr = Array.isArray(parsedResponse) ? parsedResponse : (parsedResponse?.messages ?? [parsedResponse])
if (!Array.isArray(arr)) throw new Error('Returned message history must be an array')
Defensive patterns

Strategy: validation

Validate before calling

const arr = Array.isArray(parsedResponse) ? parsedResponse
  : Array.isArray((parsedResponse as any)?.messages) ? (parsedResponse as any).messages
  : [parsedResponse]
if (!Array.isArray(arr)) throw new Error('Returned message history must be an array')

Type guard

const isMessageArray = (v: unknown): boolean => Array.isArray(v) && v.every((m) => m && typeof m === 'object' && 'role' in m)

Prevention

When it happens

Trigger: The messageHistoryCode returns `{messages: [...]}` instead of `[...]`; returns a JSON string of an object; returns null/undefined/empty string; returns a single message object not wrapped in an array.

Common situations: Authoring custom message-history fetching code (e.g. querying a DB or API) and forgetting to extract/unwrap the array; the upstream API changed its response shape.

Related errors

AI-assisted analysis of FlowiseAI/Flowise@abe4a8601a (2026-08-12). Data as JSON: /api/errors/5529b43d73255823. Report an issue: GitHub.