FlowiseAI/Flowise · error · Error

Condition function must return a string

Error message

Condition function must return a string

What it means

On the `conditionFunction` branch, user JS executes in a sandbox and its return value is used to select the next graph branch — so it must be a string matching one of the Condition's output labels. `typeof response !== 'string'` rejects numbers, booleans, objects, and undefined before they reach the graph router.

Source

Thrown at packages/components/nodes/sequentialagents/Condition/Condition.ts:287

    const selectedTab = tabIdentifier ? tabIdentifier.split(`_${nodeData.id}`)[0] : 'conditionUI'
    const variables = await getVars(appDataSource, databaseEntities, nodeData, options)

    const flow = {
        chatflowId: options.chatflowid,
        sessionId: options.sessionId,
        chatId: options.chatId,
        input,
        state,
        vars: prepareSandboxVars(variables)
    }

    if (selectedTab === 'conditionFunction' && conditionFunction) {
        const sandbox = createCodeExecutionSandbox(input, variables, flow)

        try {
            const response = await executeJavaScriptCode(conditionFunction, sandbox)

            if (typeof response !== 'string') throw new Error('Condition function must return a string')
            return response
        } catch (e) {
            throw new Error(e)
        }
    } else if (selectedTab === 'conditionUI' && conditionUI) {
        try {
            const conditionItems: IConditionGridItem[] = typeof conditionUI === 'string' ? JSON.parse(conditionUI) : conditionUI

            for (const item of conditionItems) {
                if (!item.variable) throw new Error('Condition variable is required!')

                if (item.variable.startsWith('$flow')) {
                    const variableValue = customGet(flow, item.variable.replace('$flow.', ''))
                    if (checkCondition(variableValue, item.operation, item.value)) {
                        return item.output
                    }
                } else if (item.variable.startsWith('$vars')) {
                    const variableValue = customGet(flow, item.variable.replace('$', ''))

View on GitHub (pinned to abe4a8601a)

Solutions

1. Return one of the configured Condition output labels as a literal string — e.g. `return age > 18 ? 'Approved' : 'End'`.
2. If the value is dynamic, coerce with `String(response)` and ensure it matches an output label.
3. Verify the function always hits a `return` on every code path.

Example fix

// before
conditionFunction = "return age > 18" // returns boolean -> throws [292]

// after
conditionFunction = "return age > 18 ? 'Approved' : 'End'"

function assertConditionFunctionResult(response) {
  if (typeof response !== 'string') {
    throw new Error(`Condition function must return a string label, got ${typeof response}: ${JSON.stringify(response)}`)
  }
  return response
}

const result = await executeJavaScriptCode(conditionFunction, sandbox)
assertConditionFunctionResult(result)

function isConditionResult(v): v is string {
  return typeof v === 'string' && v.length > 0
}

When it happens

Trigger: The condition function returns a boolean (e.g. the raw comparison result), a number, an object, or omits `return` (undefined).

Common situations: User wrote `return age > 18` expecting it to branch, but the router needs a label; the function returns the variable itself instead of an output label; the function returns the result of a comparison operator (boolean).

Related errors

• Condition function must return a string
• Return output must be an object
• Model is required
• Model is required
• Scenarios are required

AI-assisted analysis of FlowiseAI/Flowise@abe4a8601a (2026-08-12). Data as JSON: /api/errors/37a905ac4a333bc1. Report an issue: GitHub.