FlowiseAI/Flowise · error · Error

Invalid JSON in the Custom Function Input Variables

Error message

Invalid JSON in the Custom Function Input Variables: ${exception}

What it means

Thrown by CustomFunction_Utilities.init when JSON.parse fails on `functionInputVariablesRaw`. The field is parsed only when it is a non-object (a JSON string); objects are passed through. Any invalid JSON string aborts custom-function execution before the sandbox is built.

Solutions

1. Validate the JSON in an external linter before pasting into the node.
2. Use the Flowise JSON editor's validation (if available) or a JSON editor extension.
3. If the value is already an object at runtime, pass it as an object — the code skips JSON.parse for objects.
4. Trim/escape the string and retry.

Example fix

// before (malformed)
functionInputVariablesRaw = "{ key: 'value', }" // throws: Invalid JSON ...

// after (valid)
functionInputVariablesRaw = '{"key":"value"}'

function parseInputVars(raw: unknown): Record<string, unknown> {
  if (raw == null) return {}
  if (typeof raw === 'object') return raw as Record<string, unknown>
  try { return JSON.parse(raw as string) }
  catch (e) { throw new Error(`functionInputVariables is not valid JSON: ${(e as Error).message}`) }
}
// validate before the node runs
const inputVars = parseInputVars(functionInputVariablesRaw)

const isParsableJsonObject = (v: unknown): boolean => {
  if (typeof v === 'object' && v !== null) return true
  if (typeof v !== 'string') return false
  try { return typeof JSON.parse(v) === 'object' && JSON.parse(v) !== null } catch { return false }
}

try { JSON.parse(functionInputVariablesRaw as string) }
catch (e) { throw new Error(`Fix the Custom Function Input Variables JSON: ${(e as Error).message}`) }

When it happens

Trigger: A user enters malformed JSON in the Custom Function node's Input Variables editor (trailing comma, unquoted keys, single quotes, stray characters); a flow variable returns a partial/cut-off JSON string; copy-paste introduced smart quotes.

Common situations: Hand-typed JSON in the node config with a syntax error; JSON truncated by a length limit upstream; mixed quote styles from a word processor; a variable resolved to a non-JSON string at runtime.

Understand the failure class

• Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

Related errors

• Invalid JSON in the Chat NVIDIA NIM's baseOptions
• Invalid JSON in the IfElse's Input Variables
• Invalid JSON format in body. Original error
• Invalid JSON in Search Domain Filter
• Invalid JSON in the Additional Configuration:

AI-assisted analysis of FlowiseAI/Flowise@abe4a8601a (2026-08-12). Data as JSON: /api/errors/d55480c2e6ec9bf2. Report an issue: GitHub.

Appendix: source

Thrown at packages/components/nodes/utilities/CustomFunction/CustomFunction.ts:105

            input,
            chatflowId: options.chatflowid,
            sessionId: options.sessionId,
            chatId: options.chatId,
            rawOutput: options.postProcessing?.rawOutput || '',
            chatHistory: options.postProcessing?.chatHistory || [],
            sourceDocuments: options.postProcessing?.sourceDocuments,
            usedTools: options.postProcessing?.usedTools,
            artifacts: options.postProcessing?.artifacts,
            fileAnnotations: options.postProcessing?.fileAnnotations
        }

        let inputVars: ICommonObject = {}
        if (functionInputVariablesRaw) {
            try {
                inputVars =
                    typeof functionInputVariablesRaw === 'object' ? functionInputVariablesRaw : JSON.parse(functionInputVariablesRaw)
            } catch (exception) {
                throw new Error('Invalid JSON in the Custom Function Input Variables: ' + exception)
            }
        }

        // Some values might be a stringified JSON, parse it
        for (const key in inputVars) {
            let value = inputVars[key]
            if (typeof value === 'string') {
                value = handleEscapeCharacters(value, true)
                if (value.startsWith('{') && value.endsWith('}')) {
                    try {
                        value = JSON.parse(value)
                    } catch (e) {
                        // ignore
                    }
                }
                inputVars[key] = value
            }
        }

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