FlowiseAI/Flowise · error · Error

Invalid JSON in the PromptTemplate's promptValues

Error message

Invalid JSON in the PromptTemplate's promptValues: ${exception}

What it means

PromptLangfuse.init fetches a prompt from Langfuse then parses `nodeData.inputs?.promptValues` as JSON. If the values string is malformed, JSON.parse throws and is rethrown with this message. Note the message labels it 'PromptTemplate' which is slightly misleading for the Langfuse variant.

Solutions

1. Provide promptValues as valid JSON with double-quoted keys.
2. Pass an object from an upstream node to bypass JSON.parse.
3. Cross-check placeholder names against the Langfuse prompt via `getInputVariables(template)`.
4. Fix the misleading message text to say 'PromptLangfuse'.

Example fix

// before
throw new Error("Invalid JSON in the PromptTemplate's promptValues: " + exception)
// after
throw new Error(`Invalid JSON in PromptLangfuse promptValues: ${exception instanceof Error ? exception.message : exception}. Input: ${promptValuesStr.slice(0,120)}`)

function parsePromptValues(input: unknown): ICommonObject {
  if (input == null) return {}
  if (typeof input === 'object') return input as ICommonObject
  try { return JSON.parse(input) }
  catch (e) { throw new Error(`PromptLangfuse promptValues invalid JSON: ${(e as Error).message}`) }
}

const isPlainObject = (v: unknown): v is Record<string, unknown> => typeof v === 'object' && v !== null && !Array.isArray(v)

When it happens

Trigger: User supplies promptValues as a non-JSON string alongside a Langfuse-managed prompt; Langfuse returns a template with placeholders the user tries to fill via a malformed JSON blob.

Common situations: Mixing Langfuse-hosted prompts with locally typed prompt values; pasting values from a different prompt system that uses non-JSON syntax.

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 ChatPromptTemplate's promptValues
• Invalid JSON in the FewShotPromptTemplate's examples
• Invalid JSON in the PromptTemplate's promptValues
• Invalid JSON in the Worker's Prompt Input Values
• Returned message history must be an array

AI-assisted analysis of FlowiseAI/Flowise@abe4a8601a (2026-08-12). Data as JSON: /api/errors/37fda34b75ba63d6. Report an issue: GitHub.

Appendix: source

Thrown at packages/components/nodes/prompts/PromptLangfuse/PromptLangfuse.ts:76

        const langfuse = new Langfuse({
            secretKey: langFuseSecretKey,
            publicKey: langFusePublicKey,
            baseUrl: langFuseEndpoint ?? 'https://cloud.langfuse.com',
            sdkIntegration: 'Flowise'
        })

        const langfusePrompt = await langfuse.getPrompt(nodeData.inputs?.template as string)
        let template = langfusePrompt.getLangchainPrompt()

        const promptValuesStr = nodeData.inputs?.promptValues

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

        const inputVariables = getInputVariables(template)
        template = transformBracesWithColon(template)

        try {
            const options: PromptTemplateInput = {
                template,
                inputVariables
            }
            const prompt = new PromptTemplate(options)
            prompt.promptValues = promptValues
            return prompt
        } catch (e) {
            throw new Error(e)
        }
    }

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