windmill-labs/windmill · error

only scripts can be used as a input schema

Error message

only scripts can be used as a input schema

What it means

After locating the first module, getFirstStepSchema only supports extracting an input schema from script-like modules. If the first module's value.type is neither 'rawscript' nor 'script' (e.g., a flow input, loop, branch, or approval step), there is no script input schema to derive, so it throws.

Source

Thrown at frontend/src/lib/components/flows/flowStore.svelte.ts:57

		sendUserToast('Only scripts can be used as a input schema', true)
		return
	}
	sendUserToast('No first step found', true)
	return
}

export async function getFirstStepSchema(flowState: FlowState, flow: OpenFlow) {
	const firstModuleId = flow.value.modules[0]?.id

	if (!firstModuleId || !flowState[firstModuleId]) {
		throw new Error('no first step found')
	}

	const schema = $state.snapshot(flowState[firstModuleId].schema)
	const v = flow.value.modules[0].value

	if (v.type !== 'rawscript' && v.type !== 'script') {
		throw new Error('only scripts can be used as a input schema')
	}

	const simplifiedModule = {
		id: flow.value.modules[0].id,
		summary: flow.value.modules[0].summary,
		value: {
			type: flow.value.modules[0].value.type,
			...('path' in flow.value.modules[0].value ? { path: flow.value.modules[0].value.path } : {}),
			...('language' in flow.value.modules[0].value
				? { language: flow.value.modules[0].value.language }
				: {})
		}
	}

	return {
		schema,
		mod: simplifiedModule,
		connectFirstNode: () => {

View on GitHub (pinned to e474e8803c)

Solutions

1. Verify the first module type before calling: only 'script' or 'rawscript' first steps are supported.
2. Point the schema extraction at the first script-type module rather than strictly modules[0].
3. Fall back to a manual/empty schema when the first step is not a script.

Example fix

// before
const schema = await getFirstStepSchema(flowState, flow)
// after
const first = flow.value.modules[0]?.value
if (first?.type !== 'rawscript' && first?.type !== 'script') return null
const schema = await getFirstStepSchema(flowState, flow)

const v = flow.value.modules[0]?.value
const isScriptFirst = v?.type === 'rawscript' || v?.type === 'script'

function firstModuleIsScript(flow: OpenFlow): boolean {
  const v = flow.value.modules[0]?.value
  return v?.type === 'rawscript' || v?.type === 'script'
}

try {
  const schema = await getFirstStepSchema(flowState, flow)
} catch (e) {
  if (e.message.includes('input schema')) return buildEmptySchema()
  throw e
}

When it happens

Trigger: Calling getFirstStepSchema on a flow whose first module is a subflow, flow-input, loop, while-loop, branch, or any non-script module type.

Common situations: Reusing a schema-derivation helper built for script-first flows on flows that start with a control/IO step; reordering flow steps so a non-script module becomes first; AI-generated or templated flows with a leading flow-input module.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• Can't create script from non-inline script
• no first step found
• Module not found or is not a rawscript
• PlanWriteRefusedError
• Failed to start test run - testFlow returned undefined

AI-assisted analysis of windmill-labs/windmill@e474e8803c (2026-09-03). Data as JSON: /api/errors/e812d3d2e7288e00. Report an issue: GitHub.