What causes "Invalid key" in windmill-labs/windmill?

Use string keys only: ensure the key expression evaluates to a string, e.g. `state[String(key)] = value`

Invalid key

Error message

Invalid key

What it means

The same eval sandbox Proxy for the global state validates that every assigned key is a string before writing it and syncing the value into the World store. Symbol or non-string keys (e.g. from programmatic/spread logic, Symbol.iterator, or obfuscated expression code) throw 'Invalid key'.

Source

Thrown at frontend/src/lib/components/apps/components/helpers/eval.ts:151

	runnableComponents: Record<string, { cb?: (() => void)[] }>,
	noReturn: boolean,
	groupContextId: string | undefined,
	globalRecomputeFunction: ((excludeIds?: string) => void) | undefined
) {
	const createProxy = (name: string, obj: any) => {
		// console.log('Creating proxy', name, obj)
		if (obj != null && obj != undefined && typeof obj == 'object') {
			if (name == 'group' && groupContextId) {
				return createGroupProxy(groupContextId, obj)
			}
			return new Proxy(obj, {
				set(target, key, value) {
					if (name != 'state') {
						throw new Error(
							'Cannot set value on objects that are neither the global state or a container group field'
						)
					}
					if (typeof key !== 'string') {
						throw new Error('Invalid key')
					}
					target[key] = value
					let o = worldStore?.newOutput(name, key, value)
					o?.set(value, true)

					return true
				},
				get(obj, prop) {
					if (name != 'state' && prop == 'group') {
						return createGroupProxy(name, obj[prop])
					} else {
						return obj[prop]
					}
				}
			})
		} else {
			return obj

View on GitHub (pinned to e474e8803c)

Solutions

Use string keys only: ensure the key expression evaluates to a string, e.g. `state[String(key)] = value`
Check for accidental symbol-key assignment (Symbol.iterator, Symbol.asyncIterator) — spread those objects explicitly instead
If copying objects, assign the whole object once (`state.items = obj`) rather than copying its keys individually

Example fix

// before
state[myKey] = value // myKey may be a symbol
// after
if (typeof myKey !== 'string') throw new Error('state keys must be strings')
state[myKey] = value

Defensive patterns

Strategy: validation

Validate before calling

if (typeof key !== 'string') {
  throw new Error(`state keys must be strings, got ${typeof key}`)
}
state[key] = value

Type guard

function isStringKey(key: PropertyKey): key is string {
  return typeof key === 'string'
}

Try / catch

try {
  state[key] = value
} catch (e) {
  if (e.message === 'Invalid key') {
    state[String(key)] = value
  } else throw e
}

Prevention

Never use Symbol or non-string keys when writing to app state
Avoid spreading objects with symbol keys directly onto the state proxy
Coerce dynamic keys with String(key) or template literals before assignment
Keep state writes simple and literal in expression handlers to stay within the sandbox's assumptions

When it happens

Trigger: Assigning a computed property with a Symbol key to state inside an app expression, e.g. `state[symbolKey] = v`, or library/user code that runs Object.assign-like logic with symbol keys against the state proxy, or numeric keys coerced from exotic paths.

Common situations: Using spread or utility functions that copy symbols onto state; defining custom Symbol.toPrimitive/iterator interactions on objects assigned into state; writing expression code with dynamic bracket access where the key evaluates to a symbol.

Related errors

AI-assisted analysis of windmill-labs/windmill@e474e8803c (2026-09-03). Data as JSON: /api/errors/bf6f2d7b6f74ddb4. Report an issue: GitHub.