What causes "Frontend file '${target.path}' not found." in windmill-labs/windmill?

Call the file-listing/read tool first to confirm the exact existing path, then patch that path

Frontend file '${target.path}' not found.

Error message

Frontend file '${target.path}' not found.

What it means

The applyPatch tool in the app-edit copilot resolves the given path to a frontend file and fetches its current content via helpers.getFrontendFile. When the path is not a backend runnable path and no frontend file exists at that path, this error is thrown. It signals a bad patch target: the file the model wants to edit does not exist in the app.

Source

Thrown at frontend/src/lib/components/copilot/chat/app/core.ts:615

		showDetails: true,
		showFade: true,
		fn: async ({ args, helpers, toolId, toolCallbacks }) => {
			const parsedArgs = getPatchFileSchema().parse(args)
			const { old_string: oldString, new_string: newString, replace_all: replaceAll } = parsedArgs
			const target = resolveAppPatchTarget(parsedArgs.path)
			let currentContent = ''
			let backendRunnable: BackendRunnable | undefined

			if (target.type === 'frontend') {
				if (target.path === '/wmill.d.ts') {
					throw new Error(
						"'/wmill.d.ts' is generated automatically. Edit backend runnables instead."
					)
				}

				const frontendContent = helpers.getFrontendFile(target.path)
				if (frontendContent === undefined) {
					throw new Error(`Frontend file '${target.path}' not found.`)
				}
				currentContent = frontendContent
			} else {
				backendRunnable = helpers.getBackendRunnable(target.key)
				if (!backendRunnable) {
					throw new Error(`Backend runnable '${target.key}' not found.`)
				}
				if (backendRunnable.type !== 'inline' || !backendRunnable.inlineScript) {
					throw new Error(
						`'${target.path}' points to backend runnable '${target.key}', but only inline runnables can be patched as files.`
					)
				}

				const expectedExtension = getBackendInlineScriptExtension(backendRunnable)
				if (target.extension !== expectedExtension) {
					throw new Error(
						`'${target.path}' does not match runnable '${target.key}' language. Use backend/${target.key}/main.${expectedExtension}.`
					)

View on GitHub (pinned to e474e8803c)

Solutions

Call the file-listing/read tool first to confirm the exact existing path, then patch that path
Create the file with the create-file tool before patching it
Check for extension mismatches (e.g. .tsx vs .ts) and leading-slash normalization — resolveAppPatchTarget adds '/' if missing
If the file was intentionally new, use the creation path instead of patch

Example fix

// before
await applyPatch({ path: '/src/App.tsx', ... })
// after
const files = await listFiles()
await applyPatch({ path: files.find(f => f.endsWith('.tsx'))?.path, ... })

Defensive patterns

Strategy: validation

Validate before calling

const existing = helpers.getFrontendFile(normalizedPath)
if (existing === undefined) throw new Error(`Refusing to patch: no frontend file at ${normalizedPath}`)

Type guard

function frontendFileExists(path: string, helpers: AppHelpers): boolean {
  return helpers.getFrontendFile(path.startsWith('/') ? path : '/' + path) !== undefined
}

Try / catch

try {
  await patchTool({ path, ... })
} catch (e) {
  if (e instanceof Error && /^Frontend file '.*' not found/.test(e.message)) {
    await createFileTool({ path, content: '' }) // then retry patch
    return
  }
  throw e
}

Prevention

List files (read tool) before patching; never patch from memory of an assumed path
Create new files with the create tool, patch only existing ones
Normalize leading slashes and check extension spelling (.tsx vs .ts)

When it happens

Trigger: Calling the patch tool with parsedArgs.path that maps to type 'frontend' (anything not matching /^backend\/([^/]+)\/main\.(ts|py)$/) while getFrontendFile returns undefined — path typos, wrong extension, or a file that was never created.

Common situations: Model hallucinating a file path ('/src/App.tsx' when the app uses '/index.tsx'); deleting a file in an earlier step then patching it; case-sensitive path mismatches; patching before creating the file.

Understand the failure class

Background: "File not found" and ENOENT errors: why libraries can't find a file that should exist — this error's family across 50 libraries.

Related errors

AI-assisted analysis of windmill-labs/windmill@e474e8803c (2026-09-03). Data as JSON: /api/errors/49835125bba8219e. Report an issue: GitHub.