vercel/next.js · error · Error

Could not parse output from TypeScript's --showConfig.

Error message

Could not parse output from TypeScript's --showConfig.

What it means

Thrown by getTypeScriptConfigurationCli() when `tsc --showConfig` succeeds (exit 0) but its stdout is not valid JSON and JSON.parse throws. Next.js invokes the TypeScript CLI to expand the tsconfig into a fully-resolved config; if the CLI emits unexpected output (a banner, warning, or non-JSON text), parsing fails.

Source

Thrown at packages/next/src/lib/typescript/runTypeScriptCli.ts:253

  tscPath: string
}): Promise<{ compilerOptions: Record<string, any> }> {
  const result = await runTypeScriptCli({
    cwd: baseDir,
    tscPath,
    args: ['--showConfig', '--project', tsConfigPath, '--pretty', 'false'],
    captureOutput: true,
  })

  if (result.exitCode !== 0) {
    throw new Error(
      [result.stdout, result.stderr].filter(Boolean).join('\n').trim()
    )
  }

  try {
    return JSON.parse(result.stdout)
  } catch (cause) {
    throw new Error(`Could not parse output from TypeScript's --showConfig.`, {
      cause,
    })
  }
}

View on GitHub (pinned to 0ae8c72462)

Solutions

Run `npx tsc --showConfig --project tsconfig.json --pretty false` manually and inspect stdout — it must be pure JSON.
Disable experimental.useTypeScriptCli in next.config.js to fall back to the TypeScript compiler API path.
Remove any NODE_OPTIONS preloads or banners that write to stdout.
Ensure you are using a supported TypeScript version; if on a native preview, confirm --showConfig JSON output is stable.

Example fix

// before: experimental.useTypeScriptCli enabled with a tsc that pollutes stdout
// after (next.config.js)
module.exports = { experimental: { useTypeScriptCli: false } }

Defensive patterns

Strategy: try-catch

Validate before calling

import { execSync } from 'child_process'
function validateTscShowConfig(tscPath: string, tsConfigPath: string) {
  const out = execSync(`node ${tscPath} --showConfig --project ${tsConfigPath} --pretty false`, { encoding: 'utf8' })
  JSON.parse(out) // throws if not JSON
}

Try / catch

try {
  const config = await getTypeScriptConfigurationCli({ baseDir, tsConfigPath, tscPath })
} catch (err) {
  if (err.message.includes("Could not parse output from TypeScript's --showConfig")) {
    // disable experimental.useTypeScriptCli and fall back to the API path
  }
  throw err
}

Prevention

If you hit this, disable experimental.useTypeScriptCli.
Ensure no NODE_OPTIONS preload writes to stdout.
Pin a TypeScript version whose --showConfig emits stable JSON.
Test tsc --showConfig output manually before enabling the CLI path.

When it happens

Trigger: experimental.useTypeScriptCli is enabled and `tsc --showConfig --project tsconfig.json --pretty false` returns non-JSON stdout. Causes: a custom tsc wrapper printing to stdout, a TypeScript version whose --showConfig output differs, or stdout pollution from a Node.js preload/banner.

Common situations: Using TypeScript 7 (native preview) whose CLI output format differs; a NODE_OPTIONS preload that logs to stdout; a malformed tsconfig that makes tsc emit diagnostics to stdout instead of stderr while still exiting 0; shell wrapper aliasing tsc.

Related errors

AI-assisted analysis of vercel/next.js@0ae8c72462 (2026-08-06). Data as JSON: /api/errors/e18c4573d9f4f4b6. Report an issue: GitHub.