facebook/flow · error · Error

Flow check failed!

Error message

Flow check failed!

What it means

flow-dev-tools runs the Flow binary (execManual) and treats exit code 0 (clean) and 2 (type errors reported) as success. Any other exit code — crash, bad usage, missing binary, signal death — causes 'Flow check failed!' to be thrown with the captured stdout/stderr.

Source

Thrown at packages/flow-dev-tools/src/errors.js:51

      format(
        '%s status --no-auto-start --json %s %s',
        bin,
        flowconfigNameFlag,
        root,
      ),
  };
  const [err, stdout, stderr] = await execManual(cmd, {
    cwd: root,
    maxBuffer: Infinity,
  });

  // 0 - no errors
  // 2 - Some errors
  if (err == null || err.code === 2) {
    return JSON.parse(stdout.toString());
  }

  throw new Error(format('Flow check failed!', err, stdout, stderr));
}

async function getFlowErrors(
  bin: string,
  errorCheckCommand: ErrorCheckCommand,
  root: string,
  flowconfigName: string,
): Promise<FlowResult> {
  return getFlowErrorsImpl(bin, errorCheckCommand, root, flowconfigName);
}

function mainSourceLocOfError(error: FlowError): ?FlowLoc {
  const {operation, message} = error;
  for (const msg of [operation, ...message]) {
    if (msg && msg.loc && msg.loc.type === 'SourceFile') {
      return msg.loc;
    }
  }

View on GitHub (pinned to d1341dac89)

Solutions

1. Run the exact underlying command shown in the error output manually (e.g. `npx flow check --json --strip-root`) to see the real failure
2. Verify flow-bin is installed and runnable: `npm ls flow-bin` and `npx flow version`
3. Align versions: the number in .flowconfig's [version] section must match the installed flow-bin
4. If flow crashed, stop the server (`npx flow stop`) and retry; give CI more memory

import {execFileSync} from 'child_process';

function flowBinaryWorks(flowPath: string): boolean {
  try {
    const out = execFileSync(flowPath, ['version'], {encoding: 'utf8'});
    return /Flow\s+v?\d/.test(out);
  } catch {
    return false;
  }
}

try {
  const result = await getFlowErrors(bin, cmd, root, flowconfigName);
} catch (err) {
  // the message embeds err/stdout/stderr: surface stderr to the user,
  // distinguish 'command not found' vs flow diagnostics vs crash
  throw new Error(`Flow invocation failed: ${err.message}`);
}

When it happens

Trigger: The spawned flow command (e.g. `flow check --json` or a configured errorCheckCommand) exits with a code other than 0 or 2: flow-bin missing or not executable, a flow version incompatible with the repo's .flowconfig, a flow daemon crash or OOM, invalid flowconfig, or a custom errorCheckCommand that exits non-zero.

Common situations: Running flow-dev-tools scripts on machines without flow-bin installed; .flowconfig [version] header not matching the installed flow; a crashed flow server leaving stale state; CI containers lacking memory or native deps for flow.

Related errors

• Unexpected function parameter ${param.type}
• updatePendingStatements: Variable for dependency "${dep}" no
• updatePendingStatements: Variable parent of "${dep}" not fou
• Flow version ${version} doesn't support 'flow lsp'. Please u
• Unsupported .flowconfig option `log.file`. The VS Code exten

AI-assisted analysis of facebook/flow@d1341dac89 (2026-08-17). Data as JSON: /api/errors/0961d7fd3ef41adb. Report an issue: GitHub.