facebook/flow · critical

Failed to query mergebase: unable to find vcs

Error message

Failed to query mergebase: unable to find vcs

What it means

When mergebase tracking is enabled, the Watchman-based file watcher queries the VCS for the current merge-base of the watch root so it can re-check only files changed since the baseline. check_for_changed_mergebase() panics on ErrorStatus::NotInstalled — no usable VCS (git/hg/sapling) found for the repository — because incremental checking without a mergebase baseline would be unsound.

Source

Thrown at rust_port/crates/flow_watchman/src/lib.rs:1295

        .await
    } else {
        Ok(None)
    }
}

async fn check_for_changed_mergebase(env: &mut Env) -> Option<bool> {
    match env.mergebase.clone() {
        None => {
            flow_hh_logger::debug!(
                "Unable to check for changed mergebase: unknown previous mergebase"
            );
            None
        }
        Some(old_mergebase) => {
            let new_mergebase = match get_mergebase(env).await {
                Ok(mergebase) => mergebase,
                Err(vcs_utils::ErrorStatus::NotInstalled { .. }) => {
                    panic!("Failed to query mergebase: unable to find vcs");
                }
                Err(vcs_utils::ErrorStatus::Errored(msg)) => {
                    panic!("Failed to query mergebase: {}", msg);
                }
            };
            match new_mergebase {
                Some(mergebase) => {
                    let changed_mergebase = mergebase != old_mergebase;
                    if changed_mergebase {
                        flow_hh_logger::info!(
                            "Watchman reports mergebase changed from {:?} to {:?}",
                            old_mergebase,
                            mergebase
                        );
                        env.mergebase = Some(mergebase);
                    } else {
                        flow_hh_logger::debug!(
                            "Watchman reports mergebase is unchanged at {:?}",

View on GitHub (pinned to f88ac94bcf)

Solutions

1. Install the VCS (git or sapling/hg) and ensure it is on the daemon's PATH — verify with git -C <root> rev-parse HEAD run as the daemon user.
2. Run Flow from a real checkout, with the root at or below the repo root.
3. If the directory is intentionally not a checkout, disable mergebase tracking in the watcher settings so the non-mergebase code path is used.
4. Restart the file watcher/server afterwards — the panic tears down the watch loop.

Example fix

# before: mergebase tracking enabled in a tree without .git
flow daemon start  # later: panic: Failed to query mergebase: unable to find vcs

# after: run from a real checkout
git -C /repo rev-parse HEAD  # works
flow daemon start --root /repo

# before enabling mergebase tracking for a root
git -C "$WATCH_ROOT" rev-parse --git-dir >/dev/null 2>&1 || {
  echo "no usable VCS at $WATCH_ROOT - do not enable mergebase tracking" >&2
  exit 1
}

# returns 0 only when the root has a working VCS the daemon can call
vcs_usable() { git -C "$1" rev-parse HEAD >/dev/null 2>&1; }

When it happens

Trigger: Enabling mergebase tracking (the watcher's mergebase_with setting) for a watch root that is not a git/hg/sapling checkout, or where the VCS binary is missing from the daemon's PATH; the panic fires on a later check, once a previous mergebase has been recorded and the watcher re-queries it.

Common situations: Source trees copied out of a repo without .git; minimal containers lacking git; daemons started by launchd/systemd with a stripped PATH; the VCS being uninstalled or moved after the server started; roots nested below the actual repo root.

Related errors

• Failed to query mergebase: {}
• flow-dot-js wasm requires crypto.getRandomValues
• Unable to determine executable path: {}
• failed to read executable at {} for flow build id: {}
• failed to get current directory

AI-assisted analysis of facebook/flow@f88ac94bcf (2026-08-20). Data as JSON: /api/errors/0ecc196c51045982. Report an issue: GitHub.