What causes "cannot fetch to ' + ref + ' when checked out on ' + ref" in pcottle/learnGitBranching?

git checkout another branch first, then fetch into the ref

pcottle/learnGitBranching · error · GitError

cannot fetch to ' + ref + ' when checked out on ' + ref

Error message

cannot fetch to ' + ref + ' when checked out on ' + ref

What it means

assertNotCheckedOut throws when a fetch/push-like command would write to a remote-tracking ref that corresponds to the branch currently checked out (engine.HEAD points at it). Mirrors git's refusal to update the branch you're on via fetch.

Source

Thrown at src/js/git/commands.js:53

};

var validateOriginBranchName = function(engine, name) {
  return engine.origin.validateBranchName(name);
};

var validateBranchNameIfNeeded = function(engine, name) {
  if (engine.refs[name]) {
    return name;
  }
  return validateBranchName(engine, name);
};

var assertNotCheckedOut = function(engine, ref) {
  if (!engine.refs[ref]) {
    return;
  }
  if (engine.HEAD.get('target') === engine.refs[ref]) {
    throw new GitError({
      msg: intl.todo(
        'cannot fetch to ' + ref + ' when checked out on ' + ref
      )
    });
  }
};

var assertIsBranch = function(engine, ref) {
  assertIsRef(engine, ref);
  var obj = engine.resolveID(ref);
  if (!obj || obj.get('type') !== 'branch') {
    throw new GitError({
      msg: intl.todo(
        ref + ' is not a branch'
      )
    });
  }
};

View on GitHub (pinned to 5b09d0ff96)

Solutions

git checkout another branch first, then fetch into the ref
Fetch without a destination refspec (just git fetch origin) and let o/ tracking refs update
Push instead of fetch if your intent was to update origin

Example fix

// before
git checkout maingit fetch origin main:main
// after
git checkout -b tempgit fetch origin main:main

Defensive patterns

Strategy: validation

Validate before calling

function canFetchInto(engine, ref) { return !!engine.refs[ref] && engine.HEAD.get('target') !== engine.refs[ref]; }

Try / catch

try { fetchInto(ref); } catch (e) { if (e instanceof GitError && /checked out/.test(e.msg)) checkoutTempThenRetry(); else throw e; }

Prevention

Check engine.HEAD target before fetch-to-ref
Default to refspec-less fetch which never hits this

When it happens

Trigger: Fetching into a ref (e.g. git fetch origin main:main) while HEAD targets that same branch/ref in the simulated engine.

Common situations: Practicing refspec fetches in LearnGitBranching remote levels; forgetting to switch branches before simulating a fetch-to-ref.

Related errors

AI-assisted analysis of pcottle/learnGitBranching@5b09d0ff96 (2026-08-27). Data as JSON: /api/errors/bf7c531bcc98b87b. Report an issue: GitHub.