---
title: "MGS1101: this VCS backend cannot answer that"
description: Fires when a magus feature needs a version-control capability the configured backend has not implemented, so the answer is reported as unavailable rather than as empty.
tags: [MGS1101, capability, vcs, git, backend, branches, review]
---

# MGS1101: this VCS backend cannot answer that

magus asked the configured version-control backend a question it does not
implement, so the feature reports a gap instead of an answer.

```text
[MGS1101] hg does not report branch changes
```

Nothing failed. The command still ran, and everything that did not depend on the
missing capability is correct.

## Why

magus talks to version control through a backend registry rather than shelling
out to one tool, so a workspace can be on git, hg, or something a plugin
provides. Backends are not required to be equal: a capability exists when a
backend can answer it, and a backend that cannot is a normal state rather than a
broken install.

The reason this is a diagnostic at all is that the alternative is **silence**,
and silence is indistinguishable from good news. The branch-overlap lookup is
the clearest case:

- "No other branch is changing these files" is reassurance. You can stop
  thinking about it.
- "This backend cannot tell you which branches are changing these files" is a
  gap in what magus knows.

Both render as an empty list. Only one of them means you are safe, so magus
names the gap rather than letting an empty result speak for it. That is the same
reason a knowledge lookup distinguishes `absent` from `unknown`: a negative claim
is a claim, and magus does not make one it cannot support.

## Fix

There is usually nothing to fix in your workspace. A capability gap is a fact
about the backend, not a misconfiguration.

- **Check that the backend is the one you meant.** `magus buzz -e 'import "std";
  import "vcs"; fun main() > void { std\print(vcs\metadata()); } main();'`
  reports which backend resolved for this tree.
- **Treat the affected surface as unmeasured, not as clear.** Where magus says a
  lookup is unsupported, do the check by hand if it matters to the change in
  front of you.
- **Implement it, if you own the backend.** The capability interfaces are small
  and additive - a backend advertises what it can do by satisfying one, and
  magus reports this code for the rest. See
  [Review](../../../concepts/review.md) for how the diff surface consumes them.

## See also

- [MGS1102](MGS1102.md) - a review provider that does not implement an op.
- [Review](../../../concepts/review.md) - the surfaces that report capability
  gaps rather than hiding them.