diff --git a/rust/hg-core/src/repo.rs b/rust/hg-core/src/repo.rs
--- a/rust/hg-core/src/repo.rs
+++ b/rust/hg-core/src/repo.rs
@@ -248,6 +248,14 @@
             .contains(requirements::DIRSTATE_V2_REQUIREMENT)
     }
 
+    pub fn has_sparse(&self) -> bool {
+        self.requirements.contains(requirements::SPARSE_REQUIREMENT)
+    }
+
+    pub fn has_narrow(&self) -> bool {
+        self.requirements.contains(requirements::NARROW_REQUIREMENT)
+    }
+
     fn dirstate_file_contents(&self) -> Result<Vec<u8>, HgError> {
         Ok(self
             .hg_vfs()
diff --git a/rust/hg-core/src/requirements.rs b/rust/hg-core/src/requirements.rs
--- a/rust/hg-core/src/requirements.rs
+++ b/rust/hg-core/src/requirements.rs
@@ -88,6 +88,8 @@
     // When it starts writing to the repository, it’ll need to either keep the
     // persistent nodemap up to date or remove this entry:
     NODEMAP_REQUIREMENT,
+    SPARSE_REQUIREMENT,
+    NARROW_REQUIREMENT,
 ];
 
 // Copied from mercurial/requirements.py:
diff --git a/rust/rhg/src/commands/files.rs b/rust/rhg/src/commands/files.rs
--- a/rust/rhg/src/commands/files.rs
+++ b/rust/rhg/src/commands/files.rs
@@ -40,6 +40,11 @@
 
     let repo = invocation.repo?;
     if let Some(rev) = rev {
+        if repo.has_narrow() {
+            return Err(CommandError::unsupported(
+                "rhg files -r <rev> is not support in narrow clones",
+            ));
+        }
         let files = list_rev_tracked_files(repo, rev).map_err(|e| (e, rev))?;
         display_files(invocation.ui, repo, files.iter())
     } else {
diff --git a/rust/rhg/src/commands/status.rs b/rust/rhg/src/commands/status.rs
--- a/rust/rhg/src/commands/status.rs
+++ b/rust/rhg/src/commands/status.rs
@@ -174,6 +174,12 @@
     };
 
     let repo = invocation.repo?;
+    if repo.has_sparse() || repo.has_narrow() {
+        return Err(CommandError::unsupported(
+            "rhg status is not supported for sparse checkouts or narrow clones yet"
+        ));
+    }
+
     let mut dmap = repo.dirstate_map_mut()?;
 
     let options = StatusOptions {
diff --git a/rust/rhg/src/main.rs b/rust/rhg/src/main.rs
--- a/rust/rhg/src/main.rs
+++ b/rust/rhg/src/main.rs
@@ -588,7 +588,8 @@
     }
 }
 
-const SUPPORTED_EXTENSIONS: &[&[u8]] = &[b"blackbox", b"share"];
+const SUPPORTED_EXTENSIONS: &[&[u8]] =
+    &[b"blackbox", b"share", b"sparse", b"narrow"];
 
 fn check_extensions(config: &Config) -> Result<(), CommandError> {
     let enabled = config.get_section_keys(b"extensions");
diff --git a/tests/test-rhg-sparse-narrow.t b/tests/test-rhg-sparse-narrow.t
new file mode 100644
--- /dev/null
+++ b/tests/test-rhg-sparse-narrow.t
@@ -0,0 +1,120 @@
+#require rhg
+
+  $ NO_FALLBACK="env RHG_ON_UNSUPPORTED=abort"
+
+Rhg works well when sparse working copy is enabled.
+
+  $ cd "$TESTTMP"
+  $ hg init repo-sparse
+  $ cd repo-sparse
+  $ cat > .hg/hgrc <<EOF
+  > [extensions]
+  > sparse=
+  > EOF
+
+  $ echo a > show
+  $ echo x > hide
+  $ mkdir dir1 dir2
+  $ echo x > dir1/x
+  $ echo y > dir1/y
+  $ echo z > dir2/z
+
+  $ hg ci -Aqm 'initial'
+  $ hg debugsparse --include 'show'
+  $ ls -A
+  .hg
+  show
+
+  $ tip=$(hg log -r . --template '{node}')
+  $ $NO_FALLBACK rhg files -r "$tip"
+  dir1/x
+  dir1/y
+  dir2/z
+  hide
+  show
+  $ $NO_FALLBACK rhg files
+  show
+
+  $ $NO_FALLBACK rhg cat -r "$tip" hide
+  x
+
+  $ cd ..
+
+We support most things when narrow is enabled, too, with a couple of caveats.
+
+  $ . "$TESTDIR/narrow-library.sh"
+  $ real_hg=$RHG_FALLBACK_EXECUTABLE
+
+  $ cat >> $HGRCPATH <<EOF
+  > [extensions]
+  > narrow=
+  > EOF
+
+  $ hg clone --narrow  ./repo-sparse repo-narrow --include dir1
+  requesting all changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 2 changes to 2 files
+  new changesets 6d714a4a2998
+  updating to branch default
+  2 files updated, 0 files merged, 0 files removed, 0 files unresolved
+
+  $ cd repo-narrow
+
+  $ $NO_FALLBACK rhg cat -r "$tip" dir1/x
+  x
+  $ "$real_hg" cat -r "$tip" dir1/x
+  x
+
+TODO: bad error message
+
+  $ $NO_FALLBACK rhg cat -r "$tip" hide
+  abort: invalid revision identifier: 6d714a4a2998cbfd0620db44da58b749f6565d63
+  [255]
+  $ "$real_hg" cat -r "$tip" hide
+  [1]
+
+A naive implementation of [rhg files] leaks the paths that are supposed to be
+hidden by narrow, so we just fall back to hg.
+
+  $ $NO_FALLBACK rhg files -r "$tip"
+  unsupported feature: rhg files -r <rev> is not support in narrow clones
+  [252]
+  $ "$real_hg" files -r "$tip"
+  dir1/x
+  dir1/y
+
+Hg status needs to do some filtering based on narrow spec, so we don't
+support it in rhg for narrow clones yet.
+
+  $ mkdir dir2
+  $ touch dir2/q
+  $ "$real_hg" status
+  $ $NO_FALLBACK rhg --config rhg.status=true status
+  unsupported feature: rhg status is not supported for sparse checkouts or narrow clones yet
+  [252]
+
+Adding "orphaned" index files:
+
+  $ (cd ..; cp {repo-sparse,repo-narrow}/.hg/store/data/hide.i)
+  $ (cd ..; mkdir repo-narrow/.hg/store/data/dir2; cp {repo-sparse,repo-narrow}/.hg/store/data/dir2/z.i)
+  $ "$real_hg" verify
+  checking changesets
+  checking manifests
+  crosschecking files in changesets and manifests
+  checking files
+  checked 1 changesets with 2 changes to 2 files
+
+  $ "$real_hg" files -r "$tip"
+  dir1/x
+  dir1/y
+
+# TODO: even though [hg files] hides the orphaned dir2/z, [hg cat] still shows it.
+# rhg has the same issue, but at least it's not specific to rhg.
+# This is despite [hg verify] succeeding above.
+
+  $ $NO_FALLBACK rhg cat -r "$tip" dir2/z
+  z
+  $ "$real_hg" cat -r "$tip" dir2/z
+  z