diff --git a/tests/rig/selection.py b/tests/rig/selection.py
index a1daac001a..e67945d0dd 100644
--- a/tests/rig/selection.py
+++ b/tests/rig/selection.py
@@ -7,6 +7,10 @@
 The literal ``all`` expands to every group in the manifest. When the result is
 empty, callers should treat that as "do nothing" rather than silently running
 everything — the CLI surfaces a clear error.
+
+With ``--tier``, dep expansion never reaches outside that tier: each tier runs as
+its own CI leg, so a cross-tier ``depends_on`` would otherwise re-run the same
+group in every leg that transitively depends on it.
 """
 
 from __future__ import annotations
@@ -66,7 +70,18 @@ def resolve(
     if changed_only:
         selected.update(_groups_for_changed_paths(manifest, base=changed_base))
 
-    return manifest.expand(sorted(selected)) if selected else []
+    if not selected:
+        return []
+
+    requested = sorted(selected)
+    expanded = manifest.expand(requested)
+    if tier is not None:
+        # A tier run never pulls in another tier's groups. Tiers run as separate
+        # legs, so a cross-tier `depends_on` would re-run the same group once per
+        # leg; the owning tier's leg covers it. Explicitly requested groups stay.
+        keep = set(requested)
+        expanded = [n for n in expanded if n in keep or manifest.get(n).tier == tier]
+    return expanded
 
 
 def _groups_for_changed_paths(manifest: GroupManifest, *, base: str) -> set[str]:
diff --git a/tests/rig/tests/test_selection.py b/tests/rig/tests/test_selection.py
index d0ee7ef410..468a66667d 100644
--- a/tests/rig/tests/test_selection.py
+++ b/tests/rig/tests/test_selection.py
@@ -76,3 +76,51 @@ def test_tier_only_selects_that_tier_and_deps(tmp_path: Path) -> None:
     manifest = load_groups(_manifest(tmp_path))
     ordered = resolve(manifest, positional=[], tier="unit")
     assert set(ordered) == {"unit-a", "unit-b"}
+
+
+def _cross_tier_manifest(tmp_path: Path) -> Path:
+    p = tmp_path / "groups.yaml"
+    p.write_text(
+        """
+        version: 1
+        groups:
+          unit-a:
+            tier: unit
+            paths: [x]
+            optional: true
+          e2e-smoke:
+            tier: e2e
+            paths: [x]
+            requires_platform: true
+            optional: true
+          e2e-cross:
+            tier: e2e
+            paths: [x]
+            requires_platform: true
+            depends_on: [e2e-smoke, unit-a]
+            optional: true
+        """
+    )
+    return p
+
+
+def test_tier_run_does_not_pull_in_other_tiers(tmp_path: Path) -> None:
+    """Each tier is its own CI leg; a cross-tier dep must not re-run there."""
+    manifest = load_groups(_cross_tier_manifest(tmp_path))
+    ordered = resolve(manifest, positional=[], tier="e2e")
+    assert set(ordered) == {"e2e-smoke", "e2e-cross"}
+    # Intra-tier ordering still holds.
+    assert ordered.index("e2e-smoke") < ordered.index("e2e-cross")
+
+
+def test_explicitly_named_group_survives_tier_filter(tmp_path: Path) -> None:
+    """The filter only drops dep-expanded groups, never an explicit request."""
+    manifest = load_groups(_cross_tier_manifest(tmp_path))
+    ordered = resolve(manifest, positional=["unit-a"], tier="e2e")
+    assert set(ordered) == {"unit-a", "e2e-smoke", "e2e-cross"}
+
+
+def test_cross_tier_dep_still_expands_without_tier_filter(tmp_path: Path) -> None:
+    manifest = load_groups(_cross_tier_manifest(tmp_path))
+    ordered = resolve(manifest, positional=["e2e-cross"])
+    assert set(ordered) == {"unit-a", "e2e-smoke", "e2e-cross"}