diff --git a/muxplex/tests/test_views.py b/muxplex/tests/test_views.py
new file mode 100644
index 0000000..1396464
--- /dev/null
+++ b/muxplex/tests/test_views.py
@@ -0,0 +1,129 @@
+"""
+Tests for muxplex/views.py — views invariant enforcement.
+"""
+
+
+from muxplex.views import (
+    enforce_mutual_exclusion,
+    validate_view_name,
+)
+
+
+# ---------------------------------------------------------------------------
+# enforce_mutual_exclusion
+# ---------------------------------------------------------------------------
+
+
+def test_enforce_removes_from_hidden_when_in_view():
+    """If a session is in both hidden_sessions and a view, remove from hidden (favor visibility)."""
+    settings = {
+        "hidden_sessions": ["abc:dev", "def:build"],
+        "views": [
+            {"name": "Work", "sessions": ["abc:dev", "abc:web"]},
+        ],
+    }
+    result = enforce_mutual_exclusion(settings)
+    assert "abc:dev" not in result["hidden_sessions"]
+    assert "def:build" in result["hidden_sessions"]
+    assert "abc:dev" in result["views"][0]["sessions"]
+
+
+def test_enforce_no_change_when_no_overlap():
+    """No changes when there is no overlap between hidden and views."""
+    settings = {
+        "hidden_sessions": ["abc:old"],
+        "views": [
+            {"name": "Work", "sessions": ["abc:dev"]},
+        ],
+    }
+    result = enforce_mutual_exclusion(settings)
+    assert result["hidden_sessions"] == ["abc:old"]
+    assert result["views"][0]["sessions"] == ["abc:dev"]
+
+
+def test_enforce_handles_empty_views():
+    """Works when views is an empty list."""
+    settings = {
+        "hidden_sessions": ["abc:dev"],
+        "views": [],
+    }
+    result = enforce_mutual_exclusion(settings)
+    assert result["hidden_sessions"] == ["abc:dev"]
+
+
+def test_enforce_handles_empty_hidden():
+    """Works when hidden_sessions is empty."""
+    settings = {
+        "hidden_sessions": [],
+        "views": [{"name": "Work", "sessions": ["abc:dev"]}],
+    }
+    result = enforce_mutual_exclusion(settings)
+    assert result["hidden_sessions"] == []
+
+
+def test_enforce_deduplicates_view_sessions():
+    """Duplicate session keys within a view are deduplicated."""
+    settings = {
+        "hidden_sessions": [],
+        "views": [
+            {"name": "Work", "sessions": ["abc:dev", "abc:dev", "abc:web"]},
+        ],
+    }
+    result = enforce_mutual_exclusion(settings)
+    assert result["views"][0]["sessions"] == ["abc:dev", "abc:web"]
+
+
+def test_enforce_overlap_across_multiple_views():
+    """A hidden session appearing in multiple views is removed from hidden."""
+    settings = {
+        "hidden_sessions": ["abc:dev"],
+        "views": [
+            {"name": "Work", "sessions": ["abc:dev"]},
+            {"name": "Hobby", "sessions": ["abc:dev", "abc:printer"]},
+        ],
+    }
+    result = enforce_mutual_exclusion(settings)
+    assert "abc:dev" not in result["hidden_sessions"]
+
+
+# ---------------------------------------------------------------------------
+# validate_view_name
+# ---------------------------------------------------------------------------
+
+
+def test_validate_rejects_empty_name():
+    assert validate_view_name("", []) is not None
+
+
+def test_validate_rejects_whitespace_only():
+    assert validate_view_name("   ", []) is not None
+
+
+def test_validate_rejects_too_long():
+    assert validate_view_name("a" * 31, []) is not None
+
+
+def test_validate_rejects_reserved_all():
+    assert validate_view_name("all", []) is not None
+
+
+def test_validate_rejects_reserved_hidden():
+    assert validate_view_name("Hidden", []) is not None
+
+
+def test_validate_rejects_duplicate():
+    existing = [{"name": "Work", "sessions": []}]
+    assert validate_view_name("Work", existing) is not None
+
+
+def test_validate_accepts_valid_name():
+    assert validate_view_name("My Project", []) is None
+
+
+def test_validate_trims_whitespace():
+    """A name that is valid after trimming should pass."""
+    assert validate_view_name("  My Project  ", []) is None
+
+
+def test_validate_accepts_at_max_length():
+    assert validate_view_name("a" * 30, []) is None
diff --git a/muxplex/views.py b/muxplex/views.py
new file mode 100644
index 0000000..3a464ac
--- /dev/null
+++ b/muxplex/views.py
@@ -0,0 +1,69 @@
+"""
+Views invariant enforcement and validation for muxplex.
+
+Core invariants:
+- hidden_sessions and any views[].sessions never share a session key.
+- View names are non-empty, max 30 chars, trimmed, unique, not reserved.
+- Duplicate session keys within a view are deduplicated.
+"""
+
+RESERVED_VIEW_NAMES = frozenset({"all", "hidden"})
+MAX_VIEW_NAME_LENGTH = 30
+
+
+def enforce_mutual_exclusion(settings: dict) -> dict:
+    """Enforce that hidden_sessions and view sessions are disjoint.
+
+    If a session key appears in both hidden_sessions and any view,
+    it is removed from hidden_sessions (favor visibility over hiding).
+
+    Also deduplicates session keys within each view.
+
+    Mutates and returns the settings dict.
+    """
+    views = settings.get("views", [])
+    hidden = settings.get("hidden_sessions", [])
+
+    # Collect all session keys across all views
+    all_view_sessions: set[str] = set()
+    for view in views:
+        all_view_sessions.update(view.get("sessions", []))
+
+    # Remove overlap from hidden (favor visibility)
+    if all_view_sessions and hidden:
+        settings["hidden_sessions"] = [s for s in hidden if s not in all_view_sessions]
+
+    # Deduplicate session keys within each view (preserve order)
+    for view in views:
+        sessions = view.get("sessions", [])
+        seen: set[str] = set()
+        deduped: list[str] = []
+        for s in sessions:
+            if s not in seen:
+                seen.add(s)
+                deduped.append(s)
+        view["sessions"] = deduped
+
+    return settings
+
+
+def validate_view_name(name: str, existing_views: list[dict]) -> str | None:
+    """Validate a view name. Returns an error message string, or None if valid.
+
+    Rules:
+    - Non-empty after trimming
+    - Max 30 characters after trimming
+    - Not a reserved name ("all", "hidden") case-insensitive
+    - Unique among existing views (case-sensitive match)
+    """
+    trimmed = name.strip()
+    if not trimmed:
+        return "View name cannot be empty"
+    if len(trimmed) > MAX_VIEW_NAME_LENGTH:
+        return f"View name must be {MAX_VIEW_NAME_LENGTH} characters or fewer"
+    if trimmed.lower() in RESERVED_VIEW_NAMES:
+        return f"'{trimmed}' is a reserved name"
+    existing_names = {v.get("name", "") for v in existing_views}
+    if trimmed in existing_names:
+        return f"A view named '{trimmed}' already exists"
+    return None