diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 366379320e23..6db70293b865 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -51,6 +51,9 @@ jobs:
       - name: Validate Swift file length budget guard
         run: ./tests/test_ci_swift_file_length_budget.sh
 
+      - name: Validate auxiliary window close shortcut lint
+        run: ./tests/test_ci_auxiliary_window_close_shortcuts.sh
+
       - name: Validate CircleCI auto approval
         run: python3 tests/test_circleci_auto_approve.py
 
diff --git a/Sources/Feed/FeedTextEditorDebugWindowController.swift b/Sources/Feed/FeedTextEditorDebugWindowController.swift
index 2999642ca2e7..5ea8d6e73377 100644
--- a/Sources/Feed/FeedTextEditorDebugWindowController.swift
+++ b/Sources/Feed/FeedTextEditorDebugWindowController.swift
@@ -16,6 +16,7 @@ final class FeedTextEditorDebugWindowController: NSWindowController, NSWindowDel
             localized: "feed.textEditorDebug.windowTitle",
             defaultValue: "Feed Text Editor Lab"
         )
+        window.identifier = NSUserInterfaceItemIdentifier("cmux.feedTextEditorDebug")
         window.center()
         window.contentView = NSHostingView(rootView: FeedTextEditorDebugView())
         super.init(window: window)
diff --git a/Sources/cmuxApp.swift b/Sources/cmuxApp.swift
index d40991cd69ed..f765b1ba5dc7 100644
--- a/Sources/cmuxApp.swift
+++ b/Sources/cmuxApp.swift
@@ -1142,6 +1142,16 @@ private let cmuxAuxiliaryWindowIdentifiers: Set<String> = [
     "cmux.about",
     "cmux.licenses",
     "cmux.browser-popup",
+    "cmux.browserProfilePopoverDebug",
+    "cmux.configEditor",
+    "cmux.feedButtonStyleDebug",
+    "cmux.feedPreview",
+    "cmux.feedTextEditorDebug",
+    "cmux.fileExplorerStyleDebug",
+    "cmux.pdfPreviewChromeDebug",
+    "cmux.splitButtonLayoutDebug",
+    "cmux.tabBarBackdropLab",
+    "cmux.taskManager",
     "cmux.aboutTitlebarDebug",
     "cmux.debugWindowControls",
     "cmux.browserImportHintDebug",
diff --git a/scripts/lint_auxiliary_window_close_shortcuts.py b/scripts/lint_auxiliary_window_close_shortcuts.py
new file mode 100755
index 000000000000..cda59ae88329
--- /dev/null
+++ b/scripts/lint_auxiliary_window_close_shortcuts.py
@@ -0,0 +1,143 @@
+#!/usr/bin/env python3
+"""Require standalone cmux windows to own the standard close shortcut."""
+
+from __future__ import annotations
+
+import argparse
+import pathlib
+import re
+import sys
+
+
+DEFAULT_ROOTS = ("Sources",)
+OWNER_LIST_PATH = pathlib.Path("Sources/cmuxApp.swift")
+OWNER_LIST_NAME = "cmuxAuxiliaryWindowIdentifiers"
+
+# Hidden/internal bootstrap windows should not take Cmd+W away from the active
+# main window. Add to this set only when a window is intentionally not user
+# closable.
+IGNORED_IDENTIFIERS = {
+    "cmux.bootstrap",
+}
+
+IDENTIFIER_ASSIGNMENT_RE = re.compile(
+    r"""\b[A-Za-z_][A-Za-z0-9_]*\.identifier\s*=\s*NSUserInterfaceItemIdentifier\("(?P<identifier>cmux\.[^"]+)"\)"""
+)
+STRING_LITERAL_RE = re.compile(r'"(?P<identifier>cmux\.[^"]+)"')
+BLOCK_COMMENT_RE = re.compile(r"/\*.*?\*/", re.DOTALL)
+LINE_COMMENT_RE = re.compile(r"//[^\n]*")
+
+
+def strip_line_comments(text: str) -> str:
+    text = BLOCK_COMMENT_RE.sub(lambda match: "\n" * match.group(0).count("\n"), text)
+    return LINE_COMMENT_RE.sub("", text)
+
+
+def load_close_owner_identifiers(repo_root: pathlib.Path) -> set[str]:
+    path = repo_root / OWNER_LIST_PATH
+    try:
+        text = path.read_text(encoding="utf-8")
+    except FileNotFoundError:
+        raise ValueError(f"missing {OWNER_LIST_PATH}") from None
+
+    parse_text = strip_line_comments(text)
+    marker = f"private let {OWNER_LIST_NAME}"
+    marker_index = parse_text.find(marker)
+    if marker_index < 0:
+        raise ValueError(f"missing {OWNER_LIST_NAME} in {OWNER_LIST_PATH}")
+
+    list_start = parse_text.find("[", marker_index)
+    if list_start < 0:
+        raise ValueError(f"could not parse {OWNER_LIST_NAME} in {OWNER_LIST_PATH}")
+
+    depth = 0
+    list_end = -1
+    for index in range(list_start, len(parse_text)):
+        if parse_text[index] == "[":
+            depth += 1
+        elif parse_text[index] == "]":
+            depth -= 1
+            if depth == 0:
+                list_end = index
+                break
+    if list_end < 0:
+        raise ValueError(f"could not parse {OWNER_LIST_NAME} in {OWNER_LIST_PATH}")
+
+    list_body = parse_text[list_start:list_end]
+    return {match.group("identifier") for match in STRING_LITERAL_RE.finditer(list_body)}
+
+
+def collect_window_identifier_assignments(
+    repo_root: pathlib.Path,
+    roots: tuple[str, ...],
+) -> dict[str, list[str]]:
+    assignments: dict[str, list[str]] = {}
+    for root in roots:
+        root_path = repo_root / root
+        if not root_path.exists():
+            continue
+        for path in sorted(root_path.rglob("*.swift")):
+            rel_path = path.relative_to(repo_root).as_posix()
+            text = strip_line_comments(path.read_text(encoding="utf-8", errors="replace"))
+            for match in IDENTIFIER_ASSIGNMENT_RE.finditer(text):
+                identifier = match.group("identifier")
+                line_number = text.count("\n", 0, match.start()) + 1
+                assignments.setdefault(identifier, []).append(f"{rel_path}:{line_number}")
+    return assignments
+
+
+def main(argv: list[str]) -> int:
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument(
+        "--repo-root",
+        default=pathlib.Path.cwd(),
+        type=pathlib.Path,
+        help="repository root to scan",
+    )
+    parser.add_argument(
+        "--roots",
+        nargs="+",
+        default=list(DEFAULT_ROOTS),
+        help="repo-relative Swift roots to scan",
+    )
+    args = parser.parse_args(argv)
+
+    repo_root = args.repo_root.resolve(strict=False)
+    try:
+        close_owners = load_close_owner_identifiers(repo_root)
+    except ValueError as exc:
+        print(f"Auxiliary window close-shortcut lint could not run: {exc}", file=sys.stderr)
+        return 2
+
+    assignments = collect_window_identifier_assignments(repo_root, tuple(args.roots))
+    missing = {
+        identifier: locations
+        for identifier, locations in assignments.items()
+        if identifier not in close_owners and identifier not in IGNORED_IDENTIFIERS
+    }
+
+    if missing:
+        print("Auxiliary window close-shortcut lint failed.")
+        print("")
+        print(
+            "These cmux window identifiers are assigned to NSWindow/NSPanel "
+            f"but are missing from {OWNER_LIST_NAME}:"
+        )
+        for identifier in sorted(missing):
+            print(f"- {identifier}")
+            for location in missing[identifier]:
+                print(f"  {location}")
+        print("")
+        print(
+            f"Add each user-closable window to {OWNER_LIST_NAME} in {OWNER_LIST_PATH}, "
+            "or add a documented lint ignore for internal windows that must not own Cmd+W."
+        )
+        return 1
+
+    print("Auxiliary window close-shortcut lint passed.")
+    print(f"Checked {len(assignments)} cmux window identifier(s).")
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main(sys.argv[1:]))
diff --git a/tests/test_ci_auxiliary_window_close_shortcuts.sh b/tests/test_ci_auxiliary_window_close_shortcuts.sh
new file mode 100755
index 000000000000..96b99fb73fb7
--- /dev/null
+++ b/tests/test_ci_auxiliary_window_close_shortcuts.sh
@@ -0,0 +1,98 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+ROOT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+cd "$ROOT_DIR"
+
+python3 scripts/lint_auxiliary_window_close_shortcuts.py
+
+TMP_DIR="$(mktemp -d)"
+trap 'rm -rf "$TMP_DIR"' EXIT
+
+mkdir -p "$TMP_DIR/Sources"
+
+cat > "$TMP_DIR/Sources/cmuxApp.swift" <<'SWIFT'
+private let cmuxAuxiliaryWindowIdentifiers: Set<String> = [
+    "cmux.settings",
+]
+SWIFT
+
+cat > "$TMP_DIR/Sources/NewWindow.swift" <<'SWIFT'
+import AppKit
+
+/*
+window.identifier = NSUserInterfaceItemIdentifier("cmux.blockCommentOnly")
+*/
+
+func makeWindow() {
+    let window = NSWindow()
+    window.identifier =
+        NSUserInterfaceItemIdentifier("cmux.newWindow")
+}
+SWIFT
+
+if python3 scripts/lint_auxiliary_window_close_shortcuts.py --repo-root "$TMP_DIR" >"$TMP_DIR/missing.out" 2>&1; then
+    echo "Expected missing auxiliary-window close owner to fail" >&2
+    exit 1
+fi
+grep -q "cmux.newWindow" "$TMP_DIR/missing.out"
+grep -q "Sources/NewWindow.swift:9" "$TMP_DIR/missing.out"
+
+cat > "$TMP_DIR/Sources/cmuxApp.swift" <<'SWIFT'
+private let cmuxAuxiliaryWindowIdentifiers: Set<String> = [
+    // "cmux.newWindow",
+    /*
+    "cmux.newWindow",
+    */
+    "cmux.settings",
+]
+SWIFT
+
+if python3 scripts/lint_auxiliary_window_close_shortcuts.py --repo-root "$TMP_DIR" >"$TMP_DIR/commented-owner.out" 2>&1; then
+    echo "Expected commented-out auxiliary-window close owner to be ignored" >&2
+    exit 1
+fi
+grep -q "cmux.newWindow" "$TMP_DIR/commented-owner.out"
+
+cat > "$TMP_DIR/Sources/cmuxApp.swift" <<'SWIFT'
+private let cmuxAuxiliaryWindowIdentifiers: Set<String> = [
+    // MARK: - Main Windows [user-closable]
+    // This comment intentionally contains a lone ] bracket.
+    "cmux.newWindow",
+    "cmux.settings",
+]
+SWIFT
+
+python3 scripts/lint_auxiliary_window_close_shortcuts.py --repo-root "$TMP_DIR"
+
+cat > "$TMP_DIR/Sources/NewWindow.swift" <<'SWIFT'
+import AppKit
+
+func makeWindow() {
+    let window = NSWindow()
+    /*
+    window.identifier = NSUserInterfaceItemIdentifier("cmux.blockCommentOnly")
+    */
+    // window.identifier = NSUserInterfaceItemIdentifier("cmux.commentOnly")
+    _ = window
+}
+SWIFT
+
+python3 scripts/lint_auxiliary_window_close_shortcuts.py --repo-root "$TMP_DIR"
+
+cat > "$TMP_DIR/Sources/cmuxApp.swift" <<'SWIFT'
+private let cmuxAuxiliaryWindowIdentifiers: Set<String> = [
+    "cmux.settings",
+]
+SWIFT
+
+cat > "$TMP_DIR/Sources/NewWindow.swift" <<'SWIFT'
+import AppKit
+
+func makeWindow() {
+    let window = NSWindow()
+    window.identifier = NSUserInterfaceItemIdentifier("cmux.bootstrap")
+}
+SWIFT
+
+python3 scripts/lint_auxiliary_window_close_shortcuts.py --repo-root "$TMP_DIR"