diff --git a/docs/launcher.md b/docs/launcher.md
index de2d3077..6e6998cf 100644
--- a/docs/launcher.md
+++ b/docs/launcher.md
@@ -135,6 +135,12 @@ game/                    the built game, unless another folder was chosen (its l
 A path the Windows ANSI code page cannot hold breaks the MSYS2 tools, so the toolchain then goes to
 `C:\NX2navi\toolchain` instead (`launcher/paths.py`).
 
+Windows and the build tools stop at 260 characters for a file's whole path, and the files of a built game need up to
+128 below the game folder (the prompt folders' resource table is the longest). Begin Installation and the install
+job therefore refuse a game folder whose own path is longer than 130 characters, with both lengths
+(`paths.long_path_problem`). Longer paths are not supported: the launcher does not use Windows' long path form, and
+the compiler, the linker and `tar` would not take it.
+
 ## Installing
 
 `jobs.InstallJob`, one row per step:
diff --git a/launcher/jobs.py b/launcher/jobs.py
index 74678075..f8dfef5e 100644
--- a/launcher/jobs.py
+++ b/launcher/jobs.py
@@ -9,7 +9,7 @@ import threading
 import time
 import traceback
 
-from . import build, feed, gamefiles, locks, net, toolchain, updater, versions, winsys
+from . import build, feed, gamefiles, locks, net, paths, toolchain, updater, versions, winsys
 
 RECHECK = 3600   # seconds between update checks while the launcher stays open, hidden during play included
 
@@ -456,6 +456,9 @@ class InstallJob(Job):
         return build.install_args(self.ctx.layout, self.dump, self.game_dir)
 
     def run(self):
+        too_long = paths.long_path_problem(self.game_dir)   # the window asks first (begin_install); the command line here
+        if too_long:
+            raise build.BuildError(too_long)
         self.ensure_tools('tools')
         self.make_game(self.args(),
                        {s: s for s in ('check', 'exefs', 'romfs', 'lift', 'compile', 'shaders', 'prompts', 'scan')})
diff --git a/launcher/paths.py b/launcher/paths.py
index fd98c330..5d104233 100644
--- a/launcher/paths.py
+++ b/launcher/paths.py
@@ -38,6 +38,29 @@ def ansi_safe(path):
         return False
 
 
+# Windows: a file's whole path may be 259 characters for the programs that do not ask for more, and the build's tools
+# (the MSYS2 compiler, linker and resource compiler, tar) are among them whatever the system's long path setting.
+# LONGEST_IN_GAME is the room the game folder's own files need below it: the longest path a built game folder held
+# was 106 characters (romfs-patches-playstation-label\System\Resource\ResourceSizeTable...rsizetable.zs), and a file
+# is written under a temporary name up to 22 characters longer before it gets its own.
+WINDOWS_PATH_MAX = 259
+LONGEST_IN_GAME = 128
+
+
+def long_path_problem(game_dir):
+    """'' when the files of a game built into game_dir fit Windows' path length, else the sentence that says how long
+    the folder's path is and how long it may be. Always '' elsewhere. Without it a build into a deep folder stops
+    somewhere in the middle with "file not found" for a file that is there."""
+    if os.name != 'nt' or not game_dir:
+        return ''
+    n, most = len(os.path.abspath(str(game_dir))), WINDOWS_PATH_MAX - 1 - LONGEST_IN_GAME
+    if n <= most:
+        return ''
+    return (f'the path of the game folder is {n} characters long, and the game\'s files need {LONGEST_IN_GAME} more: '
+            f'Windows and the build tools stop at {WINDOWS_PATH_MAX + 1} for a file\'s whole path. Choose a folder '
+            f'whose path has at most {most} characters, such as D:\\Games\\NX2navi')
+
+
 def fixed_drive(drive):
     """True when drive (X:\\) is a fixed disk (GetDriveTypeW DRIVE_FIXED), not a network, removable or optical drive.
     True where that cannot be asked."""
diff --git a/launcher/ui/app.py b/launcher/ui/app.py
index 7d8439e1..977d4278 100644
--- a/launcher/ui/app.py
+++ b/launcher/ui/app.py
@@ -39,6 +39,7 @@ from tkinter import filedialog
 
 from .. import NAME, build, feed, gameini, gamefiles, jobs, locks, mods, saves, steam, versions, winsys
 from ..context import Context
+from ..paths import long_path_problem
 from . import art, picture, sound
 
 W, H = 1000, 640
@@ -966,6 +967,11 @@ class App:
             self.choose_dump()
             return
         gd = self.game_dir()
+        too_long = long_path_problem(gd)   # told here, not as a missing file halfway through the build
+        if too_long:
+            self.message = App.friendly(too_long)
+            self.render()
+            return
         drop_partial_image(gd)   # before the free space is measured
         if free_gb(gd) < 34 and not (Path(gd) / 'romfs' / 'romfs.bin').is_file():
             self.message = f'{drive_of(gd)} has {free_gb(gd):.0f} GB free; the game needs about 34 GB.'
diff --git a/tests/launcher_test.py b/tests/launcher_test.py
index d93fb8e6..1a2d8a1c 100644
--- a/tests/launcher_test.py
+++ b/tests/launcher_test.py
@@ -5130,6 +5130,38 @@ class BeginInstallTest(Tmp):
         self.assertIn('31 GB free', rows['Install Folder'].desc[0])
         self.assertIn('(31 GB free)', rows['Begin Installation'].desc[0])
 
+    def test_a_game_folder_whose_path_is_too_long_is_refused_first(self):
+        """Windows and the build tools stop at 260 characters for a file's whole path, and the game folder's own
+        files need 128 below it: a deeper folder is refused at Begin Installation and by the install job with the
+        lengths, where the build stopped in the middle with a file "not found". No limit elsewhere."""
+        from launcher import paths
+        most = paths.WINDOWS_PATH_MAX - 1 - paths.LONGEST_IN_GAME
+        fits = 'C:\\' + 'g' * (most - 3)
+        deep = fits + 'g'
+        with mock.patch.object(paths.os, 'name', 'nt'), \
+                mock.patch.object(paths.os.path, 'abspath', lambda p: p):   # as Windows gives these paths back
+            self.assertEqual(paths.long_path_problem(fits), '')
+            self.assertEqual(paths.long_path_problem(''), '')
+            said = paths.long_path_problem(deep)
+            self.assertIn(f'is {most + 1} characters long', said)
+            self.assertIn(f'at most {most} characters', said)
+        with mock.patch.object(paths.os, 'name', 'posix'):
+            self.assertEqual(paths.long_path_problem('/' + 'g' * 4000), '')
+        if os.name != 'nt':
+            return
+        App, a = mock_app(self, deep)
+        a.settings['dump'] = str(self.tmp / 'game.nup')
+        a.game_dir.return_value = deep
+        App.begin_install(a)
+        a.run_job.assert_not_called()
+        self.assertTrue(a.message.startswith('The path of the game folder is'))
+        ctx = JobTest.make_ctx(self, 'ok')
+        self.addCleanup(ctx._file.close)
+        j = jobs.InstallJob(ctx, self.tmp / 'g.nup', deep).run_inline()
+        self.assertEqual(j.result, 'failed')
+        self.assertIn('characters long', j.error)
+        self.assertEqual(j.step('tools').state, jobs.PENDING)         # before the build tools are fetched
+
 
 class UpdateTextTest(Tmp):
     def test_the_updates_row_does_not_promise_a_few_minutes(self):