diff --git a/tests/browser/workspace_system_fixture.py b/tests/browser/workspace_system_fixture.py
index 9900abc..c7d87af 100644
--- a/tests/browser/workspace_system_fixture.py
+++ b/tests/browser/workspace_system_fixture.py
@@ -35,7 +35,8 @@ read. These per-server hooks only change what a System test needs on top:
   previews for Convert outdated;
 - ``POST /__test__/system/jxl-encoder`` ``{available}`` (#317) makes the
   server read as having (``cjxl``) or lacking a JPEG XL encoder without
-  probing; reset restores the thumbnail settings and the real detection.
+  probing, whatever the host has installed (#371: ``false`` stands in for the
+  detector itself); reset restores the thumbnail settings and the real detection.
 
 - ``POST /__test__/system/duplicate-prompts`` (S7) adds two prompts whose
   text differs only in case and spacing (a duplicate prompt text group);
@@ -118,6 +119,11 @@ def register_system_commands(app, routes, api, root: Path):
     }
     from py.api import thumbnail_encode
 
+    real_detect = thumbnail_encode._detect
+
+    def _no_encoder():
+        return None
+
     monitor = getattr(image_monitor, "_monitor_instance", None)
     failures: dict[str, int] = {}
 
@@ -147,6 +153,7 @@ def register_system_commands(app, routes, api, root: Path):
         api._health_report = None
         for (owner, name), value in host_values.items():
             setattr(owner, name, value)
+        thumbnail_encode._detect = real_detect
         thumbnail_encode.reset_encoders()
         image_monitor._monitor_instance = monitor
         api.__dict__.pop("_library_resolver", None)
@@ -298,7 +305,12 @@ def register_system_commands(app, routes, api, root: Path):
         available = data.get("available")
         if not isinstance(available, bool):
             raise web.HTTPBadRequest()
-        # Test-only: stand in the cached detection (no probe).
+        # Test-only: the fixture, not the host, decides (#371). Without an encoder the
+        # detector itself stands in, so a re-probe (a "none found" is probed again on
+        # every settings read) cannot find the host's real encoder; with one the cached
+        # answer stands, and a failed write still probes the real detector as before.
+        # Reset restores the real detector.
+        thumbnail_encode._detect = real_detect if available else _no_encoder
         with thumbnail_encode._detect_lock:
             thumbnail_encode._detected = "cjxl" if available else None
         return web.json_response({"success": True})
diff --git a/tests/test_workspace_system_fixture.py b/tests/test_workspace_system_fixture.py
index a43ec85..2318956 100644
--- a/tests/test_workspace_system_fixture.py
+++ b/tests/test_workspace_system_fixture.py
@@ -1,12 +1,26 @@
-"""The System browser hooks only ever reset their own server's config.json (#249 S3)."""
+"""The System browser hooks only ever reset their own server's config.json (#249 S3)
+and decide JPEG XL availability whatever the host has installed (#371)."""
 
+import asyncio
 import os
+import sys
 import tempfile
+import types
 import unittest
 from pathlib import Path
 from unittest import mock
 
-from tests.browser.workspace_system_fixture import register_system_commands
+from aiohttp import web
+from aiohttp.test_utils import TestClient, TestServer
+
+_mock_server = mock.MagicMock()
+_mock_server.PromptServer.instance.routes = mock.MagicMock()
+sys.modules.setdefault("server", _mock_server)
+
+from py.api import thumbnail_encode  # noqa: E402
+from tests.browser.workspace_system_fixture import (  # noqa: E402
+    register_system_commands,
+)
 
 
 class TestWorkspaceSystemFixture(unittest.TestCase):
@@ -33,6 +47,48 @@ class TestWorkspaceSystemFixture(unittest.TestCase):
                     register_system_commands(None, None, None, self.root)
         self.assertEqual(elsewhere.read_bytes(), b'{"real": true}')
 
+    def test_no_encoder_holds_even_when_the_host_has_one(self):
+        """``{available: false}`` survives the settings read's re-probe (#371)."""
+        config = self.root / "config.json"
+        env = {**os.environ, "PROMPT_MANAGER_CONFIG_PATH": str(config)}
+        api = types.SimpleNamespace()
+        app, routes = web.Application(), web.RouteTableDef()
+        real_detect = thumbnail_encode._detect
+        with mock.patch.dict(os.environ, env, clear=True):
+            reset = register_system_commands(app, routes, api, self.root)
+        app.add_routes(routes)
+        self.addCleanup(thumbnail_encode.reset_encoders)
+        self.addCleanup(setattr, thumbnail_encode, "_detect", real_detect)
+
+        async def scenario():
+            async with TestClient(TestServer(app)) as client:
+
+                async def post(available):
+                    response = await client.post(
+                        "/__test__/system/jxl-encoder", json={"available": available}
+                    )
+                    self.assertEqual(response.status, 200)
+
+                await post(False)
+                status = thumbnail_encode.encoder_status()["jxl"]
+                self.assertFalse(status["available"])
+                self.assertIsNone(
+                    thumbnail_encode.detect_jxl_encoder(recheck_missing=True)
+                )
+                await post(True)
+                status = thumbnail_encode.encoder_status()["jxl"]
+                self.assertEqual(status["encoder"], "cjxl")
+                await post(False)
+
+        # The host's real detector finds an encoder (Pillow can save JPEG XL).
+        with mock.patch.object(thumbnail_encode, "_pillow_can_save_jxl", lambda: True):
+            asyncio.run(scenario())
+            # Reset gives detection back to the host.
+            reset()
+            self.assertIs(thumbnail_encode._detect, real_detect)
+            status = thumbnail_encode.encoder_status()["jxl"]
+            self.assertEqual(status["encoder"], "pillow")
+
 
 if __name__ == "__main__":
     unittest.main()