diff --git a/src/plugin_system/plugin_catalog.py b/src/plugin_system/plugin_catalog.py
index 9483630d..bd80f2ab 100644
--- a/src/plugin_system/plugin_catalog.py
+++ b/src/plugin_system/plugin_catalog.py
@@ -244,6 +244,8 @@ class PluginCatalog:
             if live and any(m.lower() == wanted for m in live):
                 return plugin_id
         for plugin_id, manifest in manifests.items():
+            if self._live_display_modes(plugin_id):
+                continue  # the display's list is the truth for this plugin
             modes = manifest.get('display_modes')
             if isinstance(modes, list) and any(
                     isinstance(m, str) and m.lower() == wanted for m in modes):
diff --git a/src/plugin_system/plugin_runtime.py b/src/plugin_system/plugin_runtime.py
index 643de58c..25f79468 100644
--- a/src/plugin_system/plugin_runtime.py
+++ b/src/plugin_system/plugin_runtime.py
@@ -161,7 +161,9 @@ def _published_modes(modes: Any) -> Optional[List[str]]:
     """The registered display modes as a snapshot carries them, or None."""
     if not isinstance(modes, list):
         return None
-    return [_clip(m, _ID_CHARS) for m in modes[:_MAX_MODES] if isinstance(m, str)]
+    # A name is a key the display matches exactly: drop one too long to
+    # carry whole rather than clip it into a different name.
+    return [m for m in modes if isinstance(m, str) and len(m) <= _ID_CHARS][:_MAX_MODES]
 
 
 def build_runtime_snapshot(state_manager: Any, *, started_at: float,
diff --git a/test/test_api_v3_display_modes.py b/test/test_api_v3_display_modes.py
index a59913f0..5258c043 100644
--- a/test/test_api_v3_display_modes.py
+++ b/test/test_api_v3_display_modes.py
@@ -7,7 +7,7 @@ manifest.json off disk and reimplemented PluginManager's own fallbacks.
 """
 
 import json
-from unittest.mock import MagicMock
+from unittest.mock import MagicMock, patch
 
 import pytest
 
@@ -155,3 +155,14 @@ class TestOneBadConfigSectionDoesNotBlankTheList:
             side_effect=RuntimeError("GET https://x/y?api_key=SEC123 failed"))
         body = api_v3_client.get('/api/v3/display/modes').get_json()
         assert 'SEC123' not in json.dumps(body)
+
+
+class TestOnDemandUsesTheRegisteredSpelling:
+    def test_a_mode_differing_in_case_is_sent_as_registered(self, client):
+        with patch('web_interface.blueprints.api_v3.display._deliver_on_demand',
+                   return_value=('socket', None)) as deliver:
+            response = client.post('/api/v3/display/on-demand/start',
+                                   json={'plugin_id': 'football-scoreboard',
+                                         'mode': 'NFL_LIVE', 'start_service': False})
+        assert response.status_code == 200, response.get_json()
+        assert deliver.call_args.args[0]['mode'] == 'nfl_live'
diff --git a/test/test_live_display_modes.py b/test/test_live_display_modes.py
index fb09f091..05e695fe 100644
--- a/test/test_live_display_modes.py
+++ b/test/test_live_display_modes.py
@@ -165,6 +165,12 @@ class TestSnapshotAndView:
         published = snapshot["plugins"]["soccer-scoreboard"]["modes"]
         assert len(published) == rt._MAX_MODES
 
+    def test_a_mode_name_is_kept_whole_or_dropped(self):
+        long_mode = "x" * (rt._ID_CHARS + 1)
+        snapshot = build_runtime_snapshot(_loaded_states(["ok", long_mode]),
+                                          started_at=1.0, now=self.NOW)
+        assert snapshot["plugins"]["soccer-scoreboard"]["modes"] == ["ok"]
+
     def test_non_strings_from_a_hand_made_snapshot_are_dropped(self):
         snapshot = {"schema": rt.SNAPSHOT_SCHEMA, "running": True,
                     "published_at": self.NOW, "plugins": {
@@ -202,6 +208,13 @@ class TestCatalog:
         assert catalog.get_plugin_display_modes("clock-simple") == ["clock"]
         assert catalog.find_plugin_for_mode("clock") == "clock-simple"
 
+    def test_a_mode_the_display_dropped_does_not_resolve_by_manifest(
+            self, plugins_dir, shared_cache):
+        display_cache, web_cache = shared_cache
+        PluginRuntimePublisher(display_cache, _loaded_states(CUSTOM)).tick()
+        catalog = self._catalog(plugins_dir, web_cache)
+        assert catalog.find_plugin_for_mode("soccer_eng.1_live") is None
+
     def test_a_stopped_display_falls_back_to_manifests(self, plugins_dir, shared_cache):
         display_cache, web_cache = shared_cache
         publisher = PluginRuntimePublisher(display_cache, _loaded_states(REGISTERED))
diff --git a/web_interface/blueprints/api_v3/display.py b/web_interface/blueprints/api_v3/display.py
index a071d1c8..74652433 100644
--- a/web_interface/blueprints/api_v3/display.py
+++ b/web_interface/blueprints/api_v3/display.py
@@ -279,6 +279,15 @@ def start_on_demand_display():
             if not resolved_plugin:
                 return jsonify({'status': 'error', 'message': f'Mode {resolved_mode} not found'}), 404
 
+    # The display matches mode names exactly: pass the registered spelling
+    # when the caller's differs only in case.
+    if api_v3.plugin_catalog and resolved_plugin and resolved_mode:
+        wanted = resolved_mode.strip().lower()
+        for registered in api_v3.plugin_catalog.get_plugin_display_modes(resolved_plugin):
+            if isinstance(registered, str) and registered.lower() == wanted:
+                resolved_mode = registered
+                break
+
     # On-demand works with disabled plugins: the running display loads one
     # for the session and unloads it afterwards, leaving config.json alone
     # (DisplayController._load_plugin_for_on_demand). Logged for debugging.