Change default Proton versions

Previously we used stable Proton as the default Proton version in case
no other configuration is found. This was changed to Proton
Experimental in Steam client at some point.

The default itself is hardcoded in one of the Steam client's shared
libraries, meaning Protontricks cannot determine it programmatically.
Instead, use Proton Experimental as the new default and fall back to the
old default of stable Proton if Proton Experimental is not available.

Fixes #435

Author: Matoking

CHANGELOG.md

@@ -7,6 +7,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 ## [Unreleased]
 ### Fixed
  - Fix Steam library folder discovery by using case-insensitive directory name matching
+ - Fix default Proton version discovery. Protontricks will now use Proton Experimental as default per Steam client's hardcoded default setting, with stable Proton as fallback.
 
 ## [1.13.0] - 2025-08-11
 ### Added

src/protontricks/steam.py

@@ -834,15 +834,18 @@ def _get_tool_app(compat_tool_name, steam_apps, steam_play_manifest):
         )
         potential_names.append(tool_name)
 
-    # Get the first name that was valid, or use stable Proton as fallback
+    # Get the first name that was valid,
+    # or use experimental or stable Proton as fallback
     try:
-        compat_tool_name = next(name for name in potential_names if name)
+        compat_tool_names = [
+            next(name for name in potential_names if name)
+        ]
     except StopIteration:
         logger.info(
             "No compatibility tool found by reading Steam configuration. "
-            "Using stable version of Proton as fallback."
+            "Using Proton Experimental or stable Proton as fallback."
         )
-        compat_tool_name = "proton-stable"
+        compat_tool_names = ["proton-experimental", "proton-stable"]
 
     # We've got a compatibility tool name,
     # now there are two possible ways to find the installation
@@ -852,30 +855,31 @@ def _get_tool_app(compat_tool_name, steam_apps, steam_play_manifest):
     #    to parse a binary configuration file to find the App ID
 
     # Let's try option 1 first
-    try:
-        app = next(
-            app for app in steam_apps if app.name == compat_tool_name
-        )
-        logger.info(
-            "Found active custom compatibility tool: %s", app.name
+    for compat_tool_name in compat_tool_names:
+        try:
+            app = next(
+                app for app in steam_apps if app.name == compat_tool_name
+            )
+            logger.info(
+                "Found active custom compatibility tool: %s", app.name
+            )
+            return app
+        except StopIteration:
+            pass
+
+        # Try option 2:
+        # Find the corresponding App ID from <steam_path>/appcache/appinfo.vdf
+        tool_app = _get_tool_app(
+            compat_tool_name=compat_tool_name,
+            steam_apps=steam_apps,
+            steam_play_manifest=steam_play_manifest
         )
-        return app
-    except StopIteration:
-        pass
-
-    # Try option 2:
-    # Find the corresponding App ID from <steam_path>/appcache/appinfo.vdf
-    tool_app = _get_tool_app(
-        compat_tool_name=compat_tool_name,
-        steam_apps=steam_apps,
-        steam_play_manifest=steam_play_manifest
-    )
 
-    if tool_app:
-        logger.info(
-            "Found active compatibility tool: %s", tool_app.name
-        )
-        return tool_app
+        if tool_app:
+            logger.info(
+                "Found active compatibility tool: %s", tool_app.name
+            )
+            return tool_app
 
     logger.error("Could not find configured Proton installation!")
 

tests/test_steam.py

@@ -333,13 +333,24 @@ def test_find_steam_deck_profile(
         assert proton_app.name == "Proton 7.77"
 
     @pytest.mark.usefixtures("info_logging")
+    @pytest.mark.parametrize(
+        "alias,name",
+        [
+            ("proton-stable", "Proton 9.0"),
+            ("proton-experimental", "Proton Experimental")
+        ]
+    )
     def test_find_steam_default_proton(
-            self, steam_app_factory, steam_dir, default_proton,
-            proton_factory, steam_config_path, caplog):
+            self, steam_app_factory, steam_dir, proton_factory,
+            steam_config_path, caplog, alias, name):
         """
-        Ensure the function returns the stable version of Proton if no
-        other configuration is available
+        Ensure the function returns the Proton Experimental or stable version
+        of Proton if no other configuration is available
         """
+        proton_app = proton_factory(
+            name=name, appid=20, compat_tool_name="proton-installation",
+            aliases=[alias]
+        )
         steam_app_factory(name="Fake game", appid=10)
 
         # Clear the 'config.vdf' and remove any tool mappings, emulating
@@ -357,17 +368,18 @@ def test_find_steam_default_proton(
             })
         )
 
-        proton_app = find_steam_compat_tool_app(
+        found_proton_app = find_steam_compat_tool_app(
             steam_path=steam_dir,
-            steam_apps=[default_proton],
+            steam_apps=[proton_app],
             appid=10
         )
 
-        assert proton_app.name == "Proton 4.20"
+        assert found_proton_app.name == proton_app.name
 
         assert any(
             record for record in caplog.records
-            if "Using stable version of Proton" in record.message
+            if "Using Proton Experimental or stable Proton as fallback"
+            in record.message
         )
 
     @pytest.mark.usefixtures("info_logging")