fix(server): use a custom model only when the Claude product model is unavailable

apps/server/src/serverSettings.test.ts

@@ -1,6 +1,7 @@
 import * as NodeServices from "@effect/platform-node/NodeServices";
 import {
   DEFAULT_SERVER_SETTINGS,
+  DEFAULT_TEXT_GENERATION_MODEL_BY_PROVIDER,
   ModelSelection,
   ProjectId,
   ProjectScript,
@@ -688,6 +689,30 @@ it.layer(NodeServices.layer)("server settings", (it) => {
       const settings = yield* serverSettings.getSettings;
 
       assert.equal(settings.textGenerationModelSelection.instanceId, "claudeAgent");
+      assert.equal(
+        settings.textGenerationModelSelection.model,
+        DEFAULT_TEXT_GENERATION_MODEL_BY_PROVIDER[ProviderDriverKind.make("claudeAgent")],
+      );
+    }).pipe(Effect.provide(makeServerSettingsLayer())),
+  );
+
+  it.effect("keeps the product text-generation model when a custom model is configured", () =>
+    Effect.gen(function* () {
+      const serverConfig = yield* ServerConfig.ServerConfig;
+      const fileSystem = yield* FileSystem.FileSystem;
+      const serverSettings = yield* ServerSettingsModule.ServerSettingsService;
+      yield* fileSystem.writeFileString(
+        serverConfig.settingsPath,
+        '{"providerInstances":{"codex":{"driver":"codex","enabled":false,"config":{}},"claudeAgent":{"driver":"claudeAgent","config":{"customModels":["z-ai/glm-5.3-flash"]}}},"defaultModelSelection":{"instanceId":"claudeAgent","model":"z-ai/glm-5.3-flash"}}',
+      );
+
+      const settings = yield* serverSettings.getSettings;
+
+      assert.equal(settings.textGenerationModelSelection.instanceId, "claudeAgent");
+      assert.equal(
+        settings.textGenerationModelSelection.model,
+        DEFAULT_TEXT_GENERATION_MODEL_BY_PROVIDER[ProviderDriverKind.make("claudeAgent")],
+      );
     }).pipe(Effect.provide(makeServerSettingsLayer())),
   );
 

apps/server/src/serverSettings.ts

@@ -338,6 +338,12 @@ function resolveTextGenerationProvider(settings: ServerSettings): ServerSettings
     : fallbackTextGenerationProvider(settings);
 }
 
+/**
+ * Pick an enabled provider when the stored text-generation selection cannot
+ * run. The model stays the product text-generation slug. A configured custom
+ * model is substituted only after that slug's one-shot attempt reports the
+ * model itself is unavailable.
+ */
 function fallbackTextGenerationProvider(settings: ServerSettings): ServerSettings {
   // Same precedence as isModelSelectionProviderEnabled: an explicit provider
   // instance wins over the legacy providers map, which decodes to defaults