fix(server): text-generation fallback uses a configured custom model

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 +689,90 @@ 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 the thread default is a built-in slug",
+    () =>
+      Effect.gen(function* () {
+        const serverConfig = yield* ServerConfig.ServerConfig;
+        const fileSystem = yield* FileSystem.FileSystem;
+        const serverSettings = yield* ServerSettingsModule.ServerSettingsService;
+        // A built-in chat model must not replace the cheap text-generation slug.
+        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":"claude-opus-4-6","options":[{"id":"effort","value":"high"}]}}',
+        );
+
+        const settings = yield* serverSettings.getSettings;
+        const productModel =
+          DEFAULT_TEXT_GENERATION_MODEL_BY_PROVIDER[ProviderDriverKind.make("claudeAgent")];
+
+        assert.isDefined(productModel);
+        assert.deepEqual(
+          settings.textGenerationModelSelection,
+          createModelSelection(ProviderInstanceId.make("claudeAgent"), productModel),
+        );
+      }).pipe(Effect.provide(makeServerSettingsLayer())),
+  );
+
+  it.effect("uses a custom thread default when falling back for text generation", () =>
+    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","options":[{"id":"effort","value":"low"}]}}',
+      );
+
+      const settings = yield* serverSettings.getSettings;
+
+      assert.deepEqual(
+        settings.textGenerationModelSelection,
+        createModelSelection(ProviderInstanceId.make("claudeAgent"), "z-ai/glm-5.3-flash", [
+          { id: "effort", value: "low" },
+        ]),
+      );
+    }).pipe(Effect.provide(makeServerSettingsLayer())),
+  );
+
+  it.effect("uses a configured custom model when falling back for text generation", () =>
+    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"]}}}}',
+      );
+
+      const settings = yield* serverSettings.getSettings;
+
+      assert.equal(settings.textGenerationModelSelection.instanceId, "claudeAgent");
+      assert.equal(settings.textGenerationModelSelection.model, "z-ai/glm-5.3-flash");
+    }).pipe(Effect.provide(makeServerSettingsLayer())),
+  );
+
+  it.effect("uses a legacy custom model when falling back for text generation", () =>
+    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":{}}},"providers":{"claudeAgent":{"customModels":["z-ai/glm-5.3-flash"]}}}',
+      );
+
+      const settings = yield* serverSettings.getSettings;
+
+      assert.equal(settings.textGenerationModelSelection.instanceId, "claudeAgent");
+      assert.equal(settings.textGenerationModelSelection.model, "z-ai/glm-5.3-flash");
     }).pipe(Effect.provide(makeServerSettingsLayer())),
   );
 

apps/server/src/serverSettings.ts

@@ -49,6 +49,7 @@ import * as Stream from "effect/Stream";
 import * as SqlClient from "effect/unstable/sql/SqlClient";
 import { writeFileStringAtomically } from "./atomicWrite.ts";
 import * as ServerConfig from "./config.ts";
+import { createModelSelection, readCustomModelEntries } from "@t3tools/shared/model";
 import { type DeepPartial, deepMerge } from "@t3tools/shared/Struct";
 import { fromJsonStringPretty, fromLenientJson } from "@t3tools/shared/schemaJson";
 import {
@@ -338,6 +339,81 @@ function resolveTextGenerationProvider(settings: ServerSettings): ServerSettings
     : fallbackTextGenerationProvider(settings);
 }
 
+/**
+ * Custom-model slugs configured on this instance.
+ *
+ * An explicit `providerInstances[id].config.customModels` array wins, including
+ * an empty one. Otherwise the legacy `providers[driver].customModels` list is
+ * used for the default instance only, matching the Settings UI.
+ */
+function configuredCustomModelSlugs(
+  settings: ServerSettings,
+  driver: ProviderDriverKind,
+  instanceId: ProviderInstanceId,
+): ReadonlyArray<string> {
+  const instanceConfig = settings.providerInstances[instanceId]?.config;
+  if (
+    instanceConfig !== null &&
+    typeof instanceConfig === "object" &&
+    "customModels" in instanceConfig
+  ) {
+    const value = (instanceConfig as { customModels?: unknown }).customModels;
+    if (Array.isArray(value)) {
+      return readCustomModelEntries(value).map((entry) => entry.slug);
+    }
+  }
+  if (instanceId !== ProviderInstanceId.make(driver)) {
+    return [];
+  }
+  const legacy = Object.entries(settings.providers).find(([key]) => key === driver)?.[1];
+  return readCustomModelEntries(legacy?.customModels).map((entry) => entry.slug);
+}
+
+/**
+ * Model for one-shot text generation when the stored selection's instance is
+ * unusable. Commit, PR, branch, and title generation all read this value.
+ *
+ * The product slug (`DEFAULT_TEXT_GENERATION_MODEL_BY_PROVIDER`, Claude's
+ * `claude-haiku-4-5`) stays the fallback when this instance has no custom
+ * model, when that slug is itself configured, or when `defaultModelSelection`
+ * for this instance is a built-in chat model. Title, branch, and commit
+ * generation keep the cheap product default for users who only picked a chat
+ * model.
+ *
+ * The hardcoded slug is replaced only on the broken fallback: the instance
+ * has custom models that do not include the product slug, and it did not pin
+ * a built-in thread default. A matching custom `defaultModelSelection` wins,
+ * options included; otherwise the first custom slug is used.
+ */
+function fallbackTextGenerationModelSelection(
+  settings: ServerSettings,
+  driver: ProviderDriverKind,
+  instanceId: ProviderInstanceId,
+): ModelSelection {
+  const productModel =
+    DEFAULT_TEXT_GENERATION_MODEL_BY_PROVIDER[driver] ??
+    DEFAULT_MODEL_BY_PROVIDER[driver] ??
+    DEFAULT_TEXT_GENERATION_MODEL;
+  const productSelection = createModelSelection(instanceId, productModel);
+  const customModels = configuredCustomModelSlugs(settings, driver, instanceId);
+  if (customModels.length === 0 || customModels.includes(productModel)) {
+    return productSelection;
+  }
+
+  const threadDefault = settings.defaultModelSelection;
+  if (threadDefault?.instanceId === instanceId) {
+    if (!customModels.includes(threadDefault.model)) {
+      return productSelection;
+    }
+    return createModelSelection(instanceId, threadDefault.model, threadDefault.options);
+  }
+
+  const customModel = customModels[0];
+  return customModel === undefined
+    ? productSelection
+    : createModelSelection(instanceId, customModel);
+}
+
 function fallbackTextGenerationProvider(settings: ServerSettings): ServerSettings {
   // Same precedence as isModelSelectionProviderEnabled: an explicit provider
   // instance wins over the legacy providers map, which decodes to defaults
@@ -353,13 +429,11 @@ function fallbackTextGenerationProvider(settings: ServerSettings): ServerSetting
 
   return {
     ...settings,
-    textGenerationModelSelection: {
-      instanceId: ProviderInstanceId.make(fallback),
-      model:
-        DEFAULT_TEXT_GENERATION_MODEL_BY_PROVIDER[fallback] ??
-        DEFAULT_MODEL_BY_PROVIDER[fallback] ??
-        DEFAULT_TEXT_GENERATION_MODEL,
-    } satisfies ModelSelection,
+    textGenerationModelSelection: fallbackTextGenerationModelSelection(
+      settings,
+      fallback,
+      ProviderInstanceId.make(fallback),
+    ),
   };
 }