fix(models): never use Fable as the default model for new tasks (#3298)

Author: tatoalo

packages/shared/src/index.ts

@@ -127,6 +127,7 @@ export {
   mentionsToPlainText,
   splitMentionSegments,
 } from "./mentions";
+export { defaultEligibleModel } from "./models";
 export {
   getOauthClientIdFromRegion,
   OAUTH_SCOPE_VERSION,

packages/shared/src/models.test.ts

@@ -0,0 +1,22 @@
+import { describe, expect, it } from "vitest";
+import { defaultEligibleModel } from "./models";
+
+describe("defaultEligibleModel", () => {
+  it.each([
+    ["claude-fable-5", undefined],
+    ["anthropic/claude-fable-5", undefined],
+    ["CLAUDE-FABLE-5", undefined],
+    ["claude-fable-5-20260601", undefined],
+    ["claude-opus-4-8", "claude-opus-4-8"],
+    ["claude-sonnet-5", "claude-sonnet-5"],
+    ["gpt-5.5", "gpt-5.5"],
+    ["@cf/zai-org/glm-5.2", "@cf/zai-org/glm-5.2"],
+    // Contains "fable" as a substring but is not the Fable family.
+    ["gpt-affable-1", "gpt-affable-1"],
+    ["", undefined],
+    [null, undefined],
+    [undefined, undefined],
+  ] as const)("%s -> %s", (modelId, expected) => {
+    expect(defaultEligibleModel(modelId)).toBe(expected);
+  });
+});

packages/shared/src/models.ts

@@ -0,0 +1,11 @@
+/**
+ * Returns the id unless it's a premium family (currently Fable) that must be
+ * an explicit per-task pick and never the implicit default for a new task.
+ */
+export function defaultEligibleModel(
+  modelId: string | null | undefined,
+): string | undefined {
+  if (!modelId) return undefined;
+  const family = modelId.toLowerCase().split("/").pop() ?? "";
+  return family.startsWith("claude-fable") ? undefined : modelId;
+}

packages/ui/src/features/home/hooks/useRunWorkstreamAction.ts

@@ -9,6 +9,7 @@ import type { TaskService } from "@posthog/core/task-detail/taskService";
 import { useService } from "@posthog/di/react";
 import {
   ANALYTICS_EVENTS,
+  defaultEligibleModel,
   getCloudUrlFromRegion,
   type TaskCreationInput,
 } from "@posthog/shared";
@@ -103,7 +104,8 @@ export function useRunWorkstreamAction(): RunWorkstreamAction {
           // honoured if the gateway still offers it (the resolver validates it),
           // so a stale persisted/pinned id can't reach the run and 403.
           const adapter = action.adapter ?? lastUsedAdapter;
-          const preferredModel = action.model ?? lastUsedModel ?? undefined;
+          const preferredModel =
+            action.model ?? defaultEligibleModel(lastUsedModel);
           let model = preferredModel;
           if (cloudRegion) {
             // The resolver swallows transient failures and returns undefined; fall

packages/ui/src/features/inbox/components/ConfigureAgentsSection.tsx

@@ -11,7 +11,11 @@ import {
 } from "@posthog/core/task-detail/taskService";
 import { useService } from "@posthog/di/react";
 import { Button } from "@posthog/quill";
-import { ANALYTICS_EVENTS, getCloudUrlFromRegion } from "@posthog/shared";
+import {
+  ANALYTICS_EVENTS,
+  defaultEligibleModel,
+  getCloudUrlFromRegion,
+} from "@posthog/shared";
 import { SELF_DRIVING_SETUP_TASK_FLAG } from "@posthog/shared/constants";
 import { useTrackAgentsViewed } from "@posthog/ui/features/agents/hooks/useTrackAgentsViewed";
 import { useAuthStateValue } from "@posthog/ui/features/auth/store";
@@ -302,17 +306,18 @@ function SetupTaskSection() {
       const settings = useSettingsStore.getState();
       const adapter = settings.lastUsedAdapter ?? "claude";
       const apiHost = getCloudUrlFromRegion(cloudRegion);
+      const preferredModel = defaultEligibleModel(settings.lastUsedModel);
       const resolvedModel = await resolveDefaultModel(
         queryClient,
         apiHost,
         adapter,
         modelResolver,
-        settings.lastUsedModel,
+        preferredModel,
       );
       // The resolver returns undefined on a transient failure; fall back to the
       // persisted id so a gateway outage degrades gracefully rather than blocking
       // setup for a user whose persisted model was valid.
-      const model = resolvedModel ?? settings.lastUsedModel;
+      const model = resolvedModel ?? preferredModel;
 
       if (!model) {
         toast.dismiss(toastId);

packages/ui/src/features/inbox/hooks/useInboxCloudTaskRunner.ts

@@ -13,6 +13,7 @@ import { useService } from "@posthog/di/react";
 import {
   type Adapter,
   ANALYTICS_EVENTS,
+  defaultEligibleModel,
   getCloudUrlFromRegion,
 } from "@posthog/shared";
 import { useAuthStateValue } from "@posthog/ui/features/auth/store";
@@ -170,16 +171,17 @@ export function useInboxCloudTaskRunner({
     // resolver keeps it only if the gateway still offers it, otherwise it falls
     // back to the server default. A stale id (e.g. one later de-listed for the
     // org) would otherwise be sent here and fail the run with a gateway 403.
+    const preferredModel = defaultEligibleModel(settings.lastUsedModel);
     const resolvedModel = await resolveDefaultModel(
       queryClient,
       apiHost,
       adapter,
       modelResolver,
-      settings.lastUsedModel,
+      preferredModel,
     );
     // The resolver returns undefined on a transient failure; fall back to the
     // persisted id so a gateway outage degrades gracefully rather than blocking.
-    const model = resolvedModel ?? settings.lastUsedModel;
+    const model = resolvedModel ?? preferredModel;
 
     if (!model) {
       toast.dismiss(toastId);

packages/ui/src/features/task-detail/hooks/usePreviewConfig.ts

@@ -8,6 +8,7 @@ import {
 import { useHostTRPCClient } from "@posthog/host-router/react";
 import {
   type Adapter,
+  defaultEligibleModel,
   GLM_MODEL_FLAG,
   getCloudUrlFromRegion,
 } from "@posthog/shared";
@@ -106,22 +107,23 @@ export function usePreviewConfig(adapter: Adapter): PreviewConfigResult {
         );
 
         // The server always returns its default model as the current value, so
-        // without this the user's last pick (e.g. fable) is lost on every
-        // refetch/remount. Restore it through applyConfigChange so the dependent
-        // effort options are recomputed for the restored model.
+        // without this the user's last (default-eligible) pick is lost on every
+        // refetch/remount. Restore it through applyConfigChange so the
+        // dependent effort options are recomputed for the restored model.
         const modelOpt = getOptionByCategory(initial, "model");
+        const restorableModel = defaultEligibleModel(lastUsedModel);
         if (
-          lastUsedModel &&
+          restorableModel &&
           modelOpt?.type === "select" &&
-          modelOpt.currentValue !== lastUsedModel &&
-          flattenConfigValues(modelOpt).includes(lastUsedModel)
+          modelOpt.currentValue !== restorableModel &&
+          flattenConfigValues(modelOpt).includes(restorableModel)
         ) {
           initial = applyConfigChange(initial, {
             adapter,
             configId: modelOpt.id,
-            value: lastUsedModel,
+            value: restorableModel,
             effortOptions:
-              getReasoningEffortOptions(adapter, lastUsedModel) ?? undefined,
+              getReasoningEffortOptions(adapter, restorableModel) ?? undefined,
             settings: {
               defaultInitialTaskMode: "",
               lastUsedInitialTaskMode: undefined,