diff --git a/.env.example b/.env.example
index a6740f7a7d..c0b913d746 100644
--- a/.env.example
+++ b/.env.example
@@ -206,6 +206,9 @@ RUST_LOG=buzz_relay=debug,buzz_datastore=info,buzz_db=debug,buzz_auth=debug,buzz
 # app launch while keeping the current identity and relay data.
 # VITE_BUZZ_FORCE_FRESH_ONBOARDING=true
 
+# Makes the default-off Bestie experiment available in eligible builds.
+# VITE_BUZZ_BESTIE=1
+
 # ── Subscription & filtering ─────────────────────────────────────────────────
 # Subscribe mode: "mentions" (default), "all", or "config" (rule-based).
 # BUZZ_ACP_SUBSCRIBE=mentions
diff --git a/desktop/src/features/settings/ui/ExperimentalFeaturesCard.tsx b/desktop/src/features/settings/ui/ExperimentalFeaturesCard.tsx
index 50052c014f..db08e8b702 100644
--- a/desktop/src/features/settings/ui/ExperimentalFeaturesCard.tsx
+++ b/desktop/src/features/settings/ui/ExperimentalFeaturesCard.tsx
@@ -1,5 +1,5 @@
 import { setAgentManagedProfiles } from "@/shared/api/tauri";
-import { desktopFeatures, useFeatureToggle } from "@/shared/features";
+import { availableDesktopFeatures, useFeatureToggle } from "@/shared/features";
 import type { FeatureDefinition } from "@/shared/features";
 import { Switch } from "@/shared/ui/switch";
 import { SettingsOptionGroup, SettingsOptionRow } from "./SettingsOptionGroup";
@@ -42,7 +42,7 @@ function FeatureRow({ feature }: { feature: FeatureDefinition }) {
 export function ExperimentalFeaturesCard() {
   // Manifest is preview-only by definition; every desktop entry is a preview
   // feature.
-  const previewFeatures = desktopFeatures;
+  const previewFeatures = availableDesktopFeatures;
 
   return (
     <section className="min-w-0" data-testid="settings-experimental">
diff --git a/desktop/src/shared/features/buildAvailability.test.mjs b/desktop/src/shared/features/buildAvailability.test.mjs
new file mode 100644
index 0000000000..046615876c
--- /dev/null
+++ b/desktop/src/shared/features/buildAvailability.test.mjs
@@ -0,0 +1,43 @@
+import assert from "node:assert/strict";
+import { describe, it } from "node:test";
+
+import {
+  filterBuildAvailableFeatures,
+  isFeatureBuildAvailable,
+} from "./buildAvailability.ts";
+
+const publicFeature = {
+  id: "projects",
+  name: "Projects",
+  description: "",
+};
+const bestie = {
+  id: "bestie",
+  name: "Bestie",
+  description: "",
+  requiredBuildFlag: "bestie",
+};
+
+describe("feature build availability", () => {
+  it("keeps features without a required build flag available", () => {
+    assert.equal(
+      isFeatureBuildAvailable(publicFeature, { bestie: false }),
+      true,
+    );
+  });
+
+  it("does not allow a required feature into an ineligible build", () => {
+    assert.equal(isFeatureBuildAvailable(bestie, { bestie: false }), false);
+  });
+
+  it("shows a required feature when its build opts in", () => {
+    assert.equal(isFeatureBuildAvailable(bestie, { bestie: true }), true);
+  });
+
+  it("filters unavailable features out of the experiment picker", () => {
+    assert.deepEqual(
+      filterBuildAvailableFeatures([publicFeature, bestie], { bestie: false }),
+      [publicFeature],
+    );
+  });
+});
diff --git a/desktop/src/shared/features/buildAvailability.ts b/desktop/src/shared/features/buildAvailability.ts
new file mode 100644
index 0000000000..3c6b3bcb44
--- /dev/null
+++ b/desktop/src/shared/features/buildAvailability.ts
@@ -0,0 +1,24 @@
+import type { FeatureBuildFlag, FeatureDefinition } from "./types";
+
+export type FeatureBuildFlags = Record<FeatureBuildFlag, boolean>;
+
+/**
+ * Build capabilities are deliberately separate from user preference. A local
+ * storage override cannot make an unavailable feature visible.
+ */
+export function isFeatureBuildAvailable(
+  feature: FeatureDefinition,
+  buildFlags: FeatureBuildFlags,
+): boolean {
+  const requiredFlag = feature.requiredBuildFlag;
+  return requiredFlag ? buildFlags[requiredFlag] : true;
+}
+
+export function filterBuildAvailableFeatures(
+  features: FeatureDefinition[],
+  buildFlags: FeatureBuildFlags,
+): FeatureDefinition[] {
+  return features.filter((feature) =>
+    isFeatureBuildAvailable(feature, buildFlags),
+  );
+}
diff --git a/desktop/src/shared/features/index.ts b/desktop/src/shared/features/index.ts
index 9f6db1b156..f476cbd14d 100644
--- a/desktop/src/shared/features/index.ts
+++ b/desktop/src/shared/features/index.ts
@@ -1,8 +1,10 @@
 export { FeatureGate } from "./FeatureGate";
 export { allFeatures, desktopFeatures, getFeature, manifest } from "./manifest";
+export { availableDesktopFeatures } from "./runtimeBuildAvailability";
 export { getOverrides, setOverride, clearOverride } from "./store";
 export type {
   FeatureDefinition,
+  FeatureBuildFlag,
   FeaturesManifest,
   FeaturePlatform,
 } from "./types";
diff --git a/desktop/src/shared/features/manifest.ts b/desktop/src/shared/features/manifest.ts
index 1e6f48ae01..8ebdae5c06 100644
--- a/desktop/src/shared/features/manifest.ts
+++ b/desktop/src/shared/features/manifest.ts
@@ -8,6 +8,7 @@ import type { FeatureDefinition, FeaturesManifest } from "./types";
 // The app keeps working; gated UI stays hidden; nothing accidentally leaks.
 
 const FeaturePlatformSchema = z.enum(["desktop", "mobile"]);
+const FeatureBuildFlagSchema = z.enum(["bestie"]);
 
 const FeatureDefinitionSchema = z.object({
   id: z.string().min(1),
@@ -15,6 +16,7 @@ const FeatureDefinitionSchema = z.object({
   description: z.string(),
   defaultEnabled: z.boolean().optional(),
   platforms: z.array(FeaturePlatformSchema).optional(),
+  requiredBuildFlag: FeatureBuildFlagSchema.optional(),
 });
 
 const FeaturesManifestSchema = z.object({
diff --git a/desktop/src/shared/features/resolveEnabled.test.mjs b/desktop/src/shared/features/resolveEnabled.test.mjs
index d1639f6e4f..dc88aa8db7 100644
--- a/desktop/src/shared/features/resolveEnabled.test.mjs
+++ b/desktop/src/shared/features/resolveEnabled.test.mjs
@@ -30,4 +30,20 @@ describe("resolveEnabled (preview-only)", () => {
   it("ignores overrides for unrelated ids", () => {
     assert.equal(resolveEnabled("workflows", { pulse: true }), false);
   });
+
+  it("requires both build availability and a user opt-in", () => {
+    const cases = [
+      { buildAvailable: false, optedIn: false, expected: false },
+      { buildAvailable: false, optedIn: true, expected: false },
+      { buildAvailable: true, optedIn: false, expected: false },
+      { buildAvailable: true, optedIn: true, expected: true },
+    ];
+
+    for (const { buildAvailable, optedIn, expected } of cases) {
+      assert.equal(
+        resolveEnabled("bestie", { bestie: optedIn }, false, buildAvailable),
+        expected,
+      );
+    }
+  });
 });
diff --git a/desktop/src/shared/features/resolveEnabled.ts b/desktop/src/shared/features/resolveEnabled.ts
index e3c544ec50..3aee2738dd 100644
--- a/desktop/src/shared/features/resolveEnabled.ts
+++ b/desktop/src/shared/features/resolveEnabled.ts
@@ -14,6 +14,8 @@ export function resolveEnabled(
   featureId: string,
   overrides: Record<string, boolean>,
   defaultEnabled = false,
+  buildAvailable = true,
 ): boolean {
+  if (!buildAvailable) return false;
   return overrides[featureId] ?? defaultEnabled;
 }
diff --git a/desktop/src/shared/features/runtimeBuildAvailability.ts b/desktop/src/shared/features/runtimeBuildAvailability.ts
new file mode 100644
index 0000000000..d0966f7095
--- /dev/null
+++ b/desktop/src/shared/features/runtimeBuildAvailability.ts
@@ -0,0 +1,21 @@
+import {
+  filterBuildAvailableFeatures,
+  isFeatureBuildAvailable,
+  type FeatureBuildFlags,
+} from "./buildAvailability";
+import { desktopFeatures } from "./manifest";
+
+const runtimeBuildFlags: FeatureBuildFlags = {
+  bestie: import.meta.env.VITE_BUZZ_BESTIE === "1",
+};
+
+export function isFeatureAvailableInThisBuild(
+  feature: Parameters<typeof isFeatureBuildAvailable>[0],
+): boolean {
+  return isFeatureBuildAvailable(feature, runtimeBuildFlags);
+}
+
+export const availableDesktopFeatures = filterBuildAvailableFeatures(
+  desktopFeatures,
+  runtimeBuildFlags,
+);
diff --git a/desktop/src/shared/features/types.ts b/desktop/src/shared/features/types.ts
index c452cd54b4..4fdf07f2af 100644
--- a/desktop/src/shared/features/types.ts
+++ b/desktop/src/shared/features/types.ts
@@ -1,6 +1,9 @@
 /** Platforms a feature is available on */
 export type FeaturePlatform = "desktop" | "mobile";
 
+/** Build-time capabilities that can make a preview available to users. */
+export type FeatureBuildFlag = "bestie";
+
 /**
  * A single feature definition from the manifest.
  *
@@ -16,6 +19,8 @@ export interface FeatureDefinition {
   defaultEnabled?: boolean;
   /** If omitted, feature is available on all platforms */
   platforms?: FeaturePlatform[];
+  /** If present, the build must opt in before the user can see or enable it */
+  requiredBuildFlag?: FeatureBuildFlag;
 }
 
 /** The root manifest schema */
diff --git a/desktop/src/shared/features/useFeatureEnabled.ts b/desktop/src/shared/features/useFeatureEnabled.ts
index b0c9878d0b..356a362cca 100644
--- a/desktop/src/shared/features/useFeatureEnabled.ts
+++ b/desktop/src/shared/features/useFeatureEnabled.ts
@@ -1,6 +1,7 @@
 import { useSyncExternalStore, useCallback, useEffect } from "react";
 import { getFeature } from "./manifest";
 import { resolveEnabled } from "./resolveEnabled";
+import { isFeatureAvailableInThisBuild } from "./runtimeBuildAvailability";
 import { getOverrides, setOverride, OVERRIDES_KEY } from "./store";
 
 type Listener = () => void;
@@ -102,7 +103,12 @@ export function useFeatureEnabled(featureId: string): boolean {
     return true;
   }
 
-  return resolveEnabled(featureId, overrides, feature.defaultEnabled);
+  return resolveEnabled(
+    featureId,
+    overrides,
+    feature.defaultEnabled,
+    isFeatureAvailableInThisBuild(feature),
+  );
 }
 
 /**
diff --git a/preview-features.json b/preview-features.json
index 388f1c39b0..79460c3cad 100644
--- a/preview-features.json
+++ b/preview-features.json
@@ -30,6 +30,13 @@
       "name": "Agent-managed profiles",
       "description": "Let agents manage their own relay name and avatar instead of restoring the desktop copy",
       "platforms": ["desktop"]
+    },
+    {
+      "id": "bestie",
+      "name": "Bestie",
+      "description": "Try a personal agent that is always close at hand",
+      "platforms": ["desktop"],
+      "requiredBuildFlag": "bestie"
     }
   ]
 }