diff --git a/utils/casting/castLoad.ts b/utils/casting/castLoad.ts
index 704f19d38..2f61c33bf 100644
--- a/utils/casting/castLoad.ts
+++ b/utils/casting/castLoad.ts
@@ -17,6 +17,7 @@ import {
 } from "@/utils/casting/capabilities";
 import { isLoadFailedError } from "@/utils/casting/castErrors";
 import { buildCastMediaInfo } from "@/utils/casting/mediaInfo";
+import { resolveDefaultAudioIndex } from "@/utils/casting/selection";
 import { getStreamUrl } from "@/utils/jellyfin/media/getStreamUrl";
 
 export interface CastLoadOptions {
@@ -42,26 +43,6 @@ export interface CastLoadParams {
 
 export type CastLoadResult = { ok: true } | { ok: false; error: unknown };
 
-/**
- * Resolve the default audio stream index for an item.
- * Fixes the previous `audioStreamIndex = 0` default, which selected the video stream.
- */
-export const resolveDefaultAudioIndex = (
-  item: BaseItemDto,
-  mediaSourceId?: string,
-): number | undefined => {
-  const source = mediaSourceId
-    ? item.MediaSources?.find((s) => s.Id === mediaSourceId)
-    : item.MediaSources?.[0];
-  if (source?.DefaultAudioStreamIndex != null) {
-    return source.DefaultAudioStreamIndex;
-  }
-  const audio =
-    item.MediaStreams?.find((s) => s.Type === "Audio" && s.IsDefault) ??
-    item.MediaStreams?.find((s) => s.Type === "Audio");
-  return audio?.Index ?? undefined;
-};
-
 const attemptLoad = async (
   params: CastLoadParams,
   caps: Parameters<typeof buildChromecastProfile>[0],
diff --git a/utils/casting/selection.test.ts b/utils/casting/selection.test.ts
new file mode 100644
index 000000000..9d6886883
--- /dev/null
+++ b/utils/casting/selection.test.ts
@@ -0,0 +1,88 @@
+import { describe, expect, test } from "bun:test";
+import type { BaseItemDto } from "@jellyfin/sdk/lib/generated-client";
+import {
+  resolveDefaultAudioIndex,
+  resolveSelection,
+  selectionsEqual,
+} from "./selection";
+
+const item: BaseItemDto = {
+  Id: "item-1",
+  MediaSources: [
+    {
+      Id: "src-a",
+      DefaultAudioStreamIndex: 2,
+      DefaultSubtitleStreamIndex: 3,
+      MediaStreams: [
+        { Type: "Video", Index: 0 },
+        { Type: "Audio", Index: 1, IsDefault: false },
+        { Type: "Audio", Index: 2, IsDefault: true },
+        { Type: "Subtitle", Index: 3 },
+      ],
+    },
+    {
+      Id: "src-b",
+      MediaStreams: [
+        { Type: "Video", Index: 0 },
+        { Type: "Audio", Index: 1, IsDefault: false },
+      ],
+    },
+  ],
+};
+
+describe("resolveDefaultAudioIndex", () => {
+  test("uses the source's DefaultAudioStreamIndex when present", () => {
+    expect(resolveDefaultAudioIndex(item, "src-a")).toBe(2);
+  });
+
+  test("falls back to the first audio stream when no default flag", () => {
+    expect(resolveDefaultAudioIndex(item, "src-b")).toBe(1);
+  });
+});
+
+describe("resolveSelection", () => {
+  test("fills every field from server defaults of the first source", () => {
+    const sel = resolveSelection(item, {});
+    expect(sel.mediaSourceId).toBe("src-a");
+    expect(sel.audioStreamIndex).toBe(2);
+    expect(sel.subtitleStreamIndex).toBe(3);
+    expect(sel.maxBitrate).toBeUndefined();
+  });
+
+  test("a partial overrides defaults and keeps the rest", () => {
+    const sel = resolveSelection(item, {
+      audioStreamIndex: 1,
+      maxBitrate: 4_000_000,
+    });
+    expect(sel.audioStreamIndex).toBe(1);
+    expect(sel.maxBitrate).toBe(4_000_000);
+    expect(sel.subtitleStreamIndex).toBe(3);
+  });
+
+  test("switching version resolves that version's defaults", () => {
+    const sel = resolveSelection(item, { mediaSourceId: "src-b" });
+    expect(sel.mediaSourceId).toBe("src-b");
+    expect(sel.audioStreamIndex).toBe(1);
+    expect(sel.subtitleStreamIndex).toBe(-1);
+  });
+});
+
+describe("selectionsEqual", () => {
+  test("true for identical selections", () => {
+    const a = {
+      mediaSourceId: "s",
+      audioStreamIndex: 1,
+      subtitleStreamIndex: -1,
+    };
+    expect(selectionsEqual(a, { ...a })).toBe(true);
+  });
+
+  test("false when any field differs", () => {
+    const a = {
+      mediaSourceId: "s",
+      audioStreamIndex: 1,
+      subtitleStreamIndex: -1,
+    };
+    expect(selectionsEqual(a, { ...a, audioStreamIndex: 2 })).toBe(false);
+  });
+});
diff --git a/utils/casting/selection.ts b/utils/casting/selection.ts
new file mode 100644
index 000000000..a950ec95d
--- /dev/null
+++ b/utils/casting/selection.ts
@@ -0,0 +1,59 @@
+/**
+ * Cast selection resolution — pure helpers, no React Native imports, so they
+ * are unit-testable under `bun test`.
+ */
+
+import type { BaseItemDto } from "@jellyfin/sdk/lib/generated-client";
+import type { CastSelection } from "@/utils/casting/types";
+
+/**
+ * Resolve the default audio stream index for an item / media source.
+ * Prefers the source's `DefaultAudioStreamIndex`, then the first audio stream.
+ */
+export const resolveDefaultAudioIndex = (
+  item: BaseItemDto,
+  mediaSourceId?: string,
+): number | undefined => {
+  const source = mediaSourceId
+    ? item.MediaSources?.find((s) => s.Id === mediaSourceId)
+    : item.MediaSources?.[0];
+  if (source?.DefaultAudioStreamIndex != null) {
+    return source.DefaultAudioStreamIndex;
+  }
+  const streams = source?.MediaStreams ?? item.MediaStreams;
+  const audio =
+    streams?.find((s) => s.Type === "Audio" && s.IsDefault) ??
+    streams?.find((s) => s.Type === "Audio");
+  return audio?.Index ?? undefined;
+};
+
+/**
+ * Complete a partial selection with the item's server defaults.
+ * Used on first load, on episode change, and when switching version.
+ */
+export const resolveSelection = (
+  item: BaseItemDto,
+  partial: Partial<CastSelection>,
+): CastSelection => {
+  const mediaSourceId =
+    partial.mediaSourceId ?? item.MediaSources?.[0]?.Id ?? "";
+  const source = item.MediaSources?.find((s) => s.Id === mediaSourceId);
+
+  return {
+    mediaSourceId,
+    audioStreamIndex:
+      partial.audioStreamIndex ??
+      resolveDefaultAudioIndex(item, mediaSourceId) ??
+      -1,
+    subtitleStreamIndex:
+      partial.subtitleStreamIndex ?? source?.DefaultSubtitleStreamIndex ?? -1,
+    maxBitrate: partial.maxBitrate,
+  };
+};
+
+/** True when two selections are equivalent — used to reconcile optimistic state. */
+export const selectionsEqual = (a: CastSelection, b: CastSelection): boolean =>
+  a.mediaSourceId === b.mediaSourceId &&
+  a.audioStreamIndex === b.audioStreamIndex &&
+  a.subtitleStreamIndex === b.subtitleStreamIndex &&
+  a.maxBitrate === b.maxBitrate;
diff --git a/utils/casting/types.ts b/utils/casting/types.ts
index 0db139fa6..4f6e04794 100644
--- a/utils/casting/types.ts
+++ b/utils/casting/types.ts
@@ -70,3 +70,18 @@ export const DEFAULT_CAST_STATE: CastPlayerState = {
   volume: 0.5,
   isBuffering: false,
 };
+
+/**
+ * What is currently loaded on the cast — the single source of truth for
+ * audio / subtitle / quality / version selection.
+ */
+export interface CastSelection {
+  /** MediaSource (version) id. */
+  mediaSourceId: string;
+  /** Absolute MediaStream index of the audio track. */
+  audioStreamIndex: number;
+  /** Absolute MediaStream index of the subtitle track; -1 = subtitles off. */
+  subtitleStreamIndex: number;
+  /** Quality cap in bits/second; undefined = unconstrained. */
+  maxBitrate?: number;
+}