diff --git a/include/libc_shim/vulkan_etc1_sky_transcoder.h b/include/libc_shim/vulkan_etc1_sky_transcoder.h
new file mode 100644
index 0000000..846705a
--- /dev/null
+++ b/include/libc_shim/vulkan_etc1_sky_transcoder.h
@@ -0,0 +1,17 @@
+#ifndef MOCKTAIL_LIBC_SHIM_VULKAN_ETC1_SKY_TRANSCODER_H_
+#define MOCKTAIL_LIBC_SHIM_VULKAN_ETC1_SKY_TRANSCODER_H_
+
+#include <cstdint>
+#include <vector>
+
+namespace libc_shim {
+
+// Converts one opaque ETC1 RGB block to an opaque BC1 RGB block.
+void ConvertEtc1RgbBlockToBc1(const uint8_t *etc1, uint8_t *bc1);
+
+bool TranscodeEtc1SkyTextureForVulkan(const char *path,
+                                      std::vector<unsigned char> *data);
+
+} // namespace libc_shim
+
+#endif // MOCKTAIL_LIBC_SHIM_VULKAN_ETC1_SKY_TRANSCODER_H_
diff --git a/src/libc_shim/vulkan_etc1_sky_transcoder.cc b/src/libc_shim/vulkan_etc1_sky_transcoder.cc
new file mode 100644
index 0000000..1867f13
--- /dev/null
+++ b/src/libc_shim/vulkan_etc1_sky_transcoder.cc
@@ -0,0 +1,294 @@
+#include "libc_shim/vulkan_etc1_sky_transcoder.h"
+
+#include <algorithm>
+#include <array>
+#include <cstddef>
+#include <cstdint>
+#include <cstdio>
+#include <cstring>
+#include <limits>
+#include <utility>
+#include <vector>
+
+namespace libc_shim {
+namespace {
+
+constexpr uint32_t kKtx1LittleEndian = 0x04030201;
+constexpr uint32_t kGlEtc1Rgb8Oes = 0x00008d64;
+constexpr uint32_t kGlCompressedRgbS3tcDxt1 = 0x000083f0;
+constexpr uint32_t kGlRgb = 0x00001907;
+constexpr size_t kMaxSkyTextureSize = 16U * 1024U * 1024U;
+constexpr std::array<uint8_t, 12> kKtx1Identifier = {
+    0xab, 0x4b, 0x54, 0x58, 0x20, 0x31, 0x31, 0xbb, 0x0d, 0x0a, 0x1a, 0x0a,
+};
+
+struct Rgb {
+  uint8_t r;
+  uint8_t g;
+  uint8_t b;
+};
+
+uint8_t ClampByte(int value) {
+  return static_cast<uint8_t>(std::max(0, std::min(255, value)));
+}
+
+Rgb ApplyModifier(const uint8_t color[3], int modifier) {
+  return {ClampByte(color[0] + modifier), ClampByte(color[1] + modifier),
+          ClampByte(color[2] + modifier)};
+}
+
+int SignExtend3(uint8_t value) {
+  return (value & 4) != 0 ? static_cast<int>(value) - 8 : value;
+}
+
+uint8_t Expand4(uint8_t value) {
+  return static_cast<uint8_t>((value << 4) | value);
+}
+
+uint8_t Expand5(int value) {
+  return static_cast<uint8_t>((value << 3) | (value >> 2));
+}
+
+// ETC decoder adapted from AssetStudio's Texture2DDecoderNative.
+// MIT License. Copyright (c) 2016 Radu and (c) 2016-2020 Perfare.
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to
+// deal in the Software without restriction, including without limitation the
+// rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
+// sell copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to inclusion of this notice. THE SOFTWARE IS
+// PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED.
+void DecodeEtc1RgbBlock(const uint8_t *data, std::array<Rgb, 16> *output) {
+  constexpr std::array<uint8_t, 16> kWriteOrder = {
+      0, 4, 8, 12, 1, 5, 9, 13, 2, 6, 10, 14, 3, 7, 11, 15,
+  };
+  constexpr uint8_t kModifier[8][2] = {
+      {2, 8},   {5, 17},  {9, 29},   {13, 42},
+      {18, 60}, {24, 80}, {33, 106}, {47, 183},
+  };
+  constexpr uint8_t kSubblock[2][16] = {
+      {0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1},
+      {0, 0, 1, 1, 0, 0, 1, 1, 0, 0, 1, 1, 0, 0, 1, 1},
+  };
+
+  uint8_t color[2][3] = {};
+  if ((data[3] & 2) != 0) {
+    for (size_t channel = 0; channel < 3; ++channel) {
+      const int base = data[channel] >> 3;
+      color[0][channel] = Expand5(base);
+      color[1][channel] = Expand5(base + SignExtend3(data[channel] & 7));
+    }
+  } else {
+    for (size_t channel = 0; channel < 3; ++channel) {
+      color[0][channel] = Expand4(data[channel] >> 4);
+      color[1][channel] = Expand4(data[channel] & 15);
+    }
+  }
+
+  uint16_t low_indices = static_cast<uint16_t>((data[6] << 8) | data[7]);
+  uint16_t high_indices = static_cast<uint16_t>((data[4] << 8) | data[5]);
+  const uint8_t code[2] = {static_cast<uint8_t>(data[3] >> 5),
+                           static_cast<uint8_t>((data[3] >> 2) & 7)};
+  const uint8_t *subblock = kSubblock[data[3] & 1];
+  for (size_t pixel = 0; pixel < 16; ++pixel) {
+    const uint8_t side = subblock[pixel];
+    const uint8_t modifier = kModifier[code[side]][low_indices & 1];
+    (*output)[kWriteOrder[pixel]] = ApplyModifier(
+        color[side], (high_indices & 1) != 0 ? -modifier : modifier);
+    low_indices >>= 1;
+    high_indices >>= 1;
+  }
+}
+
+uint16_t PackRgb565(const Rgb &color) {
+  return static_cast<uint16_t>(((color.r >> 3) << 11) | ((color.g >> 2) << 5) |
+                               (color.b >> 3));
+}
+
+Rgb UnpackRgb565(uint16_t color) {
+  const uint8_t r = static_cast<uint8_t>((color >> 11) & 31);
+  const uint8_t g = static_cast<uint8_t>((color >> 5) & 63);
+  const uint8_t b = static_cast<uint8_t>(color & 31);
+  return {static_cast<uint8_t>((r << 3) | (r >> 2)),
+          static_cast<uint8_t>((g << 2) | (g >> 4)),
+          static_cast<uint8_t>((b << 3) | (b >> 2))};
+}
+
+uint32_t ColorDistance(const Rgb &lhs, const Rgb &rhs) {
+  const int dr = lhs.r - rhs.r;
+  const int dg = lhs.g - rhs.g;
+  const int db = lhs.b - rhs.b;
+  return static_cast<uint32_t>(dr * dr + dg * dg + db * db);
+}
+
+uint32_t ReadLe32(const uint8_t *bytes) {
+  return static_cast<uint32_t>(bytes[0]) |
+         (static_cast<uint32_t>(bytes[1]) << 8) |
+         (static_cast<uint32_t>(bytes[2]) << 16) |
+         (static_cast<uint32_t>(bytes[3]) << 24);
+}
+
+void WriteLe32(uint8_t *bytes, uint32_t value) {
+  bytes[0] = static_cast<uint8_t>(value);
+  bytes[1] = static_cast<uint8_t>(value >> 8);
+  bytes[2] = static_cast<uint8_t>(value >> 16);
+  bytes[3] = static_cast<uint8_t>(value >> 24);
+}
+
+bool IsBundledSkyTexture(const char *path) {
+  constexpr char kSkyDirectory[] = "/assets/android/textures/sky/";
+  if (path == nullptr || std::strstr(path, kSkyDirectory) == nullptr) {
+    return false;
+  }
+  const size_t length = std::strlen(path);
+  return length >= 4 && std::memcmp(path + length - 4, ".tex", 4) == 0;
+}
+
+bool TranscodeKtx1Etc1(std::vector<uint8_t> *file) {
+  constexpr size_t kHeaderSize = 64;
+  if (file->size() < kHeaderSize ||
+      std::memcmp(file->data(), kKtx1Identifier.data(),
+                  kKtx1Identifier.size()) != 0 ||
+      ReadLe32(file->data() + 12) != kKtx1LittleEndian ||
+      ReadLe32(file->data() + 16) != 0 || ReadLe32(file->data() + 20) != 1 ||
+      ReadLe32(file->data() + 24) != 0 ||
+      ReadLe32(file->data() + 28) != kGlEtc1Rgb8Oes ||
+      ReadLe32(file->data() + 32) != kGlRgb) {
+    return false;
+  }
+
+  const uint32_t width = ReadLe32(file->data() + 36);
+  const uint32_t height = ReadLe32(file->data() + 40);
+  const uint32_t depth = ReadLe32(file->data() + 44);
+  const uint32_t array_elements = ReadLe32(file->data() + 48);
+  const uint32_t faces = ReadLe32(file->data() + 52);
+  const uint32_t mip_levels = ReadLe32(file->data() + 56);
+  const uint32_t key_value_bytes = ReadLe32(file->data() + 60);
+  if (width == 0 || height == 0 || depth != 0 || array_elements != 0 ||
+      faces != 1 || mip_levels == 0 || mip_levels > 32 ||
+      key_value_bytes > file->size() - kHeaderSize) {
+    return false;
+  }
+
+  size_t offset = kHeaderSize + key_value_bytes;
+  uint32_t mip_width = width;
+  uint32_t mip_height = height;
+  for (uint32_t level = 0; level < mip_levels; ++level) {
+    if (offset > file->size() || file->size() - offset < 4) {
+      return false;
+    }
+    const uint32_t image_size = ReadLe32(file->data() + offset);
+    offset += 4;
+    const uint64_t block_count = static_cast<uint64_t>((mip_width + 3) / 4) *
+                                 static_cast<uint64_t>((mip_height + 3) / 4);
+    const uint64_t expected_size = block_count * 8;
+    if (image_size != expected_size || image_size > file->size() - offset) {
+      return false;
+    }
+    for (size_t block = 0; block < image_size / 8; ++block) {
+      std::array<uint8_t, 8> converted{};
+      ConvertEtc1RgbBlockToBc1(file->data() + offset + block * 8,
+                               converted.data());
+      std::memcpy(file->data() + offset + block * 8, converted.data(),
+                  converted.size());
+    }
+    offset += image_size;
+    const size_t padding = (4 - image_size % 4) % 4;
+    if (padding > file->size() - offset) {
+      return false;
+    }
+    offset += padding;
+    mip_width = std::max(1U, mip_width / 2);
+    mip_height = std::max(1U, mip_height / 2);
+  }
+
+  WriteLe32(file->data() + 28, kGlCompressedRgbS3tcDxt1);
+  return true;
+}
+
+} // namespace
+
+void ConvertEtc1RgbBlockToBc1(const uint8_t *etc1, uint8_t *bc1) {
+  std::array<Rgb, 16> pixels{};
+  DecodeEtc1RgbBlock(etc1, &pixels);
+
+  Rgb minimum = pixels[0];
+  Rgb maximum = pixels[0];
+  for (const Rgb &pixel : pixels) {
+    minimum.r = std::min(minimum.r, pixel.r);
+    minimum.g = std::min(minimum.g, pixel.g);
+    minimum.b = std::min(minimum.b, pixel.b);
+    maximum.r = std::max(maximum.r, pixel.r);
+    maximum.g = std::max(maximum.g, pixel.g);
+    maximum.b = std::max(maximum.b, pixel.b);
+  }
+  const uint8_t inset_r = static_cast<uint8_t>((maximum.r - minimum.r) / 16);
+  const uint8_t inset_g = static_cast<uint8_t>((maximum.g - minimum.g) / 16);
+  const uint8_t inset_b = static_cast<uint8_t>((maximum.b - minimum.b) / 16);
+  minimum = {static_cast<uint8_t>(minimum.r + inset_r),
+             static_cast<uint8_t>(minimum.g + inset_g),
+             static_cast<uint8_t>(minimum.b + inset_b)};
+  maximum = {static_cast<uint8_t>(maximum.r - inset_r),
+             static_cast<uint8_t>(maximum.g - inset_g),
+             static_cast<uint8_t>(maximum.b - inset_b)};
+
+  uint16_t endpoint0 = PackRgb565(maximum);
+  uint16_t endpoint1 = PackRgb565(minimum);
+  if (endpoint0 < endpoint1) {
+    std::swap(endpoint0, endpoint1);
+  }
+  if (endpoint0 == endpoint1) {
+    if (endpoint0 < std::numeric_limits<uint16_t>::max()) {
+      ++endpoint0;
+    } else {
+      --endpoint1;
+    }
+  }
+  const Rgb color0 = UnpackRgb565(endpoint0);
+  const Rgb color1 = UnpackRgb565(endpoint1);
+  const std::array<Rgb, 4> palette = {
+      color0,
+      color1,
+      Rgb{static_cast<uint8_t>((2 * color0.r + color1.r) / 3),
+          static_cast<uint8_t>((2 * color0.g + color1.g) / 3),
+          static_cast<uint8_t>((2 * color0.b + color1.b) / 3)},
+      Rgb{static_cast<uint8_t>((color0.r + 2 * color1.r) / 3),
+          static_cast<uint8_t>((color0.g + 2 * color1.g) / 3),
+          static_cast<uint8_t>((color0.b + 2 * color1.b) / 3)},
+  };
+  uint32_t indices = 0;
+  for (size_t pixel_index = 0; pixel_index < pixels.size(); ++pixel_index) {
+    uint32_t best_distance = std::numeric_limits<uint32_t>::max();
+    uint32_t best_index = 0;
+    for (uint32_t palette_index = 0; palette_index < palette.size();
+         ++palette_index) {
+      const uint32_t distance =
+          ColorDistance(pixels[pixel_index], palette[palette_index]);
+      if (distance < best_distance) {
+        best_distance = distance;
+        best_index = palette_index;
+      }
+    }
+    indices |= best_index << (pixel_index * 2);
+  }
+  bc1[0] = static_cast<uint8_t>(endpoint0);
+  bc1[1] = static_cast<uint8_t>(endpoint0 >> 8);
+  bc1[2] = static_cast<uint8_t>(endpoint1);
+  bc1[3] = static_cast<uint8_t>(endpoint1 >> 8);
+  WriteLe32(bc1 + 4, indices);
+}
+
+bool TranscodeEtc1SkyTextureForVulkan(const char *path,
+                                      std::vector<unsigned char> *data) {
+  if (!IsBundledSkyTexture(path) || data == nullptr ||
+      data->size() > kMaxSkyTextureSize || !TranscodeKtx1Etc1(data)) {
+    return false;
+  }
+  std::fprintf(stderr,
+               "  [asset] Transcoded bundled ETC1 sky texture to BC1 for "
+               "Vulkan: %s\n",
+               path);
+  return true;
+}
+
+} // namespace libc_shim
diff --git a/stubs/CMakeLists.txt b/stubs/CMakeLists.txt
index e813b43..284e7d2 100644
--- a/stubs/CMakeLists.txt
+++ b/stubs/CMakeLists.txt
@@ -38,8 +38,12 @@ add_library(stub_sys_c SHARED empty.cc)
 configure_stub(stub_sys_c "libc.so")
 target_link_options(stub_sys_c PRIVATE "LINKER:--no-as-needed")
 
-add_library(stub_android SHARED libandroid_stub.cc)
+add_library(stub_android SHARED
+  libandroid_stub.cc
+  ${CMAKE_SOURCE_DIR}/src/libc_shim/vulkan_etc1_sky_transcoder.cc
+)
 configure_stub(stub_android "libandroid.so")
+target_include_directories(stub_android PUBLIC ${CMAKE_SOURCE_DIR}/include)
 find_package(PkgConfig REQUIRED)
 pkg_check_modules(MINIZIP REQUIRED IMPORTED_TARGET minizip)
 target_link_libraries(stub_android PRIVATE PkgConfig::MINIZIP)
diff --git a/stubs/libandroid_stub.cc b/stubs/libandroid_stub.cc
index da5dfd0..1081109 100644
--- a/stubs/libandroid_stub.cc
+++ b/stubs/libandroid_stub.cc
@@ -4,6 +4,8 @@
 // Symbols sourced from Android NDK r28 android/native_activity.h,
 // android/asset_manager.h, android/looper.h, android/configuration.h.
 
+#include "libc_shim/vulkan_etc1_sky_transcoder.h"
+
 #include <dlfcn.h>
 #include <fcntl.h>
 #include <minizip/unzip.h>
@@ -269,6 +271,9 @@ AAsset* AAssetManager_open(AAssetManager* mgr, const char* filename,
     delete asset;
     return nullptr;
   }
+  if (MocktailUsesDirectVulkan()) {
+    libc_shim::TranscodeEtc1SkyTextureForVulkan(path.c_str(), &asset->data);
+  }
   if (AssetTraceEnabled()) {
     std::fprintf(stderr, "[asset] open %s -> %s (%zu bytes)\n", filename,
                  path.c_str(), asset->data.size());
diff --git a/tests/asset_manager_test.cc b/tests/asset_manager_test.cc
index 8187c60..28cfc4a 100644
--- a/tests/asset_manager_test.cc
+++ b/tests/asset_manager_test.cc
@@ -1,3 +1,6 @@
+#include "libc_shim/vulkan_etc1_sky_transcoder.h"
+
+#include <array>
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
@@ -61,6 +64,17 @@ TEST_F(AssetManagerTest, FromJavaReturnsSingletonManager) {
   EXPECT_NE(AAssetManager_fromJava(nullptr, nullptr), nullptr);
 }
 
+TEST_F(AssetManagerTest, ConvertsSolidEtc1RgbBlockToOpaqueBc1) {
+  const std::array<uint8_t, 8> etc1 = {0x55, 0x77, 0x99, 0, 0, 0, 0, 0};
+  std::array<uint8_t, 8> bc1{};
+
+  libc_shim::ConvertEtc1RgbBlockToBc1(etc1.data(), bc1.data());
+
+  const uint16_t endpoint0 = static_cast<uint16_t>(bc1[0] | (bc1[1] << 8));
+  const uint16_t endpoint1 = static_cast<uint16_t>(bc1[2] | (bc1[3] << 8));
+  EXPECT_GT(endpoint0, endpoint1);
+}
+
 TEST_F(AssetManagerTest, OpenReadSeekAndBufferUseConfiguredAssetRoot) {
   WriteAsset("config.json", "mocktail-assets");