diff --git a/src/libANGLE/renderer/vulkan/spv_utils.cpp b/src/libANGLE/renderer/vulkan/spv_utils.cpp
index 49bf7dd571..42cd2442f8 100644
--- a/src/libANGLE/renderer/vulkan/spv_utils.cpp
+++ b/src/libANGLE/renderer/vulkan/spv_utils.cpp
@@ -6164,6 +6164,73 @@ angle::Result SpvTransformSpirvCode(const SpvTransformOptions &options,
         ASSERT(spirv::Validate(*spirvBlobOut));
     }
 
+    // ---------------------------------------------------------------------------------
+    // POC diagnostic instrumentation (not part of production code).
+    //
+    // In release builds the ASSERT(spirv::Validate(...)) above is a no-op, so the
+    // VUID-violating SPIR-V (two Input variables decorated BuiltIn FragCoord) would be
+    // handed straight to vkCreateShaderModule on the production driver. To make the bug
+    // observable on a Linux release build we unconditionally count BuiltIn FragCoord
+    // decorations in the transformed module and abort with a diagnostic if more than
+    // one is found. This does NOT change whether the bug fires; it only makes it visible.
+    // ---------------------------------------------------------------------------------
+    if (options.ditherControl != 0)
+    {
+        size_t off            = spirv::kHeaderIndexInstructions;
+        int fragCoordBuiltIns = 0;
+        uint32_t fragCoordIds[8] = {};
+        while (off < spirvBlobOut->size())
+        {
+            const uint32_t word    = (*spirvBlobOut)[off];
+            const uint32_t wc      = word >> 16;
+            const uint32_t opcode  = word & 0xFFFFu;
+            if (wc == 0)
+            {
+                break;
+            }
+            // OpDecorate <id> BuiltIn FragCoord  -> [opcode=71 wc=4] <target> 11 15
+            if (opcode == spv::OpDecorate && wc == 4 &&
+                (*spirvBlobOut)[off + 2] == spv::DecorationBuiltIn &&
+                (*spirvBlobOut)[off + 3] == spv::BuiltInFragCoord)
+            {
+                if (fragCoordBuiltIns < 8)
+                {
+                    fragCoordIds[fragCoordBuiltIns] = (*spirvBlobOut)[off + 1];
+                }
+                ++fragCoordBuiltIns;
+            }
+            off += wc;
+        }
+        if (fragCoordBuiltIns > 1)
+        {
+            fprintf(stderr,
+                    "\n========================================================================\n"
+                    "POC: SpvTransformSpirvCode emitted %d 'OpDecorate BuiltIn FragCoord'\n"
+                    "decorations on distinct Input variables. This violates\n"
+                    "VUID-StandaloneSpirv-OpEntryPoint-08721. In production this blob would\n"
+                    "be passed directly to vkCreateShaderModule on the unsandboxed Android\n"
+                    "GPU process.\n"
+                    "------------------------------------------------------------------------\n"
+                    "    OpDecorate %%%u BuiltIn FragCoord  (reserved kIdFragCoord=%u)\n"
+                    "    OpDecorate %%%u BuiltIn FragCoord  (translator-emitted generic id)\n"
+                    "------------------------------------------------------------------------\n"
+                    "Full disassembly of the transformed module follows:\n"
+                    "========================================================================\n",
+                    fragCoordBuiltIns, fragCoordIds[0],
+                    static_cast<uint32_t>(sh::vk::spirv::kIdFragCoord), fragCoordIds[1]);
+            // Dump the transformed binary so it can be inspected with spirv-dis / spirv-val.
+            if (FILE *f = fopen("/tmp/poc-transformed.spv", "wb"))
+            {
+                fwrite(spirvBlobOut->data(), sizeof(uint32_t), spirvBlobOut->size(), f);
+                fclose(f);
+                fprintf(stderr, "Transformed SPIR-V dumped to /tmp/poc-transformed.spv\n");
+            }
+            spirv::Print(*spirvBlobOut);
+            fflush(stderr);
+            abort();
+        }
+    }
+
     return angle::Result::Continue;
 }
 }  // namespace rx
diff --git a/src/tests/gl_tests/SixteenBppTextureTest.cpp b/src/tests/gl_tests/SixteenBppTextureTest.cpp
index 71a8baca57..d84c4988a3 100644
--- a/src/tests/gl_tests/SixteenBppTextureTest.cpp
+++ b/src/tests/gl_tests/SixteenBppTextureTest.cpp
@@ -933,12 +933,61 @@ TEST_P(SixteenBppTextureDitheringTestES3, SpvEmulation)
     spvEmulationTest();
 }
 
+// Regression test: a fragment shader that references gl_PointCoord lexically
+// before gl_FragCoord must not cause the dither-emulation transform to emit a
+// second BuiltIn FragCoord variable.
+TEST_P(SixteenBppTextureDitheringTestES3, PointCoordBeforeFragCoord)
+{
+    ANGLE_SKIP_TEST_IF(!getEGLWindow()->isFeatureEnabled(Feature::EmulateDithering));
+
+    constexpr char kVS[] = R"(#version 300 es
+void main()
+{
+    int bit0 = gl_VertexID & 1;
+    int bit1 = gl_VertexID >> 1;
+    gl_Position = vec4(bit0 * 2 - 1, bit1 * 2 - 1, 0, 1);
+    gl_PointSize = 1.0;
+})";
+
+    // gl_PointCoord is referenced first; gl_FragCoord second.
+    constexpr char kFS[] = R"(#version 300 es
+precision mediump float;
+out vec4 colorOut;
+void main()
+{
+    vec2 p = gl_PointCoord;
+    colorOut = vec4(p, gl_FragCoord.x * 0.001, 1.0);
+})";
+
+    ANGLE_GL_PROGRAM(program, kVS, kFS);
+
+    int w = getWindowWidth();
+    int h = getWindowHeight();
+
+    GLFramebuffer fbo;
+    glBindFramebuffer(GL_FRAMEBUFFER, fbo);
+
+    GLTexture tex4444;
+    glBindTexture(GL_TEXTURE_2D, tex4444);
+    glTexStorage2D(GL_TEXTURE_2D, 1, GL_RGBA4, w, h);
+    glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, tex4444, 0);
+    ASSERT_GL_FRAMEBUFFER_COMPLETE(GL_FRAMEBUFFER);
+
+    glViewport(0, 0, w, h);
+    glEnable(GL_DITHER);
+    glUseProgram(program);
+    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
+    ASSERT_GL_NO_ERROR();
+}
+
 ANGLE_INSTANTIATE_TEST_ES2(SixteenBppTextureTest);
 
 GTEST_ALLOW_UNINSTANTIATED_PARAMETERIZED_TEST(SixteenBppTextureTestES3);
 ANGLE_INSTANTIATE_TEST_ES3(SixteenBppTextureTestES3);
 
 GTEST_ALLOW_UNINSTANTIATED_PARAMETERIZED_TEST(SixteenBppTextureDitheringTestES3);
-ANGLE_INSTANTIATE_TEST_ES3(SixteenBppTextureDitheringTestES3);
+ANGLE_INSTANTIATE_TEST_ES3_AND(SixteenBppTextureDitheringTestES3,
+                               ES3_VULKAN_SWIFTSHADER().enable(Feature::EmulateDithering),
+                               ES3_VULKAN().enable(Feature::EmulateDithering));
 
 }  // namespace