diff --git a/MobileGL/MG_Impl/GLImpl/Sync/GL_Sync.cpp b/MobileGL/MG_Impl/GLImpl/Sync/GL_Sync.cpp
index 951f3fe7..e18b8120 100644
--- a/MobileGL/MG_Impl/GLImpl/Sync/GL_Sync.cpp
+++ b/MobileGL/MG_Impl/GLImpl/Sync/GL_Sync.cpp
@@ -8,6 +8,7 @@
 
 #include "GL_Sync.h"
 #include <MG_Backend/BackendObjects.h>
+#include <MG_State/GLState/Core.h>
 
 namespace MobileGL::MG_Impl::GLImpl {
     namespace {
@@ -35,6 +36,22 @@ namespace MobileGL::MG_Impl::GLImpl {
     } // namespace
 
     GLsync FenceSync(GLenum condition, GLbitfield flags) {
+        // GL 4.6 core 4.1.2: GL_SYNC_GPU_COMMANDS_COMPLETE is the only condition and the only
+        // legal flags value is zero; both violations return 0 rather than a handle. A caller that
+        // then hands the 0 back to glDeleteSync hits the glDeleteSync(0) no-op below.
+        if (condition != GL_SYNC_GPU_COMMANDS_COMPLETE) {
+            MG_State::pGLContext->RecordError(
+                ErrorCode::InvalidEnum,
+                MakeUnique<GenericErrorInfo>("MG_Impl/GLImpl", __func__,
+                                             "condition must be GL_SYNC_GPU_COMMANDS_COMPLETE."));
+            return nullptr;
+        }
+        if (flags != 0) {
+            MG_State::pGLContext->RecordError(
+                ErrorCode::InvalidValue,
+                MakeUnique<GenericErrorInfo>("MG_Impl/GLImpl", __func__, "flags must be zero."));
+            return nullptr;
+        }
         auto* syncObject = new SyncObject;
         syncObject->condition = condition;
         syncObject->flags = flags;
@@ -64,6 +81,18 @@ namespace MobileGL::MG_Impl::GLImpl {
     }
 
     void WaitSync(GLsync sync, GLbitfield flags, GLuint64 timeout) {
+        // GL 4.6 core 4.1.2: the server-side wait takes no flags and no finite timeout - both
+        // arguments exist only to be forward-compatible, and anything else is INVALID_VALUE.
+        // Neither backend ever honored a nonzero timeout (DirectGLES hard-codes
+        // 0/GL_TIMEOUT_IGNORED, DirectVulkan's queue ordering makes the wait implicit), so
+        // rejecting the call loses no wait that used to happen.
+        if (flags != 0 || timeout != GL_TIMEOUT_IGNORED) {
+            MG_State::pGLContext->RecordError(
+                ErrorCode::InvalidValue,
+                MakeUnique<GenericErrorInfo>("MG_Impl/GLImpl", __func__,
+                                             "flags must be zero and timeout must be GL_TIMEOUT_IGNORED."));
+            return;
+        }
         const auto* syncObject = FindSyncObject(sync);
         if (!syncObject) {
             return;
diff --git a/MobileGL/MG_Test/State/NegativeApiErrorsTest.cpp b/MobileGL/MG_Test/State/NegativeApiErrorsTest.cpp
index 58faf612..9426a7f2 100644
--- a/MobileGL/MG_Test/State/NegativeApiErrorsTest.cpp
+++ b/MobileGL/MG_Test/State/NegativeApiErrorsTest.cpp
@@ -18,6 +18,7 @@
 //     errors that guard a parameter-buffer draw.
 //   * KHR-GL43.compute_shader.api-indirect / .api-program.
 //   * KHR-GLxx.texture_storage.compressed_data - compressed formats on TEXTURE_3D.
+//   * KHR-GL32.api.coverage - glFenceSync's condition/flags and glWaitSync's flags/timeout.
 // Plus the indexed-getter parity RC-7b is about: glGetBooleani_v / glGetInteger64i_v /
 // glGetFloati_v / glGetDoublei_v must answer every pname glGetIntegeri_v answers.
 //
@@ -37,6 +38,7 @@
 #include <MG_Impl/GLImpl/Program/GL_Program.h>
 #include <MG_Impl/GLImpl/RenderState/GL_RenderState.h>
 #include <MG_Impl/GLImpl/Sampler/GL_Sampler.h>
+#include <MG_Impl/GLImpl/Sync/GL_Sync.h>
 #include <MG_Impl/GLImpl/Texture/GL_Texture.h>
 #include <MG_Impl/GLImpl/VertexArray/GL_VertexArray.h>
 #include <MG_State/GLState/Core.h>
@@ -470,4 +472,36 @@ void main() { g_color = vec4(1); }
         EXPECT_EQ(offset, 2048);
         EXPECT_EQ(GetError(), GL_NO_ERROR);
     }
+
+    // KHR-GL32.api.coverage: glFenceSync and glWaitSync took every argument they were handed and
+    // reported GL_NO_ERROR for the two calls GL 4.6 core 4.1.2 requires to fail. A rejected
+    // glFenceSync must also hand back 0 rather than a live handle.
+    TEST_F(NegativeApiErrorsTest, SyncEntryPointsRejectTheirIllegalArguments) {
+        DrainErrors();
+
+        RunRows({
+            {"glFenceSync with a condition other than GL_SYNC_GPU_COMMANDS_COMPLETE",
+             [] { EXPECT_EQ(FenceSync(GL_SYNC_FENCE, 0), nullptr); }, GL_INVALID_ENUM},
+            {"glFenceSync with nonzero flags", [] { EXPECT_EQ(FenceSync(GL_SYNC_GPU_COMMANDS_COMPLETE, 1), nullptr); },
+             GL_INVALID_VALUE},
+        });
+
+        // The legal fence still works, and with no backend function table it is the always-signaled
+        // fallback - which is all this GPU-free suite needs to reach glWaitSync's own checks.
+        const GLsync sync = FenceSync(GL_SYNC_GPU_COMMANDS_COMPLETE, 0);
+        ASSERT_NE(sync, nullptr);
+        EXPECT_EQ(GetError(), GL_NO_ERROR);
+        EXPECT_EQ(IsSync(sync), GL_TRUE);
+
+        RunRows({
+            {"glWaitSync with nonzero flags", [&] { WaitSync(sync, GL_SYNC_FLUSH_COMMANDS_BIT, GL_TIMEOUT_IGNORED); },
+             GL_INVALID_VALUE},
+            {"glWaitSync with a finite timeout", [&] { WaitSync(sync, 0, 1000000000ull); }, GL_INVALID_VALUE},
+            {"glWaitSync with the only legal argument pair", [&] { WaitSync(sync, 0, GL_TIMEOUT_IGNORED); },
+             GL_NO_ERROR},
+        });
+
+        DeleteSync(sync);
+        EXPECT_EQ(GetError(), GL_NO_ERROR);
+    }
 } // namespace