diff --git a/docs/origin-trial-changes.md b/docs/origin-trial-changes.md
index 435934ecbf..92c7a22df2 100644
--- a/docs/origin-trial-changes.md
+++ b/docs/origin-trial-changes.md
@@ -1,5 +1,12 @@
 # Tint changes during Origin Trial
 
+## Changes for M100
+
+### Breaking changes
+
+* The `@interpolate(flat)` attribute must now be specified on integral user-defined IO. [tint:1224](crbug.com/tint/1224)
+
+
 ## Changes for M99
 
 ### Breaking changes
diff --git a/src/inspector/inspector_test.cc b/src/inspector/inspector_test.cc
index 713887b00d..aeef3651cd 100644
--- a/src/inspector/inspector_test.cc
+++ b/src/inspector/inspector_test.cc
@@ -303,7 +303,7 @@ TEST_P(InspectorGetEntryPointComponentAndCompositionTest, Test) {
   std::function<const ast::Type*()> tint_type =
       GetTypeFunction(component, composition);
 
-  auto* in_var = Param("in_var", tint_type(), {Location(0u)});
+  auto* in_var = Param("in_var", tint_type(), {Location(0u), Flat()});
   Func("foo", {in_var}, tint_type(), {Return("in_var")},
        {Stage(ast::PipelineStage::kFragment)}, {Location(0u)});
   Inspector& inspector = Build();
@@ -337,9 +337,9 @@ INSTANTIATE_TEST_SUITE_P(
                                      CompositionType::kVec4)));
 
 TEST_F(InspectorGetEntryPointTest, MultipleInOutVariables) {
-  auto* in_var0 = Param("in_var0", ty.u32(), {Location(0u)});
-  auto* in_var1 = Param("in_var1", ty.u32(), {Location(1u)});
-  auto* in_var4 = Param("in_var4", ty.u32(), {Location(4u)});
+  auto* in_var0 = Param("in_var0", ty.u32(), {Location(0u), Flat()});
+  auto* in_var1 = Param("in_var1", ty.u32(), {Location(1u), Flat()});
+  auto* in_var4 = Param("in_var4", ty.u32(), {Location(4u), Flat()});
   Func("foo", {in_var0, in_var1, in_var4}, ty.u32(), {Return("in_var0")},
        {Stage(ast::PipelineStage::kFragment)}, {Location(0u)});
   Inspector& inspector = Build();
@@ -353,14 +353,20 @@ TEST_F(InspectorGetEntryPointTest, MultipleInOutVariables) {
   EXPECT_EQ("in_var0", result[0].input_variables[0].name);
   EXPECT_TRUE(result[0].input_variables[0].has_location_decoration);
   EXPECT_EQ(0u, result[0].input_variables[0].location_decoration);
+  EXPECT_EQ(InterpolationType::kFlat,
+            result[0].input_variables[0].interpolation_type);
   EXPECT_EQ(ComponentType::kUInt, result[0].input_variables[0].component_type);
   EXPECT_EQ("in_var1", result[0].input_variables[1].name);
   EXPECT_TRUE(result[0].input_variables[1].has_location_decoration);
   EXPECT_EQ(1u, result[0].input_variables[1].location_decoration);
+  EXPECT_EQ(InterpolationType::kFlat,
+            result[0].input_variables[1].interpolation_type);
   EXPECT_EQ(ComponentType::kUInt, result[0].input_variables[1].component_type);
   EXPECT_EQ("in_var4", result[0].input_variables[2].name);
   EXPECT_TRUE(result[0].input_variables[2].has_location_decoration);
   EXPECT_EQ(4u, result[0].input_variables[2].location_decoration);
+  EXPECT_EQ(InterpolationType::kFlat,
+            result[0].input_variables[2].interpolation_type);
   EXPECT_EQ(ComponentType::kUInt, result[0].input_variables[2].component_type);
 
   ASSERT_EQ(1u, result[0].output_variables.size());
@@ -371,11 +377,11 @@ TEST_F(InspectorGetEntryPointTest, MultipleInOutVariables) {
 }
 
 TEST_F(InspectorGetEntryPointTest, MultipleEntryPointsInOutVariables) {
-  auto* in_var_foo = Param("in_var_foo", ty.u32(), {Location(0u)});
+  auto* in_var_foo = Param("in_var_foo", ty.u32(), {Location(0u), Flat()});
   Func("foo", {in_var_foo}, ty.u32(), {Return("in_var_foo")},
        {Stage(ast::PipelineStage::kFragment)}, {Location(0u)});
 
-  auto* in_var_bar = Param("in_var_bar", ty.u32(), {Location(0u)});
+  auto* in_var_bar = Param("in_var_bar", ty.u32(), {Location(0u), Flat()});
   Func("bar", {in_var_bar}, ty.u32(), {Return("in_var_bar")},
        {Stage(ast::PipelineStage::kFragment)}, {Location(1u)});
 
@@ -390,6 +396,8 @@ TEST_F(InspectorGetEntryPointTest, MultipleEntryPointsInOutVariables) {
   EXPECT_EQ("in_var_foo", result[0].input_variables[0].name);
   EXPECT_TRUE(result[0].input_variables[0].has_location_decoration);
   EXPECT_EQ(0u, result[0].input_variables[0].location_decoration);
+  EXPECT_EQ(InterpolationType::kFlat,
+            result[0].input_variables[0].interpolation_type);
   EXPECT_EQ(ComponentType::kUInt, result[0].input_variables[0].component_type);
 
   ASSERT_EQ(1u, result[0].output_variables.size());
@@ -402,6 +410,8 @@ TEST_F(InspectorGetEntryPointTest, MultipleEntryPointsInOutVariables) {
   EXPECT_EQ("in_var_bar", result[1].input_variables[0].name);
   EXPECT_TRUE(result[1].input_variables[0].has_location_decoration);
   EXPECT_EQ(0u, result[1].input_variables[0].location_decoration);
+  EXPECT_EQ(InterpolationType::kFlat,
+            result[1].input_variables[0].interpolation_type);
   EXPECT_EQ(ComponentType::kUInt, result[1].input_variables[0].component_type);
 
   ASSERT_EQ(1u, result[1].output_variables.size());
diff --git a/src/inspector/test_inspector_builder.cc b/src/inspector/test_inspector_builder.cc
index 3548eea7da..734fc5a53e 100644
--- a/src/inspector/test_inspector_builder.cc
+++ b/src/inspector/test_inspector_builder.cc
@@ -55,7 +55,8 @@ const ast::Struct* InspectorBuilder::MakeInOutStruct(
     std::string member_name;
     uint32_t location;
     std::tie(member_name, location) = var;
-    members.push_back(Member(member_name, ty.u32(), {Location(location)}));
+    members.push_back(
+        Member(member_name, ty.u32(), {Location(location), Flat()}));
   }
   return Structure(name, members);
 }
diff --git a/src/program_builder.h b/src/program_builder.h
index 2d97ad628f..0fd946fb25 100644
--- a/src/program_builder.h
+++ b/src/program_builder.h
@@ -2300,7 +2300,7 @@ class ProgramBuilder {
   const ast::InterpolateDecoration* Interpolate(
       const Source& source,
       ast::InterpolationType type,
-      ast::InterpolationSampling sampling) {
+      ast::InterpolationSampling sampling = ast::InterpolationSampling::kNone) {
     return create<ast::InterpolateDecoration>(source, type, sampling);
   }
 
@@ -2310,10 +2310,23 @@ class ProgramBuilder {
   /// @returns the interpolate decoration pointer
   const ast::InterpolateDecoration* Interpolate(
       ast::InterpolationType type,
-      ast::InterpolationSampling sampling) {
+      ast::InterpolationSampling sampling = ast::InterpolationSampling::kNone) {
     return create<ast::InterpolateDecoration>(source_, type, sampling);
   }
 
+  /// Creates an ast::InterpolateDecoration using flat interpolation
+  /// @param source the source information
+  /// @returns the interpolate decoration pointer
+  const ast::InterpolateDecoration* Flat(const Source& source) {
+    return Interpolate(source, ast::InterpolationType::kFlat);
+  }
+
+  /// Creates an ast::InterpolateDecoration using flat interpolation
+  /// @returns the interpolate decoration pointer
+  const ast::InterpolateDecoration* Flat() {
+    return Interpolate(ast::InterpolationType::kFlat);
+  }
+
   /// Creates an ast::InvariantDecoration
   /// @param source the source information
   /// @returns the invariant decoration pointer
diff --git a/src/reader/spirv/parser_impl.cc b/src/reader/spirv/parser_impl.cc
index 226fa2573e..f36a8db1ee 100644
--- a/src/reader/spirv/parser_impl.cc
+++ b/src/reader/spirv/parser_impl.cc
@@ -1790,6 +1790,10 @@ bool ParserImpl::ConvertPipelineDecorations(const Type* store_type,
         }
         SetLocation(ast_decos,
                     create<ast::LocationDecoration>(Source{}, deco[1]));
+        if (store_type->IsIntegerScalarOrVector()) {
+          // Default to flat interpolation for integral user-defined IO types.
+          type = ast::InterpolationType::kFlat;
+        }
         break;
       case SpvDecorationFlat:
         type = ast::InterpolationType::kFlat;
diff --git a/src/reader/spirv/parser_impl_module_var_test.cc b/src/reader/spirv/parser_impl_module_var_test.cc
index 31007f3b9d..5e1051819e 100644
--- a/src/reader/spirv/parser_impl_module_var_test.cc
+++ b/src/reader/spirv/parser_impl_module_var_test.cc
@@ -3727,14 +3727,14 @@ fn main_1() {
 }
 
 struct main_out {
-  @location(0)
+  @location(0) @interpolate(flat)
   x_2_1 : u32;
-  @location(6)
+  @location(6) @interpolate(flat)
   x_4_1 : u32;
 }
 
 @stage(fragment)
-fn main(@location(0) x_1_param : u32, @location(30) x_3_param : u32) -> main_out {
+fn main(@location(0) @interpolate(flat) x_1_param : u32, @location(30) @interpolate(flat) x_3_param : u32) -> main_out {
   x_1 = x_1_param;
   x_3 = x_3_param;
   main_1();
@@ -4679,7 +4679,6 @@ fn main(@location(9) x_1_param : f32, @location(11) x_1_param_1 : vec4<f32>) ->
 
 TEST_F(SpvModuleScopeVarParserTest,
        EntryPointWrapping_Interpolation_Flat_Vertex_In) {
-  // Flat decorations are dropped for integral
   const auto assembly = CommonCapabilities() + R"(
      OpEntryPoint Vertex %main "main" %1 %2 %3 %4 %5 %6 %10
      OpDecorate %1 Location 1
@@ -4764,7 +4763,6 @@ fn main(@location(1) @interpolate(flat) x_1_param : u32, @location(2) @interpola
 
 TEST_F(SpvModuleScopeVarParserTest,
        EntryPointWrapping_Interpolation_Flat_Vertex_Output) {
-  // Flat decorations are dropped for integral
   const auto assembly = CommonCapabilities() + R"(
      OpEntryPoint Vertex %main "main" %1 %2 %3 %4 %5 %6 %10
      OpDecorate %1 Location 1
@@ -4855,7 +4853,6 @@ fn main() -> main_out {
 
 TEST_F(SpvModuleScopeVarParserTest,
        EntryPointWrapping_Flatten_Interpolation_Flat_Fragment_In) {
-  // Flat decorations are dropped for integral
   const auto assembly = CommonCapabilities() + R"(
      OpEntryPoint Fragment %main "main" %1 %2
      OpExecutionMode %main OriginUpperLeft
@@ -4910,7 +4907,6 @@ fn main(@location(1) @interpolate(flat) x_1_param : f32, @location(2) @interpola
 
 TEST_F(SpvModuleScopeVarParserTest,
        EntryPointWrapping_Interpolation_Floating_Fragment_In) {
-  // Flat decorations are dropped for integral
   const auto assembly = CommonCapabilities() + R"(
      OpEntryPoint Fragment %main "main" %1 %2 %3 %4 %5 %6
      OpExecutionMode %main OriginUpperLeft
@@ -5056,7 +5052,6 @@ fn main(@location(1) x_1_param : f32, @location(2) @interpolate(perspective, cen
 
 TEST_F(SpvModuleScopeVarParserTest,
        EntryPointWrapping_Interpolation_Floating_Fragment_Out) {
-  // Flat decorations are dropped for integral
   const auto assembly = CommonCapabilities() + R"(
      OpEntryPoint Fragment %main "main" %1 %2 %3 %4 %5 %6
      OpExecutionMode %main OriginUpperLeft
@@ -5221,6 +5216,162 @@ fn main() -> main_out {
   EXPECT_EQ(got, expected) << got;
 }
 
+TEST_F(SpvModuleScopeVarParserTest,
+       EntryPointWrapping_Interpolation_Default_Vertex_Output) {
+  // Integral types default to @interpolate(flat).
+  // Floating types default to @interpolate(perspective, center), which is the
+  // same as WGSL and therefore dropped.
+  const auto assembly = CommonCapabilities() + R"(
+     OpEntryPoint Vertex %main "main" %1 %2 %3 %4 %5 %6 %10
+     OpDecorate %1 Location 1
+     OpDecorate %2 Location 2
+     OpDecorate %3 Location 3
+     OpDecorate %4 Location 4
+     OpDecorate %5 Location 5
+     OpDecorate %6 Location 6
+     OpDecorate %10 BuiltIn Position
+)" + CommonTypes() +
+                        R"(
+     %ptr_out_uint = OpTypePointer Output %uint
+     %ptr_out_v2uint = OpTypePointer Output %v2uint
+     %ptr_out_int = OpTypePointer Output %int
+     %ptr_out_v2int = OpTypePointer Output %v2int
+     %ptr_out_float = OpTypePointer Output %float
+     %ptr_out_v2float = OpTypePointer Output %v2float
+     %1 = OpVariable %ptr_out_uint Output
+     %2 = OpVariable %ptr_out_v2uint Output
+     %3 = OpVariable %ptr_out_int Output
+     %4 = OpVariable %ptr_out_v2int Output
+     %5 = OpVariable %ptr_out_float Output
+     %6 = OpVariable %ptr_out_v2float Output
+
+     %ptr_out_v4float = OpTypePointer Output %v4float
+     %10 = OpVariable %ptr_out_v4float Output
+
+     %main = OpFunction %void None %voidfn
+     %entry = OpLabel
+     OpReturn
+     OpFunctionEnd
+  )";
+  auto p = parser(test::Assemble(assembly));
+
+  ASSERT_TRUE(p->BuildAndParseInternalModule());
+  EXPECT_TRUE(p->error().empty());
+  const auto got = test::ToString(p->program());
+  const std::string expected =
+      R"(var<private> x_1 : u32;
+
+var<private> x_2 : vec2<u32>;
+
+var<private> x_3 : i32;
+
+var<private> x_4 : vec2<i32>;
+
+var<private> x_5 : f32;
+
+var<private> x_6 : vec2<f32>;
+
+var<private> x_10 : vec4<f32>;
+
+fn main_1() {
+  return;
+}
+
+struct main_out {
+  @location(1) @interpolate(flat)
+  x_1_1 : u32;
+  @location(2) @interpolate(flat)
+  x_2_1 : vec2<u32>;
+  @location(3) @interpolate(flat)
+  x_3_1 : i32;
+  @location(4) @interpolate(flat)
+  x_4_1 : vec2<i32>;
+  @location(5)
+  x_5_1 : f32;
+  @location(6)
+  x_6_1 : vec2<f32>;
+  @builtin(position)
+  x_10_1 : vec4<f32>;
+}
+
+@stage(vertex)
+fn main() -> main_out {
+  main_1();
+  return main_out(x_1, x_2, x_3, x_4, x_5, x_6, x_10);
+}
+)";
+  EXPECT_EQ(got, expected) << got;
+}
+
+TEST_F(SpvModuleScopeVarParserTest,
+       EntryPointWrapping_Interpolation_Default_Fragment_In) {
+  // Integral types default to @interpolate(flat).
+  // Floating types default to @interpolate(perspective, center), which is the
+  // same as WGSL and therefore dropped.
+  const auto assembly = CommonCapabilities() + R"(
+     OpEntryPoint Fragment %main "main" %1 %2 %3 %4 %5 %6
+     OpDecorate %1 Location 1
+     OpDecorate %2 Location 2
+     OpDecorate %3 Location 3
+     OpDecorate %4 Location 4
+     OpDecorate %5 Location 5
+     OpDecorate %6 Location 6
+)" + CommonTypes() +
+                        R"(
+     %ptr_in_uint = OpTypePointer Input %uint
+     %ptr_in_v2uint = OpTypePointer Input %v2uint
+     %ptr_in_int = OpTypePointer Input %int
+     %ptr_in_v2int = OpTypePointer Input %v2int
+     %ptr_in_float = OpTypePointer Input %float
+     %ptr_in_v2float = OpTypePointer Input %v2float
+     %1 = OpVariable %ptr_in_uint Input
+     %2 = OpVariable %ptr_in_v2uint Input
+     %3 = OpVariable %ptr_in_int Input
+     %4 = OpVariable %ptr_in_v2int Input
+     %5 = OpVariable %ptr_in_float Input
+     %6 = OpVariable %ptr_in_v2float Input
+
+     %main = OpFunction %void None %voidfn
+     %entry = OpLabel
+     OpReturn
+     OpFunctionEnd
+  )";
+  auto p = parser(test::Assemble(assembly));
+
+  ASSERT_TRUE(p->BuildAndParseInternalModule());
+  EXPECT_TRUE(p->error().empty());
+  const auto got = test::ToString(p->program());
+  const std::string expected =
+      R"(var<private> x_1 : u32;
+
+var<private> x_2 : vec2<u32>;
+
+var<private> x_3 : i32;
+
+var<private> x_4 : vec2<i32>;
+
+var<private> x_5 : f32;
+
+var<private> x_6 : vec2<f32>;
+
+fn main_1() {
+  return;
+}
+
+@stage(fragment)
+fn main(@location(1) @interpolate(flat) x_1_param : u32, @location(2) @interpolate(flat) x_2_param : vec2<u32>, @location(3) @interpolate(flat) x_3_param : i32, @location(4) @interpolate(flat) x_4_param : vec2<i32>, @location(5) x_5_param : f32, @location(6) x_6_param : vec2<f32>) {
+  x_1 = x_1_param;
+  x_2 = x_2_param;
+  x_3 = x_3_param;
+  x_4 = x_4_param;
+  x_5 = x_5_param;
+  x_6 = x_6_param;
+  main_1();
+}
+)";
+  EXPECT_EQ(got, expected) << got;
+}
+
 }  // namespace
 }  // namespace spirv
 }  // namespace reader
diff --git a/src/resolver/decoration_validation_test.cc b/src/resolver/decoration_validation_test.cc
index 592ba15fc9..6f8983b49f 100644
--- a/src/resolver/decoration_validation_test.cc
+++ b/src/resolver/decoration_validation_test.cc
@@ -1332,11 +1332,10 @@ TEST_F(InterpolateTest, FragmentInput_Integer_MissingFlatInterpolation) {
        ty.void_(), {},
        ast::DecorationList{Stage(ast::PipelineStage::kFragment)});
 
-  // TODO(crbug.com/tint/1224): Make this an error.
-  EXPECT_TRUE(r()->Resolve());
-  EXPECT_EQ(r()->error(),
-            "12:34 warning: integral user-defined fragment inputs must have a "
-            "flat interpolation attribute");
+  EXPECT_FALSE(r()->Resolve());
+  EXPECT_EQ(
+      r()->error(),
+      R"(12:34 error: integral user-defined fragment inputs must have a flat interpolation attribute)");
 }
 
 TEST_F(InterpolateTest, VertexOutput_Integer_MissingFlatInterpolation) {
@@ -1350,11 +1349,11 @@ TEST_F(InterpolateTest, VertexOutput_Integer_MissingFlatInterpolation) {
   Func("main", {}, ty.Of(s), {Return(Construct(ty.Of(s)))},
        ast::DecorationList{Stage(ast::PipelineStage::kVertex)});
 
-  // TODO(crbug.com/tint/1224): Make this an error.
-  EXPECT_TRUE(r()->Resolve());
-  EXPECT_EQ(r()->error(),
-            "12:34 warning: integral user-defined vertex outputs must have a "
-            "flat interpolation attribute");
+  EXPECT_FALSE(r()->Resolve());
+  EXPECT_EQ(
+      r()->error(),
+      R"(12:34 error: integral user-defined vertex outputs must have a flat interpolation attribute
+note: while analysing entry point 'main')");
 }
 
 TEST_F(InterpolateTest, MissingLocationAttribute_Parameter) {
diff --git a/src/resolver/entry_point_validation_test.cc b/src/resolver/entry_point_validation_test.cc
index a2c0eace43..1df0c33847 100644
--- a/src/resolver/entry_point_validation_test.cc
+++ b/src/resolver/entry_point_validation_test.cc
@@ -371,9 +371,9 @@ static constexpr Params cases[] = {
 
 TEST_P(TypeValidationTest, BareInputs) {
   // @stage(fragment)
-  // fn main(@location(0) a : *) {}
+  // fn main(@location(0) @interpolate(flat) a : *) {}
   auto params = GetParam();
-  auto* a = Param("a", params.create_ast_type(*this), {Location(0)});
+  auto* a = Param("a", params.create_ast_type(*this), {Location(0), Flat()});
   Func(Source{{12, 34}}, "main", {a}, ty.void_(), {},
        {Stage(ast::PipelineStage::kFragment)});
 
@@ -386,13 +386,13 @@ TEST_P(TypeValidationTest, BareInputs) {
 
 TEST_P(TypeValidationTest, StructInputs) {
   // struct Input {
-  //   @location(0) a : *;
+  //   @location(0) @interpolate(flat) a : *;
   // };
   // @stage(fragment)
   // fn main(a : Input) {}
   auto params = GetParam();
-  auto* input = Structure(
-      "Input", {Member("a", params.create_ast_type(*this), {Location(0)})});
+  auto* input = Structure("Input", {Member("a", params.create_ast_type(*this),
+                                           {Location(0), Flat()})});
   auto* a = Param("a", ty.Of(input), {});
   Func(Source{{12, 34}}, "main", {a}, ty.void_(), {},
        {Stage(ast::PipelineStage::kFragment)});
@@ -454,9 +454,9 @@ using LocationDecorationTests = ResolverTest;
 
 TEST_F(LocationDecorationTests, Pass) {
   // @stage(fragment)
-  // fn frag_main(@location(0) a: i32) {}
+  // fn frag_main(@location(0) @interpolate(flat) a: i32) {}
 
-  auto* p = Param(Source{{12, 34}}, "a", ty.i32(), {Location(0)});
+  auto* p = Param(Source{{12, 34}}, "a", ty.i32(), {Location(0), Flat()});
   Func("frag_main", {p}, ty.void_(), {},
        {Stage(ast::PipelineStage::kFragment)});
 
diff --git a/src/resolver/resolver_validation.cc b/src/resolver/resolver_validation.cc
index f3e830228e..51e78769ec 100644
--- a/src/resolver/resolver_validation.cc
+++ b/src/resolver/resolver_validation.cc
@@ -1055,21 +1055,21 @@ bool Resolver::ValidateEntryPoint(const sem::Function* func) {
       if (pipeline_io_attribute &&
           pipeline_io_attribute->Is<ast::LocationDecoration>()) {
         if (ty->is_integer_scalar_or_vector() && !interpolate_attribute) {
-          // TODO(crbug.com/tint/1224): Make these errors once downstream
-          // usages have caught up (no sooner than M99).
           if (decl->PipelineStage() == ast::PipelineStage::kVertex &&
               param_or_ret == ParamOrRetType::kReturnType) {
-            AddWarning(
+            AddError(
                 "integral user-defined vertex outputs must have a flat "
                 "interpolation attribute",
                 source);
+            return false;
           }
           if (decl->PipelineStage() == ast::PipelineStage::kFragment &&
               param_or_ret == ParamOrRetType::kParameter) {
-            AddWarning(
+            AddError(
                 "integral user-defined fragment inputs must have a flat "
                 "interpolation attribute",
                 source);
+            return false;
           }
         }
       }
diff --git a/src/transform/canonicalize_entry_point_io_test.cc b/src/transform/canonicalize_entry_point_io_test.cc
index 0219ff2bf5..9a5f184638 100644
--- a/src/transform/canonicalize_entry_point_io_test.cc
+++ b/src/transform/canonicalize_entry_point_io_test.cc
@@ -74,7 +74,7 @@ TEST_F(CanonicalizeEntryPointIOTest, Parameters_Spirv) {
   auto* src = R"(
 @stage(fragment)
 fn frag_main(@location(1) loc1 : f32,
-             @location(2) loc2 : vec4<u32>,
+             @location(2) @interpolate(flat) loc2 : vec4<u32>,
              @builtin(position) coord : vec4<f32>) {
   var col : f32 = (coord.x * loc1);
 }
@@ -109,7 +109,7 @@ TEST_F(CanonicalizeEntryPointIOTest, Parameters_Msl) {
   auto* src = R"(
 @stage(fragment)
 fn frag_main(@location(1) loc1 : f32,
-             @location(2) loc2 : vec4<u32>,
+             @location(2) @interpolate(flat) loc2 : vec4<u32>,
              @builtin(position) coord : vec4<f32>) {
   var col : f32 = (coord.x * loc1);
 }
@@ -119,7 +119,7 @@ fn frag_main(@location(1) loc1 : f32,
 struct tint_symbol_1 {
   @location(1)
   loc1 : f32;
-  @location(2)
+  @location(2) @interpolate(flat)
   loc2 : vec4<u32>;
 }
 
@@ -145,7 +145,7 @@ TEST_F(CanonicalizeEntryPointIOTest, Parameters_Hlsl) {
   auto* src = R"(
 @stage(fragment)
 fn frag_main(@location(1) loc1 : f32,
-             @location(2) loc2 : vec4<u32>,
+             @location(2) @interpolate(flat) loc2 : vec4<u32>,
              @builtin(position) coord : vec4<f32>) {
   var col : f32 = (coord.x * loc1);
 }
@@ -155,7 +155,7 @@ fn frag_main(@location(1) loc1 : f32,
 struct tint_symbol_1 {
   @location(1)
   loc1 : f32;
-  @location(2)
+  @location(2) @interpolate(flat)
   loc2 : vec4<u32>;
   @builtin(position)
   coord : vec4<f32>;
@@ -222,7 +222,7 @@ struct FragBuiltins {
 };
 struct FragLocations {
   @location(1) loc1 : f32;
-  @location(2) loc2 : vec4<u32>;
+  @location(2) @interpolate(flat) loc2 : vec4<u32>;
 };
 
 @stage(fragment)
@@ -276,7 +276,7 @@ struct FragBuiltins {
 };
 struct FragLocations {
   @location(1) loc1 : f32;
-  @location(2) loc2 : vec4<u32>;
+  @location(2) @interpolate(flat) loc2 : vec4<u32>;
 };
 
 @stage(fragment)
@@ -302,7 +302,7 @@ struct tint_symbol_1 {
   loc0 : f32;
   @location(1)
   loc1 : f32;
-  @location(2)
+  @location(2) @interpolate(flat)
   loc2 : vec4<u32>;
 }
 
@@ -331,7 +331,7 @@ struct FragBuiltins {
 };
 struct FragLocations {
   @location(1) loc1 : f32;
-  @location(2) loc2 : vec4<u32>;
+  @location(2) @interpolate(flat) loc2 : vec4<u32>;
 };
 
 @stage(fragment)
@@ -357,7 +357,7 @@ struct tint_symbol_1 {
   loc0 : f32;
   @location(1)
   loc1 : f32;
-  @location(2)
+  @location(2) @interpolate(flat)
   loc2 : vec4<u32>;
   @builtin(position)
   coord : vec4<f32>;
@@ -1134,18 +1134,18 @@ struct VertexIn {
 };
 
 struct VertexOut {
-  @location(0) i : i32;
-  @location(1) u : u32;
-  @location(2) vi : vec4<i32>;
-  @location(3) vu : vec4<u32>;
+  @location(0) @interpolate(flat) i : i32;
+  @location(1) @interpolate(flat) u : u32;
+  @location(2) @interpolate(flat) vi : vec4<i32>;
+  @location(3) @interpolate(flat) vu : vec4<u32>;
   @builtin(position) pos : vec4<f32>;
 };
 
 struct FragmentInterface {
-  @location(0) i : i32;
-  @location(1) u : u32;
-  @location(2) vi : vec4<i32>;
-  @location(3) vu : vec4<u32>;
+  @location(0) @interpolate(flat) i : i32;
+  @location(1) @interpolate(flat) u : u32;
+  @location(2) @interpolate(flat) vi : vec4<i32>;
+  @location(3) @interpolate(flat) vu : vec4<u32>;
 };
 
 @stage(vertex)
@@ -1187,13 +1187,13 @@ fn frag_main(inputs : FragmentInterface) -> FragmentInterface {
 
 @location(3) @interpolate(flat) @internal(disable_validation__ignore_storage_class) var<in> vu_3 : vec4<u32>;
 
-@location(0) @internal(disable_validation__ignore_storage_class) var<out> i_4 : i32;
+@location(0) @interpolate(flat) @internal(disable_validation__ignore_storage_class) var<out> i_4 : i32;
 
-@location(1) @internal(disable_validation__ignore_storage_class) var<out> u_4 : u32;
+@location(1) @interpolate(flat) @internal(disable_validation__ignore_storage_class) var<out> u_4 : u32;
 
-@location(2) @internal(disable_validation__ignore_storage_class) var<out> vi_4 : vec4<i32>;
+@location(2) @interpolate(flat) @internal(disable_validation__ignore_storage_class) var<out> vi_4 : vec4<i32>;
 
-@location(3) @internal(disable_validation__ignore_storage_class) var<out> vu_4 : vec4<u32>;
+@location(3) @interpolate(flat) @internal(disable_validation__ignore_storage_class) var<out> vu_4 : vec4<u32>;
 
 struct VertexIn {
   i : i32;
@@ -1389,15 +1389,15 @@ fn frag_main(tint_symbol : tint_symbol_1) -> tint_symbol_2 {
 TEST_F(CanonicalizeEntryPointIOTest, SortedMembers) {
   auto* src = R"(
 struct VertexOutput {
-  @location(1) b : u32;
+  @location(1) @interpolate(flat) b : u32;
   @builtin(position) pos : vec4<f32>;
-  @location(3) d : u32;
+  @location(3) @interpolate(flat) d : u32;
   @location(0) a : f32;
-  @location(2) c : i32;
+  @location(2) @interpolate(flat) c : i32;
 };
 
 struct FragmentInputExtra {
-  @location(3) d : u32;
+  @location(3) @interpolate(flat) d : u32;
   @builtin(position) pos : vec4<f32>;
   @location(0) a : f32;
 };
@@ -1409,9 +1409,9 @@ fn vert_main() -> VertexOutput {
 
 @stage(fragment)
 fn frag_main(@builtin(front_facing) ff : bool,
-             @location(2) c : i32,
+             @location(2) @interpolate(flat) c : i32,
              inputs : FragmentInputExtra,
-             @location(1) b : u32) {
+             @location(1) @interpolate(flat) b : u32) {
 }
 )";
 
@@ -1433,11 +1433,11 @@ struct FragmentInputExtra {
 struct tint_symbol {
   @location(0)
   a : f32;
-  @location(1)
+  @location(1) @interpolate(flat)
   b : u32;
-  @location(2)
+  @location(2) @interpolate(flat)
   c : i32;
-  @location(3)
+  @location(3) @interpolate(flat)
   d : u32;
   @builtin(position)
   pos : vec4<f32>;
@@ -1462,11 +1462,11 @@ fn vert_main() -> tint_symbol {
 struct tint_symbol_2 {
   @location(0)
   a : f32;
-  @location(1)
+  @location(1) @interpolate(flat)
   b : u32;
-  @location(2)
+  @location(2) @interpolate(flat)
   c : i32;
-  @location(3)
+  @location(3) @interpolate(flat)
   d : u32;
   @builtin(position)
   pos : vec4<f32>;
diff --git a/src/writer/spirv/builder_entry_point_test.cc b/src/writer/spirv/builder_entry_point_test.cc
index 46d6745a48..8cc75b057f 100644
--- a/src/writer/spirv/builder_entry_point_test.cc
+++ b/src/writer/spirv/builder_entry_point_test.cc
@@ -107,13 +107,14 @@ OpFunctionEnd
 
 TEST_F(BuilderTest, EntryPoint_ReturnValue) {
   // @stage(fragment)
-  // fn frag_main(@location(0) loc_in : u32) -> @location(0) f32 {
+  // fn frag_main(@location(0) @interpolate(flat) loc_in : u32)
+  //     -> @location(0) f32 {
   //   if (loc_in > 10) {
   //     return 0.5;
   //   }
   //   return 1.0;
   // }
-  auto* loc_in = Param("loc_in", ty.u32(), {Location(0)});
+  auto* loc_in = Param("loc_in", ty.u32(), {Location(0), Flat()});
   auto* cond = create<ast::BinaryExpression>(ast::BinaryOp::kGreaterThan,
                                              Expr("loc_in"), Expr(10u));
   Func("frag_main", ast::VariableList{loc_in}, ty.f32(),
diff --git a/test/bug/tint/1081.wgsl b/test/bug/tint/1081.wgsl
index 591fb0482b..6d339a87cc 100644
--- a/test/bug/tint/1081.wgsl
+++ b/test/bug/tint/1081.wgsl
@@ -6,7 +6,7 @@ fn f(x : i32) -> i32 {
 }
 
 @stage(fragment)
-fn main(@location(1) x: vec3<i32>) -> @location(2) i32 {
+fn main(@location(1) @interpolate(flat) x: vec3<i32>) -> @location(2) i32 {
   var y = x.x;
   loop {
     let r = f(y);
diff --git a/test/bug/tint/1081.wgsl.expected.hlsl b/test/bug/tint/1081.wgsl.expected.hlsl
index 73e28307b0..ad9174c07f 100644
--- a/test/bug/tint/1081.wgsl.expected.hlsl
+++ b/test/bug/tint/1081.wgsl.expected.hlsl
@@ -1,7 +1,3 @@
-bug/tint/1081.wgsl:9:22 warning: integral user-defined fragment inputs must have a flat interpolation attribute
-fn main(@location(1) x: vec3<i32>) -> @location(2) i32 {
-                     ^
-
 int f(int x) {
   if (true) {
     if ((x == 10)) {
@@ -14,7 +10,7 @@ int f(int x) {
 }
 
 struct tint_symbol_1 {
-  int3 x : TEXCOORD1;
+  nointerpolation int3 x : TEXCOORD1;
 };
 struct tint_symbol_2 {
   int value : SV_Target2;
diff --git a/test/bug/tint/1081.wgsl.expected.msl b/test/bug/tint/1081.wgsl.expected.msl
index 478e1a54b4..3dd960160d 100644
--- a/test/bug/tint/1081.wgsl.expected.msl
+++ b/test/bug/tint/1081.wgsl.expected.msl
@@ -1,12 +1,8 @@
-bug/tint/1081.wgsl:9:22 warning: integral user-defined fragment inputs must have a flat interpolation attribute
-fn main(@location(1) x: vec3<i32>) -> @location(2) i32 {
-                     ^
-
 #include <metal_stdlib>
 
 using namespace metal;
 struct tint_symbol_2 {
-  int3 x [[user(locn1)]];
+  int3 x [[user(locn1)]] [[flat]];
 };
 struct tint_symbol_3 {
   int value [[color(2)]];
diff --git a/test/bug/tint/1081.wgsl.expected.spvasm b/test/bug/tint/1081.wgsl.expected.spvasm
index b17a7e645e..2f5ee3f55c 100644
--- a/test/bug/tint/1081.wgsl.expected.spvasm
+++ b/test/bug/tint/1081.wgsl.expected.spvasm
@@ -1,7 +1,3 @@
-bug/tint/1081.wgsl:9:22 warning: integral user-defined fragment inputs must have a flat interpolation attribute
-fn main(@location(1) x: vec3<i32>) -> @location(2) i32 {
-                     ^
-
 ; SPIR-V
 ; Version: 1.3
 ; Generator: Google Tint Compiler; 0
diff --git a/test/bug/tint/1081.wgsl.expected.wgsl b/test/bug/tint/1081.wgsl.expected.wgsl
index 34d78e1a94..3f7cc7a4d2 100644
--- a/test/bug/tint/1081.wgsl.expected.wgsl
+++ b/test/bug/tint/1081.wgsl.expected.wgsl
@@ -1,7 +1,3 @@
-bug/tint/1081.wgsl:9:22 warning: integral user-defined fragment inputs must have a flat interpolation attribute
-fn main(@location(1) x: vec3<i32>) -> @location(2) i32 {
-                     ^
-
 fn f(x : i32) -> i32 {
   if ((x == 10)) {
     discard;
@@ -10,7 +6,7 @@ fn f(x : i32) -> i32 {
 }
 
 @stage(fragment)
-fn main(@location(1) x : vec3<i32>) -> @location(2) i32 {
+fn main(@location(1) @interpolate(flat) x : vec3<i32>) -> @location(2) i32 {
   var y = x.x;
   loop {
     let r = f(y);
diff --git a/test/vk-gl-cts/spirv_assembly/instruction/float32/comparison/frexpstruct_1_frag/0.wgsl b/test/vk-gl-cts/spirv_assembly/instruction/float32/comparison/frexpstruct_1_frag/0.wgsl
index 5c734e34aa..db20f9ed7b 100644
--- a/test/vk-gl-cts/spirv_assembly/instruction/float32/comparison/frexpstruct_1_frag/0.wgsl
+++ b/test/vk-gl-cts/spirv_assembly/instruction/float32/comparison/frexpstruct_1_frag/0.wgsl
@@ -14,7 +14,7 @@ fn main_1() {
 }
 
 struct main_out {
-  @location(0)
+  @location(0) @interpolate(flat)
   x_4_1 : u32;
   @builtin(position)
   gl_Position : vec4<f32>;
diff --git a/test/vk-gl-cts/spirv_assembly/instruction/spirv1p4/hlsl_functionality1/decorate_string/0.wgsl b/test/vk-gl-cts/spirv_assembly/instruction/spirv1p4/hlsl_functionality1/decorate_string/0.wgsl
index 1ab42ae3df..ecc89f5766 100644
--- a/test/vk-gl-cts/spirv_assembly/instruction/spirv1p4/hlsl_functionality1/decorate_string/0.wgsl
+++ b/test/vk-gl-cts/spirv_assembly/instruction/spirv1p4/hlsl_functionality1/decorate_string/0.wgsl
@@ -14,7 +14,7 @@ fn main_1() {
 struct main_out {
   @builtin(position)
   gl_Position : vec4<f32>;
-  @location(0)
+  @location(0) @interpolate(flat)
   pos_1 : u32;
 };
 
diff --git a/test/vk-gl-cts/spirv_assembly/instruction/terminate_invocation/no_image_atomic/0-opt.wgsl b/test/vk-gl-cts/spirv_assembly/instruction/terminate_invocation/no_image_atomic/0-opt.wgsl
index 0e32731420..5f9494069f 100644
--- a/test/vk-gl-cts/spirv_assembly/instruction/terminate_invocation/no_image_atomic/0-opt.wgsl
+++ b/test/vk-gl-cts/spirv_assembly/instruction/terminate_invocation/no_image_atomic/0-opt.wgsl
@@ -14,7 +14,7 @@ fn main_1() {
 }
 
 struct main_out {
-  @location(0)
+  @location(0) @interpolate(flat)
   x_4_1 : i32;
   @builtin(position)
   gl_Position : vec4<f32>;
diff --git a/test/vk-gl-cts/spirv_assembly/instruction/terminate_invocation/no_image_store/1.wgsl b/test/vk-gl-cts/spirv_assembly/instruction/terminate_invocation/no_image_store/1.wgsl
index 2cd673efdf..e2a03ec766 100644
--- a/test/vk-gl-cts/spirv_assembly/instruction/terminate_invocation/no_image_store/1.wgsl
+++ b/test/vk-gl-cts/spirv_assembly/instruction/terminate_invocation/no_image_store/1.wgsl
@@ -24,7 +24,7 @@ struct main_out {
 };
 
 @stage(fragment)
-fn main(@builtin(position) x_2_param : vec4<f32>, @location(0) x_3_param : i32) -> main_out {
+fn main(@builtin(position) x_2_param : vec4<f32>, @location(0) @interpolate(flat) x_3_param : i32) -> main_out {
   x_2 = x_2_param;
   x_3 = x_3_param;
   main_1();
diff --git a/test/vk-gl-cts/spirv_assembly/instruction/terminate_invocation/no_output_write/1.wgsl b/test/vk-gl-cts/spirv_assembly/instruction/terminate_invocation/no_output_write/1.wgsl
index 07e6992bab..7b3e76a818 100644
--- a/test/vk-gl-cts/spirv_assembly/instruction/terminate_invocation/no_output_write/1.wgsl
+++ b/test/vk-gl-cts/spirv_assembly/instruction/terminate_invocation/no_output_write/1.wgsl
@@ -19,7 +19,7 @@ struct main_out {
 };
 
 @stage(fragment)
-fn main(@builtin(position) x_2_param : vec4<f32>, @location(0) x_3_param : i32) -> main_out {
+fn main(@builtin(position) x_2_param : vec4<f32>, @location(0) @interpolate(flat) x_3_param : i32) -> main_out {
   x_2 = x_2_param;
   x_3 = x_3_param;
   main_1();
diff --git a/test/vk-gl-cts/spirv_assembly/instruction/terminate_invocation/no_output_write_before_terminate/1.wgsl b/test/vk-gl-cts/spirv_assembly/instruction/terminate_invocation/no_output_write_before_terminate/1.wgsl
index a849b1d02c..cbe07a4a4e 100644
--- a/test/vk-gl-cts/spirv_assembly/instruction/terminate_invocation/no_output_write_before_terminate/1.wgsl
+++ b/test/vk-gl-cts/spirv_assembly/instruction/terminate_invocation/no_output_write_before_terminate/1.wgsl
@@ -19,7 +19,7 @@ struct main_out {
 };
 
 @stage(fragment)
-fn main(@builtin(position) x_2_param : vec4<f32>, @location(0) x_3_param : i32) -> main_out {
+fn main(@builtin(position) x_2_param : vec4<f32>, @location(0) @interpolate(flat) x_3_param : i32) -> main_out {
   x_2 = x_2_param;
   x_3 = x_3_param;
   main_1();
diff --git a/test/vk-gl-cts/spirv_assembly/instruction/terminate_invocation/no_ssbo_store/1.wgsl b/test/vk-gl-cts/spirv_assembly/instruction/terminate_invocation/no_ssbo_store/1.wgsl
index da18d1228d..7d777c91b0 100644
--- a/test/vk-gl-cts/spirv_assembly/instruction/terminate_invocation/no_ssbo_store/1.wgsl
+++ b/test/vk-gl-cts/spirv_assembly/instruction/terminate_invocation/no_ssbo_store/1.wgsl
@@ -30,7 +30,7 @@ struct main_out {
 };
 
 @stage(fragment)
-fn main(@builtin(position) x_2_param : vec4<f32>, @location(0) x_3_param : i32) -> main_out {
+fn main(@builtin(position) x_2_param : vec4<f32>, @location(0) @interpolate(flat) x_3_param : i32) -> main_out {
   x_2 = x_2_param;
   x_3 = x_3_param;
   main_1();
diff --git a/test/vk-gl-cts/spirv_assembly/instruction/terminate_invocation/ssbo_store_before_terminate/1.wgsl b/test/vk-gl-cts/spirv_assembly/instruction/terminate_invocation/ssbo_store_before_terminate/1.wgsl
index 48083ee854..f8cd63e613 100644
--- a/test/vk-gl-cts/spirv_assembly/instruction/terminate_invocation/ssbo_store_before_terminate/1.wgsl
+++ b/test/vk-gl-cts/spirv_assembly/instruction/terminate_invocation/ssbo_store_before_terminate/1.wgsl
@@ -30,7 +30,7 @@ struct main_out {
 };
 
 @stage(fragment)
-fn main(@builtin(position) x_2_param : vec4<f32>, @location(0) x_3_param : i32) -> main_out {
+fn main(@builtin(position) x_2_param : vec4<f32>, @location(0) @interpolate(flat) x_3_param : i32) -> main_out {
   x_2 = x_2_param;
   x_3 = x_3_param;
   main_1();
diff --git a/test/vk-gl-cts/spirv_assembly/instruction/terminate_invocation/terminate_loop/1.wgsl b/test/vk-gl-cts/spirv_assembly/instruction/terminate_invocation/terminate_loop/1.wgsl
index 78ca88f2f0..573f508fd1 100644
--- a/test/vk-gl-cts/spirv_assembly/instruction/terminate_invocation/terminate_loop/1.wgsl
+++ b/test/vk-gl-cts/spirv_assembly/instruction/terminate_invocation/terminate_loop/1.wgsl
@@ -34,7 +34,7 @@ struct main_out {
 };
 
 @stage(fragment)
-fn main(@builtin(position) x_2_param : vec4<f32>, @location(0) x_3_param : i32) -> main_out {
+fn main(@builtin(position) x_2_param : vec4<f32>, @location(0) @interpolate(flat) x_3_param : i32) -> main_out {
   x_2 = x_2_param;
   x_3 = x_3_param;
   main_1();