diff --git a/toolchain/sem_ir/id_kind.h b/toolchain/sem_ir/id_kind.h
index 7e079a31cf809..6c31524f2cf4b 100644
--- a/toolchain/sem_ir/id_kind.h
+++ b/toolchain/sem_ir/id_kind.h
@@ -123,12 +123,13 @@ class TypeEnum {
 using IdKind = TypeEnum<
     // From base/value_store.h.
     IntId, RealId, FloatId, StringLiteralValueId,
-    // From sem_ir/id.h.
-    InstId, AbsoluteInstId, ConstantId, EntityNameId, CompileTimeBindIndex,
-    RuntimeParamIndex, FacetTypeId, FunctionId, ClassId, InterfaceId, ImplId,
-    GenericId, SpecificId, ImportIRId, ImportIRInstId, LocId, BoolValue,
-    IntKind, NameId, NameScopeId, InstBlockId, StructTypeFieldsId, TypeId,
-    TypeBlockId, ElementIndex, LibraryNameId, FloatKind>;
+    // From sem_ir/ids.h.
+    InstId, AbsoluteInstId, AnyRawId, ConstantId, EntityNameId,
+    CompileTimeBindIndex, RuntimeParamIndex, FacetTypeId, FunctionId, ClassId,
+    InterfaceId, ImplId, GenericId, SpecificId, ImportIRId, ImportIRInstId,
+    LocId, BoolValue, IntKind, NameId, NameScopeId, InstBlockId,
+    StructTypeFieldsId, TypeId, TypeBlockId, ElementIndex, LibraryNameId,
+    FloatKind>;
 
 }  // namespace Carbon::SemIR
 
diff --git a/toolchain/sem_ir/ids.h b/toolchain/sem_ir/ids.h
index c0e25d15ae2c1..3bfebf5356abc 100644
--- a/toolchain/sem_ir/ids.h
+++ b/toolchain/sem_ir/ids.h
@@ -957,6 +957,25 @@ struct LocId : public IdBase, public Printable<LocId> {
 
 constexpr LocId LocId::Invalid = LocId(Parse::NodeId::Invalid);
 
+// Polymorphic id for `Any[...]` typed instructions. Used for fields where the
+// specific instruction structs have different field types in that position or
+// do not have a field in that position at all. Allows conversion with
+// `Inst::As<>` from the specific typed instruction to the `Any[...]`
+// instruction group.
+//
+// This participates in `Inst::FromRaw` in order to convert from specific
+// instructions, but does not participate in `Inst::ToRaw` as it's not possible
+// to convert in the other direction.
+//
+// - In the case the specific instruction has a field of some `IdKind` in the
+//   same position, the `Any[...]` type will hold its raw value in the
+//   `AnyRawId` field.
+// - In the case the specific instruction has no field in the same position, the
+//   `Any[...]` type will hold a default constructed `AnyRawId`.
+struct AnyRawId {
+  int32_t raw_id = InstId::InvalidIndex;
+};
+
 }  // namespace Carbon::SemIR
 
 #endif  // CARBON_TOOLCHAIN_SEM_IR_IDS_H_
diff --git a/toolchain/sem_ir/inst.h b/toolchain/sem_ir/inst.h
index f2f3554b8979f..e07f06bcdc01d 100644
--- a/toolchain/sem_ir/inst.h
+++ b/toolchain/sem_ir/inst.h
@@ -16,7 +16,6 @@
 #include "toolchain/base/int.h"
 #include "toolchain/base/value_store.h"
 #include "toolchain/sem_ir/block_value_store.h"
-#include "toolchain/sem_ir/builtin_inst_kind.h"
 #include "toolchain/sem_ir/id_kind.h"
 #include "toolchain/sem_ir/inst_kind.h"
 #include "toolchain/sem_ir/typed_insts.h"
@@ -267,12 +266,10 @@ class Inst : public Printable<Inst> {
   // Convert a field to its raw representation, used as `arg0_` / `arg1_`.
   static constexpr auto ToRaw(IdBase base) -> int32_t { return base.index; }
   static constexpr auto ToRaw(IntId id) -> int32_t { return id.AsRaw(); }
-  static constexpr auto ToRaw(BuiltinInstKind kind) -> int32_t {
-    return kind.AsInt();
-  }
 
   // Convert a field from its raw representation.
   template <typename T>
+    requires IdKind::Contains<T>
   static constexpr auto FromRaw(int32_t raw) -> T {
     return T(raw);
   }
@@ -280,10 +277,6 @@ class Inst : public Printable<Inst> {
   constexpr auto FromRaw<IntId>(int32_t raw) -> IntId {
     return IntId::MakeRaw(raw);
   }
-  template <>
-  constexpr auto FromRaw<BuiltinInstKind>(int32_t raw) -> BuiltinInstKind {
-    return BuiltinInstKind::FromInt(raw);
-  }
 
   int32_t kind_;
   TypeId type_id_;
diff --git a/toolchain/sem_ir/typed_insts.h b/toolchain/sem_ir/typed_insts.h
index 73469dd6e0341..59897501735ba 100644
--- a/toolchain/sem_ir/typed_insts.h
+++ b/toolchain/sem_ir/typed_insts.h
@@ -23,8 +23,6 @@
 // - Up to two members describing the contents of the struct. These are types
 //   listed in the `SemIR::IdKind` type-enum, typically derived from `IdBase`.
 //
-// TODO: There are some `int32_t` fields, which breaks the above rule, how/why?
-//
 // The field names here matter -- the fields must have the names specified
 // above, when present. When converting to a `SemIR::Inst`, the `kind` and
 // `type_id` fields will become the kind and type associated with the
@@ -83,7 +81,7 @@ struct AnyFoundationDecl {
   InstKind kind;
   InstId foundation_type_inst_id;
   // Kind-specific data.
-  int32_t arg1;
+  AnyRawId arg1;
 };
 
 // An adapted type declaration in a class, of the form `adapt T;`.
@@ -423,7 +421,7 @@ struct AnyBranch {
   // Branches don't produce a value, so have no type.
   InstBlockId target_id;
   // Kind-specific data.
-  int32_t arg1;
+  AnyRawId arg1;
 };
 
 // Control flow to branch to the target block.