diff --git a/src/ir/interp.zig b/src/ir/interp.zig
index 9f6b93a..c89b6ca 100644
--- a/src/ir/interp.zig
+++ b/src/ir/interp.zig
@@ -663,6 +663,22 @@ pub const Interpreter = struct {
             },
             .bitcast => |c| {
                 const val = frame.getRef(c.operand);
+                // Loud-fail on `.type_tag → <runtime kind>` casts. A Type
+                // value can flow through bitcast only to .any (Any-boxing)
+                // or to itself; any other destination means the lowering
+                // emitted a coercion that silently pretends the TypeId is
+                // some other shape (e.g. an int, or a string). The most
+                // likely site that would trip this: the `case type:` arm
+                // of `any_to_string` in stdlib doing `xx val to string` —
+                // which expects the value field to already be a string,
+                // a leftover from the pre-`type_tag` era when Type values
+                // were string-shaped.
+                if (val == .type_tag) {
+                    const allowed = c.to == .any or c.to == c.from;
+                    if (!allowed) {
+                        return bailDetail("comptime bitcast: Type value cast to a non-Type runtime kind — most likely a stale `xx val to string` from the pre-type_tag era; use `type_name(val)` instead");
+                    }
+                }
                 return .{ .value = val };
             },
             .int_to_float => |c| {