diff --git a/src/parser.zig b/src/parser.zig
index 7a51ae4..d3df6f1 100644
--- a/src/parser.zig
+++ b/src/parser.zig
@@ -1415,11 +1415,16 @@ pub const Parser = struct {
         // unchanged. The legacy prefix `#foreign` form still works via the
         // `is_foreign` argument; interplay/diagnostics for combining them is Phase 4.
         var is_foreign_eff = is_foreign;
-        if (self.current.tag == .kw_extern) {
-            is_foreign_eff = true;
-            self.advance();
-        } else if (self.current.tag == .kw_export) {
-            is_foreign_eff = false;
+        if (self.current.tag == .kw_extern or self.current.tag == .kw_export) {
+            // Prefix `#foreign` and the postfix `extern`/`export` keyword are two
+            // spellings of the same linkage axis — combining them is contradictory
+            // (`#foreign`=import vs `export`=define) or redundant (`#foreign … extern`).
+            // Reject at the postfix keyword rather than let it silently override.
+            if (is_foreign) {
+                const kw = if (self.current.tag == .kw_export) "export" else "extern";
+                return self.failFmt("conflicting linkage: prefix '#foreign' cannot be combined with postfix '{s}'; use either '#foreign' or postfix 'extern'/'export', not both", .{kw});
+            }
+            is_foreign_eff = self.current.tag == .kw_extern;
             self.advance();
         }