diff --git a/examples/0544-packs-imported-pack-fn-fixed-param-source-pin.sx b/examples/0544-packs-imported-pack-fn-fixed-param-source-pin.sx
new file mode 100644
index 0000000..ec2385f
--- /dev/null
+++ b/examples/0544-packs-imported-pack-fn-fixed-param-source-pin.sx
@@ -0,0 +1,22 @@
+// Regression (stdlib E4): an imported pack function whose fixed-prefix param
+// type is visible only in its defining module must resolve during pack
+// monomorphization. `lib.sx` imports `dep.sx` (which defines `Needs`) and
+// exposes `make() -> Needs` plus `use_pack(n: Needs, ..$args)`. `main` imports
+// ONLY `lib.sx`, so `Needs` is two flat hops away and not bare-visible here —
+// main never names it.
+//
+// Before the fix, `monomorphizePackFn` restored the caller's source before
+// re-binding the fixed-prefix params, so `n: Needs` was resolved in main's
+// context and rejected with "type 'Needs' is not visible" — even though the
+// control plain fn `use_plain(n: Needs)` (typed via the source-pinned call-arg
+// path) ran fine. The fixed-prefix param is now resolved under the pack fn's own
+// source (`fd.body.source_file`), matching the rest of the pack signature/body.
+#import "modules/std.sx";
+#import "0544-packs-imported-pack-fn-fixed-param-source-pin/lib.sx";
+
+main :: () -> s32 {
+    x := make();
+    print("plain {}\n", use_plain(x));
+    print("pack {}\n", use_pack(x, 1, 2));
+    return 0;
+}
diff --git a/examples/0544-packs-imported-pack-fn-fixed-param-source-pin/dep.sx b/examples/0544-packs-imported-pack-fn-fixed-param-source-pin/dep.sx
new file mode 100644
index 0000000..8cefe6c
--- /dev/null
+++ b/examples/0544-packs-imported-pack-fn-fixed-param-source-pin/dep.sx
@@ -0,0 +1,3 @@
+// `Needs` lives two flat-import hops away from `main` (main -> lib -> dep), so
+// it is NOT bare-visible at the call site under non-transitive visibility.
+Needs :: struct { v: s64; }
diff --git a/examples/0544-packs-imported-pack-fn-fixed-param-source-pin/lib.sx b/examples/0544-packs-imported-pack-fn-fixed-param-source-pin/lib.sx
new file mode 100644
index 0000000..5519bed
--- /dev/null
+++ b/examples/0544-packs-imported-pack-fn-fixed-param-source-pin/lib.sx
@@ -0,0 +1,16 @@
+// `lib.sx` imports `dep.sx`, so `Needs` is bare-visible HERE. A module that
+// imports only `lib.sx` cannot see `Needs` (non-transitive). The pack fn's
+// fixed-prefix param `n: Needs` must therefore resolve in this module's
+// context, not the caller's.
+#import "modules/std.sx";
+#import "dep.sx";
+
+make :: () -> Needs => Needs.{ v = 7 };
+
+// Control: a plain (non-pack) fn with the same fixed param already resolves in
+// its defining module — the cross-module call-arg typing path is source-pinned.
+use_plain :: (n: Needs) -> s64 => n.v;
+
+// Pack fn: the fixed-prefix param `n: Needs` is bound during pack
+// monomorphization. Its type must resolve under this module's source.
+use_pack :: (n: Needs, ..$args) -> s64 => n.v + args[0];
diff --git a/examples/expected/0544-packs-imported-pack-fn-fixed-param-source-pin.exit b/examples/expected/0544-packs-imported-pack-fn-fixed-param-source-pin.exit
new file mode 100644
index 0000000..573541a
--- /dev/null
+++ b/examples/expected/0544-packs-imported-pack-fn-fixed-param-source-pin.exit
@@ -0,0 +1 @@
+0
diff --git a/examples/expected/0544-packs-imported-pack-fn-fixed-param-source-pin.stderr b/examples/expected/0544-packs-imported-pack-fn-fixed-param-source-pin.stderr
new file mode 100644
index 0000000..8b13789
--- /dev/null
+++ b/examples/expected/0544-packs-imported-pack-fn-fixed-param-source-pin.stderr
@@ -0,0 +1 @@
+
diff --git a/examples/expected/0544-packs-imported-pack-fn-fixed-param-source-pin.stdout b/examples/expected/0544-packs-imported-pack-fn-fixed-param-source-pin.stdout
new file mode 100644
index 0000000..07a0be7
--- /dev/null
+++ b/examples/expected/0544-packs-imported-pack-fn-fixed-param-source-pin.stdout
@@ -0,0 +1,2 @@
+plain 7
+pack 8
diff --git a/src/ir/lower.zig b/src/ir/lower.zig
index b4c9890..0e17847 100644
--- a/src/ir/lower.zig
+++ b/src/ir/lower.zig
@@ -11324,8 +11324,12 @@ pub const Lowering = struct {
                 if (!p.is_comptime) {
                     // Contextually type the arg from the param (so a lambda arg
                     // `(x) => …` takes its param types from a `Closure(...)` param).
+                    // The param type is resolved under the pack fn's OWN source
+                    // (E4): a fixed-prefix type bare-visible only in the defining
+                    // module must resolve there, not the caller's. The arg itself
+                    // is lowered AFTER, in the caller's context.
                     const saved_tt = self.target_type;
-                    const pty = self.resolveParamType(&p);
+                    const pty = self.resolveParamTypeInSource(fd.body.source_file, &p);
                     if (pty != .unresolved) self.target_type = pty;
                     args.append(self.alloc, self.lowerExpr(call_node.args[ri])) catch return self.builder.constInt(0, .void);
                     self.target_type = saved_tt;
@@ -11634,7 +11638,12 @@ pub const Lowering = struct {
                 ct_arg_idx += 1;
                 continue;
             }
-            const pty = self.resolveParamType(&p);
+            // Pin to the pack fn's OWN module (E4): a fixed-prefix param whose
+            // type is bare-visible only in the defining module must resolve
+            // there, not in the caller's restored context. Mirrors the
+            // signature build above and `resolveParamTypeInSource` at the
+            // cross-module call-arg typing sites.
+            const pty = self.resolveParamTypeInSource(fd.body.source_file, &p);
             const slot = self.builder.alloca(pty);
             self.builder.store(slot, Ref.fromIndex(param_idx));
             scope.put(p.name, .{ .ref = slot, .ty = pty, .is_alloca = true });