diff --git a/examples/176-build-block-convert.sx b/examples/176-build-block-convert.sx
new file mode 100644
index 0000000..7d774bd
--- /dev/null
+++ b/examples/176-build-block-convert.sx
@@ -0,0 +1,35 @@
+// FFI plan step 5.1 — `build_block_convert(args: []Type, $ret: Type)
+// -> string` emits the per-shape source body for the generic
+// `Into(Block) for Closure(..$args) -> $R` impl that lands in step
+// 5.2. Per-call-shape monomorphisation of the impl body re-runs the
+// builder with concrete types bound, so each closure shape gets its
+// own dedicated `__invoke` trampoline + Block literal.
+//
+// This test exercises the builder directly (no `#insert`, no impl
+// wiring) — three pack shapes through the same `void`-returning
+// wrapper plus one non-void `s32`-returning wrapper to pin the
+// `return typed_fn(...)` branch. The expected output captures the
+// generated source verbatim so any formatting drift surfaces here
+// rather than as a downstream compile error inside the eventual
+// step-5.2 impl.
+
+#import "modules/std.sx";
+#import "modules/std/objc_block.sx";
+
+preview_void :: (..$args) -> string {
+    return build_block_convert($args, void);
+}
+
+preview_s32 :: (..$args) -> string {
+    return build_block_convert($args, s32);
+}
+
+run_all :: () {
+    print("--- void / 0 args ---\n{}\n", preview_void());
+    print("--- void / bool ---\n{}\n", preview_void(true));
+    print("--- void / s64, string ---\n{}\n", preview_void(42, "hi"));
+    print("--- s32 / f64 ---\n{}\n", preview_s32(3.14));
+}
+#run run_all();
+
+main :: () { print("rt\n"); }
diff --git a/tests/expected/176-build-block-convert.exit b/tests/expected/176-build-block-convert.exit
new file mode 100644
index 0000000..573541a
--- /dev/null
+++ b/tests/expected/176-build-block-convert.exit
@@ -0,0 +1 @@
+0
diff --git a/tests/expected/176-build-block-convert.txt b/tests/expected/176-build-block-convert.txt
new file mode 100644
index 0000000..e22ba0f
--- /dev/null
+++ b/tests/expected/176-build-block-convert.txt
@@ -0,0 +1,10 @@
+--- void / 0 args ---
+__invoke :: (block_self: *Block) -> void callconv(.c) { typed_fn : (*void) -> void = xx block_self.sx_fn; typed_fn(block_self.sx_env); } return .{ isa = @_NSConcreteStackBlock, flags = 0, reserved = 0, invoke = xx @__invoke, descriptor = xx @__sx_block_descriptor, sx_env = self.env, sx_fn = self.fn_ptr, };
+--- void / bool ---
+__invoke :: (block_self: *Block, arg0: bool) -> void callconv(.c) { typed_fn : (*void, bool) -> void = xx block_self.sx_fn; typed_fn(block_self.sx_env, arg0); } return .{ isa = @_NSConcreteStackBlock, flags = 0, reserved = 0, invoke = xx @__invoke, descriptor = xx @__sx_block_descriptor, sx_env = self.env, sx_fn = self.fn_ptr, };
+--- void / s64, string ---
+__invoke :: (block_self: *Block, arg0: s64, arg1: string) -> void callconv(.c) { typed_fn : (*void, s64, string) -> void = xx block_self.sx_fn; typed_fn(block_self.sx_env, arg0, arg1); } return .{ isa = @_NSConcreteStackBlock, flags = 0, reserved = 0, invoke = xx @__invoke, descriptor = xx @__sx_block_descriptor, sx_env = self.env, sx_fn = self.fn_ptr, };
+--- s32 / f64 ---
+__invoke :: (block_self: *Block, arg0: f64) -> s32 callconv(.c) { typed_fn : (*void, f64) -> s32 = xx block_self.sx_fn; return typed_fn(block_self.sx_env, arg0); } return .{ isa = @_NSConcreteStackBlock, flags = 0, reserved = 0, invoke = xx @__invoke, descriptor = xx @__sx_block_descriptor, sx_env = self.env, sx_fn = self.fn_ptr, };
+--- build done ---
+rt