diff --git a/crates/ail/tests/raw_buf_new_type_arg_pin.rs b/crates/ail/tests/raw_buf_new_type_arg_pin.rs
new file mode 100644
index 0000000..b191183
--- /dev/null
+++ b/crates/ail/tests/raw_buf_new_type_arg_pin.rs
@@ -0,0 +1,114 @@
+//! RED-pin for #51 (raw-buf fieldtest F7): the explicit element
+//! type-arg of `(new RawBuf <elem> <size>)` must be honoured.
+//!
+//! Property protected: the author's `(con Elem)` annotation on a
+//! `(new RawBuf ...)` is the binding declaration of the buffer's
+//! element type — it must NOT depend on a downstream `get`/`set` to
+//! be recovered, and it must be subject to RawBuf's `param-in`
+//! restricted set {Int, Float, Bool} at check time.
+//!
+//! Two legs, both stemming from the same defect: `(new T <type>
+//! <values>)` desugars to `(app T.new <values>)` and DROPS the
+//! leading `NewArg::Type` (crates/ailang-core/src/desugar.rs:1053).
+//!
+//!  - Leg 1 (legitimate, must build+run): `(new RawBuf (con Int) 3)`
+//!    used only via `RawBuf.size`. With the type-arg dropped, the
+//!    result's forall var `a` is never pinned by use, monomorph
+//!    defaults it to Unit (mono.rs free-fn-call arm / subst.rs), and
+//!    codegen mints `RawBuf_new__Unit`, which has no registered
+//!    intercept (intercepts.rs registers only Int/Float/Bool). Today
+//!    `ail build --alloc=rc` aborts with
+//!    `intrinsic fn RawBuf_new__Unit has no registered intercept`.
+//!    Expected post-fix: build succeeds, program prints `3`.
+//!
+//!  - Leg 2 (forbidden, must reject at check): `(new RawBuf (con
+//!    Unit) 3)`. Unit is not in RawBuf's param-in set, but because
+//!    the `(con Unit)` type-arg is dropped before check, the
+//!    `param-in` enforcement in `check_type_well_formed`
+//!    (crates/ailang-check/src/lib.rs:2084) never sees a `RawBuf<Unit>`
+//!    type, so `ail check` wrongly passes (exit 0) and the program
+//!    crashes at `ail build` with the same `RawBuf_new__Unit` error.
+//!    Expected post-fix: `ail check` fails with
+//!    `param-not-in-restricted-set` naming `Unit`; codegen unreached.
+//!
+//! These are CLI-boundary E2E tests because the defect spans the
+//! desugar -> check/codegen pipeline and the symptoms are the exit
+//! behaviour of `ail check` / `ail build` themselves.
+
+use std::path::{Path, PathBuf};
+use std::process::Command;
+
+fn repo_root() -> PathBuf {
+    let manifest_dir = env!("CARGO_MANIFEST_DIR");
+    Path::new(manifest_dir)
+        .parent()
+        .unwrap()
+        .parent()
+        .unwrap()
+        .to_path_buf()
+}
+
+/// Leg 1: a `(new RawBuf (con Int) 3)` buffer used only via
+/// `RawBuf.size` must build under `--alloc=rc` and print its size.
+#[test]
+fn rawbuf_new_int_size_only_builds_and_runs() {
+    let fixture = repo_root().join("examples/raw_buf_new_type_arg_intsize.ail");
+    let tmp = std::env::temp_dir().join(format!(
+        "ailang_rawbuf_new_type_arg_intsize_{}",
+        std::process::id()
+    ));
+    std::fs::create_dir_all(&tmp).unwrap();
+    let out = tmp.join("bin");
+
+    let build = Command::new(env!("CARGO_BIN_EXE_ail"))
+        .args(["build", fixture.to_str().unwrap(), "--alloc=rc", "-o"])
+        .arg(&out)
+        .output()
+        .expect("ail build failed to spawn");
+    assert!(
+        build.status.success(),
+        "ail build --alloc=rc failed for size-only RawBuf<Int>: stderr={}",
+        String::from_utf8_lossy(&build.stderr)
+    );
+
+    let run = Command::new(&out).output().expect("execute binary");
+    assert!(
+        run.status.success(),
+        "binary exited non-zero: {:?}",
+        run.status
+    );
+    let stdout = String::from_utf8_lossy(&run.stdout);
+    assert!(
+        stdout.trim() == "3",
+        "expected the program to print the buffer size `3`, got: {stdout:?}"
+    );
+}
+
+/// Leg 2: a `(new RawBuf (con Unit) 3)` names a forbidden element
+/// type and must be rejected at `ail check` with
+/// `param-not-in-restricted-set` naming `Unit`.
+#[test]
+fn rawbuf_new_unit_element_rejected_at_check() {
+    let fixture = repo_root().join("examples/raw_buf_new_type_arg_unit_reject.ail");
+
+    let out = Command::new(env!("CARGO_BIN_EXE_ail"))
+        .arg("check")
+        .arg(&fixture)
+        .output()
+        .expect("ail check failed to spawn");
+
+    assert!(
+        !out.status.success(),
+        "expected `ail check` to REJECT RawBuf<Unit>, but it passed: stdout={}",
+        String::from_utf8_lossy(&out.stdout)
+    );
+    let stderr = String::from_utf8_lossy(&out.stderr);
+    assert!(
+        stderr.contains("param-not-in-restricted-set"),
+        "expected diagnostic code `param-not-in-restricted-set`, got: {stderr}"
+    );
+    assert!(
+        stderr.contains("Unit"),
+        "expected the diagnostic to name the forbidden element `Unit`, got: {stderr}"
+    );
+}
diff --git a/examples/raw_buf_new_type_arg_intsize.ail b/examples/raw_buf_new_type_arg_intsize.ail
new file mode 100644
index 0000000..3b6177d
--- /dev/null
+++ b/examples/raw_buf_new_type_arg_intsize.ail
@@ -0,0 +1,8 @@
+(module raw_buf_new_type_arg_intsize
+  (fn main
+    (doc "RED-pin fixture for #51 leg 1 (raw-buf F7). A `(new RawBuf (con Int) 3)` buffer used ONLY via `RawBuf.size` — its element type is never observed by a later get/set, so the only carrier of the element type is the author's explicit `(con Int)` type-arg on `new`. Today `(new T <type> <values>)` desugars to `(app T.new <values>)` and DROPS the leading NewArg::Type (crates/ailang-core/src/desugar.rs:1053), so the result's forall var `a` is unpinned and monomorphisation defaults it to Unit (crates/ailang-check/src/mono.rs free-fn-call arm; mirror crates/ailang-codegen/src/subst.rs). Codegen then mints `RawBuf_new__Unit`, which has no registered intercept (only Int/Float/Bool are in crates/ailang-codegen/src/intercepts.rs) and `ail build --alloc=rc` aborts: `intrinsic fn RawBuf_new__Unit has no registered intercept`. Expected post-fix: the explicit `(con Int)` is carried through monomorphisation so the buffer specialises to `RawBuf_new__Int`; build succeeds and the program prints 3 (the size).")
+    (type (fn-type (params) (ret (con Unit)) (effects IO)))
+    (params)
+    (body
+      (let b (new RawBuf (con Int) 3)
+        (app print (app RawBuf.size b))))))
diff --git a/examples/raw_buf_new_type_arg_unit_reject.ail b/examples/raw_buf_new_type_arg_unit_reject.ail
new file mode 100644
index 0000000..53bb792
--- /dev/null
+++ b/examples/raw_buf_new_type_arg_unit_reject.ail
@@ -0,0 +1,8 @@
+(module raw_buf_new_type_arg_unit_reject
+  (fn main
+    (doc "RED-pin fixture for #51 leg 2 (raw-buf F7). A `(new RawBuf (con Unit) 3)` names Unit as the element type — but Unit is NOT in RawBuf's param-in restricted set {Int, Float, Bool}, so this must be REJECTED at `ail check` with `param-not-in-restricted-set` naming Unit. Today the `(new T <type> <values>)` desugar (crates/ailang-core/src/desugar.rs:1053) drops the `NewArg::Type` BEFORE check, so the `(con Unit)` element annotation never reaches the param-in enforcement in check_type_well_formed (crates/ailang-check/src/lib.rs:2084). `ail check` therefore passes (exit 0) and the program only crashes later at `ail build` with the same unregistered `RawBuf_new__Unit` intercept as leg 1. Expected post-fix: a pre-desugar reject on the Term::New shape (per the CLAUDE.md Term::New/desugar lockstep, crates/ailang-check/src/pre_desugar_validation.rs) fails `ail check` with `param-not-in-restricted-set` naming Unit; codegen is never reached.")
+    (type (fn-type (params) (ret (con Unit)) (effects IO)))
+    (params)
+    (body
+      (let b (new RawBuf (con Unit) 3)
+        (app print (app RawBuf.size b))))))