diff --git a/crates/synth-cli/tests/cabi_arena_realloc_linkability_418.rs b/crates/synth-cli/tests/cabi_arena_realloc_linkability_418.rs
new file mode 100644
index 00000000..c6d5afb2
--- /dev/null
+++ b/crates/synth-cli/tests/cabi_arena_realloc_linkability_418.rs
@@ -0,0 +1,90 @@
+//! synth#418 — embedder import `cabi-arena-realloc` binds to the native TCB
+//! arena allocator via synth's relocatable host-link path (#197).
+//!
+//! In the BYO-OS lean-MCU dissolve (gale#89), grow-free components route
+//! `cabi_realloc` to an embedder-provided `env::__cabi_arena_realloc` import
+//! (wit-bindgen `cabi-realloc-extern`). synth operates on the CORE module, so it
+//! sees the verbatim core field name `__cabi_arena_realloc` (the kebab
+//! `cabi-arena-realloc` is a component-surface concern, never reaching synth),
+//! and its `--relocatable` import-call path (#197) lowers the call to a direct
+//! `R_ARM_THM_CALL` relocation against an UNDEFINED `__cabi_arena_realloc`
+//! symbol — which the native link binds to the TCB's arena allocator. synth
+//! references the symbol; the TCB provides it; the linker binds it (the correct
+//! layering — synth never implements the allocator or the realloc contract).
+//!
+//! This locks that mechanism. The full end-to-end check on gale's real dissolved
+//! component (out of the wit-bindgen#4 → wasm-tools#2 → meld#301 pipeline) lands
+//! when that fixture arrives; this proves the synth-layer contract synthetically.
+
+use std::path::PathBuf;
+use std::process::Command;
+
+use object::read::elf::ElfFile32;
+use object::{Object, ObjectSection, ObjectSymbol, RelocationTarget};
+
+fn synth() -> &'static str {
+    env!("CARGO_BIN_EXE_synth")
+}
+
+fn fixture() -> PathBuf {
+    PathBuf::from(env!("CARGO_MANIFEST_DIR"))
+        .join("../..")
+        .join("scripts/repro/cabi_arena_realloc.wat")
+}
+
+#[test]
+fn cabi_arena_realloc_emits_undefined_symbol_and_reloc_418() {
+    let out = "/tmp/cabi_arena_realloc_418.o";
+    let status = Command::new(synth())
+        .args([
+            "compile",
+            fixture().to_str().unwrap(),
+            "--target",
+            "cortex-m4",
+            "--native-pointer-abi",
+            "--all-exports",
+            "--relocatable",
+            "-o",
+            out,
+        ])
+        .output()
+        .expect("run synth");
+    assert!(
+        status.status.success(),
+        "compile failed: {}",
+        String::from_utf8_lossy(&status.stderr)
+    );
+
+    let data = std::fs::read(out).expect("read .o");
+    let obj = ElfFile32::<object::Endianness>::parse(&*data).expect("parse ELF");
+
+    // (1) `__cabi_arena_realloc` is present as an UNDEFINED symbol — left for the
+    // native link / TCB to satisfy, not implemented by synth.
+    let sym = obj
+        .symbols()
+        .find(|s| s.name() == Ok("__cabi_arena_realloc"))
+        .expect("__cabi_arena_realloc symbol present");
+    assert!(
+        sym.is_undefined(),
+        "__cabi_arena_realloc must be UNDEFINED (embedder-bound), not defined in synth's output"
+    );
+
+    // (2) a relocation targets that symbol at the call site — so the native
+    // linker rewrites the `bl` to the TCB's arena allocator.
+    let mut reloc_targets_arena = false;
+    for section in obj.sections() {
+        for (_off, reloc) in section.relocations() {
+            if let RelocationTarget::Symbol(idx) = reloc.target()
+                && let Ok(s) = obj.symbol_by_index(idx)
+                && s.name() == Ok("__cabi_arena_realloc")
+            {
+                reloc_targets_arena = true;
+            }
+        }
+    }
+    assert!(
+        reloc_targets_arena,
+        "a relocation must target __cabi_arena_realloc (the import call site), \
+         so the native link binds it to the TCB allocator"
+    );
+}
diff --git a/scripts/repro/cabi_arena_realloc.wat b/scripts/repro/cabi_arena_realloc.wat
new file mode 100644
index 00000000..3cb790a0
--- /dev/null
+++ b/scripts/repro/cabi_arena_realloc.wat
@@ -0,0 +1,6 @@
+(module
+  (import "env" "__cabi_arena_realloc"
+    (func $arena (param i32 i32 i32 i32) (result i32)))
+  (memory 1)
+  (func (export "do_realloc") (param i32 i32) (result i32)
+    (call $arena (local.get 0) (i32.const 0) (i32.const 8) (local.get 1))))