diff --git a/compat/fixture.ts b/compat/fixture.ts
index b15f1b0..6fd9d61 100644
--- a/compat/fixture.ts
+++ b/compat/fixture.ts
@@ -7,7 +7,11 @@
 // Why the `expectTypeOf` assertions: a compile that merely succeeds is a weak
 // oracle. An `any`-typed declaration would compile, but `expect-type`'s exact
 // equality and `.not.toBeAny()` reject `any`, so the assertions prove the
-// emitted types are real. See development/ci.md § TypeScript compatibility.
+// emitted types are real. Every handler *parameter* is asserted too, not just
+// the matcher: the handler's member type comes from the `Member` inversion in
+// the declarations, and a wrong or `any` parameter would otherwise slip
+// through, since the matcher's own `(shape: T) => R` type is built from `T` and
+// the handler returns. See development/ci.md § TypeScript compatibility.
 
 import { expectTypeOf } from "expect-type";
 import {
@@ -20,8 +24,16 @@ import {
 type ResultCode = "ok" | "created";
 
 const toStatus = getPrimitiveUnionMatcher<ResultCode>()({
-    ok: () => "OK",
-    created: () => "CREATED",
+    ok: (s) => {
+        expectTypeOf(s).not.toBeAny();
+        expectTypeOf(s).toEqualTypeOf<"ok">();
+        return "OK";
+    },
+    created: (s) => {
+        expectTypeOf(s).not.toBeAny();
+        expectTypeOf(s).toEqualTypeOf<"created">();
+        return "CREATED";
+    },
 });
 
 expectTypeOf(toStatus).not.toBeAny();
@@ -29,7 +41,13 @@ expectTypeOf(toStatus).toEqualTypeOf<(shape: ResultCode) => string>();
 
 // The widening twin keeps each handler's own return type in the union.
 const toStatusW = getPrimitiveUnionMatcherW<ResultCode>()(
-    { ok: () => "OK" as const },
+    {
+        ok: (s) => {
+            expectTypeOf(s).not.toBeAny();
+            expectTypeOf(s).toEqualTypeOf<"ok">();
+            return "OK" as const;
+        },
+    },
     (rest) => {
         expectTypeOf(rest).not.toBeAny();
         expectTypeOf(rest).toEqualTypeOf<"created">();
@@ -63,7 +81,13 @@ expectTypeOf(format).not.toBeAny();
 expectTypeOf(format).toEqualTypeOf<(shape: Contact) => string>();
 
 const formatW = getTaggedUnionMatcherW<Contact>()("kind")(
-    { email: (e) => e.address },
+    {
+        email: (e) => {
+            expectTypeOf(e).not.toBeAny();
+            expectTypeOf(e).toEqualTypeOf<{ kind: "email"; address: string }>();
+            return e.address;
+        },
+    },
     (rest) => {
         expectTypeOf(rest).not.toBeAny();
         expectTypeOf(rest).toEqualTypeOf<{ kind: "phone"; number: string }>();