diff --git a/guardrails.suppressions.json b/guardrails.suppressions.json
index 68e4f96..7d466c5 100644
--- a/guardrails.suppressions.json
+++ b/guardrails.suppressions.json
@@ -6,7 +6,7 @@
     "typescript/no-unused-vars": 1,
     "unicorn/no-thenable": 1
   },
-  "tsExpectError": 2,
+  "tsExpectError": 0,
   "tsIgnore": 0,
   "safety": 5,
   "config": {},
diff --git a/knip.config.ts b/knip.config.ts
index 3e248e9..6c1fc5e 100644
--- a/knip.config.ts
+++ b/knip.config.ts
@@ -3,9 +3,15 @@ import { antelopeKnipConfig } from "@antelopejs/tooling-configs/knip";
 /**
  * `ajs module test` runs these compiled suites and builds its test project from
  * `src/antelope.test.ts`. The whole compiled tree ships in the package, and the
- * modules implementing this interface run it as their conformance suite.
+ * modules implementing this interface run it as their conformance suite. The
+ * `*.untyped.spec.js` suites feed values the types forbid, so they stay
+ * untyped JavaScript.
  */
-const conformanceSuites = ["src/tests/**/*.test.ts", "src/antelope.test.ts"];
+const conformanceSuites = [
+  "src/tests/**/*.test.ts",
+  "src/tests/**/*.untyped.spec.js",
+  "src/antelope.test.ts",
+];
 
 /**
  * No import Knip can follow reaches these files: `datasets/index.ts` is the
diff --git a/src/tests/atomic_mutation.test.ts b/src/tests/atomic_mutation.test.ts
index bead285..5f22b67 100644
--- a/src/tests/atomic_mutation.test.ts
+++ b/src/tests/atomic_mutation.test.ts
@@ -99,20 +99,6 @@ function ValidateRevision() {
       expectedRevision: { kind: "missing" },
     }),
   ).not.to.throw();
-  for (const expectedRevision of [
-    null,
-    undefined,
-    "",
-    { kind: "missing", extra: true },
-  ]) {
-    expect(() =>
-      ValidateAtomicMutation("record-a", {
-        ...request,
-        // @ts-expect-error expectedRevision is null, undefined, "" or a tag with an extra key, off-contract values that ValidateAtomicMutation must refuse with a TypeError
-        expectedRevision,
-      }),
-    ).to.throw(TypeError);
-  }
   expect(() =>
     table.atomicMutation("record-a", {
       ...request,
@@ -198,16 +184,6 @@ function ValidateEquality() {
       }),
     ).not.to.throw();
   }
-  for (const expectedValue of [null, undefined, {}, [], NaN, new Date(NaN)]) {
-    expect(() =>
-      ValidateAtomicMutation("record-a", {
-        type: "deleteIfEqual",
-        field: "title",
-        // @ts-expect-error expectedValue is null, undefined, {}, [], NaN or an invalid Date, off-contract values that ValidateAtomicMutation must refuse with a TypeError
-        expectedValue,
-      }),
-    ).to.throw(TypeError);
-  }
 }
 
 type StubbedResult =
diff --git a/src/tests/atomic_mutation.untyped.spec.js b/src/tests/atomic_mutation.untyped.spec.js
new file mode 100644
index 0000000..b98e53f
--- /dev/null
+++ b/src/tests/atomic_mutation.untyped.spec.js
@@ -0,0 +1,38 @@
+const { expect } = require("chai");
+
+const { ValidateAtomicMutation } = require("../index");
+
+const update = {
+  type: "update",
+  revisionField: "revision",
+  expectedRevision: "observed",
+  nextRevision: "fresh",
+  patch: { details: { count: 3 } },
+};
+
+describe("Atomic mutation runtime refusal of off-contract values", () => {
+  it("refuses a null, undefined, empty or extended expected revision", () => {
+    for (const expectedRevision of [
+      null,
+      undefined,
+      "",
+      { kind: "missing", extra: true },
+    ]) {
+      expect(() =>
+        ValidateAtomicMutation("record-a", { ...update, expectedRevision }),
+      ).to.throw(TypeError);
+    }
+  });
+
+  it("refuses a non-scalar or invalid expected value", () => {
+    for (const expectedValue of [null, undefined, {}, [], NaN, new Date(NaN)]) {
+      expect(() =>
+        ValidateAtomicMutation("record-a", {
+          type: "deleteIfEqual",
+          field: "title",
+          expectedValue,
+        }),
+      ).to.throw(TypeError);
+    }
+  });
+});
diff --git a/tsconfig.json b/tsconfig.json
index b9a277d..33a7e92 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -6,6 +6,7 @@
     "types": ["node", "mocha"],
     "esModuleInterop": true,
     "strict": true,
+    "allowJs": true,
     "experimentalDecorators": true,
     "emitDecoratorMetadata": true,
     "declaration": true,