diff --git a/packages/runtime/src/__tests__/computer-use-schema-parity.test.ts b/packages/runtime/src/__tests__/computer-use-schema-parity.test.ts
new file mode 100644
index 0000000000..ffb93347f2
--- /dev/null
+++ b/packages/runtime/src/__tests__/computer-use-schema-parity.test.ts
@@ -0,0 +1,152 @@
+import assert from 'node:assert/strict';
+import { describe, test } from 'node:test';
+
+import { computerUseApprovalSummary } from '@maka/core';
+import { computerParams } from '../computer-use-codec.js';
+import { computerWireParams } from '../computer-use-tools.js';
+
+/**
+ * The tool takes arguments through two schemas, and they are written by hand.
+ *
+ * `computerWireParams` is the flat object the SDK validates a model's call
+ * against — one shape covering every action, most fields optional.
+ * `computerParams` is the strict discriminated union the tool narrows to before
+ * it does anything. A field has to exist in both: the first to be accepted off
+ * the wire, the second to survive narrowing.
+ *
+ * `window_action` existed in the union and not on the wire, and was therefore
+ * unreachable from the day it shipped. Nothing failed — the SDK rejected those
+ * calls above the layer the debug journal records, the unit tests exercised the
+ * union directly, and the real-machine probe went around the tool entirely. It
+ * took a model saying it had tried the action, and a trace showing no such call
+ * had ever been made, to find it.
+ *
+ * A sample-driven test cannot catch the next one, because the next one will be
+ * an action nobody thought to sample. Everything here walks a schema.
+ */
+type ZodShape = Record<string, unknown>;
+
+function shapeOf(schema: unknown): ZodShape {
+  const shape = (schema as { shape?: ZodShape }).shape;
+  assert.ok(shape, 'the schema must expose its shape for this check to mean anything');
+  return shape;
+}
+
+function literalValue(node: unknown): string {
+  const literal = node as { value?: unknown; _def?: { value?: unknown } };
+  return String(literal?.value ?? literal?._def?.value);
+}
+
+function enumValues(node: unknown): string[] {
+  const enumerated = node as {
+    options?: unknown[];
+    _def?: { values?: unknown[]; entries?: Record<string, unknown> };
+  };
+  const values =
+    enumerated.options ?? enumerated._def?.values ?? Object.values(enumerated._def?.entries ?? {});
+  assert.ok(Array.isArray(values) && values.length > 0, 'the action field must be an enum');
+  return values.map(String);
+}
+
+function wireFields(): Set<string> {
+  return new Set(Object.keys(shapeOf(computerWireParams)));
+}
+
+function wireActions(): string[] {
+  return enumValues(shapeOf(computerWireParams).action);
+}
+
+function unionArms(): Array<{ action: string; fields: string[] }> {
+  return computerParams.options.map((option) => {
+    const shape = shapeOf(option);
+    return { action: literalValue(shape.action), fields: Object.keys(shape) };
+  });
+}
+
+/**
+ * The same comparison the real checks run, so a negative control exercises the
+ * introspection too. A zod upgrade that made `unionArms` return nothing would
+ * otherwise pass a hand-built control while the real check quietly stopped
+ * looking at anything.
+ */
+function unreachableFields(
+  wire: Set<string>,
+  arms: Array<{ action: string; fields: string[] }>,
+): Array<{ action: string; missing: string[] }> {
+  return arms
+    .map(({ action, fields }) => ({ action, missing: fields.filter((f) => !wire.has(f)) }))
+    .filter(({ missing }) => missing.length > 0);
+}
+
+describe('the two argument schemas describe the same tool', () => {
+  test('every field an action accepts can reach it through the wire', () => {
+    assert.deepEqual(
+      unreachableFields(wireFields(), unionArms()),
+      [],
+      'these fields exist in the union and not on the wire, so a model cannot send them',
+    );
+  });
+
+  test('the action names match in both directions', () => {
+    // Both directions, because each is a different failure. An arm the wire
+    // cannot name is unreachable — the `window_action` case. A wire name with
+    // no arm is accepted off the wire and then falls through narrowing, which
+    // reaches the model as a validation error naming nothing it did wrong.
+    const wire = new Set(wireActions());
+    const arms = new Set(unionArms().map(({ action }) => action));
+
+    assert.deepEqual(
+      [...arms].filter((action) => !wire.has(action)),
+      [],
+      'the union names actions the wire cannot carry',
+    );
+    assert.deepEqual(
+      [...wire].filter((action) => !arms.has(action)),
+      [],
+      'the wire accepts actions the union cannot narrow',
+    );
+  });
+
+  test('every action survives the approval summary as itself', () => {
+    // A third handwritten catalog. `computerUseApprovalSummary` downgrades an
+    // action it does not know to `unknown`, and that value is what lands in the
+    // persisted audit record and what turn-level remember is keyed on. An
+    // action added to both schemas passes the checks above while silently
+    // degrading those consumers, which is the same class of gap one layer over.
+    const degraded = unionArms()
+      .map(({ action }) => ({ action, summarized: computerUseApprovalSummary({ action }).action }))
+      .filter(({ action, summarized }) => summarized !== action);
+
+    assert.deepEqual(
+      degraded,
+      [],
+      'these actions are recorded as "unknown" in the audit trail and cannot be remembered',
+    );
+  });
+
+  test('the check would fail if a field were missing', () => {
+    // A test that cannot fail is not a check. This runs the same comparator the
+    // real check uses, against the shape of the bug it exists to catch.
+    assert.deepEqual(
+      unreachableFields(new Set(['action', 'observation_id']), [
+        { action: 'window_action', fields: ['action', 'observation_id', 'position'] },
+      ]),
+      [{ action: 'window_action', missing: ['position'] }],
+    );
+  });
+
+  test('the introspection reads real schemas, not just the objects it built', () => {
+    // The negative control above builds its own arrays. If `shapeOf` or
+    // `literalValue` stopped extracting anything, that control would still pass
+    // while the real checks compared two empty sets. These assert the readers
+    // return something from the actual schemas.
+    assert.ok(wireFields().size > 5, 'the wire schema has fields');
+    assert.ok(wireActions().length > 5, 'the wire action field is a populated enum');
+    const arms = unionArms();
+    assert.ok(arms.length > 5, 'the union has arms');
+    for (const { action, fields } of arms) {
+      assert.notEqual(action, 'undefined', 'every arm discriminates on a literal action');
+      assert.ok(fields.includes('action'), 'every arm carries the discriminant');
+    }
+  });
+});
diff --git a/packages/runtime/src/computer-use-tools.ts b/packages/runtime/src/computer-use-tools.ts
index 29e5542452..7c78e819db 100644
--- a/packages/runtime/src/computer-use-tools.ts
+++ b/packages/runtime/src/computer-use-tools.ts
@@ -83,7 +83,9 @@ export type {
 // Function-tool JSON schemas require an object at the top level.
 // Keep the wire schema as one top-level object, then apply the strict
 // discriminated union above immediately at execution.
-const computerWireParams = z
+// Exported for the parity check in `computer-use-schema-parity.test.ts`, which
+// is the only thing that can tell this schema and `computerParams` apart.
+export const computerWireParams = z
   .object({
     action: z
       .enum([