diff --git a/tests/helpers-spawn-fallbacks.test.mjs b/tests/helpers-spawn-fallbacks.test.mjs
new file mode 100644
index 00000000..9b8d3a93
--- /dev/null
+++ b/tests/helpers-spawn-fallbacks.test.mjs
@@ -0,0 +1,116 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import { runScriptWithFixtures } from './helpers.mjs';
+import { runScriptWithFetchMock } from './helpers-fetch-mock.mjs';
+import { runWithGhStub } from './helpers-gh-sandbox.mjs';
+
+// Every sandbox helper normalises the spawnSync result before handing it to a
+// test: `status ?? 1`, `stdout ?? ''`, `stderr ?? ''`. Those fallbacks only
+// run when the child never started, because spawnSync reports a failed spawn
+// as `status: null` with `stdout`/`stderr` undefined rather than throwing.
+// Until now nothing reached them, so the contract they define — a harness
+// failure surfaces as a plain non-zero run with empty output, not as a crash
+// or as `null` leaking into an assertion — was never checked. A regression
+// there would turn "the script under test failed" into "the harness could not
+// run it" without any suite noticing the difference.
+//
+// The spawn is broken by removing PATH from this process's environment, which
+// makes the bare `node` lookup every helper performs fail with ENOENT. PATH is
+// restored immediately afterwards; `node --test` runs the tests within a file
+// sequentially, so no sibling test observes the gap.
+function withoutPath(run) {
+  const saved = process.env.PATH;
+  assert.equal(
+    typeof saved,
+    'string',
+    'this check removes PATH and restores it, so it requires PATH to be set',
+  );
+  delete process.env.PATH;
+  try {
+    return run();
+  } finally {
+    process.env.PATH = saved;
+  }
+}
+
+test('runScriptWithFixtures reports an unwritten readBack path as null', () => {
+  // The documented contract of `readBack`: a path the script deleted or never
+  // wrote comes back as null, which is how write-mode tests tell "no output"
+  // apart from "empty output".
+  const result = runScriptWithFixtures(
+    'validate-awards.mjs',
+    {
+      'data/awards.json': JSON.stringify({
+        verifiedAt: '2026-08-08',
+        awards: [],
+      }),
+    },
+    { readBack: ['data/never-written.json', 'data/awards.json'] },
+  );
+
+  assert.equal(result.files['data/never-written.json'], null);
+  assert.equal(
+    typeof result.files['data/awards.json'],
+    'string',
+    'a fixture that exists must be read back as its contents, not as null',
+  );
+});
+
+test('runScriptWithFixtures degrades to status 1 and empty output when the child cannot spawn', () => {
+  const result = withoutPath(() =>
+    runScriptWithFixtures('validate-awards.mjs', {
+      'data/awards.json': JSON.stringify({
+        verifiedAt: '2026-08-08',
+        awards: [],
+      }),
+    }),
+  );
+
+  assert.equal(result.status, 1);
+  assert.equal(result.stdout, '');
+  assert.equal(result.stderr, '');
+  assert.deepEqual(result.files, {});
+});
+
+test('runScriptWithFetchMock degrades to status 1 and empty output when the child cannot spawn', () => {
+  const result = withoutPath(() =>
+    runScriptWithFetchMock({
+      script: 'validate-awards.mjs',
+      fixtures: {
+        'data/awards.json': JSON.stringify({
+          verifiedAt: '2026-08-08',
+          awards: [],
+        }),
+      },
+      outputs: ['data/awards.json'],
+    }),
+  );
+
+  assert.equal(result.status, 1);
+  assert.equal(result.stdout, '');
+  assert.equal(result.stderr, '');
+  // Outputs are still read back from the sandbox, so a spawn failure is
+  // distinguishable from a run that produced nothing.
+  assert.equal(typeof result.outputs['data/awards.json'], 'string');
+});
+
+test('runWithGhStub prepends its stub directory even when PATH is unset, and degrades to status 1', () => {
+  // With PATH absent the helper builds `<stub dir>:` from its `?? ''`
+  // fallback rather than interpolating "undefined" into the child's PATH —
+  // the stub directory stays first, and the empty tail finds no real `gh`.
+  const result = withoutPath(() =>
+    runWithGhStub({
+      script: 'pr-queue-hygiene.mjs',
+      routes: [{ match: 'pr list', stdout: '[]' }],
+    }),
+  );
+
+  assert.equal(result.status, 1);
+  assert.equal(result.stdout, '');
+  assert.equal(result.stderr, '');
+  assert.deepEqual(
+    result.calls,
+    [],
+    'a child that never started cannot have invoked gh',
+  );
+});