diff --git a/scripts/lib/ensure-test-servers.mjs b/scripts/lib/ensure-test-servers.mjs
index 6effc7fc..a273c7e8 100644
--- a/scripts/lib/ensure-test-servers.mjs
+++ b/scripts/lib/ensure-test-servers.mjs
@@ -92,9 +92,10 @@ const built = new Set();
  *
  * Unconditional per *process*, not per call: `pack-and-verify.mjs` asks twice
  * (once for the stdio entry, once for the composable one) and one `tsc` pass
- * emits both, so a second call within the same run is a no-op. A fresh process
- * always rebuilds, which is the whole point — that is what makes an edit to
- * `test-servers/src` reach the next smoke.
+ * emits both, so a second call within the same run skips the build. A fresh
+ * process always rebuilds, which is the whole point — that is what makes an
+ * edit to `test-servers/src` reach the next smoke. The *existence* assertion is
+ * not cached, because a later call can name an entry the first one didn't.
  *
  * `log` receives the bare message; the default prefixes it with `label`, which
  * is what a caller owning its own prefixed logger (pack-and-verify's `step`)
@@ -113,16 +114,29 @@ export function ensureTestServers({
   exists = existsSync,
 }) {
   const paths = requires.map((name) => testServerEntryPath(repoRoot, name));
-  if (built.has(repoRoot)) return paths;
+
+  // A cached call can name an entry the first one didn't, so the existence
+  // assertion runs on every call. Skipping it on the cache hit would return a
+  // path to a file tsc never emitted, and the consumer would then fail with
+  // exactly the opaque ERR_MODULE_NOT_FOUND this helper exists to replace.
+  if (built.has(repoRoot)) {
+    assertEmitted(paths, exists, 0);
+    return paths;
+  }
 
   log("building test-servers...");
   const status = run(repoRoot);
+  assertEmitted(paths, exists, status);
+  built.add(repoRoot);
+  return paths;
+}
+
+/** Throw the shared actionable message unless tsc succeeded and emitted each path. */
+function assertEmitted(paths, exists, status) {
   const missing = paths.filter((p) => !exists(p));
   if (status !== 0 || missing.length) {
     throw new Error(buildFailureMessage(missing));
   }
-  built.add(repoRoot);
-  return paths;
 }
 
 /** Test-only: forget which roots this process has built. */
diff --git a/scripts/lib/ensure-test-servers.test.mjs b/scripts/lib/ensure-test-servers.test.mjs
index 5f853e3e..08fb87fd 100644
--- a/scripts/lib/ensure-test-servers.test.mjs
+++ b/scripts/lib/ensure-test-servers.test.mjs
@@ -94,6 +94,18 @@ describe("ensureTestServers", () => {
     assert.deepEqual(paths, [testServerEntryPath(ROOT, "composable")]);
   });
 
+  it("still asserts existence on a cache hit that names a new entry", () => {
+    // The build is cached; the assertion is not. A second caller asking for an
+    // entry the first didn't must not be handed a path tsc never emitted.
+    const absent = testServerEntryPath(ROOT, "composable");
+    const present = (p) => p !== absent;
+    callEnsure({ requires: ["stdio"], present });
+    assert.throws(
+      () => callEnsure({ requires: ["composable"], present }),
+      (e) => e.message.includes(absent),
+    );
+  });
+
   it("builds again for a different repo root", () => {
     const calls = [];
     callEnsure({ calls });
diff --git a/test-servers/tsconfig.json b/test-servers/tsconfig.json
index 6f746dc3..9c5d964c 100644
--- a/test-servers/tsconfig.json
+++ b/test-servers/tsconfig.json
@@ -5,9 +5,14 @@
     "moduleResolution": "NodeNext",
     "outDir": "./build",
     // The script consumers rebuild this project on every run (#2111), so the
-    // up-to-date case has to be cheap. `.tsbuildinfo` lands under `build/`,
-    // which is gitignored.
+    // up-to-date case has to be cheap.
     "incremental": true,
+    // Pinned INSIDE the output dir. The default derives from `rootDir`, landing
+    // it at `test-servers/tsconfig.tsbuildinfo` — beside the config, outside
+    // `build/`. That would survive the `rm -rf test-servers/build` this project
+    // documents as the remedy for a deleted source file, leaving tsc convinced
+    // its outputs are up to date and emitting nothing.
+    "tsBuildInfoFile": "./build/tsconfig.tsbuildinfo",
     "rootDir": "./src",
     "declaration": false,
     "sourceMap": true,