diff --git a/test/gstack-brain-context-load.test.ts b/test/gstack-brain-context-load.test.ts
index b601cd798..5b9337fd8 100644
--- a/test/gstack-brain-context-load.test.ts
+++ b/test/gstack-brain-context-load.test.ts
@@ -363,7 +363,25 @@ gbrain:
     const dir = mkdtempSync(join(tmpdir(), "gstack-bcl-"));
     const binDir = join(dir, "bin");
     mkdirSync(binDir);
-    writeFakeGbrain(binDir);
+    // A SLOW fake, not the shared instant one: on a fast CI runner the
+    // instant fake answered inside even a 1ms budget (observed dur=0ms on
+    // ubicloud) and no SKIP ever printed. Sleeping makes the timeout
+    // deterministic on every machine; --version stays instant so detection
+    // has nothing to wait on.
+    const fakeBin = join(binDir, "gbrain");
+    writeFileSync(
+      fakeBin,
+      `#!/bin/sh
+if [ "$1" = "--version" ]; then
+  echo "gbrain 0.test"
+else
+  sleep 0.3
+  echo "fake gbrain $*"
+fi
+`,
+      "utf-8",
+    );
+    chmodSync(fakeBin, 0o755);
 
     try {
       const env = { ...prependPath(binDir), GSTACK_BRAIN_TIMEOUT_MS: "1" };
diff --git a/test/migrations-v1.65.0.0.test.ts b/test/migrations-v1.65.0.0.test.ts
index 3be29185e..554120513 100644
--- a/test/migrations-v1.65.0.0.test.ts
+++ b/test/migrations-v1.65.0.0.test.ts
@@ -109,6 +109,13 @@ beforeEach(() => {
   pwCache = path.join(tmpHome, 'pw-cache', 'ms-playwright');
   fs.mkdirSync(pwCache, { recursive: true });
   fs.mkdirSync(path.join(tmpHome, '.gstack'), { recursive: true });
+  // The migration gates on `uname -s` = Darwin; on the Linux CI runner the
+  // real uname made every test early-exit with empty output. Shim Darwin so
+  // the Darwin-path tests run everywhere; the non-Darwin test overwrites
+  // this shim with its own Linux uname.
+  fs.writeFileSync(path.join(fakeBinDir, 'uname'), '#!/bin/bash\necho Darwin\n', {
+    mode: 0o755,
+  });
 });
 
 afterEach(() => {