diff --git a/.buildkite/ci.mjs b/.buildkite/ci.mjs
index 224642f86f29..aba15736e6bf 100755
--- a/.buildkite/ci.mjs
+++ b/.buildkite/ci.mjs
@@ -186,8 +186,9 @@ const testPlatforms = [
   // The darwin test suite runs on real macOS agents against the Linux-built
   // artifacts from the `darwin-<arch>-build-bun` steps (the only darwin build
   // lanes — see buildPlatforms).
-  // All three darwin test lanes run on main and on opt-in only for now (see
-  // darwinTestsEnabled): the mac agent pool cannot keep up with PR volume.
+  // These three version-specific lanes run on main and on opt-in (see
+  // darwinTestsEnabled). PR builds instead get one aarch64 lane that any mac
+  // agent can take (prDarwinTestPlatforms), so the whole arm64 pool serves PRs.
   { os: "darwin", arch: "aarch64", release: "26", tier: "latest" },
   { os: "darwin", arch: "aarch64", release: "14", tier: "previous" },
   { os: "darwin", arch: "x64", release: "14", tier: "latest" },
@@ -409,7 +410,7 @@ function getTestAgent(platform, options) {
       queue: `test-${os}`,
       os,
       arch,
-      ...(arch === "aarch64" ? { "release-tier": tier } : {}),
+      ...(arch === "aarch64" && tier ? { "release-tier": tier } : {}),
     };
   }
 
@@ -1543,11 +1544,16 @@ async function getPipeline(options = {}) {
   // Tests run on main too so the canary release step below can gate on them.
   // ASAN is PR-only (see includeASAN above), so the asan test lane is dropped
   // on main along with its build.
+  // Untiered: any arm64 mac agent, whatever macOS it runs, can take it.
+  /** @type {Platform[]} */
+  const prDarwinTestPlatforms = [{ os: "darwin", arch: "aarch64", release: "any" }];
   const darwinTestsEnabled =
     isMainBranch() || isBuildManual() || /\[(macos|darwin) tests?\]/i.test(getCommitMessage());
   const relevantTestPlatforms = (
     includeASAN ? testPlatforms : testPlatforms.filter(({ profile }) => profile !== "asan")
-  ).filter(({ os }) => os !== "darwin" || darwinTestsEnabled);
+  )
+    .filter(({ os }) => os !== "darwin" || darwinTestsEnabled)
+    .concat(darwinTestsEnabled ? [] : prDarwinTestPlatforms);
   /** @type {string[]} */
   const testStepKeys = [];
   {