diff --git a/.github/workflows/test.yaml b/.github/workflows/test.yaml
index 7b97b84..38787cc 100644
--- a/.github/workflows/test.yaml
+++ b/.github/workflows/test.yaml
@@ -234,10 +234,10 @@ jobs:
           LSAN_OPTIONS: suppressions=lsan_suppressions.txt
       - name: build and run tests (Mbed TLS)
         if: matrix.tls_backend == 'mbedtls'
-        # Run mbedTLS shards with reduced parallelism — under ASAN+mbedTLS the
-        # default 4 shards overload CI runners enough that timing-sensitive
-        # ServerTest cases flake on first-request keep-alive reuse.
-        run: cd test && make test_split_mbedtls && SHARDS=2 make test_mbedtls_parallel
+        # macOS runners under ASAN+mbedTLS still flake at SHARDS=2 (rapid
+        # bind/connect on the fixture's fixed port races on the slower
+        # macos-latest runner). Serialize fully here; ubuntu stays at 2.
+        run: cd test && make test_split_mbedtls && SHARDS=1 make test_mbedtls_parallel
       - name: build and run tests (wolfSSL)
         if: matrix.tls_backend == 'wolfssl'
         run: cd test && make test_split_wolfssl && make test_wolfssl_parallel