diff --git a/esm/sdk/retry.py b/esm/sdk/retry.py
index 302d6cf..3c37c48 100644
--- a/esm/sdk/retry.py
+++ b/esm/sdk/retry.py
@@ -23,8 +23,8 @@ def retry_if_specific_error(exception):
         429,
         500,
         502,
+        503,
         504,
-        500,
     }
 
 
diff --git a/esm/sdk/retry_test.py b/esm/sdk/retry_test.py
new file mode 100644
index 0000000..882a807
--- /dev/null
+++ b/esm/sdk/retry_test.py
@@ -0,0 +1,21 @@
+"""Tests for retry.py"""
+
+from esm.sdk.api import ESMProteinError
+from esm.sdk.retry import retry_if_specific_error
+
+
+def test_retry_if_specific_error_retries_transient_statuses():
+    # Rate-limit and transient server errors are retried. 503 Service
+    # Unavailable must be retried alongside its 500/502/504 siblings.
+    for code in [429, 500, 502, 503, 504]:
+        err = ESMProteinError(error_code=code, error_msg="transient")
+        assert retry_if_specific_error(err), f"HTTP {code} should be retryable"
+
+
+def test_retry_if_specific_error_skips_non_transient_and_non_esm():
+    # A non-transient client error is not retried.
+    assert not retry_if_specific_error(
+        ESMProteinError(error_code=404, error_msg="not found")
+    )
+    # Exceptions that are not ESMProteinError are not retried.
+    assert not retry_if_specific_error(ValueError("boom"))