diff --git a/Cargo.lock b/Cargo.lock
index 0ce25a785b..3e797f1056 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1503,6 +1503,7 @@ dependencies = [
  "serde",
  "serde_json",
  "serial_test",
+ "static_assertions",
  "tempfile",
  "test-case",
  "thiserror 2.0.18",
diff --git a/src/auth/Cargo.toml b/src/auth/Cargo.toml
index 63e7c37d42..556fe5a815 100644
--- a/src/auth/Cargo.toml
+++ b/src/auth/Cargo.toml
@@ -68,6 +68,7 @@ tokio                 = { workspace = true, features = ["macros", "rt-multi-thre
 tokio-test.workspace  = true
 url.workspace         = true
 mutants.workspace     = true
+static_assertions     = { workspace = true }
 
 [features]
 default = ["default-idtoken-backend", "default-rustls-provider"]
diff --git a/src/auth/src/retry.rs b/src/auth/src/retry.rs
index 6fbd2a5c51..dbc6534d12 100644
--- a/src/auth/src/retry.rs
+++ b/src/auth/src/retry.rs
@@ -23,6 +23,7 @@ use gax::retry_loop_internal::retry_loop;
 use gax::retry_policy::{AlwaysRetry, RetryPolicy, RetryPolicyArg, RetryPolicyExt};
 use gax::retry_throttler::{AdaptiveThrottler, RetryThrottlerArg, SharedRetryThrottler};
 use std::error::Error;
+use std::panic::{RefUnwindSafe, UnwindSafe};
 use std::sync::{Arc, Mutex};
 
 #[derive(Debug)]
@@ -40,6 +41,17 @@ pub(crate) struct Builder {
     retry_throttler: Option<RetryThrottlerArg>,
 }
 
+// This is necessary because adding [Builder] to existing, released auth features
+// caused a breaking change. The containing structs would lose their automatically derived
+// `RefUnwindSafe` implementation because the dynamic trait objects (like `dyn RetryPolicy`)
+// are not `RefUnwindSafe` nor `UnwindSafe` by default.
+//
+// This is safe because in the builder, we never call or use the retry policies. We just hold them
+// until we create the client. There is no opportunity for a panic to leave them in an inconsistent
+// state.
+impl RefUnwindSafe for Builder {}
+impl UnwindSafe for Builder {}
+
 impl Builder {
     pub(crate) fn with_retry_policy(mut self, retry_policy: RetryPolicyArg) -> Self {
         self.retry_policy = Some(retry_policy);
@@ -142,6 +154,7 @@ mod tests {
     use gax::retry_state::RetryState;
     use gax::retry_throttler::RetryThrottler;
     use mockall::{Sequence, mock};
+    use static_assertions::assert_impl_all;
     use std::error::Error;
     use std::sync::Arc;
     use std::sync::atomic::{AtomicBool, Ordering};
@@ -479,4 +492,9 @@ mod tests {
             original_error_string
         );
     }
+
+    #[test]
+    fn test_unwind_safe() {
+        assert_impl_all!(Builder: std::panic::UnwindSafe, std::panic::RefUnwindSafe);
+    }
 }