diff --git a/proxy/src/lib.rs b/proxy/src/lib.rs
index 78f0c350a9..e304cfa2b9 100644
--- a/proxy/src/lib.rs
+++ b/proxy/src/lib.rs
@@ -157,12 +157,10 @@ smol_str_wrapper!(BranchId);
 // 90% of project strings are 23 characters or less.
 smol_str_wrapper!(ProjectId);
 
-// ket value neon_option fields
-smol_str_wrapper!(EndpointCacheKeyExtra);
 #[derive(PartialEq, Eq, Hash, Debug, Clone)]
 pub struct EndpointCacheKey {
     pub id: EndpointIdInt,
-    pub extra: EndpointCacheKeyExtra,
+    pub extra: Box<str>,
 }
 impl std::fmt::Display for EndpointCacheKey {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
diff --git a/proxy/src/proxy.rs b/proxy/src/proxy.rs
index 84a0aae652..76f84975ad 100644
--- a/proxy/src/proxy.rs
+++ b/proxy/src/proxy.rs
@@ -11,7 +11,6 @@ pub use copy_bidirectional::copy_bidirectional_client_compute;
 pub use copy_bidirectional::ErrorSource;
 
 use crate::intern::EndpointIdInt;
-use crate::EndpointCacheKeyExtra;
 use crate::{
     auth,
     cancellation::{self, CancellationHandlerMain, CancellationHandlerMainInternal},
@@ -413,14 +412,13 @@ impl NeonOptions {
         }
     }
 
-    pub fn get_cache_key_extras(&self) -> EndpointCacheKeyExtra {
-        // prefix + format!(" {k}:{v}")
-        // kinda jank because SmolStr is immutable
-        self.0
-            .iter()
-            .flat_map(|(k, v)| [" ", &**k, ":", &**v])
-            .collect::<SmolStr>()
-            .into()
+    pub fn get_cache_key_extras(&self) -> Box<str> {
+        let mut extras = String::new();
+        for (k, v) in &self.0 {
+            use std::fmt::Write;
+            write!(&mut extras, " {k}:{v}").unwrap();
+        }
+        extras.into_boxed_str()
     }
 
     /// <https://swagger.io/docs/specification/serialization/> DeepObject format