fix: bind hash-path attribution memos to challenge IDs

src/client/tempo/provider.rs

@@ -130,8 +130,11 @@ impl PaymentProvider for TempoProvider {
         // Auto-generate an attribution memo when the server doesn't provide one,
         // so MPP transactions are identifiable on-chain via `TransferWithMemo` events.
         if charge.memo().is_none() {
-            let memo =
-                crate::tempo::attribution::encode(&challenge.realm, self.client_id.as_deref());
+            let memo = crate::tempo::attribution::encode(
+                &challenge.id,
+                &challenge.realm,
+                self.client_id.as_deref(),
+            );
             charge = charge.with_memo(memo);
         }
 
@@ -236,7 +239,8 @@ mod tests {
 
     #[test]
     fn test_auto_generated_memo_is_mpp_memo() {
-        let memo = crate::tempo::attribution::encode("api.example.com", Some("my-app"));
+        let memo =
+            crate::tempo::attribution::encode("challenge-123", "api.example.com", Some("my-app"));
         assert!(crate::tempo::attribution::is_mpp_memo(&memo));
     }
 

src/protocol/methods/tempo/charge.rs

@@ -98,6 +98,7 @@ impl TempoChargeExt for ChargeRequest {
             .as_ref()
             .and_then(|v| v.get("memo"))
             .and_then(|v| v.as_str())
+            .filter(|s| !s.is_empty())
             .map(|s| s.to_string())
     }
 
@@ -119,6 +120,9 @@ fn parse_memo_bytes_in_context(memo: Option<&str>, context: &str) -> Result<Opti
     let Some(memo) = memo else {
         return Ok(None);
     };
+    if memo.is_empty() {
+        return Ok(None);
+    }
 
     let hex_str = memo.strip_prefix("0x").unwrap_or(memo);
     let bytes = hex::decode(hex_str)
@@ -218,6 +222,20 @@ mod tests {
             req_with_memo.memo(),
             Some("0x1234567890abcdef1234567890abcdef1234567890abcdef1234567890abcdef".to_string())
         );
+
+        let req_with_empty_memo = ChargeRequest {
+            method_details: Some(serde_json::json!({
+                "chainId": 42431,
+                "memo": ""
+            })),
+            ..test_charge_request()
+        };
+        assert!(req_with_empty_memo.memo().is_none());
+    }
+
+    #[test]
+    fn test_parse_memo_bytes_checked_treats_empty_as_none() {
+        assert!(parse_memo_bytes_checked(Some("")).unwrap().is_none());
     }
 
     #[test]